抄録
This paper presents the fatigue behaviour and fracture mechanisms of resistance spot welded joints of Al-Mg-Si alloy sheet. Prior to fatigue test, microstructure, Vickers hardness and tensile-shear strength were evaluated. Based on those results, tensile-shear fatigue tests have been performed and the fatigue behaviour and fracture mechanisms were discussed. The microstructure of a cross section of the welded joints consisted of three distinct regions of equiaxed grain structure, columnar crystal structure and base alloy structure. In the equiaxed grain structure, grains were considerably refined. The Vickers hardness in the nugget was lower than that in the base alloy because of the dissolution of precipitates. The average tensile-shear strength was 5241N and final fracture occurred by shear of the nugget. The tensile-shear fatigue strength at 107 cycles was 800N and the fatigue fracture mode was dependent on applied load. When maximum load Pmax = 2500N, final fracture took place by shear of the nugget, while when 1000N < Pmax < 2000N, it occurred in the base sheet. It was indicated that such transition of fracture modes was attributed to the stress state around the nugget induced by the crack initiation and the following crack opening displacement around the nugget.
