BS 1490 · Group B · Special Purpose
LM21
Aluminium Alloy
Al-Si6Cu4Mn0.4Mg0.2 — High Proof Stress in As-Cast Condition
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-Si6Cu4Mn0.4Mg0.2
- 5.0–7.0 %
- 2.75 g/cm³
- 170 N/mm²
- 610–525 °C
- 121 W/m·K
- 80–100 HB
Overview
High Proof Stress in As-Cast Condition
LM21 is a medium-silicon, high-copper alloy specifically formulated to deliver high proof stress and hardness in the as-cast (M) condition without the need for heat treatment. The combination of 5–7% Si and 3–5% Cu provides an as-cast Brinell hardness of 80–100 HB and good machinability, making it the choice for intricate castings requiring dimensional stability and wear resistance in service.
LM21 is only supplied in the M (as-cast) condition per BS 1490. It is not heat-treatable in the conventional T6 sense — the high Cu content means it responds minimally to precipitation hardening. The alloy is rated 3 (Good) for all gravity casting processes and is most commonly specified for hydraulic components and precision machined castings in the as-cast condition.
As-Cast Only — Good Hardness Without Heat Treatment
LM21 is BS 1490’s solution for applications requiring hardness and machinability in the as-cast state. At 80–100 HB as-cast in GDC, it delivers comparable hardness to a lightly heat-treated alloy without the cost and lead time of heat treatment. Use when dimensional stability and short delivery are priorities.
Chemical Composition
LM21 Chemical Composition —BS 1490:1988
| Element | Symbol | Min % | Max % | Role in alloy |
|---|---|---|---|---|
| Silicon | Si | 5.0 | 7.0 | 5–7% Si provides moderate fluidity. Not eutectic – wider freezing range than LM6/LM9. Controls castability at acceptable levels. |
| Copper | Cu | 3.0 | 5.0 | 3–5% Cu is the highest Cu range in Group B alloys. Contributes to as-cast hardness (80–100 HB) and excellent machinability. Significantly reduces corrosion resistance. |
| Magnesium | Mg | 0.1 | 0.3 | 0.1–0.3% Mg – minor addition. Contributes to some age-hardening response but primarily used for deoxidation in the melt. |
| Iron | Fe | — | 1.0 | Up to 1.0% Fe – higher limit reflecting its GDC/HPDC application range. |
| Manganese | Mn | 0.2 | 0.6 | Impurity limit. Controlled to maintain clean melt. |
| Nickel | Ni | — | 0.3 | Impurity limit. |
| Zinc | Zn | — | 2.0 | 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
LM21 Equivalent Grades — International Standards
BS 1490
LM21
Group B · Special Purpose Al-Si6Cu4Mn0.4Mg0.2
EN 1706
EN AC-45000
EN AC-Al Si6Cu4
ASTM / AA
308.0
308.0 — closest US equivalent
JIS H 5302
ADC8
Al-Si-Cu-Mn die casting alloy
DIN 1725-2
G-AlSi6Cu4
Superseded by EN 1706
ISO 3522
Al-Si6Cu4
ISO casting alloy designation
Mechanical Properties
LM21 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 |
|---|---|---|---|---|---|
| M | Sand / Investment | 150 | — | 1 | 75–90 |
| M | Chill / GDC | 170 | — | 1 | 80–100 |
Physical Properties
LM21 Physical & Thermal Properties
Density
2.75 g/cm³
Approx. 1/3 the density of steel
Freezing Range
610–525 °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
80–100 HB
As-cast or stated condition
| Property | Rating | Notes |
|---|---|---|
| Corrosion resistance | Fair (C) | Moderate corrosion resistance. High Cu content (3–5%) limits suitability for wet or corrosive environments. |
| Decorative anodising | Poor (D) | High Cu gives poor anodising quality. |
| Weldability | Poor | High Cu content causes hot cracking during welding. |
| Machinability group | 3 — Good | Group 3 machinability. High Cu content improves chip formation significantly. |
Casting Suitability
LM21 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 | Moderate Si and high Cu give good but not excellent fluidity. |
| Resistance to hot tearing | 3 — Good | Reasonable hot-tear resistance. |
| Pressure tightness | 3 — Good | Good pressure tightness when correctly cast. |
| Machinability (as-cast) | 3 — Good | Group 3 machinability. High Cu content improves chip formation significantly. |
Heat Treatment
LM21 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.
As-Cast (M) Condition Only
LM21 is supplied exclusively in the M (as-cast) condition per BS 1490:1988. While the Cu content would allow some precipitation hardening response, the alloy is not formally specified in any heat-treated condition. The as-cast hardness of 80–100 HB is the design-intent mechanical state.
Applications
Typical Applications of LM21
| Industry | Typical Parts | Why LM13 |
|---|---|---|
| General Engineering | Precision machined housings, valve bodies, hydraulic parts requiring as-cast hardness | High as-cast hardness without heat treatment cost |
| Automotive | Gearbox components, bearing housings, brackets requiring dimensional stability | Good machinability and as-cast hardness |
| Industrial | Machine parts requiring high precision machining, jig and fixture components | Group 3 machinability + 80–100 HB as-cast |
Precision Machined Parts
Valve Bodies
Hydraulic Parts
Gearbox Components
Bearing Housings
Machine Components
Jig Components
When NOT to Specify LM21
Do not specify LM21 when corrosion resistance is required — high Cu makes it unsuitable for marine or wet environments. Do not specify when T6 strength is needed — use LM25 or LM9. Do not specify for HPDC.
Frequently Asked Questions
Common Questions About LM21
What is the chemical composition of LM21 aluminium alloy?
LM21 per BS 1490:1988 has nominal composition Al-Si6Cu4Mn0.4Mg0.2. Silicon: 5.0–7.0%, Copper: 3.0–5.0%, Magnesium: 0.1–0.3%. The remainder is aluminium with controlled impurity limits.
What is the EN 1706 / European equivalent of LM21?
The EN 1706 equivalent of BS 1490 LM21 is EN AC-45000, with chemical symbol designation EN AC-Al Si6Cu4. This European standard supersedes the German DIN designation G-AlSi6Cu4.
What is the ASTM / AA equivalent of LM21?
The ASTM/Aluminium Association equivalent of LM21 is 308.0. 308.0 — closest US equivalent.
What is the JIS equivalent of LM21?
The JIS H 5302 equivalent of LM21 is ADC8. Al-Si-Cu-Mn die casting alloy.
What is the tensile strength of LM21?
The minimum tensile strength of LM21 is 170 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 LM21 be heat treated to T6?
No — LM21 is not normally heat-treated to T6. Available conditions: M.
What casting processes are suitable for LM21?
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 LM21 aluminium alloy?
The density of LM21 is 2.75 g/cm³ — approximately one-third the density of steel (7.85 g/cm³), making it ideal for lightweight structural applications.