Enhancing Microstructural Evolution and Mechanical Strength in Recycled AlSi7Mg Alloys Through Heat Treatment

Abstract

This study investigates the microstructural evolution and mechanical properties of AlSi7Mg alloy, processed via hot compaction and hot extrusion of recycled chips. The influence of the T6 heat treatment on the microstructure and strength was comprehensively analyzed. Hot compaction of AlSi7Mg alloy chips resulted in a microstructure featuring quasi-continuous β-Si-phase precipitates along the grain boundaries of the primary α-Al-phase, with heterogenous distribution. Subsequent T6 heat treatment refined the microstructure, reducing α-phase grain size, and promoting more regular distribution of Si eutectics, while preserving Fe-rich precipitates. The detailed microstructure analysis for the hot-extruded and heat-treated alloy revealed an increase in the prevalence of larger angle grain boundaries after heat treatment, while Si precipitates exhibited uniform dispersion facilitated by the elevated temperature. The heat treatment had the expected effect, causing an increase in hardness after hot extrusion from 51.9 HV0.5 to 82.3 HV0.5, which was attributed to the precipitation-strengthening effect. Furthermore, the investigation of the influence of heat treatment on the compression strength of the extruded material revealed a 67% increase due to heat treatment from 307.1 MPa to 513.4 MPa. These findings highlight the role of microstructural control in enhancing the performance of recycled aluminum alloys, offering valuable insights into sustainable materials processing.