Interaction of heat checks in aluminum pressure casting dies and their effect on fatigue life

摘要

Pressure casting dies are exposed to harsh service conditions consisting of cyclic thermal and mechanical loading and thus undergo thermo-mechanical fatigue. Due to cyclic plastic deformation of the material near the surface of the dies the loading conditions gradually change because of the formation of tensile residual stresses which add to the stress field from external loading. This change in the stress field influences the nucleation and the growth of cracks. Typically after a few thousand casting cycles a network of heat checks forms. In such a network crack shielding has a big influence on the evolution of the crack array. Firstly, it influences the propagation rates of the cracks and secondly it may change the propagation direction compared to the case where no neighbors are present. The crack growth rate and the length at which the cracks stop growing are also influenced by the thermo-physical and mechanical properties of the die material. It was found that the shielding effects of neighboring cracks are of equal importance. Crack deflection caused by the presence of neighboring cracks can lead to break-outs at the surface ensued by fast degradation eventually necessitating the replacement of the die. Consequently, the focus in this work is put on the investigation of the interaction of cracks in a network and their effect on the fatigue life. The problem is tackled by means of an automated strategy based on the finite element method.