Manufacturing processesReading time 11 min

Cast or Forged? Choose by Load and Volume

Casting and forging can produce similar shapes, but not the same material structure or economics. Compare strength, tooling cost, machining allowance and production volume before choosing.

01How the processes differ

	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

Forging can align grain flow with the part geometry, which may improve fatigue performance when the design uses it well.

02Choosing a casting process

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

Entrapped gas can cause trouble during later heating; confirm whether the casting and treatment route are compatible.

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.

03Hot, warm and cold forging

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

Cold forging can raise strength through work hardening, but forming limits and intermediate annealing depend on the part.

For a strength-critical forging, the drawing may specify forging ratio or grain-flow direction. Forging ratio compares the cross-sectional area before and after deformation; it is one indicator of how thoroughly the cast structure has been worked. Read it together with the required load direction and heat treatment.

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

Inspection methods depend on the defect sought, material and process; neither process has one universal test.