On the porosity induced by externally solidified crystals in high-pressure die-cast of AM60B alloy and its effect on crack initiation and propagation

The porosity induced by the externally solidified crystals (ESCs) in high pressure die casting of AM60B magnesium alloy was investigated. Attention was focused on the formation of the porosity due to the presence of ESCs and its effect on the crack initiation and propagation during tensile deformation. Based on experimental observations using optical microscopy, scanning electron microscopy and the 3-D high resolution X-ray tomography, it was found that the porosity induced by ESCs could be categorized into two types. The first type of porosity was relatively small in size, globular in geometry and dispersed inside the microstructure, whereas the second type was larger in size with an interconnected structure. During the in situ observation of tensile deformation, significant crack could be observed at grain boundaries where the second type of porosity was present. The crack propagated by connecting the neighboring porosities towards a direction perpendicular to the tensile stress. The interdendritic shrinkage during solidification at the ESC boundaries was shown to be the primary reason for the formation of the second type of porosity