microstructural property co-relation of diffusion bonded high strength steel

1

A Project Report On

Microstructural Property Co-relation of

Diffusion Bonded High Strength Steel

By

Ashish Kumar Gouda

B.Tech, 5th semester

Metallurgical and Materials Engineering

National Institute of Technology

Rourkela, Odisha -769008

Under the guidance of

Md. Murtuja Husain

Scientist, MST Division

CSIR - National Metallurgical Laboratory

Jamshedpur-831 007

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CERTIFICATE

This is to certify that Mr. Ashish Kumar Gouda, student of B.Tech, 5Th semester,

Department of Metallurgical and Materials Engineering, National Institute of

Technology, Rourkela has carried out the project work entitled “Microstructural

Property Co-relation of Diffusion Bonded High Strength Steel” in Material

Science and Technology (MST) division at our laboratory for his winter training

programme, as a trainee from 1st December 2016 to 30th December 2016 (period

of four weeks), under the guidance of Md. Murtuja Husain. During his training

period, we found him hard working, sincere and dedicated with a learning attitude.

Md. Murtuja Husain

(Scientist, CSIR-NML)

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Abstract

Similar diffusion bonding of high strength steel was carried using pure Ni inter

layer at1100oC for 30 minutes and by applying 1 tonne of load in high vacuum

brazing furnace. Optical microscope and FEG-SEM were used for microstructural

evaluation. Diffusion of chemical species was characterized by EPMA.

Microstructure of parent metal was tempered martensite. After the diffusion

bonding microstructure of the parent metal was transformed to pearlite. Line

profile of diffused elements diffusion bonded assembly was obtained by EPMA.

Micro hardness of diffusion bonded high strength steel was assembly carried out

across the joint. Micro hardness of base metal was ~320-380 VHN. The micro

hardness at interface of joint and centre of Ni interlayer were ~253 VHN and 128

VHN respectively.

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Acknowledgement

The internship has been a very good experience for me in the way that it has given

me the chance to explore the metallurgical knowledge and experimental skills. I

have learnt a lot of about the office environment research and development

laboratory. My skills and self-confidence have improved significantly. Firstly I

express my deep sense of gratitude to my honourable guide Md. Murtuja Husain

for his endeavour approach and outstanding supervision by which it has been

possible for me to make a good combination of theoretical and practical

knowledge in preparing this report. I forfeit my respect to Mr. K. L. Hansda,

training co-ordinator, CSIR-NML, for his guidance during this period, apart from

the project work he also helped me to shape my personality as a responsible citizen

of the country. I am also grateful to Mr. Raman and other lab assistants whom

helped me a lot during the internship. I thank all CSIR-NML employees for their

support and guidance. In this period I got the opportunity to learn about diffusion

bonding of steel which is totally a new concept for me as well as very interesting

one.

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Contents 1 Introduction 1

2 Literature review 4

2.1 Diffusion bonding 4

2.1.1 Basic theory of diffusion bonding 6

2.1.2 Metallurgical factors 7

2.1.3 Mechanism of DB 9

2.1.4 Parameters of DB 11

2.1.5 Interlayer importance 12

2.1.6 Advantages 12

2.1.7 Limitations 13

3 Equipments used and characterization techniques 14

3.1 High vacuum brazing furnace 14

3.2 Optical microscope 16

3.3 SEM and EDS 17

3.4 EPMA 19

3.5 Vickers micro hardness testing 20

4 Experimental procedure 21

4.1 Bonding and substrate 21

4.2 Sample preparation for SEM, EPMA study 21

4.3 EPMA study 22

4.4 Micro hardness study 22

5 Results and discussion 23

5.1 Optical microstructure 23

5.2 SEM and WDS 24

5.3 EPMA line profile 25

5.4 Vickers micro hardness 27

6 Summary 28

7 References 29

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List of figures

Sl. No. of

Figures

Caption of figure

Page No.

1 Showing various stages of diffusion bonding process 8

2 High vacuum brazing furnace 15

3 Optical microscope 17

4 Scanning electron microscope 28

5 Schematic of EPMA and WDS 19

6 Vickers micro hardness testing 20

7 Optical microstructure of diffusion bonded high strength steel (a) low magnification (b) high magnification

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8 SEM microstructure of diffusion bonded high strength steel (a) low magnification (b) high magnification

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9 Point analysis by WDS 24

10 Line profile analysis by EPMA 25

11 Concentration profile of elements by EPMA 25

12 Fe-Ni phase diagram 26

13 Vickers micro hardness study 27

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CHAPTER 1

Introduction

In the modern era the processing of various advanced materials with superior

mechanical properties has underpinned rapid progress in manufacturing of new

products. Transport industry, aerospace industry, especially car manufacturing

industry have been interested in materials with high strength-to-weight ratios as

these can provide significant performance benefits [1].Diffusion bonding process

as a state-of-the-art technology has been widely used in various fields of aviation,

aerospace, and nuclear power. In modern days, solid-state joining of similar and

dissimilar materials is a novel technique to enchant needs of various structural

applications in marine, transport, and construction industries [1]. Joining of high-

carbon/high-strength steel is challenging because of innate limitation of poor

bondability during conventional fusion welding [2, 3]. Melting and subsequent

solidification during welding results in segregation and formation of brittle

intermetallics in fusion zone, leading to premature failure. As solid-state joining,

diffusion bonding is one of the feasible solutions to eradicate limitations [4, 5].

Currently research is going on to develop the process and strong efforts are being

put to develop diffusion bonding process as it eliminates the drawbacks that

encountered during fusion welding.

Reports on diffusion bonding of carbon steel are very less, still some experiments

have been carried out, some of them are discussed below. In one of the similar

studies done by Husain et al., in which the high strength steels were bonded using

pure Ni interlayer, maximum ultimate strength of ~532 MPa was obtained along

with shear strength of ~792 MPa for the joint processed at 1050 °C, which was

higher than literature reports on martensitic steel [1].It has been reported that

joining of Armco iron to cast iron at 980 °C for 5 minutes at 4.5 MPa load

exhibited bond strength of ~215 MPa [6]. Keeping processing parameters same

when joining has been carried out for carbon steel to cast iron, tensile strength of

assembly became ~250 MPa. Both values were higher than tensile strength of cast

iron (~190 MPa) [6]. Elliot et al. [7] have attempted solid-state joining of cold-

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drawn mild steel. This study primarily highlighted fracture mechanism and extent

of bonding at faying surfaces. Maximum bond strength was to the tune of ~400

MPa for assembly welded at 900 °C for 100 min. and joint quality was controlled

by faying surface and void characteristics [7]. Low carbon steel was diffusion-

bonded using Fe–Ni–Cr–Si–B amorphous layer at temperature 1225 °C and

exhibited homogeneous microstructure without precipitation/segregation across

interface [8]. The technique showed some encouraging results in terms of bond

strength and impact toughness. This technology contained inherent limitation as

amorphous foil had a tendency of forming crystallites during bonding, ensuing in

severe embrittlement [8]. To minimize the effect of high temperature, facilitate

diffusion of alloying elements across interface, and reduce tendency of brittle

intermetallic formation, it is normal practice to use suitable interlayer during

bonding. Ni/Ni alloys can be considered as a promising candidate owing to its

substantial solid solubility in iron, high corrosion resistance, and adequate

oxidation resistance [9]. In this context, He et al. reported that nickel–stainless

steel diffusion couple was free from intermetallics [10].

Solid-state direct diffusion bonding of commercially pure Titanium (CP-Ti) and

precipitation hardening stainless steel (PHSS) has been carried out in the

temperature range of 800 °C to 1000 °C with an interval of 50 °C for 1 hour under

3.5 MPa uniaxial load in (4 to 6)×10–3 Pa vacuum. Poddar reported that transition

joints achieve tensile strength 108% and shear strength 87.6% higher compared to

pure titanium. And the ductility of the joint has been found to be 12.8% when

processed at 950 °C [11].

Solid-state diffusion bonding was done to produce transition joints between Ti–

5.5Al–2.4V and type 304 austenitic stainless steel in the temperature range of 850–

950 °C under uniaxial load for 1 hour, Ghosh et al. reported that highest bond

strength (∼272 MPa) was obtained for the diffusion-bonded assemblies processed

at 850 °C due to presence of thinner width of brittle intermetallics and the failure

occurs in the region somewhere in between FeTi and β-Ti during tensile loading

for all the diffusion-welded joints [12].

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In studies on diffusion bonding which were carried out to produce transition joints

between commercially pure titanium and 304 stainless steel (304 SS) in the

temperature range of 800-950 °C, under uniaxial pressure of 3 MPa for 1.5 hours,

Ghosh et al. reported that transition joints achieve 76% strength of pure Ti (242

MPa) along with 5% ductility when processed at 850 °C [15].

One of the widely accepted techniques is resistance spot welding of high-strength

steels. Depending on heat input and cycle time, transition joints exhibited weak

link at heat affected zone (250– 282 HV) with respect to base material (~420 HV),

and maximum shear strength was ~686 MPa [20]. Structural carbon steels were

welded through transient liquid phase bonding using 3 % Si–3.5 % B–Ni interlayer

[14]. In that case, boron did not exhibit any significant effect in improving joint

efficiency.

Conventional bonding of dissimilar material is much critical due to various

metallurgical heterogeneities like thermal expansion, large differences in melting

points, development of residual stress and formation of brittle intermetallic phases.

After much exploration of the processes, it has been suggested that solid state

joining process can give the better solution and in this respect, diffusion bonding is

convenient with minimum macroscopic deformation and reduction of mechanical

properties.

In general the diffusion bonding is carried out at the temperature range of 0.5-

0.7Tm.Whereas Tm is the absolute melting point of the material. However, the

deformation and grain growth occur in the joints bonded at such elevated

temperatures, which are main problems. However to overcome this problem and to

get optimized bonded quality lower temperature is preferred [15].

In the present investigation, to study the microstructural property co-relation of

diffusion bonded high strength steel.

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CHAPTER 2

Literature review

2.1 Diffusion bonding

“Diffusion bonding or diffusion welding is a solid state joining process. This

bonding technique is based on the atomic diffusion of elements at the joining

interface. The diffusion-welding interface has same physical and mechanical

properties as base metal. The strength of joining depends on pressure, temperature,

time of contact and the cleanness of the interface. Diffusion bonding needs longer

time than the other welding processes” [16].

The International Institute of Welding (IIW) has adopted a modified definition of

solid-state diffusion bonding, proposed by Kazakov (1985) [17].

“Diffusion bonding of materials in the solid state is a process for making a

monolithic joint through the formation of bonds at atomic level, as a result of

closure of the mating surfaces due to the local plastic deformation at elevated

temperature which aids inter diffusion at the surface layers of the materials being

joined.”

2.1.1 Basic theory of diffusion bonding

Diffusion bonding is a joining process where in the principal mechanism for joint

formation is solid state diffusion. The solid phase diffusion bonding of the product

is used in various industrial fields to make making to high performance and

making to a high function near-net shape in addition. Diffusion bonding offers

many advantages, mainly the strength of the bonding line, which is equal to the

base metals. The microstructure at the bonded region is exactly the same as the

parent metals. On the other hand, this advantage joining process requires several

strictly controlled condition clean and smooth contacting surfaces which are free

from oxides, etc., high temperature condition to promote diffusion process. In

diffusion bonding, the bond strength is achieved by pressure, temperature, time of

contact, and cleanness of the surfaces. The strength of the bond is primarily due to

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diffusion rather than any plastic deformation. Diffusion bonding is an attractive

manufacturing option for joining dissimilar metals and for making the component

with critical property continuity requirements. Unlike other joining processes the

diffusion bonding process preserves the base metal microstructure at the interface.

More importantly no localized thermal gradient is present to induce distortion or to

create residual stresses in the component. Some metals will unite to form a

homogeneous structure when placed in intimate contact under temperature and

pressure [16].

This property results in a union where the joint is metallurgically and detectable,

i.e., grain boundaries are not confined to the original joint face. For practical

purposes the intimate contact and atomic exchange is assisted by heat and pressure

from an external source, although no melting of the material takes place. Bond

strengths up to parent material properties are achievable.

The joining aspect of the process is similarly concerned with elevated temperature

flow properties and fine grain sizes. In achieving intimate contact of two originally

free surfaces, diffusion accounts for only a small, though vital amount of the mass

transport required, the majority being achieved by plastic deformation. Thus the

low flow stresses associated with fine grain sizes are desirable for bonding just as

for super plastic forming. Again, in-process grain growth can adversely affect the

diffusion bonding process and must be of special concern for bonds to be achieved

in the process cycle. Achieving high integrity joints with minimal detrimental

effects on the parent material in the bond region and also the possibility of joining

dissimilar materials are the most promising features of diffusion bonding.

Accordingly, interest in diffusion bonding has been growing in the last fifty years.

Diffusion bonding is most often used for jobs either difficult or impossible to weld

by other means, due to its relatively high cost. Examples include welding materials

normally impossible to join via liquid fusion, such as zirconium and beryllium;

materials with very high melting points such as tungsten; alternating layers of

different metals which must retain strength at high temperatures; very thin,

honeycombed metal foil structures; high-carbon and ultra-high carbon steels etc

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Several kinds of metal combination can be joined by diffusion

bonding

1. Similar metals may be joined directly to form a solid-state weld. In this

situation-required pressures, temperatures, times are dependent only account the

characteristics of the metals to be joined and their surface preparation.

2. Similar metals can be joined with a thin layer of a different metal between them.

In this case, the layer may promote more rapid diffusion or permit increased micro

deformation at the joint to provide more complete contact between the surfaces.

This interface metal may be diffused into the base metal by suitable heat treatment

until it no longer remains a separate layer.

3. Two dissimilar metals may be joined directly where diffusion-controlled

phenomena occur to form a bond.

However Diffusion Bonding can be performed with only for a limited number of

elements and some of these are Fe, Cu, Ni, Ti which are relatively easy to bond. Al

and Mg are difficult to bond, can’t dissolve their oxides, and need to add another

layer [26].

2.1.2 Metallurgical factors

Two factors of particular importance with similar metal weld are allotropic

transformation and micro structural factors that tend to modify diffusion rates.

Allotropic transformation (phase transformation) occurs in some metals and alloys.

The important of the transformation is that the metal is very plastic during that

time. This tends to permit rapid interface deformation at lower pressures in much

the same manner as does re-crystallization. Diffusion rates are generally higher in

plastically deformed metals as they re-crystallize. Another means of enhancing

diffusion is alloying or more specially, introducing elements with high diffusivity

into the systems at the interface. The function of a high diffusivity element is to

accelerate void elimination. Alloying must be controlled to avoid melting at the

joint interface. When using a diffusion activated system, it is desirable to heat the

assembly for some minimum time either during or after the welding process to

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disperse the high diffusivity element away from the interface. If this is not done,

the high concentration of the element at the joint may produce metallurgically

unstable structures [18].

Diffusion bonding is often combine with super-plastic forming (SPF) for aerospace

titanium structures. Combining the two processes of SPF and diffusion bonding in

particular as concurrent processes, provides a potential for considerable cost and

weight saving when compared with conventional fabricated structures which are

typical of aerospace structures.

Cost saving accrue from the ability to form complex structure from simple starting

blanks (in most cases flat blanks) and to form this into a complete structure in one

operation. This means of manufacture significantly reduce the parts count relative

to fabricate structures.

2.1.3 Stages/mechanism of diffusion bonding

In diffusion bonding, the nature of the joining process is essentially the

coalescence of two atomically clean solid surfaces. Complete coalescence comes

about through a three-stage metallurgical sequence of events. Each stage, as shown

in Fig .1. , is associated with a particular metallurgical mechanism that makes the

dominant contribution to the bonding process. Consequently, the stages are not

discretely defined, but begin and end gradually, because the metallurgical

mechanisms overlap in time. During the first stage, the contact area grows to a

large fraction of the joint area by localized deformation of the contacting surface

asperities. Factors such as surface roughness, yield strength, work hardening,

temperature, and pressure are of primary importance during this stage of bonding.

At the completion of this stage, the interface boundary is no longer a planar

interface, but consists of voids separated by areas of intimate contact. In these

areas of contact, the joint becomes equivalent to a grain boundary between the

grains on each surface. The first stage is usually of short duration for the common

case of relatively high-pressure diffusion bonding [19].

Fig .1. Showing various stages of diffusion bonding process

During the second stage of joint formation, two changes occur simultaneously. All

of the voids in the joints shrink, and most are eliminated. In addition, the

interfacial grain boundary migrates out of t

equilibrium. Creep and diffusion mechanisms are important during the second

stage of bonding and for most, if not all, practical applications, bonding would be

considered essentially complete following this stage. As t

remaining voids are engulfed within grains where they are no longer in contact

with a grain boundary. During this third stage of bonding, the voids are very small

and very likely have no impact on interface strength. Again, diffusion

cause the shrinkage and elimination of voids, but the only possible diffusion path

is now through the volume of the grains themselves.

Although diffusion welding is used for fabricating complex parts in low quantities

for the aerospace, nuclear, and electronics industries, it has been automated to

make it suitable and economical for moderate

process is highly automated, considerable operator training and skill are required.

8

Showing various stages of diffusion bonding process



interfacial grain boundary migrates out of the plane of the joint to lower



considered essentially complete following this stage. As the boundary moves, any



and very likely have no impact on interface strength. Again, diffusion


is now through the volume of the grains themselves.


ar, and electronics industries, it has been automated to

make it suitable and economical for moderate-volume production. Unless the


Showing various stages of diffusion bonding process



he plane of the joint to lower-energy



he boundary moves, any



and very likely have no impact on interface strength. Again, diffusional processes



ar, and electronics industries, it has been automated to

volume production. Unless the


9

2.1.4 Factors effecting diffusion bonding

The activation energy for atomic diffusion at the surface, interface and grain

boundaries is relatively low compared to the bulk diffusion due to a looser bond of

the atoms and higher oscillation frequency of the diffusing atom. This enhances

the atomic diffusion, and thus eases the diffusion bonding of two metal pieces

assuming that a perfect interface contact exists. Various parameters influencing the

mechanical properties of the diffusion bonded joint are

1. Bonding temperature

2. Bonding time

3. Bonding pressure

4. Interlayer

5. Surface roughness

6. Plastic deformation

Effect of temperature

Temperature is the most important variable in diffusion bonding. It should be

selected and controlled so as not to interfere with metallurgical changes or

transformation that may occur in materials. Bonding strength has a tendency to

increase with rise in temperature but after a maximum temperature the bond

strength starts decreasing. Temperature dependency can be shown by the

following equation D = D0 e -Q/KT where ,

D = Diffusion coefficient

D0 = Diffusion constant

Q = Activation energy

T = Absolute temperature

K = Boltzmann's constant

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Effect of time

Here also time needed to perform atomic diffusion is temperature dependent.

Longer times become less and less effective. The time needed can’t be determined

simply but has to be found experimentally.

Once welding is done longer times will not add any properties. The Expression

linking the diffusion time and distance can be shown by the following equation

X = (D*t)1/2 Where,

X = Distance.

D = Diffusion coefficient.

t = Diffusion time.

So the substitutional solid solution will be done and for a longer time, the porosity

in substrate increases which makes this region more brittle, thus the bonding

strength is decreased [18]. In this case time was 30 minutes.

Effect of bonding pressure

Pressure directly affects the outcome of diffusion welding and its importance is

great, especially in the initial stages of the process. It can be linked to the metals

involved but it is difficult to deal in theory as a predetermined value. Although

local deformation is introduced at the contact point as an essential stage in the

process, macroscopic deformations are avoided. Pressure is generally limited to the

minimum required to get good results, because of the high equipment costs

associated with high compression. Pressure and Temperature are practically

selected so that they will permit the performance of suitable welds in acceptable

time. For a Cu-Ni joint diffusion bonded at various temperatures, Kadhim et al.

reported that the maximum bonding strength is (275 MPa) at 700 C for 30 min,

surface roughness 0.2µm and at percentage of deformation 30% [17].For a high-

strength steel joint diffusion bonded by using pure Ni filler at various

temperatures, Hussain et al. reported that maximum ultimate strength of ~532 MPa

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was obtained along with shear strength of ~792 MPa for the joint processed at

1,050 °C for 30 min at uniaxial load of 40 MPa [1].

In the present sample preparation was done by the application of pressure of 1 ton.

Effect of surface roughness

Bonding strength decreases with increasing surface roughness of mating surfaces.

The reason of this decreasing is due to the gaps between the mating surface so the

contact areas between the surfaces are not perfect and thus the diffusivity is not

active.

Effect of plastic deformation

Bonding strength increases with percentage of plastic deformation. This is due to

increasing point and line defects which make diffusivity more active. However the

effect of bonding temperature, bonding pressure and bonding time for increasing

the bonding strength are greater than the surface roughness of mating surfaces with

the greatest effect of bonding temperature.

2.1.5 Importance of interlayer

Joints diffusion bonded without the use of an interlayer reveal poor bonding

characteristics due to the formation of brittle intermetallics compounds (IMCs).

These IMCs are unavoidably produced by the reaction of the elements coming

from the substrate with either the counter substrate or the fillers/interlayers used.

Most joints exhibit low strengths with a fracture in a joint due to the presence of

the brittle intermetallic phases, indicating that their brittleness is inherent and thus

difficult to avoid. A feasible way to solve such fundamental brittleness can be an

application of a proper combination of interlayer metals that are highly soluble

with each other and with both the adjacent filler and base metal, thereby realizing a

joint free from any detrimental brittle phases. Efforts have been made to avoid

such a fundamental brittleness of a high-strength steel joint with the use of a pure

Ni interlayer. Hussain et al. reported that no IMCs are formed for this joint and

very high strength is obtained for this joint [1]. Therefore the Ni interlayer plays

the role of diffusion barrier quite efficiently.

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Diffusion bonding can be achieved for materials with adherent surface oxides, but

the resultant interface strengths of these materials are considerably less than that

measured for the parent material. Aluminium alloys are prime examples of this

class of material. In practice, oxide-free conditions exist only for a limited number

of materials. Accordingly, the properties of real surfaces limit and impede the

extent of diffusion bonding. The most notable exception is titanium alloys, which,

at DB temperatures greater than 850 °C, can readily dissolve minor amounts of

adsorbed gases and thin surface oxide films and diffuse them away from the

bonding surfaces, so that they will not impede the formation of the required

metallic bonds across the bond interface [17].

Similarly, the joining of silver at 200 °C (390 °F) requires no deformation to break

up and disperse oxides, because silver oxide dissociates completely at 190 °C (375

°F). Above this temperature, silver dissolves its oxide and also scavenges many

surface contaminants. Other examples of metals that have a high solubility for

interstitial contaminants include tantalum, tungsten, copper, iron, zirconium, and

niobium. Accordingly, this class of alloy is easiest to diffusion bond [19].

2.1.6 Some of the advantages of diffusion bonding process are

Joint can be produced with properties and microstructures very similar to those of

the base metal. This is particularly important for light weight fabrications.

1. Component can be joined with minimum distortion and without subsequent

machining or forming.

2. Dissimilar alloys can be joined that are not weldable by fusion processes or by

processes requiring axial symmetry [16].

3. A large number of joints in an assembly can be made simultaneously.

4. Components with limited access to be joints can be assembled by these

processes.

5. Large components of metals that required extensive preheat for fusion welding

can be joined by these processes.

6. Defects normally associated with fusion welding are not encountered.

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7. Economic advantages

a. Simple starting blank form (particularly significant for titanium)

b. High material utilization

c. Reduces parts count

d. Process times which are insensitive to size, complexity of structural form, or

number of components manufactured in one operation.

9. Weight advantages

These weight saving occur from the ability of SPF/DB in particular, to produce

efficient structural forms with the elimination of fasteners and associated joint

flanges [20].

2.1.7 The limitations of diffusion bonding

1. Generally, the duration of the thermal cycle is longer than that of conventional

welding and brazing processes.

2. Equipment costs are usually high and this can limit the size of components that

can be produced economically.

3. The processes are not adaptable to high production applications, although a

number of assemblies may be processed simultaneously.

4. Adequate non-destructive inspection techniques for quality assurance are not

available particularly those that assure design properties in the joint.

5. Suitable filler metals and procedures have not been yet developed for all

structural alloys.

6. The surfaces to be joined and the fit-up of mating parts generally required

greater care in preparation than for conventional hot pressure welding or brazing

process.

7. The need to simultaneously apply heat and a high compressive force in the

restrictive environment of a vacuum or protective atmosphere is a major

equipment problem with diffusion welding [16].

14

CHAPTER 3

Equipments used and characterization techniques

3.1 High vacuum brazing furnace

Brazing is a metal-joining process in which two or more metal items are joined

together by melting and flowing a filler metal into the joint, the filler metal

having a lower melting point than the adjoining metal.

Brazing differs from welding in that it does not involve melting the work pieces

and from soldering in using higher temperatures for a similar process, while also

requiring much more closely fitted parts than when soldering. The filler metal

flows into the gap between close-fitting parts by capillary action. The filler metal

is brought slightly above its melting temperature while protected by a suitable

atmosphere, usually a flux. It then flows over the base metal (known as wetting)

and is then cooled to join the work pieces together. It is similar to soldering,

except for the use of higher temperatures. A major advantage of brazing is the

ability to join the same or different metals with considerable strength [22].

Vacuum brazing is carried out in the absence of air, using a specialized furnace,

and delivers significant advantages: extremely clean, flux-free braze joints of high

integrity and superior strength. Improved temperature uniformity when heating in

a vacuum, and lower residual stresses due to slow heating and cooling cycle,

results in significantly improved thermal and mechanical properties of the

material. Other benefits of vacuum brazing include heat treating or age hardening

of the work piece as part of the metal-joining process, all in a single furnace cycle.

Like conventional brazing, vacuum brazing is easily adapted to mass production.

In order to obtain high-quality brazed joints, parts must be closely fitted and the

base metals must be clean and free of oxides, normally accomplished by either

chemical or mechanical (abrasive) cleaning. In the case of mechanical cleaning,

proper surface roughness must be maintained as the capillary action of the filler

material occurs much more readily on a rough surface than a smooth surface [23].

15

Vacuum furnaces are generally classified into high vacuum furnaces and low

vacuum furnaces, depending on whether they use treatment pressures below or

above 10-2 Pa. Their essential difference lies in the type of evacuation system used:

Low vacuum furnaces mainly use a combination of mechanical booster pumps and

oil-sealed rotary pumps, while high vacuum furnaces use high vacuum evacuation

pumps, such as oil diffusion pumps, in addition to the combination of mechanical

booster pumps and oil-sealed rotary pumps [22].

The furnace used for the preparation of present study sample is shown below

Fig .2. High vacuum brazing furnace at CSIR-NML

This 5 Ton hot press can be equipped with many options: a ceramic hot zone for

operation in air (1650°C max.), a graphite or metallic zone for operation in

vacuum or inert gas (2300°C max. with press rods,2600°C in batch mode),

diffusion pumping system, turbo pumping system, roughing pump, thermocouple

or pyrometer temperature control, etc. The furnace can be equipped with classic

instruments and controls, or an intuitive and easy to use computer interface,

allowing full control of the furnace [23].

The Major Parts are:

1. Chamber and hot zone

2. Hydraulic Press

3. Power supply and Controls

16

4. Vacuum and gas system

5. Water cooling

3.2. Optical microscope

The optical microscope, often referred to as light microscope, is a type

of microscope which uses visible light and a system of lenses to magnify images

of small samples. Optical microscopes are the oldest design of microscope and

were possibly invented in their present compound form in the 17th century. Basic

optical microscopes can be very simple, although there are many complex designs

which aim to improve resolution and sample contrast [24].

In the optical microscope, when light from the microscope lamp passes through

the condenser and then through the specimen (assuming the specimen is a light

absorbing specimen), some of the light passes both around and through the

specimen undisturbed in its path, Some of the light passing through the specimen

is deviated when it encounters parts of the specimen. Such deviated light (as you

will subsequently learn, called diffracted light) is rendered one-half wavelength or

180 degrees out of step (more commonly, out of phase) with the direct light that

has passed through un-deviated. The one-half wavelength out of phase, caused by

the specimen itself, enables this light to cause destructive interference with the

direct light when both arrive at the intermediate image plane located at the fixed

diaphragm of the eyepiece. The eye lens of the eyepiece further magnifies this

image which finally is projected onto the retina, the film plane of a camera, or the

surface of a light-sensitive computer chip [26].

The microstructure generally ranges from the atomic scale (0.1nm) to 1 mm (1000

μm) with the most widely used scale of 1-1000 μm. Typical microstructural

features are grains (single crystal), precipitates, inclusions, pores, whiskers, twin

boundaries, etc.

17

Fig .3. Optical microscope at CSIR-NML

3.3 Scanning electron microscope (SEM)

The scanning electron microscope (SEM) uses a focused beam of high-energy

electrons to generate a variety of signals at the surface of solid specimens. The

signals that derive from electron sample interactions reveal information about the

sample including external morphology (texture), chemical composition, and

crystalline structure and orientation of materials making up the sample. In most

applications, data are collected over a selected area of the surface of the sample,

and a 2-dimensional image is generated that displays spatial variations in these

properties. Areas ranging from approximately 1 cm to 5 microns in width can be

imaged in a scanning mode using conventional SEM techniques (magnification

ranging from 20X to approximately 30,000X, spatial resolution of 50 to 100 nm).

The SEM is also capable of performing analyses of selected point locations on the

sample; this approach is especially useful in qualitatively or semi-quantitatively

determining chemical compositions (using EDS), crystalline structure, and crystal

orientations (using EBSD). The design and function of the SEM is very similar to

18

the EPMA and considerable overlap in capabilities exists between the two

instruments [32].

The SEM is routinely used to generate high-resolution images of shapes of objects

and to show spatial variations in chemical compositions

1. Acquiring elemental maps or spot chemical analyses using EDS,

2. Discrimination of phases based on mean atomic number (commonly related to

relative density) using BSE, and

3. Compositional maps based on differences in trace element "activators"

(typically transition metal and rare earth elements) using CL.

Fig .4. Scanning electron microscope at CSIR-NML

3.4 Electron probe micro analyser (EPMA)

Electron micro probe analyzer (EMPA), is an analytical tool used to non-

destructively determine the chemical composition of small volumes of solid

materials. It works similarly to a scanning electron microscope: the sample is

bombarded with an electron beam, emitting x-rays at wavelengths characteristic to

the elements being analyzed. This enables the abundances of elements present

within small sample volumes (typically 10-30 cubic micrometers or less) to be

determined [30]. The concentrations of elements from beryllium to plutonium can

be measured at levels as low as 100 parts per million (ppm). Recent models of

19

EPMAs can accurately measure elemental concentrations of approximately 10

ppm [31].

In the present experiment it is used to obtain the line profile of the diffusion

bonded high strength steel. Chemical composition across the interlayer boundary

and the base metal is obtained and analysed [30].

Wavelength dispersive spectrometer (WDS) Each element’s characteristic X-

ray has a distinct wavelength and by adjusting the tilt of the detecting crystal (LiF,

PET, TAP take off angle=40°), it will diffract the wavelength of specific

element’s X-ray in accordance with Bragg’s law (2dsinθ=nλ). In a fully focusing

spectrometer; sample, crystal and detector all lie on the same focusing circle called

as Rowland circle such that Bragg’s law is satisfied in all cases. The centre of the

Rowland circle lies on a linear path in order to maintain a constant take off angle

and moves on arc with X-ray source as centre. In this experiment at the centre of

the interlayer, at the base metal and at the boundary of base metal and interlayer

the WDS study was done to obtain the exact composition at that place [30].

Fig .5. Schematic of EPMA and WDS

20

3.5 Vickers micro hardness testing

Hardness is the property of a material that enables it to resist plastic deformation,

usually by penetration. However, the term hardness may also refer to resistance to

bending, scratching, abrasion or cutting. Hardness is not an intrinsic material

property dictated by precise definitions in terms of fundamental units of mass,

length and time. A hardness property value is the result of a defined measurement

procedure.

The Vickers test is often easier to use than other hardness tests since the required

calculations are independent of the size of the indenter, and the indenter can be

used for all materials irrespective of hardness. The basic principle, as with all

common measures of hardness, is to observe the questioned material's ability to

resist plastic deformation from a standard source. The Vickers test can be used for

all metals and has one of the widest scales among hardness tests. The unit of

hardness given by the test is known as the vickers pyramid number (HV)

or diamond pyramid hardness (DPH).

The DPH may be determined as the following equation

DPH = 2Psin (θ/2)/d1 X d2

Where P = applied load, kg

d1,d2 = average length of diagonals, mm

θ = angle between

opposite faces of diamond = 136̊

Fig .6. Vickers impression test

21

CHAPTER 4

Experimental procedure

4.1 Bonding and substrate

A diffusion bonded high strength steel sample joined by pure nickel interlayer

was provided. The experiment was carried out at a temperature of 1100oC, with

the application of 1 tonne load for 30 minutes (0.5 hour). After this the sample was

prepared for the study of optical microstructure, SEM, EPMA, WDS and micro

hardness testing. As high strength steel is a high carbon steel it has poor

weldability. The mechanical property of the base material is as follows.

Table 1: Composition of base material (high strength steel) [1]

Alloy Si Mn C S P Cr Mo Ni Al Fe

Carbon Steel 0.60 0.90 0.60 0.05 0.004 0.20 0.01 0.01 0.01 97.6

Table 2: Mechanical property of base material [1]

Steel Hardness (VHN) UTS (MPa) Breaking strain (%)

Carbon steel 580±5 1400±6 8±3

4.2 Sample preparation for optical microscope, EPMA and FEG-SEM

First the sample was prepared for obtaining the microstructure by optical

microscope. For this the sample was first polished in the emery papers of different

grits. After this cloth polishing was performed in alumina suspension. Subsequent

to grinding, the surfaces of the specimens were polished on rotating cloth, on

which slurry of alumina powder mixed with water was applied uniformly and

distilled water was used as lubricant.

After the desired surface was obtained etching was done, because polished metal

specimens usually show no structural characteristics. Etching of the metal surface

was done to make visible the crystalline structure of the metal and to produce

optical contrast between the various constituents.The specimens were etched with

2 % nital solution (2% conc. HNO3 + Ethanol). Etching was done by swabbing

which involves wiping the sample surface with cotton saturated with etchant. The

22

specimen was cleaned thoroughly to avoid damage on objective lenses using

ethanol first, then an ultrasonic cleaner and then dried.

4.3 Microstructure evaluation and point analysis

Metallographic samples were examined in optical (Leica DFC 295), Electron

probe micro analyzer (JEOL JXA 8230) and scanning electron microscope (Nova

nano SEM 430) to reveal finer structural details in the diffusion zone. Elemental

point analysis was done in weight % from different location of reaction zone using

energy dispersive spectroscope attached with SEM. Line analysis of diffused

elements was done at the joint on a length of 100 µm.

4.4 Vickers micro hardness testing

After the SEM, EPMA study was done the micro hardness testing was carried out

by Vickers micro hardness testing machine (LEICA VMHT AUTO) under 100gmf

(0.1kgf ) load for a dwell time of 15 seconds at an indenting speed of 30 µm/s

and a micro hardness profile was obtained across the interface. Total 16 mm

length was taken in order to perform this test with keeping interlayer as the center

8 mm before and 8 mm after the center. The distance between 2 points was 0.5

mm for indentation and hardness measurement.

23

CHAPTER 5 Result and discussion

5.1 Optical microscope analysis

The diffusion bonded high strength steel sample which was made at an

temperature of 1100 oC for 30 minutes with an applied load of 1 tonne viewed on

the optical microscope after etching was done. At various magnifications its

microstructure was observed which showed that the interlayer was totally

homogeneous and free from voids and any type of defects

Fig .7. Optical microstructure of diffusion bonded high strength steel (a) at low

magnification (b) at high magnification

The above microstructures reveals that the base material was originally was

tempered martensite which has been converted to pearlitic structure, confirmed by

the lamellar structure observed by SEM study. Joint consisted of continuous

interface without any gross heterogeneity along with the continuous reaction

layers on both sides labelled as ‘A’ as shown in Fig .7. ‘B’ represents the

unreacted/remaining interlayer.

Interlaye

Base metal Base

metal

B

A A

24

5.2Analysis by SEM and WDS

To understand the effect of diffusion of chemical species across interface, bonded

samples were examined in SEM. Pearlitic structure was also detected. Quantitative

chemical analysis was done using WDS. WDS results of the two micrographs

shown in Fig .9. and table 3.

Fig .8. SEM microstructure of diffusion bonded high strength steel (a) at low magnification (b) at high magnification

Fig .9. Points in WDS study in the diffusion bonded high strength steel

Table 3: Composition at various positions by WDS analysis of DB sample

LABEL Si C Fe Mn Cr Cu Ni Mo Position 1 0.9508 62.6589 91.3194 0.7717 0.2738 0 0.0215 0.004 Base

metal 2 0.6634 6.5484 88.9018 0.7002 0.3036 0.0412 0.0412 2.8415 Interlayer

Base metal boundary

3 0.4479 3.101 21.7488 0.3206 0.0582 0.0627 72.2544 0.0064 Centre of interlayer

Inte

rfac

e

Bas

e M

etal

Ni i

nter

laye

r

B

ase

Met

al

Pearlite Inte

rlay

er

25

5.3EPMA line profile analysis

To understand the effect of diffusion of chemical species across interface, bonded

samples were examined in EPMA. Fig.11. shows the variation of concentrations

for various major elements across the diffusion bonded interface.

Fig.10. Line profile analysis by EPMA

Fig.11. Concentration profile of present elements across the diffusion bond interface

0

0.5

1

1.5

2

2.5

3

0

10

20

30

40

50

60

70

80

90

100

0 10 20 30 40 50 60 70 80 90 100

Fe

Ni

Si

Mn

C

Cr

Cu

Mo

001

Atom

ic %

Distance in µm

26

From the above data and plot we can observe that at the diffusion boundary

regions Fe diffuses in more amount as compared to other elements, and Ni also

diffuses more into the base metal region. This concludes that Ni and Fe have very

good solid solubility where in other elements fails to achieve this. Apart from all

Mn diffuses little more higher than other elements like Si, Cr, Cu, Mo etc. The

carbon concentration at the boundary region was not too high this indicates that no

carbide formation takes place in this region. The nature of Si and Mo profiles

shows that no Si/Mo was present in the interfacial area. Some amount of Cr was

present, but this amount was not significant to predict formation of any compound.

However, as shown in table 3, concentration of iron and nickel was different

across bond line. Contribution of 2 other alloying elements was ignored in this

respect as their quantity was too small to create any effect. Binary phase diagram

of Fe–Ni did not report any intermetallic phase formation even at room

temperature [30] rather, it indicated three clear regions-bcc iron solid solution up

to ~5 wt% Ni, α-γ coexistence between ~5 and 18 wt% Ni, and γ solid solution

beyond 18 % Ni [1]. At 1100 ̊C and at room temperature Ni and Fe form only solid

solutions that can be easily demonstrated from the Ni-Fe phase diagram.

Fig.12. Fe-Ni phase diagram

27

5.4 Micro hardness evaluation

Micro hardness test was carried out at the end to obtain the hardness profile of the

diffusion bonded high strength steel sample, which is plotted in figure 13.

The micro hardness of the base metal was ~320 – 380 VHN which was lower than

the parent value of ~550 VHN with maximum micro hardness of ~380 VHN . This

was due to the transformation of tempered martensite to pearlite while cooling to

room temperature. As expected the micro hardness starts decreasing at the

interlayer base metal interface and attains a minimum value at the centre of the

interlayer i.e. ~128 VHN. Here the white interlayer was the softest region.

Interface micro hardness was ~253 VHN.

Fig.13. Micro hardness profile of the diffusion bonded joint

0

50

100

150

200

250

300

350

400

450

-9 -8 -7 -6 -5 -4 -3 -2 -1 0 1 2 3 4 5 6 7 8 9

Vick

ers M

icro

hard

ness

(VH

N)

Distance from centre (mm)

steel (Base Metal)steel (Base Metal)

Centre of Ni foil

28

CHAPTER 6

Summary

1. Microstructure of joint was pearlite, which was transformed from

tempered martensite (parent metal).

2. The thickness of the Ni interlayer was found to be around ~40µm

which was observed by optical microscope and scanning electron

microscope.

3. Sound welding of diffusion bonded high strength steel was observed

by optical microscope and scanning electron microscope. It was not

observed any defect in the joint.

4. It was observed in micro hardness that the Ni interlayer was the

softest region with micro hardness value ~128 VHN. The hardness of

substrate ~380 VHN, which was reduced as compared to parent metal

~550 VHN. Hardness at the interface was ~253 VHN.

29

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