Microstructural evolution and mechanical properties during the friction stir welding of 7075-O aluminum alloy

In the present work, the AA7075-O was subjected to friction stir welding. The rotating and welding speeds were 630 rpm and 32 mm/min, respectively. The diameters of shoulder and pin were 30 and 5.7 mm, respectively. The length of threaded pin was 4.8 mm. The tensile, bending, and hardness tests were carried out on the friction stir-welded specimens. The results show that the strength of friction stir-welded sample was about 15 % (i.e., 25 MPa) higher than that of base metal. By contrast, the ductility of base metal was as much as twice that of friction stir-welded one. According to the obtained hardness profile, the maximum hardness was obtained for the stirred zone with a continuous decrease toward the base metal. This resulted in the formation of cracks after bending specimens prepared from weldment, while there was no sign of crack in the bent specimens taken from base metal. Besides, there are microstructural evolutions from the weldment toward the base metal. These include the variations in the size and distribution of precipitates. The precipitates were coarser in the heat-affected zone, while they were finer in the stirred zone.