Aluminum 5251: Composition, Properties, Temper Guide & Applications
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Table Of Content
Table Of Content
Comprehensive Overview
5251 is a member of the 5xxx series aluminum alloys, which are magnesium-based, non-heat-treatable alloys offering a balance of strength and corrosion resistance. Its chemistry places it among the Al-Mg(-Mn) alloys where magnesium is the principal strengthening solute and manganese provides secondary grain-structure control.
Strength in 5251 is obtained primarily through solid-solution strengthening and strain hardening; the alloy cannot be significantly strengthened by precipitation heat treatment. Key traits include moderate-to-high strength for a non-heat-treatable alloy, good atmospheric and marine corrosion resistance, excellent formability in the annealed condition, and good weldability with typical aluminum fusion processes.
Typical industries using 5251 include automotive interior and exterior panels, marine structural components, architectural and building systems, and certain transport and general engineering applications. Engineers select 5251 when designers need a corrosion-resistant alloy with better strength than commercially pure aluminum and superior formability compared with many heat-treatable alloys.
5251 is chosen over other alloys when a combination of cold-forming capability, moderate strength, and durable surface performance is required while keeping fabrication costs low and avoiding age-hardening cycles. Its service profile often makes it preferable to alloys that sacrifice corrosion resistance for higher peak strength.
Temper Variants
| Temper | Strength Level | Elongation | Formability | Weldability | Notes |
|---|---|---|---|---|---|
| O | Low | High (18–30%) | Excellent | Excellent | Fully annealed; best for deep drawing and complex forming |
| H12 | Low–Medium | Moderate (12–20%) | Very good | Very good | Lightly cold-worked for slight strengthening |
| H14 | Medium | Moderate (10–18%) | Good | Very good | Quarter hard; common for sheet applications |
| H22 | Medium | Moderate (10–18%) | Good | Very good | Thermally stabilized after partial anneal |
| H24 | Medium–High | Lower (8–14%) | Fair | Good | Strain-hardened and partially annealed |
| H32 | High | Low (6–12%) | Limited | Good | Strain-hardened and stabilized; common structural temper |
| T temper (T5 / T6 / T651) | N/A | N/A | N/A | N/A | 5251 is a non-heat-treatable alloy; T tempers are not applicable |
Temper has a decisive effect on 5251’s mechanical envelope: O provides maximum ductility for forming while H-index tempers deliver progressively higher yield and tensile strengths at the expense of elongation. Weldability remains good across tempers because 5251 does not rely on age hardening, but heat-affected zones will locally soften strain-hardened conditions and can reduce local strength.
Chemical Composition
| Element | % Range | Notes |
|---|---|---|
| Si | ≤ 0.25 | Impurity; low content to preserve corrosion resistance |
| Fe | ≤ 0.40 | Common impurity that can form intermetallics affecting ductility |
| Mn | 0.20–0.80 | Controls grain structure and recovery; reduces recrystallization |
| Mg | 2.0–3.0 | Primary strengthening element via solid solution; improves corrosion resistance |
| Cu | ≤ 0.10–0.15 | Kept low to avoid reduced corrosion resistance |
| Zn | ≤ 0.20 | Minor; small amounts tolerated without large property changes |
| Cr | ≤ 0.25 | May be present to control grain growth and improve recrystallization resistance |
| Ti | ≤ 0.15 | Grain refiner when intentionally added; usually residual |
| Others | ≤ 0.05 each / ≤ 0.15 total | Trace elements and residuals limited by standards |
The composition of 5251 is engineered to exploit magnesium-driven solid-solution strengthening while minimizing elements that encourage intermetallic formation or localized corrosion. Mn and Cr act as microstructure stabilizers and help the alloy retain strength after warm working, while low Cu and controlled Fe and Si keep corrosion resistance high and maintain good ductility.
Mechanical Properties
5251 exhibits tensile behavior characteristic of the cold-worked 5xxx family: ductile and tough in the annealed condition with a pronounced increase in yield and tensile strength when strain-hardened. Yield strength increases significantly with H-tempers while tensile-to-yield ratios remain moderate, producing predictable plastic deformation characteristics useful for forming operations. Elongation drops as strength rises, so forming and final forming operations must be sequenced before or controlled during strain hardening.
Hardness correlates with temper; annealed alloys present low Brinell/Vickers hardness while H-tempers push hardness into ranges useful for structural applications. Fatigue performance is generally favorable for marine and atmospheric exposures thanks to a ductile fracture mode and resistance to localized corrosion; however fatigue limits fall as thickness increases and surface condition deteriorates. Thickness affects both yield and forming behavior: thinner gauges form more readily, whereas thicker sections retain more through-thickness strength and may be less susceptible to through-thickness cracking during bending.
| Property | O/Annealed | Key Temper (H32) | Notes |
|---|---|---|---|
| Tensile Strength | 95–155 MPa | 240–310 MPa | Range depends on cold work and thickness; H32 provides typical structural strength |
| Yield Strength | 30–80 MPa | 170–220 MPa | Substantial increase from O to H32 due to strain hardening |
| Elongation | 18–30% | 6–12% | Elongation decreases as temper hardness increases |
| Hardness | 20–45 HB | 70–95 HB | Brinell hardness approximations; varies with temper and processing |
Physical Properties
| Property | Value | Notes |
|---|---|---|
| Density | 2.68 g/cm³ | Typical for Al-Mg alloys; good strength-to-weight ratio |
| Melting Range | ≈ 600–655 °C | Solidus-to-liquidus range typical of wrought aluminum alloys |
| Thermal Conductivity | ≈ 120–150 W/m·K | Lower than pure Al but still high; useful for thermal management |
| Electrical Conductivity | ≈ 28–38 % IACS | Reduced from pure Al due to alloying; adequate for some conductive applications |
| Specific Heat | ≈ 0.90 J/g·K (900 J/kg·K) | Standard for aluminum alloys; useful for thermal mass calculations |
| Thermal Expansion | ≈ 23–24 µm/m·K | Comparable to other Al alloys; important in multi-material assemblies |
5251 retains favorable thermal and electrical transport properties compared with many structural alloys, which makes it useful in applications where both formability and heat conduction are needed. The density and thermal expansion characteristics support lightweight, thermally-stable designs, but designers should account for modest reductions in conductivity relative to pure aluminum when precise thermal or electrical performance is required.
Product Forms
| Form | Typical Thickness/Size | Strength Behavior | Common Tempers | Notes |
|---|---|---|---|---|
| Sheet | 0.3–4.0 mm | Consistent with temper; thin gauges easier to form | O, H14, H24, H32 | Most common form for automotive and architectural panels |
| Plate | 4–25 mm | Higher through-thickness strength; less formable | H22, H24, H32 | Used for structural components requiring stiffness |
| Extrusion | Variable cross-sections | Strength depends on post-extrusion cold work | O, H14 | Extruded profiles often further worked to increase strength |
| Tube | 0.5–6.0 mm wall | Strength and fatigue depend on work and wall thickness | O, H14, H32 | Used in lightweight structural systems and marine tubing |
| Bar/Rod | 6–100 mm | Bulk sections retain good machinability | O, H12, H14 | Used for machined components, fittings, and fasteners |
Differences in product form control available tempers and final mechanical properties; thinner sheets are produced and processed for maximum formability whereas plate and thick extrusions are offers for load-bearing components. Processing routes such as cold rolling, annealing, and stabilization are used to tailor ductility or strength prior to fabrication and final finishing.
Equivalent Grades
| Standard | Grade | Region | Notes |
|---|---|---|---|
| AA | 5251 | USA | Standard American Alloy designation for wrought 5251 |
| EN AW | 5251 | Europe | EN version aligns closely with AA composition and tempers |
| JIS | A5251 (approx.) | Japan | Japanese standards reference comparable Al-Mg grades; check local spec |
| GB/T | 5251 (approx.) | China | Chinese designation uses similar numbering; verify local tolerances |
Equivalency tables are approximate because regional specifications can differ in allowable impurity limits, process qualifications, and temper definitions. When substituting alloys across standards, engineers should verify chemistry, mechanical properties, and certification requirements rather than rely solely on numeric correspondence.
Corrosion Resistance
5251 offers good atmospheric corrosion resistance typical of Al-Mg alloys; a natural aluminum oxide film combined with the presence of magnesium gives enhanced performance in outdoor and mildly aggressive environments. It performs well for general marine and coastal exposure scenarios, although localized pitting resistance depends on surface finish and Mg content compared to higher-Mg alloys.
Stress corrosion cracking susceptibility in 5251 is low relative to some higher-strength Al-Mg alloys; because it is not age-hardened, it avoids precipitate-assisted SCC mechanisms that plague certain heat-treatable alloys. Contact with more noble metals or cathodic materials can introduce galvanic couples; designers should avoid direct coupling with stainless steels or copper without insulating barriers when continuous wetting is expected.
Compared with 6xxx series alloys, 5251 provides improved general corrosion resistance but lower maximum strength. Versus 3xxx and 1xxx alloys, 5251 trades slightly reduced formability and electrical conductivity for higher structural capability and better corrosion behavior in chloride environments.
Fabrication Properties
Weldability
5251 welds readily with MIG (GMAW) and TIG (GTAW) processes using aluminum filler wires in the 4xxx and 5xxx series alloys. Recommended fillers include ER4043 (silicon) for reduced cracking risk and ER5356 or ER5183 (Al-Mg fillers) when matching strength and corrosion resistance is important; minimize porosity through good joint fit-up and cleaning. Hot-cracking risk is low compared with heat-treatable alloys, but the HAZ in strain-hardened tempers will soften and reduce local strength.
Machinability
Machinability of 5251 is fair to moderate and generally better than higher-strength 5xxx alloys due to its balance of ductility and strength; it is not a free-machining aluminum. Carbide tooling with positive rake and high-feed, lower-speed strategies yield the best surface finishes and tool life. Chip control is typically achievable with sharp tools and proper coolant; avoid excessive work hardening of the immediate cut zone.
Formability
Formability in the O temper is excellent for deep drawing, bending and complex stamping operations, with relatively low springback compared with stronger alloy tempers. Recommended bend radii depend on thickness and temper but generally follow common aluminum rules of thumb (e.g., internal radius ≥ 1–2× thickness for H-tempers and ≥ 0.5–1× thickness for O temper). Cold working is the primary strengthening pathway, so sequential forming and controlled recovery anneals are normal practice to reach complex geometries without cracking.
Heat Treatment Behavior
5251 is classified as a non-heat-treatable alloy; it cannot achieve significant precipitation hardening through solution and aging treatments. Attempts to apply T-type heat treatment will not produce the strength gains seen in 6xxx and 7xxx alloys and can lead to undesirable microstructural effects.
Work hardening through cold deformation is the principal mechanism to raise yield and tensile strengths. Standard annealing (e.g., full anneal to O) will restore ductility by recovery and recrystallization, and stabilization treatments (designated H3x/H4x) are used to minimize subsequent changes in mechanical properties during fabrication or service.
High-Temperature Performance
Elevated temperatures reduce the strength of 5251 progressively, with appreciable softening often occurring above 100–150 °C depending on temper and stress level. Continuous exposure to temperatures approaching the alloy melting range is not appropriate; for intermittent elevated-temperature use, designers must allow for reduced load-carrying capacity and accelerated creep mechanisms.
Oxidation at service temperatures is limited to the formation of a stable aluminum oxide film which is protective; there is no significant scale or embrittlement as seen in some steels. The thermal stability of cold-worked tempers is moderate, so prolonged exposure to moderately elevated temperatures can relax residual stresses and lower yield strength in H-tempers, particularly near welds and high-strain regions.
Applications
| Industry | Example Component | Why 5251 Is Used |
|---|---|---|
| Automotive | Interior and exterior body panels | Good formability and moderate strength; corrosion resistance |
| Marine | Structural panels and non-critical fittings | Saltwater resistance and good fatigue performance |
| Aerospace | Secondary fittings and fairings | Favorable strength-to-weight and corrosion resistance for non-critical parts |
| Electronics | Chassis and heat spreaders | Thermal conductivity and corrosion stability |
| Architecture | Curtain wall panels and cladding | Formability for complex shapes and long-term outdoor durability |
5251 occupies a practical niche where formability, corrosion resistance, and reasonable structural strength are required simultaneously and where heat treatment would add cost or complexity. It is widely used in sheet and extruded forms that undergo significant forming operations before final deployment.
Selection Insights
5251 is a logical choice when an engineer needs more strength than commercially pure aluminum (e.g., 1100) but still requires good formability and corrosion resistance. Compared with 1100, 5251 trades higher strength and slightly reduced electrical/thermal conductivity for structural capacity and better corrosion performance in chlorinated environments.
Compared with other work-hardened alloys such as 3003 and 5052, 5251 generally sits closer to 5052 in strength and corrosion resistance, offering comparable marine performance but often better formability than highly strain-hardened variants. When designers need stronger, heat-treatable alloys like 6061, 5251 is selected if corrosion resistance, weldability, or formability in the O temper is prioritized over maximum achievable strength.
Choose 5251 when fabrication sequences involve significant cold forming or welding and when long-term atmospheric or marine exposure is anticipated; balance cost and availability against slightly lower peak strengths than heat-treatable alternatives.
Closing Summary
5251 remains a relevant and practical Al-Mg alloy for modern engineering because it combines formability, weldability, and corrosion resistance with a workable strength range obtained through cold work. Its predictable behavior across tempers, wide product form availability, and service-proven performance in automotive, marine, and architectural applications make it a dependable choice where a durable, formable aluminum is required.