Effect of the Injection Velocity and the Electromagnetic Stirring on the Mechanical Properties of a Rheo-Diecast 357 Al Alloy

Semisolid 357 Al alloy slurry was prepared in the presence and absence of electromagnetic stirring (EMS) and rheo-diecast with various injection velocities, the microstructure, porosity and mechanical properties of the 357 rheo-diecast (RDC) samples were quantitatively analysed as well as their relationships were explored. The results show that injection velocity had greater influence than EMS on the rheo-diecasting process. Injection velocity and EMS had minor influences on the secondary particles but marked influences on the primary particles. For primary particles, increasing injection velocity led to a decrease in particle diameter and an increase in shape factor. EMS can reduce the size and improve the shape factor of primary particles and large pores. The injection velocity was 0.53 m·s−1 at which the optimal values of the porosity and the shape-diameter ratio of the maximum pore for 357 RDC samples, i.e. 0.37% and 1.43 μm−1, can be obtained in the presence of EMS, corresponding to the highest tensile strength and elongation of 274 MPa and 4.85%. The tensile properties were correlated linearly with shape-diameter ratio. It was also suggested that the mechanical properties were enhanced by EMS on account of the reduction of porosity and increase in shape-diameter ratio.