Microstructure and properties of spinning deformed A356 alloy subject to the solution-DCT-aging multiplex heat treatment

The effect of DCT time (1 h,12 h, 24 h, 36 h and 48 h) on the microstructure and mechanical properties of spinning deformed A356 alloy subject to solution-DCT-aging multiplex heat treatment (hereinafter referred to as “DCT multiplex heat treatment”) was studied. The results demonstrate that the best DCT time of multiplex heat treatment is 36 h, which is the critical time for the recrystallization process. Then the recrystallization texture will appear in the 48 h DCT sample. The equal-area-circle diameter of α-Al matrix and Si phase are 24.8 μm and 1.5 μm respectively after 36 h DCT, which are 53.3% and 53.1% smaller than those after sole solution-aging treatment. After 36 h DCT, the tensile strength, yield strength, elongation and microhardness of A356 alloy arrive at the maximum simultaneously as 302.21 MPa, 272.93 MPa, 20.37% and 98.8HV separately, which are increased by 6.8%, 4.1%, 24.0% and 3.9% in comparison to the sole solution-aged sample. Short-term DCT (<36 h) can increase the residual stress and prompt the dislocation density, while long-term DCT (>36 h) is diametrically opposite. The crystal plane of DCT-ed α-Al matrix rotates towards the (200) crystal plane, and accelerates the formation of nanometer Mg2Si precipitates in the matrix, and twin structure can be observed in the Si crystal. The main strengthening and toughening mechanisms of DCT alloy concerns with fine grain strengthening, precipitation strengthening, dislocation strengthening and twin strengthening.