Aluminium Alloy Casting: How to Specify, Source and Price It Without Overpaying
Why the Same Drawing Returns Quotes That Differ by 300%
Send one aluminium component drawing to six foundries and the responses will not cluster. They will scatter — sometimes by a factor of three or more on piece price, and by an even wider margin on tooling.
This is not because five of them are dishonest. It is because a casting quotation encodes four hidden variables that never appear on the quotation itself:
Process fit. A foundry set up for high-volume Aluminum Die Casting quoting a 300-piece annual requirement will price defensively, because the job disrupts a production flow optimised for long runs. A jobbing foundry quoting the same part prices it as routine work.
Capability margin. When a part sits near the upper limit of a foundry's melt capacity, mould box size, or tolerance capability, the quotation absorbs a risk premium. The supplier knows the job will consume disproportionate engineering attention, and prices accordingly. The same part comfortably inside another foundry's normal envelope carries no such loading.
Machining assumptions. Two quotes for the same cast metal part can differ wildly simply because one assumes as-cast tolerance on non-critical features while the other machines every surface to be safe. The drawing rarely makes this explicit — so the supplier's interpretation decides the price.
Tooling amortisation policy. Some suppliers quote tooling as a separate capital line. Others bury it in the piece price over an assumed volume that may not match your actual demand. The second structure looks cheaper until year two, when volume drops and the amortisation assumption breaks.
A higher price does not reliably indicate better quality. What it usually indicates is a mismatch between your part and that supplier's operating sweet spot.
Matching Volume to Process: The Decision That Sets Your Cost Floor
Nearly every costly sourcing error in custom casting traces back to selecting a process that does not match actual annual demand. Four routes cover the practical range.
Sand Casting — 1 to 5,000 pieces
- Tolerance: ±0.8–1.5 mm, heavy machining allowance
- Minimum wall: 5–6 mm
- Lead time to first part: 2–4 weeks
Use it for: prototypes, restoration and heritage replacement parts, very large housings, machine bases, and design validation before committing die steel.
Gravity / Permanent Mould — 500 to 5,000 pieces
- Tolerance: ±0.4 mm per 25 mm
- Minimum wall: 3–4 mm
- Tooling: roughly 20–40% of comparable die tooling
Use it for: structural aluminium Casting Parts needing yield strength above 200 MPa, pressure-rated bodies, and large sections where die clamp force becomes uneconomical.
High-Pressure Die Casting — 5,000+ pieces
- Tolerance: ±0.10 mm per 25 mm as-cast
- Minimum wall: 1.2–1.5 mm
- Surface finish: Ra 1.6–3.2 µm as-cast
- Die life: 80,000–300,000 shots
Use it for: die cast housing assemblies, gearbox covers, heat sinks, telecom hardware — thin-walled, detailed geometry at volume.
CNC Machined from Billet — 1 to 200 pieces
Not a casting route, but frequently the correct answer for low volumes or when the drawing specifies a wrought alloy. No tooling, tolerances to ±0.02 mm, delivery in 1–2 weeks.
Selection Summary
| Sand | Gravity | Die Casting | CNC Billet | |
| Economical volume | 1–5,000 | 500–5,000 | 5,000+ | 1–200 |
| Tolerance (25 mm) | ±0.80 mm | ±0.40 mm | ±0.10 mm | ±0.02 mm |
| Minimum wall | 5 mm | 3 mm | 1.2 mm | Unlimited |
| Tooling cost | Low | Medium | High | None |
| T6 heat treatable | Yes | Yes | Generally no | Yes |
| First part | 2–4 wks | 6–10 wks | 10–16 wks | 1–2 wks |
Run the arithmetic before committing tooling. Divide the tooling premium by the per-piece saving to find your break-even quantity, then compare it against realistic three-year cumulative demand — not the optimistic first-year forecast in the business case. Committing die tooling for a part with genuine demand of 1,500 units per year is the most expensive mistake in metal manufacturing procurement, and it is made constantly.
Specifying the Right Casting Alloy
Aluminum alloy processing manufacturing starts with alloy selection, and this belongs in the design review — not after geometry is frozen. Alloy governs fluidity, corrosion behaviour, machinability, heat-treat response, and cosmetic appearance after anodising.
| Alloy | Si % | Cu % | Characteristics | Applications |
| A380 / ADC10 | 7.5–9.5 | 3.0–4.0 | The workhorse — balanced castability, strength, machinability | Die cast housings, gearboxes, electrical enclosures |
| A360 | 9.0–10.0 | ≤0.6 | Best corrosion resistance among HPDC alloys | Marine hardware, outdoor enclosures |
| A413 / ADC12 | 11.0–13.0 | ≤1.0 | Highest fluidity; naturally pressure-tight | Manifolds, hydraulic bodies, thin walls |
| A356 (T6) | 6.5–7.5 | ≤0.25 | Heat-treatable to 200+ MPa yield; weldable | Gravity and low-pressure structural castings |
| LM6 / LM25 (BS) | 10–13 / 6.5–7.5 | ≤0.1 | UK/European designations; LM6 for corrosion, LM25 for heat-treatable strength | Marine fittings, pump bodies, general engineering |
Three Specification Traps
Casting alloys and wrought alloys are not interchangeable. A drawing calling for 6061-T6 cannot be die cast — 6061 has poor casting fluidity and hot-tears in a steel die. It must be CNC machined from billet or built by aluminum fabrication from extruded profile.
A380 does not anodise bright. Its 3–4% copper produces a dark, mottled grey anodised finish rather than clean silver. Specify A360 or A413 at design stage if bright anodised appearance matters.
Copper drives corrosion behaviour. A380's copper improves strength and machinability but sharply reduces salt-spray resistance. For coastal, marine or road-salt exposure, A360's near-zero copper justifies the modest cost premium.
When Zinc Beats Aluminium
Zinc casting in ZAMAK 3 or ZAMAK 5 is routinely overlooked and is frequently the better engineering answer for small, detailed parts:
- Walls down to 0.4–0.6 mm versus aluminium's 1.2 mm
- Tool life of 500,000–1,000,000 shots — hot-chamber process is gentler
- Faster cycles (no ladling step) and a plating-ready as-cast surface
Choose zinc for small components under 500 g needing fine cosmetic detail. Choose aluminium when weight, thermal conductivity, or service above 100°C are drivers.
Tolerances: Specify Only What You Need
Over-tolerancing is the most reliable way to inflate a quotation without gaining function.
| Feature | HPDC As-Cast | Gravity | Sand | Machined |
| Linear (per 25 mm) | ±0.10 mm | ±0.40 mm | ±0.80 mm | ±0.02 mm |
| Across parting line | ±0.25 mm | ±0.50 mm | ±1.50 mm | — |
| Flatness (per 100 mm) | 0.15 mm | 0.40 mm | 0.80 mm | 0.02 mm |
| Hole true position | ⌀0.30 mm | ⌀0.60 mm | ⌀1.50 mm | ⌀0.05 mm |
| Surface roughness | Ra 1.6–3.2 µm | Ra 6.3 µm | Ra 12.5 µm | Ra 0.8 µm |
Porosity: Two Mechanisms, Two Different Fixes
Gas porosity forms when air or vapour is entrained during fill. The fix is vacuum assist — evacuating the cavity below 50 mbar before injection.
Shrinkage porosity forms as aluminium contracts during solidifying. The fix is localised squeeze pins or redesigning the gate and section geometry.
For pressure-tight aluminium casting products, both controls are prerequisites. Specify radiographic acceptance per ASTM E505.
Finishing, Fabrication and Assembly
A raw casting is near-net-shape, not finished. Precision machining brings critical bores and sealing faces to ±0.02 mm.
| Process | Thickness | Benefit |
| Shot blasting | — | Uniform matte surface; deflashing |
| Chemical conversion (Cr³⁺) | 0.5–2 µm | Paint adhesion; light corrosion protection |
| Anodising Type II | 5–25 µm | Corrosion and wear resistance; colour |
| Hard anodise Type III | 25–100 µm | Severe wear service |
| Powder coating | 60–120 µm | Colour, UV and impact resistance |
| E-coat | 15–35 µm | Uniform coverage in recesses |
Where Fabrication Replaces Casting
Metal fabrication is subtractive and additive. For brackets and frames built from flat stock, custom metal fabrication avoids tooling entirely and delivers in 1–3 weeks.
HAZ strength loss: welded 6061-T6 loses roughly 40% of its yield strength in the heat-affected zone.
Distortion control: weld sequence and stress relief determine whether a finished frame holds flatness.
Auditing Aluminium Die Casting Suppliers
Certification: ISO 9001:2015 is baseline; IATF 16949 for automotive. Verify directly with the registrar.
Process control: Per-cycle shot monitoring; vacuum assist; solidification simulation (MAGMA, ProCAST).
Metrology: CMM with traceable calibration; spectrometer for chemistry; X-ray for internal inspection.
Commercial terms: Request historical Cpk data. Settle tooling ownership and transfer terms in writing.
Frequently Asked Questions
Four hidden variables drive the spread: Process fit, capability margin, machining assumptions, and tooling amortisation policy. A higher price usually signals a mismatch with that supplier's operating sweet spot, not superior quality.
Under 200 pieces: CNC machining. 1 to 5,000 pieces (thick walls): sand casting. 500 to 5,000 pieces (T6 required): gravity casting. Above 5,000 pieces (thin walls): high-pressure die casting.
Sand casting can handle one piece. For die casting services, batches usually start at 500-1,000 pieces, though rapid tooling can lower the threshold for shorter runs.
Generally no, due to surface blistering from entrapped gas. Use gravity casting or vacuum-assisted die casting if T6 properties are mandatory.
Choose zinc casting for parts under 500g needing fine detail or plating. Choose aluminium for weight savings, thermal conductivity, or service temperatures above 100°C.
Audit certification, process controls (like vacuum assist), metrology capabilities (CMM/X-ray), and historical Cpk data. Ensure tooling ownership is defined in writing.
Fabricate for flat/bent geometries, volumes below 500 units, or when 1-3 week lead time is critical. Cast for internal cavities, complex curvature, or integrated bosses.