Understanding the Aluminum Alloy Designation System

All You Need To Know About Aluminum Alloy Designation System

Aluminum is typically alloyed with different elements to change and improve its chemical or mechanical properties. As a result, there is a wide range of combinations of alloying elements available. These are all categorized into a designation system to help welders make sense of them and choose the appropriate combination. Here’s what you need to know when it comes to understanding the aluminum alloy designation system, including wrought and cast alloy designations and designations for alloys and tempers.

Wrought Aluminum Alloy Designation System

Wrought alloys have a 4-digit number system. The first digit (Xxxx) indicates the principal or major alloying element, which has been added to the aluminum alloy and is often used to describe the aluminum alloy series, i.e. 1000 series, 2000 series, 3000 series, up to 8000 series (see table 1).

Table 1. Wrought aluminum alloy designation system

The second single digit (xXxx), if different from 0, indicates a modification of the specific alloy, and the third and fourth digits (xxXX) are arbitrary numbers given to identify a specific alloy in the series.

For instance, In alloy 5183, the number 5 indicates that it is of the magnesium alloy series, the 1 indicates that it is the 1st modification to the original alloy 5083, and the 83 identifies it in the 5xxx series.

The only exception to this alloy numbering system is with the 1xxx series aluminum alloys (pure aluminums). In this case, the last 2 digits provide the minimum aluminum percentage above 99%, i.e. Alloy 1350 (99.50% minimum aluminum).

Cast Aluminum Alloy Designation System

The cast alloy designation system is based on a 3 digit-plus decimal designation xxx.x (i.e. 356.0). The first digit (Xxx.x) indicates the principal alloying element, which has been added to the aluminum alloy (see table 2).

Table 2. Cast aluminum alloy designation system

The second and third digits (xXX.x) are arbitrary numbers given to identify a specific alloy in the series. The number following the decimal point indicates whether the alloy is a casting (.0) or an ingot (.1 or .2). A capital letter prefix indicates a modification to a specific alloy.

For example, Alloy - A356.0 the capital A (Axxx.x) indicates a modification of alloy 356.0. The number 3 (A3xx.x) indicates that it is of the silicon plus copper and/or magnesium series. The 56 (Ax56.0) identifies the alloy within the 3xx.x series, and the .0 (Axxx.0) indicates that it is a final shape casting and not an ingot.

The Aluminum Temper Designation System

There are considerable differences in the characteristics and consequent application of each series. For instance, there are two distinctly different types of aluminum within the series mentioned above. These are the Heat-Treatable Aluminum alloys (those which can gain strength through the addition of heat) and the Non-Heat Treatable Aluminum alloys.

The 1xxx, 3xxx, and 5xxx series wrought aluminum alloys are non-heat-treatable and are strain hardenable only. The 2xxx, 6xxx, and 7xxx series wrought aluminum alloys are heat treatable and the 4xxx series consist of both heat treatable and non-heat-treatable alloys. The 2xx.x, 3xx.x, 4xx.x, and 7xx.x series cast alloys are heat treatable. Strain hardening is not generally applied to castings.

The heat-treatable alloys acquire their optimum mechanical properties through a process of thermal treatment, the most common being solution heat treatment, and artificial aging. Solution heat treatment is the process of heating the alloy to an elevated temperature (around 990 Deg. F) to put the alloying elements or compounds into solution. This is followed by quenching, usually in water, to produce a supersaturated solution at room temperature.

Solution heat treatment is usually followed by aging. Aging is the precipitation of a portion of the elements or compounds from a supersaturated solution in order to yield desirable properties.

The aging process is divided into two types: aging at room temperature, or “natural aging,” and aging at elevated temperatures, or “artificial aging.” Artificial aging temperatures are typically about 320 Deg. F. Many heat treatable aluminum alloys are used for welding fabrication in their solution, heat-treated, and artificially aged conditions.

The non-heat-treatable alloys acquire their optimum mechanical properties through strain hardening. Strain hardening is the method of increasing strength through the application of cold working.

The Temper Designation System (see Table 3) addresses the material conditions, called tempers. The Temper Designation System is an extension of the alloy numbering system and consists of a series of letters and numbers which follow the alloy designation number and are connected by a hyphen. Examples: 6061-T6, 6063-T4, 5052-H32, 5083-H112.

Table 3. The basic temper designations

There are two subdivision categories in the basic temper designations (see Tables 4 and 5): One addressing the “H” Temper – Strain Hardening, and the other addressing the “T” Temper – Thermally Treated designation.

Table 4. Subdivisions of H Temper – Strain Hardened

The first digit after the H indicates a basic operation:

H1 – Strain Hardened Only.

H2 – Strain Hardened and Partially Annealed.

H3 – Strain Hardened and Stabilized.

H4 – Strain Hardened and Lacquered or Painted.

The second digit after the H indicates the degree of strain hardening:

HX2 – Quarter Hard
HX4 – Half Hard
HX6 – Three-Quarters Hard
HX8 – Full Hard
HX9 – Extra Hard

Table 5. Subdivisions of T Temper – Thermally Treated

T1 – Naturally aged after cooling from an elevated temperature shaping process, such as extruding.
T2 – Cold worked after cooling from an elevated temperature shaping process and then naturally aged.
T3 – Solution heat treated, cold worked, and naturally aged.
T4 – Solution heat-treated and naturally aged.
T5 – Artificially aged after cooling from an elevated temperature shaping process.
T6 – Solution heat-treated and artificially aged.
T7 – Solution heat treated and stabilized (overaged).
T8 – Solution heat treated, cold worked, and artificially aged.
T9 – Solution heat treated, artificially aged, and cold worked.
T10 – Cold worked after cooling from an elevated temperature shaping process and then artificially aged.

Additional digits indicate stress relief.

Examples:

TX51 or TXX51 – Stress relieved by stretching.
TX52 or TXX52 – Stress relieved by compressing.

Additional Information Sources

• The Aluminum Association’s “Welding Aluminum Theory and Practice”
• The Aluminum Association’s Aluminum Design Manual and Aluminum Standards and Data
• The American Welding Society Document D1.2 – Structural Welding Code – Aluminum
• The Aluminum Association – Tel: (301) 645-0756 / Web Site: www.aluminum.org
• AWS – Tel: 1 800 443 9353 / Web Site: www.aws.org