Alloy selection is one of the most consequential decisions in aluminium casting design — it affects mechanical properties, castability, machinability, corrosion resistance, surface finish, and heat treatment response simultaneously. Yet in practice, alloy selection is often inherited from a previous design or defaulted to the most familiar option rather than systematically matched to the application's requirements. Understanding the principal alloys used in gravity die casting — LM25, LM6, LM24, and ADC12 — their compositions, properties, and the applications each is best suited to is practical knowledge for anyone specifying, procuring, or producing aluminium castings.
LM25 (A356): The Automotive Standard
LM25 — equivalent to the internationally recognised A356 designation — is an aluminium-silicon-magnesium alloy (nominally Al-7Si-0.3Mg) and the dominant alloy for gravity die casting in automotive and structural applications. Its combination of good castability, excellent response to T6 heat treatment, and the superior mechanical properties achievable after T6 — tensile strength of 260–310 MPa, elongation of 3–8% — make it the default choice for safety-critical structural castings where both strength and ductility are required. The magnesium content in LM25 is the key to its heat treatability: Mg2Si precipitates formed during T6 ageing are responsible for the dramatic strength increase over the as-cast condition. LM25 requires careful magnesium control (both minimum for heat treatment response and maximum to avoid hot tearing), low sodium and hydrogen levels for adequate elongation, and clean charge materials. It is more demanding to produce well than less heat-treatable alloys, but the mechanical properties achievable justify the process discipline required.
LM6: The Corrosion-Resistant Specialist
LM6 is a high-silicon aluminium alloy (nominally Al-12Si) with no significant magnesium or copper additions. Its near-eutectic silicon content gives it outstanding castability — excellent fluidity, minimal hot cracking tendency, and good filling of thin sections — combined with very good corrosion resistance derived from the absence of copper and the low porosity that its good castability enables. LM6 does not respond meaningfully to heat treatment and is used in the as-cast (F) condition, where its mechanical properties — tensile strength of approximately 160–190 MPa, elongation of 3–5% — are adequate for applications where strength is not the primary requirement. Marine components, marine outboard housings, architectural hardware, pump bodies for water service, and general purpose covers and enclosures in corrosive environments are typical LM6 applications. Its combination of easy casting and corrosion resistance makes it a reliable choice for high-volume, moderate-complexity industrial castings where the part geometry is challenging enough to require a forgiving alloy but the service environment demands more corrosion resistance than LM25 or copper-containing alloys provide.
LM24: The General Engineering Workhorse
LM24 is an aluminium-silicon-copper alloy (nominally Al-8Si-3Cu) positioned between LM25 and ADC12 in its property and application profile. Its copper content provides a moderate strength increase over LM6 and LM25 in the as-cast condition (tensile strength 170–220 MPa depending on section) without the full heat treatment cycle that LM25 T6 requires. LM24 offers better machinability than LM6 or LM25 — the copper content improves cutting behaviour — making it a common choice for castings that require significant post-cast machining. Its castability is good, though the copper content increases hot cracking sensitivity compared to LM6, requiring attention to gating and solidification management. LM24 is less corrosion-resistant than LM6 due to its copper content, and is not the appropriate choice for marine or chemically aggressive service. Its sweet spot is general industrial castings — pump housings for non-corrosive service, gearbox covers, motor brackets, manifolds, and structural housings — where moderate strength, good machinability, and good castability are all valued and the service environment is benign.
ADC12: The High-Pressure Die Casting Standard
ADC12 (Al-11Si-1.7Cu) is the dominant alloy for high-pressure die casting in India and globally, though it is also used in gravity die casting applications where its specific properties are valued. Its high silicon content (approximately 11%) gives it excellent fluidity and die-filling capability, while the copper addition provides strength and machinability. ADC12 is not heat-treatable to the same degree as LM25 — the higher copper and silicon content limit the elongation achievable after T6 — but in the as-cast or T5 condition it provides tensile strength of 230–280 MPa with elongation of 1–3%. In gravity die casting, ADC12 is chosen when good fluidity for thin-walled or complex geometries is a priority and when the lower elongation of the as-cast condition is acceptable for the application. It is commonly used for automotive transmission cases, complex housings, and thin-walled industrial components. Its lower elongation compared to LM25 T6 makes it unsuitable for safety-critical structural applications where fatigue resistance or impact absorption is a design requirement.
Making the Selection: A Practical Framework
The alloy selection decision should flow from a structured evaluation of four factors. First, mechanical property requirements: if the application requires T6-level strength and elongation (safety-critical structural, fatigue-loaded), LM25 is the starting point; if moderate as-cast strength with good machinability is sufficient, LM24 or ADC12; if strength is secondary to corrosion resistance and castability, LM6. Second, service environment: copper-containing alloys (LM24, ADC12) are not appropriate for marine or chemically aggressive environments; LM6 or LM25 are preferred. Third, casting complexity: thin-walled, geometrically complex castings benefit from the higher fluidity of LM6 or ADC12; simpler, thicker castings are less alloy-sensitive. Fourth, post-cast operations: where significant machining is required, LM24 or ADC12's better machinability reduces tooling cost and cycle time; where T6 heat treatment is specified, LM25 is the only appropriate choice. Applying this framework systematically — rather than defaulting to a familiar alloy — produces alloy specifications that are genuinely matched to their application, reducing the risk of both over-specification (using LM25 T6 where LM24 as-cast would suffice, at unnecessary cost) and under-specification (using LM6 where the application's strength requirements demand LM25 T6).