Structure and Mechanical Properties of Hypereutectic Al-Si Alloy Produced by Thixocasting Process

The microstructure, tensile and wear properties of a A390 hyper-eutectic Al-17 mass Si-4.5 mass % Cu-0.5 mass % Mg alloy produced by thixocasting process were investigated. The microstructure of the thixocast specimen mainly consists of the primary Si, fine eutectic Si phases and α-Al phase evolved into globules after reheating to the semi-solid temperature. The average size and volume fraction of the primary Si particles were about 30μm and 13 %, respectively. At the aging temperature between 433 K and 473 K, the thixocast alloy showed higher Brinell hardness than that of the permanent mold casting, and the peak hardness of thixo- and permanent mold cast specimens aged at 453 K were HB 150 and HB 143 respectively. The ultimate tensile strength of as-thixocast and T6-treated specimens are 200 MPa and 381 MPa respectively, whose values are near those of permanent mold castings. The elongation of the as-cast and T6-treated specimens was below 2%. Wear test results indicate less wear loss of the as-thixocast and T6-treated specimens than those of the permanent mold cast specimens. This may be caused by the fact that thixocast specimen posseses more primary Si particles and their uniformer distribution, and higher hardness of the α-Al phase than those of the permanent mold casting specimen.