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CHAPTER 3

EXPERIMENTAL WORK

Friction Stir Welding is a solid state welding process and has emerged

as a welding technique to be used in high strength aluminium alloys, which are

difficult to join with conventional techniques. In this process, the external tool

has a probe and shoulder stirs the material to be joined and forges the surface.

FSW combines forging and extrusion processes. The probe and shoulder extrude

the material by stirring and the shoulder alone forges the material surface to be

joined. Aluminium material was profitably developed and the welding plates

from 1 to 75 mm thickness can be welded by FSW process. Most of the research

works considered the friction stir welding parameters of FSW machine and FSW

Tool geometry (Leal and Loureiro 2006; Rajakumar et al 2010; Merzoug et al

2010; Elangovan et al 2009). The parameters of FSW machine are tool rotation

speed, welding speed and axial force. Some of the parameters of FSW tool

geometry are probe or pin diameter, shoulder diameter to probe diameter ratio,

probe length and probe profile (Padmanaban and Balasubramanian 2009). The

experiments as per Taguchi�s L25 orthogonal array were formulated for different

thicknesses of 4 mm, 6 mm and 8 mm plates. Moreover the orthogonal array

directly gives the optimum level of the parameters. This provides the certainity

to directly study the results of optimum value of different thickness plate. The

Central Composite Design of the Response Surface Method is considered for

double side weld of 8 mm thickness plate.

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52

The main objective of the work is to perform a tensile test on friction

stir welded 6082-T6 aluminium alloy. An attempt has been made to optimize the

parameters to maximize the tensile strength of the different thickness FSW joint.

The optimum parameter level for every thickness was interpreted and projected.

Also the reduced weld zone by double side weld is discussed. The macro-

structure, micro-structure and micro-hardness of the weld joint were discussed at

different heat inputs. Numerical investigation was carried out to predict the

temperature distribution in the weldments. The flow chart described in figure 3.1

details the plan of the research work.

3.1 FRICTION STIR WELDING MACHINE AND PARAMETERS

The experiments were carried out on conventional HMT FN2V

vertical milling machine with a capacity of 7.5 hp and 1800 rpm as shown in

Figure 3.2. Process parameters such as spindle speed, welding speed, shoulder

penetration; probe profile and the shoulder profile were considered.

Figure 3.2 - Vertical milling machine

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The conventional milling (CM) machine offers the tool rotational

speed and welding speed by their spindle speed and table feed. Only the axial

force is difficult to attain in conventional milling machine (Minton and Mynors

2006). In CM machine, tool shoulder penetration executes the force parameter.

When the tool shoulder penetration increases, the axial force increases by the

resistance offered between the tool shoulder and material to be welded with

backup plate. Even in FSW machine, axial force is difficult to retain throughout

the welding. But the shoulder penetration/plunge depth can be maintained

(Zimmer et al 2010).

Trial runs are conducted to find the upper and lower limit of process

parameters, by varying one of the parameters and keeping the rest of them at

constant values. Feasible limits of the parameters are chosen in such a way that

the joint should be free from visible defects. The upper limit of a factor is coded

as +2 and lower limit as -2. The intermediate coded values are calculated from

the equation (3.1).

Xi = 2 [2X- (Xmax +Xmin)] / (Xmax - Xmin) (3.1)

Where Xi is the required coded value of a variable X; and X is any

value of the variable from Xmin to Xmax. Xmin is the lower limit of the variable and

Xmax is the upper limit of the variable.

The range of tool rotational speed and welding speed were taken from

700 rpm to 1500 rpm and 0.8 mm/sec to 4 mm/sec respectively (Minton and

Mynors 2006). Shoulder penetration was gradually increased in five steps of

0.05 mm from 0 mm to 0.20 mm for 6 mm and 8 mm thick plates. But this has

been modified for 4 mm thick plate, the shoulder penetration is considered at

five steps of 0.04 mm from 0 mm to 0.16 mm. The selected process parameters

with their limits, units and notations are given in Table 3.1.

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3.2 FRICTION STIR WELDING TOOL

High carbon high chromium steel (HCHCr) has been favoured among

high speed steel and carbides, to weld Aluminium Alloy materials due to the

Aluminium alloy�s low melting point and low hardness. Also HCHCr tool cost is

comparatively less, easy to process, easy availability and offers high strength and

high hot hardness. (Vijay and Murugan 2010). Double end usage of tool

minimizes tool material and tool cost. A typical double end tool used for trial

runs is shown in Figure 3.3. A collar or step has been provided in mid portion, to

constrain the axial movement of the tool and it also offers excess area for

transfer of torque which multiplies the force (Shigley and Mitchell 1984).

Figure 3.3 - Typical double end tool - shows (a) collar at mid and (b) square pin

(a)

(b)

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In tool geometry, shoulder profile (Badarinarayan et al 2009; Ericsson

et al 2007) is considered as a parameter. The inclination angles given for

shoulder are 5° inward and 5° outward, 10° inward and 10° outward, and with no

inclination. The inward slope extrudes the material at the nugget area of the joint

which might increase the tensile strength at joint. The outward slope closely

forges the material at the nugget area of the joint which might also increase the

tensile strength. Twenty five various FSW tools were established (Appendix 2)

by the combinations of five different pin profiles of square, pentagon, hexagon,

heptagon and octagon; for five different shoulder profiles of 10° inward, 5°

inward, flat, 5° outward and 10° outward. Few of them are shown in Figure 3.4.

Figure 3.4 - Various friction stir weld tool profiles

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Table 3.1

Process parameter with their range and values at five levels

Process Parameters Range Level 1

(-2) Level 2 (-1)

Level 3 (0)

Level 4 (1)

Level 5 (2)

Spindle Speed or Tool Rotational Speed (SS)

700 to1500 rpm

700 900 1100 1300 1500

Table Feed or Welding Speed (WS)

0.8 to 4.0 mm/sec

0.8 1.6 2.4 3.2 4.0

Shoulder Penetration (PE)

0.0 to 0.16* mm

0.00 0.04 0.08 0.12 0.16

0.0 to 0.2** mm

0.00 0.05 0.10 0.15 0.20

Probe Profile (PP)

Probe profile*

Triangle TR

Square SQ

Pentagon PN

Hexagon HX

Heptagon HP

Probe profile**

Square SQ

Pentagon PN

Hexagon HX

Heptagon HP

Octagon OC

Shoulder Profile (SP)

-10° to 10° -10° -5° 0° 5° 10°

*For 4 mm plates, **For 6 mm and 8 mm plates.

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For 4 mm thickness plate five different pin profiles of triangle,

square, pentagon, hexagon, and heptagon were considered. Friction stir tool has

two parts namely shoulder and pin. In this study pin diameter, shoulder diameter

to pin diameter (D/d) ratio and the pin length were taken as d mm, 3d and 0.95d

mm (Elangovan and Balasubramanian, 2008a) respectively. Table 3.1 shows the

process parameters in detail. The pin profiles were cut by spark erosion with

WEDM machine and others including inward and outward taper of the shoulders

was obtained in CNC turning center. The tools were oil hardened to obtain a

hardness of 60-62 HRC.

3.3 MATERIAL SPECIFICATION AND PREPARATION

The 6xxx-group is high strength AL-Mg-Si alloys that contain

manganese to increase ductility and toughness. Aluminium alloy 6082 has the

highest strength among the 6000 series alloys with excellent corrosion resistance

property. Alloy 6082 is known as a structural alloy. The addition of large amount

of manganese controls the grain structure which in turn results in a stronger

alloy. The 6082-T6 extruded medium to high strength Al-Mg-Si alloys contains

manganese to increase ductility and toughness (Appendix 3). The T6 condition is

obtained through artificial ageing at a temperature approximately 180°C

(Ericsson and Sandstorm 2003). The alloy 6082 is very common in Europe

(Alcoa, 2007) and is intended for structural applications including rod, bar, tube

and profiles.

Table 3.2

Chemical composition of the 6082 alloy (weight%)

Si Fe Cu Mn Mg Cr Zn Ti

0.7 � 1.3 0.50 0.10 0.4 � 1.0 0.6 � 1.2 0.25 0.20 0.10

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Table 3.3

Mechanical properties of the 6082 alloy

Thickness of the Material

Yield Strength (MPa)

Ultimate Strength (MPa)

Elongation (%)

Hardness (HV)

Density(g/cm3 )

Melting Point (°C)

4 mm 221.02 303.80 11.0 91-96 2.70 555

6 mm 223.25 306.85 11.0 93-98 2.70 555

8 mm 225.22 308.55 11.5 93-98 2.70 555

Composition of the AA 6082-T6 material is presented in Table 3.2.

The tensile specimen was prepared as per ASTM E8M-04 standard (2006) and

the base metal has been tested on the Universal Testing Machine. Micro Vickers

hardness of the AA 6082-T6 base material was measured. The results are shown

in Table 3.3.

3.4 MATHEMATICAL TOOLS FOR OPTIMIZING THE WELDING

PARAMETERS

3.4.1 Taguchi Experimental Design Technique

Taguchi Experimental Design Technique, Orthogonal Array, (Philip J

Rose 2005; Nicolo Belavendran, 1995; Montgomery 2001; Box et al, 1978)

assesses the influence of factors on the response, the means and the signal-to-

noise ratios (S/N) for each control factors which are to be calculated. Signals are

indicators of effect on average response and noises are a measure of deviations

from experiment output. The appropriate S/N ratio must be chosen using

previous knowledge, expertise and understanding of the process. In this study,

S/N ratio has been chosen based on the quality characteristic, �larger-the-better�

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in order to maximize the response. In Taguchi Method, S/N ratio ( j) in the jth

experiment is expressed in equation (3.2).

j =-10 log [1/n (1/Yijk2)] (3.2)

Where �n� is the number of tests and �Yijk� is an experimental value of

�i' th quality characteristics in �j� th experiment at �k� th test. In the present study,

tensile strength was measured from the samples and the data are analyzed to

determine the effect of FSW process parameters. Experimental results are

transformed into means and S/N ratio.

Analysis of the mean for each of the experiments gives better

combination of parameter levels. The mean response refers to an average value

of performance characteristics for each parameter at different levels. Means for

one level is calculated as the average of all responses that are obtained within

that level. Analyzing means and S/N ratio of various process parameters, the

larger S/N ratio corresponds to better quality characteristics. Therefore, optimal

level of process parameters is at the level of highest S/N ratio.

3.4.2 Response Surface Methodology

The statistical experimental designs most widely used in optimization

experiments are termed "response surface designs." In addition to the trials at the

extreme level settings of the variables, response surface designs contain trials in

which one or more of the variables are set at the central level of the study range

(other levels in the interior of the range may also be represented). Thus, these

designs provide information on direct effects, pairwise interaction effects and

curvilinear variable effects. Response surface methodology approach to product

and process optimization work (Box et al, 1978; Montgomery 2001).

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Figure 3.5 - Central composite circumscribed (CCC) design

A Box-Wilson Central Composite Design, commonly called a central

composite design, contains an imbedded factorial or fractional factorial design

with center points that is augmented with a group of `star points' that allow

estimation of curvature. If the distance from the center of the design space to a

factorial point is ±1 unit for each factor, the distance from the center of the

design space to a star point is ± with | | > 1. The precise value of depends on

certain properties desired in the design and on the number of factors involved.

The Central Composite Circumscribed (CCC) explores the largest

process space and the Central Composite Inscribed (CCI) explores the smallest

process space. Both the CCC and CCI are rotatable designs, but the Central

Composite Face-centered (CCF) is not. In the CCC design, the design points

describe a circle circumscribed about the factorial square. For three factors, the

CCC design points describe a sphere around the factorial cube.

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Central composite circumscribed designs are the original form of the

central composite design. The star points are at some distance from the center

based on the properties desired in the design and the number of factors in the

design. The star points establish new extremes for the low and high settings for

all factors. Figure 3.5 presents a rotatable CCD for time and temperature using

the previously defined lower and upper variable bounds.

When all independent variables are measurable, controllable and

continuous in the experiments, with negligible error, then the response surface

can be expressed by

y = f (x1, x2,��, xk)

Where, �k� is the number of independent variables. To optimize the

response �y�, it is necessary to find an appropriate approximation for the true

functional relationship between the independent variables and the response

surface. Usually a second-order polynomial equation (3.3) is used in RSM.

y = bo + biXi + biiXii2+ bijXiXj+ (3.3)

The test was designed based on a five factors at five levels; central

composite rotatable design with full replication consisting of 32 experimental

runs (Gopalakrishnan and Murugan 2011) allowed the prediction of

mathematical model and the influence of parameters on the mechanical

properties. The Friction stir welding input variables are spindle speed, welding

speed, shoulder penetration, pin profile and shoulder profile. A statistical

software �Minitab� was used to code the variables and to establish the design

matrix to obtain the required equation.

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3.5 TESTING OF WELDS

Figure 3.6 - Universal testing machine

Tensile tests provide information on the strength and ductility of

materials under uniaxial tensile stresses. This information may be useful in

comparisons of materials, alloy development, quality control, and design under

certain circumstances. The results of tension tests of specimens machined to

standard dimensions from selected portions of a part or material may not totally

represent the strength and ductility properties of the entire end product or its in-

service behavior in different environments. These test methods are considered

satisfactory for acceptance testing of commercial shipments. The test methods

have been used extensively in the trade for this purpose. The tensile specimen

was prepared as per ASTM E8M-04 standard (2006) and the base metal as well

as the welded joints has been tested on the TUE-CN-1000 Universal Testing

Machine as shown in Figure 3.6.

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Figure 3.7 - Vickers micro hardness tester

The Vickers micro hardness method is based on an optical

measurement system. The Micro hardness test procedure, ASTM E-384,

specifies a range of light loads using a diamond indenter to make an indentation

which is measured and converted to a hardness value. It is very useful for testing

a wide type of materials as long as the test samples are carefully prepared. A

square base pyramid shaped diamond is used for testing in the Vickers scale.

Typically loads are very light, ranging from a few grams to one or several

kilograms, although "Macro" Vickers loads can range up to 30 kg or more. The

Micro hardness methods are used to test metals, ceramics, and composites.

Sample preparation is necessary with a micro hardness test in order to

provide a small enough specimen that can fit into the tester. Additionally, the

sample preparation will need to make the specimen�s surface smooth to permit a

regular indentation shape and good measurement, and to ensure that the sample

can be held perpendicular to the indenter. Usually, the prepared samples are

64

mounted on a plastic medium to facilitate preparation and testing. The

indentations should be as large as possible to maximize the measurement

resolution. (The error is magnified as indentation sizes decrease) The test

procedure is subject to problems of operator influence on the test results. Micro-

hardness of the welds was measured at a test load of 5 N for 10 seconds. The

indentations were made at the midsection of the thickness of the plates across the

joint using Mitutoyo Vickers Micro hardness tester as shown in Figure 3.7.

3.6 METALLOGRAPHIC INVESTIGATION

To reveal the true microstructure of a material, the following points

must be observed during the specimen preparation. Surface undulation must be

completely removed using an appropriate etchant. Surface undulation can be

implanted into the surface of the specimen by sectioning, grinding and polishing.

Induced surface undulation falls into two categories, gross undulation and plastic

undulation. The gross undulation may be visible before surface preparation and

can be attributed to the method used to remove the sample from its original

surroundings and the planar grinding steps. The plastic undulation is hidden by

the gross undulation and is complementary to it. For successful specimen

preparation, all the stages of undulation must be removed or reduced to a

negligible level.

Metallographic analysis is an experience based practice. Many guides

for specimen preparation give a basic and universal approach to this science,

however each material is different and may require alterations to certain

preparatory steps in order to produce the correct images. The list below briefly

shows the different stages of sample preparation, in order to ensure a quality

surface finish and correct imaging of the aluminium alloy 6082 investigated.

1. Sampling - Choosing the specimen.

2. Sectioning - Removing the area of interest.

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3. Encapsulation � Mounting the specimen.

4. Grinding and polishing - Removing all the surface undulations and

irregularities to provide a undulation free surface.

5. Initial visual examination - Preparation quality assessment.

6. Etching - Highlighting of specific material characteristics.

7. Visual examination - Microstructural interpretation.

8. Quantitative Metallography - Obtaining data from the prepared

specimen.

9. Data interpretation - Drawing conclusions from the data.

The samples mounted for micro structural observation are cut from

the weld in the transverse direction to study the weld structure in detail. A piece

of 5 mm x 2 mm x 2 mm is cut from the welded sheets across the weld. The

samples are ground on a grinder, to smoothen the edges and to get an even

surface, for studying the micro structure. Mounting is done on a Buehler

automatic mounting press with black phenolic powder.

The mounted samples are polished on emery paper of grit 400, 600

and 800 with a combination of water and diamond paste for smooth finish. The

final stage of polishing is on smooth cloth with a diamond suspension liquid for

smooth finish. The samples are etched after polishing to reveal the

microstructure clearly. The acids in the etchant attack the grain boundaries and

give a clear image of the size of the grains. The agent used for aluminum alloys

is Keller�s etchant, which is prepared by adding 1 percent of hydrofluoric acid

by volume, 1.5 percent of hydrochloric acid by volume, 2.5 percent of nitric acid

by volume and 95 percent of distilled water by volume. The etched samples are

washed thoroughly to remove the carbon deposits and make dry to study the

microstructure under the optical microscope.

In this work, the welded plates were cut at mid welded portion and

specimens of size 40 mm x 10 mm x 4 mm, 40 mm x 10 mm x 6 mm and 40 mm

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x 10 mm x 8 mm were obtained for metallographic study. The samples were

prepared as per standard metallographic procedure from the welded plates and

macro etched using Keller�s solution (Beraha and Shpigler 1977). The images of

the macrograph of the etched specimen were captured using an optical scanner.

The metallographic study was carried along the various zones of the parent

metal, heat affected zone, thermo mechanically affected zone and weld nugget

across the cross sections of friction stir welded specimens using scanning

electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS)

(Liu et al 1997; Rhodes et al 1997; Flores et al 1998; Su et al 2003; Sutton et al

2002).

Scanning Electron Microscopes (SEM) are used for inspecting

topographies of materials with a magnification range that encompasses that of

optical microscopy and extends it to the nanoscale. SEM can scan the surface of

a sample with a finely focused electron beam to produce an image from the

beam-specimen interactions detected by a wide array of detectors. A variety of

detectors are available, from secondary electron detectors to provide surface

information, to backscattered detectors for compositional information in both

high and low vacuum modes.

3.7 HEAT ENERGY IN FRICTION STIR WELDING

Heat generated from friction between tool shoulder and workpiece

was implemented as the heat input (Chao and Qi 1998). The empirical equation

for calculating the heat input to the workpiece is given by equation (3.4).

Q(r) = 3Qt / 2 (ro3 ri

3) (3.4)

Where Q(r) is the rate of heat input, ro and ri are the radii of the

shoulder and the nib of the pin tool, and Qt is the total rate of heat input to the

work piece expressed as shown in equation (3.5).

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Qt = F (ro2+rori+ri

2) / 45 (r0+ri) (3.5)

Where, is the tool rotational speed, is the frictional coefficient,

and F is the downward force. The total heat developed is fully dependent on the

tool rotation speed and the axial force. The axial force is must to have the tool

material contact to generate the heat. But if the axial force is excess, the tool will

penetrate inside the work. So, the axial force is constant and negligible compared

to the tool rotation speed. The total heat generated will travel along the joint

interface to have weld. If this travelling speed is slower, then the total heat is

concentrated to that short span of the area. If the travel speed is faster, then the

total heat is dissipated to the larger span of the area. This can be expressed by

Q Spindle speed or Tool rotation speed and

Q (Welding speed or Travel speed) -1

The combination of higher spindle speed and lower weld speed

generates a more amount of heat during friction stir welding. Similarly, the lower

spindle speed and higher weld speed combination does not produce sufficient

heat. The deduced working zone or range of parameters is 700 rpm to 1500 rpm

spindle speed and 0.8 mm/sec to 4.0 mm/sec weld speed. From this the higher

spindle speed and lower weld speed combination, produces a higher amount of

heat during welding termed (Rodrigues 2009) as �hot weld� (HW) and the lower

spindle speed and higher weld speed combination produces lesser amount of

heat during welding termed as �cold weld� (CW). The optimum condition

combination obtained through experimental design technique is termed as �weld

at optimum condition� (WOC). HW is designated for the parameter condition of

1500 rpm spindle speed and 0.8 mm/sec weld speed. CW is designated for the

parameter condition of 700 rpm spindle speed and 4.0 mm/sec weld speed.