Vol. 22, No.2, August 2011A Bulletin of the Indian Laser Association

Special Issue on

Interaction Meet on Utilization of Laser Technology in Industry & Medicine at RRCAT

President

Prof. P. K. Gupta (RRCAT, Indore)

Vice President

Prof. S. K. Sarkar (BARC, Mumbai)

Gen. Sec. I

Prof. V. P. M. Pillai

Univ. of Kerala, Thiruvananthapuram

Gen. Sec. II

Prof. K.S. Bindra (RRCAT, Indore)

Treasurer

Mr. P. Saxena (RRCAT, Indore)

Regional Representatives

Dr. S. K. Bhadra,

CGCRI, Kolkata

Prof. M. P. Kothiyal,

IIT Madras, Chennai

Prof. D. Narayana Rao,

University of Hyderabad, Hyderabad

Prof. Hema Ramachandran,

Raman Research Institute, Bangalore

Dr. A. K. Razdan,

Laser Science & Technology Centre, Delhi

Editor

Prof. Manoranjan P. Singh

Editorial Board

Prof. A. K. Gupta (SCTIMST,

Thiruvananthapuram)

Dr. A. K. Maini (LASTEC, New Delhi)

Prof. S. Maiti (TIFR, Mumbai)

Prof. S. C. Mehendale (RRCAT, Indore)

Prof. V. P. N. Nampoori (CUSAT, Kochi)

Prof. B. P. Pal (IIT, Delhi)

Prof. Reji Phillip (RRI, Bangalore)

Prof. Asima Pradhan (IIT, Kanpur)

Prof. B. P. Singh (IIT, Bombay)

Prof. B. M. Suri (BARC, Mumbai)

Prof. C. Vijayan (IIT, Madras)

Editorial Committee (RRCAT, Indore)

Dr. C.P. Paul Dr. Pankaj Misra

Mr. H.S. Patel Dr. S. Verma

Dr. G.J. Singh Dr. B.N. Upadhyay

Dr. C.P. Singh

Guest Editor

Shri R. Kaul (RRCAT, Indore)

Dr. S. Sendhil Raja (RRCAT, Indore)

(RRCAT,Indore)

ILA Executive Committee Editorial Team of

Cover Photo :

Image (top left) of Laser Rapid Manufacturing (LRM) System at LMPD, RRCAT and porous structure (top right), made by cross thin wall strategy using LRM (details on page 13). Image in middle shows the laser cutting process designed and developed at SSLD, RRCAT (details on page 3). Image of Laser Surf-Check instrument (left bottom) developed at LPTD, BARC, Mumbai (details on page 24) and the laser line triangulation setup (right bottom) developed at LBAID, RRCAT, Indore (details on page 28).

A Bulletin of the Indian Laser Association

Contents

Vol. 22, No. 2, August 2011

Page No.

From The Editor 1

From The Guest Editors 2

1. High Power Nd:YAG Lasers in Indian Nuclear Power Plants 3B. N. Upadhyaya, S. C. Vishwakarma, R. Arya and S. M. Oak

2. Laser Rapid Manufacturing of Engineering Components 13C P Paul, P Bhargava, S K Mishra, C H Premsingh and L M Kukreja

3. Metallurgical Characterization of Laser Fabricated Structures of Engineering Alloys 18P Ganesh, Rakesh Kaul, Harish Kumar, C H Premsingh, S K Mishra and L M Kukreja

4. Laser Based Instruments for Measurement Applications 24Aseem Singh Rawat

5. Laser based Instrumentation 28Ishant Dave, Rohan Bhandare, Brijesh Pant, Sendhil Raja and P K Gupta

6. Application of Laser Processing of Materials for High Temperature 33Molten Chloride EnvironmentA. Ravi Shankar, Ravikumar Sole, Jagdeesh Sure and U. Kamachi Mudali

Report

7. Interaction Meet on Utilization of Laser Technology in 38Industry & Medicine at RRCAT

Announcement

8. DAE-BRNS National Laser Symposium 39

1

I feel honoured to be editor of Kiran. It has served very well in bringing

together the Indian laser community. I thank all my predecessors for their

efforts. I am sure that with active contributions from its members/readers

Kiran will attain greater heights in future.

The immense progress in laser technology in recent times has witnessed a

rapid increase in the usages of lasers in all spheres of science and

technology. In order to translate this to the well being of common man a

meaningful collaboration between the scientists/researchers and those in

the industry and Medicine is imperative. As an attempt towards this and

also to mark fifty glorious years of lasers an interaction meet on

Utilization of Lasers in Industry and Medicine was organized by Indian

Laser Association on 28th and 29th April 2011 at Raja Ramanna Centre

for Advanced Technology (RRCAT), Indore. This issue of Kiran is based

on the lectures and poster presentations during the meeting. I am thankful

to my colleagues Shri. Rakesh Kaul and Dr. Sendhil Raja for agreeing to

be the guest editors for this issue.

I look forward to receiving articles and your suggestions for further

improvement.

Best warm regards,

Manoranjan P. Singh

From the Editor....

2

Lasers have come a along way since their inception in 1960 to mature into

a reliable tool for many industrial and medical applications. The outgoing

year, being the golden jubilee year of laser invention, assumed significant

importance in the history of science and technology. In order to celebrate

50 years of invention of laser, Indian Laser Association organized a two-

day interaction meet on Utilization of Lasers in Industry and Medicine on

28th and 29th April 2011 at Raja Ramanna Centre for Advanced

Technology (RRCAT), Indore. The interaction meet was organized with

an objective to provide a platform to showcase indigenous laser

technologies developed for industrial and medical applications in major

academic and research institutions of the country and to promote closer

interaction between academic/research institutions of the country and

Indian industry. About 30 participants from 25 different companies

attended the meet. Some of the notable participants were Tata Motors Ltd.,

Bharat Heavy Electricals Ltd., Larsen & Toubro Ltd. The two day event

witnessed informative presentations, interactive technology showcase

sessions and lively group discussion sessions.

In this special issue of Kiran we are presenting selected articles based on

presentations made in the interaction meet. The selected articles cover the

application of laser in the demanding Indian nuclear industry, rapid

manufacturing of engineering components, surface treatment of plasma

sprayed thermal barrier coating for improved performance in high

temperature molten salt environment and laser based instrumentation.

We hope that you will find this issue of Kiran both interesting and useful.

Rakesh Kaul & Sendhil Raja S

From the Guest Editors....

3

radiation is within the pump band of Nd:YAG and rest goes as heat. Out of this 10% of lamp radiation is absorbed by Nd:YAG rod, only about 5-6% is emitted as laser output, the rest heats the Nd:YAG rod. In diode pumped Nd:YAG lasers, it is normally pumped at 808 nm by diode source and emission occurs at 1064 nm leading to thermal effect mainly due to quantum defect of ~ 24%. Thus, it is necessary to effectively cool the pump cavity and rod to remove the heat load. Closed loop water is circulated through the pump cavity to remove the heat load from lamp, rod and reflector. As the circulating water is in contact with the rod surface, under steady state conditions the rod center is at a higher temperature as compared to rod surface and due to temperature gradient from rod center to rod surface, the rod acts as a thermal lens. The dioptric power of pumped Nd:YAG rod increases linearly with pump power and hence acts as a

3focusing element of variable refractive power . Thus, the parameters of passive resonator are modified by the active lens element and the resonator has to be designed taking into consideration of variable lens element. Thus, it is challenging to design a resonator with single or multi-rod configuration for a long range of pump operation and to develop high power Nd:YAG lasers with a good beam quality for its fiber optic beam delivery. Then, it becomes convenient and easy to carry out material processing applications with fiber optic beam delivery and a small material processing head.

The industrial Nd:YAG laser activity of Solid State Laser Division (SSLD), Raja Ramanna Centre for Advanced Technology (RRCAT) has developed different types of highly efficient lamp pumped Nd:YAG lasers. These lasers have been extensively used to carry out various material processing applications in Indian nuclear power plants. In co-ordination with various units of Department of Atomic Energy (DAE) and Nuclear Power Corporation of India Limited (NPCIL), RRCAT explored the possibility to carry out various material processing tasks with high power Nd:YAG lasers at various nuclear power plant sites. Initially, different material processing techniques were established with high power Nd:YAG laser and based on feedback, a robust industrial laser system of 250 W average power, 2-20 ms pulse duration and 1-200 Hz repetition rate having 5 kW peak power and 100 J maximum pulse energy with time-shared multi-port optical beam delivery system was developed. This laser system is remotely operable and has been engineered for

Abstract

High power Nd:YAG lasers with fiber optic beam delivery have tremendous potential in material processing applications in the field of nuclear energy due to non-contact nature of the process, low secondary waste generation, and remote operation with flexible beam delivery through optical fibers. Robust high power Nd:YAG lasers and innovative laser material processing techniques developed at RRCAT have been successfully utilized in Indian nuclear power plants on industrial scale to enormously reduce MANREM consumption, time and cost. It has also been brought out that high power Nd:YAG lasers have potential applications in new reactor installations and in maintenance operation of running nuclear plants.

Introduction

Lamp pumped and diode pumped Nd:YAG lasers with fiber optic beam delivery have been exploited commercially for various material processing applications such as cutting, welding, drilling, etc. in

1-3harsh environments . In order to enhance quality and range of material processing applications, it is necessary to deliver the beam through an optical fiber with core diameter and numerical aperture as low as possible. Thus, to cope up with the need of material processing applications, higher and higher power Nd:YAG lasers with improved beam quality are being developed. The basic configuration of a lamp pumped Nd:YAG laser consists of a pump cavity containing a flash lamp and an Nd:YAG rod within a gold coated elliptical reflector or a close coupled diffuse reflector and an optical resonator. Similarly, the configuration of a diode pumped Nd:YAG laser consists of a Nd:YAG rod and diode pump source for end or side pumping. These lamp pumped and diode pumped systems suffer from low beam quality due to thermal lensing and stress induced birefringence. The composite effect of thermal lensing and birefringence is to limit the fundamental mode spot size within the rod and hence the beam quality. The main effort is towards reduction of thermal lensing and stress induced birefringence to improve the beam quality or alternatively to go for birefringence compensation. About 50% of the electrical input supplied to flash lamp goes as heat and the rest 50% of the electrical input is emitted as optical radiation. As the flash lamp emits in a broad spectrum, only about 9-10% of the emitted

High Power Nd:YAG Lasers in Indian Nuclear Power Plants

B.N. Upadhyaya*, S.C. Vishwakarma, R. Arya and S.M. Oak Solid State Laser Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013

*E-mail : bnand@rrcat.gov.in

4

Application of High power Nd:YAG lasers in Nuclear power plants

High power Nd:YAG lasers with fiber optic beam delivery have been utilized for various material processing applications in nuclear power plants for maintenance operations. Some of the important massive applications and related developments are as described as below:

its robustness with proper fixtures and toolings for various material processing operations on industrial scale related to nuclear field. This system is pumped with 5 kW input electrical power and provides an electrical to laser conversion efficiency of about 5%, which is the highest as compared to any commercially available lamp pumped

4Nd:YAG laser . This fiber coupled Nd:YAG laser system has four time-shared fiber ports, each of them has a fiber

having 600 mm (400 m optional) core diameter, 0.2 NA and 150 m length. Specially designed material processing nozzles of diameter in the range 13 mm to 25 mm with gas flow through the same tube containing optical fiber were developed for applications having space restrictions in nuclear power installations. Using this, cutting of stainless steel sheets up to 14 mm and welding up to depth 2 mm were established. Now, it has been scaled to 500 W average power with 2-40 ms duration and 1-100 Hz rep.

rate with 10 kW peak power and 400 mm fiber optic beam delivery for laser cutting of up to one inch SS and weld

5depths in SS up to 5 mm . A lab model of 1 kW average power and 20 kW peak power Nd:YAG laser has also been developed. Development of industrial model of 1 kW average power Nd:YAG laser with fiber optic beam delivery and its application in deep penetration welding and concrete cutting is under progress. High power lamp pumped CW Nd:YAG laser has also been developed with an output power of 880 W having 4.4% electrical to laser

6conversion efficiency . CW Nd:YAG lasers with kW level power scaling using multi-cavity design and modulation is useful in deep penetration keyhole welding and laser rapid manufacturing. Development of compact high power CW fiber lasers with all fiber integration and compact footprint has also been taken up. In initial efforts, development of 120 W single transverse mode CW fiber laser with an optical-to-optical efficiency of 75% has been carried. Its power scaling to achieve kW level will be highly useful in nuclear maintenance operations. Fig.1, 2, & 3 shows a typical view of industrial Nd:YAG lasers developed at SSLD, RRCAT. About 20 systems of industrial Nd:YAG laser have been commissioned in different DAE units for various material processing applications.

m

Fig. 1: 250 W average power and 5 kW peak power industrial Nd:YAG laser with time-shared fiber-optic beam delivery.

Fig. 2: Industrial Nd:YAG laser with 500 W average power and 10kW peak power.

Fig. 3: Industrial Nd:YAG laser with 1 kW average power and 20 kW peak power.

Fig. 4: Simplified flow diagram of PHWR with Calendria and coolant channels.

5

designed material processing nozzles of diameter 1/2 inch with gas flow through the same tube containing optical fiber were developed for applications having space restrictions in nuclear power installations. Using this system, laser cutting of 612 bellow lips during EMCCR of NAPS-1, NAPS-2 and KAPS-1 reactors has been performed successfully in May 2006, Nov. 2008 and Feb. 2009. A miniature fiber coupled laser cutting head with 1/2” diameter is mounted on the fixture in such a way that it takes care of position tolerance of bellow lip and diameter of coolant channel. It is desired to separate the bellow rings in such a way that the outer ring can be reused for welding at the time of re-commissioning. This required grooving of the ring at weld location up to a depth of ~ 4 mm. It is easy to cut through and through using laser beam while it is very difficult to make grooves in a material. The laser grooving technique for carbon steel was established specially for this purpose.

Two industrial Nd:YAG lasers with four port time shared fiber optic beam delivery and 150m long fiber optic cable were deployed for cutting of bellow lip, one on each north and south vaults of 220MW reactors and in-situ bellow lip cutting was performed and separation was ensured for all the 612 bellow lips in each reactor. The fixing of tool on any of the coolant channels requires about one minute and the cutting process takes ten minutes for each bellow lip, and total operation was completed within a few days of laser operation. Laser cutting of 18 Nos. of shock absorber yoke studs having a diameter of 16 mm was also performed during EMCCR activity to access bellow lip weld for laser cutting. These studs were jammed and could not be opened by any mechanical means. This resulted in a large MANREM saving as compared to conventional technique and also time saving of at least six months with enormous cost saving. Fig.6 shows the fixture mounted on a coolant channel performing the cutting process in mock-setup. Fig.7 shows the laser cut and separated bellow lip. Fig. 8 shows fixture mounted on end face of coolant channel for cutting of HPFC stud in mock up and fig. 9 shows laser cutting of HPFC stud. Fig.10 (a) & (b) shows the fixture mounted on E-face of one of the coolant channels in NAPS-1 and NAPS-2 reactors respectively. Fig. 10(c) shows laser cut shock absorber studs from NAPS-2 and Fig. 11 shows welded bellow lip. The same fixture was utilized for re-welding of bellow lip during re-installation of coolant channels. This fixture is able to hold laser welding nozzle as well as TIG welding torch.

Prior to bellow lip weld cutting, it is necessary to remove obstruction of all the 612 shock absorber yoke assembly and its 1224 studs. In previous EMCCR campaign at NAPS-1, it was found that in serious attempts to open a few jammed shock absorber studs, shock absorber yoke

Development of laser cutting technique and in-situ laser cutting of bellow lips during en-masse coolant channel replacement (EMCCR) campaign at NAPS-1, NAPS-2 and KAPS-1 reactors

For an introduction of nuclear applications of Nd:YAG lasers, we first look at the design of Pressurized Heavy Water Reactors (PHWRs). It is characterized by natural uranium fuel, heavy water as moderator, pressure tube containment of primary coolant, fuel bundles and ON POWER refueling. Each Reactor has typically 306 coolant channels, which are mounted horizontally within a horizontal cylindrical vessel, called Calandria and surrounded by low pressure, low temperature heavy water moderator. Fig.4 shows a simplified flow diagram of PHWR together with side-view of Calendria & coolant channels. A single coolant channel is a composite structure of end fitting, liner tube and a pressure tube. These pressure tubes, which contains fuel bundles, is made up of Zr-2 or Zr-2.5% Nb alloy and is attached with SS-403 liner tube and end fitting by means of rolled joints. Further, each end fitting is connected to a coolant pipe (feeder) by hub joint with a seal ring and high pressure feeder coupling (HPFC) studs. Annular space around the coolant channel is sealed by a metallic bellow and CO gas is circulated in it (see Fig.5). It is essential to 2

replace the pressure tubes in PHWR type of nuclear reactors after a life of 10-15 years and this replacement is performed during EMCCR campaign of such reactors. This is a complicated process due to space restrictions and high MANREM involvement. The 306 coolant channels placed in a matrix are very close to each other and bounded to the core of the reactor by means of two shrink fit welded bellow attachment rings, made up of carbon steel, one on each face of reactor core located at a distance of about 945 mm from E-face of end fittings i.e., from end point of coolant channel. These coolant channels can be replaced, if the welded bellow rings are detached at the welding point on each end. This requires grooving at the welding point up to the depth of welding (~3-4 mm) and then pulling the channel. Although, single point mechanical cutters can be utilized for this operation, but these mechanical cutters are bulky, require their frequent replacement and take long time to cut, which results in higher MANREM involvement.

The mechanism for laser cutting of bellow lip developed at RRCAT consists of a motorized circumferential rotary arrangement, which can be mounted on the E-face of coolant channel and can be fixed on it just by tightening

7of a single bolt . The tool is designed to fit on E-face of end fitting using bore of the end fitting. The locking of this tool is based on tapered ball locking grip at sealing plug position of end fitting. Tightening of a box nut of size M32x2.5 can lock the fixture at E-face. Specially

6

Fig. 5: A sketch of bellow lip cutting fixture mounted on E-face of coolant channel.

Fig. 6: Bellow lip cutting mock-up.

Fig. 7: Separated bellow lip.

Fig. 8: Fixture mounted on coolant channel for

laser cutting of HPFC studs.

Fig. 9: Laser cutting mock-up of HPFC studs.

Fig. 10(a) & (b): A site view of laser based bellow lip cutting in NAPS-1 & NAPS-2 reactors.

(b)

(a)

©

Fig. 10(c): A site view of laser cut shock absorber studs from NAPS-2 reactor.

7

miniature shielded cutting nozzle has also been developed which has a collimating and focusing lens

Oalong with a 45 bending mirror and a window to protect optics from damage during cutting operation. The cutting fixture consists of mounting, locking & holding arrangement for the laser cutting nozzle along with a motorized traversal arrangement for linear cutting of the nut. The whole mechanism has been miniaturized in size to accommodate all the components only in a total width of 27 mm, which can be easily mounted on position on the shock absorber assembly. The locking mechanism has been made to work on cam arrangement, which generates forces in a triangular frame. The cam is slightly shifted from the position of the triangular plane; due to this shift, the force generated by the cam works as a moment force, which is responsible for locking the fixture rigidly in three dimensions with the shock absorber assembly. In order to prevent yoke sock absorber stud damage, the motion of cutting nozzle has been limited by means of limit switches fitted on both of the J-hooks. Creep adjustment of H10 coolant channel at RAPS-3 was being carried out from north vault since last three biennial shutdowns due to jamming of shock absorber rear nuts, whereas creep adjustment of all the other coolant channels was being carried out from south vault. This was resulting in large MANREM consumption in creep adjustment process. The jammed nuts were cut at two locations from the same side in such a way that a piece of nut can be removed across stud diameter. Both the up and down rear nuts of H10 channel were cut successfully and creep of H10 could be adjusted from the same side after cutting of jammed nuts. Fig. 12(a), (b), (c) and (d) show the developed laser cutting fixture, a mock up of fixture mounted on yoke assembly of stud and a view of laser cut samples of rear nut.

Laser cutting of FBTR spent fuel fuel subassembly

Laser cutting for dismantling of highly radioactive fuel subassemblies of FBTR was succesfully carried out in hot cell at IGCAR, Kalpakkam using in-house build fiber coupled industrial Nd:YAG laser (250 W average power and 5 kW peak power). The Pu-U carbide fuel rods have undergone a burn-up of 154 GWd/t and had a radiation

7 level of 10 rad/hour. This fuel assembly was precisely cut at a gap of 5 mm from the position of the fuel pins for Post–Irradiation Examination (PIE) of burnt fuel with total cutting time of the subassembly ~ 2 minutes, cutting

assembly studs were broken from end shield. This required a lot of effort to make threads at proper location in end shield during re-commissioning process of reactor coolant channels. To avoid this difficulty, laser cutting technique and fixture for 18mm diameter shock absorber yoke assembly studs was also developed at KAPS site (in a similar fashion as at NAPS-2) to cut these studs near the bellow lip weld joint to remove obstruction for bellow lip weld cutting and also to avoid damage of stud threads in end shield. Out of 1224 studs, a total of 78 were found jammed and using laser technique, these studs were cut successfully and safely near the bellow lip weld joint during EMCCR campaign at KAPS-1. Now, the laser based cutting technique for EMCCR of PHWRs has been matured and can be exploited in future EMCCR campaigns.

Laser cutting of shock absorber rear nut at RAPS-3 reactor for creep adjustment

A laser based cutting technique for shock absorber rear nuts in PHWRs has also been developed. This technique has been successfully used for in-situ laser cutting of H10 rear nuts at RAPS-3 reactor in Sept. 2008. The technique consists of a motorized compact fixture, which holds a fiber optic beam delivery cutting nozzle and can be

8operated remotely .

The laser cutting system consists of our home-build 250 W average power fiber coupled industrial Nd:YAG laser having multi-port time shared fiber optic beam delivery and a specially developed remotely operable compact laser cutting fixture. The laser beam was delivered

through a 400 mm optical fiber to achieve larger depth of focus and to have high power density for grooving up to a depth of 13 mm (equal to the thickness of nut). A

Fig.11: Laser welded bellow lip.

Fig. 12: (a) Rear nut cutting fixture, (b) Mock up of fixture mounted on yoke assembly, (c) & (d) Axial and transverse view of cut sample.

(a) (b) (c) (d)

8

at IGCAR since past three years for PIE data. Fig. 13 (a), (b), and (c) show cutting of hexagonal FBTR spent fuel during mock up, cut sample and actual cutting in hot cell.

This laser was also deployed at NFC to extract fuel from rejected fuel pins of PHWRs and fuel from about 65 tons of storage was extracted within a period of one year.

Laser welding of High dose rate brachytherapy assembly

Treatment of cancer by using radiation emitted from the radio-isotopes is in practice for decades. Teletherapy and Brachytherapy are widely used for this purpose. In Teletherapy, the cancerous volume is irradiated by gamma rays emitted by radio-isotopes. Brachytherapy is one of the most efficient ways of treating cancers such as localized uterus cancer and cancers of the head and neck. Brachy is from a Greek word for "short", hence, Brachytherapy approximately means short distance therapy. This is essentially a supplementary radiotherapy, where a radioactive source is placed inside or next to the area requiring treatment. High Dose Rate (HDR) Brachytherapy is a common brachytherapy method used for treatment of a large number of cancer patients. Applicators in the form of catheters are arranged on the patient. A high dose rate source (often Iridium- 192) is then driven along the catheters on the end of a wire by a machine while the patient is isolated in a room. The source remains in a preplanned position for a preset time to allow controlled doses of radiation to be delivered to the cancerous tissues, without damaging the healthy tissues. The capsules that hold the radioactive 'seed' are only a few millimetres long, and about a millimeter in diameter and have a wall thickness of less than 150µm.

The welds that join the capsules together (five weld joints) need to produce a hermetic seal, with a smooth weld bead. Presently, hospitals in India engaged in providing Brachytherapy, use imported HDR source

9speed ~120 mm/minute, cut width of 400 m . Compared to the conventional mechanical methods there are several advantages in laser cutting like: it is fast, does not lead to contamination and secondary waste generation, does not create shape deformation and stress on surface, which is important for measuring swelling, cracks, and stress of a burnt subassembly at different locations.

The laser beam was delivered through a 400 m optical fiber with a focused spot size of 400 m on the job to minimize waste generation. The laser system has a dual port time shared fiber optic beam delivery, with one fiber port for optimization of cutting process outside the hot cell, and another for cutting inside the hot cell. A compact, shielded cutting nozzle assembly of 20 mm outer diameter containing beam delivery fiber and coaxial flow of the assist gas was specially developed for insertion through the S-bend in hot cells. As the fuel subassemblies were hexagonal in shape and sodium (the liquid coolant in the FBTR) was stuck on the inner wall of the fuel with some swelling after a huge burn-up, a loaded roller was attached to the nozzle to maintain the beam focus position. Due to the presence of highly active sodium, cutting was carried out with nitrogen as an assist

2gas at a pressure of 8 kg/cm . Now, this technique of laser cutting of FBTR spent fuel subassembly is in regular use

Fig. 13(a): Cutting of hexagonal FBTR spent fuel bundle.

Fig. 13(b): Cut sample.

Fig. 13(c): Cutting of FBTR spent fuel bundle in hot cell.

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which includes cutting of 4 mm thick liner tube and 12 mm thick end fitting made up of SS. Total cutting time was 12 min. This was cut to generate data on Zr-2.5%Nb pressure tubes used for the first time in KAPS-2 reactor after a life of 8 years.

KAPS-2 is the first reactor in which Zr-2.5%Nb pressure tubes were used and it was required to generate data on these kind of pressure tubes. It was decided to take out one of the pressure tubes after about eight years of reactor operation. To extract pressure tube, it was required to cut liner tube and end fitting from inside due to space restrictions. This cutting was performed remotely by laser cutting fixture specially designed with several innovative ideas. The coolant tube cutting fixture mechanism consisted of two disks of Alluminium, one of them gets attached at E-face and the other disk is inserted inside end fitting through a dual rod handle which comes out from two diametrically opposite holes in the first disk and holds the two disks together and can also fix the

11separation of the two disks . Fig.15 shows the developed fixture.

assembly, which consists of radioactive material, miniature housing with cover and metallic wire ropes. BARC with the help of RRCAT has developed indigenous HDR source assembly for BRIT. The quality of the indigenously developed HDR source assembly is matching with the imported ones, and will be considerably less expensive. As compared to other welding techniques, laser welding is advantageous in terms of heat affected zone(HAZ), pointed and localized heating with better bead quality. A typical HDR source assembly, has four miniature SS micro machined components viz.; machine end terminal, rope joining sleeve, source retaining capsule and cover & two SS wire ropes (dia. 0.91 and 0.73 mm). There are five laser welded joints between SS wire ropes and miniature components. Laser welding of miniature components has

10been performed without any damage . A laser welding

system with 200 mm fiber optic beam delivery and required arrangement has been developed at RRCAT, which has been commissioned at BRIT for regular production of HDR brachytherapy assembly. Fig. 14(a) and (b) show the brachytherapy assembly and laser welded sample of this assembly.

Development of laser cutting technique for in-situ cutting of a single coolant channel at KAPS-2 reactor

Laser cutting technique along with fixture was developed and deployed successfully in Jan. 2005 for in-situ cutting of single coolant channel S-7 from inside of the channel,

Fig. 14 (a): Brachytherapy Assembly having HDR Source.

Fig. 14 (b): Welded brachytherapy assembly.

Fig. 15: Coolant channel cutting fixture.

Fig. 16: Coolant channel cutting mock-up.

Machine End Terminal SS Wire Rope (dia 0.91 mm) SS Wire Rope (dia 0.73 mm)

Cover

Ir-132 Source

LW2LW3 LW5

LW4LW1

Rope Joining Sleeve

Source Retaining Capsule

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nozzle consists of a rotating disk (gear) supported by ball bearings on a vertical bracket (plate). It is having a central hole of 110mm diameter through which the tube is brought into the position by an existing ram. The tube is located and gripped by a V-block based pneumatically actuated two-piece gripper cum locator. The gripper is mounted on the backside of stationary bracket the cutting head is mounted on a compact (rectangular piston) cylinder. Rotary encoder and proximity switches monitor the position of locating cylinders, cutting head cylinders and rotary disk. The actual cutting process has been performed by initial longitudinal cutting through an axial pulling of the tube up to a length of 2.7 m, then pulling was stopped, and cutting head started rotation in circumferential direction.

Laser cutting of fuel bundle end plates

Dismantling of spent fuel bundles from PHWRs by cutting end plates is also being carried out regularly in hot cell at BARC by using laser from RRCAT. Figure 19 (a) shows a view of laser cutting of end plate of fuel bundle in hot cell. Fig. 19 (b) & (c) shows intact fuel bundle and dismantled fuel bundle.

There is a third long screw, which passes through the first disk and is attached to the second disk. Tightening of this third screw pushes movement of a button out of the disk diameter and helps in locking this disk with the inner diameter of the end fitting and the whole fixture. The motion of nozzle for circumferential cutting from inside of the tubes has been motorized by means of a DC motor and a geared coupling of fixture with the motor. Tool fixing time was about one minute and total cutting time was four minutes for liner tube and ten minutes for end fitting with enormous MANREM, time and cost savings. Fig.16 shows a mock-up of coolant channel cutting.

Development of laser cutting technique for easy storage of pressure tubes removed from reactors during EMCCR

Laser cutting technique was developed and successfully deployed for cutting of 7 pressure tubes made up of zircaloy material, which were removed from MAPS-1

12reactor . These tubes of 5m length were cut in two pieces to establish laser cutting technique for reduction in storage space.

Pressure tubes in PHWR's are about 5m in length and are highly radioactive. After EMCCR operations, these tubes are stored as such and require a large space. For initial study, a laser based cutting fixture was designed and deployed for cutting of seven pressure tubes removed from MAPS-1 in two halves to reduce storage space and found to be very useful in reducing storage space. This will be further deployed in mass cutting of pressure tubes by slotting the pressure tube linearly in three pieces using three nozzles simultaneously at 120 with each other and then cutting it circumferentially after a certain length. Fig.17 shows pressure tube cutting fixture and cutting mock up and Fig.18 shows a cut samples from pressure tube.

In this case job was fixed and laser beam was moved circumferentially. Rotation arrangement for laser cutting

Fig. 17: Pressure tube cutting fixture and mock-up.

Fig. 18: Cut sample from pressure tube.

Fig. 19: (a) A view of laser cutting of end plate of PHWR fuel bundle in hot cell, (b) intact fuel bundle, (c) dismantled fuel bundle using laser.

11

Laser cutting technique and fixture for these steam generator tubes made up of Inconel-80 from inside of the tube was developed and deployed successfully in Jan. 2009 for cutting of one of the SG tubes at SG-3 location in NAPS-2 reactor at a distance of 783 mm from the base of the tube. The base of this SG tube is welded on a base in man hole of steam generator assembly. The critical issue in cutting of SG tube was to cut blindly from inside at the desired location and that the nearby tubes should not damage. This laser cutting technique was found to be an easy technique and can be utilized in future for such leaky SG tube cutting purposes in PHWRs. Fig. 20 shows miniature cutting nozzle inserted through a dummy SG tube and fig. 21 shows laser cut sample.

Underwater cutting with laser

Underwater laser cutting and welding has many applications in nuclear facilities and is a promising technique for maintenance/dismantling operations as well as for collecting sample pieces for post-irradiation

13-14examination . In the field of nuclear decommissioning also, underwater cutting of nuclear facilities is desirable. For such operations, it is highly useful to deliver the laser beam through optical fiber because of its flexibility. During dry laser cutting process, a high-power laser beam is focused on the job so that the material reaches its melting temperature and a high-pressure active or inert gas is used to remove the molten material. During this process, a considerable amount of energy is conducted into the work piece resulting in changes in the material properties and the microstructure of the material leading to large heat affected zone (HAZ). In addition, debris and metal vapour from the cut kerf is spread in air. In cutting of irradiated material, debris and metal vapour creates airborne activity, which may be harmful for people working nearby, whereas, underwater cutting is advantageous in terms of a narrow HAZ adjacent to the laser cut surface providing better samples for the analysis of irradiated material with minimum thermal damage and effective reduction in debris spread in air.

There are several requirements from NPCIL to cut nuclear components underwater in water pool at a depth of about 8-10 m. In Dhruva reactor also, it is required to cut Aluminium racks of 3 mm thick. In this regard laser cutting technique using fiber optic beam delivery has been developed for cutting of SS up to a thickness of 12 mm and aluminium of thickness 4 mm. Development of fixture for underwater cutting is under progress. Fig. 22 shows a view of undercutting mock-up of zircaloy.

Conclusion

In conclusion, in-house built industrial Nd:YAG lasers were deployed successfully for refurbishing and maintenance operation of nuclear power plants on

Laser cutting of steam generator (SG) tube at NAPS-2 reactor

Steam generator tube assembly consists of a large number of SG tube matrix made up of Inconel-80 with inner diameter 14 mm, 1.5 mm thickness and nearby tubes are separated by a gap of 6 mm only. It is required to cut and extract leaky SG tubes for analysis purposes.

Fig. 20: Miniature cutting head inserted through SG tube.

Fig. 21: Cut sample from SG tube.

Fig. 22: Underwater cutting mock-up for 4.2 mm thick zircaloy.

12

8. S. C. Vishwakarma, R. K. Jain, B. N. Upadhyaya, Ambar Choubey, D. K. Agrawal, S. M. Oak, “Development of In-situ Laser based Cutting Technique for Shock Absorber Rear Nut in Pressurized Heavy Water Reactors”, DAE-BRNS National Laser Symposium 2007, MSU, Vadodara.

9. Ambar Choubey, D. K. Agrawal, S. C. Vishwakarma, B. N. Upadhyaya, Sabir Ali, R. K. Jain, S. K. Sah, R. Arya, Jojo Joseph, and K. V. Kashivishwanathan, S. M. Oak, “Laser cutting of Fast Breeder Test Reactor fuel subassembly in hot cell”. DAE-BRNS National Laser Symposium 2007, M. S. University of Baroda, Vadodara, Gujrat, India, Dec. 17-20, 2007, pp. 59.

10. B. N. Upadhyaya, M. K. Mishra, S. C. Vishwakarma,R. K. Jain, A. Choubey, D. K. Agrawal, D. N. Badodkar, Manjit Singh, K. V. S. Sastry, B. N. Patil, and S. M. Oak, “Laser micro-welding of Brachytherapy assembly having high dose rate source” DAE-BRNS National Laser Symposium (NLS-08), Jan. 7-10, 2009, LASTEC, New Delhi, India, SA7-022, pp. 75-76.

11. B. N. Upadhyaya, S. C. Vishwakarma, R. K. Jain, Ambar Choubey, Pankaj Gupta, S. K. Chadda, and T. P. S. Nathan, “Development of In-situ Laser Cutting Technique for Coolant Channels in Pressurised Heavy Water Reactors”, DAE-BRNS National Laser Symposium-2005, Vellore, Dec. 7-10, 2005, pp. 61.

12. B. N. Upadhyaya, S. C. Vishwakarma, P. Gaure, R. K. Jain, Amber Choubey, G. Mundra, Pankaj Gupta, S. K. Chadda and T. P. S. Nathan, “Development of Laser based Cutting Machine for Easy Storage of Irradiated PHWR Pressure Tubes”, DAE-BRNS National Laser symposium-04, BARC, Mumbai, Jan.10-13, 2005, pp. 258-261.

13. A. Kruusing, Underwater and water-assisted laser processing: Part 1—General features, steam cleaning and shock processing. Optics and Lasers in Engineering, 41(2), 307-327 (2004).

14. R. K. Jain, D. K. Agrawal, S. C. Vishwakarma, A. K. Choubey, B. N. Upadhyaya, and S. M. Oak, "Development of underwater laser cutting technique for steel and zircaloy for nuclear applications", PRAMANA Journal of Physics, 75 (6), 1253-1258 (2010).

industrial scale along with medical applications and there is a lot of scope for development of various laser systems and laser based processes which can be utilized as an advanced technique to save MANREM, time and cost for Indian nuclear power plants in future.

References

1. T. Ishide, O. Matumoto, Y. Nagura, and T. Nagashima, “Optical transmission of 2 kW CW YAG laser and its practical applications to welding”, SPIE 1277, 1990, pp.188-198.

2. W. Koechner, Solid state laser engineering, 5th ed. Berlin, Springer,1999.

3. Y. Shimokusu, S. Fukumoto, M. Nayama, T. Ishide, S. Tsubota, A. Matsunawa, and S. Katayama, "Application of 7 kW class high power yttrium–aluminum–garnet laser welding to stainless steel tanks", J. Laser Applications, 14(2), 68-72 (2002).

4. B. N. Upadhyaya, S. C. Vishwakarma, A. Choubey, R. K. Jain, Sabir Ali, D. K. Agrawal, A. K. Nath, “A highly efficient 5 kW peak power Nd:YAG laser with time-shared fiber optic beam delivery”, Optics and Laser Technology, 40(2), 337-342 (2008).

5. Ambar Choubey, S. C. Vishwakarma, B. N. Upadhyaya, R. Arya, R. K. Jain, Sabir Ali,D. K. Agrawal, S. M. Oak, “Development of 500W average power fiber coupled pulsed Nd: YAG Laser”, DAE-BRNS National Laser Symposium (NLS-09), BARC, Mumbai, India, Jan. 13-16, 2010, CP-01-11, pp. 46-47.

6. B. N. Upadhyaya, S. C. Vishwakarma, Sabir Ali, V. Bhawsar, S. K. Sah, R. Arya, D. K. Agrawal, R. K. Jain, A. Choubey, S. M. Oak, “Development of an Efficient 880 W CW Nd:YAG Laser”, DAE-BRNS National Laser Symposium 2007, M. S. University of Baroda, Vadodara, Dec. 17-20, 2007, pp. 52.

7. S. C. Vishwakarma, R. K. Jain, B. N. Upadhyaya, Ambar Choubey, D. K. Agrawal, Pankaj Gupta, S. K. Chadda, and T. P. S. Nathan, “Development of Laser based System for Cutting of Bellow lip during En-masse Coolant Channel Replacement in PHWR type of Nuclear Reactors”, DAE-BRNS National Laser Symposium-2005, Vellore, India, Dec. 7-10, 2005, pp. 61.

13

Laser Rapid Manufacturing System consists of high

power laser, integrated with a beam delivery system,

powder feeder and a 5-axis CNC workstation in a

optional controlled atmospheric chamber. A defocused

laser beam of desired diameter (0.1 - 3 mm) is used for

metal deposition at fabrication point. The metallic

powder is fed into the molten pool using either one or

both powder feeder through a co-axial powder-feeding

nozzle. Argon gas is used as shielding and powder carrier

gas. The fabrication point is moved as per the required

shape using standard numerical codes to fabricate the

component. Table I summarizes some typical examples

of industrial applications of LRM and Figure 2 shows

some of the complex shaped components laser rapid

manufactured in our laboratory.

Recent advances in high speed computers, computer

aided design (CAD), laser technologies and

layered/additive manufacturing techniques have led to

the next generation fabrication methodology, involving

''feature-based design and manufacturing''. This

fabrication procedure has been termed as Laser Rapid

Manufacturing (LRM). In this, a fully functional near-net

three dimensional (3D) object can be fabricated directly

from its CAD model by adding metallic materials (in the

form of powder) into the design domain through

sequential deposition tracks. Each track is deposited by

simultaneous laser melting and rapid solidification of fed

metallic powder on a thin wetted layer of the moving

substrate/pre-deposit surface in a predetermined shape

and dimensions. Additive nature as well as special

attributes mainly resulted from unique laser beam

characteristics has made LRM a potential candidate for

various applications such as rapid prototyping, cladding,

and parts repair especially for prime components. Small

heat affected zone (HAZ), minimal dilution, direct

deposition, and integration of CAD tools with the

production process are some of the main features of LRM

that can eliminate many manufacturing steps compared

to conventional methods, and also overcome the

limitations of existing metal manufacturing technologies

in terms of as materials-machine planning, man-machine

interaction, intermittent quality checks, reduction of

production time, enhancement of thermal controllability,

and production of functionally graded parts

(heterogeneous structures) [1]. Figure 1 illustrates one of

the LRM systems at our laboratory.

Laser Rapid Manufacturing of Engineering Components

C.P. Paul*, P. Bhargava, S.K. Mishra, C.H. Premsingh and L.M. KukrejaLaser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013

*E-mail: paulcp@rrcat.gov.in

Fig. 1: Photograph of recently commissioned Laser Rapid Manufacturing System.

Table I. Examples of Industrial applications of LRM

14

deposits, slow cooling was a prerequisite to obtain crack-

free bushes. It was achieved by placing base plate,

undergoing laser deposition, in a special sand bath

maintained at an elevated temperature of 673 K. The sand

bath consisted of an electrically heated copper plate

buried in the sand. Temperature of the bath was measured

and automatically controlled with the help of a

temperature controller. Laser rapid manufacturing of

cylindrical Colmonoy-6 bushes, with dimensions of 20

mm outer diameter, 2.5 mm wall thickness and 40 mm

length, involved depositing circular clad tracks one over

the other. Figure 3(a) shows the photograph of LRM of

Colmonoy-6 bush in progress. After LRM, the fabricated

parts were left buried in the sand bath for more than 8

hours to achieve slow rate of cooling. The resultant

Colmonoy-6 bushes were found to be crack free and their

mechanical properties were at par with the

conventionally processed bushes. The measured

dimensional tolerance using three-point method was 0.2 -

0.5 mm, while surface roughness (R ) was in the range of a

25 - 40 µm [2].

Colmonoy-6 Bushes

”Nickel-based alloys “Colmonoy , due to their

outstanding wear resistance, high hardness at elevated

temperatures and low induced radioactivity, find

applications for hardfacing of austenitic stainless steel

components of nuclear power plants. In the event of

complicated component geometry providing limited

access for hardfacing, pre-fabricated Colmonoy-6 bushes

can be used in place of local hardfacing. Conventionally,

these bushes are made by casting/weld deposition

followed by machining. However, high capital cost for

the low volume of fabrication makes it a prohibitive

option. These customized Colmonoy-6 bushes were

prepared at our laboratory by LRM as an alternative to

conventional processing. Laser rapid manufacturing of

Colmonoy-6 bushes was carried out on a sandblasted 12

mm thick plate of type 316L stainless steel as base. In

view of poor cracking resistance of Colmonoy-6

,

Fig. 2: Laser rapid manufactured (a) simple cage of Inconel-625and (b) and multiple-vane impeller of type 316L stainless steel.

(a)

(b)

Fig. 3: (a) Laser Rapid Manufacturing of Colmonoy-6 bush and (b) bushes after final machining.

(a)

(b)

15

detrimental Therefore, the selection of laser processing

parameters and material's composition play a critical role

in LRM of WC-Co. Laser rapid manufactured WC-Co

under optimized parameters were found to be free from

bulk defects such as micro-cracks, intermetallic phases

and inclusions etc. Figure 4 (a) shows the microstructure

of laser rapid manufactured WC-Co deposit showing

uniform dispersion of un-melted WC particles in Co-

matrix. The micro-hardness in the laser clad zone (1250

– 1700 HV at 1000g load) was found to be comparable to

that of conventional WC - Co specimens [3]. Laser rapid

manufacturing with optimized parameters, was

subsequently used for the fabrication of low cost tools.

Figure 4(b) shows a typical laser rapid manufactured

multi-point cutting tool. Such fabricated tools were used

for cutting of type 316 stainless steel and the cut quality

produced with these tools was found to be at par with

associated tool life of more than 80%.

Porous Structures

Until recent past, porosity was considered as one of the

harmful defects that impeded efficiency or functional

properties of the manufactured products, limiting its

applications to non-load bearing components. However,

if the porous materials could be produced with adequate

mechanical strength, they would find direct applications

as lightweight structures, functional materials,

transportation materials etc. This encouraged us to

undertake research towards the development of porous

structures with adequate mechanical strength. Laser rapid

manufacturing, being a layer-by-layer additive

manufacturing technique, has a unique capability to

selectively deposit materials at the desired points. The

loci of these desired points have been termed by us as

“LRM strategy”. Different LRM strategies can be used to

fabricate same material with different porosity contents

or the materials with same porosity content but different

mechanical properties. Various strategies are being

investigated for LRM of porous materials including

cross thin wall fabrication method, recursive ball

deposition method etc. In cross thin wall fabrication

method, clad tracks in each layer are deposited in a

direction orthogonal to its preceding layer.

Figure 5 shows optical macrographs of representative

porosities on three different cross-sections viz. plane

normal to scanning direction (X-axis), plane normal to

transverse traverse direction (Y-axis) and plane normal to

build-up direction (Z-axis) of laser rapid manufactured

structure of Inconel-625 under different deposition

.

,

,

Low-cost Cemented Carbide Tools

In the realm of the hard materials, cemented carbide

(WC-Co) is a popular choice for tools, dies and wear

prone parts that find wide applications in machining,

mining, metal cutting, metal forming, construction etc.

There is a need to develop a low cost repair technology or

new fabrication techniques for WC cutting tools because

of high cost and increasing demand of tungsten carbide

(WC) powder. In WC-Co system, the bulk hardness is

governed by WC particles, while the toughness and

strength of these materials can be tuned by adding an

adequate amount of Co. Absorption of laser radiation by

WC particles is about 1.4 times stronger than that in Co

for 1.064 µm wavelength. As a result of excessive

heating and partial melting of WC particles, WC phase

may undergo dissociation causing carbon deficiency in

WC-Co composite and precipitation of carbon as

graphite. This graphite reacts with atmospheric oxygen to

form CO and CO , which often appear as gas porosity 2

whereas availability of free carbon in the matrix leads to

formation of a brittle ternary eutectic phase of W, Co and

C; often referred as “eta” phase. Formation of both,

graphite and eta-phase in WC-Co composite, is

Fig. 4: (a) Back scattered electron image of the microstructure of WC-Co deposition, and (b) laser rapid manufactured multi-point cutting tool.

(a)

(b)

16

micrograph of cross-section corresponding to region

OA shows clearly visible pores with nearly circular

tracks. On the other hand, the macrostructure

corresponding to region AB shows elongated pores and

tracks due to compression. The pores are compressed

and almost filled due to material flow. This flow of the

material gives rise to plateau regime in the stress strain

curve. The slope and length of the curve in plateau region

depends on the rate of densification of the material, which

is primarily governed by the dynamics of compression of

pores and walls in a correlated manner within the porous

structure. The optical macrograph corresponding to

region BC shows almost completely compressed

structure. The neighboring tracks are compressed

together with completely deformed fine pores.

,

,

conditions. As seen in this figure, resultant laser rapid

manufactured specimens have pores, arranged in the

form of regular arrays. The location of these pores is at

the junctions of adjacent tracks and adjoining layers,

specifically at the track overlap region. The size of the

pores is not uniform at various locations within the same

sample and it can be seen that the average bulk porosity

increases due to increase in the pore size. The shape and

size of the pores are different on three different planes,

indicating that the resultant porous structures will have

anisotropy in mechanical properties. The shape and size

of the pores on the planes normal to X and Y axes are

nearly the same. Therefore, it is expected that the

mechanical properties along these two axes are largely

similar.

Figures 6 (a) and (b) present photograph of laser rapid

manufactured porous structure, made by cross thin wall

strategy and typical engineering stress-strain curve

obtained during compressive testing of the same,

respectively. The initial part of the curve (OA) involves

sharp increase in stress with small compressive strain.

This is a region of elastic deformation with small amount

of plastic deformation. The associated plastic

deformation in this region is responsible for mechanical

damping. After the initial sharp increase in stress, there is

a change over to a regime of plastic deformation

predominantly associated with closure of porosity where

small increase in stress is accompanied by larger

compressive strain (AB). After extended plateau regime,

the slope of the curve (BC) increased which is indicative

of densification of material in the previous regime (AB).

At this stage, the porosity is negligible with neighboring

tracks completely touching each other. The optical

,

,

Fig. 5: Optical macrographs of representative porosities on three different cross-sections of laser rapid manufactured structure of Inconel-625.

Fig. 6: (a) Laser rapid manufactured Inconel-625 porous structure and (b) typical engineering stress-strain curve obtained during compression testing of laser rapid manufactured porous structure.

17

References

1. J Lawrence, J Pou, D K Low, E Toyserkani (Eds.),

Advances in Laser Materials Processing

Technology, Research and Applications, CRC Press

and Woodhead Publishing Ltd, Cambridge, UK,

First Edition. (2010).

2. C P Paul, Amit Jain, P Ganesh , J Negi and A K Nath,

Laser Rapid Manufacturing of Colmonoy

Components, Laser and Optics in Engineering, 44,

1096-1109,(2006).

3. C P Paul, H Alemohammad, E Toyserkani, A

Khajepour, S Corbin, Cladding of WC-12Co on low

carbon steel using a pulsed Nd:YAG laser. Material

Science and Engineering-A, 464, 170-176, (2007).

4. C P Paul, S K Mishra, C H Premsingh, P Bhargava, P

Tiwari and L M Kukreja, Parametric investigations

on laser rapid manufactured porous structures of

Inconel-625 using cross-thin-wall fabrication

strategy, Int. J. Adv. Mfg. Technol. (under review).

The value of compressive yield stress is 226 MPa along

the scanning and transverse traverse directions, while it is

254 MPa along the build-up direction for laser rapid

manufactured specimens of around 12% porosity. This

difference in value is due to the LRM strategy adopted in

the present experiments. The reported tensile yield

strength of the conventionally processed Inconel-625

was in the range of 414 - 758 MPa, 414 - 655 MPa and 290

- 414 MPa in as rolled, annealed and solution treated

conditions respectively [4].

Conclusions

Our recent research presented above shows a glimpse of

the small section of the vast domain of potential

application of LRM. The results of studies demonstrated

that LRM can be adopted as an alternative fabrication

method to fabricate functional metal parts and

components. Mechanical properties of laser

processed/fabricated components are reported to be at par

or in some cases even better than their wrought

counterparts. Repair of worn metal components like

turbine blades and shafts etc may lead to large economic

incentives to various industries like power generation,

aeronautics and chemical processing etc.

Acknowledgements

The authors thankfully acknowledge the technical

support of members of Laser Materials Processing

Division, RRCAT for carrying out laser processing

experiments and characterizations.

18

manufactured structures of Inconel-625 (IN-625) and SS

316L. Different stages involved in fabrication of compact

tension (CT) test specimens are shown in Fig. 1. Fatigue

crack growth rate (FCGR) tests were conducted on the 12

and 25 mm thick Compact Tension (CT) specimens, as

per ASTM E647 standard. Subsequent to FCGR testing,

the same specimens were used for evaluation of fracture

toughness, as per ASTM E1820 standard [1].

The results of FCGR obtained in the present study were

compared with the reported data for corresponding

wrought materials. Laser rapid manufactured specimens

of IN-625 and SS 316L exhibited steady state crack

growth, referred as stage II crack growth, in the

investigated stress intensity range of 14-38 MPaÖm for

IN- 625 and 11.8-24 MPaÖm for SS 316L. Fatigue crack

growth rates for laser fabricated IN-625 were found to be

lower than the reported values in the DK range of 14-24

MPaÖm and above this range they tended to coincide as

seen in Fig. 2(a). On the other hand, FCGR in laser rapid

fabricated specimens of SS 316L were quite close to the

Abstract

Laser based manufacturing is an emerging fabrication

methodology with many unique features due to low heat

input, minimal distortion and capability to fabricate near-

net shape three dimensional (3D) components. By

adopting suitable processing methodologies and

controlling the laser processing parameters, complex

structural components (either monolithic or multi-

material) can be fabricated. The article presents the

results of the studies carried out at laser materials

processing division, RRCAT on the metallurgical

characterization of laser fabricated Inconel 625 and

bimetallic structures involving Stellite-21 and Type

316L. Laser clad composite joints of Stellite-21 and SS

316L fabricated with and without compositional grading

at joint interface were characterized to study the

influence of grading on the fracture behavior of clad joint.

Results of tensile and instrumented impact tests are

discussed in light of the compositional grading at the

substrate-clad interface.

Mechanical Properties of Laser Rapid Manufactured

Structures of Inconel 625 and Type 316L SS

A nickel based super-alloy Inconel-625 (IN-625),

because of its high temperature oxidation resistance,

mechanical strength and wide use in high temperature

applications, was used for fabrication of specimens by

LRM for evaluation of mechanical properties like tensile,

impact, fatigue and fracture toughness. An in-house

developed high power CO laser, integrated with beam 2

delivery system and CNC work station was used for

specimen fabrication. For industrial acceptability of any

new fabrication process (laser based fabrication in the

present case), it is essential to generate the structural

integrity qualification data. Present study was undertaken

to characterize the critical mechanical properties like

fatigue crack growth rate, fracture toughness and impact

toughness of laser rapid manufactured structures. Fatigue

and fracture toughness tests were performed with

compact tension (CT) and single edge notched bend

(SENB) specimens, extracted from laser rapid

Metallurgical Characterization of Laser Fabricated Structures of Engineering Alloys

P. Ganesh*, Rakesh Kaul, Harish Kumar, C.H. Premsingh, S.K. Mishra and L.M. KukrejaLaser Materials Processing Division, Raja Ramanna Centre for Advanced Technology, Indore - 452 013

*E-mail: ganesh@rrcat.gov.in

Fig. 1. Different stages involved in LRM of compact tension test specimens: (a) initial SS block with V-groove; (b) filling of groove by LRM; (c) machined CT specimen extracted from laser deposited SS block [1].

19

extensive crack branching. The crack plane followed a

tortuous path due to the layered deposition and associated

rastering pattern involved in laser fabrication process.

Instrumented Charpy impact testing of IN-625

specimens, exhibited impact energy of 46.5 - 49 J while a

post deposition annealing treatment at 1223 K brought

about 10 % improvement in impact energy. Gradual fall

in load after the peak load in load-displacement plots is

representative of ductile nature of crack propagation in

these specimens. Reported Charpy impact energy (for

keyhole specimens) of IN-625 in the as rolled condition

was in the range of 65-70 J [5].

Laser Rapid Manufacturing (LRM) of Bimetallic wall

and Tubular Bush

A bimetallic wall and a tubular bush were fabricated by

LRM with an in-house developed CW CO laser, coupled 2

with a CNC work-station. The bimetallic wall comprised

of SS 316L on one side and Stellite-21 (St-21) on the

other side, whereas the tubular structure consisted of St-

21 on the inner side and SS 316L on the outer. Chemical

composition of powders used is presented in Table I.

Table I: Chemical composition (weight %)

of powders used for LRM

Figure 3 presents schematic illustration of methodologies

adopted for fabrication of bimetallic wall and tubular

structures, along with their photographs and associated

macrostructures. Laser rapid manufacturing of bimetallic

wall involved alternate deposition of two adjacent clad

tracks of SS 316L and St-21, with overlap at the center, as

shown in Fig. 3(a) [6]. Two separate powder feeders,

positioned on either side of the laser beam, were used to

feed SS 316L and St-21 powders during the experiment.

Photograph of the bi-metallic wall and its cross-sectional

macrostructure are presented in Figs. 3(b) and 3(c),

respectively. Etching contrast between clad layers on

opposite sides of the bimetallic wall, as seen in Fig. 3(c),

is indicative of the difference in their chemical

compositions. LRM of bimetallic tube employed a co-

axial powder feeding nozzle to deposit four partly

overlapping concentric circular clad tracks in each layer.

The two inner clad tracks were deposited with St-21

powder whereas the two outer clad tracks were made with

SS 316L, as shown in Fig. 3(d). Dimensions of bimetallic

tube shown in Fig. 3(e) were: 25 mm inner diameter with

ones reported for their wrought counterparts as shown in

Fig. 2 (b)

The J-integral fracture toughness (J ) values for laser Ic

rapid manufactured specimens of IN-625 and SS 316L 2were found to be in the range of 194-254 kJ/m and 143-

2259 kJ/m , respectively. Crack tip opening displacement

(CTOD) fracture toughness values for both these

materials were found to be in the range of 0.28-0.54 mm.

Fracture toughness values (both J and CTOD) of laser IC

rapid manufactured specimens of SS 316L, although

lower than that of its wrought counterpart, are in close

agreement with the reported values for corresponding

weld metal [2].

Charpy impact energy of laser rapid manufactured

specimens of SS 316L was found to be in the range of 90-

110 J, which is at par with the wrought material in the

annealed condition[4]. Fracture surface of impact tested

specimen exhibited mixed mode fracture features with

+Fig. 2: Comparison of experimental and reported [3 , 4*] fatigue crack growth rate results for Inconel 625 and type 316L SS [1].

Material C Cr Ni Mn Si Mo Fe Co P S

SS 316L 0.025 18 12 1 0.5 2 Bal - 0.03 0.02

St-21 0.26 26.3 2.8 0.65 1.88 5.53 1.4 Bal - -

20

3.8 mm wall thickness. Sharp etching contrast developed

on the cross-section of the bimetallic tube as seen in Fig.

3(f) indicates large difference in chemical composition at

the interface.

In addition, an SS tube (post-machined dimensions: 34

mm ID and 2 mm wall thickness) with an internal step of

St-21 (height: 1.5 mm and width: 6.5 mm) has also been

fabricated by LRM, as shown in Fig. 4. This kind of

structure will be useful for fabricating components where

an insert is required to provide an internal hard-faced

lining at selective places. This demonstrates the

capability of LRM to add functional overhanging

features.

Fig. 3: Schematic illustration of methodologies adopted for LRM of (a) bimetallic wall and (d) bimetallic tube, with the images of structures (b&e) and associated macrostructures (c&f)

(a) (b)

(c)

(d) (e) (f)

Fig. 4: Laser rapid manufactured tube of SS with an internal step of Stellite 21.

21

miniature tests as they are conducted on a very local

region by compression and hence resultant values may be

higher as compared to conventional results. However the

BI test results of specimens made with 316L by LRM and

laser welding, matched well with the reported results. The

test results along with experimentally measured tensile

test results were used to formulate the empirical relations

for estimation of tensile properties of materials of

specific composition based on ShP test data from a small

volume of material [7]. It is established from the present

study that miniature specimen test techniques like BI

testing and ShP testing can be successfully used for

estimation of gradient of tensile parameters in the case of

multi-material components, where the material available

is not sufficient enough for fabrication of conventional

tensile specimens.

Laser Rapid Manufacturing of Compositionally

Graded Structures

A study on the influence of compositional grading on

fracture behavior of laser clad joint of SS 316L and St-21,

was performed with specimens fabricated using an in-

house made CW CO laser [8]. For the deposition of 2

Metallurgical characterization

Laser rapid manufactured bimetallic structures exhibited

regular pattern of clad layers of 0.6 - 0.8 mm thickness.

These components exhibited significant transition in

chemical composition and micro-hardness across their

wall thickness, as shown in Figs. 5 and 6. With respect to

bimetallic wall, tubular bush recorded gradual transition

in chemical composition and micro-hardness across its

wall thickness. Bimetallic structures of this kind may

find application in Fast Breeder Reactor, where internal

lining of Stellite is required on tubular SS components for

enhanced resistance against galling. Miniature specimen

test techniques like ball indentation (BI) testing and shear

punch (ShP) testing were used to evaluate the variation of

tensile characteristics due to associated compositional

variation across the cross-section of bimetallic structures

fabricated by LRM. Tensile strength of Stellite-21,

estimated using BI test method was about 30% higher

than the results measured from conventional tensile tests

and close to reported values for the wrought material.

Defects/discontinuities if any may show their effect in

conventional tensile tests giving a gross measure of

strength whereas no such details can be known from

Fig. 5. (a) EDS concentration profiles of Co and Fe and (b) micro-hardness profiles across wall thickness of bimetallic wall fabricated by LRM (D - distance from substrate/clad interface)

Fig. 6. (a) EDS concentration profiles of Co, Cr and Fe and (b) micro-hardness profile across wall thickness of bimetallic tube fabricated by LRM.

(a)

(b)

(a)

(b)

22

contrast, fracture surface of notched “graded clad”

specimen exhibited quasi-cleavage type fracture as seen

in Fig. 8(d).

Instrumented Charpy impact testing of laser clad

composite specimens brought out significant difference

in fracture behavior of composite specimens induced due

to compositional grading. The impact specimens were

fabricated in such a way to facilitate crack propagation

from SS 316L to St-21, as shown in the inset of figure 9.

Although, fracture of both “direct clad” and “graded

clad” specimens consumed largely similar impact

energies (32-37 J and 35-37 J, respectively), load-

displacement traces of the two specimens exhibited

distinct difference in associated modes of crack

propagation after general yield as shown in figure 9.

Failure in “direct clad” specimens was associated with

abrupt drop from peak load as crack propagated across

sharp interface between SS and St-21. On the other hand,

fracture of “graded clad” specimens was marked with

graded overlays, chemical composition of clad layers was

controlled by using pre-mixed powders of St-21 and SS

316L in predetermined ratios. Graded overlay of three

layers was deposited by cladding with premixed powders

of St-21 and SS 316L in the ratios of 30:70, 70:30 and

100:0, respectively. In the subsequent part of the text, SS

316L specimens clad with St-21 deposits and graded St-

21 deposits are referred as “direct clad” and “graded clad”

specimens, respectively. Figure 7 compares

microstructures of the interface region and associated

composition profiles of “direct clad” and “graded clad”

specimens. The cross-sections of laser clad specimens

exhibited typical cast microstructure with distinct etching

contrast with underlying base metal (SS 316L),

signifying transition in chemical composition across SS

316L-St-21 clad interface. With respect to “direct clad”

specimens, “graded clad” specimens exhibited diffused

interface involving transition from wrought

microstructure of base metal (SS 316L) to cast

microstructure of the clad layer, as shown in Figs. 7(a)

and 7(b), respectively. With respect to direct clad

specimens, graded clad specimens recorded more

gradual built up of chemical composition along the

thickness of the clad deposit. Laser rapid manufactured

composite specimens of SS 316L and St-21 were

characterized by tensile, impact and fatigue testing for

evaluating the influence of compositional grading on the

fracture behavior.

The specimens for tensile testing of “direct clad” and

“graded clad” specimens were fabricated in such a way

that the substrate/clad interface was normal to the loading

axis. In addition to smooth specimens, notched

specimens were also tested to restrict plastic deformation

to the zone of interest. The failure of smooth specimens

took place in the softest zone (viz. wrought SS) at a stress

of 600 - 630 MPa with significant amount of plastic

deformation, as manifested by its dimpled fracture

surface shown in Fig. 8(a). On the other hand, specimens

with notch in the St-21 clad region suffered brittle

fracture along inter-dendritic boundaries (refer Fig. 8(b))

at a higher stress of 950 - 968 MPa. Failure of the

specimens with notch at the interface region of took place

at an intermediate stress level (720-750 MPa) with

distinctly different modes of crack propagation in “direct

clad” and “graded clad” specimens. Fracture surface of

notched “direct clad” specimens exhibited randomly

distributed regions of ductile fracture in Fe-rich regions

(represented by dimples) and brittle fracture along inter-

dendritic boundaries in Co-rich regions (Fig. 8(c)). In

Fig. 7. Microstructure of interface region and associated EDS concentration profiles for direct clad (a, c) and graded clad (b,d) specimens. Arrows mark partially melted zone (PMZ) in figure 7(a).

Fig. 8: SEM fractographs of tensile tested specimens with SS/St-21 joint: (a) smooth specimen - failure in wrought SS; (b) notched specimens - failure in St-21 region; (c) notched “direct clad” and (d) notched “graded clad” specimens with failure in the interface region.

(a) (b)

(c) (d)

23

process which can enhance the service performance of bi-

material structures.

References

1. P. Ganesh, R. Kaul, C.P. Paul, Pragya Tiwari, S.K. Rai, R.C. Prasad, L.M. Kukreja: Fatigue and fracture toughness characteristics of laser rapid manufactured Inconel 625 structures, Materials Science and Engineering-A 527(29-30) (2010) 7490–7497.

2. W. J. Mills, in: S. R. Lampman et al, (Eds.), ASM handbook Vol. 19: Fatigue and fracture, ASM international, materials park, OH, 1997, pp.733-735.

3. h t t p : / /www.ascgenoa . com/ma in /news le t t e r / 9 /%5B2%5D6-07_AIAA_2007_2381-meta l -Fatigue.pdf

Bahram Farahmand, Charlie Saff, De Xie and Frank Abdi, Estimation of Fatigue and Fracture Allowables For Metallic Materials Under Cyclic Loading, report No. AIAA-2007-2381, American Institute of Aeronautics and Astronautics.

4. S Lampman, in: S. R. Lampman et al, (Eds.), ASM handbook Vol. 19: Fatigue and Fracture, ASM international, materials park, OH, 1997, pp.725-727.

5. http://www.specialmetals.com/documents/Inconel %20alloy%20625.pdf on June 29, 2010

6. P. Ganesh, R. Kaul, S. Mishra, P. Bhargava, C.P. Paul, Ch. Prem Singh, P. Tiwari, S.M. Oak and R.C. Prasad: Laser rapid manufacturing of bi-metallic tube with stellite-21and austenitic stainless steel, Transactions of The Indian Institute of Metals 62(2) (2009) 169-174.

7. P. Ganesh, V. Karthik, R Kaul, C. P. Paul, P. Tiwari, S. K. Mishra, C. H. Prem Singh, T. Reghu, S. S. Sheth, K. V. Kasiviswanathan, R. C. Prasad and L.M. Kukreja: Fabrication of Multi-material Components by Laser Rapid Manufacturing and their Characterization, Proc. International symposium on Processing and fabrication of Advanced Materials-XVII, Vol. I, N Bhatnagar and T. S. Srivatsan, Eds. (I.K. Internal publishing house, New Delhi, India, December-2008) pp. 97-108.

8. P. Ganesh, A. Moitra, P. Tiwari, S. Sathyanarayanan, H. Kumar, S.K. Rai, R. Kaul, C.P. Paul, R.C. Prasad, L.M. Kukreja: Fracture behavior of laser-clad joint of Stellite 21 on AISI 316L stainless steel, Materials Science and Engineering-A 527(16-17) (2010) 3748–3756.

Acknowledgements

Authors are thankful to the members of Laser Materials Processing Division, RRCAT for their technical support for carrying out laser processing experiments. Thanks are due to Smt Pragya Tiwari of ISUD, RRCAT for extending SEM facilities. Authors gratefully acknowledge the support from Shri V. Karthik and Dr A Moitra from IGCAR Kalpakkam for extending the testing facilities for carrying miniature specimen tests and instrumented impact tests.

gradual drop in load from its peak as crack propagated

through graded interface, indicating plastic deformation

accompanying crack propagation. Compositional

grading across SS/St-21 interface brought about an

increase in the fraction of crack propagation energy at the

expense of initiation energy. In the light of the results of

the study it is inferred that compositional grading brought

about a change in the mode of crack propagation (from SS

to St-21) from initiation-controlled fracture in “direct

clad” specimens to propagation-controlled fracture in

“graded clad” specimens. Scanning electron microscope

examination of the associated fracture surfaces revealed

ductile fracture in the SS 316L clad region and brittle

fracture along inter-dendritic boundaries similar to that in

tensile fracture surface (Fig 8b).

Conclusions

The present article briefly reviewed the recent results of

the metallurgical characteristics of the laser fabricated

engineering alloys. It is evident from the results of FCGR

and fracture toughness that the laser fabricated structures

of IN-625 and SS 316L possessed adequate toughness

and fatigue crack growth rate at par with the wrought

counterparts. The results of the study on laser rapid

manufacturing of bi-metallic structures demonstrated

that near-net-shaping multi-material metal parts with

engineered compositional heterogeneity is feasible with

proper control on processing parameters. The use of

miniature specimen test techniques like ball indentation

testing and shear punch testing was found to be effective

in estimating the gradient in tensile properties of multi-

material structures fabricated by LRM. The results of

instrumented impact tests conducted on direct and graded

clad SS- St-21 Charpy specimens revealed that

compositional grading at substrate-clad interface can

alter the mode of crack propagation and delay the fracture

Fig. 9: Load-displacement plot of instrumented Charpy impact specimens (shown in inset) [8].

24

s = optical RMS roughness o

l = Wavelength of the incident beam

a = incident angle

Ra is calculated from s assuming sinusoidal profile for o

normal machined surfaces and using calibration curve.

This technique is useful for surfaces whose roughness is

very less than the wavelength, l, of the incident beam.

Thus if a diode laser at 670 nm is used then surfaces of Ra

less than 250 nm i.e. 0.25 µm can be measured.

The instrument consist of three parts(Fig 1)

(I) Sensor head unit

(ii) Monitoring Unit

(iii) Power Supply unit

The sensor head unit carries a Laser diode as light source

and two photodiodes for measurement of incident and

specularly reflected intensities. An amplifier card is also

there to amplify the photodiode signals. The monitor unit

receives the output from the sensor head. It carries a

microcontroller (87c552) which has integrated 10 bit

ADC. The two inputs are fed directly to the analog inputs

of µc , which converts them into digital value for use by

the microcontroller for calculation of s & Ra values o

using look-up table. The µc is also interfaced to LCD and

keyboard for user interface. Through keyboard it is

possible to select either Ra or s for display or voltages o

corresponding to specular and incident intensities. The

power supply unit carries linear regulated power supply

for the Laser diode and for electronic circuit. + 3 V for

Laser diode and ± 5 V for electronic circuit.

Salient features

Measurement Range : Average roughness (Ra) :

0.05 µm to 0.22 µm

RMS roughness (σ ) :0

0.05 µm to 0.15 µm

Accuracy : ±20 %

Measurement time : < 1 sec

Laser source : Diode Laser at 670 nm, 4 mW

Beam spot size : 1 mm

Measurable surface : Flat

shape

The following three instruments have been developed at

Laser and Plasma Technology Division, Bhabha Atomic

Research Centre, Mumbai and can be taken up by

industry for production:

LASER SURF-CHECK

Introduction

It is a Laser based non-invasive, hand-held, stand-alone

Roughness measuring instrument. The measuring probe

here is a low power Laser beam from diode Laser, which

falls on the sample surface. The incident beam and the

specularly reflected beam intensities are measured and

the ratio of the two is used to calculate the optical RMS

(σ ) and the average roughness (Ra) of the sample. In the 0

mechanical stylus based roughness measuring

instrument, a diamond tip on a stylus moves along the

surface and leaves a mark on the surface while measuring

its profile. This spoils a soft surface of very low

roughness value. Also it is very slow. Many times it is not

required to know the fine details but to obtain the average

parameter like Ra or RMS roughness of the surface. In

such cases, Laser based parametric technique like the one

of specular reflectance measurement used in Laser Surf-

Check gives directly the roughness parameter i.e. Ra or

σ , very fast.0

Being non-invasive in nature, it is very useful for

roughness measurement of soft surfaces. The fast

measurement capability makes it ideal for routine

comparison of similar surfaces.

Method

This optical technique for roughness measurement is

based on measurement of specular reflectance and then

using following equation:

2 Ix/Io = exp [ - (4ps /l) cos a] o

Where,

Ix = specularly reflected intensity from relatively

smooth surface

Io = specularly reflected intensity from perfectly

smooth surface

2

Laser Based Instruments for Measurement Applications

Aseem Singh RawatLaser and Plasma Technology Division, BARC, Mumbai - 400 085

E-mail : aseem@barc.gov.in

25

mechanical) are placed at known separation along the

path of the projectile and time taken by the projectile to

move the distance from one sheet to another is measured.

When optical sheets are used for measurement, it

becomes non-contact and non-destructive in nature.

The Laser Velocity Meter instrument uses diode lasers

with optics to generate light sheets. It has microcontroller

based circuit for velocity calculation along with LCD and

switches for user interface. It can be used as a tabletop

standalone single instrument or with two parts having

sensor unit near the projectile path and display unit at

control room connected through fiber optic link. It has

measurement range from 25 m/sec to 5000 m/sec with an

accuracy better than ±2 % of measured value.

Method

In LVM, two parallel light sheets are generated using

diode lasers at 670 nm with line generating optics. They

are separated, by fixed known distance and kept

perpendicular to the direction of projectile motion. When

a horizontal moving projectile crosses the light sheet,

shadow is generated on photodiode detector at each sheet

(Fig 2) that creates electrical pulses. The time between the

two pulses is electronically measured using high speed

Type of surface finish : Polished, Grinded

Surface material : Electroformed nickel alloy

For other materials, calibration is required

Micro-controller based circuit

LCD display

Keyboard interface for Selecting Ra or σ for display ,0

Diagnostic testing by selecting intensity values for

display

Resetting the instrument

RS 232 interface for data logging in PC

Fig 1 Photograph of Laser Surf-Check instrument

Application areas

• For actual measurement of roughness of metallic

mirrors.

• For routine comparison of similar surfaces of any

finish process like grinding, lapping, polishing.

LASER VELOCITY METER

Introduction

Measurement of speed of a projectile is required in

various material study experiments for determination of

parameters like relationship between material surface

deformation with impact, coefficient of restitution,

viscosity of a liquid, testing quality of a bullet in firing

range etc.

Laser Velocity Meter (LVM), is a non-contact, stand-

alone velocity measuring instrument based on time of

flight principle. In time of flight based measurement, the

time taken by a projectile to move a known distance is

measured and used to calculate the speed. For

determining time, two parallel sheets (optical or Fig. 3 : Block Diagram of LVM from top

Fig. 2 Light sheet Generation ( front view)

26

Applications

1) For study of material properties like deformation

with known impact

2) To find Coefficient of restitution of a material

3) In Defense material test labs (One unit has been

supplied to DMRL, Hyderabad)

REBOUND VELOCITY METER

Introduction

Rebound Velocity Meter (RVM), is a non-contact, stand-

alone velocity measuring instrument based on time of

flight principle. It is similar to LVM, except that here

instead of two light sheets, only one light sheet is used due

to which measurement range has reduced. It can be used

as a tabletop standalone single instrument. It has

measurement range from 1 m/sec to 400 m/sec with an

accuracy better than 2 % of measured value.

Method

Rebound velocity meter is also a velocity measuring

instrument based on time of flight principle, but here the

difference is in its time measurement method, unlike the

conventional method of measuring time taken by the

object to travel between two optical sheets separated by a

known distance, here a single narrow light sheet is used.

For known dimension of the object, the time taken by the

object to cross the thin sheet ( Fig 5) is measured to

calculate its velocity. The advantage of this method is that

the optical sheet can be placed very near to target and it

measures more accurate velocity for smaller object at

lower speed than two sheet method. The other important

feature of Rebound Velocity meter is that the electronic

circuit in it is designed such that instead of single velocity

it can measure two consecutive velocities and display

them.

Salient features

Measurement range : 1 m/sec to 400 m/sec

Accuracy : 2% of measured value

Size/Shape of : Preferably spherical, of diameter

counter to give time of flight. Thus time taken by a

projectile to move from one sheet to another sheet

measured electronically is used to calculate the velocity

of the projectile. The instrument has microcontroller

based circuit for velocity calculation(Fig 3).

Salient Features

Measuring Range : 25-5000 m/sec

Accuracy : better than ±2% of

measured value

Measurement Time : 10 msec

(max.)

Projectile Size : > 2 mm

Laser Source : Diode Laser (670nm, 5mWatt)

Width of light sheet : 50 mm

Thickness of : 1 mm

light sheet

Distance between two parallel light sheets : Adjustable

Detector used : PIN Photodiode

Remote operation possible with sensor unit near the

target and monitor unit with display can be upto 50 meters

away

Sensor unit & monitor unit connected using optical fiber

cable to avoid EMI

Stand-alone : Microcontroller based circuit

instrument

Display on monitor : Liquid Crystal Display unit

Speed display in m/sec.

PC-Connectivity : Serial Port, RS-232 interface at

1200 baud

Fig 4 Photograph of Sensor Unit of Laser Velocity Meter

Fig.5 Block diagram of RVM

27

RS 232 interface

Power input : 230 Vac, 20 mA

Power consumption : < 5 Watts

Dimensions : 74cmx15cmx16cm

Fig 6 Photograph of Rebound Velocity Meter

Applications

1) For study of material properties like deformation

with known impact

2) To find Coefficient of restitution of a material

3) In Defense material test labs

between 2 - 40 mm

Measurement time : Few msecs (depends on speed and

size of projectile)

Laser Source : Diode Laser at 670 nm of

5 mW power

Thickness of : 1 mm

light sheet

Width of light sheet : 40 mm

Suitable for opaque as well as transparent objects

Can be configured for horizontal and/or vertical moving

projectiles

Stand alone instrument with microcontroller based

circuit and LCD display (Fig 6)

Keyboard interface for

Resetting the instrument (required after each reading to

avoid multiple readings)

Entering projectile dimensions (if different from default )

Function key to select display the ratio of rebound to

falling velocity

Function key to select decimal resolution of reading

LED indication for function selected

Switch selectable measurement of falling velocity only or

with rebound

the projectile

28

The PSD consists of a uniform resistive layer formed on the surface of a high resistivity semiconductor substrate, and a pair of electrodes formed on both ends of the resistive layer for extracting the position signal currents. The schematic of the PSD is shown in figure 2.

The active area, which is a resistive layer generates photocurrent when light falls on it. When the laser spot is incident on the PSD, an electric photo-current is generated at the incident position. This electric photo-current generated is divided by the resistive layer and collected by output electrodes X and X as photocurrents 1 2

Ix and Ix , while is divided in inverse proportion to 1 2

distance between incident position and each electrode. The two current outputs of PSD Ix and Ix are converted 1 2

into voltages V and V by trans-impedance amplifiers. 1 2

Thus we have X = (V -V )/(V +V )×Lx/2A 2 1 2 1

The calculated position X is proportional to displace-A

ment of target from reference point x and is given by

x = sinγ × X /M × sinαA

where γ is angle between focal plane of focusing lens and plane of PSD while α is angle between translation axis of target and focal plane of focusing lens and M is the magnification of the lens.

Introduction

Lasers due to their unique properties find ready applications in metrology and inspection of precision mechanical components. Moreover laser based metrology and inspection systems are non-contact in nature making them suitable for use in the nuclear industry where the components to be measured or inspected could be radioactive. Various laser based instruments and systems have been developed at RRCAT for the metrology and inspection of nuclear fuel components. Some of the techniques developed for these applications could be adapted for use in industrial applications. We present here three such developments that could find wide application in industrial use. The first is a laser triangulation probe developed for non-contact profiling of precision mechanical components, the second is a laser line-triangulation based 3D digitization system developed for accurate form measurement and the third is a fiber optic proximity sensor that can be used for position measurement at high speeds such as in turbine and motor shaft position and rpm.

Triangulation Sensor

The triangulation sensor is a position sensing system that uses a non-contact measurement technique to determine position of an object with respect to certain reference point. The schematic of the sensor is shown in figure 1. The laser diode module shown in the schematic illuminates the target whose position is to be measured. The light scattered from the target is re-imaged on to a position sensing detector (PSD) using a lens assembly with a magnification M. The laser diode module, imaging lens assembly and position sensing detector all are encased in a sensor head and forms the triangulation senor probe. As the target position changes along the axis shown, the scattered light is imaged on different positions along the PSD. The PSD generates two outputs in form of currents which are proportional to the position of incident spot. The trans-impedance amplifier converts these currents into voltages. These voltages are digitized and processed by a microcontroller to determine the position of the target. The calculated position is displayed on LCD after linearization and multiplied by the calibration factor.

Laser based InstrumentationIshant Dave*, Rohan Bhandare, Brijesh Pant, S. Sendhil Raja and P.K. Gupta

Laser Biomedical Applications and Instrumentation Division

Raja Ramanna Centre for Advanced Technology, Indore - 452 013

*E-mail : ishantdave@rrcat.gov.in

Fig. 1: Schematic of a laser triangulation sensor probe.

Fig. 2: Schematic of a typical position sensitive detector

29

IDE provided by Keil development Tools. The compiler for code was provided by Keil development Tools. It generates both object file (OMF2 format) as well as hex file. The generated hex file is downloaded into flash of micro-controller using USB debug adaptor provide by Silicon Laboratories. The microcontroller C8051F120 contains an on chip industrial standard JTAG interface for in circuit testing. Due to this provision the microcontroller device can be debugged in actual application system without the use of external hardware.

For calibration, the system was mounted on an optical bread board. The target was mounted on a linear translation stage with digital readout, in front of the sensor. Zero reading of the stage was preset at stand-off distance from the sensor. The target was moved over the sensor range (±10mm) and the stage position from the digital readout and the sensor readings were noted. The calibration curve is shown in figure 4 below. For more accurate results a second order curve was fitted and same equation was implemented in firmware for compensation of the PSD non-linearity and to extend the linear range of the sensor.

The developed sensor can be used for position or displacement measurement in systems where non contact measurement is necessary, like in nuclear industry, where conventional techniques like LVDT fails. The sensor in conjunction with a X-Y positioning stage can be used for

Thus position of the target surface x is given by

x = sinγ/Msinα × (V -V )/(V +V )×Lx/22 1 2 1

The two current outputs of the PSD are converted into voltages by current to voltage converters using op-amps as shown in the circuit diagram in figure 3. These two voltages are digitized by an on chip ADC of the microcontroller. The logged values are averaged for 1000 samples to reduce the scatter in the position data due to random noise in the signal. The microcontroller processes these outputs using the formula discussed above to calculate the position of target .The calculated value is displayed on a liquid crystal display, with an update rate of 2Hz. To minimize the error due to ambient light the PSD output is measured with the laser diode in the “on” condition and in the “off” condition to correct for the positional error due to ambient light. The TTL modulation input of the laser diode module is connected to port pin of microcontroller, pulling this pin high or low switches laser diode on or off. The laser diode is switched on and off with square a wave generated by microcontroller on the pin with a frequency of 2 KHz. The micro controller logs both the outputs of PSD with laser diode on and off. Then it subtracts the value with laser diode off from laser diode on and the subtracted value is averaged for 1000 samples.

The system also compensates for varying reflectivity of target using an automatic gain control technique. The on chip DAC output of the microcontroller is amplified and used to control the analog modulation input of the laser diode module. The output of DAC is adjusted such that sum of current outputs of PSD remains within the specified band of predefined values irrespective of the reflectivity of the target surface.

The firmware for the system is developed in C language. The editing and listing of code was done in an integrated development environment provided by Silicon Laboratories. It can also be done using Keil Micro-vision

Fig. 3: Schematic of the front end circuit of the sensor

Fig. 4: The calibration curve for the developed sensor

Fig. 5: Photograph of the developed sensor head & the display unit

30

illuminated by the laser module. The object is moved along different orientations, either rotated or linearly translated to obtain the desired 3D information. Alternatively the laser line can be scanned across the object to generate the 3D data points. In the current system scanning is achieved using a FPGA based stepper motor controller card which rotates a mirror for the purpose of scanning the laser line output as shown in figure 7, a photograph of the setup. The developed system is portable and tripod mountable and can be used to scan objects of size 100mm X 100mm X 100 mm with a resolution of 10 microns. This system needs one time calibration for mapping the coordinate frames and generating the mathematical framework which is needed for extracting the 3D Data.

The calibration of the scanner is done using a calibration grid as proposed by Zhang et al. The camera parameters and the extrinsic parameters viz. translation and rotation for the object frame are calculated using the images shown below in figure 8.

surface profiling of various components for form measurement. The sensor can be used as a replacement for a mechanical dial gauge to measure thickness, height and run out etc.

The developed laser based triangulation sensor is shown in figure 5. The developed sensor has a linearised operating range of ±10 mm and a resolution of 20µm. The sensor operates with a stand-off distance of 100mm.

Laser based 3D Digitization System

Three dimensional digitization of real objects is of interest in various fields such as engineering design & proto-typing, reverse engineering, metrology, orthopaedics and dentistry. The field is ever-growing since the advancement of laser based digital scanners which are capable of generating very accurate data at high scanning rates. The system developed here has a typical range of 100x100x100mm with a resolution of 10microns.

The laser triangulation probe described in the previous section can be extended to the other dimension by replacing the laser spot with a laser line and the PSD with a CCD camera as shown in figure 6. In laser line triangulation we have a plane described by the laser line generator and a unique direction identified by the camera, the intersection of which is a unique point in three dimensions Intersection of a line and a plane is a unique point which can be easily computed if the equations of the line and the plane are known. In three dimensions, solving for the coordinate of the point implies solving three equations in three respective dimensions. The system developed consists of a laser line generator designed with reflective optics which illuminates the object of interest, a high resolution CCD Camera (1200 X 1600 pixels is used to grab frames of the object

Fig. 6: Schematic of a Laser line triangulation setup

Fig. 7: Photograph of the laser line triangulation setup

Fig. 8: Calibration images used for the laser line triangulation setup

31

show the point cloud generated from the scanning of a dental mold and the subsequent meshed point cloud to generate the CAD model of the object.

Reconstruction of 2.2 million points is done in a minute by the developed software. The system can be scaled to digitize large objects such as in archeological artifacts or other similar objects of interest.

Fiber-optic Proximity Sensor: Fiber optic sensors are very popular in instrumentation where there are constraints such as contactless, non-electrical in nature, noise/radiation immune, small in size etc. A fiber optic based sensor probe was developed for measurement of vibrations and RPM (Revolutions Per Minute) of turbo machinery. It is possible to measure the displacements in the range of about 3 to 5 mm with a resolution better than 10 µm and RPM of up to 2,50,000. The output is transmitted as an industry standard 4 to 20 mA current signal used in industrial PLC control.

The schematic of the probe is shown in figure.11 above. The probe tip contains a bundle of multimode fibers in the centre called the illuminating fibers. These are coupled to a high brightness LED at the end. The illuminating fiber illuminates the measuring surface. A bundle of fibers concentric to the illuminating bundle collects the back reflected light. These fibers are coupled and focused onto a photo detector which converts the optical signal into an electrical signal. The intensity of the reflected light depends on the distance between probe tip and measuring surface, which is measured in terms of displacement.

The laser line is scanned across the object of interest and the point cloud for the illuminated pixels is calculated. The image shown in figure 9 is one of hundreds of images which are acquired with the help of the rotating mirror scanner and the synchronized CCD camera of the laser line triangulation setup.

The point clouds generated from the laser line scanning are post-processed in a 3D meshing software viz. 3D Reshaper from Technodigit Corp. which is capable of meshing point clouds, in order to generate a CAD file of the acquired point cloud data. The images 10a and 10b

Fig. 9: A typical image of the laser line triangulation output.

Fig. 10a: The point cloud generated by the system for a dental mold.

Fig. 10b: The rendered CAD model generated by the system for a dental mold.

Fig 11: Schematic of the fiber optic proximity sensor

Fig. 12: Position-response characteristic

curve of the developed probe.

32

The figure 14 shows the electrical interface diagram for both RPM and displacement measurement. The back coupled light from receiving fibers is focused on a photodiode. The Op Amp based signal conditioning circuitry provides necessary gain and offset adjustments. The processed signal is transmitted as a standard instrumentation signal of 4 to 20 mA. For RPM measurement a comparator circuit converts the optical pulses into a train of electrical pulses. The pulses are counted by the CPLD based counter in specified time and hence the frequency/RPM is measured. It also generates necessary interface signal for a “serial 4 to 20mA” transmitter. The threshold and hysteresis adjustment is provided to compensate for surfaces of different reflectivity and roughness

There are several limiting factors in use of this kind of fiber optic displacement sensors. The measuring surface must be scattering type (Lambertian surface) as the reflected light is measured in terms of distance. The surface and tip of the probe must be free from any contaminations which obstructs light. Any change in reflectivity or any obstruction of light to and from the sensor affects the accuracy of the measurement. Due to typical characteristics of the probe it produces the same output for two different locations on each slope which may be misleading. Inspite of these limitations due to the probes simplicity and robustness it can find applications in health monitoring of rotating machines where contact probes wear out frequently and electrical sensors are prone to EMI noise from the motors. The probe can also be used over a large temperature variation such as in turbo-machinery since the probe front-end has no electronic components.

Conclusion: We have presented here three different laser based instruments developed as spin-offs from the various instruments developed for the nuclear industry. The developed instruments are to be considered as complementary to the existing mechanical techniques and not as replacements for them. The use of laser based instruments comes at an added penalty of requiring a greater level of sophistication of the operators and cleaner operating environment. The instruments should be used where the benefits offered such as high accuracy, wear-free, robustness etc by the laser based instruments greatly offset the penalties. The three instruments described here have been prototyped and tested in field applications and are at a level where they can be readily transferred to interested industrial partners who are interested in taking up production of these instruments.

The figure.12 shows the typical characteristic curve of the developed probe. The intensity of the reflected light measured by the photodiode is plotted against the distance of the probe tip from the measuring surface. It depicts two semi-linear regions with two different slopes called the “front slope” and “back slope” with an optical plateau in between. The shape of the characteristic curve depends on various parameters like distance between illuminating and receiving fibers, diameter of the fibers, numerical aperture of the fibers, arrangement of the fibers etc and can be manipulated by the appropriate design of the probe.

The figure.13 shows how both displacement and RPM of a rotating spindle can be measured using this kind of probe. Above portion shows the displacement part in which any deflection of a spindle changes the distance between tip and surface causing change in intensity as shown in shaded region. The change in intensity is measured in terms of displacement. For RPM measurement a notch is provided on the spindle. Every time the notched passes the tip the distance changes. This produces optical pulses at the output which is converted into electrical signal and counted in terms of RPM.

Fig. 13: Displacement and RPM measurement with the probe

Fig. 14: Schematic of the front-end signal conditioning circuit.

33

corrosive environments. A successful coating should be

refractory, chemically inert, possess good mechanical

strength and thermal shock resistance, have low thermal

conductivity and exhibit similar thermal expansion

coefficient to that of the substrate [4-6]. These

requirements have led to the development of partially

stabilized zirconia coatings as TBC's for its high thermal

expansion coefficient close to that of many metals and

alloys used in engine applications [5,7]. Plasma sprayed

yttria stabilized zirconia coatings (PSZ) are considered as

one of the potential options for the salt purification vessel

and electrorefiner application in reprocessing plant and

the coating has shown excellent corrosion resistance in molten LiCl-KCl salt [8,9]. Therefore ceramic oxide

coatings increase inertness, corrosion resistance and

durability in aggressive molten LiCl-KCl salt. The

plasma spray process is an economical method for

producing reproducible and durable thick thermal barrier

coatings. It is a well established technique for applying

ceramic coatings to protect the surface of engineering

components against corrosion, erosion and wear at high

temperatures [10]. During the plasma spraying process,

residual stresses generated within the coating system by

the rapid cooling of molten droplets are relieved by

through-thickness microcracking [3]. The presence of

micro cracks and interconnected porosity in the coating

affects the mechanical properties and deteriorates the

oxidation and corrosion resistance. These micro cracks

and pores are considered to be path for molten salts and

corrosive gases to attack the TBC system [2,3,11]. Lasers

have been used for the modification of surfaces and

elimination of such defects.

Lasers are promising technological tool for surface

modification [12], due to their characteristics speedy

treatment, simple process control, and delivery of high

energy density to a localized surface area without significantly heating up the whole body [12,13]. Laser

processing has been attempted for surface modification

of PSZ coatings to improve the wear resistance, thermal- shock resistance [2], corrosion resistance [2,14], and life

Abstract

Thermal barrier coatings are well known for the protection of high temperature components. Plasma

sprayed yttria stabilized thermal barrier zirconia coating

(PSZ) has been proposed for protection of various

components in pyrochemical reprocessing plants

involving molten LiCl-KCl salt for temperatures up to o600 C. The pores and micro-cracks present in the as-

sprayed coatings may cause corrosion of the substrate as

molten LiCl-KCl salt can penetrate through these defects

on prolonged exposure. Laser processing has been

attempted as a promising technique for surface

modification of as-sprayed PSZ coatings. Laser

remelting of the as-sprayed PSZ coatings resulted in a 50

µm thick densified layer on the surface and eliminated

microstructural inhomogeneities like pores and voids,

however, segmented cracks were formed. The

microhardness of the laser remelted surface increased

and surface roughness was reduced. The beneficial non-

transformable tetragonal phase was formed after laser re-

melting while the as sprayed coating consisted of

insignificant monoclinic phase. Distinct polygonal grains

with interface separating fine and coarse grains were

observed. Laser processing by addition of silica

decreased segmented cracks; however, further

optimisation is required to achieve dense and crack-less

surface. The paper highlights the results obtained for

application of laser remelting on plasma sprayed PSZ

coating.

Introduction

Thermal barrier coatings (TBC's) are being increasingly

used in the turbine section of advanced gas turbine

engines and aerospace industry [1-3]. For pyrochemical

reprocessing of spent metallic fuels, by electrorefining

process, molten LiCl-KCl salt is used as electrolyte at o600 C. The material used for fabrication of

electrorefining vessels and components should have high

corrosion resistance. High performance corrosion

resistant coatings are essential for such severely

Application of Laser Processing of Materials for High Temperature Molten Chloride Environment

A. Ravi Shankar, Ravikumar Sole, Jagdeesh Sure and U. Kamachi Mudali*Corrosion Science and Technology Group

Indira Gandhi Centre for Atomic Research, Kalpakkam – 603 102

*E-mail: kamachi@igcar.gov.in

34

employed are given in Table 3. The as sprayed and laser

remelting samples were characterised by XRD, Optical

microscopy and SEM.

Results and Discussion

The surface morphology of as-sprayed partially

stabilised zirconia (PSZ) coating applied over 316L SS is

shown in fig 2a. The typical morphology of as sprayed

coating consists of completely melted splat regions,

cracks within the splats, unmelted particles, and pores

between the splats (fig 2a). These types of complex

microstructures form due to rapid solidification of molten

and semi molten particles impinging on the substrate

during plasma spraying. It has been reported that thermal

barrier PSZ coating by plasma spraying posess upto 10%

pores, and form microcracks due to splat type of melting

and solidification, consisting of unmelted particles,

partially melted particles etc. [18] which are detrimental

for corrosion. The SEM micrograph of laser remelted

surfaces is shown in figures 2b-d. The porous as sprayed

coating shown in fig 2a became smooth and dense after

laser remelting as shown in fig 2b-d. After laser melting,

of the coating [15]. Laser remelting has been considered

as a potential process for the improvement of plasma

sprayed TBC properties by reducing surface roughness,

eliminating open porosity on the surface [3,16], and

generating a fully dense layer with homogeneous

microstructure [2]. In order to reduce the corrosion attack

and to seal open pores on the TBC surface [17], and to

enhance the thermal shock resistance and life time of

zirconia coatings, laser remelting process is a promising

technique [1]. Numerous studies on laser surface

treatment of zirconia were performed to eliminate cracks

and porosity using CW and multimode, CO laser, high 2

power diode laser, Nd: YAG laser, etc. [1-3,12,13]. A four

fold improvement in life after laser glazing zirconia

based TBC's was observed in cyclic corrosion tests [11].

In addition to eliminate cracks and porosity on the

surface, lasers have been used to improve resistance to

mechanical erosion and chemical corrosion of plasma

sprayed zirconia coatings [13]. The present paper

discusses laser remelting of plasma sprayed partially

stabilized zirconia surface on type 316L SS to achieve a

smooth and pore free surface.

Experimental Details

AISI Type 316L SS discs of 25 mm diameter and 5 mm in

thickness were used as substrates. A bond coat of 50 µm

thick NiCrAlY was coated on 316L stainless steel

substrates to provide good adhesion between substrate

and ceramic coat. Over the bond coat 300 µm thick

partially stabilised zirconia (ZrO -8 wt %Y O ) was 2 2 3

coated by Air Plasma Spraying (APS) process. The

coated layers were produced by M/s Plasma Spray

Processors, Mumbai, with a METCO 9MB type plasma

gun with optimized spray parameters. The laser remelting

process was carried out with Continuous Wave (CW)

multi beam CO laser (wave length 10.64 µm) with a 3 2

axis Computer Numerical Controller (CNC) workstation

as shown in fig 1. The laser processing parameters

Fig. 1. Schematic diagram of multi beam CW CO laser system.2

Table 1. Laser processing parameters.

Laser processing parameters

CO laser active CO + N + He gases 2 2 2

medium (15: 26: 61 mbar)

Material ZrO + 8 wt% Y O2 2 3

Power (W) 50, 75, 100

Beam diameter (mm) 1.5

Scan speed (mm/sec) 1, 2.5, 5

Interaction time (sec) 1.5, 0.6, 0.3

Track shift (mm) 0.5

Shield gas Argon

Fig 2. SEM micrographs of partially stabilized zirconia coating over type 316L SS (a) as-sprayed (b-d) laser remelted.

35

showed well delineated columnar grain structure

[8,19,20]. As the solidification was directional and

vertically from the substrate to the remelted area,

formation of columnar dendritic structure takes place. In

comparison to the lamellar structure resulting from

thermal spraying, the columnar structure provides a

better thermomechanical resistance during thermal

cycling. Figure 4b shows the interface separating coarse

and fine grain structure [20]. The formation of sharp

interface with development of coarse grains was

attributed to the (additional heat entrapment) reheating of

the solidified material where the ripples coalesce, during

rastering of the laser beam with a shift. The combination

of coarse and fine grain structure offers optimum

properties. Fig 5. SEM micrographs of partially

stabilized zirconia coating (a) laser remelted (b) laser

remelted with silica overlay (c & d) laser co-deposition of

silica and corresponding EDX [20,21].

Chen et.al, [17] reported that a dense and crack-less thin

layer can be achieved on the surface of 3wt% SiO -doped 2

ZrO coating using laser re-melting. Based on the 2

literature laser remelting with ZrO + SiO overlay and 2 2

laser co-deposition of silica were carried out [20,21]. The

porosity was eliminated completely and smooth and

dense surface was obtained. SEM micrograph after laser

treatment of the coating showed preferred well delineated

grains of zirconia (fig 2 c&d). However, the segmented

crack morphology was observed in the laser treated

region (fig 2b). The well defined segmented crack

network all along grain boundaries (fig 2d) was formed

due to shrinkage and relief of thermal stresses during

cooling [8,19].Fig 3. Cross section micrographs of

partially stabilized zirconia coating over type 316L SS

(a&c) as-sprayed (b&d) laser remelted.

The cross section SEM micrograph of as-sprayed and

laser remelted partially stabilised zirconia (PSZ) coating

over 316L SS with intermediate bond coat is shown in fig

3 a&b respectively. A 50 µm dense laser remelted layer is

clearly seen in the micrograph (fig 3b). Fig 3 c&d shows

the cross section optical micrographs of as-sprayed and

laser remelted regions respectively depicting the porosity

present. Area percentage porosity of coatings were

determined as per ASTM E 2109 method A, which is a

manual, direct comparison method utilizing standard

images given in the standard which depict typical

distributions of porosity in TBCs. The porosity in the

partially stabilized zirconia coating is reduced from 10%

in the as-sprayed coating (fig 3c) to 0.5% after laser

treatment (fig 3d). The microhardness measurement

indicated that the hardness increased significantly from

664 VHN in the as-sprayed region to 1230 VHN in the

laser treated region (fig 3d) probably due to significant

decrease in the porosity and microstructural changes.

The optical micrographs showing laser remelted

microstructure with fully dense coarse and fine polygonal

grains of zirconia is shown in fig 4. The optical

micrograph of the laser remelted PSZ coated surface

Fig 3. Cross section micrographs of partially stabilized zirconia coating over type 316L SS (a&c) as-sprayed (b&d) laser remelted.

Fig 4. Optical micrographs of laser remelted partially stabilized zirconia coating over type 316L SS.

Fig 5. SEM micrographs of partially stabilized zirconia coating (a) laser remelted (b) laser remelted with silica overlay (c & d) laser co-deposition of silica and corresponding EDX [20,21].

36

Typical splat type of surface morphology of plasma

sprayed Al O -40wt%TiO coating on high density 2 3 2

graphite is shown in fig 7a [22]. Plasma spray coatings

contain completely melted splats, unmelted and partially

melted particles and in addition to that phase separation

could occur in Al O -40wt%TiO which results in 2 3 2

inhomogeneous surface as shown in fig 7b containing

60%Ti. In order to achieve homogeneous surface and

reduce roughness plasma sprayed Al O -40wt%TiO 2 3 2

coatings were subjected to melting using a micro pulsed

Nd:YAG laser at RRCAT, Indore, with power densities of 20.64 and 0.8 MW/cm . Microstructural examination

showed that inhomogeneities were eliminated and a

smooth surface was achieved (fig 7c). However, network

of cracks was formed irrespective of power density. XRD

results indicated that stable α-Al O and Al TiO phase 2 3 2 5

was more predominant in laser remelted coatings [22].

Conclusions

Laser consolidation of plasma sprayed PSZ coatings over

316L SS has been attempted to produce dense and smooth

surface for application in molten chloride environment.

Laser remelting of the coatings resulted in smooth and

glossy surface with fully dense well delineated columnar

grain. The porosity in the as-sprayed coating decreased

and hardness increased significantly after laser treatment.

Segmented crack network along grain boundaries in laser

treated samples are formed due to shrinkage and relief of

thermal stresses. The formation of coarse and fine grains

with sharp interface was attributed to the reheating of the

solidified material. The addition of silica during laser

processing decreased the segmented cracks and resulted

in dense and pore free surface, however further

optimisation of parameters are required. Beneficial non-

transformable tetragonal phase was present in the

remelted layer as determined by XRD analysis. Laser

remelting of Al O -40wt%TiO on high density graphite 2 3 2

eliminated surface inhomogeneities and smooth and

dense surface was formed.

optical microstructure of laser re-melted PSZ surface is

shown in fig 5a while fig 5b shows the optical

microstructure after giving an overlay of ZrO + SiO and 2 2

then laser re-melting. The purpose of SiO is to alter the 2

surface composition and thereby decrease the segmented

cracks [20]. As shown in fig 5b, there is decrease in the

segmented cracks with formation of large cells, however,

the cross section optical micrograph indicated partial

delamination of the ZrO + SiO layer. In order to achieve 2 2

dense and crack less layer, laser parameters and silica

content need to be optimised. An attempt was made by

laser co-deposition of silica with 3.5 kW CO laser at 2

RRCAT, Indore, in pulse modulated mode. Figure 5 c&d

shows the surface morphology of silica deposited sample

with corresponding EDX spectra which shows PSZ

containing 40 wt% silica with dense and fine spherical

agglomerate deposited during laser processing without

any cracks [21]. This type of dense structure without

pores and cracks are desirable for molten chloride

application and further fine tuning of parameters are

required to achieve completely dense and crack free

surface.

The presence of minor amounts of detrimental

monoclinic phase present in the as sprayed coatings as

shown in fig 6 is eliminated on laser remelting. X-ray

diffraction pattern of the as sprayed and laser remelted

surfaces indicated the presence of beneficial metastable

tetragonal phase in the laser treated sample (fig 6)

[8,19,20] which is in good agreement with the literature.

The presence of t' phase in the thermal barrier coatings

was expected to be highly beneficial because of its

thermal stability and structural hardening.

Fig 6. XRD spectra for as coated and laser re-melted PSZ at 50W at varying scan speeds.

Fig 7. (a & b) Surface morphology of plasma sprayed Al O -40 2 3

wt%TiO coating (c) after laser remelting.2

37

10. R. Siva Kumar, and B.L. Mordike, Surf. Eng., 3

(1987) 299-305.

11. I. Zaplatynsky, Thin Solid Films, 95 (1982) 275.

12. Sang Ok Chwa and Akira Ohmori, Surf. Coat.

Technol., 148 (2001) 88-95.

13. Xing Wang, Ping Xiao, Marc Schmidt and Lin Li,

Surf. Coat. Technol., 187 (2004) 370-376.

14. C. Batista, A. Portinha, R.M. Ribeiro, V. Teixeira,

C.R. Oliveira, Surf. Coat. Technol., 200 (2006)

6783-6791.

15. P. C. Tsai, C. S. Hsu, Surf. Coat. Technol., 183

(2004) 29-34.

16. K.M. Jasim, R.D. Rawlings and D.R.F. West, J.

Mater. Sci., 26 (1991) 909-916.

17. H.C. Chen, E. Pfender, and J. Heberlein, Thin Solid

Films, 315 (1998) 159-169.

18. J. R. Davis, Davis and Associates, Microstructural

Characterization of Thermal Spray Coatings, in

'ASM handbook', 9 (2004) 1038.

19. A. Ravi Shankar, B. Jagdeesh Babu, Ravikumar

Sole, U. Kamachi Mudali and H.S. Khatak, Surf.

Eng., 23 (2007) 147-154.

20. A. Ravi Shankar and U. Kamachi Mudali, Surf.

Eng., 25 (2009) 241-248.

21. A. Ravi Shankar et.al unpublished (2011).

22. Jagadeesh Sure et al, unpublished (2011).

Acknowledgements

The authors thank scientists at RRCAT, Indore for their

support in part of the laser processing work carried out at

RRCAT, Indore.

References

1. G. Antou, G. Montavon, F. Hlawka, A. Cornet, C.

Coddet and F. Machi, Surf. Coat. Technol., 172

(2003) 279-290.

2. A. Petitbon, L. Boquet and D. Delsart, Surf. Coat.

Technol., 49 (1991) 57-61.

3. C. Batista, A. Portinha, R.M. Ribeiro, V. Teixeira,

M.F. Costa and C.R. Oliveira, Surf. Coat. Technol.,

200 (2006) 2929-2937.

4. J. R. Brandon and R. Taylor, Surf. Coat. Technol.,

39/40 (1989) 143-151.

5. R. Taylor and J.R. Brandon, Paul Morrell, Surf.

Coat. Technol., 50 (1992) 141-149.

6. J.F. Li, H.L. Liao, C.X. Ding and C. Coddet, J.

Mater. Process. Technol., 160 (2005) 34-42.

7. P. Scardi, M. Leoni and L. Bertamini, Thin Solid

Films, 278 (1996) 96-103.

8. A. Ravi Shankar, U. Kamachi Mudali, Ravikumar

Sole, H.S. Khatak and Baldev Raj, J. Nucl. Mater.,

372 (2008) 226-232.

9. A. Ravi Shankar, U. Kamachi Mudali, Mater.

Corros., 59 (2008) 878- 882.

IONS-India

IIT Delhi student chapter of OSA is happy to host IONS (International OSA Network of Students) conference for the first time in India at Indian Institute of Technology Delhi (IIT Delhi) 1-2 December 2011. Program information will be posted at

http://ions-project.org/?id=4&topic=ionsdelhi.

For more informationcontact Ms. Kanchan Gehlot at

gehlot.kanchan@gmail.com

Important Information

38

Atomic Energy to industrial partners. In the afternoon,

the technology showcase session provided a platform for

one-on-one technical discussions between researchers

and industrial delegates. This was followed by

presentations by industrial participants. Interesting

presentations were made by delegates from Tata Motors

Ltd. Pune, Bharat Heavy Electricals, Hyderabad, Larsen

& Toubro, Mumbai and Archaeological Survey of India.

The proceedings of the day were concluded with a group

discussion session chaired by Dr P. K. Gupta. The

session witnessed lively discussions with industrial

delegates on the ways to strengthen interaction between

laser research community and indigenous industries. On

the second day the first talk was delivered by Prof B. D.

Gupta of Indian Institute of Technology, Delhi on optical

sensors for process monitoring. This was followed by a

presentation by Dr Sunita Belgamwar of Nexus

Mechatronics, Pune on lasers in therapy. Subsequent

talks on optical spectroscopy and imaging for bio-

medical diagnosis were delivered by Dr. Diwakar Rao &

Dr. S. K. Majumder of RRCAT and on laser based

metrology & inspection by Dr. Sendhil Raja of RRCAT.

These presentations were followed by an informative

presentation by Dr P. S. Raju of Technology

Development Board (TDB) of Department of Science

and Technology (DST) on various funding schemes

offered by TDB. The meet was concluded with a group

discussion session chaired by Dr. P. S. Raju.

By:

Rakesh Kaul & S. Sendhil Raja.Raja Ramanna Centre for Advanced Technology, Indore

On the occasion of 50th year of invention of laser, Indian

Laser Association organized a two-day interaction meet

on Utilization of Lasers in Industry and Medicine on 28th

and 29th April 2011 at Raja Ramanna Centre for

Advanced Technology (RRCAT), Indore. The

interaction meet aimed to provide a platform to showcase

indigenous laser technologies developed for industrial

and medical applications in major academic and research

institutions of the country and to promote closer

interaction between academic/research institutions of the

country and Indian industry.

The interaction meet was inaugurated by Dr. P. K. Gupta,

President, Indian Laser Association. The inaugural talk

on technology generation and incubation was delivered

by Shri A. M. Patankar Head TT& CD of BARC. The key

features of the meet were presentations by laser experts

explaining rudiments of laser applications, presentations

by industrial delegates, outlining their experiences and

prospective requirement of laser technology in their

industry and technology showcase sessions involving

presentations of indigenous laser technologies. The meet

was attended by about 30 participants from 25 different

companies. About 35 posters on various laser-based

technologies developed at different R&D centers in India

were presented in the meet.

In forenoon session of the first day, presentation made by

Dr. L. M. Kukreja and Shri Rakesh Kaul included

fundamentals of laser material processing and overview

of related presentations in technology showcase session.

Shri A. M. Patankar, delivered an informative talk on the

modalities of Technology transfer from Department of

Interaction Meet on Utilization of Laser Technology in Industry & Medicine at RRCAT, Indore

Report

MEMBERSHIP FORM

INDIAN LASER ASSOCIATION

Announcement

For circulation among ILA members only.(Not for sale)

Printed by : Rohit Offset Pvt. Ltd. Indore. 2422201-02A Bulletin of the Indian Laser Association

Prof. B. D. Sharma, Dr. Sunita Belgamwar, Dr. V.K. Saxena and Dr. P. S. Raju delivering their lectures during the meet.

Technical Sessions of Interaction Meet on Utilization of Laser Technology in Industry & Medicine, RRCAT, Indore, April 28-29, 2011.

Participants of the meet discussing during the tea break and the poster session.