On the characterization of microstructure and fracture in a high-pressure die-casting Al-10 wt%Si alloy

Microstructure and fracture behavior in a high-pressure die-casting Al-10 wt%Si alloy have been investigated using optical microscope (OM), scanning electron microscope (SEM) and a high-resolution laboratory computed tomography (CT). The results showed that a typical heterogeneous microstructure of the alloy comprised α-Al rich region, eutectic silicon band region and porosity. The microstructure patterns highly dependent on fluid convection and rapid solidification. Under high filling speed, externally solidified crystals (ESCs) and the growing dendrites migrated in center and formed α-Al rich region. Si particles was discharged and enriched in the final solidified liquid, forming eutectic silicon band. Hard Si particles and brittle Fe-rich phases served as obstacles prevented dislocation migration, causing local stress concentration. Due to large movable slip systems in α-Al rich region, the propagation path of the crack was greatly extended. Net-shrinkage that induced by dense impinging dendrites led to the microcracks along the boundary of dendrites which promoted intergranular fracture.