BS 1490 · Group C · Special Purpose — Limited Application
lm28
Aluminium Alloy
Al-Si18Cu1.5Mg1Ni1 — Hypereutectic Piston Alloy — Minimum Thermal Expansion
Sand Casting
Gravity Die Casting
Low Pressure Die Casting
T6 Heat-Treatable
ISO 9001:2015
At a Glance
- Nominal composition
- Si content
- Density
- Best tensile (min)
- Freezing range
- Thermal conductivity
- Hardness
- Al-Si18Cu1.5Mg1Ni1
- 17.0–20.0 %
- 2.65 g/cm³
- 190 N/mm²
- 580–520 °C
- 109 W/m·K
- 120–140 (TE), 100–144 (TF) — mandatory per BS 1490 HB
Overview
Hypereutectic Piston Alloy — Minimum Thermal Expansion
LM28 is a hypereutectic Al-Si alloy with 17–20% silicon — far above the eutectic composition (12.6%). In hypereutectic alloys, primary silicon crystals solidify first as hard, angular particles dispersed throughout the aluminium matrix. This microstructure delivers exceptional wear resistance, the lowest coefficient of thermal expansion of any standard LM alloy, and strength retention at elevated temperatures — making LM28 a specialised piston alloy for high-performance engines.
The high Si content requires phosphorus modification (as specified in BS 1490 clause 5.4) to produce a fine, well-distributed primary Si structure. Without modification, coarse primary Si particles cause poor machinability and early fatigue fracture. Machining requires diamond tools. LM28 is a Group C alloy of limited application and restricted foundry availability.
Mandatory Metallographic Requirements — BS 1490 Clause 5.4
LM28 is subject to mandatory microstructure testing: average size of primary silicon crystallites ≤40µm, maximum individual particle ≤70µm. These requirements apply to separately cast chill test samples for each cast. Microstructure verified by metallurgical microscope at Creative Alucast.
Chemical Composition
LM28 Chemical Composition — BS 1490:1988
| ELEMENT | SYMBOL | MIN % | MAX % | ROLE IN ALLOY |
|---|---|---|---|---|
| Silicon | Si | 17.0 | 20.0 | 17–20% Si — hypereutectic composition. Primary silicon crystals form as hard particles before the eutectic. These particles provide wear resistance and reduce thermal expansion. Responsible for LM28's unique properties but also its machining difficulty. |
| Copper | Cu | 1.3 | 1.8 | 1.3–1.8% Cu contributes to elevated-temperature strength. |
| Magnesium | Mg | 0.8 | 1.5 | 0.8–1.5% Mg works with Ni to form complex precipitates stable at elevated temperature. |
| Iron | Fe | — | 0.7 | Controlled to ≤0.7%. Lower Fe than HPDC alloys, reflecting the quality requirements of piston applications. |
| Manganese | Mn | — | 0.6 | Impurity limit. Controlled to maintain clean melt. |
| Nickel | Ni | 0.8 | 1.5 | Impurity limit. |
| Zinc | Zn | — | 0.2 | Impurity limit. Higher Zn increases hot-cracking risk. |
| Lead | Pb | — | 0.1 | 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
LM28 Equivalent Grades — International Standards
BS 1490
LM28
Group C · Special Purpose — Limited Application Al-Si18Cu1.5Mg1Ni1
EN 1706
EN AC-48000
EN AC-Al Si18Cu1Ni1Mg1
ASTM / AA
—
No direct ASTM equivalent
JIS H 5302
—
No direct JIS equivalent
DIN 1725-2
G-AlSi18CuMgNi
Approximate equivalent — superseded
ISO 3522
Al-Si18CuNiMg
ISO casting alloy designation
Mechanical Properties
LM28 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 | 170 | — | — | 90–130 |
| TF | Chill / GDC | 190 | — | — | 100–144 |
Physical Properties
LM28 Physical & Thermal Properties
Density
2.65 g/cm³
Approx. 1/3 the density of steel
Freezing Range
580–520 °C
Solidification temperature range
Thermal Conductivity
109 W/m·K
Heat dissipation capability
Electrical Conductivity
— % IACS
Relative to copper standard
Linear Expansion
19 ×10⁻⁶/K
20–300°C range
Brinell Hardness
120–140 (TE), 100–144 (TF) — mandatory per BS 1490 HB
As-cast or stated condition
| PROPERTY | RATING | NOTES |
|---|---|---|
| Corrosion resistance | Fair (C) | Moderate corrosion resistance. Primarily used in internal engine environments. |
| Decorative anodising | Poor (D) | Not suitable for anodising. |
| Weldability | Poor | High Si hypereutectic alloy — not weldable. |
| Machinability group | 1 — Difficult | Group 1 — very difficult to machine. Diamond tooling required. High Si particles cause extreme tool wear. |
Casting Suitability
LM28 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
1
Gravity Die Casting (GDC)
2
Low Pressure (LPDC)
1
High Pressure (HPDC)
n
| CASTABILITY PROPERTY | RATING | EXPLANATION |
|---|---|---|
| Fluidity | 2 — Fair | Very high Si gives some fluidity but hypereutectic composition has castability challenges. |
| Resistance to hot tearing | 3 — Good | Reasonable hot-tear resistance from high Si. |
| Pressure tightness | 2 — Fair | Moderate pressure tightness. |
| Machinability (as-cast) | 1 — Difficult | Group 1 — very difficult to machine. Diamond tooling required. High Si particles cause extreme tool wear. |
Heat Treatment
LM28 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.
Hardness Testing Mandatory — BS 1490 Table 5
Brinell hardness testing is mandatory for all LM28 castings. TE condition: 90–130 HB. TF condition: 100–144 HB. Testing is performed on the actual castings whenever possible — otherwise on separately cast test samples before the tensile test. Microstructure is also mandatory to verify phosphorus modification effectiveness.
Applications
Typical Applications of LM28
| INDUSTRY | TYPICAL PARTS | WHY LM13 |
|---|---|---|
| Automotive Racing | High-performance engine pistons, racing engine components | Lowest thermal expansion + maximum wear resistance |
| Aviation (piston engines) | Aircraft piston engine components | Elevated temperature strength + minimal expansion |
| High-Performance Industrial | Compressor pistons, high-speed reciprocating components | Wear resistance + thermal stability |
High-Performance Pistons
Racing Engine Parts
Aviation Pistons
Compressor Pistons
High-Speed Components
Wear-Resistant Parts
When NOT to Specify LM28
Do not specify LM28 for general engineering — it is a highly specialist alloy requiring diamond machining and mandatory microstructure control. For standard elevated-temperature applications, LM13 is more practical. LM28 is reserved for applications where minimum thermal expansion is the overriding requirement.
Frequently Asked Questions
Common Questions About LM28
What is the chemical composition of LM28 aluminium alloy?
LM28 per BS 1490:1988 has nominal composition Al-Si18Cu1.5Mg1Ni1. Silicon: 17.0–20.0%, Copper: 1.3–1.8%, Magnesium: 0.8–1.5%. The remainder is aluminium with controlled impurity limits.
What is the EN 1706 / European equivalent of LM28?
The EN 1706 equivalent of BS 1490 LM28 is EN AC-48000, with chemical symbol designation EN AC-Al Si18Cu1Ni1Mg1. This European standard supersedes the German DIN designation G-AlSi18CuMgNi.
What is the ASTM / AA equivalent of LM28?
The ASTM/Aluminium Association equivalent of LM28 is —. No direct ASTM equivalent.
What is the JIS equivalent of LM28?
There is no direct JIS H 5302 equivalent for LM28. No direct JIS equivalent.
What is the tensile strength of LM28?
The minimum tensile strength of LM28 is 190 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 LM28 be heat treated to T6?
Yes — LM28 is T6 heat-treatable (TF condition). Solution treat at 515–525°C, water quench, artificially age at 155–175°C. Full in-house T6 available at Creative Alucast with batch hardness verification.
What casting processes are suitable for LM28?
Per BS 1490:1988 Table 8: Sand casting (1), Gravity Die Casting (2), Low Pressure Die Casting (1), High Pressure Die Casting (n). Scale: 4=Excellent, 3=Good, 2=Fair, n=Not recommended.
What is the density of LM28 aluminium alloy?
The density of LM28 is 2.65 g/cm³ — approximately one-third the density of steel (7.85 g/cm³), making it ideal for lightweight structural applications.