BS 1490 · Group C · Special Purpose — Limited Application
LM31
Aluminium Alloy
Al-Zn5Mg0.7Cr0.5Ti — Highest Strength Without Heat Treatment — Excellent Corrosion Resistance
Sand Casting
Gravity Die Casting
Low Pressure Die Casting
High Pressure Die Casting
ISO 9001:2015
At a Glance
- Nominal composition
- Si content
- Density
- Best tensile (min)
- Freezing range
- Thermal conductivity
- Hardness
- Al-Zn5Mg0.7Cr0.5Ti
- ≤0.25 %
- 2.81 g/cm³
- 215 N/mm²
- 615–570 °C
- 138 W/m·K
- 65–80 HB
Overview
Highest Strength Without Heat Treatment — Excellent Corrosion Resistance
LM31 is the only major aluminium casting alloy in BS 1490 based on the Al-Zn-Mg system rather than Al-Si. The absence of silicon entirely, combined with 4.8–5.7% zinc, 0.5–0.75% magnesium, and 0.4–0.6% chromium creates an alloy that naturally age-hardens at room temperature without any heat treatment. BS 1490 requires that LM31 castings in the M condition be naturally aged for 3 weeks before testing.
LM31 delivers 215 N/mm² minimum tensile in the as-cast (M) condition — the highest as-cast tensile of any LM-series alloy without heat treatment. Combined with excellent corrosion resistance (matching LM5) and good decorative anodising capability, it is specified for large sand castings requiring structural strength without the dimensional risk of heat treatment quenching, and for applications demanding both strength and colour-anodised appearance.
Natural Age-Hardening — No Furnace Required
LM31 develops its strength through natural ageing at room temperature — no solution treatment or artificial ageing furnace is needed. The 215 N/mm² minimum tensile is achieved simply by allowing the casting to age naturally for 3 weeks per BS 1490. This makes LM31 the preferred specification when heat treatment distortion risk is unacceptable or when heat treatment facilities are not available.
Chemical Composition
LM31 Chemical Composition — BS 1490:1988
| Element | Symbol | Min % | Max % | Role in alloy |
|---|---|---|---|---|
| Silicon | Si | — | 0.25 | Strict impurity limit ≤0.25%. Silicon is not an alloying element in LM31 – it is an impurity that would impair the corrosion resistance and anodising quality if allowed to exceed this limit. |
| Copper | Cu | — | 0.10 | Strict impurity limit ≤0.10%. Like LM5, LM31 has the tightest Cu limit of any LM alloy to preserve maximum corrosion resistance. |
| Magnesium | Mg | 0.5 | 0.75 | 0.5–0.75% Mg is the primary strengthening element in combination with Zn. Forms MgZn2 precipitates during natural ageing – the mechanism for age-hardening without furnace treatment. |
| Iron | Fe | — | 0.5 | Controlled to ≤0.5% – tight limit reflecting the structural and corrosion-resistant application of this alloy. |
| Manganese | Mn | — | 0.1 | Impurity limit. Controlled to maintain clean melt. |
| Nickel | Ni | — | 0.1 | Impurity limit. |
| Zinc | Zn | 4.8 | 5.7 | Impurity limit. Higher Zn increases hot-cracking risk. |
| Lead | Pb | — | 0.05 | Impurity limit. |
| Tin | Sn | — | 0.05 | Impurity limit. |
| Titanium | Ti | 0.1 | 0.25 | 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
LM31 Equivalent Grades — International Standards
BS 1490
LM31
Group C · Special Purpose — Limited Application Al-Zn5Mg0.7Cr0.5Ti
EN 1706
EN AC-71000
EN AC-Al Zn5Mg
ASTM / AA
712.0
712.0 — Al-Zn-Mg casting alloy, T1 condition
JIS H 5302
—
No JIS die casting equivalent
DIN 1725-2
G-AlZn5Mg
Superseded by EN 1706
ISO 3522
Al-Zn5Mg
ISO casting alloy designation
Mechanical Properties
LM31 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 | 215 | — | 4 | 65–80 |
| TE | Sand / Investment | 215 | — | 4 | 65–80 |
Physical Properties
LM31 Physical & Thermal Properties
Density
2.81 g/cm³
Approx. 1/3 the density of steel
Freezing Range
615–570 °C
Solidification temperature range
Thermal Conductivity
138 W/m·K
Heat dissipation capability
Electrical Conductivity
35 % IACS
Relative to copper standard
Linear Expansion
24 ×10⁻⁶/K
20–300°C range
Brinell Hardness
65–80 HB
As-cast or stated condition
| Property | Rating | Notes |
|---|---|---|
| Corrosion resistance | Excellent (A) | Equal to LM5 — best corrosion resistance in the LM series. Zero Si, zero Cu. Suitable for marine, chemical, and outdoor environments. |
| Decorative anodising | Good (A) | Good decorative anodising — the best of the structurally significant alloys. Clear or colour anodising possible. LM5 remains the benchmark for anodising quality. |
| Weldability | Fair | Weldable with care. Al-Zn-Mg alloys can be stress-corrosion sensitive at welds — consult metallurgist for welded structural applications. |
| Machinability group | 3 — Good | Group 3 machinability. No hard Si particles — relatively easy to machine for an aluminium casting alloy. |
Casting Suitability
LM31 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)
2
Low Pressure (LPDC)
2
High Pressure (HPDC)
2
| Castability Property | Rating | Explanation |
|---|---|---|
| Fluidity | 3 — Good | Al-Zn-Mg alloys have acceptable fluidity but wider freezing range than eutectic Al-Si alloys. |
| Resistance to hot tearing | 2 — Fair | Wider freezing range and no eutectic Si means higher hot-tear risk than Al-Si alloys. Requires careful mould and feeder design. |
| Pressure tightness | 2 — Fair | Moderate pressure tightness. |
| Machinability (as-cast) | 3 — Good | Group 3 machinability. No hard Si particles — relatively easy to machine for an aluminium casting alloy. |
Heat Treatment
LM31 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.
Natural Ageing — No Furnace Required
LM31 in the M condition must be naturally aged for a minimum of 3 weeks at room temperature before testing or service per BS 1490 clause 5.2. This is not a furnace treatment — it simply requires adequate storage time after casting and fettling. The TE (artificially aged) condition gives the same minimum mechanical properties as M but with improved dimensional stability. At Creative Alucast, LM31 castings are held for the full natural ageing period before despatch.
Applications
Typical Applications of LM31
| Industry | Typical Parts | Why LM13 |
|---|---|---|
| General Engineering | Large structural castings, sand-cast frames and housings requiring strength without heat treatment | 215 N/mm² tensile without furnace treatment — no distortion from quenching |
| Marine & Outdoor | Decorative marine fittings, outdoor architectural components | Best corrosion resistance + good decorative anodising |
| Decorative | Colour-anodised architectural fittings, decorative structural castings | Good decorative anodising capability in Group C alloys |
Large Structural Castings
Sand-Cast Frames
Marine Fittings
Architectural Components
Colour-Anodised Parts
Outdoor Hardware
Structural Housings
When NOT to Specify LM31
Do not specify LM31 for thin-wall intricate GDC or LPDC castings — castability is only Fair (2) for these processes. Do not specify for high-pressure die casting applications — use LM2 or LM24. Do not specify when T6 strength above 215 N/mm² is required — use LM25 TF (280 N/mm² GDC).
Frequently Asked Questions
Common Questions About LM31
What is the chemical composition of LM31 aluminium alloy?
LM31 per BS 1490:1988 has nominal composition Al-Zn5Mg0.7Cr0.5Ti. Silicon: —–0.25%, Copper: —–0.10%, Magnesium: 0.5–0.75%. The remainder is aluminium with controlled impurity limits.
What is the EN 1706 / European equivalent of LM31?
The EN 1706 equivalent of BS 1490 LM31 is EN AC-71000, with chemical symbol designation EN AC-Al Zn5Mg. This European standard supersedes the German DIN designation G-AlZn5Mg.
What is the ASTM / AA equivalent of LM31?
The ASTM/Aluminium Association equivalent of LM31 is 712.0. 712.0 — Al-Zn-Mg casting alloy, T1 condition.
What is the JIS equivalent of LM31?
There is no direct JIS H 5302 equivalent for LM31. No JIS die casting equivalent.
What is the tensile strength of LM31?
The minimum tensile strength of LM31 is 215 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 LM31 be heat treated to T6?
No — LM31 is not normally heat-treated to T6. Available conditions: M, TE.
What casting processes are suitable for LM31?
Per BS 1490:1988 Table 8: Sand casting (3), Gravity Die Casting (2), Low Pressure Die Casting (2), High Pressure Die Casting (2). Scale: 4=Excellent, 3=Good, 2=Fair, n=Not recommended.
What is the density of LM31 aluminium alloy?
The density of LM31 is 2.81 g/cm³ — approximately one-third the density of steel (7.85 g/cm³), making it ideal for lightweight structural applications.