In the absence of gross defects such as cold shuts, the fatigue properties of castings are largely determined by the sizes of microstructural defects, particularly pores and oxide films. In contrast, the effects of grain size, second-phase particles and non-metallic inclusions are insignificant. We review the fatigue properties of castings made by gravity die casting, sand casting, lost-foam casting, squeeze-casting and semisolid casting, and compare 356/357 alloys with 319-type alloys. The application of fracture mechanics enables the properties to be rationalized in terms of the defects characteristic of each casting process, noting both the sizes and types of defect. The differences in the properties of castings are entirely attributed to their different defect populations: no single process is inherently superior. Four types of defects are recognized: thick and thin oxide films, and gas and shrinkage pores. For defects of the same size (in terms of projected area), oxide films have the smallest effect on fatigue life and pores, the greatest. Suggestions for further work are made.
展开▼
