Decoding the Aluminum Alloy Numbering System

Originally published on fastradius.com on June 1, 2021

Aluminum is a lightweight yet incredibly strong metal, making it an excellent choice for many applications in the automotive, aerospace, and electronics industry. Along with its durability and physical lightness, aluminum is corrosion-resistant due to a strong layer of oxide film covering its surface.

Since aluminum easily forms compounds with other chemical elements, a large number of aluminum alloys have been developed over the years. To create an aluminum alloy and improve certain qualities of base aluminum, you must add a chemical element to pure aluminum. This requires a thorough mixing of these elements — such as magnesium, silicon, zinc, or copper — with aluminum while the metal is molten. These elements can increase aluminum’s strength, density, workability, electrical conductivity, and more.

Aluminum alloys can vary significantly depending on their composition and tempering. To prevent confusion, aluminum alloys are named and categorized according to an aluminum alloy numbering system. These systems help designers and engineers familiarize themselves with various alloys, their characteristics, and common applications. This helps product teams choose the right aluminum alloy and manufacturing method for a particular part.

Wrought Aluminum Series Numbers

The Aluminum Association established the wrought alloy description system in 1954. When the system was first implemented, it listed 75 chemical compositions — today, there are more than 530 registered active chemical elements, and this number continues to grow.

The wrought aluminum series names elements with four numerical digits where the first digit represents the principal alloying element, the second digit indicates a modification of a specific alloy, and the third and fourth digits are arbitrary numbers assigned to specific alloys in the series.

Here are the primary alloying agents in the wrought aluminum series:

1xxx — 99.000 Minimum Aluminum

It’s impossible for aluminum to be 100% pure, but aluminum in this wrought series category contains at least 99% aluminum. For all intents and purposes, 1xxx alloys are considered pure aluminum. Notably, this alloy is the exception to the wrought series naming rule — in 1xxx alloy names, the last two digits stand for the minimum percentage of aluminum above 99. For example, Alloy 1350 consists of at least 99.50% aluminum.

Pure aluminum has great corrosion resistance and workability, plus high electrical and thermal conductivity. For this reason, this alloy is often used for electrical and chemical applications. Pure aluminum is not very strong and is seldom used for structural applications, but strain hardening can moderately increase material strength.

2xxx — Copper

This wrought series alloy offers high strength and performance over a wide range of temperatures and is regularly used in aerospace applications. One well-known aircraft aluminum alloy is Alloy 2024. However, some copper aluminum alloys are susceptible to heat and stress corrosion cracking and are considered non-weldable, whereas other 2xxx alloys can be welded using the right methods. 2xxx decreases the elongation and tensile strength of aluminum and doesn’t offer as good corrosion resistance as other alloys in the wrought series.

3xxx — Manganese

3xxx aluminum alloys were first used only in pots and pans, but are now widely used in heat exchanger components for vehicles and power plants. With good temperature stability and corrosion resistance, alloys in this category are suitable for use in extreme conditions. 3xxx also allows for good formability and workability. 3003 is a popular manganese alloy used for moderate strength applications that require complex shapes.

4xxx — Silicon

Silicon reduces aluminum’s melting point and improves its fluidity when molten. For this reason, 4xxx alloys are often used in fusion welding wire and as brazing alloys. Silicon on its own is non-heat-treatable, but a number of 4xxx alloys respond well to heat treatments thanks to added copper or magnesium.

5xxx — Magnesium

Aluminum alloys in this category are easily weldable and widely used for applications in shipbuilding, transportation, bridge building, and construction. 5xxx alloys offer good corrosion resistance in marine environments and have the highest strength of all non-heat-treatable alloys. However, 5xxx alloys with more than 3 to 3.5% magnesium are not recommended for elevated temperature service above 65.6°C (150°F) due to the possibility of stress corrosion cracking.

6xxx — Magnesium and Silicon

6xxx alloys usually consist of around 1.0% magnesium and silicon each, which produces magnesium-silicide. Magnesium-silicide can support solution heat treatments that improve strength, formability, and corrosion resistance. This wrought series number is used throughout the welding fabrication industry, primarily incorporated into structural components and extrusions.

These alloys are sensitive to solidification cracks, which means they should not be welded without filler materials — oftentimes, 6xxx is welded with 4xxx or 5xxx filler materials to increase weldability. A major magnesium-silicide alloy is 6061, which is one of the most versatile heat treatable aluminum alloys.

7xxx — Zinc

This wrought series number contains some of the highest strength aluminum alloys that are best-suited for high-performance applications in competitive sporting equipment or the aircraft and aerospace industries. Zinc additions range from 0.8 to 12% in 7xxx alloys and can be coupled with smaller percentages of magnesium, copper, and chromium for heat treatability.

Like 2xxx, 7xxx contains alloys both suitable and non-suitable for welding — one commonly welded alloy is 7005, which is primarily used with 5xxx alloy fillers. One of the highest-strength aluminum alloys available is 7075, which is often used in air-frame structures and for other high-stress applications.

Considerations for the Wrought Aluminum Series

It’s important to note which aluminum alloys are heat treatable and which are non-heat-treatable. This will help product teams determine appropriate applications for specific aluminum alloys, and protect components from high temperatures when necessary.

• 2xxx, 6xxx, and 7xxx series alloys are heat treatable.
• 1xxx, 3xxx, and 5xxx series wrought aluminum alloys are non-heat-treatable and only allow for strain hardening.
• 4xxx series contains some heat-treatable alloys, but mostly non-heat-treatable alloys. However, most 4xxx non-heat-treatable alloys can respond to heat treatments when mixed with other heat-treatable alloys.

In the United States, we mainly use the wrought aluminum alloy series, but there are other aluminum naming and organization systems, including the cast aluminum alloy series. Cast aluminum series numbers look similar to the numbers above. The cast aluminum series uses a 3-digit number and one decimal place (xxx.x), where the first digit (Xxx.x) indicates the principal alloying element.

Understanding Aluminum Alloys and Tempers

Tempers indicate whether an aluminum alloy has undergone any processing to increase mechanical properties like tensile strength, hardness, or heat resistance. Tempers are shown in the wrought aluminum series as a tagged-on letter after an alloy number, e.g. 3003-H.

Strained-Hardened Tempers (-H)

The addition of an “-H” after alloys 1xxx, 3xxx, 5xxx, and sometimes 4xxx indicates that the alloy has been strained-hardened since they cannot be heat treated. The number immediately following the H indicates its processing:

• H1 — Strain-hardened
• H2 — Strain-hardened and partially annealed
• H3 — Strain-hardened and stabilized
• H4 — Strain-hardened and lacquered or painted

Strained hardened aluminum alloys are followed by two numbers, where the second number indicates a scale designation ranging from 0 (fully annealed, softest) to 8 (hardest). For example, Aluminum 5052-H32 has been strained-hardened plus stabilized and is relatively soft.

Thermal and Heat-Treated Tempers (-T)

The addition of a “-T” after alloys 2xxx, 6xxx, 7xxx, and some 4xxx indicates that the alloy has been heat-treated, rapidly cooled or quenched, or precipitation hardened. The number following the T indicates which thermal process it has undergone.

• T1 — Cooled from an elevated temperature and naturally aged
• T2 — Cooled from an elevated temperature, cold-worked, and naturally aged
• T3 — Solution heat-treated, cold-worked, and naturally aged
• T4 — Solution heat-treated and naturally aged
• T5 — Cooled from an elevated temperature and artificially aged
• T6 — Solution heat-treated and artificially aged
• T7 — Solution heat-treated and stabilized
• T8 — Solution heat-treated, cold-worked, and artificially aged
• T9 — Solution heat-treated, artificially aged, and cold-worked
• T10 — Cooled from elevated temperature, cold-worked, and artificially aged

Precipitation hardened or heat-treated alloys sometimes contain an additional digit, which indicates specific end properties such as stress relief by stretch (-T51) or compression (-T52).

Reading the Wrought Aluminum Series With SyBridge

While it is important to be familiar with the aluminum alloy numbering system, we don’t expect you to know all aluminum alloys by heart. To ensure you’re choosing the right aluminum alloy for your project’s application, enlist the help of a seasoned manufacturing partner.

SyBridge can help you understand the aluminum alloy wrought series and determine the best possible aluminum alloy for a given part. Our team of expert manufacturers has in-depth industry knowledge and experience, which we’ll dedicate to making your next project a success. Contact us today to begin your aluminum alloy journey.