Effects of Welding Process Parameters and ResidualStresses on the Fatigue Life of Friction Stir Spot WeldedJoints

Recently, Friction Stir Spot Welding (FSSW) has been widely used and developed by many aerospace and automotive companies such as Boeing, Toyota, General Motors, Mazda and Kawasaki Motor Corporation. In this investigation, the influence of welding process parameters including tool rotation speed, plunge depth, indentation traverse speed, and tensile residual stresses around the nuggets of aluminum 7075-T6 tensile-shear friction stir spot welded specimens on the fatigue strength of the joints have been studied, experimentally and numerically. The values of residual stresses have been measured experimentally and compared with those obtained from the Finite Element (FE) analysis. Fatigue tests have been conducted on the welded specimens using a 250kN Zwick/Amsler servo-hydraulic fatigue testing machine and the results have been compared with those predicted based on the Smith-Watson-Topper (SWT) damage equation. Welded specimens have been simulated using three dimensional finite element model using Abaqus software. Only a half of model was meshed and analyzed, taking advantages of the plane of symmetry in the longitudinal direction of the welded joints. The results show that, with increasing tool rotation speed and plunge depth, the fatigue life of the joints would increase. However, with increasing the indentation speed, a relatively reduction in fatigue life has been observed. Tensile residual stresses near the nuggets have reduced the fatigue life of the specimens; hence, in finite element analysis must be taken into account.