J-integral based correlation evaluation between microstructure and mechanical strength for FSSW joints made of automotive aluminum alloys

Abstract

Through using the crack resistance energy evaluated by a fracture parameter of J-integral, a correlation evaluation approach for potential relationships between post-weld microstructure and resultant mechanical properties was proposed. The present article, based on this method, discussed the effects of welded base metal combinations, AA 5052-H32 and 6061-T6 while considering their dissimilar configuration, on microstructure forming and further tensile strength of friction stir spot-welded (FSSW) joints, in the hope of micro-action mechanism. With the developed numerical evaluation model considering the weakened binding capability caused by inherent Hook defect, the cracking process during the initial and Hook propagation was simulated utilizing Virtual Crack Closure Technique (VCCT) to achieve a more accurate prediction of mechanical response. Meanwhile, both J-integral values of main kinked cracks was numerically calculated to study the influence of changes in local microscopic features on the overall performance. Consequently, morphological size of the Hook and amount of precipitated particles (Mg2Si mainly) in the stir zone (SZ) play key roles in determining the shear strength. Additionally, the dissimilar 5052/6061 joints exist a sudden change in Mg and Si concentrations at lap interface, thus making the integrity structure more prone to damage during loading. In terms of the two opposite lap orders while bonding dissimilar alloys, configuring the 5052 with greater plastic fluidity to the upper plate will yield a greater degree of embedding and mixing, accompanied by higher joint strength.