Setting of Natural Fracture Splitting Surface on Connecting Rod and Its Formation Mechanism

Abstract

To break through the limitation of fracture splitting process on material selection and solve problems during fracture splitting such as parts tearing, failing to split, dropping dregs, fracture surface deformation and so on, a new technique of setting natural splitting surface in casting blank is proposed, aiming to achieve brittle fracture along pre-set surface during fracture splitting process. In this research, casting blanks are produced with metal molds. A layer of AZ31 foil is set in advance before casting, the layer interacts with liquid LD10 aluminum alloy, forming a brittle interface layer across the whole casting, then a fracture splitting hole is machined in the middle of the casting blank and cracking grooves are machined on the inner hole near the interface to achieve fracture splitting. Experiment revealed that the initial crack on the specimen starts from the root of the cracking groove, and the crack basically expands along the pre-set fracture splitting surface. The fracture surface is characterized by flaky brittle fracture. There is residual magnesium and pellumina, which have characteristics of melt with low-melting point, and micro-porosity in the fracture. Further analysis suggests that the formation mechanism of a natural fracture splitting surface can be described as follows: the magnesium foil with strong oxidation in high-temperature alloy liquid interacts with the pellumina at the front of liquid flow, which forms a interface. Meanwhile a layer of melt with a low-melting point forming as a result of interface reaction is pushed to the edge of the grain boundary, and surface liquid film shrinks to be micro-porosity. With such a combined effect it finally forms the brittle surface, which provides the condition for conducting subsequent fracture splitting process.