Aluminium Casting Alloys Explained
LM2, LM4, LM6, LM25, ADC12, A380, A356 — seven alloys, one complete guide. Understand composition, mechanical properties, heat treatability, and the right alloy for your application, from RR Enterprises' 20+ years of casting experience in Coimbatore.
Best Aluminium Alloys for Casting – LM6, LM25, ADC12, A380, A356 and More Explained
Selecting the right aluminium alloy for casting is one of the most consequential decisions in the design and procurement of cast components. Choose the wrong alloy and you risk poor castability, inadequate mechanical strength, failed pressure tests, or components that cannot be heat treated as required.
At RR Enterprises, Coimbatore, we have been casting aluminium components since 2002 — working with the full range of aluminium casting alloys across die casting, gravity casting, and sand casting processes. This guide covers every major casting alloy we use: its composition, mechanical properties, heat treatability, corrosion resistance, and the applications it is best suited for.
Quick Reference: For high-volume die casting, specify ADC12 or A380. For structural gravity castings requiring T6 heat treatment, specify LM25 or A356. For maximum corrosion resistance in gravity or sand casting, specify LM6. For general-purpose die casting where cost is a priority, LM2 is a reliable choice.
Why Alloy Selection Matters
Aluminium casting alloys are primarily Al-Si (aluminium-silicon) alloys — the silicon content dramatically affects the alloy's fluidity, porosity, shrinkage behaviour, machinability, corrosion resistance, and mechanical properties. Secondary elements — copper (Cu), magnesium (Mg), zinc (Zn), manganese (Mn) — further tailor properties for specific applications.
The key properties you need to balance when selecting an aluminium casting alloy are:
- Castability — how easily does the alloy fill the mould, and how prone is it to shrinkage porosity and hot cracking?
- Mechanical strength — tensile strength (MPa), yield strength (MPa), and elongation (%) in both as-cast and heat-treated conditions
- Heat treatability — can the alloy be T5 or T6 treated to significantly increase strength? Not all alloys can.
- Corrosion resistance — essential for marine, chemical, food-contact, and outdoor applications
- Machinability — how easily can cast features be post-machined to final dimensions and tolerances?
- Cost — alloy and processing costs vary significantly across the range
Detailed Alloy Profiles – The 7 Most Important Casting Alloys
Below we profile each major aluminium casting alloy used by RR Enterprises — with actual mechanical property data, composition, process compatibility, and the applications each alloy is best suited for.
LM6 is a near-eutectic Al-Si12 alloy with the best castability in the BS range. Its high silicon content (approximately 12%) gives exceptional fluidity — ideal for thin-wall castings and complex geometries. LM6 is not heat treatable, but its as-cast corrosion resistance is outstanding, making it the preferred alloy for marine components, chemical equipment, and thin-wall decorative or structural parts that must survive in corrosive environments without post-treatment.
LM25 is the workhorse of premium gravity and sand casting — and the British equivalent of the American A356 alloy. The addition of 0.25–0.45% magnesium makes LM25 fully heat treatable. After T6 treatment (solution heat treatment + artificial ageing), tensile strength increases dramatically from approximately 180 MPa as-cast to 280–310 MPa T6. This makes LM25/A356 the preferred alloy for aerospace structural castings, automotive safety-critical parts, and pressure-tight applications where maximum integrity is required.
ADC12 (Japanese Industrial Standard, equivalent to A383) is the dominant die casting alloy across Asia — used in automotive, electronics, and general engineering high-pressure die casting. Its high silicon content with copper addition gives excellent fluidity for thin-wall die casting, good dimensional stability, and adequate strength for most non-structural applications. ADC12 is not heat treatable due to its copper content increasing porosity susceptibility during solution treatment, but its as-cast mechanical properties are sufficient for the vast majority of die casting applications.
A380 is the most widely used die casting alloy globally — the international benchmark for high-pressure die casting. It offers an excellent combination of castability, mechanical properties, and cost. Compared to ADC12, A380 has slightly lower silicon content but higher tensile strength in the as-cast condition. A380 maintains good dimensional stability across a wide temperature range — making it ideal for automotive under-hood components, electronic housings, and industrial equipment where thermal cycling occurs. Like ADC12, it is not heat treatable in standard HPDC form.
A356 is the American ASTM designation for the same base alloy as British LM25 — an Al-Si7Mg0.3 alloy that is the premier choice for aerospace, defence, and automotive structural gravity castings. After T6 heat treatment (solution at 540°C, water quench, artificial ageing at 155°C), A356 achieves the best combination of strength, ductility, and pressure tightness available from any standard gravity casting alloy. A356-T6 is specified for safety-critical castings where the component must withstand both static and dynamic loads — including wheel hubs, brake callipers, and aerospace frames.
LM2 is a cost-effective general-purpose die casting alloy — the British Standard equivalent widely used for non-critical die castings where cost is a primary driver and maximum mechanical properties are not required. Its good fluidity and die castability make it suitable for a wide range of housing, bracket, and cover castings in the automotive, electrical, and consumer goods industries. LM2 offers good machinability compared to higher-silicon alloys.
LM4 is a highly versatile general-purpose aluminium casting alloy compatible with sand casting, gravity die casting, and pressure die casting. Its lower silicon content (4–6%) compared to LM6 or ADC12 gives better machinability and makes it heat treatable — T5 treatment improves mechanical properties over the as-cast condition. LM4 is a cost-effective choice for general engineering components, automotive parts, and mechanical equipment castings where a single alloy must work across multiple casting processes.
Full Alloy Comparison Table
The table below gives a side-by-side comparison of all seven alloys across the key selection criteria:
| Property | LM6 | LM25 / A356 | ADC12 | A380 | LM2 | LM4 |
|---|---|---|---|---|---|---|
| Casting Process | Gravity / Sand | Gravity / Sand | Die (HPDC) | Die (HPDC) | Die (HPDC) | All processes |
| Si Content (%) | ~12% | 6.5–7.5% | 9.6–12% | 7.5–9.5% | 9–11.5% | 4–6% |
| Tensile Strength | 160 MPa | 310 MPa (T6) | 240–280 MPa | 280–320 MPa | 220–260 MPa | 200–260 MPa |
| Heat Treatable | No | T5 / T6 | No | No | No | T5 |
| Corrosion Resistance | Excellent | Good | Moderate | Moderate | Moderate | Moderate |
| Castability / Fluidity | Excellent | Good | Excellent | Excellent | Very Good | Good |
| Porosity Risk | Low | Low | Moderate | Moderate | Moderate | Low–Moderate |
| Machinability | Moderate | Good | Moderate | Good | Good | Very Good |
| Pressure Tightness | Excellent | Excellent | Moderate | Moderate | Moderate | Good |
| Typical Cost | Medium | Medium–High | Low–Medium | Medium | Low | Low–Medium |
Not Sure Which Alloy to Specify?
Share your component drawings and requirements with RR Enterprises — our metallurgy team will recommend the optimal alloy and provide a detailed quote within 24 hours.
Property Ratings by Alloy
Here's a visual summary of how the three most widely used alloys compare across five key properties — useful for a quick selection at a glance:
Alloy Selection by Casting Process
Different casting processes are compatible with different alloy groups. Here's a quick guide to which alloys work best with each process:
High-Pressure Die Casting
- ADC12 — most common in Asia
- A380 — most common globally
- LM2 — cost-effective general use
- A360 — where corrosion resistance matters
Gravity Die Casting
- LM25 / A356 — structural, T6 parts
- LM6 — corrosion-resistant, thin-wall
- LM4 — general engineering parts
- A357 — premium aerospace gravity cast
Sand Casting
- LM6 — best sand castability
- LM25 / A356 — heat-treated structural
- LM4 — general-purpose flexibility
- LM9 — good for complex sand cores
Alloy Applications by Industry
Different industries have specific alloy preferences driven by their component requirements — here's how the major sectors align with specific aluminium casting alloys:
Automotive
Structural: LM25/A356-T6. Housings: ADC12/A380. Brackets: LM2/LM4
ADC12 / LM25Aerospace
Safety-critical structural parts requiring maximum T6 strength and fatigue life
A356-T6 / A357Marine & Offshore
Maximum corrosion resistance in salt spray and immersion environments
LM6Pumps & Hydraulics
Pressure-tight, low-porosity castings for fluid-handling systems
LM25 / LM6Electrical & EV
High-volume thin-wall die castings for motor and battery housings
A380 / ADC12Oil & Gas
Pressure-rated valve bodies, manifolds and instrumentation housings
LM25 / A356General Engineering
General-purpose housings, brackets, and covers — cost-driven
LM4 / LM2Export (ME/Europe)
Export clients often specify international designations — A380, A356, A383
A380 / A356Heat Treatment of Aluminium Casting Alloys
Heat treatment can dramatically improve the mechanical properties of certain aluminium casting alloys — but only those whose composition includes the necessary precipitation-hardening elements (primarily magnesium, Mg).
The two most important heat treatment conditions for casting alloys are:
- T5 (Artificial Ageing) — parts are cooled from an elevated casting or solution temperature and then artificially aged at 150–180°C. This improves hardness and strength moderately without a separate solution treatment step. Applicable to LM25, LM4, A356.
- T6 (Solution Treatment + Artificial Ageing) — parts are solution heat-treated at 520–540°C (dissolving Mg₂Si precipitates), rapidly water-quenched, then artificially aged at 150–165°C (re-precipitating fine Mg₂Si). This gives the maximum possible strength increase — up to 70% increase in tensile strength for LM25/A356 T6 compared to as-cast. Applicable to LM25, A356, A357.
Critical Point: Heat treatment cannot be applied to standard high-pressure die cast components made from ADC12, A380, or LM2 — the trapped gas pores expand at solution treatment temperatures, causing surface blistering and dimensional distortion. If you need a heat-treated die casting, discuss vacuum-assisted die casting (VADC) options with RR Enterprises' engineering team.
How RR Enterprises Recommends Alloys
When a client sends us drawings for aluminium casting, our metallurgy engineers assess the following before recommending an alloy:
- Casting process compatibility — the alloy must be fully compatible with the intended process (die, gravity, or sand casting)
- Mechanical property requirements — tensile strength, yield strength, hardness, and fatigue life targets from the drawing or specification
- Heat treatment feasibility — is T6 treatment achievable given the component geometry and porosity risk?
- Environmental conditions — will the component be exposed to corrosive environments, elevated temperatures, or cyclic stress?
- Post-casting operations — will the component be pressure-tested, welded, anodised, or heavily machined after casting?
- Customer specification or standard — if the buyer specifies a grade by name (e.g., ADC12, A380, LM6), we match it exactly including chemistry verification
- Cost optimisation — where mechanical requirements allow, we recommend the most cost-effective alloy that meets the specification
Frequently Asked Questions
ADC12 and A380 are the leading die casting alloys. ADC12 is dominant in Asia and offers excellent fluidity and dimensional stability. A380 is the most popular globally and provides slightly higher tensile strength in the as-cast condition. Both are Al-Si-Cu alloys optimised for high-pressure injection — neither is heat treatable in standard HPDC form, but both deliver consistent, cost-effective results for high-volume production.
LM25 (equivalent to A356) is the preferred gravity casting alloy when high strength after T6 heat treatment is required. LM6 is preferred where maximum corrosion resistance is the priority — especially for marine, chemical, and thin-wall applications. LM4 is a versatile, cost-effective choice for general-purpose gravity castings. The correct choice depends on the mechanical property requirements, operating environment, and whether heat treatment is feasible.
LM25 is the British Standard (BS 1490) designation and A356 is the American (ASTM/AA) designation for essentially the same Al-Si7Mg alloy. Both have approximately 6.5–7.5% silicon and 0.25–0.45% magnesium. Minor compositional differences exist between the national standards, but both alloys behave identically in gravity casting and heat treatment. RR Enterprises can supply components to either designation as required by the customer's specification.
Standard high-pressure die castings in ADC12 or A380 cannot be T6 heat treated because the trapped gas pores in HPDC castings expand at solution heat treatment temperatures (520–540°C), causing blistering and dimensional distortion. Vacuum-assisted die casting (VADC) significantly reduces porosity and can sometimes allow T5 treatment. If heat treatment is required, gravity casting with LM25 or A356 is the recommended alternative.
Yes. RR Enterprises provides material test certificates (MTCs) confirming alloy chemistry for every production batch on request. For export orders and safety-critical applications, we can provide certificates from NABL-accredited third-party laboratories. All raw material ingots are sourced from certified suppliers with traceability, and ingot certificates are maintained for each production run.
Ready to Order Aluminium Castings?
RR Enterprises casts all major aluminium alloys — LM6, LM25, ADC12, A380, A356 and more. Send your drawings for a free alloy recommendation and competitive quote from Coimbatore.
Related Articles
