Aluminum 5251 - 3.3525 - EN AW-5251 - EN AW-AlMg2 - EN AW-AlMgMn0,3

Origins, standards and designations of 5251 aluminium alloy (EN AW-5251)

5251 aluminium alloy belongs to the 5000 series (Al-Mg), developed in the 20th century, in which magnesium is the primary alloying element. It corresponds to the former British NS4 and to Birmabright BB2 (1929), notably used on early Land Rover vehicles from 1948 for its light weight and corrosion resistance. In Europe, common designations include EN AW-5251, AlMg2Mn0.3, 3.3525, and BS N4 in British specifications. All refer to an Al-Mg grade (Mg ≈ 2%, typically 1.7 to 2.4%) known for its formability and marine-environment corrosion resistance.

What is the role of the alloying elements in 5251?

The composition of this alloy (often referred to as AlMg2 or AlMg2Mn0.3 depending on the standard) is based on aluminium alloyed with around 2% magnesium, which promotes strain hardening. It is a non-heat-treatable alloy (no precipitation hardening). A moderate manganese content (0.10 to 0.50%, typically ~0.3%) refines the grain structure and supports strength. Finally, the very low copper content significantly improves corrosion resistance, especially compared with 2000/7000 series alloys.

Mechanical properties of 5251 aluminium by temper

As noted above, 5251 gains strength through strain hardening and does not respond to precipitation hardening. A slight softening may occur in work-hardened tempers over time or under moderate heating, without a significant effect in typical service conditions.

Typical values by temper (sheet)

In O (annealed), typical values are yield strength Rp0.2 ≈ 68 MPa, ultimate tensile strength Rm ≈ 180 MPa, and Brinell hardness HBW ≈ 44. To compare the tempers from softest to hardest, see the table below.

Mechanical properties by temper and product form (sheet, indicative values)

Comparison of different tempers (from softest to hardest) in the longitudinal direction. These indicative (average) values depend on thickness, orientation (L/T) and applicable specifications.

Elongation at break A is high in O (~18%), then drops in H24/H26 (~5 to 8%) due to strain hardening. Fatigue performance is fair to good for a medium-strength grade and is higher than that of near-pure aluminium. 5251 is therefore suitable for structures subjected to repeated vibratory loading (a typical use case).

Corrosion resistance of 5251 aluminium alloy

5251 shows excellent performance in neutral to mildly aggressive environments (typically pH 4 to 9). However, strongly acidic or strongly alkaline media can dissolve the protective oxide film. Anodising is very effective for this alloy: it increases protection and improves surface appearance.

Marine environments and sensitisation

In seawater, performance is very good thanks to its Mg content and very low Cu content. Unlike higher-Mg 5000 series alloys (Mg > 3%, e.g., 5083), 5251 is less prone to sensitisation (precipitation of β-Al3Mg2 at grain boundaries) at normal service temperatures. Its relatively moderate magnesium level helps limit the risk of stress corrosion cracking under usual conditions. Nevertheless, unfavourable galvanic couples should be avoided and dissimilar-metal joints should be protected.

Using 5251: forming, welding and machining

Its formability is excellent: deep drawing, bending, rolling and spinning. Severe forming operations are typically performed in O, followed by re-work-hardening to recover part of the strength. Even in H22/H24, there is still useful forming margin.

Weldability is very good using MIG/TIG processes and resistance spot welding. For similar-alloy joints, 5356 filler (Al-5% Mg) is often preferred to maintain Mg continuity; 4043 fillers (Al-Si) may be selected for specific compatibility requirements. The HAZ causes local annealing (loss of work hardening): strength decreases locally, but this remains acceptable in most cases. However, its brazability is poor.

Finally, its machinability is average. There is a risk of built-up edge / galling in the O temper, reduced by sharp tooling, suitable coolant/lubrication and controlled cutting speeds. H24/H26 generally machine slightly better, without reaching the “crisp” machinability of high-Si/high-Pb alloys (e.g., 2011).

How does 5251 compare with competing alloys?

Compared with 5052 (AlMg2.5)

5052 aluminium typically contains slightly more Mg and sometimes Cr, resulting in higher mechanical properties. In H32, minimum yield strength Rp0.2 ~ 160 MPa (ASTM), noticeably above the ~120 MPa (typ.) of 5251 H22; in H34, Rm and Rp0.2 can increase by a further ~20 to 30 MPa. 5251 often retains a formability advantage at equivalent temper. The two grades are not interchangeable from a standards perspective.

Compared with 5754 (AlMg3)

With an Mg level around 3%, 5754 aluminium reaches slightly higher strengths while remaining highly weldable and corrosion resistant. For instance, a 5754 H14 can target Rm ≈ 240 MPa (typ.), versus ≈ 220 MPa for 5251 H24. Formability of 5754 can be slightly lower at comparable temper due to its higher strength levels.

Industrial and aerospace applications of 5251 aluminium alloy

Aerospace

Under the BS EN 3L81 (L81) designation, with tighter requirements, it is used for secondary structures: non-load-bearing panels, fairings, cowlings and auxiliary tanks. Its corrosion resistance and adequate fatigue behaviour suit marine and marine-atmosphere environments. It is still found in light aircraft and gliders where formability is critical.

Marine and saline environments

This is a widely used marine alloy. Typical uses include hulls, decks, offshore equipment and accessories (ladders, railings). Welded 5251 sheet offers a long service life in salt water, with simplified maintenance.

Industry, transport and construction

A very common grade in fabrication and sheet metal work, it is used for silos, tanks, cisterns and food-processing equipment (compatible and easy to clean). It is also used in bodywork and exterior cladding for buses and trains. In architecture, it is widely used for facades, cladding, roofing and anodised panels; however, for highly uniform decorative anodising, 5005 aluminium is often preferred.