Flat friction stir spot welding of three 6061-T6 aluminum sheets

Three 6061-T6 aluminum sheets with a thickness of 1 mm for each were subjected to the flat friction stir spot welding technique, which contains two steps of welding process. In step 1, a specially designed back plate with a pre-drilled dent was used in order to form a protuberance on the bottom side of the joint. In step 2, the keyhole and the protuberance were removed by using a probe-less rotating tool and a smooth back plate. The optimized welding condition for the three sheet spot welding was 700 kg/550 rpm/2 sin step 1 and 1000 kg/600 rpm/2 sin step 2. Two sheet/sheet interfaces were formed in the bowl-like stir zone. The stir zone showed a grain size varied from 4.2 to 8.6 mu m, much smaller than the 17.6 mu m of the base metal. Three different tensile test modes were used to evaluate the failure behavior of the joints. The interface between the top and middle sheet showed a longer bonding length of 11.5 mm and a higher fracture load of about 6438 N. While the interface between the middle and the bottom sheet showed a bonding length of 5.2 mm and a fracture load of about 4450 N. The step 2 joints showed plug-type failure mode during the shear tensile tests.