Gui-ju Zhang ,Cai-yuan Xiao ,**,Olatunj Oladimeji Ojo

a Key Laboratory of Hunan Province for Efficient Power System and Intelligent Manufacturing,Shaoyang,422000,Hunan,China

b College of Mechanical and Energy Engineering,Shaoyang University,Shaoyang,422000,Hunan,China

c Department of Industrial and Production Engineering,Federal University of Technology Akure,Akure,Nigeria

Keywords:Friction stir spot welding Underwater friction stir spot welding Nugget characterization Microstructure Mechanical properties Fracture

ABSTRACT This paper studies the friction stir spot welding of AA2024-T3/AA7075-T6 Al alloys in the ambient and underwater environments by clarifying the nugget features,microstructure,fracture and mechanical properties of the joints.The results show that the water-cooling medium exhibits a significant heat absorption capacity in the AA2024-T3/AA7075-T6 welded joint.Nugget features such as stir zone width,circular imprints,average grain sizes,and angular inter-material hooking are reduced by the watercooling effect in the joints.Narrower whitish(intercalated structures)bands are formed in the underwater joints while Mg2Si and Al2CuMg precipitates are formed in the ambient and the underwater welded joints respectively.An increase in tool rotational speed(600-1400 rpm)and plunge depth(0.1-0.5 mm)increases the tensile-shear force of the welded AA2024-T3/AA7075-T6 joints in both the ambient and underwater environments.The maximum tensile-shear forces of 5900 N and 6700 N were obtained in the ambient and the underwater welds respectively.

1.Introduction

Effective control of the weld thermal cycle via external medium has been reported as a novel approach for improving the efficiency and the mechanical properties of friction stir(spot)welded heattreatable aluminum alloys and other heat-sensitive alloys.Cooling media(such as water and liquid nitrogen,Fig.A1)enforce a high cooling rate and less(reduced)thermal gradient in the in-process water/nitrogen-cooled joints.This phenomenon reduces the volume fraction and the average sizes of intermetallic compounds in underwater welds[1]and it is equally adjudged to be capable of inhibiting atmospheric oxidation(oxides)responsible for the formation of S-line defect in the friction stir welded Al joints[2].Precipitate dissolution,coarsening of strengthening phases,weld softening,undesirable surface texture and large heat affected zone are salient attributes that can also be impeded in underwater joints[2,3]in order to improve mechanical properties of joints.Hui-Jie et al.[4]revealed that underwater cooling enhanced the strength of the friction stir welded AA2219 Al alloy from 324 MPa to 341 MPa while plasticity of the welded joint deteriorated.This paper aims to further clarify underwater friction stir spot welds of dissimilar Al alloys by providing requisite information on morphological nugget features,microstructure,fracture and mechanical properties of welds.

Wang et al.[5]studied the strengthening mechanism of the underwater friction stir welded Al-Zn-Mg-Cu alloy joints.It was revealed that the cooling medium eliminated the hard-etched area in the weld nugget of the alloy.In fact,the rapid water-cooling of the Al-Zn-Mg-Cu joint introduced Guinier-Preston(GPII)zone and retained a substantial amount of needle-like semi-coherent MgZn2structure in the joint.This microstructural adjustment is identified to have boosted coherency strengthening of the joint under axial loading conditions.Rouzbehani et al.[6]identified that a direct correction existed between the mean grain and precipitate sizes and the ratio of tool rotational and travel speeds of the AA7075 Al alloy joints fabricated in an underwater environment.Tan et al.[7]discovered that a decrease in the ambient temperature caused an equivalent decrease in the recrystallized grains and the volume(amount)of the homogeneously dispersed secondary phases(MnAl6and(Fe,Mn)Al6particles)in the underwater friction stir welded AA3003 Al alloy.The water-cooling environment caused the formation of fine grain structure with minimal intermetallic precipitates and weld porosity in the works of Mehta et al.[8],Zhang et al.[9],Heirani et al.[10],and Zhang et al.[11]respectively.Other works on the thermal effect of dissimilar welded joints have been studied in the literature[12].

Wang et al.[13]revealed that the fluidity of metal is severely weakened during the underwater welding process.This phenomenon enables the possibility of metal joining at a higher tool rotational speed without the concern for overheating.Zhang et al.[14]conducted 3-dimensional thermal modeling of an underwater FSW process.It was reported that peak temperature was significantly lowered in the underwater welds as compared with the ambient welded joint.However,a higher surface heat flux(of the shoulder)was recorded in the underwater welds.The in-process water cooling route was reported to drastically narrow the hightemperature affected/distributed area and the thermal cycle(of different zones)in the underwater welds.The role of in-nugget/residual Alclad on the ambient friction stir spot welded AA2219 Al alloy has been characterized by Ojo et al.[15]but the impact of rapid cooling on such nugget properties is yet to be expounded in literature.The influence of welding parameters on properties and microstructure of friction stir spot welded joints has been well reported in the literature[15,16].Ojo et al.[16]reported that tool rotational speed had the dominant effect on the material flow and strength of the friction stir spot welded AA2219 Al alloy.It was revealed that the percentage contributions of tool rotational speed,plunge depth and dwell time on the failure load of the joint were 53.47%,35.12%,and 10.64% respectively.However,there is still a need to further experimentally clarify the role of rapid cooling on nugget features(such as inherent flow-induced variation),microstructure and other properties of dissimilar AA2024-T3/AA7075-T6 Al alloys.These alloys are used for structural applications in the aerospace and automotive industries.This paper investigates the nugget properties of underwater(UFSSW)and ambient friction stir spot welded(FSSW)joint of AA2024-T3/AA7075-T6 aluminum alloys obtained under different process parameter combinations.The microstructure,mechanical properties and fracture of the underwater joints are also investigated and compared with the joints fabricated under normal ambient conditions.

2.Experimental procedure

The base metals used for this research are 1.6 mm thick AA2024-T3 and 2 mm thick AA7075-T6 aluminum alloy plates.The ultimate tensile strength,yield strength,and elongation of the latter are 560 MPa,485 MPa,and 12%respectively while those of the former are 448 MPa,317 MPa,14% respectively.Table 1 shows the compositions of the base materials obtained via quantometry.The asreceived base materials were cut into the dimensions of 100 mm×25 mm and subsequently cleaned with acetone in order to remove dirt and marks(paint).

Table 1Chemical composition of the base materials(wt%).

Friction stir spot welding(FSSW)processes were carried out on the base materials in an overlapped geometry with an overlapped area of 25 mm×25 mm under different environmental conditions(in air and underwater).The AA2024-T3 Al alloy was placed on top of the AA7075-T6 Al alloy.The welding/joining processes performed under the ambient and water conditions were referred to as friction stir spot welding(FSSW)and underwater friction stir spot welding(UFSSW)processes respectively in this paper.The UFSSW process was carried out in a rectangular chamber filled with tap water.A water height of about 4 cm was left on the surface of the clamped work-pieces prior to the welding process.A K-type thermocouple was embedded underneath the workpiece set up in order to measure peak temperature during the welding process.Fig.1 illustrates the schematic location of thermocouple during the welding process.An HSS cylindrical pin tool was employed for the welding processes.The tool has a shoulder diameter,pin diameter,and a pin length of 10 mm,4 mm and 2.4 mm respectively while the plunge rate of the tool and dwell time were set as 2 mm/min and 5 s for the entire experiments(see Fig.2).The tool rotational speed(630-1400 rpm)and the shoulder plunge depth(0.1-0.5 mm)were varied during the two welding processes.The selected process parameter ranges were based on the outcomes of the preliminary experimental runs.

Fig.1.Location of thermocouple during the process.

Fig.2.UFSSW process with tool profile.

The cross-sections of the joints were produced,ground,polished(with a diamond paste)and etched(with Keller’s reagent-2 ml(HF)+3 ml(HCL)+20 ml(HNO3)+175 ml(H2O))according to the standard metallographic procedures.The specimens were examined via the use of an optical microscope(Olympus CK40)and a scanning electron microscope(VEGA TESCAN)equipped with an energy dispersive X-ray(EDX).Fig.3 shows the pictorial image of the as-welded sample that was used for the tensile test according to AWS C1.1:2007 standard.The tensile samples were subjected to tensile loading on a computer-controlled tensile machine(INSTRON 5500R)at a constant cross-head speed of 2 mm/min.

Fig.3.Tensile shear sample used for the tensile test(according to AWS C1.1:2007 standard).

3.Results and discussion

3.1.Macrostructure and peak temperature

Fig.4 shows the top surfaces of the AA2024-T3/AA7075-T6 joints obtained under different welding conditions and process parameters.The morphological appearance of the joints produced by FSSW and UFSSW processes(see Fig.5)appears to be the same as circumferential expelled flashes are found around the weld nuggets irrespective of the level of the tool rotational speed.The volumetric amount of the expelled flash increases(at the peripheral weld nugget)as the tool rotational speed is increased from 630 rpm to 1400 rpm in both weld categories.This finding corroborates the works of Ojo and Taban[16],and Heydari et al.[17].Oladimeji et al.[18]revealed that high frictionally induced heat input increased flowability and reduced viscosity of the plasticized materials(at high tool rotational speed).This phenomenon enforces the expulsion of plasticized materials around the tool per unit time.Fig.5 shows the magnified view of the FSSW and UFSSW joints.Fig.5 reveals that the surface appearance of the underwater welded joints appears darker as compared to that of the joints produced under ambient conditions.This observation was also reported in the works of Rouzbehani et al.[6].The darker appearance in the underwater welds was attributed to finer grains(within the weld nugget),and temperature-induced changes(by the cooling medium)in the UFSSW joints.Water was reported to have a higher heat absorption capacity than the atmospheric air.This occurrence causes a significant differential thermal cycle in the underwater welds as compared to that of the joint fabricated under ambient conditions.This consequently facilitates the formation of darker weld nugget appearance in the UFSSW joints.

Fig.4.Top surface appearance of AA2024-T3/AA7075-T6 joints,(a),(b),(c)FSSW,(d),(e)and(f)UFSSW at various tool rotational speed(630 rpm,1000 rpm and 1400 rpm).

Fig.5.Surface appearance of AA2024-T3/AA7075-T6 joints produced by(a)FSSW(b)UFSSW(bright surface appearance)process.

Fig.6 shows the back surface appearances of the AA2024-T3/AA7075-T6 joints.These regions are the AA7075-T6 Al alloy(back)sides that are in contact with the backplate during the friction stir spot welding process.Clear circular imprints are observed at the back of the AA7075-T6 Al alloy side(see Fig.6).The circular impression takes the profile of the tool shoulder and it is positioned directly opposite/underneath the impinged tool shoulder surface.The width/diameter of the back surface(circular)impression increases with the level of the tool rotational speed.The width of the circular impression increases in both the ambient welded joints(from 7.89 mm to 10.49 mm)and the underwater welded joints(from 7.5 mm to 9.57 mm)as the tool rotational speed increased from 630 rpm to 1400 rpm.This occurrence indicates that the amount of frictionally induced heat input directly influences the size(width)of the bottom-plate circular impression(as a direct relationship exists between weld heat input and tool rotational speed).However,the width of the circular impression in the underwater welds is lesser than that of the ambient welded joints at all levels of tool rotational speed.This occurrence is owing to the water cooling effect on the underwater welds.Some fraction of the frictional induced heat input(during the FSSW process)is lost to the water environment and this is adjudged to have prevented the direct transfer of the total frictionally induced thermal energy to the area underneath the shoulder surface or the bottom AA7075-T6 Al plate during the joining process.Subsequently,the width of the thermally aided circular impression at the AA7075-T6 Al plate is reduced as compared to that of the joint obtained under the ambient condition.

The coloration of the back circular imprinted section/surface of the AA7075-T6 Al alloy(after underwater welding)does not significantly change when compared to that of the base metal(see Fig.6(d)-6(f)).Dark coloration is observed at the circular imprinted section of the ambient FSSW welded joints(Fig.6(a)-6(c)).In another perspective,an increase in tool rotational speed(or heat input)increases the flowability and intermingling of materials during the FSSW process.Under this condition,the bulk penetrative flow of plasticized materials underneath the tool shoulder surface(during shoulder plunging action)is adjudged to have enforced a compressive effect on the lower plate(AA7075-T6 Al alloy side).The synergy of compressive material flow effect(as a result of the tool plunging effect)and thermal input underneath the tool shoulder surface is considered to have produced the circular impression at the back of the bottom plates in both FSSW and UFSSW welded joints.

Fig.6.Back surface appearance of AA2024-T3/AA7075-T6 joints,(a),(b),(c)FSSW,(d),(e)and(f)UFSSW at various tool rotational speed(630 rpm,1000 rpm and 1400 rpm).

Figs.7 and 8 show the cross-sections of the FSSW and UFSSW joints.The cylindrical pin of the tool leaves pin-holes in all weld categories.The whitish(light)region in the joints(close to the pinholes and the areas beneath the shoulder)represents a region that has been subjected to the dominant thermal input and material flow.The shoulder under-surface area of the tool has been reported to generate more than 70% of the frictionally induced heat input during the FSSW process[19,20].Based on this understanding,the material close to the tool shoulder surface is exposed to intense/higher heat input,shear,and compressive deformation.This phenomenon facilitates severe plastic deformation,dynamic recrystallization and increases the width of the stir zone(SZ)around the shoulder area.The SZ width(nugget feature)decreases from the tool shoulder-material contact area towards the vortex region(pin tip)in Figs.7 and 8.This occurrence is attributed to the decline in the frictionally induced heat input and the volumetric material flow from the tool shoulder area to the pin tip.As a result,the largest SZ width is observed around the tool shoulder and this region of the SZ is taken as the SZ width for all joints in this paper.The assessment of Figs.5 and 6 shows that the SZ width of the joint increases with an increase in both the levels of plunge depth and tool rotational speed.Narrower SZ width(whitish/light region)is noticeable in the UFSSW joints as compared to that of the FSSW joints at different parameter combinations.This observation is attributed to the water cooling effect of the joints.The SZ width of the joints is thus measured(as indicated in Fig.7)in order to numerically clarify the effect of water cooling and different parameter combination on the SZ width of the AA2024-T3/AA7075-T6 joints.

Fig.7.Macrostructure of the AA2024-T3/AA7075-T6 joints fabricated by the FSSW process.

Fig.8.Macrostructure of the AA2024-T3/AA7075-T6 joints fabricated by the UFSSW process.

Fig.9 shows the correlation between the SZ width(nugget feature),the welding condition(FSSW and UFSSW),and the welding parameter levels observed in the AA2024-T3/AA7075-T6 joints.At a constant plunge depth of 0.5 mm,an increase in the tool rotational speed(from 630 rpm to 1400 rpm)causes a rise in the SZ width of the FSSW joints(from 4.85 mm to 7.1 mm)and the UFSSW joints(from 1.5 mm to 4.2 mm).This occurrence is attributed to the increase in the induced heat input and flowability(reduced viscosity).Similarly,the SZ width of the ambient welded AA2024-T3/AA7075-T6 joints increases from 2.85 mm to 4.85 mm(at 630 rpm),1.6 mm-6.3 mm(1000 rpm)and 6.2 mm-7.1 mm(at 1400 rpm)as the plunge depth is increased from 0.1 mm to 0.5 mm.This occurrence is also observed in the underwater joints as the(SZ width)increment of 1-1.5 mm(at 630 rpm)and 4.2 mm-4.8 mm(at 1400 rpm)was obtained.This confirms that the water-cooling environment narrows the SZ width of the AA2024-T3/AA7075-T6 joints.

Fig.9.Correlation between SZ width and welding parameters of AA2024-T3/AA7075-T6 joints.

Fig.10 reveals the effect of an increase in the tool rotational speed on the peak temperature of the friction stir spot welded AA2024-T3/AA7075-T6 Al joints.The peak temperatures of 390,425°C and 478°C were obtained in the FSSW joints as the tool rotational speed was increased from 630 rpm to 1400 rpm.This implies that the increase in the tool rotational speed generates an equivalent increase in the frictionally induced heat input,materials stirring and flow within the weld nugget of the joint.This phenomenon causes a rise in both the overall heat input and the peak temperature of the AA2024-T3/AA7075-T6 Al joints.The result of the comparison between the peak temperatures of the UFSSW and FSSW joints is shown in Fig.11.The peak temperatures of 300°C and 425°C were obtained in the UFSSW and FSSW joints respectively at 1000 rpm.The water environment acts as a heat sink during the welding process and causes a decrease in the peak temperature of the underwater(SZ)weld by about 29% with respect to the ambient welded joint.Sabari et al.[21]reported that the heat absorption attribute of water induces a higher temperature gradient in the transversal and longitudinal axes of the underwater joint.Zhang et al.[11]reported that heat dissipation(from water)is enhanced via the occurrence of non-static annular flow around the welding tool.

Fig.10.Peak temperature attained at different tool rotational speed and at a 0.5 mm shoulder penetration depth.

Fig.11.Peak temperature attained at 0.5 mm shoulder penetration depth.

Figs.12 and 13 show the close-up images of the stir zone(SZ)of the FSSW and the UFSSW joints respectively.The appearance of the magnified SZ of the UFSSW joint(see Fig.13)is darker than that of the FSSW joint(see Fig.12).In addition,the magnified sections of the SZ show that the intermingling of the base materials(during the welding process)leads to the formation of an intercalated structure(see the whitish bands in Figs.12 and 13).Narrower whitish bands are predominant in the SZ of the underwater welded joint as compared with the ambient welded joint.This implies that grain coarsening is impeded in the UFSSW with respect to the FSSW owing to the reduced thermal input and peak temperature.The assessment of the faying region between the upper and the lower plates(Figs.12 and 13)shows that flow-induced bonding occurs at the regions close to the probe-hole(pin peripheral region),and away from the bonded region,palpable contact line is observed in both welds.This is an indication that the entire faying region beneath the tool shoulder was not completely bonded.

Fig.12.Closed-up assessment of weld nugget in the FSSW joint(1000 rpm,0.1 mm).

Apart from the differential SZ width effect,the water-cooling medium also significantly controls the angular variation between the plasticized AA2024-T3 and AA7075-T6 alloys(within the weld nugget)as revealed in Fig.14.The inherent angular variation within the weld nugget is attributed to the materials flow phenomenon which is dependent on the induced heat input.The flow-induced nugget angle(indicated in Fig.14)is higher in FSSW joints(see Fig.14(c))as compared to that of the UFSSW joint(see Fig.14(b)).The plunging effect of the welding tool into the work-piece setup is adjudged to have enforced the upward flow of the lower plate materials into the stir region of the upper plate during the welding process.The upward(plasticized)flow develops into such an angular variation between the AA2024-T3 and AA7075-T6 alloys(see Fig.14)and grows into forming hook profiles within the weld nugget due to higher heat input.Thus,lesser angular variation(or hook profile)is formed in UFSSW when compared with that of the ambient FSSW joint owing to the reduced viscosity or flowability that ensues in the UFSSW(as a result of the water-cooling effect).Thermal dissipation into the water environment reduces the inherent frictionally induced heat input,and this consequently reduces the materials flowability and the formation of a highlysteep hook profile(intense hook angle)during the underwater friction stir spot welding of the AA2024-T3/AA7075-T6 Al alloys.

Fig.13.Closed-up assessment of weld nugget in the UFSSW joint(1400 rpm,0.1 mm).

Fig.14.The flow-induced angular variation between the AA2024-T3 and AA7075-T6 alloys in(a),(b)UFSSW joints and(c)FSSW joint.

3.2.Microstructure

Fig.15 shows the EBSD images of the stir zone(SZ)of the welded AA2024-T3/AA7075-T6 Al alloys in ambient and underwater environments.Severe plastic deformation(shearing action of the tool)and dynamic recrystallization takes place at the SZ of the join.However,finer grains are present in the UFSSW joint when compared to that of the FSSW.Fig.16 illustrates the variation of the average grain sizes of the weld SZ(s)at different process parameters under ambient and underwater environments.An increase in both the tool rotational speed and the plunge depth causes a rise in the average grain sizes of the SZ.The increase in tool rotational speed(from 600 rpm to 1400 rpm)at a constant plunge depth(0.1 mm)increases the average grain sizes of the underwater welds from 2.6μm to 5.2μm and that of the ambient joints from 3.3μm to 7.8μm.Similarly,the increase in plunge depth(from 0.1 to 0.5 mm)causes a growth in the recrystallized grains of the SZ from 2.6μm to 3.3μm(in UFSSW joints)and 3.3μm-5.0μm(in FSSW joints).The reason for this occurrence is associated with the impact of the frictional and deformational heat input on grain coarsening.An increase in the tool plunge depth is expected to produce a more deformational thermal effect on the SZ of the joint with respect to the joint produced at a lower depth level.This occurrence is adjudged to have promoted the formation of coarsened grain sizes in the joint obtained at a higher plunge depth irrespective of the welding environment.The water-cooling environment reduces the amount of heat required to cause severe grain growth in the UFSSW and as a result,finer grains are obtained in UFSSW joints as compared to that of the FSSW.In the same perspective,there is a rise in the frictionally induced heat input at a high level of tool rotational speed(in the joint).This incidence facilitates grain growth in the SZ of the AA2024-T3/AA7075-T6 Al joints irrespective of the welding environment because the water environment only takes away a fraction of the total induced heat input in the weld during the joining process.This finding corroborates the works of Tan et al.[7]as it was reported that higher heat input was produced in the joint fabricated in the normal ambient condition as compared to that of the underwater welded joint.The inherent lower recrystallization temperature and high dislocation density(in the underwater weld)were stated to be responsible for the finer(recrystallized)grains in the underwater weld nugget of AA3003 Al alloy.

Fig.17 reveals the elemental distribution in the nugget zone of the friction stir spot welded AA2024-T3/AA7075-T6 joint.The EDS mapping of the SZ shows a dense agglomeration of Al,Mg,Cu,Zn,and Mn which is a confirmation of refined grains at the SZ.The analysis of the salient features on the obtained FSSW and UFSSW joints is carried out in order to identify the possible phases formed in the AA2024-T3/AA7075-T6 Al alloys under different welding environments.Fig.18 shows that Mg2Si(black dots)and Al2CuMg(white dots)phases are precipitated in the ambient(FSSW)and underwater(UFSSW)welded AA2024-T3/AA7075-T6 joints respectively.The formation of these phases is obviously due to the disparity in the heat input or peak temperature in the respective joints.However,the presence of such white dots(in Fig.18(b)and the appendix)is reported in the works of Zhao et al.(2014).This precipitate is stated to hinder dislocation movement in the underwater friction stir welded AA7055 Al alloy.On the other hand,Paidar et al.[22]had recently worked on modified(ambient)friction stir clinching of AA2024-T3/AA7075-T6 Al alloy.It was reported that atomic diffusion ensued during the double-sided intermaterial mixing of the weld nugget and phases of Al2CuMg,Mg2Si and MgZn2were subsequently formed in the stir zones of the joint.A comparison of the observed phases(in the present work)with that of Paidar et al.[22]indicates that the underwater cooling condition affects the type of precipitate formed in AA2024-T3/AA7075-T6 Al alloy.

Fig.15.EBSD images of SZ obtained at 1000 rpm rotational speed and 0.1 mm plunge depth,(a)FSSW and(b)UFSSW.

Fig.16.The interrelationship between average grain size and welding parameters.

3.3.Tensile shear force

Figs.19 and 20 show the interrelationship between the welding parameters and the tensile-shear force of the AA2024-T3/AA7075-T6 joints obtained under ambient and underwater environments respectively.The tensile-shear force of the underwater welds is higher than that of the ambient welded joints,Fig.A2.The maximum tensile-shear forces of 5900 N and 6700 N were obtained in the ambient and the underwater welds respectively at 1400 rpm tool rotational speed and 0.5 mm penetration depth(see the appendix for the scatter plot).Figs.19 and 20 also indicate that an increase in tool rotational speed(600 rpm-1400 rpm)and plunge depth(0.1 mm-0.5 mm)increases the tensile-shear force of the welded AA2024-T3/AA7075-T6 joints in both ambient and underwater environments.This occurrence may be attributed to the increase in the SZ width,bonded length at the faying region and bulk compressive effect(owing to the increased plunging depth).The improvement in the tensile-shear force of the underwater weld is also attributed to the reduced average grain sizes found in its weld nugget as compared to that of the ambient weld(see Fig.16).The finer grains in the underwater welds imply an increase in the number of grain boundaries.The grain boundaries are adjudged to have acted as strong obstacles to the transmission/propagation of slip from a crystal to its neighboring ones during axial loading(deformation)of the weld nugget.Paidar et al.[23,24]reported that fine grain sizes enforce less dislocation pile-up or require a greater amount of(axial loaded)stress/force in order to push dislocation to the neighboring grains(to cause deformation).This phenomenon is adjudged to have improved the tensile shear failure load of the underwater joints as compared to that of the ambient welded joint.In addition,the presence of a misorientation angle across low-angle grain boundaries is also reported by Tan et al.[7]to be responsible for the improved tensile strength of underwater welds.A decrease in the volume fraction of secondary phases within the weld nugget is obtained in the underwater welds[7].The presence of white dots(Al2CuMg)in the underwater welds(see Fig.18)is adjudged to have further strengthened the grain boundaries of the underwater joint by inhibiting dislocation movement[25].The emergence of this phenomenon in the underwater welds could be strongly responsible for the improved tensile shear failure load of the underwater AA2024-T3/AA7075-T6 Al alloy joints.

On the other hand,Ojo et al.[16]reported that the presence of hook in the FSSW joint divides the stir zone(at the interfacial region)into an effective bonded section(active metallurgical bonding of the upper and lower plates)and an inactive zone.The increase in hook angle(growth of hook profile into the stir zone)reduces the size of the effective bonded section.This attribute lowers the loadbearing capacity of welds.This phenomenon is adjudged to have caused the lower tensile shear force in the ambient welded joints as compared to the underwater welds(with smaller flow-induced angle).

According to Tan et al.[7],the decrease in the tensile properties of AA3003 Al(fabricated in air)was attributed to a decrease in the initial dislocation density of the weld nugget while the presence of inherent grain refinement and a high density of dislocation was reported to be responsible for the improved tensile properties of the underwater joints.Similarly,Sabari et al.[21]reported that the heat conduction and dissipation during the underwater welding process reduce the width of the thermomechanical affected zone(TMAZ)and the heat-affected zone(HAZ).The marginal increase in the tensile strength of the underwater welds was attributed to the reduction in the over-aging effect of the weld nugget,narrow HAZ and reduced width of the weaker(stress-concentration)zone in the works of Sabari et al.[21].This occurrence is considered to have contributed to the improved tensile-shear force of the AA2024-T3/AA7075-T6 Al alloy joint.

Fig.17.EDS mapping of the stir zone of AA2024-T3/AA7075-T6 joint(a)selected SZ,(b)mapped SZ showing all elemental compositions,(c-g)individual element mappings.

Fig.18.Phases at the stir zone of AA2024-T3/AA7075-T6 joints produced by,(a)FSSW and(b)UFSSW process.

3.4.Fracture location and fractography

Fig.21 illustrates the various fracture morphologies of the welded AA2024-T3/AA7075-T6 joints subjected to axial loading.Although the fracture morphologies of the welded AA2024-T3/AA7075-T6 joints are similar,the stir zone and the nature of the hook profiles are the salient features that govern the nature of the joint failure.Hook profile is a major stress concentration factor of the FSSW joints and a higher flow-induced angle(hook)lowers the load-bearing capacity of the joints.Crack propagation occurs through the hook path into the effective bonded section of the joint prior to the final failure(crack growth into the keyhole)[16].The lower hook profile supports fracture resistance in underwater welds as compared to the FSSW joint.The synergy of tool plunging and rotational effects influences the SZ width and the hook angle in the weld nugget of the AA2024-T3/AA7075-T6 joint.Increased SZ width is obtained in the ambient welded joint while a lower angular difference or hook angle is obtained in underwater welds.Fig.21(a)illustrates the type of failure that ensues as a result of a high hook height and angle in the FSSW joint.While Fig.21(b)shows the failure that occurs in the joint with intermediate and low hook angle respectively.Indeed,Fig.21(a)shows that crack initiation ensues around the interfacial region(partially bonded)of the AA2024-T3/AA7075-T6 joint.The crack grows along the hook path(see the blue dotted lines in Fig.21)into the tip of the hook profile and propagates into the stir zone(see the region enclosed with red dotted lines in Fig.21)before the final failure at the stir zone close to the peripheral region of the probe-hole.The refined grain structure of the SZ is adjudged to have offered some resistance to crack propagation during the axial loading of the joint.The finer grains have more grain boundaries(at the SZ)and they are expected to impede dislocation movement in the underwater joints.Whereas at a higher penetration depth of shoulder,the crack first initiated at the interface of the upper and lower sheets,then grows into the upper sheet thickness beneath shoulder indentation,and finally,the fracture occurred in the shoulder indentation.Similar fracture modes under the tensile shear loading were observed by Fujimoto et al.[26,27].

Fig.19.Interrelationship between welding parameters and tensile-shear force of ambient welded AA2024-T3/AA7075-T6 joints(FSSW)(a)contour plot,(b)surface plot.

Fig.20.The interrelationship between welding parameters and tensile-shear force of underwater welded AA2024-T3/AA7075-T6 joints(UFSSW)(a)contour plot,(b)surface plot.

Fig.21.Fracture morphologies of the AA2024-T3/AA7075-T6 joints at various shoulder penetration depths,(a)0.1 mm,(b)0.5 mm.

Figs.22 and 23 show typical fracture surfaces of the AA2024-T3/AA7075-T6 joint obtained under ambient and underwater conditions respectively.It is evidence that plasticization is apparently intense in Fig.22 as compared to that of Fig.23 due to higher heat input in the ambient weld.Crack propagates from the intermediate region between the AA2024-T3 and AA7075-T6 Al alloys in both cases(see the red arrows in Figs.22 and 23).However,the high heat input(of the ambient weld)implies that the HAZ will be larger in Fig.22.This occurrence causes the crack to propagate downwards(where coarsened grains are present)before the final failure around the outside region of the vortex zone.In addition,different grain orientations at the interface between the weld zones(SZ/TMAZ/HAZ)is adjudged to cause inhomogeneous deformation resistance during axial loading[4]in order to cause the observed fracture morphology in Fig.22.Hui-Jie et al.[4]reported that HAZ was the inherent weakest zone(fracture location)of the ambient welded joint while the interfacial region between the weld nugget and the TMAZ was the fracture location of the underwater joint.Fig.23 shows that crack propagates from the interfacial region(between the upper and the lower plates)into the weld zone.The fracture surfaces of the failed joints are examined in order to further under the role of water cooling effect on the joints.Figs.24 and 25 show the SEM images of the failed AA2024-T3/AA7075-T6 joints under axial loading condition.The nature of dimples on the fracture surfaces is expected to vary with the welding environments(ambient and underwater(0°C)conditions).More shallow dimples are present on the fracture surfaces of the underwater welds(Fig.24)as compared to that of the ambient welded joint(Fig.25).This implies that an improved ductility ensues in the UFSSW joint.This observation corroborates the works of Tan et al.(2017)that a decrease in ambient temperature increases the ductility of friction stir welded joints of AA3003 Al alloy.The orientation of the fine dimples in Fig.24 is adjudged to be towards the axial loading direction(Sabari et al.,2016).The underwater joints failed via ductile fracture mode while mixed fracture mode ensues in the FSSW joints.

Fig.22.Fracture mode of AA2024-T3/AA7075-T6 joint obtained under ambient condition(a)cross-section of joint,(b)upper sheet surface,(c)lower sheet surface.

Fig.23.Fracture mode of AA2024-T3/AA7075-T6 joint obtained in the underwater condition(a)cross-section of joint,(b)upper sheet surface,(c)lower sheet surface.

Fig.24.SEM images of the failed UFSSW joint(a)7075-T6 Al alloy side,(b)2024-T3 Al alloy side.

Fig.25.SEM images of the failed FSSW joint(a)7075-T6 Al alloy side,(b)2024-T3 Al alloy side.

4.Conclusions

The underwater and ambient friction stir spot welding of AA2024-T3/AA7075-T6 Al alloys have been successfully carried out.Nugget features,microstructure,mechanical and fracture behaviors of the underwater welds were compared with that of the ambient welded joints.The findings of the work are summarized as follow:

1.The water environment causes about a 29%decrease in the peak temperature of the AA2024-T3/AA7075-T6 welded joint with respect to the ambient welded joint.

2.Water cooling reduces the stir zone width,the width of circular impression(at the back of bottom plate),and the angular variation(hooking)between the plasticized AA2024-T3 and AA7075-T6 alloys owing to reduced viscosity and flowability.

3.An increase in the tool rotational speed(6300 rpm-1400 rpm)increases the SZ width of the underwater(1.5 mm-4.2 mm)and the ambient(4.85 mm-7.1 mm)welded joints.A rise in the tool rotational speed from 630 rpm to 1400 rpm increases the width of the circular impression in the underwater(7.5 mm-9.57 mm)and the ambient(7.89 mm-10.49 mm)welded joints.

4.Finer grains are formed in the underwater welds.The increase in plunge depth(0.1 mm-0.5 mm)causes a growth in the recrystallized grains of the SZ from 2.6 to 3.3μm in UFSSW joints and 3.3μm-5.0μm in the FSSW joints.The increase in tool rotational speed(600 rpm-1400 rpm)at a constant plunge depth(0.1 mm)increases the average grain sizes of the underwater welds from 2.6μm to 5.2μm and that of the ambient joints from 3.3μm to 7.8μm.

5.Improved tensile-shear failure loads are obtained in the underwater welds due to the induced finer grain sizes and strengthening Al2CuMg precipitate in the weld.An increase in tool rotational speed(600 rpm-1400 rpm)and plunge depth(0.1 mm-0.5 mm)increases the tensile-shear force of the welded AA2024-T3/AA7075-T6 joints in both ambient and underwater environments.The maximum tensile-shear force of 5900 N and 6700 N were obtained in the ambient and underwater welds respectively.

Acknowledgment

Scientific Research Fund of Hunan Provincial Education Department(No.15C1240)and Innovation platform open fund Project(No.16K080).

Appendix

Fig.A1.EDS mapping of the key feature in the underwater welded AA2024-T3/AA7075-T6 joint showing the possible Al2CuMg phase.

Fig.A2.The tensile-shear force of underwater and ambient welded AA2024-T3/AA7075-T6 Al joints(scatter plot).