When selecting an aluminum alloy for a casting application, understanding the varying properties of specific graded materials is vital. Three alloys regularly considered for their castability and mechanical attributes are LM6, LM25, and LM24. While all are in the LM series designation standard, each offers distinct characteristics influencing their suitability for different fabrication needs. This article provides the differences between aluminum LM6, LM25, and LM24.
What’s Aluminum Alloy LM6
LM6 has a chemical composition of 10-13% silicon, with smaller amounts of other elements like copper, magnesium, iron, manganese, and nickel. It is suited for die casting processes like sand casting, pressure die casting, and gravity die casting. Mechanical properties include a tensile strength of 160-190 MPa for sand casting and 280 MPa for pressure die casting, with an elongation of 5% for sand casting and 2-5% for pressure die casting. Its strength decreases at temperatures above 250°C. LM6 exhibits excellent corrosion resistance under atmospheric and marine conditions and can be anodized in various colors. It is widely used for marine components, automobile fittings, housings, and other applications where ease of casting and corrosion resistance are important. However, it has relatively low machinability due to its silicon content.
LM6 Chemical Composition
LM6 is a high-silicon aluminum alloy primarily composed of Aluminum (Al) as the base metal, with approximately 10–13% Silicon (Si) for enhanced fluidity and corrosion resistance. It typically contains small amounts of Copper (Cu) up to 0.1%, Magnesium (Mg) up to 0.1%, Iron (Fe) up to 0.6%, Manganese (Mn) up to 0.5%, Nickel (Ni) up to 0.1%, Zinc (Zn) up to 0.1%, and Titanium (Ti) up to 0.2%. The high silicon content makes LM6 particularly suitable for complex castings requiring good surface finish, excellent corrosion resistance, and low shrinkage during solidification.
What’s Aluminum Alloy LM25?
LM25 is an aluminum casting alloy that conforms to British Standard BS 1490:1988. It has a chemical composition of 0.2% maximum copper and 0.2-0.6% magnesium, with 6.5-7.5% silicon. LM25 can be produced through sand casting, gravity die casting, and high pressure die casting processes. It exhibits higher strength levels than other aluminum alloys when heat treated, with the precipitation treated (TE) condition, solution treated and stabilized (TB7) condition, and fully heat treated (TF) condition offering increasing mechanical property improvements. Some key properties include a tensile strength of 160-200 MPa in the as-cast condition increasing to 220-260 MPa when fully heat treated. LM25 also displays good corrosion resistance and machinability. It finds wide application in automotive components, transportation, industrial machinery, and other situations where higher strength combined with ease of casting is required.
lM25 Chemical Composition
LM25 is a heat-treatable aluminum casting alloy primarily composed of Aluminum (Al) with around 6.5–7.5% Silicon (Si) and 0.2–0.6% Magnesium (Mg) to enhance strength and hardness after heat treatment. It also includes controlled amounts of Iron (Fe) up to 0.5%, Copper (Cu) up to 0.2%, Manganese (Mn) up to 0.5%, Zinc (Zn) up to 0.1%, Nickel (Ni) up to 0.1%, Lead (Pb) and Tin (Sn) each up to 0.05%, and Titanium (Ti) up to 0.2%. LM25 offers a good balance of mechanical properties, excellent corrosion resistance, and high strength when heat-treated, making it ideal for automotive, aerospace, and marine applications.
What’s Aluminum Alloy LM24?
LM24 is an aluminum alloy that conforms to British Standard BS 1490:1988. It has a chemical composition containing 3.0-4.0% copper, 0.3% maximum magnesium, and 7.5-9.5% maximum silicon, and can be used for both gravity die casting and high pressure diecasting processes. Mechanical properties include a tensile strength of 180 MPa for gravity die castings and 320 MPa for high pressure diecast parts, with retention of most room temperature strength up to around 150°C. Machinability is similar to other aluminum alloys containing silicon, requiring carbide tools. Corrosion resistance is fair under normal conditions but protection is needed in marine environments. LM24 principally provides excellent casting characteristics for pressure die casting applications and higher mechanical properties than LM6 in die cast components, making it well-suited for high strength die casting applications.
lM24 Chemical Composition
LM24 is a pressure die casting aluminum alloy primarily composed of Aluminum (Al) and Copper (Cu), with Copper content ranging from 2.0–3.5%, which provides high strength and good wear resistance. It also contains Silicon (Si) between 9.0–11.0% to improve castability, along with Iron (Fe) up to 1.3%, Magnesium (Mg) up to 0.10%, Manganese (Mn) up to 0.5%, Nickel (Ni) up to 0.5%, Zinc (Zn) up to 1.0%, Lead (Pb) and Tin (Sn) each up to 0.1%, and Titanium (Ti) up to 0.2%. LM24 is widely used in the automotive and general engineering sectors due to its excellent casting characteristics, good mechanical properties, and pressure tightness.
Difference Between LM6, LM25 & LM24
| LM6 | LM25 | LM24 | |
| Composition | 0.1% max Cu | 0.2-0.6% Mg | 3.0-4.0% Cu |
| Strength | Moderate | High, enhanced by heat treatment | High for die casting |
| Machinability | Difficult, high Si | Fair | Similar to other Al-Si alloys |
| Heat Treatment | Generally not | Can be strengthened through precipitation hardening | Generally not |
| Corrosion Resistance | Excellent | Good | Fair, needs protection in marine environments |
| Casting Process | Sand, die casting | Sand, gravity die casting | Die casting |
| Applications | Marine, auto fittings | General fabrication, transportation | High strength pressure die casting |
-LM6 has best corrosion resistance
-LM24 provides advantages for die casting applications requiring maximum properties
-Machinability is poorest for LM6 and best for LM24
-Composition, properties and processing suit different specialized application needs for each alloy.
LM6, LM25 & LM24 Aluminum Alloy, Which is the Best?
LM6 is best for corrosion resistance and fluidity. With its high silicon content (10–13%), LM6 is ideal for intricate castings and marine or chemical environments. It is non-heat-treatable but offers excellent surface finish and dimensional stability, making it ideal for non-structural components exposed to harsh conditions.
LM25 is best for mechanical strength and versatility. It is heat-treatable and contains 6.5–7.5% silicon and 0.2–0.6% magnesium, making it suitable for structural components in automotive, aerospace, and marine applications. LM25 offers a strong balance of toughness, corrosion resistance, and machinability.
LM24 is best for pressure die casting and high strength in non-corrosive environments. It has higher copper (2.0–3.5%) and silicon (9.0–11.0%) content, making it stronger and more wear-resistant than LM6 or LM25 but less corrosion-resistant. It is commonly used for automotive engine parts and housings where strength and precision are key.