Aluminum EN AW-5251: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

EN AW-5251 is a member of the 5xxx series aluminum-magnesium family, defined by magnesium as the principal alloying element. This series is known for non-heat-treatable, strain-hardenable alloys where strength is gained predominantly through cold work rather than solution and precipitation heat treatments.

Typical major alloying elements in EN AW-5251 include magnesium (primary strengthening element), trace manganese for grain structure control, and small amounts of iron and silicon as residuals. The alloy balances moderate strength with very good corrosion resistance, especially in atmospheric and mildly marine environments, alongside good weldability and fair formability in softer tempers.

The alloy is chosen across industries that require a mix of formability, corrosion resistance and moderate strength without the need for heat treatment, such as automotive body components, architectural panels, marine fittings and some electronic housings. Designers prefer EN AW-5251 when a cost-effective, weldable alloy with better strength than commercially pure aluminum and improved marine performance over some 3xxx alloys is required.

Compared with high-strength heat-treatable alloys, EN AW-5251 offers simpler processing (no solution/age steps) and more predictable behavior in welded structures because it does not suffer the same degree of HAZ embrittlement as certain age-hardenable alloys. This makes it attractive for welded assemblies, formed sheetwork and extrusions where in-service corrosion resistance is important.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High (20–35%)	Excellent	Excellent	Fully annealed, maximum ductility for deep drawing
H12	Low–Moderate	Moderate (10–20%)	Very Good	Very Good	Light strain hardening, good for moderate forming
H14	Moderate	Moderate–Low (8–15%)	Good	Very Good	Quarter hard, balance of formability and strength
H16	Moderate–High	Low–Moderate (6–12%)	Fair	Very Good	Half hard, typical for exposed panels
H18	High	Low (4–10%)	Limited	Very Good	Full hard, for high stiffness sheet applications
H22	Moderate	Moderate–Low	Good	Very Good	Strain-hardened then stabilized; dimensional stability improved
H24	Moderate–High	Low–Moderate	Fair	Very Good	Strain-hardened and artificially aged (stabilized) for improved yield
H111	Low–Moderate	High	Excellent	Excellent	Slightly worked after anneal, good formability with some strength

Tempering in 5xxx-series alloys is primarily a matter of cold work rather than classical heat-treatment sequences. The O temper delivers maximum ductility for stamping and deep drawing, while increasing H-numbers indicate greater cold work and higher strength at the expense of elongation and formability.

Stabilized tempers (H22/H24 and H111) are commonly used when forming followed by light thermal exposure or welding is expected, because they provide more consistent mechanical properties with reduced risk of undesired softening during fabrication.

Chemical Composition

Element	% Range	Notes
Si	≤ 0.25	Controlled impurity from processing; can slightly reduce ductility
Fe	≤ 0.40	Typical intermetallic former; excess can degrade corrosion resistance
Mn	≤ 0.40	Microstructure control; improves strength and recrystallization behavior
Mg	2.0–3.0	Principal solid solution strengthener and corrosion-resistance contributor
Cu	≤ 0.10	Kept low to avoid susceptibility to stress corrosion cracking
Zn	≤ 0.25	Minor residual; higher levels not characteristic for this series
Cr	≤ 0.15	Added in some variants for grain structure control and to limit recrystallization
Ti	≤ 0.15	Grain refiner in cast products and some wrought forms
Others (each)	≤ 0.05	Other elements present as impurities or controlled additions

The magnesium content is the dominant factor controlling yield and tensile strength in EN AW-5251, through solid-solution strengthening and interaction with dislocations. Manganese and chromium at low levels refine grain structure and improve strength retention during thermal exposure, while iron and silicon are residuals that form intermetallic particles and can influence fatigue and pitting behavior.

The composition is deliberately constrained to limit elements (notably copper and zinc) that would increase susceptibility to stress-corrosion cracking or reduce generalized corrosion resistance, making 5251 a reliable choice for exposed applications.

Mechanical Properties

EN AW-5251 exhibits classical 5xxx-series tensile behavior: ductile in annealed condition and progressively stronger with cold work while elongation drops. In the O temper the alloy shows a wide uniform elongation and a low yield to tensile ratio, making it favorable for forming operations that need large plastic strains. Under typical H tempers the yield strength increases substantially while tensile ductility becomes limited, with the onset of localized necking occurring earlier.

Hardness tracks with cold work and is a useful in-process metric for locating temper targets after rolling or drawing. Fatigue performance is sensitive to surface condition, thickness and the presence of intermetallic particles; polished or anodized surfaces substantially improve fatigue life versus as-rolled finishes. Thickness has a significant effect on strength and formability — thinner gauges cold-roll to higher work-hardening through the forming sequence and are easier to weld without edge distortion.

When designing components, engineers must account for the alloy’s non-heat-treatable nature: peak strength is achieved by mechanical deformation and stabilization, not by thermal aging. For welded assemblies, localized softening may occur near the HAZ but is generally less severe than in precipitation-hardening alloys if the chosen temper and filler match are appropriate.

Property	O/Annealed	Key Temper (H14/H24 Typical)	Notes
Tensile Strength	120–155 MPa	200–260 MPa	Values depend strongly on cold work and thickness
Yield Strength	50–90 MPa	140–210 MPa	Yield rises markedly with strain-hardening; H24 shows stabilized yield
Elongation	20–35%	6–16%	Ductility reduces as temper hardens; annealed shows best formability
Hardness (HB)	30–45 HB	60–95 HB	Hardness correlates with strength and cold-work level

Physical Properties

Property	Value	Notes
Density	2.68–2.70 g/cm³	Typical for Al–Mg wrought alloys
Melting Range	~570–650 °C	Alloy solidus/liquidus spread; use conservative design margins
Thermal Conductivity	120–150 W/m·K	Slightly lower than pure Al due to alloying additions
Electrical Conductivity	~28–38 % IACS	Reduced from pure Al as magnesium increases
Specific Heat	~900 J/kg·K	Typical for aluminum alloys at ambient temperature
Thermal Expansion	23–24 µm/m·K (20–100 °C)	Important for bonded assemblies and multi-material joints

The physical constants place EN AW-5251 close to other Al–Mg alloys in thermal and electrical behavior; magnesium lowers conductivity compared with pure aluminum but retains excellent overall thermal performance for heat-spreading applications. Designers should account for thermal expansion when mating 5251 with dissimilar materials, particularly in structural joints and bonded assemblies.

Melting and softening ranges mean that welding and any post-weld thermal cycles must be managed to avoid excessive local softening; heat input controls and fixturing to limit distortion are standard practice for tight-tolerance panels.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Exhibits strong thickness-dependence; thin gauges cold work readily	O, H12, H14, H24	Most common for body panels, facades and marine deck plating
Plate	6–50 mm	Lower ductility in thicker plates; used where stiffness is required	H16, H18	Often used for structural components where bending stiffness is key
Extrusion	Cross-sections up to several hundred mm²	Properties depend on extrusion ratio and subsequent cold work	O, H111, H14	Good for profiles with moderate strength and complex geometry
Tube	Diameters 6–200 mm, wall 0.5–6 mm	Welded and seamless options; properties vary by manufacturing	O, H14, H16	Used in fluid lines, handrails and structural members
Bar/Rod	Diameters up to 50 mm	Produced by extrusion or drawing; strength increases with drawing	O, H12, H14	Typical for fabricated fittings and machined parts

Sheet and plate processing routes differ in rolling schedules and subsequent cold-working steps; sheets are routinely coil-processed and then cut and formed, while plates are rolled for thicker sections with different thermal-mechanical histories. Extrusions require attention to billet temperature and die design to control surface finish and residual stresses; post-extrusion stretching and aging (stabilization) are common to reduce distortion.

Welded tubular forms and machined bar stock are often produced from the same base alloy but processed to different tempers; choosing the right intermediate temper and machining allowance can reduce scrap and rework in production environments.

Equivalent Grades

Standard	Grade	Region	Notes
AA (Aluminum Association)	5251	USA	Common wrought designation aligned with EN AW-5251 chemistry and properties
EN AW	5251	Europe	Standard European nomenclature for wrought Al–Mg alloy
JIS	— (closest: A5052)	Japan	No direct one-to-one JIS equivalent; A5052 often considered the nearest commercial match
GB/T	5251	China	Chinese standard often lists 5251 as the corresponding alloy; check manufacturer certification

Direct one-to-one equivalents are not always exact because regional standards permit slightly different impurity limits and certification practices. Cross-referencing should be done using specific chemical and mechanical property requirements rather than only by alloy number.

When substituting, engineers should compare tensile/yield ranges, tempers available and surface treatments; 5052 and 5154 are commonly used alternatives with slightly different Mg contents and thus different strength/corrosion trade-offs.

Corrosion Resistance

EN AW-5251 offers very good atmospheric corrosion resistance typical of Al–Mg alloys, forming a stable protective oxide layer that limits general corrosion in urban and industrial environments. The magnesium content improves resistance to pitting in chloride-containing atmospheres versus many 1xxx and 3xxx family alloys, making 5251 a frequent choice for exterior architectural and marine-adjacent applications.

In marine immersion or splash environments the alloy performs well, but localized pitting can occur on rough or damaged surfaces and in stagnant crevices. Design details such as drainage, avoidance of crevices, and appropriate surface finishes (anodizing, conversion coatings or paint) significantly improve service life.

Stress corrosion cracking risk for Al–Mg alloys increases with higher magnesium levels and elevated tensile stresses; at the Mg levels typical for 5251 the risk is moderate and can be mitigated by selecting lower-strength tempers in highly stressed welded assemblies. Galvanic interactions must be evaluated: when electrically coupled to more noble materials (stainless steels, copper alloys), 5251 will act anodically and requires isolation or protective coatings to prevent accelerated corrosion. Compared to 6xxx and 7xxx heat-treatable alloys, EN AW-5251 generally offers superior generalized corrosion resistance but lower achievable peak strength.

Fabrication Properties

Weldability

EN AW-5251 is highly weldable by common fusion processes such as TIG and MIG, showing good fusion characteristics and low hot-cracking tendency when proper filler metals are used. Typical filler choices are Al–Mg alloys in the 4–5% Mg range (e.g., ER5356) to maintain corrosion resistance and to minimize weld zone softening. Heat input should be controlled to limit HAZ softening, and pre- or post-weld treatments (e.g., light cold work or stress relief) may be applied to stabilize properties.

Machinability

Machining of EN AW-5251 is moderate in difficulty; it machines more easily than high-strength age-hardened alloys but is less free-cutting than older leaded alloys. Carbide tooling with positive rake geometry, appropriate chip breaking, and moderate cutting speeds produce good surface finishes. Work-hardening near the cut can occur if feeds are too light, so consistent feed and coolant application are recommended to avoid built-up edge and tool vibration.

Formability

Formability in annealed (O) condition is excellent, enabling deep drawing, roll forming and complex stamping operations with small bend radii. As tempers increase (H12–H18), bend radii need to be increased and springback becomes more pronounced, so tooling compensation is necessary. For cold forming, aim to start with the softest available temper practical for the application and use progressive forming steps to minimize fracture risk.

Heat Treatment Behavior

EN AW-5251 is a non-heat-treatable alloy; mechanical strength is achieved through cold working and microstructural control rather than solution and precipitation hardening. Full annealing to restore ductility is achieved by heating into the 350–415 °C range and holding long enough for recrystallization, followed by slow cooling to avoid residual stresses. Temper transitions are therefore described in terms of cold-work levels and stabilization cycles (H22/H24) rather than classical T-temper sequences.

Artificial aging is not effective for increasing strength in 5xxx alloys, but controlled thermal exposure at moderate temperatures can alter ductility and reduce residual stresses. Designers should avoid service temperatures and manufacturing steps that inadvertently anneal or over-agework-hardened parts unless a controlled softening is intended. For components that will undergo welding, selecting a temper that tolerates partial thermal exposure (H22/H24, H111) reduces the risk of unacceptable property changes post-fabrication.

High-Temperature Performance

EN AW-5251 maintains useful mechanical properties up to modest elevated temperatures, but strength reductions become significant above approximately 100–150 °C, and long-term exposure above ~200 °C is generally not recommended for load-bearing applications. Oxidation is limited due to the protective aluminum oxide layer, but prolonged high-temperature exposure can accelerate magnesium diffusion and coarsening of microstructural features, which decreases mechanical performance.

Welded regions and HAZs are sensitive to thermal cycles; excessive heat input during fabrication or service can reduce local yield strength and increase susceptibility to creep under sustained loads. For applications with cyclic thermal or mechanical loading at elevated temperatures, selecting a more thermally-stable alloy or designing for additional margin is prudent.

Applications

Industry	Example Component	Why EN AW-5251 Is Used
Automotive	Inner body panels, trim	Good formability in O/H12; weldability and corrosion resistance
Marine	Decking, fittings	Mg-rich composition gives improved pitting resistance in marine atmospheres
Aerospace	Secondary structures, fairings	Favorable strength-to-weight and good fatigue behavior for non-primary parts
Electronics	Enclosures, heat spreader panels	Good thermal conductivity and corrosion resistance for outdoor enclosures

EN AW-5251 occupies a useful middle ground where moderate strength, excellent corrosion resistance and good fabrication characteristics are required. Its combination of properties supports broad use across transportation, architecture and marine industries where cost-effective, weldable and formable material is needed.

Designers often choose 5251 for components that will be fabricated using standard sheet metal operations and then exposed to outdoor or coastal environments without the complexity of solution/age processing.

Selection Insights

EN AW-5251 should be selected when you need better strength and corrosion resistance than commercially pure aluminum (1100), while maintaining good formability and weldability. Compared with 1100, 5251 trades off some electrical and thermal conductivity for significantly higher yield and tensile strength, which enables lighter gauge designs for the same stiffness.

Compared with work-hardened alloys such as 3003 and 5052, 5251 typically provides higher strength at similar or improved corrosion resistance in marine and atmospheric service. If you need the highest possible Mg-related corrosion resistance or a particular temper availability, compare 5251 carefully with 5052/5154 since chemistry and processing differences shift the balance of properties.

Compared with heat-treatable alloys such as 6061 or 6063, EN AW-5251 is preferred when fabrication involves extensive welding or forming without the ability or desire to perform solution/aging steps. Although 6061 will reach higher peak strengths after heat treatment, 5251 offers more predictable welded joint performance and simpler processing for large, formed structures.

Closing Summary

EN AW-5251 remains a practical, widely used Al–Mg wrought alloy that delivers a balance of corrosion resistance, weldability and moderate strength without the need for heat treatment. Its versatility across sheet, plate and extrusion forms, together with predictable fabrication behavior, keeps it relevant for automotive, marine, architectural and general engineering applications where durable, cost-effective aluminum solutions are required.