Aluminum Alloys for Casting: A Practical Guide to Grades, Properties, and Selection

A drawing lands on your desk with a material block that reads only "aluminum alloy." That single line can mean A356 for a structural bracket, A380 for a high-volume die-cast housing, or B390 for a compressor cylinder that must survive years of sliding contact. Choose the wrong grade and the first prototypes may look flawless—then cracks appear after machining, pressure-test leaks surface in a batch of 2,000 parts, or anodized finishes come out patchy and dark.

Here is the direct answer before the detail. For high-pressure die casting, the practical defaults are A380 and A383 , with ADC12 as the Asian designation for a similar alloy. For low-pressure, gravity, and sand casting, the standard choices are A356 and 356 . If sliding wear dominates the design—cylinder liners, compressor cylinders, bearing surfaces— B390 deserves evaluation. If corrosion resistance and cosmetic finish outweigh raw strength, 518 is the alloy to test. The rest of this article explains those conclusions with numbers you can bring into a supplier conversation.

How Cast Aluminum Alloy Designations Work

Cast aluminum alloys in North America follow the Aluminum Association four-digit system with a decimal point. The first digit identifies the principal alloying element, and that digit alone tells you most of what you need to know about the alloy's behavior in a mold and in service.

• 1xx.x — 99.0% or higher aluminum, used mainly for electrical conductors and chemical equipment.
• 2xx.x — copper as the main element. This family produces the highest strength of all cast aluminum alloys after heat treatment, but corrosion resistance suffers.
• 3xx.x — silicon with added copper and/or magnesium. This is the workhorse family for both die casting and gravity casting.
• 4xx.x — silicon as the principal element, offering excellent fluidity and pressure tightness for leak-critical parts.
• 5xx.x — magnesium, giving the best corrosion resistance and the most consistent anodized finish.
• 7xx.x — zinc, age-hardenable with high as-cast strength but less common in commercial casting work.

What the numbers, letters, and decimals actually mean

The second digit indicates a modification of the basic alloy; the last two digits identify the specific alloy within the family. The decimal matters too: x.x is casting form, x.xx is ingot form. The letter prefix, such as the A in A356, means tighter impurity limits than the unlettered baseline alloy. In practice, a specification like A356-T6 is a complete instruction: alloy family 3xx.x, modified 356 composition, tighter impurities, and T6 heat treatment.

Temper designations explained briefly

Temper is written after the alloy, separated by a hyphen. T6 means solution heat treatment followed by artificial aging—the temper most often specified when a structural casting must carry load. T5 means the part is cooled from casting and then artificially aged, skipping the solution step, which is commonly used for die castings to avoid distortion and blistering. F means as-fabricated, which is how most die castings are supplied. These distinctions directly affect yield strength: A356-T6 reaches roughly 186 MPa yield strength , while as-cast A356 sits around 125 MPa .

The Most Common Aluminum Alloys for Casting

According to the Metal Casting Institute, the most common alloys for sand casting, gravity casting, and low-pressure casting are 356 and 357 , containing about 6% and 7% silicon respectively. A materials-science overview published on ScienceDirect confirms the pattern, noting that A319 (Al-6Si-3Cu-1Fe-1Zn) and A356 (Al-7Si-0.25Cu-0.2Fe-0.4Mg) account for most industrial applications in gravity casting. For high-pressure die casting, alloy selection guidance shows the 3xx.x series dominating, followed by 4xx.x, 5xx.x, and 2xx.x.

Property values above follow published ASM International data for separately cast test bars. Treat them as the starting point for engineering evaluation, not as a guarantee for a specific part geometry.

How the Casting Process Restricts Alloy Choice

The casting process is the first filter in alloy selection. Each process fills the mold differently, and the alloy has to tolerate the filling conditions.

High-pressure die casting: speed and thin walls

The aluminum die casting process injects molten metal into a hardened steel die under pressures between 10 and 175 MPa. Fill times are measured in milliseconds, so the alloy must be extremely fluid at the casting temperature. A380 and A383 are the dominant choices because their silicon content gives the fluidity needed for thin walls and complex ribs. A356 can be die-cast, but its lower silicon content makes it harder to fill thin sections cleanly and increases the risk of die soldering.

Low-pressure casting: clean fill and fewer pores

Low-pressure casting fills the die from a furnace below, pushing the melt upward through a riser tube under controlled gas pressure. The flow is steady and laminar compared with high-pressure injection. Porosity levels tend to be lower, and mechanical properties are more repeatable—which is why wheels, suspension arms, and structural brackets are made this way. A356 in T6 is the standard combination for these components. When porosity is the main rejection reason, it is worth verifying with a supplier that low-pressure die casting can reduce gas holes enough to meet your leak-test standard.

Gravity and sand casting: versatility with thicker sections

Gravity permanent-mold casting uses simple bottom-fill or tilt-pour geometry, while sand casting offers the widest shape freedom. Both processes are more forgiving of lower fluidity, so A356, 356, 319, and 357 see the most use. Because cooling is slower than in die casting, the microstructure is coarser, and you compensate by specifying a heat treatment rather than trying to hold tight as-cast properties.

Which Alloy Properties Matter in Real Components

Beyond tensile strength, a casting alloy has to survive the casting process itself and perform in the final environment. These six properties decide most real-world selections.

Hot cracking resistance

Cracks form during solidification when the alloy has a wide freezing range and insufficient feeding of liquid metal between dendrites. Copper-rich 2xx.x alloys are the worst offenders here. A356 solidifies over a narrow range, which gives it strong hot-cracking resistance and explains why it dominates structural gravity castings.

Die soldering tendency

Molten aluminum can react with steel die surfaces, welding itself to the tooling. Iron in the alloy mitigates this by forming iron-rich intermetallic phases that reduce wetting. Die-casting alloys therefore specify iron up to 1.3%, while gravity-cast alloys like A356 keep iron down to 0.20% because there is no die to solder to. If you take a gravity-cast alloy and try to die-cast it, expect soldering problems.

Pressure tightness

Hydraulic valve bodies, pump housings, and transmission cases all face leak-test requirements. A413 with roughly 12% silicon has excellent pressure tightness. A356 and A380 also perform well. B390, with 16–18% silicon, is less reliable for pressure-tight castings because the primary silicon particles disrupt the continuity of the matrix and can create micro-porosity networks.

Corrosion resistance

518 is the strongest candidate in most outdoor or saltwater environments among die-casting alloys, thanks to its 7.5–8.5% magnesium content. Copper-bearing alloys like A380 and 319 corrode faster in salt-laden air. A356 sits in the middle and is acceptable for most machinery and automotive under-hood applications if proper surface protection is applied.

Machinability

A380 and 319 machine well, producing short broken chips and giving good tool life at high cutting speeds. The high magnesium content of 518 produces stringy chips that can tangle around tooling, so sharper inserts and chip-breaker geometry are necessary. If your part will need extensive drilling and tapping, factor that into the alloy decision.

Thermal conductivity

Thermal management parts such as heat sinks and motor housings depend on how quickly heat spreads through the metal. A356 in T6 reaches roughly 151 W/(m·K) , while A380 measures about 96 W/(m·K) . If a part removes heat from an electric motor or power electronics, that difference of about 55 W/(m·K) changes the junction temperature significantly.

Application-Driven Alloy Selection

Alloy selection is usually an intersection of structural requirements and the economics of the casting process. The table below maps common components to the grades that casting shops actually run for them.

For powertrain and structural applications, suppliers commonly produce aluminum die-cast gearbox housings in A380 or A383 because these alloys deliver the stiffness and pressure tightness needed in transmission service. When a gearbox housing also has to survive vibration fatigue, A356 with a T6 temper is worth considering even if the cycle time is longer.

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In the automotive engine category, aluminum auto engine cylinder parts are usually specified in A356-T6 when structural integrity and cooling performance matter, or A380 when the production volume demands the lowest possible cost per piece. B390 is selected when the cylinder bore area needs high resistance to sliding wear without a separate liner.

Custom Aluminum Auto Engine Cylinder Parts Manufacturers. OEM Suppliers - Ningbo Beilun Youyuan Machinery Manufacturing Co., Ltd is a China OEM/ODM Aluminum Auto Engine Cylinder Parts manufacturers and Aluminum Auto En... View Product →

For fluid-power components, aluminum die-cast hydraulic valve bodies are typically cast in A380 because it machines cleanly into flat sealing faces and holds pressure without macro-porosity. For moderate series with higher fatigue demand, A356 in T6 remains the benchmark.

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Design Considerations That Affect Alloy Choice

The alloy is only half the decision. Part geometry, tolerance expectations, and downstream surface treatment all influence which grade is realistic to produce at the quoted price.

Wall thickness

• Die casting — minimum walls of 0.8–1.5 mm for small components; 2–3 mm is the practical floor for medium housings with ribs and bosses.
• Low-pressure casting — minimum walls of 2.5–4 mm depending on the alloy's freezing range.
• Gravity and sand casting — minimum walls of 3–5 mm to avoid misruns and shrinkage defects.

Dimensional tolerances

• Die casting holds about ±0.1 mm on small dimensions and ±0.3–0.5 mm across 100 mm. This is why most precision housings and brackets are die-cast.
• Low-pressure and gravity casting typically run ±0.5 mm over 100 mm. A356-T6 structural parts are machined after heat treatment if tighter accuracy is required.
• Sand casting gives ±0.8 mm or more, and machining is almost always necessary on functional faces.

Draft angle

Die casting needs at least 1–2 degrees of draft on external walls and 1.5–2.5 degrees on internal cores to extract the part cleanly. Low-pressure and gravity tools also need draft, but slightly less because filling and ejection forces are lower. If your design allows zero draft because of assembly requirements, expect a conversation about machining allowances or slide actions that add cost.

Heat treatment and distortion

T6 heat treatment raises yield strength substantially, but it also leads to distortion from the quench step. On thin-walled castings, a straightening operation is often required after T6. Die-cast aluminum alloys are usually used in the F temper because the gas content trapped by high-pressure injection can cause surface blistering during solution heat treatment. Confirm whether your part really needs T6; upgrading from F to T6 can add both process cost and scrap rate.

Surface treatment compatibility

Powder coating and painting work on every alloy in this article. Anodizing is the exception. Silicon levels above about 7% produce a darker, patchy anodic coating, and die-cast skins often contain a fine porosity layer that looks bad after anodizing. If you need a decorative anodized finish, either specify a low-silicon alloy like 518, or plan to machine off the as-cast surface before anodizing. The example references for a clean production finish show how aluminum die-casting applications increasingly rely on surface-prep steps to meet cosmetic standards.

Frequently Asked Questions About Aluminum Alloys for Casting

What grade of aluminum is best for casting?

There is no single best grade. Use A356 or 356 for low-pressure, gravity, and sand casting when strength and ductility matter. Use A380 or A383 for high-pressure die casting when production volume, thin walls, and cost efficiency dominate the decision. The best grade is the one that matches your process first, then your load case and surface requirements.

Is ADC12 the same as A383?

They are close but not identical. ADC12 is the Japanese JIS designation for a die-casting alloy with roughly 9.5–11.5% silicon and 1.5–3.5% copper. A383 is the Aluminum Association designation for a similar alloy with tight iron and manganese limits. In practice, many Asian die casters use ADC12 nomenclature even when exporting parts drawn to A383, so confirm the exact chemistry on the material certificate rather than assuming they are interchangeable.

Can die-cast aluminum parts be anodized?

Yes, but the result depends on silicon content. A380 and A383 anodize to a dark, patchy finish that does not match architectural specifications. Low-silicon die-casting alloys such as 518 produce a much better anodic coating. For consistent appearance, machine or grind the surface before anodizing, and test pre-production samples from the actual die before committing to a finish spec.

What does T6 mean in A356-T6?

T6 is the temper designation for solution heat treatment, quenching, and artificial ageing. For A356, this raises yield strength from roughly 125 MPa in the as-cast condition to about 186 MPa, while retaining 5–10% elongation. It is the default temper for structural castings that will face significant mechanical load.

Which aluminum casting alloy has the best pressure tightness?

A413, with around 12% silicon, is widely cited as the most pressure-tight die-casting alloy. A356 also performs well for gravity and low-pressure castings that need to hold oil or hydraulic fluid. Avoid 2xx.x copper alloys for pressure-tight applications, and design the part with uniform wall thickness to prevent shrinkage porosity regardless of alloy.

Why is B390 used for compressor and engine cylinder applications?

B390 contains 16–18% silicon and 4–5% copper. The high silicon content forms hard primary silicon particles that give exceptional sliding wear resistance, which is essential in cylinder bores and compressor cylinders. The trade-offs are lower ductility, more difficult machining, and poorer pressure tightness, so it is selected only when wear is the dominant failure mode.

A Practical Closing Recommendation

Put the alloy decision on the table before the die is cut. Ask your casting supplier which alloys they run regularly on their equipment—a foundry that produces A380 parts every day will quote them with tight cycle times and predictable scrap rates, while a first-time A356 die-cast part carries development risk that the buyer eventually pays for. On the other hand, if your part gets certified for fatigue or leak-tightness, shifting from your supplier's preferred alloy to a closer fit for the application could reduce field failures enough to justify the higher piece price. Verify the chemical composition through a certified material test report, machine a test batch under production conditions, and check that the mechanical properties from the actual part geometry meet the drawing requirements. That sequence will always be cheaper than discovering an alloy mismatch during a customer audit.