Casting produces a metal part by filling a shaped cavity with liquid metal and allowing it to solidify.

The method can form geometries that would be difficult or wasteful to create by removing material from a solid block.

Basic process

A generic casting sequence is

melt alloy
   ↓
prepare mould
   ↓
fill cavity
   ↓
solidify and cool
   ↓
remove casting
   ↓
trim and finish

The mould can be expendable, as in many sand-casting processes, or reusable, as in permanent-mould and die-casting processes.

Filling the mould

Liquid metal must reach the required regions before it freezes. Filling behavior depends on geometry, temperature, flow resistance and the way metal enters and vents the cavity.

Poor filling can produce incomplete regions such as misruns or cold shuts.

Solidification

As heat leaves the liquid metal, solid material nucleates and grows. Cooling conditions influence grain structure and therefore material properties.

Solidification is not simply a geometric shrinking of a perfectly uniform liquid. Alloys can pass through a temperature range containing both liquid and solid phases, and solute can redistribute as the microstructure develops.

Shrinkage

Most metals contract during cooling and solidification. If liquid metal cannot feed regions that are still shrinking, voids or shrinkage porosity can form.

Process design therefore tries to control where solidification progresses and how remaining liquid can supply contracting regions.

Gas and inclusions

Gas dissolved or entrained in the melt can form pores as pressure and solubility change. Oxides, slag and other nonmetallic material can become inclusions if melt handling and filling are poorly controlled.

Process, structure and properties

Casting connects manufacturing to materials science:

process conditions
      ↓
solidification and defects
      ↓
microstructure
      ↓
properties and performance

Selecting a casting process therefore requires considering not only whether the cavity can be filled, but also dimensional accuracy, surface finish, production volume, tooling cost and the required integrity of the final material.