Effect of pouring and cooling temperatures on microstructures and mechanical properties of as-cast and T6 treated A356 alloy

The A356 castings were fabricated using a well-developed temperature controlled permanent mold. To improve the strength and hardness of cast A356, the microstructures and mechanical properties of as-cast and T6 heat treated A356 alloy with various mold and pouring temperatures were studied. The results reveal that the undercooling is closely related to the mold and pouring temperatures. As the mold/pouring temperature changed from 258 °C / 680 °C and 270 °C / 680 °C to 288 °C / 650 °C, the in-situ undercooling is 12 °C, 17 °C and 11 °C, respectively. It is observed that the Si phase changes from long continuous flake to discontinuous globular-fibrous morphology after T6 heat treatment as the mold and pouring temperature is 270 °C / 680 °C, and the T6 heat treated specimens exhibit better mechanical properties in comparison to those as-cast ones with an increase of 162% and 102% in yield strength and elongation, which are 34.6% and 190% higher than the ASTM B108-03a standard, respectively. As a result, the tensile fracture morphology of the as-cast A356 alloy shows quasi-cleavage fracture and the T6 heat treated A356 alloy shows ductile fracture.