Comparison of Mechanical Properties and Metallurgical Characterization of

Dissimilar TIG Welded Joint of AA5083-H111 and AA6061-T6 Aluminium

Alloy Using ER5356 and Scandium Added ER5356 Filler Rods.

Sankaralingam. T*1 and Subbaiah. K2

1 Senior lecturer, Mechanical Department, Sri Durgadevi polytechnic college RSM

Nagar, Kavaraipettai, Tamil Nadu, India. 2 Professor, Department of Mechanical Engineering, Sri Sivasubramaniya Nadar

College of Engineering, Kalavakkam, Chennai, Tamil Nadu, India.

E-mail: 1 tsankaralingam5@gmail.com, 2 subbaiahk@ssn.edu.in

Abstract

This work focus on comparing the mechanical properties and microstructures of

dissimilar Tungsten Inert Gas weld of AA6061-T6 and AA5083-H111 aluminium

alloys with commercial ER5356 filler and Scandium added ER5356 filler Rod with

0.25% Scandium composition. The microstructural characterization of the welded

samples is studied by means of optical microscope and scanning electron

microscope. The mechanical property is characterized by means of (UTM)

universal testing machine to find out the effect of scandium on tensile strength and

percentage elongation of the welded samples. The fractured surface is analyzed

using scanning electron microscope to find out the type of failure that has occurred

in the welded samples. The hardness analysis is done along the transverse

direction of the weld to find out the hardness values at the various zones of the

weld.

Keywords: AA5083-H111, Tungsten Inert Gas Welding, , AA6061-T6,

Mechanical Properties, Microstructures.

1. Introduction

AA5083 aluminium alloy, which fits into the family of non heat-treatable

alloys. It embraces magnesium (Mg) as the major alloying element. [1]. 5xxx alloy

are utilized in marine applications, cryogenic tanks to name a few [2, 3]. Matter-of-

fact, realistic application of 5083 Al alloys includes structures of fast ferries and

fishing boats which is first and foremost because of admirable corrosion resistant

property in the marine environment. Additionally 5083 display high specific

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strength, ductility, excellent formability and good weldability [4, 5]. AA6061

alloys are used in marine frames, storage tanks applications. These alloys are

readily weldable by fusion welding; 5xxx alloys are generally welded with filler

metals ER5356 filler rod due to similar chemical composition. Choice of proper

materials with appropriate mechanical properties based on their utilization is very

much vital. Engineer’s knowledge about these standards before selecting the fitting

aluminium alloys for their usage is expected.

Even though AA5083-H111 and AA6061-T6 can be welded by solid state

and fusion welding techniques, where earlier literatures highlights that friction stir

welding (FSW) gives better mechanical properties [6]. But fusion welding is in

general preferred for commercial applications because of low cost and easy of

helpfulness. When assessment are drawn among a variety of fusion welding

techniques, TIG welding is favored because of better-quality weld properties of the

specimen [7,8]. Further to strengthen the FZ of the TIG weld joint various

techniques are utilized. One such method to improve the strength of weld is adding

grain refiners. Scandium (Sc) an effective grain refiner is a rare earth material

which can be used successfully to increase the strength of the weld by retarding

grain growth [9]. Sc improves the corrosion resistance and improves strength. Also

Sc when added to Al improves the weldability [10]. Since Sc is costly its

utilization is least. Along with Sc, Erbium (Er), Zirconium (Zr) are generally

mixed to reduce Sc consumption, thereby gain required improvement strength of

the weld [11].

In this work comparison is drawn between the microstructural and

mechanical properties of TIG welded joints of dissimilar AA5083-H111 and

AA6061-T6 fabricated with commercial ER5356 filler rod and 0.25% scandium

added ER5356 filler rod . The fusion zones of the welds are studied, and the effect

of scandium addition is documented.

2. Materials and Methods

The base materials used in this study are AA5083-H111 and AA6061-T6

Aluminium alloys. Along with the base materials commercial filler rod ER5356

and 0.25% Sc added filler rods are used. Their chemical composition of the base

materials and fillers rods are given in Table 1. The mechanical properties of the

base materials are listed in Table 2. The scandium added filler rod is chill-casted

by adding master alloy (Al-2wt%Sc) to commercial ER5356 standard filler wire.

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The constant current TIG welding process is adopted for the experimentation. The

single side V butt joint is fabricated and the details of TIG welding parameters are

presented in Table 3. Microstructures at various zones of the joints were inspected

using OM, SEM analysis. Vickers Hardness measurements were carried on the

under the load of 1 Kgf for 15 dwell time at 1 mm intervals using an automatic

microhardness tester for the welds. The averages of 3 readings are taken for each

welded samples.

Table 1. Chemical composition (CC) of the base metals (BM) (wt %)

Base material Mg Mn Fe Si Cu Cr Zn Ti Sc Al

AA5083-H111 4.25 0.52 0.26 0.98 0.35 0.11 0.10 0.01 - Bal

AA6061-T6 0.81 0.06 0.32 3.01 1.14 0.18 0.07 0.02 - Bal

ER5356 5.17 0.07 0.15 0.43 0.13 0.12 0.39 - - Bal

ER5356 + (0.25% Sc) 4.56 0.07 0.15 0.43 0.13 0.12 0.39 - 0.25 Bal

Table 2. Mechanical properties (BM)

Base materials Yield Strength

(MPa)

Ultimate tensile Strength

(MPa)

Elongation

%

AA5083-H111 197.4 321.3 22.25

AA6061-T6 265.9 325 15.9

Table 3. Process parameters (PP) of TIG welding

Process TIG welding (or) GTAW

Welding machine Miller Synocroware

Voltage (v) 16

Current (A) 170

Welding speed (mm/min) 150

Shielding gas Argon

Gas flow rate(L/min) 11

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3. Results and Discussions

3.1 Optical Microstructures

The optical microstructures AA6061-T6 and AA5083-H111 are shown in Fig

1a and 1b. The microstructures of weld joints fabricated using commercial ER5356

filler rod (weld 1) and weld with 0.25% scandium added ER5356 filler rod (weld

2) are shown in the Fig 2a and Fig 2b. The optical microstructures at weld 1 fusion

zone expose course dendrites. However weld 2 fusion zone show significant

transformation of microstructure to fine dendrites. From this it is evident that the

addition of Sc to the filler effected the refinement of the grains which is clearly

manifested, which result in upgrading of mechanical properties and strength of the

weld. Similar observation been made were enhanced properties due to fine

dendrites formation due to Sc added to Al-Mg alloy [12, 13].

Fig. 1 Optical microstructures of base materials a) AA6061-T6; b AA5083-H111

Fig. 2 Optical microstructures a) weld 1; b weld 2

3.2 SEM Microstructures

Scanning electron microstructures of the base materials AA6061-T6 and

AA5083-H111 are shown in Fig 3a and 3b. The weld fusion zones of weld 1 and

weld 2 are shown in Fig 4a and 4b. Course grain structures which are observed

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from the SEM microstructures at the weld FZ revealed continuous eutectic for

weld 1. The SEM microstructure for weld 2 shows relatively very fine grains and

discontinues eutectic at the FZ due to addition of grain refiner that is Sc. Similar

grain refinement behavior due to addition of Sc was reported for TIG weld of

AA6061 Al alloy with ER4043 filler rod modified with addition of Sc [14].

Fig. 3 SEM microstructures of BM a) AA5083-H111; b AA6061-T6

Fig. 4 SEM microstructures a) weld 1; b weld 2

3.3 Tensile Properties

The tensile properties of weld 1 and weld 2 are listed in table 4. The fracture

locations of the samples shown in Fig 6a for weld 1 and Fig 6b for weld 2. The

dimensions of tensile specimens are shown in Fig 5. Both the welds failed at

AA6061-T6 side. From the values recorded 91 MPa improvement in UTS, 3 MPa

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enhancement in YS was observed for weld 2 to that of weld 1. The welds are

completely intact for all three Sc welds , this show that the weld is stronger than

the parent materials. The joint efficiency of weld joints is 45% and 73.5% for weld

1 and weld 2. Similar results were been observed on cast Al–Mg alloy with

0.17wt%Sc added to it. Al–Mg–Sc alloy had lower ductility in contrast with cast

Al-Mg alloy without Sc [5]. The enhanced UTS, YS of the joints may be due to the

finer dendrites formed due to Sc addition to the weld. Similiar observation were

made on AA5083 alloy, with addition of Sc the dendrites structures are suppressed

resulting in enhanced UTS and YS [13].

Fig.5 Dimensions of Tensile specimen

Fig. 6 Tensile specimen after testing a weld 1; b weld 2

Table. 4 Tensile properties of TIG weld samples

Process Specimen –

ID

FILLER

ROD

UTS,

MPa

YS,

MPa % EI

Fracture

location

TIG

WELD

Weld – 1 ER 5356 145 123 6.5 Weld center

Weld – 2 ER5356 +

0.25%Sc 236 126 11

AA6061-T6

side

3.4 Fractographic studies

The SEM fractography of base materials are shown in Fig 7a and Fig 7b

subjected to tensile testing which clearly shows large dimple, with different size of

cavities indicating ductile mode of fracture. The SEM fractography of the weld 1

and weld 2 are shown in Fig 8a and Fig 8b. From the image it is clear that the

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cracks commenced from several crack initiating sites . The fractured samples

investigated at weld 1 shows flat face and course dimples which results in

reduction of ductility in the weld when compared with the BM which is to justify

the fact that the ductility is reduced by TIG weld. Similar results where reported

for TIG weld of AA5083-H111 with different filler rod diameters [15]. Weld 2

shows fine dimples which reveals the mode of failure is ductile similiar to the

observation made previously on AA5083 alloy TIG weld joints with Sc added

filler [13].

Fig. 7 Fractographs of BM a AA6061-T6; b AA5083-H111

Fig.8 Fractographs of welds a weld 1; b weld 2

3.5 Hardness Curves

The hardness measurement was taken along the cross section in transverse

direction parallel to the weld surface and base material. The hardness distribution

for weld 1 and weld 2 are shown in Fig 9. The base material hardness for the alloys

was also plotted alongside. From the graph the average hardness of weld 2 is

100Hv1 for that with that of weld 1 is around 80Hv1. The gain of hardness is

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approximately 20Hv1 for the weld produced with Sc added filler weld. The graphs

plotted across the AA6061-T6 side for weld 1 and weld 2 give you an idea about

the hardness which diminishes below the hardness of AA6061-T6 base material

side. The graph of weld 1 show gradual increase in values from AA6061-T6 side

below 60Hv1 followed with increase at weld FZ around 80Hv1, which is almost

equivalent to the hardness of AA5083-H111 and further moving to AA5083-H111

side 85Hv1 is achieved. Similarly the hardness survey conducted for weld 2

exhibits noticeable increase of hardness in the weld FZ region with maximum

value identified is 104Hv1. The hardness values at AA6061-T6 side of weld almost

pursues similiar pattern to that of weld 1 whereas at the AA5083-H111 side of the

weld accounts the hardness value of 90Hv1. Increase in hardness in weld 2 FZ is

due to finer grains formed due to grain refinement by Sc and formation of Al3Sc

precipitate present. Similar hardness enhancement were been observed due to

addition of 0.25wt%Sc to AA5083 TIG weld joints [13]. The formation of course

dendrites at weld 1 FZ result in reducing the hardness when compared to that of

weld 2 FZ were fine dendrites are evident from the optical microscopic imaging .

the fracture at weld 2 joint occured at 6061-T6 side where least hardness is

observed.

Fig. 9 Hardness survey of weld 1 and weld 2

3.6 Correlation between microstructures tensile property and hardness

The presence of course dendrites present in weld 1 FZ influences the

reduction of strength of the weld considerably. Addition of Sc upto 1 atomic

percentage increases the tensile properties by 1000 MPa [12]. The cast ER5356

filler rod contains around (0.5wt%Sc) added from the master alloy, which contains

(2wt%Sc) and 98wt% of Al in it. From the trails conducted using filler rod ER5356

with (0.25wt%Sc) significant mechanical property enrichment was observed due to

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Sc doping [13]. At weld 2, noteworthy increase in tensile property is observed.

The suppression of dendrites growth due to addition of Sc and finer dendrites at

weld 2 FZ influence weld joint efficiency [12]. By literatures the weld 2 FZ which

has Sc added results in formation of Al3Sc a second-phase particle. These particles

improves the strength further by precipitate strengthening mechanism. Also Sc

particle there refines the grains at the weld which results in strengthening of the

weld joint thereby assisting in the enhancement of weld strength further by Hall-

Petch effect. From the optical microstructures the course dendrites are observed at

weld 1 FZ and the fine dendrites are observed at weld 2 FZ. The hardness values of

Sc added filler rod weld is higher in contrast to the hardness of Sc free filler rod

welded joints. From the literatures study, finer dendrites influences the

enhancement of hardness as well as tensile properties discussed above [12]. The

other significant factor influencing higher hardness of weld 2 FZ is the presence of

Al3Sc precipitates formed at the weld 2 FZ [13].

4. Conclusions

The comparison study of aluminium alloys AA5083-H111 and AA6061-T6

are TIG welded with 0.25%Sc ER5356 filler rod and commercial ER5356 fillers

rod is done. The conclusions made in this study are given below:

1. The fusion zone of weld 2 shows refined grains compared to the grains

present at the weld 1 fusion zone

2. The tensile properties of weld 2 is superior compared to weld 1 which is

primarliy due to scandium addition.

3. From the fractographic image of the welded samples showed brittle mode of

failure.

4. The hardness of the fusion zone is enriched by scandium addition and

strengthens the weld Fusion zone.

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