Competition between root defects and non-uniform properties in governing the tensile deformation behavior of friction stir welds: a finite element analysis

Root defects and non-uniform material properties are two key factors governing the tensile deformation behaviors of friction stir welds, yet the competitive mechanism between these two factors remains unclear. In this study, we propose a finite element model that incorporates both variable geometric features of root defects and the continuous spatially non-uniform distribution of material properties, to investigate the tensile deformation behavior of 6082-T6 aluminium alloy friction stir welds. It is found that the non-uniform material properties result in inhomogeneous tensile direction strain and stress distributions. As root defect length increases or defect orientation approaches perpendicular to the workpiece bottom surface, the location of maximum strain and stress concentration shifts from the heat affected zone (HAZ) to the weld nugget zone (WNZ), indicating a corresponding change in the tensile fracture location. A competition between the root defect and non-uniform properties in governing the spatial concentration of strain is revealed, which leads to two tensile failure modes. Mode I failure occurs in the HAZ, similar to defect-free welds, implying negligible defect effects; whereas Mode II failure occurs in the WNZ, where root defects exert a significant impact. The critical root defect length for the mode transition is inferred to be 340.1 μm, and the critical orientations are 37.9° and 140.9°. Finally, a novel approach is proposed to probe the tensile deformation behavior by analyzing strain and stress on the weld top surface, enabling an evaluation of the influence of internal defects on the in-service performance.