Effect of thermal exposure on microstructure and mechanical properties of friction stir welding 7B50-T7451 aluminium alloy thick plate joint

Abstract

In the present work, thermal exposure was performed on the 7B50-T7451 aluminium alloy friction stir welding (FSW) joints at different temperatures (150 °C, 175 °C and 220 °C) for 200 h. The results show that the microstructure and mechanical properties of as-welded joints along the thickness direction of the plate are heterogeneous. The hardness and strength of FSW joints decrease while the elongation continuously increases with increasing thermal exposure temperature. Compared with the base material, the joints present much higher hardening capacity which is further increased with increasing temperature. The precipitates in the heat-affected zone (HAZ) exhibit continuous coarsening and the precipitate distribution becomes broader as the temperature rises. The transformation from η′ phase to η phase is obvious at 220 °C. The weld nugget zone (WNZ) re-precipitates a large number of nano-scale η particles subjected to 200 h thermal exposure at 150 °C and re-precipitated η phase in WNZ of the double-sided joint has reached micron-sized at 175 °C and 220 °C. When exposed at different temperatures, all FSW joints show typical ductile fracture features. Fracture positions at different thermal exposure temperatures are inconsistent and this variation is closely related to different microstructural characteristics.