CHAPTER 4 MECHANICAL TESTING AND METALLURGICAL...

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CHAPTER 4

MECHANICAL TESTING AND

METALLURGICAL CHARACTERIZATION

OF THE FRICTION WELDED JOINTS

4.1 INTRODUCTION

As the welded joints find applications in critical components

(whose failure results could lead to catastrophe), the acceptance standards are

continually rising. Acceptance standards represent the minimum weld quality

and are based upon test of welded specimens containing some discontinuities;

usually a safety factor, which was added to yield the final acceptance

standard. A good research effort is to correlate the discontinuities with the

performance.

Small imperfections, which cause a variation in the average

properties of the weld-metal are called discontinuities. When the discontinuity

is large enough to affect the function of the joint it is termed a defect (Khan,

2007).

Standard codes do permit a limited level of defects based on

principles of fracture mechanics, taking into consideration, the service

conditions of the fabrication. In spite of this, the fabricator must strive to

prevent the occurrence of weld defects in the first instance and to rectify them

if they do occur. There are many types of defects such as undercuts, cracks,

porosity, slag inclusions, lack of fusion and lack of penetration.

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All types of welded structures which experience high pressure are

expected to perform certain functions. The joints comprising these structures

must possess certain service-related capabilities. To test that the required

function will be met, different types of tests were conducted. The ideal test is

the observance of the structure in actual practice. This is usually not possible.

Therefore, some destructive tests and non-destructive tests were performed on

the standard specimens to assess the behavior of the structure in service.

Laboratory tests should be carried out with caution because the size,

configuration, environment, types of loading may not be identical to the

actual situation. When selecting a test, its function, time and cost factors

should be considered (Khan, 2007).

This chapter presents a brief description of various destructive and

non-destructive tests carried out on the friction-welded joints (as part of the

investigation) and metallurgical characterization.

As part of destructive testing, the following tests were carried out:

Tensile test;

Bend test;

Hardness test; and

Impact test.

Only radiography test was performed on the welded specimen

under the non-destructive tests. Further, metallographic examinations

(micrograph, macrograph and SEM) were carried out on the welded

specimens. Finally, XRD examination and Williamson Hall plot (WH plot)

were performed on the friction-welded specimens to observe the chemical

composition of the weld and the base metals and the strain developed at the

welded joint.

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The test standards specified for welded specimens with solid state

welding process were adopted for the tests on friction welded specimens to

assess quality and strength.

4.2 EVALUATION OF MECHANICAL STRENGTH THROUGH

DESTRUCTIVE TESTING

Any test which alters the shape, form, size, or structure of the

material being tested is termed a destructive test. These are service-

weldability tests that are designed to measure mechanical properties vital to

the satisfactory performance of the welded joint in service. Out of the various

service-weldability tests, transverse tensile test and bend test are found to be

more suitable. For all the mechanical tests, the specimen should be prepared

such that the welded joint is in the middle of the specimen.

4.2.1 Tensile Tests

Tensile tests were carried out to determine the ultimate tensile

strength on the weld section that is heterogeneous in nature, containing base

metal, weld metal and the welded joint. Various types of tensile tests carried

out to evaluate the weldment include, all-weld metal test, longitudinal butt-

weld test, transverse butt-weld test, transverse butt weld with notch test and

tension-shear test. The transverse tensile test is a standard test for procedure

qualification and is also used to indicate whether the weld strength equals that

of the base metal tensile strength or less (Anderson, 1988).

To find the tensile strength of the welded joint the samples were

prepared as per ASTM standards (AWS B4.0). Tensile test was carried out by

gripping one end of the specimen in a universal testing machine (UTM)

shown in Figure 4.1 and applying an increasing pull on to the specimen ends

till it fractures. During the test, the tensile load and the elongation of a

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previously-marked gauge length in the specimen were measured using the

load dial of the machine and extensometer, respectively. Interpretation of the

test results for the welded joint as a whole is complicated due to the different

strengths and ductility in the various regions of the joint. This lack of uniform

properties is particularly important with the transverse tensile test.

Figure 4.1 Universal Testing Machine

The five weld samples were prepared by using the optimized

friction welding parameter values as per the Table 3.7 (trials based on design

of experiments) of the chapter 3. The Friction-welded specimens were

prepared as per the specifications provided by AWS B4.0 standard (Figure

4.2).

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Figure 4.2 Friction welded transverse tensile test specimen

If the strength of the weld metal exceeds that of the base metal, a

resultant necking and failure will occur outside the weld area. When the weld

strength is considerably lower than that of the base metal, most of the plastic

strain occurs in the weld. The tensile test results are given in Table 4.1, which

indicates that the weld strength is slightly above the ultimate tensile strength

in the base material of the specimen. Failure in the base material of the

specimens in trials indicates that the weld joint is stronger than the base

material. The failure of specimen in the weld means that the weld is weaker

than the base material. This may be possibly due to the insufficient heating

time or pressure. From the tensile tests, it was observed that there was no

fracture in the weldment, because friction welds were conducted as per the

trials based on design of experiments.

Table 4.1 Results of tensile test on the friction-welded specimens

Specimen/ Material

Specimen size in mm

U.T.S. in MPa

Position of fracture

1 12.7 × 6.00 456 Base Metal 2 12.7 × 6.00 506 Base Metal 3 12.7 × 6.00 498 Base Metal 4 12.7 × 6.00 493 Base Metal

5 12.7 × 6.00 448 Base Metal

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4.2.2 Bend Tests

The bend test is a ductility test employed to evaluate the ability of

the weld to undergo plastic deformation in bending. The quality of the weld,

in terms of ductility of the weld metal as well as tests for opening defects,

particularly lack of side-wall fusion, root fusion and penetration of welded

joint are most frequently revealed by means of a bend test (Anderson, 1988).

-homogeneity of the joint, there is a tendency for free

bend test specimens to take-up an irregular shape. This defect is overcome in

guided bend test.

The top and bottom surfaces of a specimen are designed as the face

and root surface respectively. In face-bend test, face-side is under tension. To

be acceptable, a test piece must be capable of bending through 180º with no

surface fissures. Bend test may be carried out on a universal testing machine

with the help of certain attachments. Any cracks of the metal indicate false

fusion or defective penetration. The stretching of the metal indicates its

ductility to some extent. Friction welding parameters are optimized to an

extent to have maximum penetration. Bend tests are used primarily to

determine the degree of weld penetration. In a bend test the specimen is

placed in the jig with the face or root down depending on whether it is face-

or root-bend test. The mandrel is depressed until the piece becomes U-shaped

in the die (Figure 4.3).The diameter of the mandrel is four times the thickness

of the specimen and angle of the bend is 180º.

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Figure 4.3 Bend test of friction-welded specimen

For conducting the bend tests, the five weld samples were prepared

by using the optimized friction welding parameter values as per the Table 3.7

(trials based on design of experiments) of the chapter 3. A specimen for bend

test prepared as per AWS 4.0 standard is shown in Figure 4.4. The results of

bend test are given in Table 4.2.

Figure 4.4 Friction-welded bend test specimen

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Table 4.2 Bend test results

Specimen Bend Test Remarks

1 No open discontinuity observed Passed


3 1 mm open discontinuity observed Passed


5 1 mm open discontinuity observed Passed

It was observed that all the specimens passed the bend tests and

showed no or 1 mm open discontinuity, respectively. Specimen failure in the

bend test implies an insufficient forging pressure or presence of impurities

due to improper cleaning. In this case, the failure of bend may be due to

excess melting of metal and increase of brittleness and hardness at the weld.

As the welds were prepared as per the trials based on design of experiments,

no failure was observed during bend tests.

4.2.3 Micro-Hardness Test

The term micro-hardness test usually refers to static indentations

made with loads not exceeding 0.5 kg. The indenter is either the Vickers

diamond pyramid or the Knoop elongated diamond pyramid. The procedure

for testing is very similar to that of the standard Vickers hardness test, except

that it is done on a microscopic scale with higher precision instruments. The

surface being tested generally requires a metallographic finish; the smaller the

load used, the better the surface finish required. Precision microscopes are

used to measure the indentations; these usually have a magnification of

around X500 and measure to an accuracy of +0.5µm. Also, with the same

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observer, differences of +0.2µm can usually be resolved. The equipment used

for measuring the micro-hardness of friction-welded specimen is as shown in

Figure 4.5.

Figure 4.5 Vickers micro-hardness measuring instrument

The Vickers hardness test consists of indenting the friction-welded

test material with a diamond indenter, in the form of a right pyramid with a

square base and an angle of 136 degrees between opposite faces (Figure 4.6)

subjected to a load of 0.1 to 1 kg. The full-load is normally applied for 15

seconds. The two diagonals of the indentation left in the surface of the

material after removal of the load are measured using a microscope and the

average is calculated. The area of the sloping surface of the indentation is

calculated. The Vickers hardness is the ratio of load and the area of

indentation (Anderson, 1988).

2

2136sin2

d

FHV

o

(4.1)

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where F = Load in kg

d = Arithmetic mean of the two diagonals, d1 and d2 in mm

HV = Vickers hardness

HV = 1.854 (F/d2) approximately

When the mean diagonal of the indentation is determined the Vickers

hardness may be calculated.

Figure 4.6 Vickers hardness test

Preparation of the specimen for micro-hardness test involves the

following processes:

Cutting and burring;

Mounting of the specimen for handling during preparation;

Grinding;

Polishing; and

Etching.

When the specimens are mounted and polished, consideration

should be given to edge support if impressions are to be made near edges. If

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the edges are rounded off even slightly during metallographic preparation,

due to soft mounting compounds, the micro-hardness impressions will be

distorted. Figure 4.7 illustrates the micro-hardness levels at the interface of

the welds.

Figure 4.7 Micro-hardness measured along the weld-line

During the past decades, many efforts have been made to correlate

the tensile strength with microhardness. The relationship between the tensile

u

HV*3.33(en.wikipedia.org/wiki/vickers-hardness-test). Here, C is a constant.

By using this equation, tensile strengths were calculated and compared with

measured tensile strengths. Since the measured tensile strength values (456

MPa, 506 MPa, 498 MPa, 493 MPa and 448 MPa) were higher than the

calculated values (280.38 MPa, 263.07 MPa, 247.08 MPa, 272.06MPa and

258.07 MPa), the welds had good strength.

4.2.4 Charpy Impact Test

This is a dynamic test in which the friction-welded test-piece, v-

notched in the middle and supported at each end, is broken by a single blow

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of a freely swinging pendulum. The energy absorbed is a measure of the

impact strength of the material. The test is carried out at room temperature

(Nayar, 2005). The equipment used for charpy impact test of the friction-

welded specimen is shown in Figure 4.8.

Figure 4.8 Charpy impact test equipment

The standard test bar is 10 mm x5 mm x 55 mm as per AWS B.4. A

specimen for charpy impact test prepared as per AWS B.4 standard is shown

in Figure 4.9. The results obtained are presented in Table 4.3.

Figure 4.9 Specimen dimensions for charpy impact test

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Table 4.3 Impact test results

Specimen Energy absorbed (J)

1 29

2 29

3 31

4 30

5 31

The test results indicate variation of impact strength for base

material, welds with varying range of parameters. The impact toughness at 0º

was found to be nearly 24J or the base metal. The impact strengths for

specimens are 29, 29, 31, 30 and 31, respectively. It was observed that

specimen 3 and 5 yielded the highest impact strength. Since test standards are

not yet available for welded specimens with interlayer, the pass or failure of

specimen for impact test cannot be specified.

4.3 NON DESTRUCTIVE TESTING (NDT)

NDT is an examination of a component in any manner which does

not impair its future use.

Several objectives of NDT are:

Material sorting;

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Materials characterization;

Property monitoring( process control);

Thickness measurement;

Defect detection/location; and

Defect characterization.

The major task for NDT is to detect and identify the range of

defects ( Hellier, 2003).

4.3.1 Radiography Test

Radiography is one type of a non-destructive testing technique

which may be employed to examine the defects in friction welded joints.

Radiography is essentially a shadow pattern created when certain types of

radiations penetrate an object. The patterns may be different depending on

variations of thickness, density or chemical composition of the material. The

transmitted radiation is commonly registered on a photographic film to

provide a permanent record.

There are three common techniques used in radiography viz.

i) Single-wall exposure, single-view technique;

ii) Double-wall exposure, single-view technique; and

iii) Double-wall exposure, double-view technique.

For the friction welded specimens, the third technique of

radiography was employed (Figure 4.10).

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The third radiography technique requires the radiation to pass

through both the walls of the object and both walls are evaluated. In this

technique, the source of radiation may be positioned directly over the area of

interest, thus superimposing the top portion with the region directly under it.

As an alternative, the source may be offset by an angle of approximately 15º

in order to observe both as the top and bottom walls. When the source is

technique.

Figure 4.10 Schematic representation of double-wall exposure, double- view technique

Radiography test of welded specimens does not demand any

preparation. The weld region must be cleaned before test is performed.

The quality of the friction welded specimens is assessed using

radiography test. Radiography using X-ray is employed to identify and

analyze the defects at the friction welded joint. From the test results, it is

observed that there are only few pores at the weld joint which do not directly

affect the quality and is well within the acceptable limits.

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4.3.2 X-Ray Diffraction Technique

X-ray diffraction technique plays a vital role in determining the

grain size on the welded specimen both for the base metal as well as the weld

joint. Further, the chemical composition and strain can also be determined

from the results of the X-

helps in estimating the increase in strain at the weld metal as compared to the

base metal. X-rays are short wavelength electromagnetic radiations. The

wavelength range of X-rays is from 10-5Å to 100 Å approximately.

The X-ray diffraction experiment requires the following: a

radiation, a sample and a detector for the reflected radiation. In each of these

cases, there can be several variations (Hebbar, 2007). For example, the

radiation can be of many kinds, a single monochromatic variable frequency

source. The sample for friction welding analysis consists of a solid piece. The

detector can be of several kinds, ranging from a simple photographic plate to

a sophisticated counter or an area detector.

Figure 4.11 X-ray diffraction machine

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For the friction welded specimen, a diffraction method called

diffractometer is employed, in which a convergent beam strikes the sample

and the intensity as a function of diffraction angle is measured. The position

of the diffraction peak and the intensity at this point are the two factors used

in the determination. From these results, the strain variation from the weld

metal to base metal can be estimated. The X-ray diffraction machine

employed for this study is shown in Figure 4.11.

If there is no inhomogeneous strain, the particle sized can be

estimated from the peak formula (Cullity, 1978).

cos (2 sin )Kd

(4.2)

where is Full-width half maximum (FWHM) in radians

is the X-ray wavelength in Å (1.5406 Å)

is the strain induced on the particle

is the Bragg angle

K is a correction factor which was taken as unity and

d is the grain size in nm

X-ray diffraction patterns are obtained using Rigaku x-ray

diffractometer with CuK Å) w

with the

standard JCPDS (Joint committee powder diffraction standards) values. Semi-

quantitative analysis by scanning linescan EDX, was performed at the

central region of bonding interface and also at the ends. The results were very

similar with little variation in the diffusion layer between the main chemical

elements that make up the aluminum alloy and steel, like Al and Fe.

Figure 4.12 shows the interdiffusion between Fe and Al, characterizing the

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diffusion as the main bonding mechanism in the rotary friction welding

process according to Fukumoto et al. (1997, 1999), Fuji et al. (1997), Kimura

et al. (2003) and Ylbas et al. (1995). The Al diffused less in Fe than Fe in Al,

and a reason for this is the smallest diameter of Fe atom in relation to Al.

Another reason for the different distances from the diffusion zone is the

different concentrations of Fe and Al contained in each material. EDX

analysis confirmed that the particle-dispersed region (PD) comprised a

mixture of granular silver, Ag3Al, Al and Fe particles.

Figure 4.12 EDX showing the interdiffusion of the main elements of aluminum alloy and steel

4.3.2.1 Williamson Hall Plot (WH PLOT)

The shift in X-ray diffraction peak position can also occur from the

strain induced in the unit cell. This effect of strain can be investigated using

Williamson Hall plot (Tan et al., 2005). The particle size and strain can be

obtained from the intercept at the Y axis and the slope, respectively. Figures

4.13 and 4.14 show the W-H plot of base metal and weld region, respectively.

Al

Ag3Al

Fe

Ag

Ag3Al

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From the W-H plot (Figure 4.13), the strain at the base metal is about

0.00438.

Figure 4.13 W-H plot for the base metal

Also, from the W-H plot (Figure 4.14), the strain at the weld region

is observed to be about 0.0054. There is an increase in the strain in the weld

region as compared to that in the base metal. This increase in the strain could

be due to significant shrinkage forces in the weld area as compared to those in

the base metal, as this portion is subjected to expansion and contraction due to

localized heating.

0.000438

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Figure 4.14 W-H Plot for the weld region

is estimated from the line broadening of the X-ray diffraction peaks. The

maximum intensity peak occurs at 2 = 45º. To find the average particle size

(d) of the base metal and weld region and samples, the high intensity peak is

applying the X-ray diffraction line broadening. The average crystallite size

varies from 20 to 24 nm for the base metal. For weld metal the average

crystalline size varies from 19 to 22 nm.

4.4 METALLURGICAL ANALYSIS

The micrographic examination of grain size in base metal and the

weld region of friction welds consist of examining the polished surface of a

specimen which has been appropriately treated to reveal the grain boundaries

and estimating the grain size by comparison or measurement method. Image

analyzer was used to capture the macrographs of the friction-welded

specimens. Scanning electron microscope was used to capture the images of

the friction welded joints to analyze the fractured surface (if any) and to find

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the possible causes to the fracture. A few weld samples are completed by

using the optimized friction welding parameters as shown in Table 4.4,

according to the trials based on design of experiments for metallurgical

examination.

Table 4.4 Friction welded specimens with friction welding parameters for metallurgical examination

Specimen

Factor 1 Factor 2 Factor 3 Factor 4 Factor 5 Factor 6

A:friction time (sec)

B:forging time (sec)

C:forging pressure

(Mpa)

D:friction pressure (Mpa)

E:thickness (µm)

F:rotational speed (rpm)

1 4 1.2 50 260 12 1500 2 4 1.2 50 260 8 1500 3 4 1.2 50 250 12 1500 4 4 1.5 60 260 8 1500 5 4.25 1 55 255 10 1500

From the extensive metallurgical examinations on the friction

welded specimens (Figure 4.15 to 4.29), the microstructure of the fusion zone

consisting of fine grained dendritic microstructure precipitates at the grain

boundaries is observed. The reason for the formation of this fine grain

microstructure in the FZ is the high solidification rates involved which helps

retaining the strength of the re-solidified aluminum. No solidification

cracking or liquation cracking occurred in the FZ or in the HAZ. A narrow

HAZ was formed and the coarsening of the strengthening precipitates in the

HAZ was not that pronounced. There was, however, some liquation at the

grain boundaries in the FZ and HAZ. Although grain boundary liquation was

detected in the FZ and HAZ, no liquation cracking was observed. The joints

were stronger than the aluminum parent metal with only a thin diffusion layer

at the interface.

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Another important observation to note is that the grain boundary

liquation becomes more pronounced as the amount of Si along the grain

boundaries increased. The overaging in the HAZ region of the welding leads

to a decrease in the strength of HAZ while the strength of the fusion zone is

restored to the level of the heat affected zone. The acceptable level of

cracking, debonding and porosity were determined in the joints produced.

However, no weld-defect formation took place in the weld regions of all the

joints produced.

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4.4.1 Metallurgical examination on Specimen 1



C:forging pressure (Mpa)


E:thickness (µm)


4 1.2 50 260 12 1500

Figure 4.15 Fusion line of the friction welded specimen 1

Figure 4.16 Weld interface of the friction welded specimen 1

Figure 4.17 SEM micrograph of the friction welded specimen 1

100 µm

100 µm

100 µm

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D:friction pressure

(Mpa)

E:thickness (µm)


4 1.2 50 260 8 1500




100 µm

100 µm

100 µm

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4.4.3 Metallurgical examination on specimen 3




D:friction pressure

(Mpa)

E:thickness (µm)


4 1.2 50 250 12 1500



Figure 4.23 SEM Micrograph of the friction welded specimen 3

100 µm

100 µm

100 µm

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E:thickness (µm)


4 1.5 60 260 8 1500




100 µm

100 µm

100 µm

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E:thickness (µm)


4.25 1 55 255 10 1500




100 µm

100 µm

100 µm

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4.5 SUMMERIZING THE CHAPTER - MECHANICAL

TESTING AND METALLURGICAL CHARACTERIZATION

A few friction-welded specimens were prepared by using the

optimized friction welding parameters as per the trials based on the design of

experiments, which was discussed in the chapter 3 to conduct mechanical

testing which includes destructive testing & non-destructive testing and

examines the metallurgical charecterization. From the tensile test, it was

observed that there was no fracture in the weldment whereas fractures were

found in the base metals. In the bend tests, all the specimens passed the bend

tests and showed no or 1 mm open discontinuity. The micro-hardness test

showed the hardness in the aluminum alloy side is between 1350-1500 VHN

and the hardness in the stainless steel side was around 150-300 VHN. The

weld had good strength because the hardness in the weld zone was higher

than the hardness in base metal. The impact toughness at 0º was found to be

nearly 24J. The impact strengths for specimens are 29, 29, 31, 30 and 31,

respectively.

Radiography test showed only few pores at the weld joint, which do

not directly affect the quality and they are within the acceptable limits. From

the X-ray diffraction test, it was noted that Al diffused was less in Fe than Fe

in Al, and a reason for this was the smallest diameter of Fe atom in relation to

Al. Another reason for the different distances from the diffusion zone was the

different concentrations of Fe and Al contained in each material. Williamson

Hall plot showed the average crystallite size varying from 20 to 24 nm for the

base metal and for weld metal, the average crystalline size varied from 19 to

22 nm.

From the metallurgical examinations, the acceptable level of

cracking, debonding and porosity were determined in the joints produced.

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However, no weld-defect formation took place in the weld regions of all the

joints produced.

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