Casting

The production of most useful metallic objects involves casting, whether in final form or as ingots that are later shaped as solids by rolling, extruding, or forging. Cast metal components include engine blocks and suspension parts for railcars, trucks, and autos; valves, pumps, faucets, pipes, and fitting equipment for drilling oil wells; surgical equipment and prosthetic devices; and components for household and electronic devices. Injection molding, which is a form of casting, is used to produce many polymer objects.

A number of considerations are important in casting. These include liquid-to-solid shrinkage that requires a reservoir or riser of liquid to prevent void formation; thermal shrinkage of the solid that must be accounted for in designing molds; thermal gradients that can cause warping and residual stresses; segregation of components in solution and gas evolution during freezing; and surface appearance. Technological advances in the past few decades have improved the quality and decreased the cost of castings. Computer analysis has allowed prediction and control of the flow of molten metal in the mold, the temperature profiles, and the position of the solid-liquid interface during solidification. The result is the possibility of elimination of internal voids. The use of styrofoam patterns has increased productivity by simplifying mold making.

Macrostructure of Castings

Typically the outside skin of a casting is composed of fine grains of random orientation. As freezing progresses inward, grains that are more favorably oriented for growth crowd out less favorably oriented grains and form columnar crystals.