Don't Lose Your Temper-Temper
Aluminum producers go to great lengths to process the product so it is useful in the application intended. This requires the material to have a commonly recognized name for its final condition, called temper.
It used to be easy to keep track of the various tempers in the early days of aluminum production, because there were so few options. As the number of alloys and different combinations of processing multiplied, a system was set up to try to make sense of it all. The Aluminum Association was chartered to make the rules and keep a master list of tempers and their properties. A committee from aluminum producers meets regularly and maintains discipline in the system — though they have not always been successful in reining in the aluminum industry. A few odd temper call-outs got some use and made their way into specification prints before corrections were attempted.
The temper -T411 was often used by the forging industry to describe heating the aluminum to the normal heat-treat temperature and then air cooling — done to "stabilize" the part and improve the ability of ultrasonic inspection to find possible internal flaws. Because it's a form of hardening, this condition had very low strength and was never intended for use in service. The danger of using a -T4 designation for this special condition became clear when producers were often asked how to age-harden the -T411 to the -T611 condition. The Aluminum Association Technical Committee on Product Standards fixed this by changing the temper to the -01 designation instead.
Basic Rules for the Temper Designation System
Nothing was done to the part after it was made by the producer, following the last fabrication operation. In reality, nothing is ever actually put into service in this temper condition — it's just a convenient way to label or name forgings that are in an intermediate stage of processing.
Means the material has been hardened by heating to a high temperature and then slow cooling, so as not to produce a hardening effect. This temper is used when forming is scheduled later in the manufacturing process. For structural aluminum, heat treating to a -T temper (example -T6) to improve strength is normally planned after a forming operation.
Assigned (example: H111) to alloys that can't really be heat treated to higher strengths. Hardening is accomplished by "strain hardening" — the aluminum product is stretched, compressed, or rolled a substantial amount during the final manufacturing step. Some commercial alloys respond to this process and make useful products by this strengthening method alone. For hand forgings, it's accomplished by carefully compressing between the dies at room temperature — an example would be 5083H112.
Assigned to aluminum "waiting" to age-harden to a stable condition after solution heat treat — think of "W" as waiting for something to happen. For example, 2014-W is the proper call-out just after solution heat treat, and after roughly four days it transforms into 2014-T4; during that in-between period it's referred to as 2014-W. Certain alloys, such as 7075-W, never become stable without a final thermal process after solution heat treat. The W temper is simply a convenient way to describe material that isn't yet finished in its hardening process.
This is the basic designation to which most hand forgings are ultimately heat treated. The official definition is "thermally treated to produce stable tempers other than F, O, or H." The T is always followed by one or more digits to indicate the more specific condition, such as -T6, -T74, or T7452.
Even with these basic rules, things get complicated once you get into the variations of the basic tempers. For hand forgings, the important temper designations are -T6 and -T7. You never see a stand-alone -T7 temper, because -T7 means the part has been over-aged — which raises the question of how much. That's why you see -T73, -T74, and similar call-outs associated with -T7, describing the degree of over-aging. Often you'll see -T7X used to describe the multiple conditions possible, rather than just -T7 alone. To fully understand all the variations, refer to the industry standards.
Hand Forgings
For hand forgings, a particularly useful temper is the stress-relieved temper, indicated by adding "52" to the basic -T6 or -T7X tempers — yielding, for example, -T652, -T7352, and -T7452. Another common temper is -01, described above — this intermediate temper is selected whenever extensive machining is scheduled and the part will be fully heat treated later.