Alloy selection is the decision that happens before the die is designed, before the process is set, and before a single kilogram of metal is melted. It sets the ceiling on every mechanical property the casting can achieve, determines how cleanly the alloy will fill the die geometry, dictates what post-cast treatment is possible, and influences every downstream operation from machining to surface finishing. Yet in practice, alloy selection in many foundries and design offices is made on the basis of habit, incomplete information, or a misapplied specification from a reference document that was not written for the specific application at hand.
The three aluminium casting alloys that dominate gravity die casting across the Indian foundry industry — LM6, LM25, and LM24 — are genuinely different in their properties, their casting behaviour, and their application fit. Understanding those differences in practical terms is the starting point for getting the selection right.
What Alloy Selection Actually Determines
The alloy determines the composition of the metal being cast — the percentages of silicon, magnesium, copper, and other elements that define the alloy family. From this composition flow a set of physical and mechanical properties: density, thermal conductivity, solidification behaviour, as-cast mechanical properties, and response to heat treatment. What alloy selection does not control is casting quality in the sense of soundness — porosity, inclusions, and cold shuts are process defects, not alloy defects. A well-processed LM6 casting will outperform a poorly processed LM25 casting on every quality metric despite LM25's superior mechanical properties. Alloy selection sets the potential. Process execution determines how much of that potential is realised.
LM6 — Al-12Si — Understanding Why It Casts So Well
LM6 sits at or near the eutectic composition of the aluminium-silicon binary system — approximately 12 percent silicon. The eutectic composition solidifies at a single temperature rather than over a range, meaning LM6 transitions from fully liquid to fully solid over a very narrow temperature interval. The practical foundry implications are significant: low shrinkage porosity tendency, low hot tearing susceptibility, and excellent fluidity that enables fill in thin sections and complex geometries before freezing.
These characteristics make LM6 genuinely forgiving to cast. A foundry with imperfect gating design or a die that runs slightly too cold will produce acceptable LM6 castings where the same conditions would produce misruns or cold shuts in LM25. The corrosion resistance of LM6 is also outstanding — the high silicon content passivates the alloy surface effectively, making it well-suited for marine environments, food processing applications, and outdoor exposure without surface treatment.
The limitation is mechanical performance. LM6 in the as-cast condition achieves tensile strength of approximately 160 to 190 MPa. More importantly, it is not heat-treatable — the absence of magnesium means precipitation hardening is not possible, and the alloy cannot be T6 tempered. The as-cast strength is the ceiling. For any component where the design load requires more than this, LM6 is the wrong alloy regardless of how well it casts.
LM25 — Al-7Si-0.3Mg — The Alloy That Earns Its Complexity
LM25 is the dominant structural aluminium casting alloy in gravity die casting globally. Magnesium enables precipitation hardening — during T6 heat treatment, fine precipitates of magnesium silicide form within the aluminium grain structure, obstructing dislocation movement and dramatically increasing yield strength and tensile strength relative to the as-cast condition. LM25 in the as-cast condition has tensile strength of approximately 130 to 160 MPa. In the T6 condition, tensile strength rises to 230 to 280 MPa with yield strength of 200 to 220 MPa — nearly double the as-cast value.
The practical consequence is that LM25 imposes higher demands on melt quality than LM6. The magnesium addition makes the alloy more sensitive to oxide bifilm defects — bifilms that would be tolerated in LM6 without measurable property loss will reduce elongation in LM25 T6 below specification. Degassing must be thorough, fluxing must be consistent, and RPT acceptance criteria must be set and enforced. Machinability of LM25 is significantly better than LM6 — the lower silicon content reduces abrasive tool wear, and the surface finish achievable in turning and boring is superior.
LM24 — Al-8.5Si-3.5Cu — The Specialist Alloy
LM24 is defined by its copper content — approximately 3.5 percent — which gives it a specific set of properties suited to a narrower but important range of applications. Copper increases hardness and wear resistance in the as-cast condition: LM24 achieves a Brinell hardness of approximately 75 to 85 HB — higher than as-cast LM25 — making it the natural specification for components experiencing surface contact stress, abrasive wear, or fretting in service. Pressure tightness is the other defining characteristic — LM24 consistently produces castings with low interconnected microporosity, making it the specification of choice for hydraulic components, valve bodies, and pressure vessel applications.
The significant limitation is corrosion resistance. The copper content makes LM24 susceptible to galvanic corrosion in humid, marine, and chemically active environments — far more so than either LM6 or LM25. LM24 components in corrosion-risk environments require protective coating — painting or powder coating is the standard approach since copper content interferes with anodising quality.
The Selection Decision in Practice
The practical selection logic is direct. If castability and corrosion resistance are the primary requirements with modest strength needs, specify LM6. If the component is structural, will be heat treated to T6, and requires good machinability alongside the highest achievable casting alloy strength, specify LM25. If pressure tightness and wear resistance are the primary technical requirements and the component will be protected from corrosion by coating, specify LM24.
Where the application falls between these categories — a structural component in a corrosive environment, or a pressure-tight component that also needs T6 strength — the selection requires a genuine engineering evaluation. The alloy that casts best is not always the alloy that performs best in service, and the alloy that performs best in service is not always the alloy that the foundry can process most reliably. Getting this three-way balance right is what alloy selection is actually about.