Aluminum Alloy

Category Scope and Definition

Aluminum alloy is a metal-material category sitting under Materials & Raw Stock › Metal Material, alongside sibling families such as alloy steel, stainless steel, copper material, and carbon steel. It is not a single product type but a material family that spans the full wrought and cast aluminum landscape, from rolled sheet and extruded profiles to gravity and high-pressure die castings. The defining characteristic is that aluminum is the predominant element, with alloying additions tuned to a target balance of strength, corrosion resistance, formability, and cost.

Pure aluminum is soft and low in strength, so deliberate alloying with copper, magnesium, silicon, manganese, zinc, and lithium is what makes the family useful for structural engineering. The same additions that raise strength can also lower corrosion resistance or weldability, which is why the category fragments into many series, each optimized for a different service envelope. Understanding the category therefore means understanding the trade-offs each alloying element introduces, not memorizing a single grade. Where the strength-to-weight ratio of even the 7xxx grades is not enough, engineers step up to titanium alloy; for sustained high-temperature service, where aluminum loses strength, the move is to nickel alloy.

Several intrinsic properties make aluminum alloys the most widely used non-ferrous structural metals. The high strength-to-weight ratio comes from a density of about 2.70 g/cm3, roughly one-third that of steel at 7.85. Corrosion resistance is largely passive and self-healing: exposure to air forms a thin, adherent Al2O3 oxide film that re-forms if scratched. The metal is also highly conductive electrically and thermally, fully recyclable, and exceptionally formable, which is why it appears everywhere from beverage cans to aircraft spars to EV battery enclosures.

The baseline physical properties of aluminum, cross-verified, frame every selection decision in this category. Density is about 2.70 g/cm3. The elastic (Young's) modulus is about 69-70 GPa, roughly one-third of steel's 200 GPa, which means aluminum structures deflect more for the same section and load, so stiffness-driven designs use more material or deeper sections. The melting point of pure aluminum is about 660 degrees C, while alloys melt over a range, typically about 475-660 degrees C. Thermal conductivity ranges from about 120 to 235 W/m.K depending on alloy and temper (pure aluminum about 235; 6061-T6 about 167). Electrical conductivity of pure aluminum is about 61% IACS, and the 1350 conductor grade reaches about 61.8% IACS. The coefficient of thermal expansion is about 23-24 x 10^-6 per K, roughly double that of steel, a fact that matters in mixed-metal assemblies and at temperature.

Because the category is broad, this guide is organized top-down: first the two families (wrought vs cast), then the series within each, then the temper that finishes a part, then the standards that govern composition and properties, and finally the cross-cutting selection logic engineers use to map a service requirement onto a specific alloy and temper. Specific aluminum product-type leaf pages (such as aluminum extrusion, aluminum sheet, or aluminum die casting) sit one level below this category and are cross-linked where they exist.

The Two Major Families: Wrought vs Cast

Aluminum alloys split into two top-level families, each with its own Aluminum Association designation system. Wrought alloys are mechanically worked into shape and carry a 4-digit number; cast alloys are poured molten into a mold and carry a 3-digit-plus-decimal number. Getting the family right is the first selection decision, because it determines both the achievable properties and the available manufacturing routes.

Wrought alloys use a 4-digit system (e.g., 6061, 7075). The first digit names the principal alloying element. The strengthening mechanism splits the series into heat-treatable alloys, which are strengthened by precipitation (solution treatment plus aging), and non-heat-treatable alloys, which can only be strengthened by strain (work) hardening. Heat-treatable series are 2xxx, 6xxx, 7xxx, and some 8xxx; non-heat-treatable (work-hardenable only) series are 1xxx, 3xxx, 4xxx, and 5xxx. The 9xxx group is reserved and unused in the wrought system.

The 6xxx series deserves special note because it is the commercial backbone of the category: the Mg + Si combination forms a strengthening Mg2Si precipitate, gives heat-treatable strength, and extrudes superbly, which is why 6061 and 6063 dominate the world's structural and architectural profile output. The 7xxx series sits at the top of the strength ladder but is the most demanding to use, since its peak-aged T6 condition is susceptible to stress-corrosion cracking under sustained tensile load.

Cast alloys use a 3-digit-plus-decimal system (e.g., 356.0, 380.0). The first digit indicates the principal element; the digit after the decimal indicates form, where ".0" denotes a casting and ".1" or ".2" denote ingot. A leading letter (e.g., A356) denotes a composition modification of the base alloy. Cast alloys generally carry more silicon than wrought alloys because silicon dramatically improves molten fluidity and castability, at the cost of ductility.

In practice two cast families carry most of the volume. The 3xx.x family (silicon with copper and/or magnesium) is the workhorse: 356.0 and A356 (about 7% Si) give ductile, heat-treatable structural castings for gravity die and sand casting, while 380.0 and A380 are the most common high-pressure die-casting alloys. The 4xx.x family (high silicon, e.g., 413) is chosen when maximum fluidity and corrosion resistance matter more than strength. Choosing the family and series is the start; the temper applied afterward, covered next, finishes the property set.

Tempers and Verified Spec Reference

A grade number alone does not define mechanical properties; the temper does. The temper designation is the suffix after a hyphen (e.g., 6061-T6) and records how the material was hardened. The basic tempers are F (as-fabricated), O (annealed, lowest strength and maximum ductility), H (strain-hardened, used on non-heat-treatable alloys), W (solution heat-treated but unstable and naturally aging), and T (thermally treated to a stable temper, used on heat-treatable alloys).

For H tempers, a two-digit suffix follows the H. The first digit is the process: H1 is strain-hardened only, H2 is strain-hardened then partially annealed, and H3 is strain-hardened then stabilized. The second digit is the degree of hardening: 2 = quarter hard, 4 = half hard, 6 = three-quarter hard, 8 = full hard, and 9 = extra hard. So H14 is half-hard, H18 is full-hard, and H32 is quarter-hard-stabilized. For T tempers, the key subdivisions are T3 (solution-treated, cold-worked, naturally aged), T4 (solution-treated, naturally aged), T5 (artificially aged from elevated-temperature shaping), T6 (solution-treated and artificially aged, giving peak strength), T7 (overaged or stabilized, for example T73 which trades some strength for stress-corrosion-cracking resistance in 7xxx), and T8 (solution-treated, cold-worked, and artificially aged).

The table below is a verified spec reference for common alloy-temper combinations, cross-verified across Wikipedia, ASTM, and supplier datasheets. Values are typical or minimum at room temperature and should be treated as nominal: published values vary by product form and thickness, so always confirm against the governing standard and the mill certificate for the exact form you are buying.

Reading the table top to bottom shows the strength ladder of the category. Conductivity and formability grades (1100, 3003) sit lowest in strength; marine 5xxx grades occupy the moderate band; the 6xxx structural and architectural alloys form the commodity middle; aerospace 2024 and 7075 reach the top, with 7075-T6 the strongest common alloy at up to about 572 MPa ultimate. Cast alloys trade ductility for shape: A356-T6 retains useful elongation for structural castings, while die-cast A380/ADC12 reaches good strength but only about 1-3.5% elongation. Note also the temper effect within one grade: 7075-T73 sacrifices a slice of strength versus T6 in exchange for far better stress-corrosion-cracking resistance.

Alloying Elements and Forming Technologies

The behavior of every series traces back to what each alloying element does inside the aluminum matrix. Understanding these effects lets an engineer predict, rather than look up, why a grade behaves as it does and which grade to reach for when a datasheet is missing.

• Copper (Cu) raises strength through precipitation of Al2Cu and Al-Cu-Mg phases but lowers corrosion resistance and weldability; it defines the 2xxx series and many cast alloys.
• Magnesium (Mg) provides solid-solution strengthening plus excellent corrosion resistance, especially in seawater; it defines the 5xxx series.
• Silicon (Si) lowers melting point and dramatically improves fluidity and castability; it defines the 4xxx series and the cast 3xx/4xx families, and with magnesium forms the strengthening Mg2Si phase used in 6xxx.
• Magnesium plus silicon together form the Mg2Si precipitate that is the basis of the heat-treatable, extrudable 6xxx series.
• Zinc (Zn), with magnesium and copper, gives the highest strength via the MgZn2 (eta) phase; it defines the 7xxx series.
• Manganese (Mn) adds modest strength with good formability and corrosion resistance; it defines the 3xxx series.
• Iron (Fe) is usually an impurity that forms brittle intermetallics and lowers ductility, but it is tolerated or used in die-cast alloys (it reduces die soldering) and in Al-Fe conductor foil.
• Lithium (Li) lowers density about 3% and raises modulus about 6% per 1% Li, the basis of Al-Li 8xxx aerospace alloys.
• Chromium, zirconium, scandium, and titanium act as grain refiners that control recrystallization and improve toughness.

Forming technology is the second cross-cutting axis of the category, because the chosen route constrains which alloys are practical. Wrought routes include hot and cold rolling (for sheet, plate, and foil), extrusion (where 6xxx alloys dominate thanks to their extrudability), forging, and drawing (for tube and wire). These are followed by heat treatment (solutionizing, quench, then natural or artificial aging) in a heat treatment furnace for the heat-treatable series, or by strain hardening with annealing or stabilizing to reach H tempers on the non-heat-treatable series.

Casting routes include high-pressure die casting (HPDC), run on a die casting machine, for thin-wall, high-volume parts in alloys such as A380 and ADC12; permanent-mold (gravity die) casting; sand casting; low-pressure die casting; and investment casting, with A356 and A357 dominating quality structural castings. The industry is increasingly adopting vacuum HPDC and structural "megacasting" for EV bodies, which consolidates dozens of stamped parts into a single large casting.

Joining spans TIG welding and MIG welding (using 4xxx or 5xxx filler; 6xxx is weldable, while 7xxx and 2xxx are generally not arc-welded for structure), friction stir welding (FSW, excellent for 2xxx, 7xxx, and large panels), brazing with a 4xxx filler, riveting, adhesive bonding, and self-pierce riveting in automotive assembly. Surface treatment includes anodizing (which thickens the protective oxide for decorative or hard-coat duty), chromate and chromate-free conversion coating, powder coating, and painting. Together, alloy chemistry and forming route define what is buildable; the next chapter covers the standards that make those properties contractual.

Governing Standards Landscape

Aluminum is governed by two layers of standards: those that define designation and chemical composition, and those that define product form and mechanical properties. Quoting the correct standard number on a drawing or purchase order is what makes a spec contractual and traceable to a mill certificate.

On the designation and composition layer, the originating systems come from ANSI H35.1 and The Aluminum Association, which register the wrought 4-digit and cast 3-digit-decimal systems (the Teal Sheets and Pink Sheets). In Europe, EN 573 (Parts 1-4) covers chemical composition and designation of wrought aluminum and its alloys, with EN 573-3 listing composition and alloys written in the form EN AW-6061; EN 1706 covers the composition and mechanical properties of cast aluminum alloys (for example EN AC-42100 is an A356-type, EN AC-42000 a 356.0-type, and EN AC-46000 an ADC12/A380-type). ISO 209 is the international wrought composition and designation standard. In China, GB/T 3190 sets the chemical composition of wrought aluminum and alloys and mirrors the AA numbering. In Japan, JIS H 4000 covers rolled sheet, strip, and plate (extruded shapes fall under JIS H 4100) and JIS H 5302 defines die-casting alloys such as ADC12.

On the product-form and mechanical-property layer, the US ASTM standards and their European EN counterparts run in parallel:

• ASTM B209 covers aluminum and aluminum-alloy sheet and plate; the European counterpart is EN 485 (485-1 conditions, 485-2 mechanical properties, 485-3/-4 tolerances).
• ASTM B221 covers extruded bars, rods, wire, profiles, and tubes; the European counterparts are EN 755 (extruded) and EN 12020 (precision profiles in 6060/6063).
• ASTM B210 and B241 cover drawn and extruded seamless tube.
• ASTM B247 covers die, hand, and rolled-ring forgings; EN 586 covers forgings.
• ASTM B179 covers ingot for castings, with ASTM B26 (sand castings), B85 (die castings), and B108 (permanent-mold castings) for the cast products.
• AMS series (SAE Aerospace Material Specifications) cover aerospace alloys and tempers (for example AMS-QQ-A-250 and AMS 4045 for 7075 sheet).

A critical caveat governs cross-standard substitution. Common tempers (O, H14, H18, T4, T6) and compositions are broadly comparable across the AA/ASTM, EN, GB/T, and JIS systems, but EN standards such as EN 485 generally specify tighter strength and flatness/thickness tolerances than ASTM B209. Equivalents are therefore "near-equivalent," not identical. When substituting, verify chemistry limits and minimum properties: 6061 is near EN AW-6061 and GB 6061, 5052 is near EN AW-5052, and A380 is near ADC12 and EN AC-46000. The standards numbers most worth getting exactly right on a drawing are ASTM B209 (sheet/plate), ASTM B221 (extrusion), EN 573-3 (composition), EN 485 (sheet/plate), EN 755 (extrusion), EN 1706 (castings), GB/T 3190, ISO 209, ANSI H35.1, and JIS H 5302.

Selection and Sourcing

To map a service requirement onto a specific alloy and temper, engineers work through a cross-cutting set of selection criteria rather than starting from a favorite grade. The criteria below summarize how the category is navigated in practice.

1. Strength requirement: low → 1xxx/3xxx; moderate → 5xxx/6xxx; high → 2xxx; highest → 7xxx.
2. Corrosion environment: marine or chemical service → 5xxx (especially 5083/5052) or anodized 6xxx; avoid bare 2xxx/7xxx in corrosive or marine service (use clad, coated, or T7-temper material).
3. Forming or fabrication method: extrusion → 6xxx (6061/6063); deep drawing or forming → 3003/5052; die casting → A380/ADC12; quality castings → A356.
4. Weldability needed: yes → 1xxx, 3xxx, 4xxx, 5xxx, 6xxx; generally no for structural welds → 2xxx, 7xxx.
5. Machinability: 2011, 6061, and 7075 machine well; high-silicon cast alloys are abrasive on tooling.
6. Conductivity (electrical/thermal): 1xxx (1350) for conductors; 6101/6063 for bus bars.
7. Service temperature: most aluminum alloys lose strength above about 150-200 degrees C; for elevated temperature consider 2xxx (e.g., 2618) or specialty alloys.
8. Cost and availability: 6061 and 6063 (extrusion), 5052 and 3003 (sheet), and A380 (die cast) are the high-availability commodity choices; 7075/2024 and Al-Li carry aerospace premiums.
9. Stress-corrosion-cracking risk: for sustained tensile load in 7xxx, specify T73 or T76 overaged tempers rather than T6.
10. Recycled content and carbon footprint: increasingly specified; secondary (recycled) aluminum needs only about 5% of primary smelting energy, and low-carbon or recycled grades now command a market premium.

On the sourcing side, the category is served by a tiered supply chain. Primary producers smelt alumina into ingot and include China Hongqiao Group (the world's largest, about 6.7 Mt/yr), Aluminum Corporation of China / Chalco (about 6.2 Mt/yr), UC Rusal (about 3.8 Mt/yr), Alcoa, Rio Tinto's aluminium division, Norsk Hydro, Emirates Global Aluminium (EGA), and Hindalco Industries. Rolled, extruded, and value-added product makers include Novelis (part of Hindalco; the world's largest rolled-aluminum producer and recycler), Hydro Extrusions, Constellium, Arconic, Kaiser Aluminum, and Aleris (now part of Novelis). Recycling and low-carbon supply is led by Novelis (targeting 75% recycled content by 2030), Hydro (whose CIRCAL line uses 75% or more post-consumer scrap), Alcoa, and Real Alloy.

A growing third sourcing dimension is recycled content and carbon footprint. Primary aluminum is smelted from bauxite-derived alumina by electrolysis, which is energy-intensive, whereas secondary (recycled) aluminum is remelted from scrap and needs only about 5% of the energy of primary smelting. Because aluminum is fully recyclable without loss of properties, low-carbon and high-recycled grades are increasingly specified and now command a market premium: Hydro's CIRCAL line uses 75% or more post-consumer scrap, and Novelis targets 75% recycled content by 2030. For most structural duty the recycled metal performs identically once it meets the governing composition and property standard, so it should be verified against the mill certificate exactly as primary metal is.

Two sourcing dimensions are easy to overlook at the purchasing stage but decide total cost. The first is form-and-temper availability: a commodity grade in a non-stock temper or thickness can carry a long lead time, so confirm the exact alloy-temper-form combination is stocked before designing around it. The second is documentation: a mill certificate that states the governing standard, heat number, and measured chemistry and properties is what lets you verify the metal you received matches the metal you specified, especially when substituting across the ASTM, EN, GB/T, and JIS systems described in Chapter 5. Taken together, the selection criteria, the verified supply tiers, and disciplined documentation are what turn the broad aluminum alloy category into a defensible, traceable procurement decision.