Investigation on the process parameters of TIG-welded aluminum alloy through mechanical and microstructural characterization

Muhmm Smiuin ,Jing-long Li ,Muhmm Timoor ,Mohmm Noumn Siiqui ,Sumir Uin Siiqui ,Jing-to Xiong

a State Key Laboratory of Solidi fication Processing,Northwestern Polytechnical University,Xi’an,710072,PR China

b Shaanxi Key Laboratory of Friction Welding Technologies,Northwestern Polytechnical University,Xi’an,710072,PR China

c Metallurgical Engineering Department,NED University of Engineering and Technology,Karachi,75850,Pakistan

d Department of Aeronautics and Astronautics,Institute of Space Technology,Islamabad,44000,Pakistan

Keywords: Tungsten inert gas welding(TIG) Heat input Welding defects Tensile strength Charpy impact strength Micro-vicker hardness SEM

ABSTRACT Multi-pass TIG welding was conducted on plates(15×300×180 mm3)of aluminum alloy Al-5083 that usually serves as the component material in structural applications such as cryogenics and chemical processing industries.Porosity formation and solidi fication cracking are the most common defects when TIG welding Al-5083 alloy,which is sensitive to the welding heat input.In the experiment,the heat input was varied from 0.89 kJ/mm to 5 kJ/mm designed by the combination of welding torch travel speed and welding current.Tensile,micro-Vicker hardness and Charpy impact tests were executed to witness the impetus response of heat input on the mechanical properties of the joints.Radiographic inspection was performed to assess the joint’s quality and welding defects.The results show that all the specimens displayed inferior mechanical properties as compared to the base alloy.It was established that porosity was progressively abridged by the increase of heat input.The results also clinched that the use of medium heat input(1-2 kJ/mm)offered the best mechanical properties by eradicating welding defects,in which only about 18.26%of strength was lost.The yield strength of all the welded specimens remained unaffected indicated no in fluence of heat input.Partially melted zone(PMZ)width also affected by heat input,which became widened with the increase of heat input.The grain size of PMZ was found to be coarser than the respective grain size in the fusion zone.Charpy impact testing revealed that the absorbed energy by low heat input specimen(welded at high speed)was greater than that of high heat input(welded at low speed)because of low porosity and the formation of equiaxed grains which induce better impact toughness.Cryogenic(-196°C)impact testing was also performed and the results corroborate that impact properties under the cryogenic environment revealed no appreciable change after welding at designated heat input.Finally,Macro and micro fractured surfaces of tensile and impact specimens were analyzed using Stereo and Scanning Electron Microscopy(SEM),which have supported the experimental findings.

1.Introduction

Being a lightweight metal,Aluminum Alloys find its application in almost every field such as aerospace manufacturing,defense,automobiles,food processing,structural and cryogenic applications,etc.[1,2].In the category of all non-heat treatable aluminum alloys,Al-5083 is well suited for cryogenic applications e.g.Brazed Aluminum Heat Exchangers(BAHX),which is at the heart of cryogenic processes in terms of its minimal installation cost and high productivity with low maintenance.Al-5083 alloy possesses the highest strength among others non-heat treatable aluminum alloys.It is capable of resisting exceptionally low temperatures without brittleness or loss of properties.It is indicated that Al-5083 welds processed with GTAW are mechanically more dependable than GMAW[3].It has also been reported that TIG welding is the best-suited process for joining Al-5083 alloy[4,5].There are some key problems associated with the welding of aluminum alloys due to its high conductivity and ability to form a brittle oxide layer,especially during fusion welding processes.Welding of thick plates of 5xxx series alloys also causes challenges due to the formation of different weld defects like induced porosity,spatter,and lack of fusion which severely affect the mechanical performance of the joint[6,7].Welding speed,welding current,and its polarity,heat input,shielding gas purging are among the TIGwelding parameters that control joint quality.A lot of literature is available addressing the effect of the above-mentioned parameters on the joint’s performance.Kim,D.,et al.conducted experiments on Al-5083 using different filler materials with varying Mg contents under different heat input values,results indicated that filler material with high Mg contents(5.9 wt%)provide 300 MPa tensile strength while the same strength achieved with low Mg filler material(5.1 wt%)under low to medium heat input values[8].Cevik,B.et al.studied mechanical and microstructural properties of laser-welded Al-Mg alloy by changing welding speed.Results clinched that escalation in welding speed promoted micropores and hot cracking which deteriorated tensile and impact properties[9].Capace,M.C.et al.discussed the in fluence of process parameters with the combination of current welding speed at various percent cold work values.He concluded that strength decreases with fast speed and high percent cold work due to grain growth[10].Singh,L.,et al.applied Taguchi method to optimize process parameters of TIG-welded Al-5083.240 A welding current,7 lit/min gas flow rate with 98 mm/min welding speed were found optimum parameters that provide 129 MPa tensile strength.Moreover,tensile strength decayed with a further increase in welding current beyond optimum value[11].Zhu.C et al.did some experiments on Al-5083 alloy using narrow gap GMA welding and he found that welding current and travel speed had a great in fluence in the formation of sidewall pores.He recommended that pre heating at 250°C with proper arc oscillation provide weld strength up to 90%of base alloy[12,13].Liang,Yet al.examined the effect of current on microstructure and mechanical properties of 6061 alloy using a hybrid welding process,he concluded that high welding current weakened the joint strength due to lengthy solidi fication time which produced coarse grain in HAZ and consequently,a drop in hardness observed[22].Porosity inducement is one of the perplexing problems that often occurred in the welding of Al-5083 alloys.Heat input,speed,welding current shielding gas,and weld grove geometry are the main parameters through which porosity can be controlled[2,13,14].Zhu,C.,et al.conducted simulations on the weld pool behavior of Al-5083 alloy by changing the welding current.He veri fied his simulation results through experiments and concluded that current variation triggered alteration in the fluid flow pattern,which ultimately produced weld pool defects like lack of fusion and sidewall porosity.Results indicated that an upward fluid flow pattern facilitates a lack of fusion defect while downward and backward fluid flow patterns produced porosity near the sidewall of the weld pool due to high current[12].Huang et al.characterized the joints of laser-GMAW hybrid welding of Al-5083 alloy,he concluded that mechanical properties are largely dependent on porosity formation and vaporization of Mg content,some other factors like uniform distribution of precipitates and dislocation density also governed the joints strength[2].Welding of thick aluminum alloy plates can be done through different methods like Friction stir welding,NG Laser and NG GMAW welding,electron-beam welding[15-17].But arc welding i.e.GMAWand TIGwelding are economically more feasible options due to less energy consumption and more flexibility.During Fusion welding,arising of pore defect is a key problem in the welding of thick plates of Al-5083 alloys,which occurred owing to hydrogen solubility.Beytullah et al.also considered the porosity effect,particularly on mechanical and microstructural examination of CMT,welded Al-5083,and Al-6082 aluminum alloy.He found that the pore formation severely deteriorated fatigue strength of the weld joint[18].Yao Liu et al.compared two fusion welding processes i.e.TIG and GMAW in terms of the propensity to pore formation.Results showed that TIG welding produced lesser pores compared to the GMAWprocess[5].

Al-5083 attains its strength from strain hardening phenomenon which is lost when welded due to recrystallization of grains,softening occurred in the partially melted zone(PMZ)or HAZ along with a grain growth in the fusion zone.However,strength and corrosion resistance can be improved by controlling the grain size and distribution of intermetallics[5-9].Alloying elements like Mn,Si,and Fe present in Al-5083 are used to improve mechanical properties by forming precipitates of Mg2Si,Al6Mn,and Al6(Mn,Fe)[8,9].Recently,researchers are incorporating nanomaterial e.g.CNTs,TiO2,and Al2O3in TIG welding of Al alloys to improve the microstructure and better mechanical properties by eradicating welding defects[19,20].Kumar.P.,et al.and several researchers reported that due to change in welding parameters i.e.welding current,speed,and heat input accompanied changes in microstructure including HAZ grain size,distribution of precipitates.Mustafa,U.,et al.also studied mechanical and corrosion properties of TIG-welded Al-5083 and he reported the presence of small precipitates of Al6(Fe,Mn)along the grain boundaries which causes high hardness value[21].Guo,Y.,et al.investigated welding speed,plasma current and gas flow rate on Al-5083 and established that a localized change in microstructure was observed in the upper and lower part of the joint due to different heat cycles,bead width and penetration increased with lower welding speed(10 mm/s)and high welding current(up to 250 A)[11].She,X.,et al.studied microstructure and fracture characteristics of a thick plate of Al-5083 which containedɑ-Al,Al6(Fe,Mn),and Mg2Si phases.He pointed out two forms of Mg2Si,one is formed along the Fecontaining intermetallics i.e.Al6(Fe,Mn)and the other grown independently in the Al matrix.Furthermore,amid solidi fication Fe and Mn tend to adhere to the grain boundaries,Mg dissolved in the Al matrix and Si dispersed near the grain boundaries.Thus,the formation of Mg2Si transpired in the lean regions of Fe and Mn.Furthermore,Mg2Si phase adhered to Fe based intermetallics can be easily separated from the Al matrix thus providing a crack initiation site during fracture[22].Several other researchers also investigated the microchemistry of Al-5083 during homogenization and established similar results that Mg2Si particles directly form on the constituent phases i.e.Al6(Fe,Mn)[23-25].

In the experimental scheme,heat input was chosen,since by analyzing the heat input one can scrutinize the effect of several process parameters of the TIG welding process like weld speed,weld current and voltage directly by altering the heat input.Thus,different heat input values linked with the change in welding parameters escorted some metallurgical disparities.Heat input plays important role amid TIG welding in determining joint’s mechanical performance,it was observed that by increasing the heat input,joint strength becomes inferior due to the formation of wide heat affected zone and coarsening of precipitation size[26].Thereby,grain morphologies,HAZ/PMZ width,precipitate size and its nature,and associated welding defects like porosity&solidi fication cracks are the key metallurgical features that can be controlled and improved through an optimum heat input value.Variation of hardness across the weld bead has also been reported by various researchers.Leo et al.concluded that change of hardness occurred due to the segregation of alloying elements.He also found that the maximum drop of hardness occurred in the weld centerline and increased at the interface between fusion zone and HAZ[27]since the weld cycles tend to promote precipitate dissolution and grain re finement which in fluences hardness to be maximized.Variation of welding current in the TIG welding process also in fluences the mechanical properties of the joint.

Most of the available literature on TIG welding Al-5083 alloy only focused on thin sheets.There is pintsize information available for thick plates utilizing a multipass TIG welding process to determine the mechanical properties[4].Therefore,this study focuses to examine the behavior of heat input in a multiple pass weld of thick Al-5083 plate(15 mm)by probing its metallurgical and mechanical properties.Porosity formation is the most common problem associated with Al-5083 alloys,hence by utilizing different heat input values,an optimum value has been found out which produced low porosity and subsequently,the mechanical properties of the joint is enhanced.Plenty of experimental trials were carried out with a range of torch traveling speed and welding current causing heat input to change.Resultant variations in heat inputs were categorized as low,medium,and high.Each category relates two echelons of currents and torch travel speed so that the maximum data set can be attained and optimized parameters were narrowed down by probing out mechanical and microstructural characterization techniques.In response to heat input,a correlation between microstructural variations and mechanical behavior was also explored by probing microscopic technique to fetch the information about changes in weld bead pro file,grain size,and phase distribution.Additionally,Macro and micro fractured surfaces of tensile and impact specimens were observed via Stereo and Scanning Electron Microscopy(SEM).All the experimental results are braced in the light of published literature with the intention that the research finding can be exploited in the welding of Al-5083 alloy.

2.Experimental procedure

The experimental procedures were planned in such a manner that the optimal parameters can be implemented to weld a thick plate of Al-5083 alloy used in structural and cryogenic components e.g.cold box in BAHX.Al-5083-H111 alloy plates having dimensions of 15×300×180 mm3were purchased from local aluminum manufacturer to make butt joints.As most of the components used in cryogenic applications(e.g.nozzles of the cold box in BAHX)are in the range 10-15 mm thickness.Therefore,15 mm thick plates were intentionally selected to take the full thickness response amid TIG welding.Width of 180 mm was selected so that tensile specimens with an overall length of 300 mm(as per ASTME8 standard)can be extracted as shown in Fig.1.Large grip lengths were purposely used to avoid slip and to prevent failure in the grip section.Moreover,all the weld runs of 300 mm were exploited which was achieved using a 300 mm plate length.

A schematic understanding of weld groove preparation and other details are provided in Fig.1.

Fig.1.Schematic of joint con figuration and other experimental details.

Commercially available argon gas protected the molten weld pool.Al-5183 filler rod which contains more magnesium content than Al-5083 was selected for the process.According to previous studies[1,8,21,28],the finest strength joints of Al-5083 is achieved when the filler material holds more magnesium content as compared to the base metal.Therefore,Al-5183 filler rod which contains more magnesium was selected for the process.Moreover,additional Mg content can compensate for the loss of Mg during the fusion welding process.The elaborated chemical composition of the materials used during experimentation is shown in Table 1.

Table 1 Chemical composition of as received plates and filler material(wt.%).

All the process parameters were selected in the light of literature survey,as discussed previously[4,5,8-10,26,28-35]and according to these studies,welding current and torch traveling speed is among the most in fluential parameters that affect the joint’s properties.Thus,in the experimental scheme,heat input was chosen,So that,one can scrutinize the effect of several other process parameters of the TIG welding process(in this work torch speed and welding current)by changing the heat input.Consequently,six test pieces were welded among the data set by varying the torch travel speed and weld current to establish a range of heat input values.(i.e.Low,Medium,and High)which were determined according to the European standard EN ISO-1011[8,14,32].All the relevant speci fications and experimental schemes are mentioned in Tables 2 and 3 respectively.

Table 2 Welding speci fications used in this study for the preparation of TIG-welded joints.

High heat input specimenss(HHI)were prepared with relatively slow welding speed(<1 mm/s)and low current(175 and 220 A)for each pass.These parameters were intentionally established to achieve higher heat inputs.Medium heat input specimens(MHI)were welded with comparatively high torch travel speed(2-4 mm/s)and current(270 and 320 A)for each pass.Since the speed of torch is inversely proportional to the heat input.Therefore,heat input value dropped as compared to HHI.Relatively higher welding speed and a large amount of weld current were required in preparing the low heat input test piece(LHI).EN ISO 1011-1 standard was used to calculate the heat input at each pass and an average value was considered after executing multipass TIG welding.Parameters like current,voltage,torch travel speed,and heat input were measured for every single pass from the welding site and are mentioned in Table 3.In this paper,HHI,MHI,and LHI codes were used for high heat input,medium heat input and low heat input welded specimens respectively.

Table 3 Experimental scheme followed in preparing weld coupons.

2.1.Radiographic examination

After welding,the foremost objective was to examine welding defects.In this regard,all the test pieces were analyzed by Radiographic testing.A portable X-ray system(Control EVO,COMET,Denmark)with 15 kV and 1.2 mA operating parameters was used to diagnose the welded portion along its length.Digital X-ray scans are displayed in Fig.2 with their corresponding panoramic bead images.Results revealed that extensive pore formation occurred in HHI specimens during TIG welding and the intensity of pore formation diminished as the heat input decreases with the increase in weld current.Through this nondestructive testing,an initial guess about suitable weld parameters can be estimated.Plates welded at high heat input revealed severe pore formation as displayed in Fig.2(a)and(b)while plates welded at medium heat input(MHI)revealed no visible porosity.Low heat input(LHI)specimenss exhibited some unfused region along with some undercuts marked in Fig.2(d)and(e)Thus;from the radiographic examination,only MHI plates were found quali fied among all the welded plates.Reasons for such defect formations are discussed in subsequent sections.

2.2.Mechanical characterization

2.2.1.Tensile test

A universal tensile testing machine(American Instron Corporation,model-3382)was used to assess elongation and Ultimate Tensile Strength(UTS)of all the weld coupons.All the specimens were tested at room temperature with a strain rate of 5 mm/min.Tensile specimens were skimmed perpendicularly from the welding direction in such a manner that the fusion zone positioned within the gauge length.ASTM E8 standard was used in the specimen’s preparation.The location of tensile specimens taken from the welded plates is shown in Fig.1.

2.2.2.Impact test

Charpy Impact test was done using the PIT-H series(Shenzhen Wance Testing Machine Co.,Ltd.,China)machine with a striking edge of 2 mm radius(R2)was used to determine the toughness of weld joints both at room temperature and cryogenic temperatures.Liquid nitrogen was used to conduct low temperatures impact test.All the specimens were made as per the ASTM E−23 standard.For low-temperature testing,samples were immersed in liquid nitrogen at-196°C for about 5 min to ascertain temperature homogenization.Three specimens were prepared for each plate and a mean of the three was considered.AV-notch was engraved at weld faces using a dedicated notch broaching machine(Wance group).

2.2.3.Micro Vickers hardness test

Hardness pro file was mapped out using HMV-G21(Shimadzu,Japan)Microhardness tester to determine the hardness of different regions like weld bead,HAZ,and base metal.Dwell time was set to be 10 s and a force of 500 gm was applied for each indentation.Hardness test was carried out on two different locations to take the full-thickness response of hardness variation due to the formation of different characteristic zones.Starting from the weld center,hardness was measured at regular intervals of 2 mm along a straight line as demonstrated in Fig.1.

2.3.Macro and microstructure characterization

Metallography was done to study the descriptive science between process attribution and resultant microstructural changes.In this regard,all the specimens were prepared by following standard metallographic steps i.e.grinding,polishing,and etching.Ultrasonic cleaning was done using an ethanol solution to circumvent image artifacts.Poulton’s reagent was used to produce image contrast and revealing different microstructural features like grain boundary,intermetallic phases,and precipitates,etc.Lastly,Olympus GX51 microscope was used to capture the images.Since the specimens were made using multiple-pass TIG welding.Therefore,there was a need to study microstructures at different depths.In this regard,microstructures of different locations were examined as shown in Fig.6.In the macrostructural examination,different characteristics of weldment were studied using stereomicroscope(MTI Corporation).All the specimens were sliced from the joints,and then ground and polished until a reasonably smooth surface obtained.Caustic etching was used for macro examination.Specimens were immersed in a freshly prepared 10%NaOHsolution heated at 65°C with 20 min immersion time.All the reagents were prepared following the ASM handbook 9th Edition;p.354-355.Tensile and impact specimens with fracture surfaces were studied from the stereo and Scanning Electron Microscope(Quanta 200S),operated at 15 kV-20 kV with the support of EDX detector.All the optical microstructures,stereo images,and SEM scans are presented in Figs.7-10 respectively.

3.Results &discussion

3.1.Effect of heat input on tensile properties

The changes made in heat input brought a prodigious impact on the joint’s mechanical properties.Thus,tensile testing was done to uncover the behavior of welded joints upon load.Fig.3 represents the stimulus-response of heat input on mechanical properties.Three specimens were tested from each category(i.e.low,medium,and high)and average values were considered in the analysis.Fig.3 shows that the samples from MHI treated at 270 and 320 A welding current with 1-2 mm/s torch traveling speed offered maximum tensile strength that is 77.59%and 81.74%respectively,to its base metal connoted as a datum in Fig.3.Resultant elongation also found a maximum(16.8%)to its reference value(24.67%).LHI specimens received inferior mechanical properties possibly due to weld defects as depicted in Fig.2(d).Visual examination of LHI specimens suggested that high amperage(i.e.>350 A)should be avoided as it produces signi ficant weld discontinuities and flaws like cold laps,undercutting and excessive weld burns as depicted in Fig.2(d)and(e).When welding current and torch travel speed changed to 175 A and 0.75 mm/s respectively,it carried excessive pore formation due to high heat concentration and be fitted by increasing current to 220 A.A slight increase in tensile strength and elongation was observed with 220 A current,which was mainly attributed due to the reduction in porosity as identi fied from the radiographic scans.It can also be shown from the results that yield strength did not signi ficantly change with the insurgence of the heat input value and it remained unaffected within the category.

Fig.2.Radiographs of welded plates.(a)High Heat input,175 A,(b)High Heat input,220 A,(c)Medium Heat input,320 A(d)Low Heat input,360 A,(e)Low Heat input,385 A.

Fig.3.TIG-welded Joints with the effect of heat input on mechanical properties.

In summary,all the specimens exhibited comparatively lower tensile strength and elongation to the reference base metal value.Reason to exhibited inferior joint strength relative to base metal is obvious since TIG welding causes local melting and solidi fication which results in recrystallized grains,as non-heat treatable aluminum like Al-5083 catches its strength from strain hardening[36],hence local re-solidi fication degrades the mechanical properties of the joint by changing its microstructure which eradicates the attributed features of strain hardening.S.L.Xia et al.as well discussed the in fluence of the microstructural change on mechanical properties of Al-5083 aluminum alloy.He concluded that the tensile properties of Al-5083 alloy got affected by the heating rate.High heating rate produced fine equiaxed grains with the highest tensile strength while coarse elongated grains foster by low heating rate offered low strength[37].Moreover,subsequent joint strength of the specimens processed at medium heat input was very much closer to the experimental value found by several researchers[2,34,38].More discussion on the variation of tensile strength is explained in subsequent sections.

3.2.Effect of heat input on charpy impact energy

Fig.4 depicted the impact energy of specimens processed at several heat inputs,controlled through changing torch travel speed and weld current.The test was conducted on both at the room and cryogenic temperature(-196°C)to assist the temperature dependency of joints’impact strength.For obtaining low temperature,liquid nitrogen was used,which has a temperature of-196°C.Three specimens were tested from each category and their average was used in plotting a graph.Results showed Al-5083 alloy impact toughness was not much affected by the test temperature.A minimal change was noticed in the toughness of base metal at room temperature and at-196°C,which con firmed that toughness does not change with temperature,as it was expected due to FCC crystal structure.The impact strength reduced severely after the commencement of welding in comparison to the base metal[39].Additionally,the impact energy of the welded samples also did not change appreciably in cryogenic impact testing(see Fig.4).However,all the specimens exhibited a substantial change in impact energy due to heat input.It was noticed that the absorbed energy by HHI welded specimens(welded at low speed)was lower than that of LHI welded specimens(welded at higher speed).A slight improvement in impact energy was observed at 220 A welded samples owing to the reduction of porosities as discussed previously.While the impact energy of the MHI specimens was found maximum,ranging from 22 to 24 J.The amount of absorbed impact energy by welded specimens(in each category),at room and the cryogenic temperature,was found to be almost equal within the scope of instrumental error.This behavior dictates that absorbed impact energy of Al-5083 alloy is impervious to the cryogenic temperature[9,33].In consultation with Fig.6,it was noticed that with the increase in heat input value,the severity of the pore formation also intensi fied as depicted in Fig.2.The distribution and size of precipitates were also affected by heat input,escalations in heat input intensify pore formation due to the vaporization of low melting constitutes like Mg contents which in-turn decrease the impact strength because pores act as stress risers facilitating crack growth during fracture[14,40].On the contrary,LHI promotes grain coarsening with some weld irregularities accentuating the loss in tensile strength.However,the Impact energy of LHI welded specimens were comparatively higher than HHI.Two factors contributed to an improvement in the impact energy of LHI specimens,firstly,the weld reinforcement which was slightly greater than HHI specimens with a paradigm transition of columnar to equiaxed grain morphology(discussed in section 3.5)and second,the most important factor was low porosity[29,35,41].These two factors contributed to escalating the impact energy.Refer to Fig.6 alteration of heat input changed the bead shape(transverse to weld direction),it became widened as the heat input decreased which is the reason for getting high impact energy,even though the same sample contained micro porosities and cold laps[42].

Fig.4.Effect of heat input on impact energy of TIG-welded Joints at ambient and cryogenic temperature.

3.3.Effect of heat input on weldment hardness

Results of traverse micro Vickers hardness tests at different altitudes on the weld cross-section are presented in Fig.5 for some representative specimens marked in Fig.6(b),(c),and(e).It was noticed that hardness of weld bead found to be lowest in all the specimens thus contemplating the weakest region of all the weldments.Different hardness responses were observed at the two locations.At the bottom surface,approximately 65%loss of hardness was observed in the weld bead of LHI welded samples which reached down to 22%hardness reduction with HHI welded specimens.While on the top surface only 23%declination was noticed with LHI welded samples and it progressively decreased to 11.4%hardness loss occurred when treated with HHI welding.

The results were interpreted as a vindication of the change in heat input.Such dwindling in bead hardness manifests the increase in weld current as argued by several researchers discussed previously[3,20,29,34,39,41].Moreover,a deep well was formed in the hardness pro file mapped near the bottom surface of the weld bead while such kind of reciprocation was not recognized at the top surface.Multiple pass TIG welding brought a change in hardness pro file across the weld cross-section since an overlay of the weld deposit was made which produced an aided heat that consumed in altering the microstructure[4,6].Consequently,grain size coarsened and porosity inducement transpired as a result of a decrease in heat input or an increase in weld current,optical microstructures analysis also supports this argument.Hardness contour at the top surface became flattened at the weld fusion zone making a roughly “U” type contour.Moreover,near the top surface,a smooth transition in hardness pro file from weld bead to HAZ can be easily observed.No sharp discontinuity of hardness was observed except the specimen with high heat input.

Fig.5.Hardness pro file across weld bead cross-section at two different locations(a)near weld root(b)near weld reinforcement.

Fig.6.Weld beads in the transverse direction of welded joints(a)High Heat input,175 A,(b)High Heat input,220 A,(c)Medium Heat input,320 A(d)Low Heat input,360 A,(e)Low Heat input,385 A,(f)Schematic showing the effect of heat input on bead morphology.

Considering Fig.5(b),the hardness pro file of HHI samples,a small protuberance in hardness pro file was established adjacent to the weld bead(hatched region)which can be perceived as a region of Heat Affected Zone(PMZ).In this vicinity,a sudden drop of hardness occurred relative to the weld bead,indicated the presence of micro-pores along the PMZ of HHI welded specimens as shown in Fig.7(c).Since high heat input causes “Mg” vaporization along with the dissolution of second phase particles that embraced such hardness variation in high heat input samples[4,43].On comparing the hardness curves,it can be realized that all samples possess variation in hardness across its weld bead due to different cooling rates,precipitate size and its distribution,porosity formation,and off-course change in grain size.All these factors are discussed in sections 3.5 and 3.6.

Fig.7.Optical microstructures of designated specimens at different locations.Exaggerated images shown in figs g,h and i are at high magni fication.

3.4.Effect of heat input on weld bead appearance

Different bead shape and weld penetration can be easily grasped from Fig.6 which were attributed to the concurrent change of welding parameters i.e.heat input,weld current,and weld speed.It is discerned that as the heat input decreased,it broadened the weld bead and induced a high degree of dilution as shown in Fig.6(a-e).At high heat input,shallow dilution was ensued owing to low welding current and dilution augmented with the hike of welding current[29,41].This phenomenon described schematically in Fig.6(f)in which arrows indicate flaring out of the weld interface in response to heat input.Some welding defects can also be seen includes weld cracking,cold laps,pores,and undercutting marked by arrows(see Fig.6(d)and(e)).Since the 5xxx series,aluminum alloys are susceptible to form porosity in weld metal because of the tendency of dissolving a large number of gases in the molten pool.Weld porosity is greatly dependent on solidi fication kinetics.Specimens at high heat input(welded at low welding speed)promote grain coarsening and maximize the magnesium evaporation which ultimately produces porosity as evident from Figs.7 and 9[4].

It was noticed that the high cooling rate generates a large degree of porosity and vice versa.It is because,at low cooling rates,gas bubbles have sufficient time to float,amalgamate,and escape from the molten pool.Samples treated at medium and low heat input produced a lesser number of pores.Porosity can be diminished by setting up some compositional changes in shielding gas together with Argon.The fluidity of the molten pool will eventually rise which will increase the probability of the entrapped gases to escape that leads to better mechanical properties owing to the low degree of porosity[14,38].

3.5.Effect of heat input on microstructural features

Al-5083 attains its strength from strain hardening phenomenon which is lost after welding due to the recrystallization of grains in the fusion zone accompanied by grain growth.However,improvisation in mechanical properties was governed by controlling the grain size and distribution of intermetallic[5,44,45].Fig.7(a to i)indicated microstructures of the weldments observed at different positions marked in Fig.6(b),(c)and(e).Since,joints properties,like hardness,varied across its length(both transverse and longitudinal)due to change of heat transfer owing to varying welding parameters was the reason for choosing these multiple locations.Therefore,corresponding microstructural changes need to be addressed,thus,ascribed in Fig.7.

It was noticed from Fig.7(a to c)that heat input and other welding parameters have had a prominent role in the broadening of PMZ and its grain size(see Table 4).As weld heat input increases the width of PMZ flared away also the grain size turns out to be coarsened[28],this ultimately caused low tensile strength and drop in the hardness of samples at the weld fusion zone(i.e.weld bead).Image analysis software in Olympus GX51 was used to determine the grain size.Approximately 15-28μm PMZ Grain size was measured in low heat input specimens,80 to 95μm grains were present in medium heat input specimens,while high heat input specimens contained 60 to 105μm.Table 4 speci fied the comparison of different microstructural features.In comparison,high heat input HHI welded specimens possess coarse PMZ grains because of the slow cooling rate,thus reducing the weld tensile strength as shown in Fig.3.Small PMZ grain size was observed in low heat input LHI welded specimens but the presence of welding defects i.e.lack of fusion and internal cracks(see Fig.6(d)and(e))in the weld fusion zone degraded its mechanical properties.

Table 4 Comparison of different microstructural features.

Reasonable low degree of micro-porosity existed in medium heat input MHI specimens with relatively small grain size making it robust in terms of joints mechanical properties.Moreover,there was also a difference in grain size between the weld fusion zone and PMZ attributed to the difference in heat transfer rate.Grains of the PMZ was found to be coarser than weld metal(WM)and(BM)within the specimen category.Variation in grain size was responsible for the change of hardness pro file in both locations.It can also be noted that the grain size of the PMZ region of HHI specimens was comparatively coarser than the other specimens.Also,the PMZ of such samples was decorated with the maximum number of pores as depicted in Fig.7(c).It was also recognized that fine equiaxed grains started to appear on the weld face(near the top surface),which is the last pass of the weldment.Whereas,coarser grains were established in the lower regions of weld metal as shown by inscribed images in Fig.7(g to i).The reason for this coarseness is the reheating,which occurs because of subsequent weld passes.Thus,providing sufficient heat to the underlying weld passes for the re-growth of grains.It was also evident from Fig.7(g to i)that the degree of porosity became less severe at the bottom i.e.at weld root due to reheating during multiple weld passes[43].

3.6.Effect of heat input in the formation of intermetallic precipitates

The principal strength in the Al-5xxx series comes from magnesium,which forms a solid solution with Aluminum or other constituents in the alloy,such as with Silicon it forms Mg2Si.Notably,the solubility of Mg in Al is around 2%at room temperature and 15%at about 450°C.Therefore,most of the magnesium is in dissolved form and predominantly a single-phase must exist until and unless a non-equilibrium condition prevails or an alloy contains more Mg than its solubility limit at a given temperature.When such conditions emerged,some new phases formed within grains or along the grain boundaries.As in welding,under nonequilibrium condition Al-5083 alloy tend to form Mg2Si,Al6(Mn,Fe),Other compounds,such as Al12(Fe,Mn)3Si,Al6(Mn-Fe-Cr)and Al-Mg-Si intermetallic compounds have also been witnessed to form by several researchers[23,24,46,47].

Exaggerated images in Fig.7,showed dissemination of eutectic constituent of Mg2Si and Al6(Mn,Fe)second phase particles[9,23]both the phases are different in terms of refractive index,Mg2Si gave dark claw-like appearance while Al6(Mn,Fe)revealed as gray bulks in the form of flakes and fine particles[23,47]as indicated in Fig.7(g to i).Although the formation of Al3Mg2akaβ-phase along with some other second phase particles cannot be overruled[43,48].It can be seen from Fig.7(g to i)that the alteration of heat input affected the precipitate size and its distribution.The rise of heat input facilitates the diffusion of Mg atoms towards grain boundary as it offers low energy nucleation site for precipitation compared to intra-grain regions[43],thus,owing to the higher welding speed in LHI welded specimens,which promotes large undercooling,resulting in intermittent grain boundary structure throughout the weld region.As the heat input increased,accompanied with small undercooling(i.e.slow cooling rate),constituent phases like Mg2Si and Al6(Mn,Fe)decorated the grain boundary regions,marked by arrows in by Fig.7(i).It is an interesting fact found by some researchers[23,24,46,47]that in 5xxx series aluminum alloys Al6(Mn,Fe)provides a nucleation site for Mg2Si and both the phases precipitate out on each other like alternative chains/bands of phases.Also,it is quite cognizant that as the heat input increase,Al6(Mn,Fe)phase began to diminished out and partially used up by dark nucleated Mg2Si phase due to its solidification rate sensitivity[23,49].Additionally,the precipitate size of Mg2Si coarsened with evolution from LHI to HHI and Al6(Mn,Fe)phase became fine nodular due to the surplus of heat in multi-pass TIG welding which posed a negative effect on joint mechanical strength.Since large precipitate size along grain boundary weakens the tensile strength by providing ease of grain boundary sliding was also evident in Fig.3[4].

In summary,it is known that the strength of Al-5083 is because of solid solution strengthening mechanism in which solute constituents readily dissolve in the solution matrix.At LHI specimens,higher welding speed causes a rise in undercooling,thus promotes micro porosities(Fig.7(d)and(g))and dissolution of solute from the solution matrix.Due to which strength from solid solution strengthening mechanism is preferentially decreased propelling to a decrease in tensile strength and toughness.

Whereas,high heat input produces grain coarsening with the formation of porosities and large precipitate size which brings lower tensile strength due to weak grain boundary region[50].Thus,improved mechanical properties can be obtained by utilizing medium heat input values with appropriate weld parameters.For Al-5083 alloy,optimum parameters were found with MHI welded specimens having 2-4 mm/s torch traveling speed accompanied with 270-320 A welding current.

Thermal gradient “G” and growth rate “R” of the weld metal are the two aspects that dictate the weld morphology under the influence of welding parameters.The ratio ofG/Ris useful in predicting solidi fication morphologies i.e.columnar,dendritic,and equiaxed.The solidi fication front changes its morphology from planar to equiaxed grain as theG/Rratio drops.A correlation among grain morphology,Thermal gradient(G),and grain growth(R)can be well explained from Fig.8,which pictorially de fines an exaggerated portion of the weld pool marked with a dotted rectangle.It is eminent that the thermal gradient becomes low at the weld pool center and increases towards the weld trailing edges because of weld pool temperature,which is high at the center and low at the fusion boundaries.While grain growth occurred in the direction of maximum heat extraction as shown in Fig.8.Reason in the morphological variation across the weld trailing edge can be easily grasped by considering the two vectors i.e.weld velocity(V)and grain growth(R).While moving from the bottom trailing edge,the angle(θ)changes from maximum to minimum(i.e.90 to 0°)facilitating grain growth due to cosine vector projection(cos 0°=1),on the contrary,growth rate becomes zero at the fusion boundary(cos 90°=0).Several researchers observed the in fluence of heat input and weld speed on the grain structure of commercial aluminum alloys and it was observed that equiaxed grains formation occurred with the increase of weld speed[51].Similar behavior is observed amid TIG welding 0f Al-5083 alloy at different heat input values.

As described in Fig.8,G/Rratio plays a signi ficant role in determining weld morphology across the fusion line,which changes as this ratio varies.Specimens with high heat input cause large undercooling that leads to trivial grain growth(R)and promoting highG/R.Eventually,this deciphers to columnar dendritic structure across the fusion boundary(ref to Fig.8(c)).Moreover,with the decreasing torch travel speed and increasing heat input value,theG/Rratio became low thus resulting in equiaxed grains formation evident in Fig.8(a).Therefore,weld process parameters have a direct in fluence on fusion zone microstructure.Apart from porosity generation,grain morphologies also in fluence on joint mechanical properties.It was found that columnar and dendritic morphologies are not good for welded joints,as these morphologies tend to deteriorate the mechanical properties.This argument is well supported by the results obtained by tensile and impact testing.Notice that despite signi ficant weld flaws in LHI specimens,their impact energies stands substantially high compared to HHI welded specimens.An escalation in impact properties of LHI welded specimens accentuated by equiaxed grain structure,formed at low heat input parameters.Although,among different heat input values,MHI welded specimens exhibited the highest tensile and impact properties due to equiaxed grains formation and zilch amount of welding defects accentuated enhanced mechanical properties[51,52].

Fig.8.Variations transpired in Grains morphologies w.r.t thermal gradient “G” and growth rate “R” across fusion line and their corresponding optical microstructures.

3.7.Macro-fractured surfaces of impact and tensile specimens

Fig.9(a to d)illustrated fractured surfaces of base metal and TIG welded impact and tensile specimens.Since the appearance of the fractured surfaces elucidates the nature of fracture/failure.In this regard,an inspection of the fracture surfaces was made under a stereomicroscope.Fig.9(a)revealed macro fracture surface of the parent metal,indicated planer shiny facets and fibrous surface features along with the appreciable amount of straining leads to form shear lips at the edges of the specimens,signi fied towards a ductile failure.However,in cases of welded impact Fig.9(b to f)and tensile Fig.9(g to k)samples,the fractured surfaces revealed severe porosities,particularly with HHI welded specimens treated at 175 A.Both types of specimens(i.e.impact and tensile)displayed porosities ranging from 1 to 3 mm in diameter,illustrated in Fig.9(b)and(g)resulting in poorer mechanical properties as discussed previously.However,by utilizing MHI welding parameters(1-2 kJ/mm),porosities completely wiped-out as shown in Fig.9(d)and(i).Some cold laps were formed,when specimens processed at LHI(<1 kJ/mm)by utilizing high current(360-385 A)and fast torch speed(>5 mm/s).LHI welded samples also displayed arrays of micropores indicated in Fig.9(e)and(j)which leads to formed unfused regions with the rise in LHI weld current(385 A)marked in Fig.9(f)and(k).Such features were responsible for low energy absorbance during plastic deformation as compared to the parent metal.Since,welded zone contained a lot of inhomogeneities i.e.intermetallic precipitates and porosities,thereby promoting multiple crack initiation sites,which can be revealed from Fig.9(b to k).Thus,no particular chevron patterns were formed in the welded samples.

Fig.9.Macro-fractured surfaces of impact and tensile specimens.(a)Base metal(b,c)High heat input,impact(d)Medium heat input,impact(e,f)Low heat input,impact(g,h)High heat input,tensile(i)Medium heat input,tensile(j,k)Low heat input,tensile.

3.8.SEM examination of fractured specimens

To grasp a detailed study on fractured surfaces,SEMscans were made at the fractured locations shown in Fig.9.EDX was also performed in support of SEM to corroborate the intuition developed in section 3.6.All the Fractographs are discerned in terms of dimple size and its shape,porosity,grains distortion,sliding marks,etc.Fig.10(a to c)indicated SEMscans of tensile fractured surfaces of welded specimens,showing the presence of various sizes of dimples,speci fied in the tensile mode of fracture in which grains were pulled against the tensile load.The presence of dimples is a characteristic property of a ductile fracture,since;dimple size signi fies the strength and ductility of the material.Fine dimples display high strength materials whereas low strength material possesses coarse dimples[38].Fig.10(a)represents the fractured surface of high heat input welded specimens recognized by the embedded porosity which can also be seen in Figs.2(a)and 9(g).A key feature displayed in Fig.10(b)showed grain boundary segregation which occurred due to Mg diffusion in response to high heat input[34]and accompanied by progressive growth of intermetallic phases along the grain boundary as shown in Fig.7(i).Apart from porosities,these overgrown precipitates along grain boundaries expedite crack propagation,thereby,a weldment experiences a quasi-brittle fracture,consequently,high heat input welded specimens fractured at a low-stress level compared to medium heat input specimens.One prominent feature regarding dimple size can be observed in Fig.10 that large and shallow dimples were exhibited by HHI welded specimens and the dimple size reduces with the decrease of heat input,thereby improving the tensile strength with fairly good impact property.Although LHI welded specimens also revealed fine and deep dimples,extensive voids formation and weld irregularities worsened its joint strength.

Fig.10.SEM scans of designated fractured specimens of tensile specimens(a)Low heat input(b)Medium heat input(c)High heat input(d)EDX analysis of selected regions.

An EDX scan namely,spectrum 1 elucidates the presence of intermetallic compound which includes 48%Mg,21.4%Si,6.5%Mn,and 2.2%Fe as chief elements at the pinpoint region marked as “+” in Fig.10(b).This con firmed the presence of elements that formed the intermetallic phases i.e.Al6(Mn,Fe)as described in Fig.7(i).Likewise,Spectrum 2 was taken at the porosity region represented in Fig.11(b)indicated 98%Al,0.5%Mg,and the trace elements con firmed the “Mg” evaporation which turned out to be porosity[14].Spectrum 3 indicated the presence of Mg2Si particles which were disseminated in the matrix and beside grain boundaries,as displayed in Fig.10(b),(c),and Fig.11(d)[23,50].

Fig.11(a to f)represents impact Fractographs,unlike tensile mode fracture impact specimens,contained twisted and sheared grains in the direction of impact load.An appreciable distinction exists among all the fractographs that were endorsed for variation in welding heat input.Although fracture surfaces of cryogenic impact specimens displayed similar features like room temperature impact specimens due to irresponsive behavior of this aluminum alloy under low temperature[53].Specimens at high heat input contained porosities connoted by Fig.11(a)and(b),these escalations in pore formation are responsible for the inferior impact properties(see Fig.4).The de ficiency of Mg contents con firmed by EDX analysis at the surface of microvoid elicited in Fig.11(b)is considered to be the reason for the deterioration of properties[7,22].

Additionally,overgrowing intermetallics i.e.Al6(Mn,Fe)and other precipitates as shown in Fig.11(d),in the welded zone backs the argument that these particles provide nucleating sites for voids coalescence and crack growth[22].These precipitates tend to promote grain sliding owing to grain boundary weakening[50,54].Medium heat input specimens exhibited fine dimples compared to the other specimens as shown in Fig.11(c)and(d).Some fine and elongated dimples also appear in Fig.11(e)and(f)for LHI welded specimens along with smooth facets,which are attributed to brittle fracture.Cleavage facets are prominent in brittle dominant fracture surfaces i.e.Fig.11(e)and(f),while the presence of dimples with fibrous surface implied ductile fracture.Therefore,it can be concluded that fine,well-dispersed precipitate produced small dimples while large and overgrown precipitates formed coarse dimples and facilitate grain sliding with cleavage fracture as evident by high heat input specimens,(ref to Fig.11(e)and(f)).Thereby,characteristic surface attributes of the aforementioned fractographs justi fied the judgment made in the previous section.

Fig.11.SEM scans of designated fractured specimens of Charpy Impact specimens at room and cryogenic temperature:(a&b)Low heat input(c&d)Medium heat input(e&f)High heat input.Images at left are of room temperature.

4.Conclusion

A study was made to select suitable welding procedures and to develop a set of adequate welding parameters for the welding/repairing of components used in structural applications,chemical processing industries,and cryogenic applications e.g.Brazed Aluminum Heat Exchanger(BAHX).Hence,in the light of experimental results and extensive literature review,the subsequent conclusions can be made:

?The optimum heat input value to weld a thick plate of Al-5083 alloy was found to be 1-2 kJ/mm with 270-320 A welding current and 2-4 mm/s torch traveling speed.Above and below these parameters,joint properties were declined.

?Radiographic examinations revealed that extensive pore formation occurred in HHI specimens during TIG welding and the intensity of pore formation diminished as the heat input decreases with the change of welding parameters.This intuition was authenticated by analyzing fractured surfaces.

?All the specimens displayed inferior mechanical properties as compared to the reference value.Medium heat input welded specimens exhibited the highest tensile strength;only about 18.26%of datum strength was lost after welding.A minimum tensile strength was perceived in LHI welded specimens translating a decline in 65%strength and an intermediate strength was achieved in HHI welded specimens with a drop of 36%strength.A similar trend was also observed in elongation.Yield strength of all the welded specimens did not change appreciably indicated no in fluence of heat input.

?Impact results showed that there wasn’t a signi ficant difference observed in the room and cryogenic temperature which indicated that the subject material doesn’t possess temperaturedependent impact properties.Although the absorbed energy by low heat input specimen LHI(welded at high speed)was greater than that of high heat input HHI(welded at low speed)because of low porosity and formation of equiaxed grains resulting better impact toughness.

?Variation in hardness occurred across the weld bead both longitudinally and in the transverse direction.Minimum hardness value witnessed in the entire weld bead region.Hardness values severely decreased by lowering heat input values.A signi ficant drop of hardness occurred in the welds bottom region compared with the top surfaces due to different heat input values triggering a change in precipitate size,its distribution,porosity formation,and the anticipated change in grain size.

?PMZ width also affected by Heat input and it became widened with the increase of heat input,also the grain size of PMZ was found to be coarser than the respective grain size in the fusion zone of the weldments.Moreover,Fine equiaxed grains were established on weld face pass of all the weldments.Whereas,coarser grains were established in the lower regions of weld metal.The reason for this coarseness was the reheating which occurred because of subsequent weld passes.

?Thermal gradient “G” and grain growth rate “R” also signi ficantly changed which accompanied a paradigm transition of columnar to equiaxed grain morphology as heat input varied from high to low respectively.

?Presence of intermetallics like Mg2Si and Al6(Mn,Fe)was con firmed by EDX analysis and found that with the increase of heat input,coarse precipitates were tend to form along the grain boundaries which ultimately deteriorate the mechanical properties of the joint by grain sliding and cleavage fracture(ref to Figs.7(i)and 10(c)and Fig.11(e)and(f)).It was also established that as the heat input increases,Al6(Mn,Fe)phase began to diminished out and partially used up by dark nucleated Mg2Si phase due to its solidi fication rate sensitivity.

Declaration of competing interest

Article that is submitted toDefence Technologyfor review is original,has been written by the stated authors,and has not been published elsewhere.

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Acknowledgments

The authors gratefully acknowledge the assistance of Mr.Jin Fei(Wuhan Sanlian Speci fic Tech Co.Ltd.)for providing the Radiographic testing facility.