Quick Answer
A356 is the default structural aluminum casting alloy — roughly 7% silicon for castability, 0.3% magnesium for heat-treatability. In the T6 condition it reaches about 230–280 MPa tensile with good elongation, welds and machines cleanly, and takes pressure tightness well. Wheels, suspension parts, pump housings, valve bodies and structural brackets all default to A356 unless something specific pushes them elsewhere.
Composition and Why It Works
| Element | Range | Role |
|---|---|---|
| Silicon | 6.5–7.5% | Fluidity, low shrinkage — the castability engine |
| Magnesium | 0.25–0.45% | Forms Mg₂Si for T6 strengthening |
| Iron | ≤ 0.20% (A356) | Kept low for ductility — the key difference from 356 |
| Titanium | ≤ 0.20% | Grain refinement |
The “A” prefix means tighter iron control than plain 356 — iron forms brittle needles that cut fatigue life, so structural and safety parts specify A356 rather than 356.
T6: Where the Strength Comes From
As-cast A356 is modest (~160 MPa). The T6 cycle — solution treat ~540°C, water quench, age ~155°C — precipitates fine Mg₂Si and lifts properties to:
| Condition | Tensile (typical) | Yield | Elongation |
|---|---|---|---|
| As-cast (F) | ~160–180 MPa | ~90 MPa | 5–8% |
| T6 | 230–280 MPa | 165–210 MPa | 3–10% |
Actual numbers depend on process (permanent mold beats sand for properties) and section thickness. Specify “A356.0-T6” plus the test-bar type your standard requires.
Process Fit
- Gravity die casting: the natural home — repeatable structural parts at medium volume with good properties
- Low-pressure casting: wheels and pressure-tight housings; quiet filling means low porosity
- Sand casting: large or low-volume parts; slightly lower properties from slower cooling
- Not high-pressure die casting — HPDC uses ADC12/A380 class; entrained gas makes those parts non-heat-treatable
A356 vs the Alternatives
- vs ADC12/A380 (die casting): A356-T6 is stronger per gram, heat-treatable, weldable and pressure-tight; ADC12 wins on thin walls and high-volume piece price
- vs 6061 (wrought): similar strength class; choose by geometry — complex one-piece shape favors casting, plate-and-machine favors 6061; our cast vs wrought aluminum guide covers this decision
- vs A357: higher magnesium for ~10–15% more strength in aerospace-class parts, at tighter process control
FAQ
Is A356 weldable?
Yes, readily (4043/5356 filler). Weld zones lose T6 strength locally; re-aging restores most of it — plan the sequence for structural welds.
Does A356 corrode?
It resists atmospheric and fresh-water corrosion well. For marine service, protect or anodize it — and watch galvanic pairs with steel fasteners.
What surface finish can I expect?
Gravity/low-pressure castings: smooth enough for paint or machining datum faces directly; class-A cosmetic surfaces need polishing or coating.