Microporosity in cast alloys: simple considerations on its formation

A few examples are chosen on the basis of the significance of the effects of microporosity on the solidification or post-solidification behaviour and properties of cast products. The paper begins with a short survey of the main casting variables that affect the formation of microporosity in different types of cast alloys: aluminium-based alloys, nickel-based alloys in investment casting, and spheroidal graphite cast iron. The second part summarises the most useful and comprehensive physical ideas that allow understanding of the formation of microporosity for various cast alloys and casting processes. Three main causes of microporosity formation are pointed out, i.e. local chemistry of liquid, local dendritic microstructure, and local liquid pressure within the mushy zone. In the last part, emphasis is put on the influence of another important cause of microporosity: the mechanical behaviour of the solid in the mushy zone during solidification. Similarities and differences between microporosity and hot tearing are tentatively drawn.