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Chapter 7

COMPARISON FSW WELD WITH TIG WELD

7.0 Introduction

Aluminium welding still represents a critical operation due to its complexity

and the high level of defect that can be produced in the joint. The main problems

related to the welding of aluminium are due to its high thermal conductivity, high

chemical reactivity with oxygen, and high hydrogen solubility at evaluated

temperature. All these factors lead defects in the weld bead. If fusion welding is

performed on aluminium alloy in a normal atmosphere, oxidization readily occurs and

this results in both slag inclusion and porosity in the weld, greatly reducing the

strength of the joint. TIG welding is one of the most common method used for

welding aluminium alloy. This method produces good welds, but more recently solid-

state methods for welding the material have been developed, one of these being

friction stir welding. So the comparative study conducted for the welding of

aluminium alloy 6082 using TIG welding process and then compare mechanical and

metallurgical properties of TIG welded joints with the mechanical and metallurgical

properties of FS Welded joints is discussed in this chapter.

7.1 Preparation of material

It is imperative to carry out welding of aluminium alloy sheet of same grade as

was used for friction stir welding. Aluminium alloy plates having dimensions 300 x75

x 6 mm were prepared for joining by TIG process.

7.2 Selection of process parameter and experimentation

It was decided to adopt manual welding technique for preparation of test

pieces as in actual practice, for welding industry. Before welding all the edges were

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thoroughly cleaned mechanically and chemically in order to avoid any source of

contamination like rust, scale, dust, oil, moisture etc. that could creep into the weld

metal and later on, could result possibly into a weld defect. The welding parameters

selected were based on extensive trials runs. It was found that for sheet of thickness of

6 mm, the current rating may be varied from around 80 Amp to 200 Amp. It was

further observed that below 80 Amp, it was difficult to maintain the arc and the joint

obtained was poor having uneven height. The current more than 200 Amp caused

burn through. So based on extensive trial runs a current range from 120 Amp to 180

Amp was selected. This range will give a steady arc with least spatters. So it was

decided to make minimum of four joints within the range of welding current. The

experiments were repeated thrice to reduce experimental error. The photo graphs of

TIG welded specimens shown in the Figure 7.1. Other details related to the process

and procedures used in the present work include:-

Weld joint was prepared by using commercial 4043 filler wire whose diameter

is 2.4 mm. The chemical composition of the filler wire is presented in the Table 7.1.

The commercially available argon was used as shielding gas at flow rate 15 l/min.

The electrode was the commercially available Tungsten EW-Th-2 (Thoriated

tungsten) of 3 mm diameter. Electrode to work angle was 45º.

7.3 Specimen sampling and testing

The specimens for tensile testing, micro hardness testing and microstructural

studies were taken from the weld pads as schematically illustrated in Figure 7.2.

Sample test pieces were prepared for mechanical and metallurgical testing and tests

were conducted as discussed in the chapter 4.

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7.4 Results and discussion

7.4.1 Tensile properties

The transverse tensile strength of all the joints prepared using TIG welding

process at different welding current conditions was evaluated. In each condition three

specimens were tested and the average tensile strength of three specimens and their

corresponding percentage elongations thus obtained are mentioned in Table 7.2. The

photo graph of some tensile test specimens before fracture is shown in Figure 7.3. The

tensile results show that maximum tensile strength of 204 MPa was possessed by the

specimens made using 140 Amp current and minimum tensile strength of 161 MPa

was possessed using 120 Amp welding current. This indicates that there was a 35% to

49% reduction in the strength due to the change in the micro structure. As shown in

Figure 7.5 to 7.8, the micro structures of the weld metal, the dendrite size and cell

spacing, indicate that high tensile strength and ductility was possessed by the joints at

140 Amp current. This can be attributed to smaller dendrite sizes and lesser inter-

dendritic spacing in the fusion zone. Relatively lower tensile strength and ductility

was possessed by the joints with long dendrite sizes and large inter-dendritic spacing

in the fusion zone of the joint welded using higher welding current (180Amp) and

lower welding current (120Amp). The joint efficiencies [defined as (UTS weld

joint)/(UTS base metal) x 100] of 55%, 66%, 59% and 56% were achieved at 120

Amp, 140 Amp, 160Amp and 180 Amp current respectively.

7.4.2 Comparison of tensile strength of FSW and TIG welded joints

The average transverse tensile strength of TIG welded joints is presented in

Table 7.2. The tensile strength of base metal was 310 MPa. The maximum tensile

strength obtained in TIG welded joint was 204 MPa. This indicates 35% reduction in

tensile strength of TIG welded joint. FSW joints showed the highest tensile strength

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of 271 MPa, this indicates 13% reduction in tensile strength. Though the tensile

strength of friction stir weld joints and TIG weld joints was lower than the base metal,

but tensile strength of friction stir weld joints was 22% higher than TIG welded joints.

This suggests that friction stir welded joints were stronger than TIG welded joints.

The elongation of parent metal was 14%. The TIG welded joints showed the

maximum elongation of 6.5%. This indicates that there was a 50% reduction in

ductility in TIG welded joints. The elongation of FSW joints was 9.5%. This indicates

that there was a 30% reduction in ductility in FS weld joints. Though, percentage

elongations of FS welded joints were lower than the base metal, but were 20% higher

than TIG welded joints. The comparative analysis shows that the TIG welding process

has strong influence on the tensile strength and elongation which decreases 35% and

50% respectively in comparison to that of the base material. If FSW was applied the

tensile strength and percentage elongation of the weld decreases 13% and 30%

respectively in comparison to that of the base material. Thus, the tensile strength of

aluminium alloy 6082 was affected by both the welding processes but affect is more

in the TIG as compared with FS welded joints.

7.4.3 Impact toughness of TIG welded aluminium alloy

The charpy test sample method of impact testing does possess certain

advantage, these include ease of preparation, simplicity of test method, and low cost

per test. The results of impact tests of TIG weld aluminium alloy 6082 joints are

presented in Table 7.2. Impact toughness of base material was 14 J. It was observed

that the impact toughness of the TIG welded aluminium alloy joints was lower as

compared to the base metal and varied from 4J to 8 J, by changing the welding current

as shown in the Table 7.2. Impact strength of TIG welded joints was lower due to

larger grain size of the welded joints and precipitate distribution. During the welding,

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liquid films were formed at grain boundaries adjacent to the fusion boundary. These

liquid films lead to the formation of microscopic inter granular cracks which, after

welding may provide path for subsequent brittle inter granular fracture. The variation

in the strength values is mainly due to the formation of these micro cracks. Grains of

the joint welded at 140 Amp current was uniformly distributed and the spacing

between the grains was lower as compared to the other microstructures. It may be the

reason of higher impact toughness of the joints welded at 140 Amp current.

7.4.4 Comparison of Impact strength of FSW and TIG welded joints

Friction stir welds showed a very interesting trend on impact toughness. The

mean results of the Charpy Impact test of FSW and TIG joints carried out at room

temperature are given in Table 7.2. The impact toughness of FSW was greater than

TIG joints and base material. In contrary to most of mechanical properties of welds,

that were either not altered or slightly deteriorated during FSW of AA 6082-T651

alloy, impact toughness was the property which improved significantly. The measured

value of impact toughness base material was 14 Joules, whereas the welds made by

FSW process showed impact toughness values much greater than base material (26 J).

This situation can be explained with the grain refinement of the stirring effect in the

FSW process. The impact toughness of the FSW joint was 48% greater than that of

the parent material but impact toughness of TIG weld joints was 40% lesser than that

of the base metal. This impact toughness difference is related to the weld structures

obtained with FSW and TIG processes. The stirring effect of FSW improves the

microstructure of the weld and increases the resistance to impact.

7.4.5 Micro hardness of TIG welded specimens

The micro hardness survey of TIG welded specimens at different welding

current from cross section is presented in Figure 7.4. In all the weld joints there was a

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hardness loss in the HAZ, fusion line area and weld zone. The average micro hardness

was 48 Hv for 120 Amp current, 58 Hv for 140 Amp current, 53 Hv for 160 Amp

current and 49 Hv for 180 Amp current in the weld zone. While moving from the

weld centre to base metal, an increasing trend in the order of weld metal, HAZ,

unaffected base metal for all the joints made at different welding currents was

observed. In all the joints, HAZ area adjacent to the fusion boundary was coarse

grained which possessed low hardness whereas the HAZ area adjacent to the base

metal was fine grained which possessed high hardness. The reason for this trend of

micro hardness in the HAZ of all the joints was that the area adjacent to the weld zone

experiences relatively slow cooling rate and hence has coarse grained microstructure,

whereas the area adjoining the base metal undergoes high cooling rate due to steeper

thermal gradients and consequently has fine grained microstructure. This is evident

from the trend depicted by the micro hardness survey within the HAZ of each of these

joints.

7.4.6 Comparison of micro hardness of FSW and TIG welded joints

The base metal in its initial condition showed a hardness value of 94 Hv. The

hardness was greatly reduced in the weld region of both the welding processes. This

was one of the reasons for the location of failure of all the tensile test specimens at the

weld region in TIG welding process but in FSW all the tensile test specimens failure

location was TMAZ of advancing side. TIG weld joints showed the highest average

micro hardness of 58 Hv at weld centre. This indicates that there was 40% reduction

in micro hardness in the weld zone due to welding heat. FS weld joint showed highest

average micro hardness of 65 Hv at the weld zone and this indicates that there was

30% reduction in the weld zone. Though micro hardness in both welding processes

was lower than the base metal, but micro hardness of the friction stir weld joint was

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10% higher than TIG weld joints. However friction stir weld specimens showed

higher micro hardness as compared to TIG weld joints and this was due to shear

stresses induced by tool motion which leads to generation of very fine grain structure

which allows partial recovery of hardness. In case of TIG welding, very high

temperature increases the peak temperature of the molten weld pool causing slow

cooling rate, in turn causes relatively wider dendritic spacing in the fusion zone.

These microstructures generally offer lower resistance to indentation and this may be

one of the reason for lower hardness and inferior tensile properties as compared to

FSW joints.

7.4.7 Microstructure of TIG welded specimens

Optical micro graphs of the TIG welded joints showing the micro structure of

fusion boundary along with HAZ, weld metal and heat affected zone, at different

welding currents are shown in Figure 7.5 to Figure 7.8. From the micrographs, it was

observed that there was an appreciable difference in grain size of the weld zone and

HAZ regions with different welding currents. This may be due to the rapid cooling

induced by good thermal conductivity and low thermal capacity of aluminium. The

grain size of the fusion zone and HAZ are influenced by the heat input of the welding

process. In the HAZ, the grains next to the fusion boundary were found to be grown

larger due to the intensive heat and high temperature experienced during welding.

Grain structure at the fusion boundary was very coarse and columnar showing that

epitaxial growth has taken place. Liquation can be seen along the grain boundary in

the region close to the fusion boundary. Fusion zone shows equiaxed grain structure

consisting of grains of aluminium solid solution and eutectic mixture and beta phase

along the grain boundary [Koteswara et al., 2008]. The fusion zone of TIG welded

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joints contain dendrites structure and this may be due to the fast heating of base metal

and fast cooling of molten metal due to welding heat [Venugopal et al., 2004].

7.4.8 Comparison of microstructure of FSW and TIG welded joints

Optical micrographs of the base metal and friction stir welded joints are

displayed in Figure 6.43. The base metal contains coarse and elongated grains in the

rolling direction. The weld region of FSW joint contains finer grains compared to TIG

welded joints. The weld region of TIG welded joints shows coarse and elongated

grains normal to the welding direction. However, the size and distribution of

strengthening precipitates are different in TIG welded joints and FSW joints. Fine and

evenly distributed precipitates, observed in friction stir welding were one of the

reasons for higher strength of the joints. The comparative analysis shows that the weld

region of FSW joint contains very fine, equiaxed grains and this may be due to

dynamic recrystallisation that occurred during FSW process [Barcellona et al., 2006].

The weld zone of TIG welded joints contain dendritic structure and this may be due to

fast heating of base metal and fast cooling of molten metal due to welding heat.

7.4.9 Fractographic observations of TIG welded joints

The fractured surfaces of the tensile specimens were analyzed using SEM and

are presented in Figure 7.9 (a) to Figure 7.9 (d). Dimples of varying size and shape

were observed in all the fractured surfaces which indicate that major fracturing

mechanism was ductile. From Figure 7.9 (b) it was observed that fractured surface of

the specimen at 140 Amp current contains a large population of small and shallow

dimples which was indicative of its relatively high tensile strength and ductility. From

Figure 7.9 (c) and Figure 7.9 (d) it was observed that as heat input increases, coarse

and elongated dimples are observed. It is also observed that small dimples are

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surrounded by the large ones in all the specimens and a small quantity of tearing ridge

is also present

7.4.10 Comparison of fractograph of FSW and TIG welded joints

The fracture surface of tensile test specimens, as observed under the

Scanning Electron Microscope (SEM) are displayed in Figure 5.50 and Figure 7.9 for

FSW and TIG joints. The fractrography consists of dimples, indication the failure of

the tensile specimen in a ductile manner under the action of tensile loading both in

TIG welding process and FS welding process. An appreciable difference exists in the

size and shape of the dimples with respect to welding processes. The dimple size has

directly proportional relationship with strength and ductility. If the dimple size is

finer, then the strength and ductility of the respective joint is higher. Coarse dimples

are seen in TIG joint and fine dimples in FSW joint. Since fine dimples are

characteristic feature of ductile failure, FSW joints have shown higher ductility as

compared to TIG joints. So the fine dimples in FSW are the reason of higher strength

properties as compare to TIG welded joints.

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Table 7.1: Chemical composition of the filler wire 4043

Si Fe Cu Mn Mg Cr Zn Ti Balance

4.5-6.0 0.8 0.3 0.05 0.05 0.0 0.10 0.20 Al

Table 7.2: TIG welding process parameters and experimental results

Specimen

Designation

Current

(Amp)

Gas flow

{lit/min)

Tensile strength

(MPa)

Impact

toughness (J)

A1 120 15 166 6

A2 140 15 205 8

A3 160 15 185 5

A4 180 15 174 4

Figure 7.1: Photographs of TIG welded specimens

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Figure 7.2: Schematic illustration of the specimen sampling from the weld pads.

Figure 7.3: Photographs of some tensile test specimens of TIG welded joints before

fracture

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Figure 7.4: Micro hardness profile showing micro hardness of different zones of the

weld joints at different welding currents

40

50

60

70

80

90

100

-20 -15 -10 -5 0 5 10 15 20

Mic

ro h

ard

ne

ss (

Hv)

Distance from the weld centre line (mm)

120 Amp 140 Amp 160 Amp 180 Amp

BM HAZ Weld metal HAZ BM

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(a) Fusion zone (b) weld bead

(c) HAZ

Figure 7.5: Optical micrograph showing microstructure of TIG welded specimen (a)

Fusion boundary (b) centre of the weld (c) HAZ ( 120 Amp, at 200X)

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(a) Fusion zone (b) weld bead

(c) HAZ

Figure 7.6: Optical micrograph showing microstructure of TIG welded specimen (a)

Fusion boundary (b) centre of the weld (c) HAZ (140 Amp, at 200X)

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(a) Fusion zone (b) Weld bead

(c) HAZ

Figure 7.7: Optical micrograph showing microstructure of TIG welded specimens (a)

Fusion boundary (b) centre of the weld (c) HAZ (160 Amp, at 200X)

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(a) Fusion zone (b) Weld bead

(c) HAZ

Figure 7.8: Optical micrographs showing microstructure of TIG welded specimens (a)

Fusion boundary (b) centre of the weld (c) HAZ (180 Amp, 200X)

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Figure 7.9(a): SEM fractograph showing fracture surface of tensile test TIG welded

specimen (120) Amp current

Figure 7.9 (b): SEM fractograph showing fracture surface of tensile test TIG welded

specimen (140) Amp current

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Figure 7.9 (c): SEM fractograph showing fracture surface of tensile test TIG welded

specimen (160) Amp current

Figure 7.9 (d): SEM fractograph showing fracture surface of tensile test TIG welded

specimen (180) Amp current