Characterization of externally solidified crystals in a high-pressure die-cast AlSi10MnMg alloy and their effect on porosities and mechanical properties

The characterization of externally solidified crystals (ESCs) and porosities in a high-pressure die-cast AlSi10MnMg alloy was investigated using optical microscope (OM) and computed tomography (CT) with particular attention on the effect of shot speeds on the ESCs and fracture behavior. Results showed that ESCs tended to agglomerate at the center region and changed from fine globular grains into large size dendrites from surface to center. The formation of the ESCs is found to be coarser as the "slow shot speed" in the first stage of high-pressure die casting (HPDC) or the "fast shot speed" in the second stage of HPDC is reduced, consequently, the corresponding ESC area fraction increased. From the fractured morphology, the plate specimen with a higher ESC content exhibited a rough fracture and a large volume fraction of shrinkage porosities was discovered beneath the fracture surface especially when a lower fast shot speed was applied. In in-situ tensile test, the large size shrinkage porosity served as a crack source and it further expanded in an inter-granular mode along ESC boundaries/other porosities (which caused a tortuous crack route) or in a trans-granular mode across the ESC grains/fine (α-Al) II grains (which caused a flat crack route). In addition, the gas porosities in the specimen promoted the crack propagation as a crack concentration area and experienced almost no deformation during the tensile process.