BS 1490 · Group C · Special Purpose — Limited Application
lm26
Aluminium Alloy
Al-Si10Cu3Mg1 — Elevated Temperature Piston Alloy — Alternative to LM13
Sand Casting
Gravity Die Casting
Low Pressure Die Casting
ISO 9001:2015
At a Glance
- Nominal composition
- Si content
- Density
- Best tensile (min)
- Freezing range
- Thermal conductivity
- Hardness
- Al-Si10Cu3Mg1
- 8.5–10.5 %
- 2.72 g/cm³
- 210 N/mm²
- 580–520 °C
- 121 W/m·K
- 90–120 (mandatory per BS 1490 TE condition) HB
Overview
Elevated Temperature Piston Alloy — Alternative to LM13
LM26 is a Group C special-purpose alloy in the Al-Si-Cu-Mg system, designed as an alternative piston alloy to LM13 with improved resistance to elevated temperatures. The higher silicon content (8.5–10.5%) combined with 2–4% Cu and 0.5–1.5% Mg delivers good strength at elevated temperatures and excellent wear resistance, with a lower coefficient of thermal expansion than general-purpose alloys.
LM26 is primarily used for diesel and petrol engine pistons and is produced in small volumes — foundries offering LM26 castings are more limited than those offering Group A alloys. Like LM13, LM26 is subject to mandatory hardness testing per BS 1490. Machining requires carbide or diamond tooling due to the high Si content.
Group C — Limited Availability
LM26 is a Group C alloy of limited application. Prospective users should confirm availability and foundry experience with this alloy before specifying it. For most elevated-temperature applications where LM26 is considered, LM13 and LM29 are the primary alternatives.
Chemical Composition
LM26 Chemical Composition — BS 1490 : 1988
| Element | Symbol | Min % | Max % | Role in alloy |
|---|---|---|---|---|
| Silicon | Si | 8.5 | 10.5 | 8.5–10.5% Si — near eutectic. Provides good fluidity and wear resistance through primary Si particles in service. |
| Copper | Cu | 2.0 | 4.0 | 2–4% Cu contributes to elevated-temperature strength through CuAl2 precipitation. |
| Magnesium | Mg | 0.5 | 1.5 | 0.5–1.5% Mg — forms Mg2Si precipitates, adding to the complex precipitation system. |
| Iron | Fe | — | 1.2 | Up to 1.2% Fe. Forms intermetallic compounds that contribute to elevated-temperature strength. |
| Manganese | Mn | — | 0.5 | Impurity limit. Controlled to maintain clean melt. |
| Nickel | Ni | — | 1.0 | Impurity limit. |
| Zinc | Zn | — | 1.0 | Impurity limit. Higher Zn increases hot-cracking risk. |
| Lead | Pb | — | 0.2 | Impurity limit. |
| Tin | Sn | — | 0.1 | Impurity limit. |
| Titanium | Ti | — | 0.2 | Grain refiner when added as TiB2 master alloy. |
| Aluminium | Al | Remainder | Base metal. Composition verified by OES (ARUN Technology UK, MERLIN-4 ULTRA). | |
| Other elements | — | — | 0.05 each / 0.15 total |
Each unspecified element ≤0.05%, total ≤0.15%. |
Global Standards
LM26 Equivalent Grades — International Standards
BS 1490
LM26
Group C · Special Purpose — Limited Application Al-Si10Cu3Mg1
EN 1706
EN AC-48200
EN AC-Al Si10Cu3Mg
ASTM / AA
—
No direct ASTM equivalent
JIS H 5302
—
No direct JIS equivalent
DIN 1725-2
G-AlSi10Cu3Mg
Approximate — superseded by EN 1706
ISO 3522
Al-Si10Cu3Mg
ISO casting alloy designation
Mechanical Properties
LM26 Mechanical Properties — BS 1490:1988 Table 4
Minimum values from separately cast test samples per BS 1490:1988. These are guaranteed minimums — not average values. Properties in actual castings vary with section thickness, cooling rate, and local geometry.
| Condition | Process | Tensile N/mm² min | 0.2% Proof N/mm² | Elongation % min | Brinell HB |
|---|---|---|---|---|---|
| TE | Chill / GDC | 210 | — | — | 90–120 |
Physical Properties
LM26 Physical & Thermal Properties
Density
2.72 g/cm³
Approx. 1/3 the density of steel
Freezing Range
580–520 °C
Solidification temperature range
Thermal Conductivity
121 W/m·K
Heat dissipation capability
Electrical Conductivity
— % IACS
Relative to copper standard
Linear Expansion
21 ×10⁻⁶/K
20–300°C range
Brinell Hardness
90–120 (mandatory per BS 1490 TE condition) HB
As-cast or stated condition
| Property | Rating | Notes |
|---|---|---|
| Corrosion resistance | Fair (C) | Moderate corrosion resistance. Cu content limits suitability for corrosive environments. |
| Decorative anodising | Poor (D) | Not suitable for decorative anodising. |
| Weldability | Poor | High Cu and complex alloy system — not recommended for welding. |
| Machinability group | 3 — Good | Group 3. Carbide tooling required due to Si content. |
Casting Suitability
LM26 Casting Process Suitability — BS 1490:1988 Table 8
Ratings from BS 1490:1988 Table 8. Scale: 4 = Excellent, 3 = Good, 2 = Fair, 1 = Poor, n = Not normally recommended.
Sand Casting
3
Gravity Die Casting (GDC)
3
Low Pressure (LPDC)
3
High Pressure (HPDC)
n
| Castability Property | Rating | Explanation |
|---|---|---|
| Fluidity | 3 — Good | Near-eutectic Si gives good fluidity. |
| Resistance to hot tearing | 4 — Excellent | Good resistance from near-eutectic composition. |
| Pressure tightness | 2 — Fair | Moderate pressure tightness. |
| Machinability (as-cast) | 3 — Good | Group 3. Carbide tooling required due to Si content. |
Heat Treatment
LM26 Heat Treatment — BS 1490:1988 Conditions
M
As Cast
No heat treatment. Used as-cast. Dimensional stability is the priority.
TS
Stress Relieved
200–250°C, 2–4 hours. Reduces residual casting stresses without significant property change.
TB
Solution + Natural Age
Solution treat, quench, age at room temperature. Strength develops over days to weeks.
TB7
Solution + Stabilise
Solution treat, quench, then stabilise at low temperature. Good ductility and dimensional stability.
TE
Artificially Aged Only
Age at 160–180°C, 6–12 hours. Moderate strength improvement without solution treatment.
TF
T6 — Peak Strength
Solution treat + quench + artificial age. Peak strength condition. Full in-house at Creative Alucast.
TF7
T7 — Over-aged / Stabilised
Solution treat + quench + over-age. Slightly lower strength than TF but superior dimensional stability.
TE Condition Only — Hardness Testing Mandatory
LM26 is specified only in the TE (artificially aged, no solution treatment) condition. Brinell hardness testing is mandatory per BS 1490 Table 5: 90–120 HB for TE condition. Verified in-house at Creative Alucast.
Applications
Typical Applications of LM26
| Industry | Typical Parts | Why LM13 |
|---|---|---|
| Automotive | Engine pistons, cylinder components, elevated temperature structural parts | Elevated temperature strength above 200°C |
| Industrial Engines | Diesel pistons, high-temperature valve bodies | Alternative to LM13 for some piston applications |
Engine Pistons
Elevated Temperature Parts
Diesel Pistons
High-Temp Valve Bodies
Cylinder Components
When NOT to Specify LM26
Do not specify LM26 for general engineering applications. For most piston applications, LM13 or LM29 may be more appropriate. Confirm foundry availability before specifying.
Frequently Asked Questions
Common Questions About LM25
What is the chemical composition of LM26 aluminium alloy?
LM26 per BS 1490:1988 has nominal composition Al-Si10Cu3Mg1. Silicon: 8.5–10.5%, Copper: 2.0–4.0%, Magnesium: 0.5–1.5%. The remainder is aluminium with controlled impurity limits.
What is the EN 1706 / European equivalent of LM26?
The EN 1706 equivalent of BS 1490 LM26 is EN AC-48200, with chemical symbol designation EN AC-Al Si10Cu3Mg. This European standard supersedes the German DIN designation G-AlSi10Cu3Mg.
What is the ASTM / AA equivalent of LM26?
The ASTM/Aluminium Association equivalent of LM26 is —. No direct ASTM equivalent.
What is the JIS equivalent of LM26?
There is no direct JIS H 5302 equivalent for LM26. No direct JIS equivalent.
What is the tensile strength of LM26?
The minimum tensile strength of LM26 is 210 N/mm² in the best condition and process per BS 1490:1988 Table 4 (separately cast test samples). Properties vary with casting process and heat treatment condition.
Can LM26 be heat treated to T6?
No — LM26 is not normally heat-treated to T6. Available conditions: TE.
What casting processes are suitable for LM26?
Per BS 1490:1988 Table 8: Sand casting (3), Gravity Die Casting (3), Low Pressure Die Casting (3), High Pressure Die Casting (n). Scale: 4=Excellent, 3=Good, 2=Fair, n=Not recommended.
What is the density of LM26 aluminium alloy?
The density of LM26 is 2.72 g/cm³ — approximately one-third the density of steel (7.85 g/cm³), making it ideal for lightweight structural applications.