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. The official definition of an alloy is:
"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 trying to make it more useful than either basic metal. In aluminum without alloys, we would only know it to be primarily useful in wrapping turkey sandwiches or covering a bowl of chili in the refrigerator. In the history (~1890's) of inventing aluminum, when the euphoria of developing the first commercial batches wore off, the next problem was how to make it useful. The early developers having background in copper and other metals knew if you mixed pure aluminum with certain other metals, you will probably make something much more useful. This of course, fostered a lot of activity and the experimenters developed a growing list of valuable aluminum materials. With each new material, you are presented with the problem of giving it a name. Since most of the activity was centered on the Alcoa laboratories, there was a bit more discipline in this process and in the beginning the materials were labeled simply 2S, 3S, 17S, 24S for example. Marketing got involved in the process and, for example, gave 17S the name "Duraluminum" and it was the preferred material for early aircraft. Aluminum alloys with a few exceptions, did not get recognized "names" compared to other materials found, for example, with steel alloys.
After WWII a Trade Group known as the Aluminum Association emerged as the "Official" Spokesperson" for aluminum industry and established discipline into the alloy naming process and creation. It was their influence that founded the current system of alloy designation. With much thought and discussion at the time, the known aluminum alloy products were broken into nine categories depending on the major alloying ingredients. The series started at 1000, and wound up to 9000. Existing alloys were fitted into this new system by adding additional letters, such as 3S, became 3003 and 24S was assigned 2024 for examples. The temper system was given a similar "makeover" and emerged as the familiar –TXXX (and others) designation. Specific information can be found under Tempers link. Various alloys have become useful depending upon some unique characteristic and the following brief summary will highlight some of the "performers" in each alloy system.
Starting with the 1000 or the 1XXX series, (replace the X's with your choice of alloy) we discover it is what aluminum is called when it is basically all aluminum. If the product has more than 1% of "other" metals you can't fit it into the 1XXX series. The 1XXX series is the "raw" or "pure" state where most of the aluminum begins, and is delivered from the reduction facilities. So the materials that are in the 1XXX series are low strength, highly formable products such as wire, foil and sheet. Household aluminum foil, if you found a designation on the label, would read Aluminum alloy 1023. Forgings in this alloy series are quite few as only specialized applications have utilized this low strength material. To strengthen the sheet material, cold working (aka cold forging) is necessary by passing it through a series of rollers.
The 2XXX series is where the forging products begin to emerge. This alloy series is distinguished from the others because the major ingredient is copper. Here we have a number of useful forging alloys. Of the modern general purpose alloys, 2014 has been the "workhorse" for the aircraft industry after WWII. It featured high strength, excellent corrosion, fatigue resistance and generally well behaved in heat treatment. Most of the product forms have been die forgings in relatively thinner sections. 2014 alloy is being phased out of aircraft use by the more advanced alloys. The 2024 alloy while being very common in sheet and plate forms, never found much use in forgings as it tended to be brittle in the highest strength forms and 2014 tended to be preferred except when higher temperatures were encountered. Moving from the mainstream, we get to some specialized applications. 2618 and 2219 are common choices where specialized tasks are required. 2618 is used for pistons and where high temperatures are involved. Developed later, 2219 is similarly useful in higher temperatures and/or where welding may be involved.
Looking at the next series, the 3XXX classification uses small amounts of manganese for hardening and we find nothing commercially used in the forging industry. Since it can't be heat treated, it never develops strengths useful for aircraft use. Mostly you will see alloys such as 3003 in sheet and tubing applications. It finds many uses where moderate strength and formability is required.
The 4XXX series is a similar orphan with regard to the forging industry. Silicon can be dissolved easily in pure aluminum and the 4XXX series may use typically more than 10%. This large amount of silicon makes for good pistons in the die forged state as the particles of silicon embedded provide good wear resistance against the cast iron cylinders. The high concentration of silicon has a downside however, as fabrication (sawing and machining) of parts (with the hard silicon particles) made from for example, 4032 material, is difficult, due to short tool life.
As we move to the next series,-5XXX, we begin to find some forging applications. The 5xxx series uses magnesium as its primary alloying material. The addition of magnesium tends to lighten the combination but because it does not respond to heat treat; 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 and weldability is required. This alloy because of its chemistry is not heat treatable.
The 6XXX series is the all-purpose aluminum alloy. If you want to use aluminum alloy and don't know too much about it, -- pick 6061 for example. Classifying the current 6XXX series alloys which have a unique chemistry such as 6061, 6063, and 6151 posed a problem for the Aluminum Association group assigned to such matters. These alloys have silicon and magnesium in "substantial percentages". Neither Si nor Mg dominates or is the "major" alloying ingredient. This dual nature caused a deviation to their ordered scheme of assigning a particular series to one specific metallic additive. These chemistries did not fit into any other classification so after debating the issue, they received their own series and were assigned 6XXX. Alloys such as 6061 and 6063 are most likely to be found on your car or around your home. They are the first choice of most common commercial applications such as screen doors, windows, and building structural applications. 6061 is a remarkable alloy as it has the strength exceeding most common steels on a weight basis, is weldable, has great corrosion resistance and, if aluminum is on your agenda,--affordable.
As we move to the 7XXX series, we begin to depart from most commercial alloys and enter the aerospace materials. Zinc is the major alloying and "classifying" element in this group. The high strength aerospace materials also need an inclusion of magnesium and copper to achieve their strength. When all the elements are added up you have an alloy of aluminum that may be less than 90% aluminum. The 7XXX series debuted at the end of WWII and have been the focus of most developments for airframe application. Alloys such as 7075, 7178, 7049, and 7050 are the bulk users in this series. If there are newer advanced alloys to be developed for aerospace applications, it probably will be within this series where the formulation will emerge.
The list of alloys populating the 8XXX series is very short. The 8XXX series alloys are where you put them if they don't fit into the other alloy series slots. The only recent activity concerning this alloy series was when the Aluminum-Lithium alloys (Al-Li) were about to emerge into commercial production. One group wanted to assign these unique lithium containing alloys to the 8XXX series, but the final decision put them under the 2XXX series. The lithium was not the predominate alloy ingredient and it would have set a bad precedent to do this.
The 9XXX series list will be even shorter as it is designated as "unused" series. In other words, the Aluminum Association task group covered future events by leaving this open for inventions of things they could not think of at the time.