Manufacturing processesReading time 11 min

Casting and forging differ by more than cost

The same part cast or forged can differ twofold in strength, and the cost structures are nothing alike.

01The fundamental difference

	Castings	Forgings
State of the material	Melted, then poured into a cavity	Deformed under pressure while solid
Grain flow	No direction	Continuous along the profile; high strength
Internal defects	Shrinkage, porosity, inclusions	Laps, cracks (less common)
Freedom of shape	High; hollow sections and complex cavities possible	Low; limited by tooling and material flow
Typical accuracy	CT8 ～ CT12	Better, but still needs machining
Minimum wall thickness	3 to 5 mm in sand casting	Limited by forging ratio
Tooling cost	Low for sand, high for die casting	High

Grain flow in a forging follows the profile, which is why parts under cyclic stress are almost always forged.

02Casting: which mould

Method	Materials	Accuracy	Batch	Character
Sand casting	Iron, steel, copper, aluminium	Low	1 to a few thousand	Cheap tooling, rough surface
Investment casting	Almost any alloy	High (CT5 to CT7)	Hundreds upward	Thin-walled complex parts, good surface
Die casting	Aluminium, zinc, magnesium	High	Tens of thousands	Fast cycles, but porosity prevents heat treatment
Gravity casting	Aluminium	Medium	Hundreds to thousands	Denser structure than die casting
Low-pressure casting	Aluminium	Medium-high	Thousands	Structural parts such as wheels

Die castings trap gas inside, which blisters on heating, which is why they are generally not T6 treated.

Clues on a casting drawing

Draft angles, usually 1° to 3°, are the most obvious sign.
Fillets everywhere two faces meet, to avoid shrinkage and stress concentration.
Uniform wall thickness; abrupt changes produce shrinkage cavities.
Machining allowance, shown as two outlines: as-cast and as-machined.
Parting line and gate positions, often with a note that no remnant may be left.

03Forging: hot, warm and cold

Device	Temperature	Accuracy	Typical use
Hot forging	Above the recrystallisation temperature	Low; needs machining	Large parts, complex shapes
Warm forging	In between	Medium	A compromise
Cold forging	Room temperature	High; near net shape	Screws, fasteners, small parts

Work hardening in cold forging raises strength, but the amount of deformation is limited and intermediate annealing is often needed.

Forging ratio, the ratio of cross-sectional area before and after deformation, is the measure of forging quality. A ratio of 3 or more is generally required to break down the cast structure and refine the grain. If a drawing calls out a forging ratio or a grain flow direction, it is a strength part.

04Defects and inspection

Defect	Common in	Detection
Shrinkage cavity / porosity	Thick sections of castings	X-ray, ultrasonic
Gas porosity	Castings, die castings	X-ray, sectioning
Inclusions	Castings	X-ray, magnetic particle
Lap	Forgings	Magnetic particle, dye penetrant
Forging crack	Forging surface	Magnetic particle, dye penetrant
Decarburised layer	Hot forging surface	Metallographic section, hardness traverse

Castings are mostly checked internally by X-ray; forgings mostly on the surface by magnetic particle and penetrant.