A356-T6 and A380 sit on opposite branches of the cast-aluminum decision tree: A356-T6 is a heat-treatable Al-Si-Mg alloy with 6.5–7.5% Si and 0.25–0.45% Mg, while A380 is a non-heat-treatable Al-Si-Cu alloy carrying 7.5–9.5% Si and 3.0–4.0% Cu [S2][S4].
Selection hinges on three hard constraints: casting process, post-cast heat treatment access, and the ductility versus productivity trade, all of which line up cleanly against aluminum alloy families used in structural castings.
Mechanical Properties Side by Side
Typical ultimate tensile strength lands at 37 ksi (255 MPa) for A356-T6 permanent-mold castings and 47 ksi (324 MPa) for A380 die castings, with yield strengths of 26 ksi (180 MPa) and 23 ksi (160 MPa) respectively on the 0.2% offset method [S1]. A380 wins on raw UTS because of its copper content, but A356-T6 returns a higher yield-to-UTS ratio, a more useful figure for design allowables.
Elongation over 2 inches splits the same way: 5% for A356-T6 versus 3.5% for A380 [S1]. Independent tabulations show A356.0-T6 at 6.0% elongation, 270 MPa UTS, and 200 MPa yield, with A380.0 at 3.3%, 290 MPa, and 160 MPa, indicating that source-to-source variance is roughly 10–20 MPa on strength and 1–2 percentage points on elongation [S2].
Brinell hardness (500-kgf load, 10-mm ball) is 70–100 HB for A356-T6 permanent mold and 80–85 HB for A380 die cast, with makeitfrom.com listing both at 80 HB [S1][S2]. For a deeper dive into how these numbers feed into the broader aluminum die casting machine selection logic, the process window is the deciding factor, not the alloy in isolation.
Thermal, Electrical, and Physical Numbers
Thermal conductivity is where A356-T6 pulls clearly ahead: 150 W/m-K versus 96 W/m-K for A380, a 56% gap that matters for heat-exchanger housings and LED heat sinks [S2]. Specific heat capacity is 900 J/kg-K against 870 J/kg-K, and the liquidus/solidus range sits at 610/570 °C for A356 versus 590/550 °C for A380, giving A356 a wider freezing interval that helps feed complex permanent-mold geometry.
Electrical conductivity tells the same story: 40% IACS by volume for A356-T6 versus 25% IACS for A380, with the equal-weight figure at 140% versus 78% IACS [S2]. Maximum mechanical service temperature is 170 °C for both alloys, and thermal expansion is 21 µm/m-K for A356 against 22 µm/m-K for A380, essentially interchangeable for thermal-cycling design.
Density is 2.6 g/cm³ for A356.0-T6 and 2.9 g/cm³ for A380.0 [S2]. Permanent-mold A356-T6 typically comes in at 0.097 lb/in³, a small but real stiffness-to-weight advantage of 7–8% over A380 [S1].
Chemistry and Why A380 Cannot Be T6 Heat Treated
A356.0 holds 0.25–0.45% Mg, the element that forms Mg₂Si precipitates during T6 aging, which is why A356 responds to solution treat, quench, and artificial age [S2]. A380.0 holds 0–0.1% Mg and 3.0–4.0% Cu, with the copper locked into Al₂Cu and Al-Cu-Mg phases that do not respond to a T6 schedule in the same way [S2][S4].
A standard T6 cycle for A356 runs solution treat at 535±5 °C for 8–12 hours, water quench in under 45 seconds, then age at 180±5 °C for 4–8 hours before air cooling [S4]. A380 cannot reach T6 because the magnesium content is below the threshold needed for precipitation strengthening, and die-cast porosity worsens any quench attempt by blistering the surface [S4].
Alloy families such as Aural 2, 365, and Silafont 36 are the NADCA-recommended substitutes when designers need a die-castable alloy that can still be T6-aged, per the 2021 NADCA Product Specification Standards for Die Castings [S3].
Which Casting Process Fits Which Alloy
A356 is specified for permanent-mold and sand casting, with good fluidity, low hot-cracking tendency, reduced wire shrinkage, and high air tightness, but it is not die-castable in a productive way [S4]. A380 is the default for high-pressure die casting because it flows aggressively, fills thin walls, and gives up some ductility and corrosion resistance for that productivity [S3][S4].
NADCA rates A380 (ANSI/AA A380.0) as by far the most widely cast of the aluminum die-casting alloys, specified for most general-purpose product applications including electronic and communications equipment, automotive components, engine brackets, transmission and gear cases, appliances, lawn-mower housings, furniture components, and hand and power tools [S3]. Alternatives 383 and 384 trade some strength for better die filling; A360 trades for higher corrosion resistance and elevated-temperature strength; A413 is the pressure-tight choice; B390 is the wear-resistant choice for valve bodies; and 518 gives the best strength-ductility-corrosion-finishing package at the cost of castability [S3].
For permanent-mold and gravity pours where T6 is on the table, A356 dominates; for high-volume die-cast housings, A380 dominates. Process selection is upstream of alloy selection and is rarely reversible without re-tooling.
Application Map and Use-Case Recommendations
A356-T6 is the right call for aviation pump accessories, automotive transmissions, pulleys, casings, volutes, brackets, and any high-stress structural casting where weldability, corrosion resistance, and 5% elongation matter [S4]. Its corrosion resistance and weldability are widely cited advantages, and after T6 it reaches mechanical properties that gravity-cast A380 cannot match.
A380 is the right call for die-cast enclosures, motor housings, brackets, and gear cases where dimensional repeatability across 100,000+ parts and fast cycle time outweigh the loss of ductility. The 3.0–4.0% Cu also drives the elevated-temperature strength that puts A380 in transmission cases and engine brackets [S3].
Hot-cracking is a real risk on A380 in metal-mold gravity casting, traced to the copper content, and is the main reason die-casting is the default for A380 [S4]. The trade chain on the aluminum die casting machine side mirrors this: faster cycle and thinner walls, at the cost of porosity and no T6.
Limitations, Failure Modes, and Standards Anchors
A356-T6 limitations center on casting process: it cannot run in a standard cold-chamber die-casting cell without blistering at the solution-treat step, and it is more expensive per kg than A380 [S4]. A380 limitations center on ductility (3.3–3.5% elongation), elevated copper-driven hot-cracking in permanent-mold gravity pours, and the inability to T6-age for higher yield [S2][S4].
Fatigue strength is a meaningful divergence: 90 MPa for A356.0-T6 versus 140 MPa for A380.0, a 56% advantage to A380 that does not show up in static UTS comparisons and matters for cyclic-loaded brackets [S2]. For procurement, the chemistry envelope comes from the Aluminum Association designation system; A380 is registered as A380.0 (also UNS A13800, former SC84C) and A356 as A356.0, per the cross-references in the makeitfrom.com data set [S2].
For process heat-source planning on the foundry side, a gas aluminum melting furnace sized to the 550–610 °C liquidus window covers both alloys with margin.
Decision Matrix in One Pass
On four decision criteria, A356-T6 leads on three and A380 leads on one. Strength-to-weight (axial): A356-T6 29 points, A380 28 points. Thermal conductivity: A356-T6 150 W/m-K, A380 96 W/m-K. Weldability and corrosion resistance: A356-T6 yes on both, A380 limited. Cycle time and die-cast thin-wall productivity: A380 wins decisively because A356 is not die-castable in a production cell [S2][S3][S4].
Choose A356-T6 when the part is permanent-mold or sand-cast, needs T6 properties, requires welding in assembly, or runs in a corrosion-sensitive service envelope. Choose A380 when the part is die-cast in high volume, needs a 47 ksi UTS, and does not require post-cast heat treatment or field welding. Mixed requirements, such as die-cast geometry with T6 strength, route to Aural 2, 365, or Silafont 36 per NADCA Publication #403 [S3].
Trackable next signals: NADCA Publication #404 (High Integrity Die Castings) for the latest T6-capable die-cast alloy data, and the Aluminum Association's A360 and 518 revisions when structural die-cast performance is the priority.
For related coverage, see Steel vs Aluminum Enclosed Track: Light-Duty Conveyor Decision Guide.