How to Choose the Right Aluminum Alloy for Your Casting Process
The cost of choosing the wrong alloy
Choosing an aluminum alloy for a casting project is not a trivial decision. A wrong selection can manifest in various and all costly ways: parts that fail to meet mechanical specifications after machining, elevated scrap rates due to porosity or hot tearing, inability to achieve the required surface finish, or unnecessarily high raw material costs for the application.
In our experience advising over 200 foundries in Mexico and Latin America, we have identified that approximately 30% of quality problems in aluminum castings originate from inadequate alloy selection, not from the casting process itself. The problem is that the alloy decision is frequently made by the purchasing department based solely on price, when it should be a technical decision involving product engineering, process engineering, and metallurgy.
The most costly error we see repeatedly is using 380 alloy for parts that require T6 heat treatment or weldability. 380 contains 3-4% copper which makes it practically impossible to weld without cracking and which generates blisters during T6 treatment due to gas trapped during the die casting process. If your part needs T6 or welding, 380 is not the right alloy, regardless of how good a price you get.
Decision criteria for alloy selection
Optimal alloy selection must consider multiple factors simultaneously. A perfect alloy rarely exists; the decision is always a balance between properties, processability, and cost. The main criteria are:
- Casting process: This is the most restrictive factor. The process determines which alloy families are viable. You cannot use a gravity alloy (A356) in conventional die casting, nor expect a die casting alloy (380) to achieve gravity casting properties.
- Required mechanical properties: Tensile strength, yield strength, elongation, hardness, fatigue resistance. Each application has a minimum specification that must be met with a safety margin.
- Surface finish: Some applications require anodizing, chrome plating, or painting. Alloys with high silicon (> 10%) and high copper make uniform anodizing difficult.
- Machinability: If the part requires extensive machining, copper-containing alloys (319, 380) produce shorter chips and better CNC finish.
- Corrosion resistance: Outdoor applications, marine environments, or fluid contact require copper-free alloys (A356, AlSi10MnMg).
- Weldability: If the part will be welded in subsequent assembly, copper-free alloys with narrow solidification range are preferred (A356, 413).
- Alloy cost: Ingot price varies significantly between alloys. High-purity alloys (A356, A357) cost 15-30% more than standard-grade 380.
- Local market availability: Not all alloys are available in all markets. In Mexico, 380 and A356 are the most readily available.
Decisión flowchart
The following interactive diagram guides you through a series of questions to arrive at the recommended alloy for your application. Start by selecting your casting process and answer the requirements questions to get a personalized recommendation.
Recommendations by casting process
Below we present specific recommendations for each casting process, with preferred alloys and their alternatives.
Permanent mold gravity casting
Permanent mold gravity casting produces parts with excellent surface finish and good mechanical properties. The solidification rate is moderate (faster than sand, slower than die casting), allowing for a refined microstructure without the entrapped gas complications of die casting. This process is ideal for medium production volumes (500-50,000 parts/year) and parts requiring structural integrity.
Primary recommended alloy: A356.0 (AlSi7Mg0.3). This is the worldwide reference alloy for gravity casting. Its solidification range of approximately 60 degrees C provides good feeding, and its Al-7Si-0.3Mg composition makes it ideal for T6 heat treatment. In T6 condition, it typically achieves 262 MPa tensile strength, 186 MPa yield strength, and 5% elongation. It is weldable, anodizable, and has excellent corrosión resistance.
High-strength alternative: A357.0 (AlSi7Mg0.6). When mechanical specifications exceed A356-T6 capabilities (typically when more than 280 MPa yield strength is required), A357 is the next option. Its higher magnesium content (0.45-0.70%) allows achieving 310 MPa tensile and 250 MPa yield strength in T6. Cost is approximately 10-15% higher than A356, and it requires tighter process control, particularly in degassing (hydrogen levels < 0.10 mL/100g Al) and solidification rate.
Machinability alternative: 319.0 (AlSi6Cu3). When the part requires extensive machining (engine blocks, cylinder heads, valve bodies), 319 offers the best machinability among gravity alloys thanks to its 3-4% copper. The trade-off is lower corrosión resistance and lower elongation (2% vs 5% for A356). It is heat-treatable to T6, achieving 250 MPa tensile strength.
Regardless of the alloy chosen for gravity casting, the addition of strontium (150-250 ppm) as an Al-Si eutectic modifier is virtually mandatory to maximize mechanical properties. Strontium transforms silicon particles from acicular (brittle) to fibrous (ductile) morphology, increasing elongation by 50-100%. For A356, the difference between modified and unmodified can be 3% vs 7% elongation.
Sand casting
Sand casting offers the greatest flexibility in part size and geometry with minimal tooling costs. However, the solidification rate is the slowest of all processes, resulting in a coarser microstructure and mechanical properties inferior to permanent mold for the same alloy and heat treatment. The same alloys apply (A356, A357, 319), but mechanical properties will be typically 10-15% lower than permanent mold.
To improve properties in sand casting, the use of grain refiners (Ti-B master alloys, typically 0.10-0.20% Ti) and eutectic modifiers (strontium) is critical. Metal chills can also be placed in critical sections of the sand mold to locally accelerate solidification and refine the microstructure where it matters most.
High-pressure die casting (HPDC)
High-pressure die casting is the highest-productivity process, with cycle times of 30-120 seconds and the capacity to produce hundreds of thousands of parts per year with a single die. The extremely high injection velocity (30-60 m/s at the gate) and intensification pressure (50-120 MPa) allow filling walls as thin as 1.5 mm with excellent surface detail. However, these same conditions trap gas (air, lubricant vapor) inside the part, which limits usable alloys and eliminates the possibility of conventional T6 heat treatment.
Recommended alloy for general use: 380.0 (AlSi9Cu3). The most produced die casting alloy worldwide, representing approximately 85% of die casting volume in North America. Its combination of 8.5% Si and 3.5% Cu offers excellent fluidity for filling complex geometries, good strength (317 MPa as-cast), and the permitted iron content (up to 1.3%) prevents die soldering. Ideal for housings, brackets, covers, and general automotive and electrical components.
Thin-wall alternative: A383.0 (AlSi10Cu2). With higher silicon content (10.5%) and lower copper (2.5%) than 380, this alloy offers better hot tearing resistance and superior fluidity. Preferred when the design includes thin walls (< 2 mm), tall ribs, or long flow paths. Mechanical properties are similar to 380.
Pressure-tightness alternative: 413.0 (AlSi12). Eutectic alloy with the best fluidity of all commercial alloys and excellent pressure tightness. Its singular solidification point (vs. the range of 380 and A383) minimizes shrinkage microporosity, making it ideal for hydraulic, pneumatic, and any application that must contain pressurized fluids. Its low copper content also gives it better corrosión resistance than 380.
Conventional die cast parts should NOT be subjected to T6 heat treatment. Gas entrapped during injection expands during the T6 solution stage (at 540 degrees C), forming surface blisters that ruin the part. Exceptions are vacuum die casting and squeeze casting, where entrapped gas is dramatically reduced, allowing a modified T6. Mega casting alloys (AlSi10MnMg) are specifically designed to achieve high properties in F condition, without T6.
Squeeze casting
Squeeze casting is a hybrid process that combines die casting pressure with slow, controlled fill speeds. Metal fills the die cavity at low speed (avoiding turbulence and gas entrapment) and then high pressure (70-140 MPa) is applied during solidification to feed shrinkage and eliminate porosity. The result is parts with near-100% density, mechanical properties superior to any other casting process, and suitability for T6 heat treatment.
Recommended alloys: A356.0-T6 and A357.0-T6. In squeeze casting, these alloys achieve properties that rival forgings: A356-T6 can reach 290-310 MPa tensile with 8-12% elongation (vs. 262 MPa and 5% in conventional gravity). A357-T6 in squeeze casting can exceed 340 MPa with 5-7% elongation. These properties make squeeze casting a competitive alternative to forging for automotive suspensión parts (control arms, knuckles) and aerospace components.
Mega casting (large-format structural die casting)
Mega casting uses die casting machines with 6,000-9,000 tons of clamping force to produce automotive structural body parts up to 1.5 meters long in a single operation, replacing assemblies of 70-100 stamped and welded parts. This process requires very specific alloys that combine excellent fluidity to fill enormous cavities, high elongation in as-cast condition (no T6, because quenching would distort such large parts), and good weldability for joining to the rest of the body structure.
Recommended alloy: AlSi10MnMg (EN AC-43500). This alloy family (Silafont-36, Castasil-37, Aural-2, Aural-5) was developed specifically for this application. Key characteristics are: ultra-low iron (< 0.15%) to maximize elongation; virtually no copper (< 0.03%) to guarantee weldability and corrosión resistance; manganese (0.5-0.8%) that compensates for the absence of iron as an anti-soldering agent; and a balanced Si-Mg content that provides 8-15% elongation in F condition with no heat treatment whatsoever.
The role of iron in alloy selection
Iron content is perhaps the most underappreciated factor in aluminum alloy selection, yet it has an enormous impact on mechanical properties, particularly elongation and fatigue resistance. Iron forms intermetallic compounds with aluminum and silicon that, depending on their morphology, can be benign or devastating to properties.
Low iron for structural parts (Fe < 0.20%)
In low-iron alloys (such as A356 with Fe max 0.20%), the few iron intermetallics that form are small and dispersed, without significantly affecting properties. This is why "A" type alloys (A356 vs 356, A357 vs 357) are specified for structural applications: the only difference between 356 and A356 is the iron limit (0.6% vs 0.20%), but this difference doubles the elongation and multiplies fatigue life by a factor of 3-5x.
Primary aluminum P1020 (99.7% purity) typically contains 0.10-0.15% iron, which easily meets "A" type alloy limits. The use of scrap in the charge can rapidly increase iron to unacceptable levels. To consistently produce A356, the metallic charge must be at least 70% primary or clean returns of the same alloy.
Higher iron acceptable in die casting (Fe 0.8-1.3%)
In die casting, iron serves a vital function: preventing molten aluminum from soldering to the die steel. Without a minimum iron content (typically > 0.7%), aluminum chemically attacks the die surface, creating a layer of Al-Fe intermetallics that grows progressively and ruins both the part and the die. For this reason, die casting alloys (380, A383, 413) specify maximum iron of 1.3-2.0%.
The high solidification rate in die casting (10-100 degrees C/second vs. 1-5 degrees C/second in gravity) refines the iron intermetallics to a size that has little impact on as-cast mechanical properties. However, this limits the part to F condition: if T6 were attempted, the partial dissolution and re-precipitation of these phases during heating would produce coarse particles that would degrade properties.
In mega casting alloys (AlSi10MnMg), iron is kept ultra-low (< 0.15%) but manganese (0.5-0.8%) is added as an anti-soldering agent. Manganese forms Al15(Fe,Mn)3Si2 compounds with Chinese script (compact) morphology instead of the beta-Al5FeSi platelets that iron alone forms. This compact morphology does not act as a stress concentrator and allows maintaining the high elongation required for structural parts.
Heat treatment response
Heat treatment is a powerful tool for increasing the mechanical properties of certain aluminum alloys, but not all alloys respond to treatment and not all casting processes allow its application. Understanding this interaction is fundamental to correct selection.
T6 treatment (solution + artificial aging)
T6 is the most common heat treatment for aluminum casting alloys. It consists of three stages: (1) solution at high temperature (typically 535-540 degrees C for A356) for 6-12 hours, which dissolves Mg2Si precipitates into the aluminum matrix; (2) rapid water quench (from 540 degrees C to < 80 degrees C in seconds), which freezes Mg and Si atoms in supersaturated solid solution; and (3) artificial aging at intermediate temperature (typically 155-160 degrees C for 4-6 hours for A356), which precipitates nanoscopic Mg2Si clusters that block dislocation movement and harden the material.
| Alloy | F condition (as-cast) | T6 condition | UTS increase | Notes |
|---|---|---|---|---|
| A356.0 | 172 MPa / 6% | 262 MPa / 5% | +52% | Standard T6, industry reference |
| A357.0 | 200 MPa / 4% | 310 MPa / 3% | +55% | Requires strict degassing |
| 319.0 | 186 MPa / 2% | 250 MPa / 2% | +34% | Good response from Cu + Mg |
| 380.0 | 317 MPa / 3.5% | Not recommended | N/A | Entrapped gas causes blisters in T6 |
| A383.0 | 310 MPa / 3.5% | Not recommended | N/A | Same as 380, not T6 suitable |
| 413.0 | 296 MPa / 2.5% | Not recommended | N/A | No significant Mg, not treatable |
| AlSi10MnMg | 250-310 MPa / 8-15% | Possible in vacuum DC | Variable | Designed for high F performance |
F condition (as-cast) and when it is enough
F condition means the part is used as it comes out of the mold, without any subsequent heat treatment (except possible natural aging at room temperature). For many die casting applications, F condition is perfectly adequate. 380-F alloy achieves 317 MPa tensile strength, which is sufficient for housings, covers, non-structural brackets, and thousands of industrial applications.
The trend in the automotive industry is to design alloys that achieve high properties in F condition, eliminating the need for the costly and complex T6 treatment. The AlSi10MnMg family is the best example: with 250-310 MPa strength and 8-15% elongation in F condition, these alloys meet automotive structural specifications without any post-treatment, reducing part cost by 20-30% compared to A356-T6.
T5 treatment (direct artificial aging, without prior solution) is an alternative for die cast parts that need a moderate strength increase. The part is aged at 160-200 degrees C for 2-4 hours immediately after casting, taking advantage of the partial solid solution that naturally forms during the rapid solidification of die casting. T5 increases strength by 10-15% without blister risk, but does not reach T6 levels.
Frequently asked questions
Can I switch from 380 to A356 to improve properties of an existing part?
Generally you cannot make a direct switch without redesigning the die and possibly the process. A356 has a different solidification range, lower fluidity at typical die casting temperatures, and requires low iron (< 0.20%) which can cause die soldering in conventional HPDC. If you need to improve the properties of a die cast part, consider these alternatives in order of complexity: (1) switch to A383 or 413, which are compatible with the same process; (2) migrate to vacuum die casting with AlSi10MnMg; (3) redesign for squeeze casting with A356-T6.
How does recycled scrap affect alloy choice?
Recycled scrap progressively increases iron, copper, and other trace element levels. For 380 alloy, this is relatively tolerable: you can produce 380 with up to 100% selected scrap. For A356, control is much tighter: iron must be kept below 0.20%, which limits scrap use to in-house returns of the same alloy and classified scrap with analysis certificates. For AlSi10MnMg, the 0.15% maximum iron makes it nearly impossible to use external scrap; P1020 primary metal is required as the base.
What alloy should I use if the part will be anodized?
Anodizing aluminum castings is inherently more difficult than anodizing extrusions due to the silicon phases and intermetallics present in casting alloys. For the best anodizing result, choose alloys with lower silicon content and no copper: A356 produces better results than 380 or 413. Silicon is exposed as dark spots in the anodic layer, and copper causes yellowish discoloration. If decorative anodizing is a critical requirement, consider 5xx.x series alloys (Al-Mg, no silicon) or extrusion processes with 6063 alloy.
How can I validate that the selected alloy meets my specifications before production?
The recommended approach has three stages: (1) Filling and solidification simulation (software such as MAGMASOFT, ProCAST, or Flow-3D CAST) to verify that the selected alloy, with its specific thermophysical properties, fills the mold correctly and solidifies without predicted defects. (2) Separately cast test bars under representative solidification conditions to validate the mechanical properties of the alloy with its heat treatment. (3) Pre-production pilot lot (typically 50-200 parts) with complete inspection: radiography, tensile testing of specimens cut from the part, metallographic microstructure analysis, and pressure-tightness testing if applicable. Only after passing all three stages should series production be committed.
Conclusión: there is no universal alloy
The perfect alloy for all applications does not exist. 380 dominates production volume due to its versatility and low cost, but it is not the answer for structural, welded, or heat-treated parts. A356 is the reference for quality and properties, but it is not economically viable for general-purpose housings. AlSi10MnMg is the technological frontier for mega casting, but it requires die casting equipment and liquid metal control that are not within reach of every foundry.
The correct decisión requires an honest assessment of the technical requirements of the part, the capabilities of the available process, and the project budget. If you need advice to select the optimal alloy for your specific application, our technical team can help with simulation, alloy selection, and process optimization.
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