Husain Mehdi,R.S.Mishra

Department of Mechanical Engineering,Delhi Technological University,Delhi,India

Keywords:Residual stress TIG+FSW Heat transfer Micro-hardness Tensile strength

ABSTRACT Tungsten inert gas(TIG)welding is the most commonly used joining process for aluminum alloy for AA6061 and AA7075 which are highly demanded in the aerospace engineering and the automobile sector,but there are some defects occur during TIG welding like micro-crack,coarse grain structure,and porosity.To improve these defects,the TIG welded joint is processed using friction stir processing(FSP).

1.Introduction

Friction stir welding(FSW)is a solid-state joining process that uses a non-consumable tool to join similar or dissimilar materials.The FSW does not involve the melting of parent material,which have many advantages on conventional fusion welding processes[1].The FSW process involves the heat generation,which produced by pressure and friction between the tool and the workpiece.As a result of the generated heat,the weldment surfaces in contact soften become plasticized[2].The modern development in the automobile sector and defense sector have been transformed from conventional materials to light material such as aluminum alloy.Due to excellent physical and mechanical properties of Al-alloy such as high corrosion resistance,low density,high strength to weight ratio and high thermal conductivity,it is mostly used for making various components such as military aircraft,rocket,and rocket launcher,axle shafts,rims,bumpers,and car bodies.FSW is used in the fabrication of automotive and shipbuilding components,aircraft structural components and reducing structural weight[3-7].To study material flow and temperature distribution in friction stir welding of similar or dissimilar aluminum alloys many authors developed the CFD model to analyze the plastic flow,heat transfer and heat generation in the welding process[8-10],and the effect of tool profile on the material flow[11-13].Due to low heat generation in the friction stir welding,the residual stresses are also low in the weldment.The transverse force of friction stir welding tools plays an important role in stress measurement[14-16].The residual stresses in the weldment have a big impact on the performance of the welded structure.So far,information on the residual stress distribution of FSW has been limited[17].The effects of tool feed rate on residual stresses of FSW of Al-alloy joints were studied with synchrotron X-ray measurement and analyze the residual stresses in longitudinal and transverse directions[16].Many researchers have used finite element methods for the residual stress distribution in FSW.The computational fluid dynamic supports the heat transfer model and heat source of friction stir welding.The residual stress depends on the empathetic of heat transfer during FSW,although the correct prediction of temperature distribution depends on the heat source[18-20].FSW can affect the performance of the residual stresses in the weldment and the processing parameters such as rotating tool may affect the development of residual stresses[21,22].The mechanical properties,grain size are dependent on tool geometry,processing parameters and chemical composition of alloying elements.A new grain structure could be formed in friction stir welding by controlling the process parameter and heat input[23-27].Comparison of residual stresses of TIG and FSW has been done for AA6061 and AA7075,and found that the longitudinal residual stress is larger than the traverse residual stress in the welded joint[28].There are no successful investigations have been done to find out residual stress and heat transfer in TIG welded joint with friction stir processing.The small plate having the dimension of 150 mm×25 mm×0.8 mm,have been investigated,as a result,the properties at the beginning and end of the welded joints can be significantly different.The tensile strength first increases and then decreases with the growth of rotational speed.Three layers i.e.top,middle,and bottom,have considered measuring the microhardness of the welded joint.The top layer of the weld has a higher recrystallization degree and dynamic recovery and the bottom layer is strongly stirred under mechanical action of the tool pin such that each layer of welded joint shows the different mechanical properties[29].A new welding approach of TIG+FSP was successfully applied to the AA2024 to enhance the mechanical properties of the TIG welded joints.They conclude that the defects and porosities in the TIG welded joints are completely reduced by the FSP process and modified the microstructure and mechanical properties of the TIG+FSP welded joint[30].The application of FSP on the TIG welded joint improved the ductility and tensile strength of the FSW and TIG welded joint of AA5083-H111 and results revealed best-reduced dimple size[31].

In this work,the effect of FSP on TIG welding for dissimilar Alalloy AA6061 and AA7075 with filler wire ER4043 and ER5356 was carried out and analyze the mechanical properties,residual stress distribution and heat transfer in(TIG+FSP)welded joint.The present study was focus on the improvement of mechanical properties and the residual stress distribution of the TIG-welded joint with different filler wire(ER4043 and ER5356)by using a friction stir processing approach.This topic is carefully selected after noticing that there is little literature review available on the TIG+FSP welding approach.The outcomes of this paper will give the new approach for enhancing the mechanical properties and heat transfer of TIG+FSP welded joints which play a huge role in welded structures.

2.Materials and methods

Two AA6061 and AA7075 aluminum alloy plates having dimensions(150 mm×40 mm×6.2 mm)were welded together using the TIG+FSP welding approach.V butt joints of 45°were prepared for TIG welding with the help of milling cutter and power hacksaw.There are two types of samples were prepared for TIG welded joint.In the first sample,ER4043 filler wire was used and for the second sample,ER 5356 filler wire was used.The chemical composition of filler rod and base materials are shown in Table 1.The processing parameters and filler wire are strappingly affected by the mechanical properties of the welded joint.In this work,the filler wire of ER4043 and ER5356 of diameter 2.4 mm were used to join a single V groove plate at voltage 22 V and current 50 amp for all welding processes.Argon(25 L/min)is used as a shielding gas and travel speed 3.6 mm/s to fabricate the joint.

After TIG welding,the FSP technique was used on TIG weldment to improve the welding quality and mechanical properties.The non-consumable H13 steel tool with pin diameter,shoulder diameter and a pin length of 3 mm,19.5 mm,and 5.5 mm respectively was used.0°tilt angle was used during friction stir processing on the AA6061 and AA7075 plate at a rotational speed of 800 rpm-1600 rpm and a traverse speed of 63 mm/min is shown in Fig.1(a).The tool nomenclature is shown in Fig.1(b).The processing parameters of TIG welding and friction stir processing are shown in Table 2 and Table 3.

Fig.2 demonstrate the dimension of the tensile sub test specimen as per ASTM E8.The tensile test specimens were sliced and machined from the welded joint using a milling cutter and shaper machine.Single-pass FSP is used to fabricate TIG+FSP welded joint.Three sub tensile specimens were tested on a UTM machine atroom temperature and the average of these three results is taken.Vicker microhardness machine was used for measuring the hardness across the welded joints with a load of 100 g and dwell time 30 s.Microstructure observations were carried out using a scanning electron microscope(SEM)for TIG and TIG+FSP welded joints.

Table 1Chemical composition of filler wire and parent material.

Fig.1.(a)FSP process after TIG welding,(b)Friction stir processing tool of H13 tool steel.

3.Numerical modeling[10]

For incompressible flow,the continuity equation in the direction ofx,yandzis given by

The momentum conservation equation of the heat source is given as

Fig.2.Dimension of tensile test specimen as per ASTM E8.

The flow stress can be calculated as

The effective strain may be written as

Whereεijis given by

So,the viscosity can be calculated as

The thermal energy conservation equation at steady state is given by

Table 2Processing parameters of TIG welded joint[32].

Table 3Processing parameters of friction stir processing[33].

Then Simay be written as

During friction stir welding,mixing is not atomic,only grains structure is deformed not mixing of atoms.

3.1.Boundary conditions

When the work-piece top surface away from the tool shoulder edge,the boundary condition for heat exchange involves both convection and radiation heat transfer.However,at the sides and the bottom surface of the work-piece,the boundary condition for heat exchange involves only convection heat transfer.The heat generation rate at the interface between tool and the work-piece can be given be given by

WhereTandWdenote the tool and work piece respectively.The heat flux continuity on the shoulder interface yields

WhereJwandJTare the heat conducted to the work-piece and tool respectively

WhereCpis heat capacity,ρandKare density and thermal conductivity respectively.

The total heat generation rate may be written as

The heat transfer coefficient may be calculated as.

Value ofhbcan be calculated as

The heat transfer due to radiation and convection is written as

In this model,the computational region is considered as a single-phase visco-plastic non-Newtonian fluid and the FSW tool is considered as rotating in a fixed position as shown in Fig.3.The material flows into the computational domain from the inlet velocity and out from outlet velocity at the welding speed(traverse speed).Top,bottom and side surfaces of the work-pieces are considered equivalent to the wall surface,having the same velocity as welding speed but opposite in direction.

Fig.3.Boundary condition.

Velocity at the tool pin periphery have been defined in terms of tool translation velocity and the tool pin angular velocity

Similarly,at the shoulder contact,the velocity boundary condition may be written as

3.2.Material properties

The density of AA7075 and AA6061 is taken as constant,equals to 2810 kg/m3and 2700 kg/m3respectively,while the specific heat and thermal conductivity are considered as temperature dependent properties[34]as given below

3.3.Residual stress measurement by cosαmethod

A mini portable X-ray diffraction apparatus(pulstecμ-X360)used for analyzing the residual stresses in welded joint of AA6061 and AA7075 samples.The X-ray incident angle was set 35°and±5°oscillation was applied.The X-ray incident time was 4-5 min throughout this process for each sample.Under these conditions,a diffracted beam from the workpiece surface has captured the images of the welded plates at 50μm resolution,the size of the beam spot is approximately 2.5 mm for 1.2 mm pinhole collimator.

For residual stress determination,the cosαmethod was described[35].The translation from the diffractometer space to the sample inherently more complex due to the 2D planar geometry of the measurement and can be expressed as

The strain projection along(η,α)coordinates can be written as in terms of scattering vector and strain component as

So,the strain projection may be written as

Now,defining two parametersa1anda2for linear determination ofσ11andσ22

After re-expressing of Eq.(26)and Eq.(27)to lead the final relationship for this method.

atφ0=0,the above equations will be

Thus the term cosαin Eq.(30)is the origin of the name for this method.

The value of stresses after re-expression maybe written as

4.Results and discussion

4.1.Tensile strength

In order to analyze the effect of friction stir processing on the TIG-welded joint,the tensile load applied on TIG and TIG+FSP welded joint by universal testing machine(UTM)at room temperature and fractured surfaces obtained from the tensile test were compared with one another.Three test specimens were tested at each condition and the average of these three results are presented in Tables 4-10.The tensile strength of the weldment made by TIG and TIG+FSP techniques are compared.Due to the absence of porosity,small grain size and presence of extra material in the welded region with filler wire,the tensile strength of TIG+FSP obtained higher value than the conventional TIG joining process.Filler ER5356 present in the welded joint makes more compact pressure leading to increase bond strength instead of filler ER4043.The grain size in the fusion zone for ER5356 is smaller than the ER4043.The tensile strength with filler ER5356 was obtained higher than ER4043[36].According to Hall Petch equation σ1=σi+kd(-1/2),the tensile strength is inversely proportional to the grain size[37].The tensile strength of the TIG-welded joint using filler 4043 and 5356 was calculated 158.6 MPa and 172.2 MPa respectively which is less than the TIG+FSP as shown in Table 4.The tensile strength of the TIG+FSP welded joint increased with increasing tool rotational speed.The highest tensile strength(233.17 MPa)was observed in TIG+FSP with filler ER5356 at tool rotational 1600 rpm with feed rate 63 mm/min.Three statistical parameters are investigated i.e.,standard deviation(SD),standard error(SE)and 95% confidence interval for TIG+FSP welded joint with filler ER4043 and ER5356 as shown in Tables 5-10.The standard deviation(SD)provides the deviance of the experimentalvalues from the mean it may be calculated asSD=[Σ(Хi-M)2/(N-1)]1/2and the standard error is a quantity used to measure how to close the prediction values to the experimental values.SEis calculated asSE=SD/N1/2,whereN=No of observation andM=mean.

Table 4Mechanical properties of TIG welded joint.

Table 5Tensile strength of TIG+FSP welded joint with filler ER4043.

Table 6Hardness of TIG+FSP welded joint with filler ER4043.

Table 7Residual Stress of TIG+FSP welded joint with filler ER4043.

Table 8Tensile strength of TIG+FSP welded joint with filler ER5356.

Table 9Hardness of TIG+FSP welded joint with filler ER5356.

Table 10Residual Stress of TIG+FSP welded joint with filler ER5356.

The confidence interval has shown that tensile strength and hardness increased with increasing tool rotation,whereas residual stress decreased with increasing tool rotation.

4.2.Micro-hardness

The micro-hardness distribution of the TIG-welded joint of AA6061 and AA7075 with different filler wire and TIG+FSP weldments with different processing parameters were analyzed by Vickers hardness testing results are shown in Fig.4.Vicker microhardness machine was used for measuring the hardness across the welded joints with a load of 100 g and dwell time 30 s.The hardness distributions are asymmetrical in the weld center due to the microstructure of the advancing side and retreating side introduced by unsteady plastic flow from the base metal to the weld center[38-40].The hardness slopes downward from the base metal to the welded region.The hardness fluctuates largely on the advancing side while the hardness holds steady in the retreating side[41].

Fig.4.Comparison of micro-hardness of TIG and TIG+FSP,(a)filler wire ER4043,(b)filler wire ER5356.

The alloying elements such as Si and Mg existing in the weld center make precipitation reaction and form a strong precipitate of Mg2Si to give a higher strength as shown in Fig.5(a).Fig.4 shows the micro-hardness profile at the heat-affected zone of aluminum alloy 6061 and 7075.It showed a significant difference,where the welded joint using filler ER4043 showed a lower average hardness value compared to filler ER5356.The hardness value at the center of weldment for filler ER4043 and ER5356 are 96 HV and 102 HV respectively,during welding,the filler wire ER4043 shows the columnar grains while fine equiaxed grains are found in ER5356.It may be analyzed that the fine equiaxed grains improve mechanical properties than the columnar grains[42].

4.3.XRD and EDX analysis

X-ray diffraction(XRD)analysis has been taken on transverse cross-sections of dissimilar TIG and TIG+FSP welded joint of AA6061 and AA7075 with filler ER4043 and ER5356.The corresponding pattern for the welded joints are shown in Fig.5 and found four major phases i.e.Al,Al2CuMg,MgZn2,and Mg2Si.Magnesium(Mg)and Silicon(Si)elements were found in the weldment besides the aluminum(Al),it is found that Mg and Si created the phase after the precipitation reaction in the weldment.The very high intensity was found from aluminum,because of the fragmentation of precipitates the intensity of MgZn2was decreases after friction stir processing on TIG welded joint with filler ER5356.The alloying elements such as Si and Mg existing in the weld center make precipitation reaction and form a strong precipitate of Mg2Si to give a higher strength.The same phase of Al2CuMg was detected in both the cases(filler ER4043 and ER 5356).

The hardness of the FSP joints is based on boundary energy,brittle intermetallic formation,strain hardening and precipitates formation in the joint.Fine recrystallized grains and increase of grain boundaries in the(SZ)of AA6061 and AA7075 joints predict higher hardness.Because of fine precipitates and fine grains structure,the stir zone is associated with plastic deformation and high temperature,due to precipitates formation at high temperature along the grain boundaries the hardness value of TIG+FSP at 1600 rpm with filler ER5356 recorded higher values than the TIG welded joints.The energy-dispersive X-ray spectroscopy(EDX/EDS)of the TIG-welded joint with filler ER4043 and ER 5356 welded joint has been analyzed.It was found that the atomic percentage of Si and Mg in the welded joint is higher than the other elements.Fig.5(b)and(c)illustrates the EDX image and percentage of element concentration at fusion zone(FZ)in the TIG welded joint with filler ER4043 and ER5356.Zinc(Zn),Magnesium(Mg)and Silicon(Si)elements were found in the weldment besides the aluminum(Al),it is found that Mg,Zn,and Si created the phases after the precipitation reaction in the weldment.

4.4.Residual stresses measurement

A mini portable X-ray diffraction apparatus(Pulstecμ-X360)at Delhi technological university,Delhi,India was used to determine the residual stresses in weldment of AA6061 and AA7075 by the cosαmethod.The cosαmethod was introduced in japan for residual stress analysis in 1978[43].This method is allowed the stress analysis by capturing the results by a single incident X-ray beam via a 2D detector.It shows the peak center is very stable throughout the welding.Debye ring and distortion ring is obtained by using cosα method.An investigation has been done to analyze the residual stress.The 3D Debye ring and distortion ring of welded samples are shown in Fig.7.The X-ray incident angle was set 35°and±5°oscillation was applied.The variation of residual stresses in the transverse direction of weldment with filler ER4043 and ER5356 are shown in Fig.6(a)and(b).Residual stresses(compressive or tensile)will influence the mechanical behavior of the welded joint.It can reduce brittle fracture strength,buckling strength and cracking in the weldment.Residual stress is also influenced by the prediction of brittle failure and affect the lifetime prediction of the component[44-47].Residual stress contributes both negative and positive effects to the weldment,generally,the tensile residual stress leads to a negative effect on the weldment[48].When the fixtures are unconstrained and the welded plate temperature is reduced to room temperature,the material in the nugget zone(NZ)tends to recover,but the material in the heat-affected zone(HAZ)has smaller deformation and will prevent the recovery process in NZ,so the maximum residual stress occurs in the boundaries of the HAZ with a minimum in the NZ.The deformation on the advancing side and retreating side differ,causing the recovery processes to differ too.So the residual stress cannot be symmetry to the welding line[49].The base material AA7075 on LHS of the weldment shows a minimum compressive residual stress,however,the residual stress gradually increases from the base material to towards the weldment and then decreases till second base material AA6061.The maximum residual stress occurs in the location where the equivalent of the plastic strain is decreased suddenly.The maximum compressive residual stress 71 MPa were located at the fusion zone(FZ)of the TIG weldment with filler ER4043,whereas minimum compressive residual stress 37 MPa was obtained at stir zone(SZ)of the TIG+FSP with filler 5356,the residual stresses profile of TIG+FSP were not symmetrical about the centerline of the weldment for both the cases.The left side peak value of the weldment was greater than the right side because the forming processes are different for the retreating side(RS)and advancing side(AS).

Fig.5.(a)XRD peaks of TIG and TIG+FSP welded joint,(b&c)EDX analysis of TIG welded joint in fusion zone with filler ER4043 and filler ER 5356.

4.5.Model validation

To confidently use CFD results for investigating the TIG+FSP process,the CFD model has to be correctly defined and a thorough validation has to be achieved.The model was first applied to simulate the experimental work on friction stir welded joint of Alalloy 6061 carried out by Hwang el al[50].The experimental temperatures were measured by thermocouple units placed equally at a distance of 5 mm,along the traverse direction of the rotating tool.The tool rotational speed and traverse speed,was 920 rpm and 20 mm/min respectively.The present simulation result was validated by this experimental results which gives the satisfactory amount of assurance in the fidelity of the simulation of welded joints as shown in Table 11.

4.6.Temperature variation in TIG+FSP process

The experimental temperature results of TIG+FSP measured by thermocouples.The advancing side temperatures in the transverse direction are A1,A2,A3,and A4and the retreating side has R1,R2,R3,and R4.The peak temperature of advancing side is marginally higher than the retreating side[51,52].The temperature of FSP tool is symmetric about the tool axis.The high temperature in the vicinity of the welding tool is attributed to the localized heat generation.During the TIG+FSP process,the temperature of the base plate around the welding tool is around the 765 K.It is still lower than the melting point of AA6061 and AA7075.When the position weld bead is far away from the FSP tool,the temperature drops quickly as shown in Fig.9(a)and(b).Fig.8 shows the temperature distribution plot for TIG+FSP welding at 160 s.During this simulation,eight points were observed to obtain the temperaturetime curves which can be compared with the experimental results.The area around the FSP tool reached the maximum temperature.The maximum temperature was observed 760 K at the advancing side whereas minimum temperature was observed 307 K at the retreating side.Higher heat is generated in the SZ at high tool rotation,the prevailing thermal conditions are controlled by the distribution and availability of precipitates in the matrix.Fig.10 shows the temperature variation profile on the mid-section of the top surface of(TIG+FSP)welded joint with different processing parameters.The region where its peak temperature is higher almost 450°C in(TIG+FSP)of AA6061 and AA7075 aluminum alloy at tool rotational speed of 1600 rpm.The heat is transmitted to the aluminum alloy which is preheated during tool rotation.The initial heating was predicted as the monitoring thermal contour of the rotating tool.When the rotating tool moves on the monitoring location,the temperature contour leads to slow cooling.Due to the relation between tool pin rotation and material flow,the higher shearing rate was observed at(A.S).Thus the temperature of(RS)is slightly lower than the(AS),while almost the symmetric temperature was observed at the bottom of the welded joint.

Fig.6.Variation of transverse compressive residual stress at the welded joint,(a)filler ER4043,(b)filler ER5356.

Fig.8.Temperature contour of TIG+FSP at 160 s.

The estimated maximum temperature about 515°C was calculated in the stir zone of the TIG+FSP welded joint at tool rotation of 1600 rpm whereas 408°C was calculated in the stir zone at 800 rpm.The temperature and heat flux distribution with different processing parameters are shown in Figs.10 and 11.In general,the temperature values at the(A.S)about 10-25°C greater than the(R.S),When the tool approaches the target location then there is a rapid change in the temperature,while the slower cooling rate was observed when the tool moves away from the target location[53].The variation of heat flux at the tool workpiece interface is shown in Fig.11.It was observed that the heat flux is directly proportional to the tool rotational speed.The maximum heat flux about 5.33×106w/m2was obtained in the SZ at 1600 rpm,whereas minimum heat flux was obtained at 1000 rpm.The non-uniformity was observed in the heat flux pattern at different processing parameters of the tool,because the rapid recirculation of plasticized material,the heat flux rate does not lead to the variation of local temperature.

4.7.Microstructure analysis of TIG and TIG+FSP welded joint

Fig.7.(a)3D Debye ring at the center of the TIG+FSP welded joint,(b)2D Distortion ring at the center of the TIG+FSP welded joint.

Table 11Validation of temperature distribution of friction stir welded joint of AA6061[50].

Fig.9.Variation of temperature during TIG+FSP welding:(a)Advancing side,(b)Retreating side.

For metallography observation,samples were first ground and mechanically polished and then etched in keller reagent(HNO3,HF and HCL)and observed by the microscopic machine.The microstructure of TIG welded joint of Al-6061 and 7075 with filler ER4043 and ER 5356 were observed at fusion zone as shown in Fig.12.The dispersed precipitates of Al2CuMg intermetallic compound were observed in welded region and coarse equiaxed grains were also observed in fusion zone with filler ER4043.The equiaxed grains is absent due to the material close to the fusion line provides plenty of sources for the crystal nucleation of the liquid metal during the TIG welding[54].Fine grains with small precipitates was reported in TIG welded joint with filler ER5356 as compare to ER4043.

Three different zones have been recognized in TIG+FSP weldment at low magnification due to mechanical and thermal stresses caused by the processing parameters.These zones are nugget zone(NZ),thermo-mechanically affected zone(TMAZ),and heat affected zone(HAZ).The formation of nugget zone shape in TIG+FSP welded joint is recognized to the maximum deformation and plasticization in the material which shows the fine recrystallized equiaxed grains.The formation of nugget shapes depends on thermal gradient,processing parameters and tools geometry in the work-piece[55].Therefore,coarse grain structure of TIG welded joint is transformed into the uniform and fine grains structures in the weld nugget zone due to adequate softening of material revealed the maximum tensile strength and microhardness of the TIG+FSP welded joint as shown in Fig.13.At high welding speed,the weld nugget zone is more homogenous than those produce low welding speed because high heat input gives the effective recrystallization and more homogenous temperature distribution in weld nugget zone.The grains size in stir zone change crucially which is depend on the heat input and processing parameters[56,57].

The grain size of TIG weldment were analyzed by the image J software and observed grain size in fusion zone with filler ER4043 and ER5356 are 20.4μm,and 18.2μm.Fig.14 shows the effect of tool rotational speed on grain size of TIG+FSP welded joint at nugget zone.It can be conclude that the rotational speed has a significant effect of grain size of the welded joint.When the rotational speed increases,grain size decreases,this observation give the satisfactory amount of assurance with Yupeng Li[58].The average minimum grain size i.e.3.4μm was observed in TIG+FSP with filler ER5356 at tool rotational speed 1600 rpm.

Fig.15 shows the fractured surface after the tensile test.In this analysis two mode of failure were observed.The fractured occurs in retreating side at low rotational speed(800 rpm),on the other hand failure was observed in advancing side at high tool rotational speed(1600 rpm).This observation gives the satisfactory amount of assurance with Mishra[5].SEM fractograph has been taken from the fractured tensile test.fracture morphology between the TIG and TIG+FSP shows that TIG welded portion shows the large voids whereas TIG+FSP welded portion shows fine dimples,this is the evidence of crack nucleation and growth 4 mm away from the weld line.The ductile fracture of welded joint occurs,an improvement in ductility may be achieved when the cavity nucleation could be suppressed[59].The maximum inter-facial normal stress is depends upon the grain particle size and the volume fraction of the grain particles[60].The fractured surface at room temperature were shown in Fig.15.

Fig.10.Temperature distribution at the center of the TIG+FSP welded joint,(a)800 rpm,(b)1000 rpm,(c)1250,(d)1600 rpm.

Fig.11.The heat flux at the center of TIG+FSP welded joint,(a)800 rpm,(b)1000 rpm,(c)1250 rpm,(d)1600 rpm.

Fig.12.SEM images of TIG welded joint at fusion zone(a)filler ER 4043,(b)filler ER 5356.

Fig.13.SEM images of TIG+FSP welded joint at tool rotation(a)800 rpm,(b)1000 rpm,(c)1250 rpm,(d)1600 rpm.

Fig.14.Effect of tool rotational speed on grain size of TIG+FSP welded joint.

The large dimples and quasi cleavage with sharp edge and various depth were found on the fractured tensile specimen surface of TIG welded joint with filler ER4043 and ER5356as shown in Fig.15(a)and(b).Many large and equiaxed dimples were observed in TIG welded joint with filler ER4043 whereas the main fractured mode was quasi cleavage with lot of tiny dimples gathered around the large and quasi cleavage dimples found in filler ER5356 which shows the ductile fracture.The FSW processing parameters effect the temperature distribution and material flow across the TIG welded joint.Fig.15(c)shows the SEM micrograph of factored surface of TIG+FSP welded joint,finer dimples are observed in TIG+FSP weldment as compare to TIG welded joint,which shows the better mechanical properties.

5.Conclusion

A new welding approach has been investigated to see the effect of FSP on TIG welding for dissimilar alloy AA6061 and AA7075 followed by the different filler wire ER4043 and ER5356.The mechanical characterization,finite element formulation and mathematical equations of heat transfer of TIG+FSP welded joints are investigated using ANSYS Fluent software by adjusting process parameters of FSP.The confidence interval has shown that tensile strength and hardness increased with increasing tool rotation,whereas residual stress decreased with increasing tool rotation.The maximum compressive residual stress 71 MPa were located at the fusion zone(FZ)of the TIG weldment with filler ER4043,whereas minimum compressive residual stress 37 MPa was obtained at stir zone(SZ)of the TIG+FSP with filler 5356.The large dimples and quasi cleavage with sharp edge and various depth were found on the fractured tensile specimen surface of TIG welded joint with filler ER4043 and ER5356 whereas fine dimples are observed in TIG+FSP weldment,which shows the better mechanical properties.TIG+FSP resulted in generated fine grains of 3.2-4μm.The grain size decreases by increasing the tool rotational speed.The minimum grain size was obtained in TIG+FSP welded joint with filler ER5356 at 1600 rpm.

Fig.15.SEM images of fractured tensile specimen,(a)TIG welded joint with filler ER4043,(b)TIG welded joint with filler ER4043,(c)TIG+FSP welded joint.

The asymmetry of the temperature distribution during TIG+FSP welding of AA7075 and AA6061 determined by numerical simulation corresponded with the experimental results.The peak temperatures on AS were higher than the RS of 20 K.The heat transfer is analyzed by the effect of different processing parameters of FSP tool.The temperature at advancing side is higher than the retreating side.The predicted peak values of temperature at the weld region was calculated by the ANSYS software and found the maximum temperature about 515°C at tool rotation of 1600 rpm,whereas the maximum heat flux rate about 5.33×106was observed at 1600 rpm.

Declaration of competing interest

The author declare that they have no conflict interest.