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.

Friction stir keyholeless spot welding of AZ31 Mg alloy-mild steel

Friction stir keyholeless spot welding（FSKSW） using a retractable pin for 1.0 mm thick galvanized mild steel and 3 mm thick AZ31 B magnesium alloy in a lap configuration was investigated.The process variables were optimized in terms of the joint strength.The effects of the stacking sequence on joint formation and the joining mechanism of FSKSW AZ31B-to-mild steel joints were also analyzed.It shows that the process window and joint strength are strongly influenced by the stacking sequence of the workpieces.While the process window is narrow and unstable for FSKSW of a magnesium-to-steel stack-up,a desirable process was established for the steel-to-magnesium stacking sequence,a desirable process and higher strength joint can be got when the steel-to-magnesium stacking sequence.XRD phase and EPMA analyses of the FSKSW joint showed that the intermetallic compounds are formed at the steel-to-magnesium interface,and the element diffusion between the mild steel and AZ31 B magnesium alloy revealed that the joining methods for FSKSW joints is the main mechanical joining along with certain metallurgical bonding.

Galvanic corrosion of AZ31B joined to dual-phase steel with and without Zn layer by ultrasonic and friction stir welding

Galvanic corrosion of AZ31B joined with bare or Zn-coated DP590 steel by ultrasonic spot welding or linear friction stir welding was quantitatively studied by pre-defining anode and cathode in the lap joint samples. Corrosion volume and depth from Mg anode surfaces exposed to 0.1 M sodium chloride solution was analyzed as functions of cathode surface type and welding method. Characterization of as-welded joints was performed to identify any microstructural feature of the bonding zone that could impact galvanic corrosion behavior.COMSOL modeling with modified user subroutine was conducted to simulate the progression of Mg corrosion in the same joint and electrode configurations used for the corrosion experiments. The experimental results indicated that Zn-coated cathode surface can reduce Mg galvanic corrosion significantly as galvanic polarization and cathodic current on Zn-coated surface remained relatively low for Mg in the weld joints.COMSOL modeling described the growth of Mg galvanic corrosion in a reasonable manner but showed limitation by underestimating the corrosion volume as it did not capture self-corrosion.

Microstructure and properties of probeless friction stir spot welding of AZ31 magnesium alloy joints

The AZ31 magnesium alloy with a thickness of 1.8 mm was welded by the probeless friction stir spot welding process without Zn interlayer.The influence of process parameters on joint microstructure and mechanical properties was investigated by using different rotating speeds and dwell time.Microstructure of joints is divided into three regions:stir zone,thermomechanically-affected zone and heat-affected zone.With the increase of rotation speed and dwell time,the depth of stir zone gradually increases,and hook defects extend from the interface of two plates to the surface of the upper plate.The tensile shear strength of joints and two fracture modes(shear fracture and plug fracture)are closely related to hook defects.The maximum tensile shear strength of the joint is 4.22 kN when rotation speed and dwell time are 1180 r/min and 9 s,respectively.Microhardness value and its fluctuation in upper sheet are evidently higher than those of the lower sheet.

Development of liquid-nitrogen-cooling friction stir spot welding for AZ31 magnesium alloy joints

A liquid-nitrogen-cooling friction stir spot welding(C-FSSW) technology was developed for welding AZ31 magnesium alloy sheets. The liquid-nitrogen cooling degraded the deformability of the welded materials such that the width of interfacial cracks increased with increasing cooling time. The grain size of the stirred zone(SZ) and the heat-affected zone(HAZ) of the C-FSSW-welded joints decreased, whereas that of the thermomechanically affected zone(TMAZ) increased with increasing cooling time. The maximum tensile shear load of the C-FSSW-welded joints welded with a cooling time of 5 or 7 s was larger than that of the friction stir spot welding(FSSW)-welded joint, and the tensile shear load decreased with increasing cooling time. The microhardness of the C-FSSW-welded joints was greater than that of the FSSW-welded joint. Moreover, the microhardness of the SZ and the HAZ of the C-FSSW-welded joints increased, whereas that of the TMAZ decreased, with increasing cooling time.

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.

Effect of pin penetration depth on double-sided friction stir welded joints of AA6061-T913 alloy

Friction stir welding (FSW) of aluminum alloys is currently utilized in several modern industries. The joints must have sufficient elastic?plastic response and formability levels similar to that of the base metal. In this work, double-sided FSW of AA6061 sheet was compared with its conventional single-sided one. An adjustable tool with different pin lengths (50%?95% of the sheet thickness) was used to perform the double-sided welds. Macro- and micro-structures, strength, and hardness of the joints were investigated to determine the optimum pin penetration depth. The best results were obtained for a double-sided joint made by a pin length equal to 65% of the sheet thickness, which showed an increase of 41% in the ultimate tensile strength compared with the single-sided joint.

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.

Development of processing windows for friction stir spot welding of aluminium Al5052/copper C27200 dissimilar materials

Friction stir spot welding technique was used to join dissimilar combinations of aluminium alloy(Al5052)with copperalloy(C27200)and friction stir spot welding windows such as tool rotational speed–dwell time and tool rotational speed?plungedepth diagrams for effective joining of these materials were developed.Using a central composite design model,empirical relationswere developed to predict the changes in tensile shear failure load values and interface hardness of the joints with three processparameters such as tool rotational speed,plunge depth and dwell time.The adequacy of the developed model was verified usingANOVA analysis at95%confidence level.Response surface methodology was used to optimize the developed model to maximizetensile strength and minimize interface hardness.A high tensile shear failure load value of3850N and low interface hardness valueof HV81was observed for joints made under optimum conditions,and validation experiments confirmed the high predictability ofthe developed model with error less than2%.The operating windows developed shall act as reference maps for future designengineers in choosing appropriate friction stir spot welding process parameter values to obtain good joints.

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.

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.

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.

Microstructure evolution along thickness in double-side friction stir welded 7085 Al alloy

The microstructure evolution in the weld zone of double-side friction stir welded（DS-FSWed） 7085-T7452 Al alloy was investigated by the electron backscatter diffraction method.The results indicate that DS-FSW process results in substantial grain refinement.The misorientation angle distribution shows a very high volume fraction of high angle grain boundary（HAGB）（above 75%） under DS-FSW condition at rotational rate of 300 r/min.The fraction of HAGB rapidly decreases with increasing the rotational rate from 300 to 950 r/min,and the obvious growth of grain in the weld nugget zone（WNZ） is presented.The average grain sizes in the elongated grains of thermal-mechanical affected zone（TMAZ） and partially equiaxed and coarser grains of thermal affected zone（HAZ） are 7.3 and 15.7 μm with the fractions of HAGBs less than 43% and 30%,respectively.The intensities of（100）,（110） and（111） pole figures in the WNZ obviously decrease when compared with those in the BM and present significantly difference along the thickness direction of plate.

Fatigue behavior of friction stir spot welded AZ31 Mg alloy sheet joints

The fatigue behavior of friction stir spot welded （FSSW） AZ31 magnesium alloy sheet joints was investigated by tension- compression of fatigue test. The results suggest that all the fatigue failures occur at the stir zone of the FSSW AZ31 sheet joints, and all cracks initiate at the stir zone outer edge between the upper and lower sheet. When the cycle force equals 1 kN, the crack propagates along the interface of heat-affected zone and thermo-mechanical zone, simultaneously across the direction of force; while the cycle force equals 3 kN, the crack propagates along the diameter of stir zone and shear failure occurs finally. Moreover, the transverse microsections indicate that there is a tongue-like region at the outer edge of stir zone between the two AZ31 sheets, and the direction of tongue-like region is toward outside of the stirred zone and all fatigue cracks initiate at the tongue-like region.

Evolving properties of friction stir spot welds between AA1060 and commercially pure copper C11000

Friction stir spot welding technique was employed to join pure copper （C11000） and pure aluminium （AA1060） sheets. The evolving properties of the welds produced were characterized. The spot welds were produced by varying the rotational speed, shoulder plunge depth using different tool geometries. The presence of a copper ring of different lengths was observed on both sides of the welds indicating that Cu extruded upward into the Al sheet which contributed to obtaining strong welds. The microstructure showed the presence of copper particles in the aluminium matrix which led to the presence of various intermetallics observed by the energy dispersive spectroscopy and X-ray diffraction. The maximum tensile failure load increases with an increase in the shoulder plunge depth, except for the weld produced at 800 r/min using a conical pin and a concave shoulder. A nugget pull-out failure mode occurred in all the friction stir spot welds under the lap-shear loading conditions. High peaks of Vickers microhardness values were obtained in the vicinity of the keyhole of most of the samples which correlated to the presence of intermetallics in the stir zone of the welds.

Microstructure and mechanical properties of dissimilar pinless friction stir spot welded 2A12 aluminum alloy and TC4 titanium alloy joints

The microstructure and mechanical properties of dissimilar pinless friction stir spot welded joint of2A12aluminum alloy and TC4titanium alloy were evaluated.The results show that the joint of Al/Ti dissimilar alloys can be successfully attained through pinless friction stir spot welding(FSSW).The joint can be divided into three zones(SZ,TMAZ and HAZ).The microstructure of joint in Al alloy side changes significantly but it basically has no change in Ti alloy side.At the same rotation speed,the maximum load of welded joints gradually rises with the increase in dwell time.At the same dwell time,the maximum load of the welded joint increases with the increase of the rotational speed.In addition,optimal parameters were obtained in this work,and they are rotation speed of1500r/min,plunge speed of30mm/min,plunge depth of0.3mm and dwell time of15s.The fracture mode of welded joints is interfacial shear fracture.The microhardness of the joint on the Al side distributes in a typical“W”type and is symmetry along the weld center,but the distribution of the microhardness on the Ti side has no obvious change.

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.

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.

Comparison of microstructure and mechanical properties of conventional and refilled friction stir spot welds in AA6061-T6 using filler plate

Friction stir spot welding with refilling by friction forming process （FSSW-FFP） was successfully modified using filler plate. Both of this new refilling technique and conventional friction stir spot welding （FSSW） process were used to weld A1 6061-T6 lap shear specimens and the results were compared. Effects of tool rotational speeds on mechanical and metallurgical properties in both the cases were studied. Static shear strength of refilled weld samples was found to be better than those welded by conventional FSSW process at all tool rotational speeds. This is explained in terms of effective increase in cross-sectional area of weld nugget due to addition of more material from filler plate, thereby eliminating the probe hole. Failure mechanisms were discussed and fracture surfaces were analyzed through scanning electron microscopy （SEM）. The hardness profile of the welds exhibited a W-shaped appearance in both the processes and the minimum hardness was measured in the HAZ.

Material properties of friction stir spot welded joints of dissimilar aluminum alloys

Mechanical properties and material mixing patterns of friction stir spot welded (FSSW) joints of dissimilar aluminum alloys were investigated.Two aluminum alloys typically used in automotive applications,5052-H32 and 6061-T6,were selected.During the experiment,the process parameters including the z-axis force and torque histories were measured as a function of the tool displacement.The mechanical properties were investigated by microhardness measurements of the joint,and the material mixing in the stir zone was investigated by EPMA.The experimental results illustrate different process parameter histories,material mixing in the stir zone and material properties including microhardness distributions for FSSW joints of dissimilar aluminum alloys,likely due to different mechanical behaviors of the selected aluminum alloys in the FSSW process temperature range.