Microstructure characteristics and solidification behavior of wrought aluminum alloy 2024 rheo-diecast with self-inoculation method

One important problem in casting wrought aluminum alloys is the high tendency to the formation of hot tears in the solidification process. By using semi-solid metal (SSM) processing, the hot tearing tendency of alloys can be minimized during casting. In the present research, the semi-solid slurry of wrought aluminum alloy 2024 was firstly prepared with a novel self-inoculation method (SIM), and then the microstructure characteristics of the semi-solid slurry and the rheo-diecastings cast with the semi-solid slurry were investigated. The results indicate that finer and more uniform globular primary α-Al particles can be obtained when the semi-solid slurry are isothermally held for a short period within the semi-solid temperature range, and the primary α-Al particles without entrapped liquid are uniformly fine, globular grains in the rheo-diecastings. The holding temperature and time affect the solid fraction, particle size, and shape factor. After the semi-solid slurry is held at 625 ℃ for 3 min and 5 min, the optimal values for the average equivalent diameter are 70.80 μm and 74.15 μm, and for the shape factor are 1.32 and 1.42, respectively. The solidification process of the rheo-diecastings is composed of the following two distinct stages: primary solidification process and secondary solidification process. The secondary solidification process consists further of the following three stages: (1) direct growth of secondary primary (α2) phase from the surface of the primary α-Al phase particles without re-nucleation, (2) independent nucleation and growth of α3 phase from the residual liquid, and (3) eutectic reaction at the end.