A356 is an aluminium-silicon-magnesium casting alloy commonly evaluated for heat-treatable gravity, permanent-mold, low-pressure and sand-cast parts. The designation alone does not guarantee one strength, elongation, porosity level or dimensional capability. Casting route, section thickness, local solidification, heat treatment, specimen location and acceptance specification all matter.
A356 and A356-T6: alloy versus condition
A356 identifies the alloy; T6 identifies a heat-treatment condition involving solution treatment and artificial aging. They are not competing alloy names. Compare a property value only after matching the alloy, casting route, temper and specimen basis. A test-coupon result should not be presented as the measured strength of every wall in a finished housing.
For heat-treatment scope, see ASTM B917/B917M. Furnace settings and acceptance values must come from the approved process and controlling specification, not a generic online recipe.
| Question about A356 properties | What to compare before choosing a value |
|---|---|
| Tensile or yield strength | Temper, material specification, specimen location and lot |
| Elongation | Same specimen basis and heat-treatment condition as the strength result |
| Hardness | Test method, location and agreed acceptance range |
| Pressure tightness | Finished-part leak-test method and acceptance; not inferred from tensile strength |
| Dimensional stability | Machining and heat-treatment sequence plus inspection of the final delivery state |
If you are selecting a supplier rather than researching properties, use the A356 casting material and product overview. For drawing limits, see the casting tolerance guide.
Cooling rate and microstructure
Metal molds generally remove heat faster than sand molds. Higher local cooling rates can reduce secondary dendrite-arm spacing and refine the as-cast structure. That is the correct direction of the relationship.
It is not reliable to claim that gravity casting produces a finer structure than high-pressure die casting simply because gravity filling is slower. HPDC differs in fill velocity, pressure, alloy choice, section thickness, gas entrapment and heat-treatment constraints. Compare the actual alloy-process-temper combination and the functional requirement instead of using one cooling-rate slogan.
Use the right standard for the right job
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| Document | What it controls |
|---|---|
| ASTM B108/B108M | Material specification for aluminium-alloy permanent-mold castings |
| ASTM B26/B26M | Material specification for aluminium-alloy sand castings |
| ASTM B557/B557M | How tensile tests are performed on wrought and cast aluminium- and magnesium-alloy products |
| ASTM B917/B917M | Heat-treatment practice for aluminium-alloy castings from multiple casting processes |
ASTM B557 is a tensile test method. It is not the source of a universal A356 property table. Mechanical-property requirements should come from the controlling material specification, drawing, purchase order and agreed specimen basis. B557 then defines how the tensile test is performed.
Why a property number needs context
Before putting tensile or yield values on a drawing, define:
- •casting process and applicable material specification;
- •alloy designation and temper;
- •whether acceptance uses separately cast coupons, specimens cut from the casting, or both;
- •specimen orientation and location when cut from a part;
- •section thickness and heat-treatment lot representation;
- •required test frequency and retest rules;
- •whether the property is a design value, qualification value or lot-acceptance value.
Results from a standard tensile specimen may not represent every local section of the finished component. ASTM's own B557 description warns that specimens from selected portions may not fully represent the complete end product or service behaviour.
Sampling follows the specification, not a universal weight rule
There is no defensible universal 500 kg-based sampling rule for every A356 project. Sampling frequency must follow the controlling specification, customer requirement and approved control plan. For permanent-mold work, the applicable B108 edition and the purchase order may tie testing to melt and heat-treatment combinations; safety-critical or customer-specific programs can require additional evidence.
Write the frequency explicitly. If it is undecided, ask the supplier to propose a lot definition and sampling plan for approval rather than calling a weight-based rule an industry standard.
RFQ and drawing checklist
For an A356 casting quote, provide:
- •revision-controlled 2D drawing and 3D model;
- •process route or permission for the supplier to propose it;
- •alloy, temper and controlling material specification with edition;
- •functional datums, machined surfaces and pressure boundaries;
- •mechanical-property requirements and specimen basis;
- •heat-treatment, inspection and traceability requirements;
- •prototype quantity, annual volume and target timing;
- •acceptance criteria for porosity, leak testing and dimensional reports.
Use the drawing-based RFQ route for a process review.
*This guide does not reproduce copyrighted property tables and does not replace the current purchased standard, customer drawing or engineering validation.*
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