Casting

Molten metal is poured or forced into a mould and solidifies in its shape. It is the most direct route from metal to component, it makes internal cavities and complex geometry that no other process reaches, and it is the only economic way to produce a shape like an engine block.

What it produces is a solidification structure, and that is the whole of its metallurgy. A casting has the grain structure the cooling gave it — coarse where it cooled slowly, fine where it cooled fast — plus whatever porosity the shrinkage and the dissolved gas left. Every mechanical property follows from that, which is why a cast part is generally less strong and much less ductile than the same alloy forged.

Not every alloy casts. Good castability means a low melting point, a narrow freezing range and low shrinkage, and it is a different property from strength — which is why the alloys used for casting and for wrought products are usually different alloys entirely.

Sand casting for large and low-volume work — engine blocks, pump housings, machine bases — where a mould is destroyed with each part and the tooling is a pattern rather than a die. Die casting for high volume in zinc, aluminium and magnesium, forcing metal into a steel die under pressure at cycle times of seconds. Investment casting, where a wax pattern is coated in ceramic and melted out, for the accuracy and surface finish that turbine blades and surgical instruments need.

Continuous casting for the semi-finished forms — slab, bloom and billet — that rolling and forging start from. Almost all steel and aluminium now begins this way rather than as an ingot.

Older than writing. Copper was cast in the fifth millennium BC and bronze from the fourth, and the lost-wax process is at least six thousand years old — the same principle as modern investment casting, and for the same reason.

Iron casting requires far higher temperatures and arrives much later in the west, though China was casting iron by the fifth century BC, roughly seventeen hundred years before Europe managed it. The blast furnace made cast iron ordinary in Europe from the fifteenth century, and it is what made cannon, cooking pots and eventually the structural iron of the industrial revolution.

It is the cheapest route to a complex shape and remains so. A cast component that would need dozens of machining operations from solid is the standard argument, and it holds from a door handle to a ship's propeller.

Continuous casting is the quieter economic story: introduced widely from the 1960s, it removed a whole reheating and rolling stage from steelmaking and is one of the larger single efficiency gains in the industry's history.