Key points
- Choose the alloy from what the part must survive: castability, whether T6 is needed, corrosion behaviour in the service medium and machinability of sealing features, then match it to the casting route.
- A356 is an Al-Si-Mg casting alloy with copper held to 0.2% or less; the magnesium is what makes it respond to T6.
- Published A356-T6 squeeze-cast reference values are 260-320 MPa tensile and 220-260 MPa yield (company-reported); these are references for first feasibility, not acceptance criteria.
- Copper raises strength in die casting alloys such as A380, AlSi9Cu3 and ADC12, but lowers corrosion resistance.
- A380 is used as cast; conventional high-pressure die castings blister when someone tries to T6 them, because trapped gas expands during solution heat treatment.
- An alloy name does not make a part pressure-tight; leak paths come from connected porosity, shrinkage at hot spots, cold shuts and machining breakouts.
- Corrosion performance must be specified by the buyer and depends on the alloy selected, the surface finish applied and the validation programme agreed for that part.
- Published example (company-reported): a boat part squeeze cast in A356.2-T6 replaced a conventional aluminum high-pressure die casting with at least 50% higher strength reported.
A356-T6 vs A380 on the factors a marine drawing has to fix
The table compares the two on the points that usually decide a marine or outboard casting. Where no figure is published, the cell says so instead of inventing one.
| Factor | A356-T6 | A380 |
|---|---|---|
| Alloy family and copper | Al-Si-Mg casting alloy; copper held to 0.2% or less | Copper-bearing general-purpose die casting alloy; copper lowers corrosion resistance. Check exact chemistry against the governing standard |
| Usual route | Squeeze casting; a squeeze casting is often ordered to a sand or permanent mould standard by agreement | High-pressure die casting; also listed as a squeeze casting alloy |
| Heat treatment | Heat-treatable to T6 | Used as cast, not T6; die castings tend to blister because trapped gas expands during solution heat treatment |
| Strength reference | 260-320 MPa tensile, 220-260 MPa yield, published squeeze-cast reference (company-reported), for feasibility only | No figure given here; compare against the drawing's required property and the agreed verification method |
| Corrosion starting point in water or outdoor service | Low copper keeps corrosion behaviour good, including when bolted to a dissimilar metal; low-copper alloys are evaluated first for water, coolant or outdoor service | Copper lowers corrosion resistance, so low-copper alloys are evaluated first for water, coolant or outdoor service |
| Pressure tightness | Route-dependent: A356 relies on the squeeze route and pressure during solidification | Route-dependent: high-pressure die casting traps gas, and connected pores opened by machining become a leak path |
| Anodising and coating | Anodising result depends on alloy chemistry | Confirm the anodised appearance on representative A380 samples, since high-silicon die casting alloys can anodise grey and uneven; powder coating cure heat can raise blisters on gas-bearing castings |
| Welding | Weldable; the heat-affected zone loses T6 strength and welding is not in CEX's published in-house scope | Not a published reason for choosing A380; check any weld requirement at drawing review |
| Where it is usually the better choice | Where T6 or welding is required, or a leak or failed mechanical test cannot be solved on the existing route | Thin walls, fine detail, high volume and cosmetic parts where internal soundness has never been the problem |
Why copper matters in seawater and outdoor service
For a wet part, copper is the clearest difference between the two alloys. In die casting alloys such as A380, AlSi9Cu3 and ADC12, copper raises strength and lowers corrosion resistance; where parts see water, coolant or outdoor service, the low-copper alloys A356, A360 and AlSi10Mg are evaluated first. A356 keeps copper at 0.2% or less, which keeps corrosion behaviour good when the casting is bolted to a dissimilar metal. Galvanic contact needs its own line on the drawing: when dissimilar metals share an electrolyte, the anodic member is at risk of galvanic corrosion, so isolation, coating coverage and fastener material belong in the specification, not in a verbal note.
Finish and validation remain part of the corrosion specification, so the buyer states the medium, the environment and the acceptance criteria. Corrosion performance is not reduced to a single figure in a quotation. If A380 has to stay for other reasons, A360 offers better corrosion resistance and pressure tightness than A380 on the die casting route, and AlSi10Mg is a low-copper die casting and squeeze casting option that is heat-treatable only on a qualified route. Route and finish trade-offs are covered in squeeze casting vs die casting for marine parts.
Heat treatment, strength and the route
A380 has no T6 step. It is used as cast. A T6 attempt on a conventional high-pressure die casting tends to raise blisters, because gas trapped during high-speed filling expands during solution heat treatment. A356 responds to T6 because of its magnesium, and the squeeze route fills at low velocity and holds the metal under pressure while it solidifies, so there is less gas to trap.
Strength figures for A356-T6 squeeze castings are quoted as 260-320 MPa tensile and 220-260 MPa yield (company-reported). Treat them as typical references for first feasibility: guaranteed minimums come from the named standard, or from minimums agreed for the part with a defined specimen location. "A356-T6" alone is a grade name, not a purchase specification. Name the governing standard with its edition, the condition and how properties are verified. A356.0 is the casting designation; A356.2 is an ingot designation with tighter limits, especially on iron. Detail on grade, temper and verification is on the A356-T6 casting specification page, with process background on A356-T6 squeeze casting.
Sealing, porosity and machined faces
Specifying A356 or A380 does not by itself give a pressure-tight part. Leak paths come from connected porosity, shrinkage at hot spots, cold shuts and machining breakouts, and on the squeeze route the wall design and the location of sealing faces matter as much as the grade. Machining can also open porosity at a sealing face, and connected pores then become a leak path. Squeeze casting reduces the amount of gas to trap and the shrinkage to connect, but no process guarantees zero porosity. Plan when bores, sealing faces and datums are machined relative to T6.
Keep die casting where it already works: a part that passes leak test after a gating fix does not need a new process, and squeeze casting is evaluated when a leak, a void opened by machining or a failed mechanical test cannot be solved on the existing route. For the decision path, see how high-pressure die casting porosity leads to squeeze casting and the inspection and leak testing approach.
When A380 die casting is the right call
Die casting remains the better route for thin walls, fine detail, high volume and cosmetic parts where internal soundness has never been the problem, and A383 fills thin or complex sections more easily than A380. If the part sits in a protected location, needs no T6 and no welding, and corrosion is carried by the finish and an agreed validation programme, A380 is a practical choice, with A360 as the die casting alternative where better corrosion resistance and pressure tightness are needed.
Decide on total finished-part cost (tooling, yield, machining, inspection and volume) rather than casting price per piece, because squeeze casting has a different economic window from die casting at very high volume. The squeeze vs die casting selection worksheet sets out that comparison factor by factor.
Finish and corrosion validation
Anodising results depend on alloy chemistry, and high-silicon die casting alloys can anodise grey and uneven; confirm the look on representative samples before committing to a cosmetic anodised part. Powder coating gives durable colour and weather resistance, but its curing heat can raise blisters on gas-bearing castings. Mask sealing lands, threads and fitted bores; finish after machining, with impregnation only if permitted; and agree which leak test is the release test.
Corrosion tests are not included automatically. If the drawing requires salt-spray or another corrosion test, state the method and the acceptance criteria in the RFQ so test responsibility is agreed in writing before tooling, and do not assume an in-house corrosion-test capability unless it is confirmed in writing. CEX's published in-house finishing covers blasting (tumbling, hanging shot blast, handheld sandblasting), vibration tumbling, grinding, polishing and ultrasonic cleaning, and published parts include powder-coated components, though whether coating is in-house is not stated.
What to put on the drawing and RFQ
- Service medium and environment: seawater immersion, splash, washdown, fuel, coolant, outdoor exposure. The alloy follows the medium.
- Alloy designation exactly (A356.0 or A356.2; A380), the governing standard with edition, the condition (T6, or as cast for A380), and how properties are verified.
- Mechanical minimums, the specimen location they apply to and the test method; material tests state alloy, temper and specimen.
- Soundness: which zones are X-rayed, with the limit and the record agreed before production.
- Leak test, if the part is sealed: medium, pressure, units, allowable leak rate, after final machining.
- Finish specification with masking of sealing lands, threads and fitted bores, finishing after machining, and whether impregnation is permitted.
- Corrosion test method and pass/fail criteria, with test responsibility agreed in writing.
- Mating metals and fasteners in contact with the casting (general good practice; dissimilar-metal contact is part of the corrosion review).
- Weld locations if any, with a note that the heat-affected zone loses T6 strength and that procedure and responsibility are agreed per project.
- The note "No repair welding or impregnation without written approval."
- Quantity and volume basis, stated as total finished-part cost rather than piece price.
- For each inspected characteristic: method, sample size, limit and record, agreed before production.
- Or say the material is open: send function, loads, medium, pressure and environment, and CEX proposes an alloy and route with the reasoning for the customer to approve before tooling.
What CEX can and cannot confirm
CEX lists squeeze casting alloys A356 / A356.2, A380, A383, AlSi9Mg, AlSi10Mg and AlSi9Cu3, with squeeze presses from 300-1,100 t, die casting machines from 160-1,100 t, wall thickness 4-15 mm and an envelope up to 1.2 x 0.8 m; as-cast blank tolerance is +/-0.2 mm and machined tolerances are agreed per feature (company-reported). Those limits are not all reachable on one geometry and are confirmed per part at drawing review. Because CEX runs both die casting and squeeze casting, a die-casting fix and a squeeze-cast sample can be compared on the same drawing.
The in-house lab covers an optical emission spectrometer for melt and incoming ingot, a tensile testing machine, a hardness tester, X-ray, helium leak detection by sniffer, a water immersion tank and a CMM, with characteristic, method, sample size, limit and record agreed per part before production. Tooling, heat treatment including T6, and CNC machining are in-house; CEX has cast aluminum since 1995 and developed its squeeze casting process in-house since 2015 (both company-reported). The IATF 16949:2016 certificate 0494978 covers manufacturing of aluminum alloy die-cast parts with clause 8.3 product design excluded, valid until 2027-01-07; design authority and drawing approval stay with the customer.
Corrosion-test responsibility is agreed in writing before tooling. Whether coating is done in-house is not stated, so coating scope is confirmed per project. Weld procedure and welding responsibility are agreed per project; see the notes on welding squeeze cast aluminum parts before adding weld callouts. If the existing die casting route is the better answer for your part, you get a documented reason.