Microstructure and Mechanical Properties of Friction Stir Welded Similar and Dissimilar Joints of Al and Mg Alloys

Similar joints of 2024-T3 Al alloys and dissimilar joints of 2024 Al alloy to 7075 Al alloy and AZ31 Mg alloy were produced by friction stir welding (FSW). Effects of FSW parameters such as welding speed, rotation speed and backing materials on microstructure and mechanical properties of similar 2024-T3 Al alloy joints and dissimilar joints of 2024-T3 Al alloy to 7075-T6 Al alloy and AZ31 Mg alloy aluminum alloy. Scanning electron microscopy, with energy dispersive X-ray spectroscopy (EDS) was carried to analyze the homogeneity of constituents in the stir zone (SZ) of dissimilar joints. An increase of equiaxed grain size in the SZ with increasing rotation speed was observed for all similar and dissimilar joints. Reduction of grain size in the SZ was obtained for similar 2024 Al alloy joints when using type 2 of backing material at the same welding conditions. For dissimilar joints between 2024 and 7075 Al alloys, at a rotation speed of 400min -1 there was no mixing of two alloys in the SZ and a border between them was observed. Increase of rotation speed more to than 400min -1 brought about a mixed structure resembling onion rings with periodic change of equiaxed grain size and heterogeneous distribution of alloying elements in the SZ. In the case of dissimilar joints between 2024 Al alloy and AZ31 Mg alloy a laminated structure was formed in the SZ near the boundary between SZ and TMAZ on the advancing side of 2024 Al alloy regardless of the welding speed. Hardness values increased with increasing rotation speed and fluctuated in the SZ of dissimilar joints due to formation of onion rings and intermetallic compounds. In the case of defect-free joints, tensile strength increased with increasing rotation speed and backing plate of type 3 brought about the best tensile properties for similar 2024 Al alloy joints.