Selecting the right aluminium casting alloy is one of the most consequential decisions in the design of a structural casting. The choice determines not only the mechanical properties of the finished part but also its castability, response to heat treatment, machinability, corrosion behaviour, and ultimate suitability for the application. Three alloys dominate structural aluminium casting applications in India and globally: LM25 (equivalent to A356/AlSi7Mg), A356 itself in its international designation, and ADC12 (equivalent to A383 in ASTM). Each occupies a distinct position in the performance-castability-cost space.
LM25 (BS 1490 / AlSi7Mg0.3)
LM25 is a hypoeutectic aluminium-silicon-magnesium alloy containing approximately 7% silicon and 0.3% magnesium. It is the workhorse alloy for structural gravity die castings and sand castings where mechanical performance matters. Its defining characteristic is heat treatability: LM25 responds well to T6 heat treatment (solution treatment followed by artificial ageing), which raises tensile strength from around 160–180 MPa in the as-cast condition to 260–290 MPa, with proof strength (0.2% offset) reaching 220–250 MPa. Elongation typically runs at 3–5% in T6 condition, giving the alloy enough ductility for structural applications where impact or load redistribution is a consideration. LM25 castings also machine cleanly, respond well to anodizing, and exhibit good corrosion resistance in most service environments. The limitation is that LM25 requires careful feeding and solidification control — its narrower freezing range than eutectic alloys means shrinkage porosity is a real risk if riser design and gating are not optimized.
A356 (ASTM / AlSi7Mg0.35)
A356 is effectively the international equivalent of LM25, with slightly tighter compositional controls — particularly on iron content, which is held to a maximum of 0.2% in premium A356 versus up to 0.5–0.6% in standard LM25. This tighter iron control is significant because iron forms needle-like intermetallic phases (β-AlFeSi) in aluminium-silicon alloys that reduce ductility and fatigue life. Low-iron A356 — often designated A356.2 — is specified for safety-critical structural castings such as automotive suspension components, steering knuckles, and aerospace brackets where elongation and fatigue performance are critical. In practice, for many Indian foundry applications, LM25 and A356 are used interchangeably, with the distinction becoming relevant only when the customer specifies a particular standard or when the application demands premium mechanical properties. A356 in T6 condition can achieve elongations of 6–8% with appropriately controlled iron levels, compared to 3–4% for standard LM25.
ADC12 (JIS / AlSi11Cu3)
ADC12 is a fundamentally different alloy from LM25 and A356, and comparing them requires understanding the different casting processes they are designed for. ADC12 — containing approximately 11% silicon and 3% copper — is optimized for high-pressure die casting (HPDC). Its high silicon content gives it excellent fluidity and minimal solidification shrinkage, both critical for filling thin-walled HPDC dies under high injection pressure. The copper addition improves machinability and elevated-temperature strength. However, ADC12 is not heat treatable to the same degree as LM25/A356 — the T6 response is limited, and many HPDC ADC12 castings are used in the as-cast or T5 (artificially aged only) condition. In GDC or sand casting, ADC12 can be used but its high copper content increases susceptibility to corrosion, and the alloy's mechanical properties in gravity-cast form are generally inferior to T6-treated LM25 for structural applications.
Head-to-Head Comparison for Structural GDC Applications
For a structural aluminium part produced by gravity die casting — a gearbox housing, a pump body, a structural bracket — LM25/A356 in T6 condition is almost always the correct alloy choice. The combination of adequate strength, good elongation, heat treatability, and corrosion resistance makes it fit for purpose in a way that ADC12 is not. ADC12 belongs in HPDC applications: thin-walled automotive body parts, transmission covers, and electronic housings where volume and cycle time are paramount. The error that occasionally appears in Indian foundry practice is attempting to use ADC12 in GDC for structural parts because it is more commonly available or cheaper — the result is typically a casting that meets dimensional requirements but underperforms on mechanical testing, particularly elongation and impact resistance.
Practical Guidance for Alloy Selection
The decision framework for alloy selection in structural aluminium castings should begin with the mechanical property requirements — specifically tensile strength, proof strength, elongation, and fatigue life if cyclic loading is present. If T6 heat treatment is feasible in the production process, LM25 or A356 is the correct starting point. If tighter iron control is needed for safety-critical applications, specify A356.2 or equivalent with iron capped at 0.15–0.20%. If the part is produced by HPDC and mechanical performance requirements are moderate, ADC12 is appropriate. If corrosion resistance in marine or outdoor environments is a priority alongside strength, consider LM6 (AlSi12) for castability or LM25 with appropriate surface treatment. The alloy datasheet is a starting point — the final properties depend equally on casting process control, heat treatment execution, and the specific section thicknesses of the part being produced.