Investigation of microstructure and properties in an aged ultra-high strength Al alloy and its friction stir welded joints

An Al-Zn-Mg-Sc-Zr alloy with high Zn was prepared to obtain ultra-high strength in the present study. The microstructure of the aged samples and the friction stir welding (FSW) joints were investigated by optical microscopy, electron backscatter diffraction and transmission electron microscopy. The mechanical properties were evaluated by microhardness and tensile tests. Results show that the peak microhardness of the aged Al-Zn-Mg-Sc-Zr alloy was ∼212.2 Hv. Strengths up to 697.2 MPa in the yield strength and 721.8 MPa in the ultimate tensile strength were obtained after ageing at 120 °C for 24 h, and the elongation was ∼9%. The ultra-high strength was mainly attributed to the precipitate strengthening from η′ and Al3ScxZr1-x precipitates, solid solution strengthening and fine grain strengthening. Soft regions with hardness of ∼122 Hv were developed near the interfaces between heat-affected zone and thermo-mechanically affected zone due to the coarsening and/or dissolution of strengthening precipitates. The joint coefficient was ∼69.5% for the Al-Zn-Mg-Sc-Zr alloy. Fine recrystallized grains with an average diameter of ∼1.8 μm were formed in the nugget zone (NZ), and these newly formed fine grains experienced further deformation and resulted in high fractions of substructure and deformed grain. The size and shape of Al3ScxZr1-x precipitates remain almost unchanged during ageing at 120 °C and the FSW processing. The existence of η phase in the NZ was associated with the Al3ScxZr1-x particles, indicating the heterogeneous precipitation of η precipitates on the Al3ScxZr1-x precipitates during FSW.