A novel investigation on the influence of Cu–P modifier on the microstructure, mechanical performance, and melting process of Al–50Si alloys treated by overheating

Abstract

As an efficient modifier, phosphorus (P) can significantly reduce the size of primary silicon (Si) crystals. However, modifier can aggregate when the P content exceeds a certain proportion, which decreases refining efficiency. In this study, the evolution of microstructures and mechanical properties of Al–50Si alloy have been studied by combining superheating and modification treatments. After modification, the mean size of the primary Si phase increased with superheating temperature, while the hardness decreased with increased superheating. Although the alloy was in an over-modified state when the modified content was 7 ​wt%, the overall alloy hardness was the highest. Nanoindentation tests were used to examine the mechanical properties of each alloy phase and the results showed that the nano-hardness of Al2Cu phase was 5.52 ​GPa, three times that of an Al matrix, and was considered to be the reason for increased alloy hardness after over-modification. This phenomenon indicated that, except for size and distribution of the primary Si phase, intermetallic compounds, such as Al2Cu, also played important roles in modifying alloy mechanical properties. The alloy melting process was observed in-situ using a laser confocal high-temperature scanning microscope and results indicated that the melting temperature of α-Al decreased with increased matrix Cu content. A model for calculating distortion energy was proposed and its rationality verified. Calculation results showed that, with increased matrix Cu content, the distortion energy gradually increased, which was the main reason for decreased matrix melting temperature. Moreover, the existence of distortion energy was verified using the EBSD method.