The Research On Dynamic Properties Of Dissimilar Metals Friction Stir Spot Welding

In this paper,Keyhole-free FSSW joint of 1mm DP600 galvanized steel sheet and 2mm AA6082-T6 aluminum alloy sheet are successfully welded with pin retracting technique,and the original keyhole is eliminated.The thermocouple,OM,SEM,EDS and XRD were used to study temperature field,microstructure morphology,flow field,interface behavior,element diffusion,intermetallic compounds?IMCs?and fracture morphology of spot welded joint,and the dynamic properties were also investigated,including corrosion properties,tensile and shear properties,impact properties and dynamic fatigue properties.The structural damage process and failure behavior law of spot welded joint are analyzed under dynamic conditions.The failure mechanism is studied under dynamic conditions and the fracture failure model is further established.In the process of FSSW,the change of the temperature curve completely coincides with the welding process,and the peak temperature around shoulder reaches about 500°C.WNZ of spot welding joint forms a cloud-like chaotic structure with fine grain structure,and its grain size is reduced from 50?m of original base metal to less than 1?m.TMAZ has an elongated texture with a texture layer thickness of about 0.2 mm,while microstructure of HAZ is slightly larger than BM.The Al/steel interface forms a non-uniform transition layer with a thickness of approximately 30?m.Spot welded joints have good corrosion resistance in simulated seawater.Corrosion characteristics are mainly exfoliation and pitting corrosion on surface of aluminum alloy.The corrosion resistance sequence of different areas of spot welded joints?decreasing corrosion resistance?is WNZ>TMAZ>BM>HAZ.The Mg and Al elements of Mg₂Si and the silicon-containing solid solution??Al?preferentially form an anodic dissolution reaction,resulting in exfoliation corrosion around Si element.The iron-rich phase??Al₃Fe?acts as a cathode,causing corrosion of the Mg element in the magnesium-rich phase??Al₃Mg₂?surrounding the anode and forming pitting corrosion.The spot welding samples with a welding parameter of 1000 rpm have the best tensile strength and impact toughness,which are 9kN and 42J,respectively.For the tensile shear performance,the spot welded joints have typical monotonic load-displacement corresponding curve,especially in the fracture failure stage,with obvious fracture failure characteristics,and in particular,the fracture failure stage has an obvious fracture failure characteristic with mutation transition.In terms of impact performance,to some extent,the maximum impact deformation directly reflects the post-crack propagation energy,which significantly affects the impact toughness.The larger the crack propagation work,the longer the crack propagation path,the greater the impact deformation,and the better the impact toughness,and vice versa.The dynamic fatigue performance of spot welding joints shows a series of special phenomenons.The high frequency fatigue load will increase the deformation u and fatigue fracture absorption energy E_a of spot welding joint.The deformation u and fatigue fracture absorption energy E_a of spot welding joint under asymmetrical fatigue load are larger than that under symmetric fatigue load.At the same time,the symmetrical fatigue load can form a steady state hysteresis loop,and the asymmetric fatigue load forms displacement increment of fatigue softening.And when the loading frequency is constant,the fatigue limit F_-1 of spot welding joints under symmetry load is less than the fatigue limit F_f under asymmetric load.When the stress ratio R is certain,the fatigue limit F_{f H} under high frequency load is almost the same as the fatigue limit F_fL under low frequency load,and the difference is only the speed of reaching the fatigue limit F_f.This shows that the loading method,this is,the stress ratio R determines the fatigue limit F_f,while the fatigue limit F_f is less sensitive to the loading frequency.The spot welding joint can be divided into four regions which are STZ,SAZ,NAZ and NWZ,according to the welding process and fracture morphology.The fracture modes of tension,impact and fatigue are the same.The cracks crack from the gap and extend to STZ along the interface.Due to the presence of structural defects and brittle phases within the STZ,the fracture mechanism of multiple crack sources and ductile-brittle mixture is formed eventually.