5251 Aluminium

Plain English is a quick buyer's guide. Engineer view adds the full property tables, composition and design-for-laser figures.

Overview

5251 is an aluminium–magnesium alloy with around 2% magnesium, giving a good balance of medium strength, excellent corrosion resistance — especially in marine and coastal conditions — and good formability. It is supplied work-hardened, so it holds more strength than the pure 1xxx grades while staying easy to bend and weld. It is a workhorse for marine, vehicle and outdoor panel work. Where you need higher strength use heat-treatable 6082-T6 or marine 5083; where you want a bright, highly reflective finish use pure 1050A.

Best for

Marine and coastal parts and fittings
Outdoor panels, enclosures and signage
Vehicle, trailer and bodywork panels
Folded brackets and trays (medium duty)
Welded fabrications

Less ideal for

Highly-stressed structural members (use 6082-T6)
Bright, mirror-reflective trim (use 1050A)
High-wear surfaces (moderate hardness)
Easy machining of complex profiles

Properties

BS EN 485-2:2008

Physical

Density	2.69	g/cm³
Melting range	600–650	°C
Thermal expansion	24	×10⁻⁶/K
Modulus of elasticity	70	GPa
Thermal conductivity	138	W/m·K
Electrical conductivity	~30	% IACS

Mechanical

Tensile strength	190–230	MPa
Proof stress (0.2%)	130 min	MPa
Hardness	~55	HB
Elongation A50	4 min	%

Sheet (typical; figures vary with thickness)

Chemical Composition

BS EN 573-3 · Alloy 5251 (AlMg2)

Al Mg Mn Fe Si Cr Zn Cu Ti —

Element		% present
Aluminium	Al	Balance
Magnesium	Mg	1.7 – 2.4
Manganese	Mn	0.1 – 0.5
Iron	Fe	0.0 – 0.50
Silicon	Si	0.0 – 0.40
Chromium	Cr	0.0 – 0.15
Zinc	Zn	0.0 – 0.15
Copper	Cu	0.0 – 0.15
Titanium	Ti	0.0 – 0.15
Other (each)	—	0.0 – 0.05

Formability & Fabrication

5251 forms well cold — it bends and folds cleanly and tolerates moderate internal radii, though it is a little less ductile than pure 1050A. It welds readily by TIG and MIG and keeps good corrosion resistance in the weld zone. Machinability is fair, and being a 5xxx alloy it is not suited to brazing or soldering.

Cold working

Good

Bending

Good

Gas welding

Good

Arc / TIG / MIG

Excellent

Resistance weld

Good

Brazing

Poor

Soldering

Poor

Machinability

Fair

Recommended filler wire: 5356 for most 5251 welds and when joining to other 5xxx/6xxx alloys; 4043 where a smoother bead or closer anodising match is wanted, noting it can lower as-welded strength on 5xxx alloys.

Corrosion Resistance

The magnesium content gives 5251 excellent resistance in marine and coastal atmospheres — better than the pure 1xxx grades in salt — which is why it is a classic choice for boats, trailers and seaside fittings. It resists fresh water and most atmospheres readily, and like all aluminium should be isolated from dissimilar metals to avoid galvanic attack.

Environment	Suitability
Atmospheric / outdoor	Excellent
Marine splash / coastal air	Excellent
Fresh water	Excellent
Industrial / chemical	Good
Salt-water immersion	Good
Strong alkalis / caustic	Poor

Typical Applications

Boat fittings & light hulls Vehicle & trailer panels Fuel & water tanks Outdoor signage & enclosures Architectural cladding Chemical & food equipment Folded brackets & trays Marine deck hardware

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Laser Cutting & Ordering

at Metal Parts Direct

Thicknesses we cut

0.9 mm1.2 mm1.5 mm2 mm2.5 mm3 mm4 mm5 mm6 mm

Cut tolerance

±0.25 mm up to 20 mm, ±0.5 mm above

Max part size

1000 × 1000 mm

Max part weight

No maximum

Estimated delivery if ordered today

Friday 28th August 2026 – Monday 31st August 2026

UK-made Laser cut to your spec No minimum order Material certs on request

Design-for-laser quick reference

These are the exact minimums the Part Builder enforces as you design, relative to sheet thickness — so your part and your quote never disagree.

Feature	Minimum
Min hole / bore Ø	0.7 × thickness (min 1 mm)
Min slot / opening width	0.7 × thickness (min 1 mm)
Min web / wall between cuts	0.7 × thickness (min 1 mm)
Min cut-to-edge distance	0.7 × thickness (min 1 mm)

Frequently Asked Questions

Is 5251 strong enough for a structural bracket?

For medium-duty work, yes — it is noticeably stronger than the pure 1xxx grades. For heavily-loaded structural parts choose heat-treatable 6082-T6, or marine 5083 where you also need high strength with weldability.

What is the difference between 5251 and 1050A?

1050A is commercially pure aluminium — soft, very formable and bright. 5251 adds about 2% magnesium, which raises its strength and gives markedly better resistance to salt and marine conditions, at the cost of a little ductility and reflectivity.

Is 5251 good for marine use?

Yes — it is one of the standard marine aluminium grades. The magnesium gives excellent resistance to coastal air and salt spray, so it is widely used for boats, trailers and seaside fittings.

Can it be anodised?

Yes, mainly for protection and a uniform matt finish. Cosmetic clear anodising is less bright than on pure 1050A, and the magnesium can shift the shade slightly — fine for most jobs, worth a sample for critical appearance work.

Will my laser-cut part be marked or scratched?

It is harder than 1050A so stands up better to handling, but it is still aluminium and can mark. If surface finish is critical, ask about PVC-film-coated sheet and handle the part carefully after cutting.

Equivalents

Also designated

EN AW-5251 AW-AlMg2 AA5251 BS N4 3.3525

Cross-references are indicative and may not be exact equivalents — confirm against the certificate for critical work.

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This data is indicative and provided in good faith from recognised EN standards and industry references. It is not a substitute for the full specification or material certificate; mechanical properties in particular vary with temper, form and dimension. Material supplied will conform to the relevant applicable standards but may differ from these figures. No liability is accepted for actions taken in reliance on this guide. Confirm critical values against the certificate for your delivery.