Aluminum 518: Composition, Properties, Temper Guide & Applications

Comprehensive Overview

Alloy 518 is a member of the 5xxx (Al-Mg) series of aluminum alloys, characterized primarily by magnesium as the principal alloying element. It belongs to the non-heat-treatable family where strength is developed predominantly through solid-solution strengthening and strain (work) hardening rather than precipitation heat treatment.

Typical major alloying additions in 518 include magnesium in the mid single-digit percent range, with controlled quantities of manganese and trace elements such as chromium and titanium to stabilize grain structure and control recrystallization. These elements combine to deliver a balance of moderate to high strength, good ductility in annealed tempers, and improved performance in marine and atmospheric environments compared with many Al-Si or Al-Mn alloys.

Key traits of 518 are its favorable strength-to-weight ratio, good resistance to general corrosion and pitting in seawater environments, and excellent cold formability in annealed conditions. Weldability is generally good using conventional fusion processes, though local softening in the heat-affected zone (HAZ) and some susceptibility to stress-corrosion cracking in specific conditions should be considered in design.

Industries that commonly use alloys like 518 include automotive, truck trailers, marine structures and panels, architectural cladding, and certain structural components in transportation and energy sectors. Engineers select 518 where a combination of formability, corrosion resistance, and moderate strength is required and where heat-treatmentable alloys are either unnecessary or disadvantageous for forming and joining operations.

Temper Variants

Temper	Strength Level	Elongation	Formability	Weldability	Notes
O	Low	High	Excellent	Excellent	Fully annealed, best for complex forming
H12	Low-Medium	Medium	Very good	Excellent	Quarter hard; moderate increase in strength
H14	Medium	Low-Medium	Good	Excellent	Half hard; common for sheet that needs some stiffness
H16	Medium-High	Low	Fair	Excellent	Three-quarter hard; used where higher strength without heat treatment is needed
H18	High	Low	Limited	Excellent	Full hard; limited forming but highest cold-work strength
H111	Low-Medium	Medium-High	Very good	Excellent	Slightly worked after anneal, unspecified degree of strain hardening
H32	Medium	Low-Medium	Good	Excellent	Strain-hardened and stabilized to retain formability after limited annealing

Temper has a first-order effect on the mechanical balance and fabrication behavior of 518. Annealed (O) gives maximum ductility for deep drawing and complex stamping while H‑tempers provide increasing strength at the expense of elongation and tightness of bend radii.

Because 518 is not strengthened by conventional precipitation heat treatment, tempering through controlled cold work and stabilization treatments is used to tailor properties for end-use. Designers must account for HAZ softening after welding, and select tempers that match forming sequences and post-fabrication requirements.

Chemical Composition

Element	% Range	Notes
Si	0.10 max	Impurity; kept low to preserve ductility and corrosion resistance
Fe	0.40 max	Typical impurity; influences strength and intermetallic formation
Mn	0.20–0.80	Controls grain structure and inhibits recrystallization
Mg	3.5–5.0	Principal strengthening element; improves corrosion resistance and strength
Cu	0.10 max	Minimized to retain corrosion resistance; elevated Cu reduces SCC resistance
Zn	0.25 max	Kept low to maintain anodic behavior versus cathodic metals
Cr	0.05–0.25	Grain refiner and improves resistance to recrystallization and grain boundary corrosion
Ti	0.05–0.15	Grain refinement agent during cast and wrought processing
Others (Al balance)	Balance	Aluminum forms the matrix; other trace elements controlled per specification

The magnesium content is the primary determinant of strength and corrosion performance in 518, with increments generally increasing tensile properties while also influencing susceptibility to stress-corrosion cracking at high service concentrations. Manganese and chromium act as microalloying elements to control grain size and to reduce the extent of softening during thermal exposure and welding. Impurity elements such as iron and silicon are limited to avoid coarse intermetallics that can degrade toughness and formability.

Mechanical Properties

In tensile loading, 518 shows a broad range of behavior depending on temper and thickness. Annealed (O) material exhibits relatively low yield with high elongation suitable for deep drawing and stretch forming, while H‑tempers show progressive increases in proof and yield with decreases in ductility and bendability. Yield and tensile strength show a dependence on thickness and processing history; thinner gauge sheet and more heavily cold worked tempers achieve substantially higher room-temperature strength.

Hardness trends mirror tensile properties and are used as an expedient shop-floor metric to verify temper. Fatigue performance is strongly correlated to surface condition, residual stress, and microstructure; polished and cold-worked surfaces tend to have improved fatigue life, whereas notches, weld toes, and coarse intermetallics reduce it. Design practice requires accounting for HAZ softening adjacent to welds when fatigue-critical detail is present.

Property	O/Annealed	Key Temper (e.g., H14 / H32)	Notes
Tensile Strength	130–200 MPa	220–320 MPa	Wide overlap; tensile depends on cold work level and thickness
Yield Strength	60–140 MPa	150–260 MPa	Yield increases markedly with H‑tempers and strain hardening
Elongation	20–35%	6–15%	Ductility is highest in annealed material and falls with work hardening
Hardness	30–55 HB	60–95 HB	Brinell values approximate relative temper strength

Physical Properties

Property	Value	Notes
Density	~2.66 g/cm³	Typical for wrought Al-Mg alloys; enables high specific strength
Melting Range	~555–650 °C	Solidus-liquidus range dependent on alloying and trace impurities
Thermal Conductivity	~130–160 W/m·K	Lower than pure Al due to alloying; still good for heat dissipation
Electrical Conductivity	~30–45% IACS	Moderately conductive compared with other structural aluminum alloys
Specific Heat	~0.9 J/g·K	Useful for transient thermal calculations
Thermal Expansion	~23–24 µm/m·K	Typical coefficient for aluminum alloys; important for joint design

The physical properties position 518 as an attractive material for applications where weight reduction and heat dissipation are needed together, such as body panels and certain heat spreader components. The thermal conductivity and electrical conductivity are sufficient for many thermal-management tasks while being inferior to pure aluminum and select heat-treatable alloys; designers should account for this when specifying thicknesses for thermal conduction paths.

Product Forms

Form	Typical Thickness/Size	Strength Behavior	Common Tempers	Notes
Sheet	0.3–6.0 mm	Thickness-dependent; cold work raises strength	O, H12, H14, H32	Widely used in body panels, cladding, and interior panels
Plate	6–50 mm	Lower cold work potential; properties controlled by rolling	O, H111	Used where thicker section strength and corrosion resistance are required
Extrusion	Wall thickness 1–25 mm	Strength influenced by cold-drawing and aging of billets	O, H11, H22	Structural extrusions for frames and stiffeners
Tube	Ø 6–200 mm	Rolling and drawing impact mechanical anisotropy	O, H14, H16	HVAC, structural tubing, and marine tubing applications
Bar/Rod	Ø 3–80 mm	Cold work increases hardness and yield	O, H12, H14	Machined components and fasteners where moderate strength is acceptable

Form factor directly affects the attainable strength and microstructure due to differences in rolling, drawing, and cooling rates. Thin sheet can be heavily worked to achieve high H‑tempers inexpensively while plate and thick extrusions rely more on controlled rolling and thermomechanical processing to achieve target properties.

Processing differences imply application choices: sheet is optimized for forming and surface finish, plate for structural load-carrying parts, extrusions for complex cross-sections, and bar/rod/tube for machined and fabricated components.

Equivalent Grades

Standard	Grade	Region	Notes
AA	518	USA	Wrought Al-Mg alloy; often referenced in supplier catalogs
EN AW	5182 (closest)	Europe	5182 is commonly used in Europe and is compositionally similar to AA 518 variants
JIS	A5182 (closest)	Japan	Japanese practice often references 5182 for similar Al-Mg compositions
GB/T	5182 (closest)	China	Chinese standards have equivalents in the 5xxx family; direct one-to-one numbering varies

Direct one-to-one equivalents can be elusive because alloy numbering systems include family-level and vendor-specific variants; 518 is typically aligned with 5182-style chemistries in international standards. Minor tolerances, impurity limits, and mandated tempers can differ between specifications, so buyers must verify mill certificates and mechanical test requirements when substituting cross-region grades.

Corrosion Resistance

Alloy 518 provides good general atmospheric corrosion resistance and is often specified for marine and coastal applications where chloride exposure is a concern. The magnesium content confers galvanic behavior that is favorable relative to more cathodic aluminum alloys, but surface protection and anodizing practices are commonly applied to enhance long-term performance.

In marine environments 518 shows good resistance to pitting and crevice corrosion provided that chlorides are managed and protective coatings or sacrificial anodes are incorporated in the design. Localized corrosion tends to be aggravated by impurities, rough surfaces, or disbonded coatings, so surface finish control and proper sealing of joints are important design measures.

Stress corrosion cracking (SCC) susceptibility increases with magnesium content and applied tensile stress at elevated temperatures or in aggressive chloride environments; alloys with Mg above approx. 5% are notably more at risk. For 518-grade chemistries that stay within mid-Mg ranges, SCC is controllable through material selection, design to reduce residual tensile stresses, and post-weld treatments such as mechanical stress relief or appropriate cladding when critical service dictates.

Fabrication Properties

Weldability

518 is readily welded by MIG (GMAW), TIG (GTAW), and resistance processes with conventional filler alloys compatible with Al-Mg systems. Typical filler selections are 5xxx-series fillers that match or slightly overmatch base-metal magnesium content to reduce susceptibility to corrosion and HAZ softening. HAZ softening is inherent in Al-Mg alloys after welding, so designers often specify post-weld mechanical design allowances or select temper/filler combinations to mitigate loss of local strength.

Machinability

Machinability of 518 is moderate and generally favorable compared with higher-strength Al alloys; it machines cleaner than many Al-Mn alloys but is softer than heat-treatable alloys like 6061. Carbide tooling with positive rake geometry, rigid workholding, and controlled chip evacuation are recommended to avoid built-up edge and surface galling. Tool speeds should be set for aluminum (high SFM) combined with appropriate feed to prevent chatter and generate uniform chip formation.

Formability

Formability of 518 in the O temper is excellent for deep drawing, stretch forming, and hemming; typical minimum bend radii depend on temper and thickness but can often approach 1–1.5× thickness in annealed conditions. Cold working increases strength and reduces allowable bend radii; springback must be accounted for in tooling design when working in H-tempers. Warm forming can extend formability limits slightly but is rarely needed unless extreme shapes or high springback compensation are required.

Heat Treatment Behavior

518 is classified as a non-heat-treatable wrought aluminum alloy; bulk strength improvements are achieved through solid-solution effects (from Mg) and strain-hardening rather than precipitation hardening. There is no useful T6-style precipitation aging path for sustained strength increases, and attempts to apply conventional heat treatment will typically produce softening rather than strengthening.

Typical thermal processing focuses on annealing to recover ductility (e.g., anneal at temperatures near 345–415 °C depending on product form) and stabilization treatments to reduce residual stresses and control dimensional stability. Where higher strength is required, work hardening sequences (rolling, drawing) combined with controlled temper designations (H‑tempers) are the industrial route to reach target properties.

High-Temperature Performance

At elevated temperatures, 518 experiences gradual strength loss due to recovery and recrystallization phenomena, with usable mechanical properties typically limited to service temperatures below roughly 100–150 °C for load-bearing applications. Oxidation is minimal in most atmospheric environments, but prolonged exposure at higher temperatures or in oxidizing, chloride-bearing atmospheres accelerates microstructural change and can compromise corrosion resistance.

Close attention should be paid to welded assemblies since the HAZ can experience additional softening under thermal cycles, and elevated-temperature creep performance is limited; design allowances and testing are recommended for components subjected to sustained loads at moderate temperatures.

Applications

Industry	Example Component	Why 518 Is Used
Automotive	Body panels, inner liners	Excellent formability in O; good corrosion resistance and dent resistance when strain-hardened
Marine	Cabin components, structural panels	Good seawater corrosion resistance and weldability for welded assemblies
Aerospace	Secondary structure, fairings	Good strength-to-weight and formability for non-primary structural parts
Architecture	Cladding and roofing panels	Weathering resistance and ease of fabrication for aesthetically finished surfaces
Electronics	Heat spreader panels	Adequate thermal conductivity with lower density for weight-sensitive enclosures

518 is used where a balance of formability, corrosion resistance, and moderate strength is required and where weldability and surface finish are important. Its adaptability across product forms and tempers makes it a practical choice for midsize structural and enclosure components in multiple industries.

Selection Insights

For a designer comparing 518 to commercially pure aluminum like 1100, expect 518 to trade off some electrical and thermal conductivity for substantially higher strength and much better load-bearing capability. If conductivity is the primary requirement, 1100 or high-purity alloys remain preferable; choose 518 when structural performance and corrosion resistance take priority.

Compared with work-hardened alloys such as 3003 or 5052, 518 typically sits higher in strength due to its elevated Mg content while retaining competitive corrosion resistance; however, 5052 may have superior formability in some deep-draw applications. When deciding between 518 and common heat-treatable alloys like 6061/6063, select 518 if extensive cold forming or superior marine corrosion performance is required despite its lower peak strength; 6061 is preferable when higher heat-treatable strength and machinability are necessary.

Considerations for procurement should include local supplier availability of desired tempers and thicknesses, weld filler compatibility, and the potential need for post-weld or post-forming treatments to ensure the final component meets fatigue and dimensional stability requirements.

Closing Summary

Alloy 518 remains relevant because it combines the intrinsic advantages of Al-Mg systems—good corrosion resistance, weldability, and high formability in annealed condition—with the ability to achieve useful strengths through economical cold working. Its balanced set of properties makes it a versatile choice for transportation, marine, and architectural applications where reliable performance, manufacturability, and cost-effectiveness are required.