Microstructure and mechanical properties of friction stir welded dissimilar aluminum joints of AA2024-T3 and AA7075-T6

Dissimilar aluminum alloys such as 2024-T3 and 7075-T6 plates 3 mm thickness were friction stir butt welded. The welding was carried out at a constant welding speed of 100 mm/min and rotation speeds of 400, 800, 1200 1600 and 2000 min(-1). Effects of rotation speeds and fixed location of two alloys on microstructures especially the homogeneity of elemental distribution in the stir zone (SZ), hardness distributions, and tensile properties of the joints were investigated. The homogeneity of constituents of the two alloys in the SZ was analyzed by a scanning electron microscope (SEM) equipped with an energy dispersive X-ray spectroscopy (EDS). At the lowest rotation speed of 400 min(-1) there was no mixing of two alloys in the SZ and a border between them was observed regardless of the fixed location. Increase of rotation speed more than 400 min(-1) brought about a mixed structure likewise onion ring with periodic change of equiaxed grain size and heterogeneous distribution of alloying elements in the SZ. At 2000 min(-1) of rotation speed the SZ mainly composed of the material fixed on the advancing side. The hardness increased in the zones occupied by 2024-T3 Al alloy and fluctuations took place in the SZ due to the onion ring at the higher rotation speeds till 1600 min(-1). At 2000 min(-1) of rotation speed the hardness of SZ mainly depend on the material fixed on the advancing side. Hardness minima was in the heat affected zone (HAZ) on the side of AA2024-T3 Al alloy and the values slightly increased as the rotation speed increased. In the case of transverse tensile test, defect-free joints were fractured at the HAZ on 2024-T3 side and a maximum tensile strength of the joints was achieved at 1200 mint of rotation speed when 2024-T3 Al alloy was located on the advancing side. On the other hand. the tensile properties of SZ in the longitudinal direction showed higher values when 7075 Al alloy was located on the advancing side.