Influence of pore characteristics and eutectic particles on the tensile properties of Al-Si-Mn-Mg high pressure die casting alloy

Heterogeneous microstructure and unpredictable porosity distribution in Al-Si high-pressure die-castings give rise to a significant variation in their mechanical properties. This study investigated the influence of porosity distribution, volume of the largest pore, eutectic phase morphology and Mn-containing intermetallics on the tensile properties of Al-Si-Mn-Mg high-pressure die-castings. The as-cast material has a "network" of fine coralloid eutectic Si as well as blocky AlSiFeMn intermetallics in interdendritic area. The solution treatment step of the T7 heat treatment spheroidized the eutectic silicon. Tensile samples were tested in both the as-cast and T7 conditions. The yield strength (YS), ultimate tensile strength (UTS) and elongation (%E) of as-cast samples were in the ranges of 129-146 MPa, 242-282 MPa and 2.6-4.7%, respectively. After T7 heat treatment the YS, UTS and %E had ranged from 124-142 MPa, 195-207 MPa and 4.6-12.1%, respectively. The T7 heat treatment had little effect on the YS but reduced the UTS considerably. The enhanced %E can be ascribed to the spheroidization of eutectic Si. The largest pore by volume in the sample also plays an important role in determining the ductility according to three-dimensional (3D) pore morphologies obtained via computed tomography (CT). The sample with largest pore by volume has the lowest %E, indicating the %E can be estimated prior to tensile test based on the size of the largest pore by volume in the sample.