Search Results

Failure modes of spot friction welds in cross-tension specimens of aluminum 6061-T6 sheets are first investigated based on experimental observations. Optical and scanning electron micrographs of the welds before and after failure under quasi-static and cyclic loading conditions are examined. Experimental results show that the failure modes of the welds under quasi-static and cyclic loading conditions are quite different. Under quasi-static loading conditions, the failure mainly starts from the necking of the upper sheet outside the weld. Under low-cycle loading conditions, the dominant fatigue cracks are the kinked cracks growing into the upper sheet from the crack tips; hence, the upper nugget pullout failure mode can be seen. Under high-cycle loading conditions, the dominant fatigue cracks are kinked cracks growing into the lower sheet from the crack tips; subsequently, the lower nugget pullout failure mode can be seen.

Effects of tool geometry on mechanical properties of spot friction welds in lap-shear specimens of 6061-T6 aluminum sheets are investigated based on experimental observations. Three tools with shoulder diameters of 8 mm (small), 10 mm (medium) and 12 mm (large) were used to join aluminum sheets, respectively. The experimental results indicate that the failure strengths of the welds show significant dependence on the tool size. Optical and scanning electron micrographs of the welds made by three tools before and after failure were examined and micro indentation tests of these welds were conducted. The results show that the weld geometry, microstructure distributions and micro hardness distributions of the three welds are quite similar. However, the material flow trends of these welds appear to be quite different. The small and medium failed welds show the circumferential/shear failure mode and the large failed weld shows the nugget pullout failure mode.