Improving tensile-shear properties of friction stir lap welded dissimilar Al/Mg joints by eliminating hook defect and controlling interfacial reaction

Yuqing MAO, Ping YANG, Wenyan ZHANG, Ning LI, Hao NIE,Danyang LIN, Liming KE

National Defence Key Discipline Laboratory of Light Alloy Processing Science and Technology, Nanchang Hangkong University,Nanchang 330063, China

KEYWORDS Dissimilar Al/Mg alloys;Friction stir lap welding;Hook defect;Interface microstructure;Tensile-shear properties

Abstract To improve tensile-shear properties of fiction stir lap welded (FSLW) dissimilar Al/Mg joints, pin-tip profiles were innovatively designed and welding speed was optimized, and effects of them on formation, interface microstructure and mechanical properties of different FSLW joints were investigated.With increasing the welding speed, the tensile-shear load of FSLW joints produced by three pins presents an increasing firstly and then decreasing trend.Compared with Rpin, the hook and hole defect in the joints made by S-pin and T-pin are eliminated owing to additional eccentric force.Moreover, the joints obtained by T-pin at 75 mm/min have the highest tensile-shear load, and a maximum value of 3.425 kN is produced, which increases by 96.8%.Meanwhile, the pin-tip profile improves significantly the interface reaction depending on the welding temperature.For R-pin, thick brittle intermetallic compounds of about 6.9 μm Al3Mg2 and 13.3 μm Al12Mg17 layers at the welding interface derived from diffusion reaction are formed,resulting in continuous cracks.However, using T-pin can raise the interface temperature, and which makes the interface liquefy locally to generate only 2.2 μm Al3Mg2 layer and dispersive (Al12-Mg17+Mg)eutectic structure.This can release high residual stress and remove welding crack,consequently enhancing the interface properties of T-pin joints.

1.Introduction

Structural lightweight has an incessant demand in the transportation and aerospace industries.Accordingly, the composite application of Al/Mg alloy, making the structure design more flexible and giving better play to the performance of each material,has aroused wild attention in these fields.However,it is difficult to obtain dissimilar Al/Mg alloys composite structure with higher strength by welding due to the huge differences between aluminum (fcc) and magnesium (hpc) in physical and chemical properties.1–3Correspondingly, various defects including tunnel,pores,element loss and large amounts of brittle intermetallic compounds (IMCs) are easy to form in the joint during fusion welding.But, friction stir welding(FSW)with low heat input can reduce adverse effect on undesirable metallurgical reaction in the fusion welding for almost every dissimilar pair of alloys,4–7which is regarded as one of the most suitable welding methods for Al/Mg alloys.

For friction stir lap welded(FSLW)dissimilar metals,it is a huge challenge to eliminate hook defect and reduce brittle IMCs in the joint in order to improve the strength of the joint,and many appropriate measures should be taken to control the formation of defects.For some academics, changing process parameter is one of ways to improve the joint formation,including welding speed and rotating speed.He et al.8selected different rotating speeds (700 r/min ～1500 r/min) and placement positions of base material for FSLW Al/Mg alloys, and observed obvious hook defects and IMCs at the lap interface.Kwon et al.9reported that there were not noteworthy changes in the tensile strength as a function of the tool rotation speed.Lv et al.10found that the type of new formed IMCs was not sensitive to the welding parameters,but the thickness and morphology were affected.Therefore, the welding parameter has little influence exerted in the thermodynamics of interface reaction, and it is unlikely to radically alter the interface structure of FSLW Al/Mg joints.

Tool profile significantly affects the interface morphology,and which can play an important role in the FSW process.Relevant studies about the influence of pin profile on microstructure evolution and mechanical properties of FSW joint are reported.Elangovan et al.11–12tested various pin profiles(cylindrical, tapered, threaded, triangular and square) and rotation speed, the results showed that regardless of the rotating speed, the square pin profiles created sound joints, and which had better tensile properties at 1600 r/min.Similarly,Ravindra and Surjya13studied effects of pin profile and welding parameters on FSW AA5083 joints,and confirmed that the joints obtained by a cylindrical pin had the maximum strength with a rotational velocity of 1400 r/min.Moreover, Chen et al.14and Xu et al.15found that pin profile had an active affect on the plastic material flow, and the weld possessing the best quality was obtained by thread tapered pin.But,Rao et al.16noticed that the triangular pin profile provided smaller grains compared to that of the comical pin.According to the above-mentioned results,it is proved that the pin profile can change thermal cycle and force filed during FSW process,resulting in affecting plastic flow, microstructure and mechanical performances of FSW joints.However,there is not a consistent agreement with effect mechanism of the pin profile depending on specific process parameters to achieve better joint.Meanwhile, for FSLW dissimilar alloys, the strength of the joint is mainly determined by the welding interface structure.Although our previous studies have found that the pintip profile can improve local metal flow of thick FSW joint17,18,it is rarely reported the effect of pin-tip profile on the welding interface of FSLW dissimilar metals joints.

Therefore,Al/Mg alloys sheets are subjected to lap welding in the present study,and the aim is to improve the tensile-shear properties of FSLW dissimilar Al/Mg joints by designing innovatively pin-tip profiles and optimizing welding speed.In details, effects of welding speed and pin-tip profile on macrostructure, interface characteristic, microstructure evolution and mechanical behaviors of the joints are further investigated.

2.Experimental details

5A06 Al alloy and AZ80 Mg alloy plates,with the dimensions of 120 mm (length) × 100 mm (width) × 5 mm (thickness),were used as base material for welding.Before welding test,the oxide layer of the plates was cleaned by mechanical polishing.Al alloy plate was placed on the upper while Mg alloy plate was on the lower.The experiments were performed on the X53K vertical FSW machine, and the process parameters were selected at rotational speed of 1180 r/min, traverse speed from 23.5 mm/min to 118 mm/min, tilt angle of 2° and pin plunge depth of 0.2 mm.Schematic diagram of FSLW process of dissimilar Al/Mg alloys is shown in Fig.1(a).Three tools with different pin-tip profiles were adopted for welding,including round pin-tip(R-pin),square pin-tip(S-pin)and triangular pin-tip (T-pin), as shown in Fig.1(b).The tool shoulder was concave with a diameter of 21 mm, and the length and diameter of tool pin were 4.6 mm and 7 mm, respectively.Also,the length of special pin-tip profile was 1.2 mm.The measurement position (point A in Fig.1(a)) of welding thermal cycle was located at the bottom of Al plate close the welding interface.

Fig.1 Schematic diagram of welding process and FSLW tool.

The metallographic specimens and tensile-shear specimens were cut along the cross-sectional perpendicular to the welding direction.The metallographic specimens were mechanically ground and polished.The Keller’s reagent (2 mL HF, 5 mL HNO3, 3 ml HCl and 190 mL distilled water) was used to reveal the microstructure in 5A06 Al alloy side.Whereas the etchant in AZ80 Mg alloy side consisted of 5 ml acetic acid,4.2 g picric acid, 10 mL distilled water and 100 ml ethanol.The microstructure and element distribution of IMCs were identified by SEM-EDS (SU1510), XRD (D8ADVANCEA25)and EMPA(JXA-8230).The tensile-shear tests of FSLW joints were conducted on a universal testing machine (WDW-50) with a speed of 0.5 mm/min, and the dimensions of the tensile-shear specimens were 160 mm (length) × 10 mm(width).The SEM technology was used to observe and analyze the fracture location and fracture surface of the weld samples.

3.Results

3.1.Tensile-shear properties

Fig.2 show average tensile-shear load of FSLW joints produced by R-pin,S-pin and T-pin under various welding speeds of 23.5 mm/min to 118 mm/min.It is clearly found that, with increasing the welding speed from 23.5 mm/min to 118 mm/min, the average value of the tensile-shear load of the joints obtained by all three pins present an increasing firstly and then decreasing trend when the rotation speed is kept at 1180 r/min.Moreover, at the invariable welding speed, the tensile-shear loads of the joints obtained by S-pin and T-pin are obviouslyhigher than that of R-pin joints, and which indicates that pin tip-profile has an active effect on tensile-shear properties of FSWL joints.It is because that the pin profile can significantly influence material flow behavior, plastic deformation and welding temperature during the FSW process.19–20Compared with other FSLW joints, the tensile-shear properties the joint fabricated by T-pin is the best at a welding speed of 75 mm/min, and the maximum tensile-shear load is up to 3.425 kN.

Fig.2 Average tensile-shear load of FSLW joints produced by different welding speeds and pins.

3.2.Macrostructure

Fig.3 shows cross-sectional macrostructures of FSLW joints produced by three pins with different welding speeds.Seen from Fig.3, it is clearly found that there are apparent hook defects at the lap interface of R-pin joints.In detail, some Rpin joints are well formed and have no obvious other welding defects when the welding speeds of 23.5 mm/min and 47.5 mm/min are used.However,the hole defect starts to generate in the middle-upper part of the stirring zone (SZ) with continuously increasing the welding speed from 75 mm/min to 118 mm/min.For S-pin and T-pin joints,the hook at the lap interface disappears and there exists no obvious hole defect, and there are only some differences in the macrostructure of the SZ for various joints.

Generally,using a low welding speed,more welding input is generated by rotating tool and the degree of plastic deformation of weld material is greater in the SZ.With gradually increasing welding speed, the welding input decreases and the plastic flow of weld metal along the vertical direction is insufficient, resulting in enlarging the area of the hole defect and reducing effective lap width of the stirring zone.

Fig.4 shows cross sections of various joints welded by three pins with a welding speed of 75 mm/min.For R-pin joint,it is seen from Fig.4(a) that Mg alloy metal is extruded by weld material and then migrates upward from the lower part of the lap interface in the thermo-mechanical affected zone, and which makes the hook defect and cold lap interface are formed.Meanwhile, obvious hole defect is observed in the upper of the SZ,and which may be prone to crack propagation under an external load.However,there is a huge change of the macroscopic feature for S-pin and T-pin joints, as shown in Fig.4(b)-4(c).A bread-shaped SZ is formed in the Al side,and a transition zone is created and connected with Mg side.In addition, the hook defect at the interface and the hole region are eliminated.Compared to S-pin and T-pin joints,the shape and structure of the transition zone layer under the SZ are significantly various.Therefore,it can be concluded that the pin-tip profile has a significant influence on driving force and temperature field of weld metal during FSLW process, and which can alter plastic flow and metallurgical reaction at the interface for different joints.

3.3.Interfacial microstructure

Fig.3 Cross-sectional macrostructures of FSLW joints fabricated produced by different welding speeds and pins.

Fig.4 Cross sections of FSLW joints welded by different pins at 75 mm/min.

Fig.5 Microstructure and EDS result at interface of R-pin joint.

Fig.5(a)-5(d)show SEM and EPMA images of microstructure at Al/Mg lap interface of R-pin joint, while Fig.5(e) shows EDS results of the interface region.In detail, seen from Fig.5(a) (SEM image of region 1 in Fig.4(a)), it is clearly observed that the hook defect on the advancing side (AS) of the joint and a running crack occurs close to Al side away from the SZ.Further,a continuous Al3Mg2phase layer identified by EDS analysis with about 3.3 μm thickness near the SZ is formed,and a large area of Al12Mg17phase in the middle area of the hook is found, as shown in Fig.5(e).According to EPMA results of Mg and Al elements distribution in Fig.5(b)-(c), there is an obvious IMCs layer on both sides of the hook, and which is indicated that constitutional liquefaction occurs in the middle region.Fig.5(d) shows SEM image of region 2 at the lap interface in Fig.4(a).In the transition zone located between the SZ and Mg alloy side,the IMCs layer is obviously divided into two layers.The lower layer close to Mg alloy side with a thickness of about 13.3 μm is composed of Al12Mg17phase characterized by EDS,while the upper layer is Al3Mg2phase with about 6.9 μm thickness.Also, a continuous crack is observed below the SZ, and which suggests a greater stress concentration exists in this area due to suffering different thermal history during welding process.

Fig.6 shows interfacial microstructure of S-pin joint corresponding to region 3 in Fig.4(b)and EDS results.It is clearly found that the interface morphology is visibly different compared with R-pin joint, and a new eutectic structure layer is formed in the transition zone except for the upper IMCs layer.Based on EDS element analysis of position 2 in Fig.6(a), the block structure is classified as the primary Al12Mg17phase.In the meantime, local region of the eutectic structure layer is magnified, as shown in Fig.6(d).The island structure of point 3 mainly consists of Al12Mg17phase, and lamellar interdendritic structure of point 4 is characterized as Mg + Al12-Mg17phase.The formation of the eutectic structure proves that the constitutional liquefaction occurs at the bottom of the interface during FSLW process,21–23so the solution structure is formed near the interface due to the eutectic reaction:L → Mg + Al12Mg17.Furthermore, observed from Fig.6(b)-6(c), the EPMA results display that the content of Mg element in the SZ is higher, and this phenomenon shows that Mg element in base material spreads into the SZ.Compared with R-pin, used S-pin can induce additional eccentric force during FSLW process, and which increases the stirring force of rotating pin.12Thus,the total welding input including frictional heat and deformational heat may be higher, and which is conducive to Mg element diffusion into the upper SZ induced from bigger driving force and higher temperature.

Fig.7 shows interfacial microstructure of T-pin joint marked in region 4 in Fig.4(c) and EDS result.Clearly, the interface structure obviously varies, as shown in Fig.7(a).The transition layer between the SZ and Mg side is mainly comprised of Al12Mg17phase, and where it is distributed in a network.What is more, the thickness of the transition layer significantly decreases to 27.9 μm.Through surface scanning of EPMA and EDS analysis,it can be found that a IMCs layer of new Al3Mg2phase with only 2.2 μm thickness is generated in Fig.7(c), and which is beneficial to improve the joining strength of the Al/Mg interface.However, a small number of discontinuous microcracks also appear on the upper of IMCs layer, possibly accelerating the crack propagation of T-pin joint.In comparison to Fig.6,the phenomenon of Mg element diffusion into the SZ disappears for T-pin interface seen from Fig.7(b)-7(c),and which implies that the driving force or temperature of interface diffusion reaction could reduce.

Fig.6 Interfacial microstructure and EDS result of S-pin joint.

Fig.7 Interfacial microstructure and EDS result of T-pin joint.

Fig.8 Stress strain curves of tensile-shear tests of FLSW joints welded by different pins.

3.4.Fracture mechanism

Fig.8 presents stress strain curves of tensile-shear tests of FLSW joints produced by three different pins.It is clearly seen that the tensile-shear properties of R-pin joints are the worst,and the minimum tensile-shear load of only 1.74 kN is obtained.As mentioned above, this may be due to bad formation such as holes, hook defects and brittle IMCs in the joint.Under the same welding parameters, the mechanical properties of S-pin and T-pin joints are obviously improved.Especially welding with T-pin, the joints perform the highest tensile-shear performances, and a maximum value of average tensile-shear load reaches 3.425 kN.For FSLW dissimilar Al/Mg alloys, the main factors influencing joint properties are hook defects and microstructure in IMCs layer.The formation of the hook will decrease effective lap height of FSLW Al/Mg joint and induce cracking derived from large stress concentration.On the other hand,when the thickness of the IMCs layer is less than 10 μm, the interface strength of FSLW joint can be significantly enhanced.

In order to analyze deeply effect of pin-tip profile on fracture behavior of FSLW joint, the fracture locations and morphologies of different joints are observed.Fig.9 shows fracture behavior of different FSLW joints after tensile-shear testing.For R-pin joint,under continuous external tensile-shear force,crack initiates from the unbonded interface and expands along the direction of hook defect on the AS.Then the holes in the SZ continue to produce cracks, and which eventually causes the joint failing.On the basis of fracture morphology of local region, it is confirmed that the failure of R-pin joint occurs in the holes area observed from regions Ι-1 and Ι-2, although there exist IMCs layers.However, the fracture paths of S-pin and T-pin joints have a huge change.On one hand, the crack initiation still starts at the unwelded Al/Mg interface,but then the crack propagates along the direction of brittle IMCs layer above the eutectic structure,and which can be clearly observed from fracture magnification of regions Π-1 and Π-2.On another hand, for T-pin joints the crack expands through IMCs layer of regions Ш-1 and Ш-2, and which indicates that the crack propagation rate is reduced, resulting in notably improving bonding strength of the interface.Moreover, it is worth noting that the interface crack expands mainly near the Al3Mg2layer.This is due to the fact that the residual stress of Al3Mg2phase is higher than that of Al12Mg17phase,reducing the cracking resistance.24

Fig.9 Fracture locations of FSLW joints fabricated by different pins.

Fig.10 shows surface morphologies and XRD results of the weld center of different fractured joints.It is seen from fracture morphology of R-pin joint that the fracture surface of R-pin joint is smooth,and obvious river patterns are observed.Also,the microstructure consists of Al3Mg2phase characterized by XRD result.However, for S-pin joint the fractured surface presents a quasi-cleavage fracture by SEM images, and only Al3Mg2phase analyzed from XRD analysis is found.Meanwhile, many tiny cracks appear on the fractured surface of T-pin joint,and Al12Mg17phase is also detected except for Al3-Mg2phase.Therefore,the results mean that the fracture mode of all joints presents a typical brittle fracture, and which is agreement with the above results of Figs.5–7.

4.Discussion

4.1.Material flow behavior

Fig.11(a)-11(c) show physical models of plastic flow of weld metal around three different pins during FSLW process.Generally,the plasticized metal derived from the combined force of the shoulder and pin will migrate downwards along the thread direction until releasing from the pin root when the pin with left thread is used,and accumulates under the pin root to form the extruded zone to move outwards.Then the extruded zone becomes bigger and bigger since more weld metals move downwards, and squeeze surrounding cold metal to move outward and upward.As a result, two hooks on the AS and RS are formed, as shown in Fig.11(a).However, For S-pin and Tpin the flow path of the plasticized metal releasing from the pin thread will change.The triangle and square flats at the pin tip can produce an additional eccentric force,25making the pin-tip increase significantly stirring action.Moreover,the eccentric force drives the plasticized metal flowing transversely around the pin root to form the dynamic path which is related to the ratio of static volume and dynamic volume of the plasticized metal in the weld, and the ratio is equal to 1 for R-pin, 1.56 for S-pin and 2.3 for T-pin.26–27On another hand,the thread path of the S-pin and T-pin is shorted due to the existence of the triangular and square flats, and which can prevent the plasticized metal from migrating downwards.Therefore, the hook at the interface of the S-pin and T-pin joints is eliminated in Fig.11(b)-11(c).

For further analyze the effect of the pin-tip profile, the microstructure of the plasticized metal in the extruded zone under pin root is characterized in Fig.11(d)-11(f).Clearly seen from Fig.11(d), the R-pin mainly squeezes the plasticized metal to move outward and upward,forming elongated structure.However, the plasticized metal releasing from the pin thread in the S-pin and T-pin lap joints is sufficiently broken up, and the massive structure is found, as shown in Fig.11(e)-11(f).In addition, the massive structure in the Tpin joint is finer than that of the S-pin joint.This can prove that a bigger stirring force derived from the pin-tip is obtained attributing to producing an extra eccentric force for the S-pin and T-pin, promoting the weld material flow horizontally around the pin tip.

Fig.10 SEM images of surface morphologies in different fracture joints and XRD results.

4.2.Interfacial reaction

As mentioned in the above results, the pin-tip profile not only changes the stirring force for the plasticized metal, and also may change the welding temperature field.For all FSLW experiments, the Al and Mg sheets are welded in lap configuration, and the pin does not penetrate into the lower sheet.This means that the IMCs layer and eutectic structure at the interface are formed by mutual diffusion reaction of Al and Mg elements at the metastable equilibrium interface, and which can be influenced obviously by welding thermal history.10In order to evaluate effect of pin-tip profile on the interfacial diffusion reaction, the welding thermal cycle curves at the interface of FSLW joints made by three various pins are measured,as shown in Fig.12.It is found that there is a significant difference in welding peak temperature.In comparison to the R-pin, the welding peak temperature at the Al/Mg interface fabricated by S-pin and T-pin increases due to the additional eccentric force, and a maximum value of 431.5 °C is obtained by S-pin.What is more,the dwelling time is about 55 s in a range of above 100°C,and the cooling rate is 6°C/s,which indicates that there is enough time for Al/Mg atoms at the lap interface to conduct metallurgical diffusion reaction to form IMCs layer.However, some differences exist between peak temperature and composition affected by different pintip profiles, and the formation of new phases at the interface is varying, as shown in Figs.5–7.

In theory,there are two sources of Al-Mg IMCs such as diffusion reaction and eutectic reaction according to Al-Mg binary phase diagram.One is diffusion-induced reaction,and Mg atom diffuses into Al atom to form the IMCs layer.28Also,its thickness can be theoretically calculated on the basis of Fick´s first law of Eq.(1):

where d is the thickness of the IMCs layer(m),D0is the diffusion constant (m2/s), Q is the activation energy (J/mol), R is the gas constant (8.314 J/(mol∙K)), T is the absolute temperature (K), t is the reaction time (s).

Fig.11 Physical models of material flow at the FSWL interface and local microstructure for different pins.

Taking an example of the mutual diffusion of Al and Mg atoms under 400 °C (673 K) temperature, the diffusion reaction time is about 2.4×105s calculated by equation(1)to form the IMCs layer of 15 μm thickness.However, observing from the welding thermal cycle in Fig.12, the time above 400°C is only about 10 s,and which states that the IMCs layer of 15 μm thickness cannot be taken place under conventional steady-state condition.In fact, the weld metal during the actual FSLW process may experience a higher strain rate in comparison of in the steady-state condition, resulting in much higher vacancy concentration.Moreover, the dislocation density after FSLW increases obviously, and which can provide much faster diffusion channels.Therefore, the practical diffusion rate of Al and Mg atoms during FSLW process is much higher than that of under the steady-state condition.

On the other hand,the eutectic-induced reaction can occur with decreasing the welding temperature,29and the IMCs are formed from the liquid, as shown the following Eqs.(2)-(3):

Fig.13 shows schematic diagram of interfacial reaction depending on the welding temperature.Based on the above result of the welding thermal cycles in Fig.12, the maximum peak temperature near the welding interface obtained by Spin is 431.3 °C (close to liquefaction temperature of 437 °C),suggesting that the diffusion reaction of Al and Mg atoms mainly takes place at the interface.Welding by common Rpin, the interfacial temperature is low, when the content of Al and Mg atoms at the welding interface exceeds the solid solubility limit during the diffusion process, a new Al12Mg17phase starts to nucleate and grow close to Mg side interface due to low Gibbs free energy,and then a continuous Al12Mg17IMC layer is formed.Meanwhile, Mg atom gradually diffuses into the Al side, and another new Al3Mg2phase begins to nucleate.As the diffusion reaction continues, an Al3Mg2IMC layer is generated by the reaction into the Al matrix enrichment area.Eventually,the Al12Mg17layer is formed near Mg side interface and the Al3Mg2layer is near Al side in Fig.13(a), and which is consistent with the IMCs layer observed from Fig.5(d).Furthermore, in friction stir welded dissimilar Al/Mg alloys joint,28the stability of Al12Mg17phase is better than that of Al3Mg2, so a large amount of Al12Mg17phase is formed, reducing the precipitation of Al3Mg2phase.With increasing the interfacial temperature, the liquefaction at FSLW interface may appear,30and which makes the eutectic reaction occur in the liquid phase on the Al12Mg17side (the Mg-rich region) during solidification process, forming the eutectic structure of Al12Mg17and Mg solid solution on the Mg side.Due to the difference of the interfacial temperature,the S-pin interface can completely liquefy to form a lamellar eutectic structure, while the T-pin interface only forms the local liquefaction, as shown in Fig.13(b)-13(c).

Fig.13 Schematic illustration of welding interface reaction for different pins.

5.Conclusions

In the present study,Al/Mg alloys sheets were successfully friction stir lap welded (FSLW), and effects of welding speed and pin-tip profile on macrostructure, microstructure and tensileshear properties were investigated, main conclusions can be as follows:

(1) With increasing welding speed from 23.5 mm/min to 118 mm/min, the tensile-shear load of all joints produced by three pins firstly increases and then decreases.With a welding speed of 75 mm/min,the joints obtained by T-pin perform the highest tensile-shear properties and the average tensile-shear load reaches 3.425 kN.

(2) Compared with common R-pin, the hooks at the interface of FSLW joints fabricated by S-pin and T-pin are eliminated due to changing flow path of the plasticized metal.Additional eccentric force induced from the triangle and square flats at the pin-tip can significantly increase the stirring action of the pin,and drive the plasticized metal flowing transversely around the pin root to prevent from moving downwards.

(3) The welding heat input and interfacial reaction are obviously affected by pin-tip profile.The interface structure of R-pin joint is composed of thicker IMCs of about 6.9 μm Al3Mg2and 13.3 μm Al12Mg17layers derived from diffusion reaction,and the hook with many cracks and hole defect exist.Due to the increase of the interfacial temperature,the welding interface of S-pin joint can completely liquefy to form Al3Mg2layer and lamellar eutectic structure (Al12Mg17+ Mg).However, the T-pin interface only forms the local liquefaction to generate thinner Al3Mg2layer of only 2.2 μm and dispersive eutectic structure, and which is beneficial to improve the interface performances of FSLW Al/Mg joint.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (NSFC) (Nos.52005240 and 52164045), Young Talent Program of Major Disciplines of Academic and Technical Leaders in Jiangxi Province (No.20212BCJ23028), and Key Laboratory Fund Project (No.EG202180417).