Fatigue Mechanism of Internal Casting Defect-Originating Fracture of Aluminum Alloy Die Casting

Abstract

The mechanism of internal casting defect-originating fracture of aluminum alloy die-casting in the ultra-high cycle regime was studied from the observation of fracture surface and cross section perpendicular to fracture surface. The main results were as follows : (1) Fine irregular fracture surface around the crack initiation point was clearly observed after 108 cycles, and the boundary between porosity and fine irregular fracture surface was not clear beyond 109 cycles, and a partial chip of spherical porosity was observed at crack initiation point. (2) From SIM observation of cross section perpendicular to fracture surface, fine structure of nanocrystalline was observed at fine irregular fracture surface and fine structure of nanocrystalline was not limited to fine irregular fracture surface. Fine structure of nanocrystalline was also observed at the vicinity of fine irregular fracture surface. (3) The boundary between fine structure of nanocrystalline and matrix structure was clear. This feature was similar to the feature of fine structure of nanocrystalline obtained from the severe plastic deformation process. (4) From the observation of fracture surface and cross section perpendicular to fracture surface, it seems to be that the contact between the fracture surface could occur at the crack initiation point and around the crack initiation point of the internal casting defect-originating fracture, and the repetition of the contact between the fracture surface seems to influence the formation of fine structure of nanocrystalline.