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

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 diecast Al alloys.For instance,the tensile strength increased from 224.3 to 249.5 MPa;the hardness increased from HV110.5 to HV 124.6,and the elongation increased from6.28%to 7.72%,which in relative terms corresponds to11.2%,12.8%and 22.9%,respectively.The compound heat treatment process of the alloy led to significant refinement of its a-Al phases.In addition,Si phases tended to be more ellipsoidal or granular,while the tips of Fecontaining phases became rounded,which played a key role in enhancing the mechanical properties and microstructure stability of the alloys.