A356-T6 Aluminium Wheel Castings

Why A356 is the wheel alloy

A356 is a hypoeutectic aluminium-silicon-magnesium casting alloy. About 7 % silicon gives the melt the fluidity to fill rims and spokes; about 0.3 % magnesium forms Mg₂Si, which precipitates during ageing and doubles the yield strength. Tight limits on iron and copper keep the casting ductile and corrosion resistant.

In the trade, "wheel hub" is often used for the whole road wheel, and that is what this page covers: the cast rim-and-disc wheel that carries the tyre. Low-pressure die casting (LPDC) is the dominant process for original-equipment wheels because molten metal rises into the die from below, without turbulence, and solidifies directionally toward the feeding hub. Gravity casting is used for some aftermarket designs, and flow-formed wheels combine a cast centre with a rim that is spun and stretched to refine its grain and thin the wall.

A356 composition and equivalents (wt %)

A356.2 ingot is the usual charge for wheel foundries because its lower iron leaves room for the small pick-up during melting. Raising magnesium toward the top of the range increases strength and hardness but reduces elongation; copper and zinc lower ductility and corrosion resistance and are kept to a minimum. Melts are normally grain-refined with titanium-boron and modified with strontium so that the eutectic silicon becomes fine and fibrous instead of coarse plates.

Mechanical properties of A356-T6 wheels

Wheel producers usually specify properties on test bars cut from the wheel, with separate limits for rim and spoke. Use the values on your drawing or OEM standard for design.

Elongation is as important as strength. A wheel must deform rather than crack when it strikes a kerb, which is what the 13° impact test simulates. High iron, coarse silicon, oxide films and shrinkage porosity all cut elongation, so melt treatment and degassing matter as much as alloy chemistry.

T6 heat treatment of cast wheels

1. Solution treatmentTypically about 535 – 545 °C for 4 – 8 hours, enough to dissolve Mg₂Si and spheroidise the eutectic silicon.
2. QuenchInto water at about 60 – 80 °C within seconds of leaving the furnace; warm water limits distortion and residual stress in the rim.
3. Artificial ageingAbout 150 – 165 °C for 3 – 6 hours to precipitate the strengthening phase. Exact cycles are tuned to wheel mass and target properties.

After heat treatment the wheel is X-rayed, machined on the hub face, bolt holes and bead seats, leak-tested for tubeless sealing, balanced, and painted or diamond-cut and clear-coated.

Inspection and wheel test standards

• Radiography: every wheel or a defined sample is X-rayed; gas porosity, shrinkage and inclusions are graded against reference radiographs such as ASTM E155, with stricter limits in the spoke-to-rim junction and bead seats.
• Surface: no cold shuts, cracks, laps or unfilled areas; machined faces free of exposed porosity.
• Leak test: helium or air-under-water test of the rim for tubeless tyres.
• Dimensional: radial and lateral run-out, bolt-circle position and centre-bore diameter.
• Mechanical: test bars from the wheel for tensile and elongation, plus hardness on each heat-treatment batch.

Cast A356 wheels compared with forged 6061 wheels

Flow-formed wheels sit between the two: a cast centre with a work-hardened rim, saving several percent of weight over a fully cast wheel at a moderate cost increase.

Related products and applications

A356 aluminium (aluminum) wheel castings: FAQ