Liquidus semi-continuous casting, reheating and thixoforming of a wrought aluminum alloy 7075

Abstract

Abstract The as-cast microstructures, the evolution of the microstructures during reheating and the mechanical properties of thixoformed products of 7075 aluminum alloy cast by liquidus semi-continuous casting (LSC) were studied in this paper. When the melt was held for 30 min at the temperature (638 °C) closely near its liquidus temperature, then was cast semi-continuously, a microstructure of fine, uniform and net-globular grains was obtained, which was satisfactory for meeting the microstructural requirement of thixoforming. Decrease first cooling and the casting velocity, and increase second cooling were propitious to the formation of the fine, uniform and net-globular grains. The formation of the net-globular grains was resulted from the increase in the number of crystal nuclei and the decrease in growth velocity of grains during LSC. The net-globular and rosette-shaped grains obtained by LSC became spheroids during reheating. The optimum microstructure was obtained when the reheating temperature was near 580 °C and the holding time was 15–30 min, or the reheating temperature was near 600 °C and the holding time was 5–15 min. It was very easy to thixoform when the ingots were reheated to 600 °C for 15 min. The ultimate strength of thixoformed products was 357.9 and 468 MPa before and after T 6 treatment, respectively.