Abstract
The effects of eutectic Si phase, Al-Fe system compounds and precipitates on the tensile properties of Al-7 mass % Si-0.5 mass % Mg 357 casting alloy produced by the thixocasting process were investigated. The tensile strength, 0.2 % proof stress and elongation of T6-treated thixocastings decrease from 360 MPa, 296 MPa and 13.6 % to 324 MPa, 274 MPa and 7.2 % with an increase in eutectic Si particle size from 2.3μm to 4.1μm, respectively. Relationships between tensile properties and eutectic Si particle size of thixocasting alloys were very similar with those of permanent mold castings. Two kinds of Al-Fe system compounds identified as β and π phases were observed at eutectic regions in both produced alloys. The needle-shaped precipitates were found lying along three [100] directions in the Al-matrix phase in thixoand permanent mold cast specimens aged at 433 K for 21.6 ks. Two types of precipitates with irregular and parallelogram network arrangement of bright dots in their cross-section were observed using high resolution transmission microscopy. These needle-shaped precipitates result in stronger Al-matrix in both alloys after T6-heat treatment. The morphology and size of precipitates of thixocast specimens were similar to those of permanent mold cast specimens. From results in this study, it was found that the tensile properties of the 357 thixocast alloy are mainly influenced by the size and shape of eutectic Si particles.
