Dissimilar friction stir spot welding of AA2024-T3/AA7075-T6 aluminum alloys under different welding parameters and media

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(600e1400 rpm)and plunge depth(0.1 e0.5 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.

Filling technique for keyhole of friction stir spot welding based on the principles of resistance spot welding

Keyhole at the end of a weld prepared by friction stir welding(FSW)is one of the major issues that impede the application of FSW.To address this issue,a keyhole filling technique was proposed in this paper,which is based on the principles of resistance spot welding(RSW).A three-phase secondary rectifier resistance spot welder was applied as the experimental instrument for filling the keyhole in the center of friction stir spot weld(FSSW).The test sheet is a 2024-T4 aluminium alloy with a thickness of 6.0mm.The experiments results show that the filled joint strength is improved by 26.12%since the area is increased for the plug in the keyhole.And there are two kinds of dimples in the tensile fracture-equiaxial dimples and long dimples.The filled joint involves the fusion welding zone(FWZ),pressure welding zone(PWZ),melted plug zone(MPZ),and plastic deformation zone(PDZ).The FWZ and the PWZ is the melting bond and diffusion bond between the plug and keyhole,respectively.The MPZ is the center part and the PDZ is upper or lower part of the plug.

Refill Friction Stir Spot Welding Al Alloy to Copper via Pure Metallurgical Joining Mechanism

The current investigation of refill friction stir spot welding(refill FSSW)Al alloy to copper primarily involved plunging the tool into bottom copper sheet to achieve both metallurgical and mechanical interfacial bonding.Compared to conventional FSSW and pinless FSSW,weld strength can be significantly improved by using this method.Nevertheless,tool wear is a critical issue during refill FSSW.In this study,defect-free Al/copper dissimilar welds were successfully fabricated using refill FSSW by only plunging the tool into top Al alloy sheet.Overall,two types of continuous and ultra-thin intermetallic compounds(IMCs)layers were identified at the whole Al/copper interface.Also,strong evidence of melting and resolidification was observed in the localized region.The peak temperature obtained at the center of Al/copper interface was 591℃,and the heating rate reached up to 916℃/s during the sleeve penetration phase.A softened weld region was produced via refill FSSW process,the hardness profile exhibited a W-shaped appearance along middle thickness of top Al alloy.The weld lap shear load was insensitive to the welding condition,whose scatter was rather small.The fracture path exclusively propagated along the IMCs layer of Cu_(9)Al_(4) under the external lap shear loadings,both CuAl_(2) and Cu_(9)Al_(4) were detected on the fractured surface on the copper side.This research indicated that acceptable weld strength can be achieved via pure metallurgical joining mechanism,which has significant potential for the industrial applications.

Material flow behavior and microstructural evolution during refill friction stir spot welding of alclad 2A12-T4 aluminum alloy

In this study, we used the stop-action technique to experimentally investigate the material flow and microstructural evolution of alclad 2A12-T4 aluminum alloy during refill friction stir spot welding.There are two material flow components, i.e., the inward-or outward-directed spiral flow on the horizontal plane and the upward-or downward-directed flow on the vertical plane.In the plunge stage, the flow of plasticized metal into the cavity is similar to that of a stack, whereby the upper layer is pushed upward by the lower layer.In the refill stage, this is process reversed.As such, there is no obvious vertical plasticized metal flow between adjacent layers.Welding leads to the coarsening of S(Al2CuMg) in the thermo-mechanically affected zone and the diminishing of S in the stir zone.Continuous dynamic recrystallization results in the formation of fine equiaxed grains in the stir zone, but this process becomes difficult in the thermo-mechanically affected zone due to the lower deformation rate and the pinning action of S precipitates on the dislocations and sub-grain boundaries, which leads to a high fraction of low-angle grain boundaries in this zone.

Effect of rotary direction and speed on mechanical properties in friction stir spot welding of A6061 sheets e

Friction stir spot welding of A6061 sheets was conducted using a tool with thread pin. The hook geometries, hook formation and mechanical properties of the joints welded with different rotary directions and speeds were investigated. The results show that the hook in the joint welded in clockwise was curved upwards and that in anticlockwise was curved downwards. The hook formation was related to the plastic material flow in the joint. With increasing the rotary speed in clockwise direction, the hook moved upwards and far way from the center of the keyhole, resulting in an increase in the effective weld width and a decrease in the effective sheet thickness. Three types of fractuces were observed and they were affected by the hook geometries. The tensile shear load increased firstly and then decreased when the rotary speed increased in clockwise direction, which was related to the hook geometries.

Pinless Friction Stir Spot Welding of Mg-3Al-1Zn Alloy with Zn Interlayer

With the addition of a thin Zn interlayer, 2.4 mm thick Mg-3AI-1Zn alloy sheets were friction stir spot welded （FSSW） using a pinless tool with fiat, convex and concave shoulder shapes. The results showed that an alloying reaction took place between the Mg substrate and Zn interlayer during FSSW, forming a discontinuous intermetallics layer composed of dispersive （α-Mg ＋ MgZn） eutectic structure under- neath the shoulder and a Mg-Zn intermetallics bonding zone at the outside of the joints. This alloying reaction increased the bonded area and eliminated the hook defects, thereby producing sound FSSWjoints with a shallow keyhole without hook defects. The increase of plunge depth was beneficial to the Mg-Zn diffusion, thereby increasing the tensile-shear load of the joints. However, excessive plunge depths re- sulted in a decrease of the effective sheet thickness, reducing the strength of the joints. At a small plunge depth, the convex and concave shoulders were more beneficial to the interface reaction than the fiat shoul- der. The maximum joint load of 6.6 kN was achieved by using the concave shoulder at a plunge depth of 1.0 mm. A post-welding heat treatment promoted the dissolution of the discontinuous reaction layer in the joints; however, it led to the occurrence of void defects, influencing the bonding strength.

Refill friction stir spot welding of 5083-O aluminum alloy

In this work,refill friction stir spot welding（RFSSW） was used to weld 2 mm-thick 5083-O alloy.The Box–Behnken experimental design was used to investigate the effect of welding parameters on the joint lap shear property.Results showed that a surface indentation of 0.3 mm effectively eliminated the welding defects.Microhardness of the stir zone（SZ） was higher than that of the base material（BM） and the hardness decreased with increasing the heat input during welding.The optimum failure load of 7.72 k N was obtained when using rotating speed of 2300 rpm,plunge depth of 2.4 mm and refilling time of 3.5 s.Three fracture modes were obtained during the lap shear test and all were affected by the hook defect.

Flat friction stir spot welding of low carbon steel by double side adjustable tools

2 mm low carbon steel plates were successfully welded by the flat friction stir spot welding(FSSW) using double side adjustable tools, by which the keyhole formed in the conventional FSSW was eliminated and a flat surface on both the top and bottom sides of the welded joints was obtained. In addition, the hook shape usually generated in the conventional FSSW was eliminated by this technique, and the unbonded interface was parallel to the surface of the sheets. Owing to the enlarged bonded interface width by eliminating the keyhole and the intermixed interface by the adjustable probe, the plug fracture occurred under all the welding conditions in the present study. Due to the suppression of the thickness thinning and elimination of the hook shape, the joint performance was improved in the plug fracture mode. The shear tensile performance was considered to strongly depend on the microstructure in the tip area of the unbonded interface and the maximum shear fracture load of 23.0 kN was achieved in this study.

Interface strengthening in dissimilar double-sided friction stir spot welding of AZ31/ZK60 magnesium alloys by adjustable probes

Dissimilar welding of AZ31/ZK60 magnesium alloys with a thickness of 2 mm was successfully carried out by the double-sided friction stir spot welding with adjustable probes.A dissimilar joint bearing flat surfaces on both sides without a keyhole was obtained and the shear failure load of 8.7±0.5 kN was reached.The role of the adjustable probes has been revealed in detail.In the center of the stir zone,the welding interface structure was heterogeneous around which some distinct oxides still remained,leading to a weak interface strength.On the contrary,the welding interface structure around the shoulder/probe interface was homogeneous with no oxides giving rise to a strong interface strength,which is attributed to the severe material flow introduced by the adjustable probes.In addition,the vicinity outside the shoulder/probe interface,where the fracture occurred during the shear tensile tests,was also strengthened owing to the shearing and torsion by the adju stable probes.Therefore,a stable plug failure can be obtained and the joint properties can be improved.

Effects of pin geometry on the material flow behavior of friction stir spot welded 2A12 aluminum alloy

A three-dimensional finite volume model was established by the ANSYS FLUENT software to simulate the material flow behavior during the friction stir spot welding （FSSW） process. Effects of the full-threaded pin and the reverse-threaded pin on the material flow behavior were mainly discussed. Results showed that the biggest material flow velocity appeared at the outer edge of the tool shoulder. The velocity value became smaller with the increase of the distance away from the tool surface. In general, material flows downwards along the pin thread when the full-threaded pin is used. Meanwhile, both the materials of the upper and the lower plates flow towards the lap interface along the pin thread when the reverse-threaded pin is used. The numerical simulation results were investigated by experiment, in which 2A12 aluminum alloy was used as the research object. The effective sheet thickness （EST） and stir zone （SZ） width of the joint by the reverse-threaded pin were much bigger than those by the full-threaded pin. Accordingly, cross tension failure load of the joint by the reverse-threaded pin is 23% bigger than the joint by the full-threaded pin.

Effect of tool plunge depth on the microstructure and fracture behavior of refill friction stir spot welded AZ91 magnesium alloy joints

We used refill friction stir spot welding(RFSSW)to join 2-mm-thick AZ91D-H24 magnesium alloy sheets,and we investigated in detail the effect of tool plunge depth on the microstructure and fracture behavior of the joints.A sound joint surface can be obtained using plunge depths of 2.0 and 2.5 mm.Plunge depth was found to significantly affect the height of the hook,with greater plunge depths corresponding to more severe upward bending of the hook,which compromised the tensile-shear properties of the joints.The hardness reached a minimum at the thermo-mechanically affected zone due to the precipitation phases of this zone as it dissolved into theα-matrix during the welding process.The fracture modes of RFSSW joints can be divided into three types:shear fracture,plug fracture,and shear–plug fracture.Of these,the joint with a shear–plug fracture exhibited the best tensile-shear load of 6400 N.

Bonding interface morphology of keyholeless friction stir spot welded joint of AZ31B Mg alloy and DP600 galvanized steel

Because the bonding interface of dissimilar metal joint between AZ31 B Mg alloy and DP600 galvanized steel by keyholeless friction stir spot welding(KFSSW)is permanent bonding,the interface morphology cannot be directly observed.If the joint is separated by external force,the original features of bonding interface of joint will be destroyed,which has influence on the accuracy for observation and analysis of the result.In this paper,the coordinates of the key point at the interface of every cross-section at intervals of 0.2 mm were measured and connected into an outline.The outline of all interfaces makes up the three-dimensional morphologies of bonding interface between AZ31 B Mg alloy and DP600 steel by KFSSW,which was constructed by Solidworks software to restore the real mechanical bonding state of joint.Combined with the microhardness analysis of cross-section and results of in-situ tensile test,the unique bonding state and morphology of Mg and steel in the welded joint were confirmed.

Clarifying effect of welding conditions on microstructure and mechanical properties of friction stir spot-welded DH590 automotive high-strength steel plates

Friction stir spot welding was successfully applied to the 1.2-mm-thick DH590 dual-phase steel plates by using a polycrystalline cubic boron nitride rotating tool.During welding,the rotation speed ranged from 600 to 1000 r/min and the penetration depth ranged from 0.1 to 0.3 mm.In the spot joints,the size of the stir zone increased with the increase in rotation speed as well as the penetration depth of the tool.Comparing with the banded ferrite and martensite structure of the base metal,a mixed microstructure of ferrite and tempered martensite,refined bainite structure and coarse martensite structure were found in the heat-affected zone,thermomechanically affected zone and stir zone of the joints,respectively.Two kinds of interfacial shapes were formed between the upper and lower steel plates,and the formation of the bonded interface was dominated mainly by the penetration depth of the rotating tool rather than the rotation speed.It was revealed that the joints with straight interfaces showed higher shear tensile loads comparing with those with hook-like interfaces.Shear tensile tests showed that the maximum shear tensile load reached about 15.56 kN when the rotation speed and penetration depth were set as 800 r/min and 0.3 mm,respectively.The specimen was fractured through plug failure mode with a total elongation of about 5.6 mm.

Microstructure and mechanical optimization of probeless friction stir spot welded joint of an Al-Li alloy

In this work, a third generation AI-Li alloy has been successfully spot welded with probeless friction stir spot welding （P-FSSW）, which is a variant of conventional friction stir welding. The Box-Behnken exper-imental design in response surface methodology （RSM） was applied to optimize the P-FSSW parameters to attain maximum tensile/shear strength of the spot joints. Results show that an optimal failure load of 7.83 kN was obtained under a dwell time of 7.2 s, rotation speed of 950 rpm and plunge rate of 30 mm/rain. Sufficient dwell time is essential for heat conduction, material flow and expansion of the stir zone to form a sound joint. Two fracture modes were observed, which were significantly affected by hook defect. In addition to mechanical testing, electron backscattering diffraction （EBSD） and differential scanning calorimetry （DSC） were used for microstructure evolution and property analysis. The precipitation of GP zone and AI3Li as well as the ultrafine grains were responsible for the high microhardness in the stir zone.

Influencing mechanism of pre-existing nanoscale Al5Fe2 phase on Mg-Fe interface in friction stir spot welded Al-free ZK60-Q235 joint

Al-free ZK60 magnesium (Mg) alloy sheet was selected as substrate material of Mg-steel pinless friction stir spot welding (FSSW), avoiding the effect of the Al element in the substrate on the alloying reaction of Mg-iron (Fe) interface. The sound FSSW joint of ZK60 Mg alloy and Q235 steel with a hot-dipped aluminum (Al)-containing zinc (Zn) coating was successfully realized. The detailed microstructural examinations proved that Al5Fe2 phase at the Mg-Fe interface came from the pre-existing Al5Fe2 phase in the coating and acted as the transition layer for promoting the metallurgical bonding of Mg and Fe. The interfaces with well-matched lattice sites among Fe, Al5Fe2 and Mg were formed during FSSW. A low energy interface with good match of lattice sites ((002)Al5Fe2//(110)Fe, [110]Al5Fe2//[113]Fe) between Al5Fe2 and Fe was identified. For the interface between Al5Fe2 and Mg, an orientation relationship of (622)Al5Fe2//(3112)Mgand[158]Al5Fe2//[2423]Mg was observed. The tensile-shear load of the ZK60-steel joint could reach 4.6 kN. Moreover, the joint fracture occurred at the interface between the Al5Fe2 layer and the Mg alloy substrate, suggesting the brittle fracture characteristic.

Effect of connection processes on mechanical properties of 7B04 aluminum alloy structures

The static and fatigue properties of 7B04 aluminum alloy structures connected by riveting and refill friction stir spot welding(refill FSSW)were compared and analyzed.Results show that the static compression load of the typical structure connected by riveting and refill FSSW fluctuated in the range of 117-124 kN,and the shear load was in the range of 89-95 kN.Welds spacing had a small influence on the static load of the structures joined by refill FSSW.However,the fatigue life of riveted structures was lower than that joined by refill FSSW.For the welded structure,the heterogeneous microstructures of the welded joint led to the uneven microhardness,and the hook at the lap interface bent upwards at the same time.These factors made the welded structures during the fatigue test failure along the path of sleeve moving.