What Are Alloys?
The word "alloy" is tossed about in various conversations when we discuss aluminum and its applications. The word alloy is reserved in the English language to describe things made from metals.
A homogeneous mixture or solid solution of two or more metals, the atoms of one replacing or occupying interstitial positions between the atoms of the other.
In simpler terms, an alloy is a deliberate mixture of two (or more) metals, aiming to make the result more useful than either basic metal on its own. Without alloys, aluminum would only be useful for wrapping a sandwich or covering a bowl of leftovers. In the history of inventing aluminum in the 1890s, once the excitement of developing the first commercial batches wore off, the next problem was how to make it useful. Early developers with backgrounds in copper and other metals knew that mixing pure aluminum with certain other metals would probably make something much more useful — and that fostered a lot of activity, with experimenters developing a growing list of valuable aluminum materials.
What's in a Name?
With each new material came the problem of giving it a name. Since most of the early activity was centered at the Alcoa laboratories, there was a bit more discipline in the process, and in the beginning materials were labeled simply 2S, 3S, 17S, and 24S, for example. Marketing got involved and gave 17S the name "Duraluminum," which became the preferred material for early aircraft. Aluminum alloys — with a few exceptions — never developed recognized "names" the way some steel alloys did.
The Aluminum Association
After WWII, a trade group known as the Aluminum Association emerged as the official spokesperson for the aluminum industry and established discipline in the alloy naming and creation process. It was their influence that founded the current system of alloy designation. With much thought and discussion, known aluminum alloy products were broken into nine categories depending on the major alloying ingredient, starting at 1000 and running through 9000. Existing alloys were fitted into the new system by adding digits — 3S became 3003 and 24S was assigned 2024, for example. The temper system received a similar makeover and emerged as the familiar –TXXX designation.
Pure Aluminum
The 1XXX series is what aluminum is called when it is basically all aluminum — if a product has more than 1% of "other" metals, it can't fit into the 1XXX series. It's the raw or pure state where most aluminum begins, delivered directly from the reduction facilities. Materials in this series are low strength, highly formable products such as wire, foil, and sheet. Household aluminum foil, if it carried a designation, would read Aluminum alloy 1023. Forgings in this series are quite rare — only specialized applications use this low-strength material. To strengthen the sheet material, cold working (cold forging) through a series of rollers is necessary.
Copper-Based — Where Forging Begins
The 2XXX series is where forging products begin to emerge. This series is distinguished by copper as the major alloying ingredient, and includes a number of useful forging alloys.
Of the modern general-purpose alloys, 2014 has been the workhorse for the aircraft industry since WWII — high strength, excellent corrosion and fatigue resistance, and generally well-behaved in heat treatment. Most product forms have been die forgings in relatively thin sections, though 2014 is being phased out of aircraft use by more advanced alloys. 2024, while very common in sheet and plate forms, never found much use in forgings, as it tended to be brittle in its highest-strength forms — 2014 was generally preferred except at higher temperatures.
Moving to specialized applications: 2618 is used for pistons and other high-temperature work, and 2219, developed later, is similarly useful at higher temperatures and/or where welding is involved.
Manganese — Not Heat Treatable
The 3XXX series uses small amounts of manganese for hardening, and there is nothing commercially used in the forging industry within this series. Because it can't be heat treated, it never develops strengths useful for aircraft applications. Alloys such as 3003 are mostly seen in sheet and tubing applications, where moderate strength and formability are required.
Silicon — Wear-Resistant, Hard to Machine
The 4XXX series is another series with no significant forging use. Silicon dissolves easily in pure aluminum, and the 4XXX series may use typically more than 10%. This high silicon content makes for good pistons in the die-forged state, as embedded silicon particles provide good wear resistance against cast-iron cylinders. The downside is that fabrication — sawing and machining — of parts made from alloys such as 4032 is difficult, due to the short tool life caused by the hard silicon particles.
Magnesium — Corrosion and Weld Resistant
The 5XXX series is where forging applications begin to reappear. This series uses magnesium as its primary alloying material — the addition of magnesium lightens the combination, but because it does not respond to heat treatment, really high strengths are not developed.
The most common product form for the 5XXX series is sheet and plate. Forgings are used primarily in cold-temperature (cryogenic) applications and in marine applications, where corrosion resistance and weldability are required. This alloy series is not heat treatable.
The All-Purpose Alloy
The 6XXX series is the all-purpose aluminum alloy — if you want to use an aluminum alloy and aren't sure which one, 6061 is a common default. Alloys such as 6061, 6063, and 6151 have silicon and magnesium in substantial percentages, with neither dominating as the single "major" alloying ingredient. This dual nature didn't fit the Aluminum Association's ordered scheme of one series per additive, so after debate, these chemistries received their own series: 6XXX.
Alloys such as 6061 and 6063 are the ones most likely to be found on your car or around your home — the first choice for common commercial applications such as screen doors, windows, and building structural applications. 6061 is a remarkable alloy: it exceeds the strength of most common steels on a weight basis, is weldable, has great corrosion resistance, and is affordable.
Zinc — The Aerospace Alloys
The 7XXX series departs from most commercial alloys and enters the aerospace materials. Zinc is the major alloying and classifying element in this group. High-strength aerospace materials also need magnesium and copper to achieve their strength — when all the elements are added up, a 7XXX alloy may be less than 90% aluminum.
The 7XXX series debuted at the end of WWII and has been the focus of most development for airframe applications. Alloys such as 7075, 7178, 7049, and 7050 are the bulk users in this series — if newer advanced alloys are developed for aerospace applications, they will most likely emerge within this series.
The Catch-All Series
The list of alloys populating the 8XXX series is very short — it's where alloys are placed if they don't fit into the other series. The only recent activity in this series came when Aluminum-Lithium (Al-Li) alloys were about to enter commercial production. One group wanted to assign these lithium-containing alloys to the 8XXX series, but the final decision placed them under the 2XXX series instead, since lithium was not the predominant alloying ingredient and assigning it to 8XXX would have set a bad precedent.
Reserved for the Future
The 9XXX series is designated as "unused." The Aluminum Association task group left this series open to cover future inventions — materials they could not anticipate at the time the system was established.