A356 is one of the most used aluminum casting alloys in engineering. It shows up in automotive structural parts, aerospace components, pump housings, and anything else where the combination of good castability, heat-treatability, and corrosion resistance matters. If you are evaluating materials for a cast aluminum part — or your supplier has recommended A356 — this guide covers what you actually need to know.
What Is A356 Aluminum?
A356 is an aluminum-silicon-magnesium alloy (Al-Si-Mg family) developed specifically for casting. The "A" prefix in the designation indicates a tighter control on impurities compared to the base 356 alloy — particularly lower iron content — which results in better mechanical properties and casting consistency.
It is classified under UNS A03560 and is recognized by the Aluminum Association as a standard casting alloy. The key engineering advantage of A356 is its response to heat treatment: in T6 condition, it achieves a combination of tensile strength, ductility, and corrosion resistance that few other casting alloys can match.
Chemical Composition
A356 aluminum alloy chemical composition per Aluminum Association standards:
| Element | Content | Role |
|---|---|---|
| Aluminum (Al) | 91.1 – 93.3% | Base metal |
| Silicon (Si) | 6.5 – 7.5% | Improves fluidity, reduces shrinkage, lowers hot cracking tendency |
| Magnesium (Mg) | 0.20 – 0.45% | Primary strengthening element; enables precipitation hardening in T6 |
| Iron (Fe) | ≤0.20% | Controlled impurity; higher Fe reduces ductility |
| Copper (Cu) | ≤0.20% | Controlled impurity; low Cu preserves corrosion resistance |
| Manganese (Mn) | ≤0.10% | Trace |
| Zinc (Zn) | ≤0.10% | Trace |
| Titanium (Ti) | ≤0.20% | Grain refiner; improves ductility and strength consistency |
The low iron limit (≤0.20%) is what distinguishes A356 from standard 356. Iron forms brittle intermetallic phases in aluminum-silicon alloys. Keeping it below 0.20% prevents these phases from forming in quantities that compromise elongation and fatigue performance.
Mechanical Properties
A356 properties vary significantly by casting process and heat treatment condition. The following table covers the most common tempers:
| Condition | Tensile Strength | Yield Strength | Elongation | Hardness (HB) |
|---|---|---|---|---|
| As-cast (F) | ~160 MPa | ~80 MPa | 3 – 6% | ~55 |
| T5 (artificial age only) | ~185 MPa | ~130 MPa | 3 – 5% | ~65 |
| T6 (solution + age) — Sand Cast | 234 – 262 MPa | 165 – 186 MPa | 3 – 5% | 70 – 85 |
| T6 — Permanent Mold | 255 – 310 MPa | 180 – 230 MPa | 3 – 6% | 70 – 100 |
| T61 (modified T6) | ~250 MPa | ~200 MPa | 5 – 8% | ~75 |
| T7 (overaged) | ~235 MPa | ~190 MPa | 4 – 7% | ~70 |
T6-treated permanent mold castings typically achieve the best combination of strength and ductility. The elongation values (3–6%) indicate meaningful ductility — the alloy can absorb load without sudden fracture, which matters for structural and fatigue-loaded applications.
Density is approximately 2.67 – 2.68 g/cm³, consistent with most aluminum-silicon alloys. Thermal conductivity is around 150 W/m·K in T6 condition.
A356 vs. A356.2: What's the Difference?
A356.2 is a higher-purity sub-grade of A356 with even tighter limits on iron and other trace elements. The practical effect: better elongation and fatigue resistance compared to standard A356. It is the preferred specification for aerospace structural castings and high-cycle fatigue applications such as automotive wheels and suspension components. A356.2 costs more due to tighter incoming material controls, but the mechanical property premium is real.
Casting Processes: Where A356 Fits
A356 is primarily used with three casting processes. The choice affects not just cost, but the mechanical properties you actually get from the finished part.
Sand Casting
Sand casting is the most flexible and lowest-tooling-cost process for A356. Slow solidification gives the alloy time for gas to escape, resulting in low porosity — which is why A356 sand castings respond well to T6 heat treatment. Used for large, complex shapes, prototypes, and low-to-medium production volumes. As-cast surface finish is rough (Ra 6–12 µm typical), so machining of functional surfaces is normally required.
Permanent Mold Casting (Gravity Die Casting)
Permanent mold casting produces better mechanical properties than sand casting because faster cooling from the metal mold creates finer grain structure and lower porosity. T6 heat treatment on permanent mold A356 parts typically achieves the best strength-elongation balance. This is the dominant process for A356 in automotive structural components — control arms, knuckles, brackets — where fatigue performance matters.
Low-Pressure Die Casting (LPDC)
LPDC feeds molten aluminum from a sealed crucible through a riser tube into the mold under low pressure (typically 0.03 – 0.15 MPa). This controlled fill significantly reduces turbulence and oxide entrapment compared to gravity casting. The result is high-density, low-porosity castings that respond reliably to T6 heat treatment. LPDC is the standard process for aluminum alloy wheels worldwide — most are A356-T6 — and is increasingly used for EV battery housings and structural underbody components where both strength and pressure tightness are required.
High-Pressure Die Casting (HPDC): A Caution
A356 can be cast by HPDC, but it is generally not the right choice for this process. High-pressure injection creates turbulent filling that entraps air and oxide films inside the casting. These pockets expand during solution heat treatment (typically around 538°C), causing blistering or dimensional distortion. For this reason, A356-T6 parts produced by HPDC are rare in practice. If your application requires both HPDC and T6 heat treatment, discuss this explicitly with your casting supplier — some vacuum-assisted HPDC processes can achieve this, but it requires controlled process conditions and is not standard.
T6 Heat Treatment: What It Does and How It Works
T6 is the heat treatment condition that unlocks A356's full mechanical potential. Without T6, A356 performs similarly to many generic aluminum alloys. With T6, tensile strength can increase by 50–80% over the as-cast condition.
The T6 process for A356 consists of two stages:
- Solution heat treatment: The casting is heated to approximately 538°C ±6°C and held for 4–12 hours. At this temperature, the magnesium and silicon dissolve into a uniform solid solution in the aluminum matrix. Time at temperature depends on casting wall thickness and section geometry.
- Quenching: The casting is rapidly cooled — typically into hot water (65–100°C) or a polymer quench solution. This locks the magnesium and silicon in supersaturated solution, preventing them from precipitating out prematurely.
- Artificial aging: The casting is reheated to 155°C ±5°C for 2–5 hours (per ASTM B917). During aging, fine Mg₂Si precipitates form throughout the aluminum matrix. These nanoscale particles block dislocation movement, which is the mechanism that produces high strength.
T5 heat treatment skips the solution step — the casting goes directly to aging after cooling from casting temperature. This is faster and cheaper, but results in lower strength than T6 because the precipitates are coarser and less uniformly distributed.
T7 (overaging) uses longer aging time or higher temperature than T6. Strength is slightly lower than T6 peak, but dimensional stability and stress corrosion resistance are better. Some aerospace applications specify T7 for this reason.
A356 vs. A380: Choosing the Right Alloy
This is the most common alloy selection question in aluminum casting. A356 and A380 are both widely used, but they are designed for different processes and applications:
| A356 | A380 | |
|---|---|---|
| Alloy family | Al-Si-Mg | Al-Si-Cu |
| Casting process | Sand, permanent mold, LPDC | HPDC (primary); sand casting |
| Heat treatable | Yes — T5, T6, T7 | Limited — T5 only in HPDC |
| Tensile strength (T6) | 255 – 310 MPa | ~320 MPa (as-cast HPDC) |
| Elongation | 3 – 6% (better ductility) | ~3.5% (as-cast) |
| Corrosion resistance | Excellent | Moderate (copper reduces resistance) |
| Weldability | Good | Poor (copper content) |
| Surface finish (as-cast) | Moderate (sand/PM) to good (LPDC) | Excellent (HPDC smooth surface) |
| Best for | Structural, safety-critical, fatigue-loaded parts | Complex geometry, thin walls, high-volume HPDC production |
The short version: use A356 when the part needs heat-treated strength, good corrosion resistance, or downstream welding. Use A380 when the priority is HPDC thin-wall geometry, production efficiency, and as-cast surface finish.
At Meituo, we verify alloy composition by spectrometer before every production batch. For structural and safety-critical applications — where alloy purity directly affects fatigue performance — this incoming material verification is a non-negotiable part of our process.
Common Applications
A356 appears across industries wherever the combination of heat-treatable strength, castability, and corrosion resistance is needed:
- Automotive: Aluminum wheels (most are A356.2-T6 by LPDC), suspension knuckles, control arms, engine brackets, structural crossmembers
- Aerospace: Engine control components, airframe brackets, impellers, structural fittings — applications that require both precision casting and certified mechanical properties
- Industrial machinery: Pump housings, impellers, compressor bodies, valve housings — applications requiring pressure tightness and corrosion resistance in fluid contact
- Power equipment: High-velocity blowers, generator housings, motor end bells
- EV and electrification: Battery enclosure components, power electronics housings — where corrosion resistance and pressure tightness are engineering requirements
Machinability, Weldability, and Surface Finishing
Machinability
A356-T6 machines well with carbide tooling. The silicon content (6.5–7.5%) is abrasive, so tooling life is shorter than with wrought aluminum alloys like 6061, but manageable with appropriate cutting parameters. Surface finish of Ra 0.8 – 1.6 µm is achievable on machined surfaces without difficulty.
Weldability
A356 is weldable — TIG welding with 4047 or 4043 filler is the standard approach. The low copper content is important here: copper in alloys like A380 significantly reduces weld quality and increases cracking tendency. For structural weld repair or assembly welding, A356 is the preferred casting alloy. Post-weld heat treatment to T6 is possible but requires controlled conditions to avoid distortion.
Surface Finishing
A356 accepts most common aluminum surface treatments:
- Anodizing: Good results, uniform oxide layer. Silicon particles in the alloy can create slight gray toning in the anodized film — not a concern for functional anodizing, but relevant for decorative applications where color consistency matters.
- Powder coating and painting: Standard processes apply without issue.
- Chromate conversion: Common for corrosion protection in industrial applications.
- Electroplating: Possible but requires careful surface preparation due to silicon content.
Working with A356 in OEM Programs
For OEM sourcing teams specifying aluminum casting parts in A356, a few practical notes:
- Always specify the temper alongside the alloy: A356-T6, A356.2-T6, A356-T5, etc. The temper determines the mechanical properties you actually receive.
- Specify the casting process in your drawing or RFQ. A356 sand cast and A356 permanent mold cast will have different mechanical properties even in the same temper — this matters for fatigue-loaded structural parts.
- For safety-critical applications (suspension, structural), require material certification with chemical composition report and mechanical test results per ASTM B108 (permanent mold) or ASTM B26 (sand casting).
- Incoming alloy verification by spectrometer at the casting supplier is a basic quality expectation. Ask whether this is standard practice.
Meituo's team can support material selection discussions for aluminum casting programs — if you are deciding between A356, A380, ADC12, or other alloys for your application, contact us with your part requirements and we'll provide a recommendation with technical rationale.
