Four properties, and which one to design on
Tensile strength is the load-bearing limit. Yield strength is where the part starts to deform permanently and is the usual basis for structural design. Elongation is the capacity to deform before fracture; it drives impact behaviour and is the property most sensitive to porosity. Hardness is resistance to indentation, a quick lot check after heat treatment rather than a design value. A drawing that names only tensile strength leaves the other three open.
Test bar values vs values from the part
Most datasheets publish separately cast test-bar values. A production die casting, measured on specimens cut from parts, sits lower: the gap is porosity, flow scatter and section thickness, not chemistry. Both numbers are correct; they answer different questions. When you agree acceptance, state which basis applies.
| Alloy | Tensile (MPa) | Yield (MPa) | Elongation (%) | Hardness (HB) |
|---|---|---|---|---|
| A380 | 200–250 | 110–130 | 1.5–3.0 | 75–85 |
| ADC12 | 190–240 | 100–130 | 1.0–3.0 | 80–95 |
| A360 | 210–260 | 120–140 | 2.0–4.0 | 70–80 |
| A383 | 200–250 | 110–130 | 2.0–4.0 | 70–80 |
| AlSi9Cu3 | 190–240 | 100–130 | < 1.5 | 85–95 |
| AlSi10Mg | 190–240 | 100–130 | 1.0–2.0 | 80–90 |
| A356-T6, squeeze cast (reference) | 260–320 | 220–260 | See A356 page | See A356 page |
Why conventional die castings hit a ceiling
High-pressure die casting fills turbulently and traps gas. The trapped gas limits elongation and, more importantly, prevents T6: during solution treatment the gas expands and blisters the surface. So a conventional die casting is normally used as cast, and its as-cast strength is the ceiling. When a design needs more, the options are a different route (squeeze casting, which allows T6), a different alloy, or a design change, not a stricter number on the same route.
What the drawing should state
- Property and minimum. Tensile, yield, elongation as needed; minimums, not typicals.
- Condition. As-cast, T5, T6, T61.
- Specimen basis. Separately cast bar, cast-on, or cut from a named zone of the part, with orientation.
- Method. ASTM B557 or ISO 6892-1; gauge length for elongation.
- Sampling. Per heat-treatment batch, per lot, first article only.
- Response. Retest rules and what a failure means for the lot.
A result below the requirement: check in this order
- Specimen location: was it cut from a thick, porous zone the requirement did not intend?
- Fracture surface: porosity or oxide films on it explain low elongation
- Condition: was the heat-treatment batch record complete?
- Chemistry: magnesium and iron against the limits
- Test method and machine calibration
Material properties are not component validation
A coupon result does not replace component testing for a loaded or safety-relevant part. Where a casting replaces a forging or a weldment, the design authority defines the component tests; the material properties support, but do not establish, equivalence.