The improvement of mechanical properties of friction-stir-welded A356 Al alloy

The joint characteristics of friction-stir-welded A356 alloys, especially concerning the improvement of mechanical properties at the weld zone, were observed with various FSW (friction stir welding) speeds. The microstructures of the weld zone are composed of SZ (stir zone), TMAZ (thermo-mechanical affected zone) and BM (base metal). The BM shows the hypoeutectic Al–Si dendrite structure. The microstructure of the SZ is very different from that of the BM. The eutectic Si particles are dispersed homogeneously in primary Al solid solution. TMAZ, where the original microstructure was greatly deformed, is characterized by dispersed eutectic Si particles aligned along the rotational direction of the welding tool. These regions are mainly formed at the advancing side and the upper region of the retreating side. The mechanical properties of the weld zone are greatly improved in comparison to that of the BM. The hardness of the weld zone shows more uniform distribution than that of the BM because some defects are remarkably reduced and the eutectic Si particles are dispersed over the SZ. The tensile strength of the SZ is also greatly elevated and shows over 178 MPa, i.e. almost 120% that of the BM. Therefore, the FSW is well applied to the joining of the cast Al alloys.