🔩 ALUMINUM
Aluminum Casting: Alloy Selection, Process Routes, and Real Tolerances
Aluminum is the workhorse of nonferrous casting because it pours at roughly 660 C, fills thin sections cleanly, and takes heat treatment that pushes yield strength past 300 MPa. The catch buyers miss is that the wrought grades they know from machining stock, the 6061-T6, 7075-T73, 2024, and 5052 listed on a print, are not the alloys a foundry actually pours. Understanding the swap from wrought designations to casting alloys is the difference between a clean quote and a part that cracks in the gate.
ISO 9001AS9100NADCAP
Why your 6061 and 7075 prints become A356 and A206 on the foundry floor
The wrought alloys called out in this run, 6061-T6, 7075-T73, 2024, and 5052, are rolling and extrusion alloys. They contain too little silicon to flow well in a mold and they hot-tear badly when cast because their freezing range is wide and they lack the eutectic that feeds shrinkage. Foundries do not pour them. When a buyer hands a foundry a 6061-T6 drawing, the realistic substitution is A356.0-T6, an Al-7Si-0.3Mg alloy that delivers comparable strength (around 230 MPa yield in T6) with excellent castability.
For 7075-level strength the cast equivalent is A206.0-T7 or 201.0-T7, high-copper alloys that reach 280 to 350 MPa yield but are notoriously hot-tear prone and demand tight gating and chill placement. There is no true cast 2024; buyers wanting that copper-bearing strength get 201/206 with the same caveats. For 5052, a marine-grade alloy chosen for corrosion resistance, the cast analog is 535.0 (Almag 35) or a 356 variant if some seawater resistance can be traded for castability.
The practical takeaway for procurement: specify the cast alloy by its Aluminum Association casting designation, not the wrought number. If the print says 6061, confirm with the foundry whether A356-T6 meets the mechanical minimums before tooling is cut. This single clarification prevents the most common requote on aluminum casting jobs.
Sand vs. permanent mold vs. high-pressure die: matching process to volume
Process choice drives cost more than alloy does. Green sand and resin-bonded (no-bake) sand casting carry the lowest tooling cost, often a few thousand dollars for a pattern, and suit volumes from one prototype to a few thousand parts. As-cast linear tolerance runs about plus or minus 0.5 mm on the first inch plus 0.003 in/in, with surface finish around 250 to 500 microinch Ra. Sand is the only economical route for large parts over 50 lb or one-offs.
Permanent mold (gravity die) uses a reusable steel mold, raising tooling into the $10,000 to $40,000 range but improving tolerance to roughly plus or minus 0.4 mm and finish to 150 to 250 microinch. The faster chill from the steel mold also refines grain and lifts mechanical properties 10 to 15 percent over sand. It is the sweet spot for 500 to 50,000 parts in A356 and 356.
High-pressure die casting (HPDC) injects molten aluminum at 1,500 to 25,000 psi into hardened H13 dies. Tooling runs $25,000 to $150,000+, but per-part cost at scale is the lowest of any route and tolerances tighten to plus or minus 0.1 to 0.2 mm with as-cast finishes near 32 to 63 microinch. The trade-off is alloy: HPDC uses high-iron alloys like A380 and ADC12 that resist die soldering but cannot be reliably T6 heat treated because entrapped gas blisters on solution heat. If you need both die-cast economics and heat treatment, ask about vacuum-assisted or squeeze casting.
Heat treatment, porosity, and the defects that actually fail parts
The dominant failure mode in aluminum castings is porosity, and it comes in two flavors. Gas porosity is rounded and caused by hydrogen dissolved in the melt; it is controlled by degassing with rotary impellers and argon, and by keeping the melt clean and dry. Shrinkage porosity is jagged and follows the last region to solidify; it is fought with chills, risers, and good gating that promotes directional solidification toward the riser.
T6 heat treatment, solution at roughly 540 C, water quench, then artificial age at 155 to 175 C, is where A356 earns its strength. But solution treatment will blister any part with trapped air, which is why standard HPDC parts skip it. For pressure-tight castings, hydraulic manifolds, pump housings, the spec usually demands X-ray or CT inspection per ASTM E155 reference radiographs, and porosity above a graded level is cause for rejection.
Secondary operations to budget: hot isostatic pressing (HIP) closes internal porosity in aerospace A206 and 357 parts but adds $200 to $1,000+ per part and a week of lead. Impregnation with resin seals microporosity for pressure-tight low-criticality castings at a few dollars per part. Always clarify which finish datum and which porosity grade govern before tooling, because retrofitting a chill or adding HIP after first articles is the most expensive way to fix a casting.
Frequently Asked Questions
No reputable foundry pours wrought 6061 or 7075. Both are extrusion and rolling alloys with wide freezing ranges that hot-tear and feed poorly in a mold. The standard substitution for 6061-T6 is A356.0-T6, which delivers about 230 MPa yield and 290 MPa ultimate with excellent castability. For 7075-level strength (roughly 430 MPa yield in wrought T73), no cast alloy matches it exactly; the closest are A206.0-T7 and 201.0-T7 at 280 to 350 MPa yield, and these demand careful gating because they hot-tear. If your application genuinely needs 7075 properties, the honest answer is to machine from wrought billet or forge rather than cast. On any quote, confirm the cast alloy substitution against your mechanical minimums in writing before tooling is committed, because this is the single most common cause of requotes and scrapped first articles.
It depends heavily on process. Sand casting holds about plus or minus 0.5 mm on the first 25 mm plus 0.003 in/in thereafter, with 250 to 500 microinch Ra surface finish and a draft of 1.5 to 3 degrees. Permanent mold tightens to roughly plus or minus 0.4 mm and 150 to 250 microinch. High-pressure die casting reaches plus or minus 0.1 to 0.2 mm and 32 to 63 microinch as-cast. Any feature tighter than these, a bearing bore at H7, a sealing face flat to 0.05 mm, a threaded hole, gets machined as a secondary operation, so design with machining stock of 1.5 to 3 mm on critical surfaces. Flatness and true position on cast datums are looser than machined datums, so pick your datum scheme around features you intend to machine. Wall thickness minimums are about 3 mm for sand, 2.5 mm for permanent mold, and 1 to 1.5 mm for die casting.
Tooling dominates upfront cost: a sand pattern runs $2,000 to $8,000, a permanent mold $10,000 to $40,000, and a hardened die-cast die $25,000 to $150,000+ depending on cavity count and complexity. Per-part cost is driven by weight (aluminum runs roughly $1.20 to $2.00 per lb of finished casting in alloy, but you pay for the gating and risering scrap too, often 1.5 to 2x part weight in poured metal), plus machining, heat treat, and inspection. A simple sand-cast bracket might be $40 to $120 in low volume; a die-cast housing at 10,000 units can drop under $8 each. Heat treatment adds $1 to $5 per part, X-ray inspection $5 to $30 per part, and HIP $200+ for aerospace work. The biggest hidden cost is scrap from porosity on pressure-tight parts, which is why degassing and gating quality matter to your unit economics.
For sand casting, expect 4 to 8 weeks from PO to first articles: 1 to 3 weeks for pattern build, then molding, pouring, cleaning, and inspection. Permanent mold tooling adds 6 to 12 weeks of tool build before first samples. Die-cast tooling is the longest at 12 to 20 weeks because hardened H13 dies require machining, heat treat, and tryout. Once tooling is proven, production lead times drop to 2 to 4 weeks for sand and a few days to two weeks for die casting depending on volume. Add 1 to 2 weeks for T6 heat treatment if it is outsourced, and another week if HIP or full X-ray per ASTM E155 is required. Aerospace AS9100 and NADCAP first-article qualification can add 2 to 6 weeks of paperwork and source inspection on top of the physical lead time, so build that into program schedules.
Casting is wrong when you need the full mechanical properties of wrought 7075 or 2024, when the part is a simple prism better served by sawing and machining from plate, or when annual volume is below roughly 25 to 50 parts and tooling cannot amortize. For low volumes of complex geometry, plate-and-weld fabrication or CNC machining from billet is often faster and cheaper than cutting a pattern. For thin, flat, or extrusion-friendly profiles, an extrusion plus machining beats casting on both cost and properties. Casting wins decisively when geometry is complex (internal passages, organic shapes, integral ribs and bosses), volume is in the hundreds to millions, and the modest property hit from A356-T6 versus 6061-T6 is acceptable. If a buyer is fighting porosity rejects on a thin pressure-tight part, the honest alternative is often a machined-from-billet or forged part, not a better casting process.
Last updated: July 2026
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