Mechanism of cryogenic, solid solution and aging compound heat treatment of die‐cast Al alloys considering microstructure variation

Abstract

Abstract To develop a new compound heat treatment process for improving the mechanical properties of die‐cast Al alloys, this study investigated the effects of cryogenic, solution and aging compound treatment on the microstructure and mechanical properties of die‐cast Al alloys. The characterization methods used were optical microscopy (OM), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron backscatter diffraction (EBSD), and tensile tests; and the Image Pro Plus software was used for statistical analysis. The results indicated that compared with T6 heat treatment, the compound heat treatment process consisting of cryogenic treatment (−196 °C for 12 h), solid solution treatment (476 °C for 22 min), and aging (159 °C for 403 min) significantly enhanced the mechanical properties of the die‐cast Al alloys. For instance, the tensile strength increased from 224.3 to 249.5 MPa; the hardness increased from HV 110.5 to HV 124.6, and the elongation increased from 6.28% to 7.72%, which in relative terms corresponds to 11.2%, 12.8% and 22.9%, respectively. The compound heat treatment process of the alloy led to significant refinement of its α‐Al phases. In addition, Si phases tended to be more ellipsoidal or granular, while the tips of Fe‐containing phases became rounded, which played a key role in enhancing the mechanical properties and microstructure stability of the alloys.