No abstract

A magnesium alloy was adopted to an evaporative pattern casting (EPC) process to combine advantages of each. In the present study, foam patterns were cast at a top gating system under atmospheric and reduced pressure to evaluate casting characteristics of AZ91D to develop a complete EPC process for high productivity of magnesium alloy castings. Filling time and temperature of molten metal were measured during mold filling. It was recognized that the average filling velocity was affected by the difference of the coating material and the degree of reduced pressure. The grain size of the magnesium alloy was slightly dependent on the degree of reduced pressure. It was considered that the application of the high reduced pressure, which changed the shape of melt surface from convex to concave, was related to the occurrence of internal defects such as cold shut laps or folds in the casting.

Traditional fatigue fracture theory and practice focus principally on structural design. It is thus too conservative and inappropriate when used to predict the high-cycle fatigue life of dies used for metal forming, especially cold forging. We propose a novel mean stress correction model and diagram to predict the high-cycle fatigue lives of cold forging dies, which focuses on the upper part of the equivalent fatigue strength curve. Considering the features of die materials characterized by high yield strength and low ductility, a straight line is assumed for the tensile yield line. To the contrary, a general curve is used to represent the fatigue strength. They are interpolated, based on the distance ratio, when finding an appropriate equivalent fatigue strength curve at the mean stress and stress amplitude between the line and curve. The approach is applied to a well-defined literature example to verify its validity and shed light on the characteristics of die fatigue life. The approach is also applied to practical forging and useful qualitative results are obtained.

No abstract