Aluminum Etc.: Explaining aluminum series numbers for cast alloy designation

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In 1954, 75 chemical compositions were identified as part of the introduction of series numbers to differentiate aluminum and aluminum alloys. More than 600 new chemical compositions have been added since then. Currently, there are 400 wrought aluminum and aluminum alloys and 200 aluminum alloys in the form of ingots and castings, with more added every year. Aluminum is separated into wrought and cast aluminum categories and differentiated using a temper designation system.

A quick overview:

While this system is very helpful to a welding engineer, it can be a bit overwhelming for the typical welder just trying to get a project completed. But as technology advances and more procedures and processes are discovered and used, understanding aluminum alloys and applications will become more important to the future of manufacturing.

So how do you know which aluminum series to use?

First, let’s break down the four-digit identification system specific to wrought (cast has a three-digit system and a decimal—more on that later). The first digit (XXXX) is the principal element that was added to the aluminum. The second digit (XXXX) is a modification of the specific alloy, unless it is 0. The third and fourth digits (XXXX) identify a specific alloy and are considered arbitrary numbers.

Using 5356, a commonly used aluminum series, as an example, the 5 indicates that magnesium is the principal element added to the aluminum; the second digit, 3, lets us know that manganese was added to the magnesium; and the last two digits, 5 and 6, identify the 5000 series.

Alloy series 1XXX has 99% aluminum as its principal alloying unit—this is essentially pure aluminum. In 2XXX, the principal alloying unit is copper; in 3XXX, it’s manganese; in 4XXX, it’s silicon; in 5XXX, it’s magnesium; in 6XXX, it’s magnesium and silicon; in 7XXX, it’s zinc; and in 8XXX, it’s “other elements.”

In today’s welding world, some series are more user friendly than others. Hot cracking and stress corrosion are huge reasons why we can arc weld some series while others are considered unweldable.

Parts makers use cast aluminum because it produces complex parts with high accuracy, enabling manufacturers to replicate the exact design and reduce the need for machining. Materials made from this metal are lightweight and offer outstanding strength. If we consider its weight-to-strength ratio, cast aluminum parts fare better than any other casting process, including the stronger cast iron. As a result of this, the automobile and aerospace industries use cast aluminum extensively. Many industries prefer parts made from this metal because it is very affordable.

So much of our daily lives revolves around cast aluminum. From pots and pans to engine pistons, there isn’t an industry I can think of that doesn’t use it in some way.

The cast alloy designation system is based on three digits plus a decimal designation. Much like with wrought aluminum, the first digit in cast indicates the principal alloying agent. The second and third digits represent arbitrary numbers that identify a specific alloy in the series. In both designation systems, the 1, 2, 4, 5, and 7 series used as that first digit are identical primary alloy elements.

In the cast designation system, the remaining four numbers (3, 6, 8, and 9) mean something else:

Where this designation system gets interesting is the number following the decimal point. A 0 after the decimal indicates the aluminum was cast into a working mold. A 1 or a 2 indicates an ingot (pure aluminum mold) and will be reworked into its final product.

Understanding the identification system for wrought and cast aluminum and what each series number says about the different characteristics will help you make good choices for your application. From here you can find the letter specifying temper designations, which are especially helpful when welding on heat-treatable and nonheat-treatable aluminum alloys.