May 2021 Volume 3

MATERIALS

Heat Treatment ofWrought Aluminum Alloys: Tempers andCritical Requirements By Jorge A. Manriquez, Ph.D. and Miguel A. Bocanegra, M.Sc.

This article describes three elements relevant to the heat treating of wrought aluminum alloys: (1) common tempers and alloys; (2) critical features and testing of thermal processing equipment; and (3) material and process specifications. Series 2XXX, 6XXX, and 7XXX are used as a way to emphasize challenges and best practices. Introduction Many forgings and partsmade fromaluminumalloys need to be heat treated to develop physical and mechanical properties. Depending on the alloy and the intended application, forged aluminum components are heat treated typically in agreement with one or several industry material specifications. For instance, aluminum components for the Aerospace industry are commonly heat treated in furnaces complying with AMS 2750, AMS2772/2770, and other Nadcap requirements. Table I lists a series of common aluminum alloys and typical heat treating and strength classifications. Tempers A temper is a code added to the alloy´s name to indicate the condition or state of the alloy in terms of thermal processing. Material Al 6061- T6 for instance, means that an alloy 6061 has been heat treated to attain a T6 temper and is expected to show a defined set of mechanical and physical properties associated to that particular temper. In some cases, a more detailed description might include the specification used to execute the heat treatment, for instance Al 6061-T6 per AMS 2772. In other cases a full code will also include a manufacturing spec, such as in 6061-T6 per ASTM B247 (with AMS 2772 being required by ASTMB247). Tracking of the temper code throughout the processing of a forging and the life of a part is of major relevance for suppliers and users.

Heat treatable grades. Alloys from series 2XXX, 6XXX and 7XXX can be heat treated to a variety of tempers. The basic tempers used for aluminum are (description is focused on heat treated tempers): F As Fabricated. This temper means the material has received no heat treatment processing after fabrication. O Annealed. This temper is achieved by heating the material to high temperatures and then cooling slowly to produce a soft state. This is considered a stable temper. H StrainHardened. This is a temper achieved by cold working the material to increase hardness and tensile properties. This temper considers different levels of cold work and the option of applying an annealing step after cold working. T Solution and Aged. This is a temper achieved by heating to high temperatures followed by a quench (rapid cooling) step and then aging for a specified time. The quench media can be water or a polymer solution. The aging period is realized more often at a specified medium temperature and time in a process known as artificial aging, but aging at room temperature (known as natural aging) is also required for some particular applications. Temper T is considered a stable temper. Below is a summarized list of T tempers (see AS1900) that are commonly applied to aluminum alloys. T1: Cooled from a high-temperature forming operation and natural aged. T2: Cooled from a high-temperature forming operation + cold worked + natural aged. T3: Solution treated + cold worked + natural aged. T4: Solution treated and natural aged. T5: Cooled from a high-temperature forming operation + artificial aged. T6: Solution treated and artificial aged. T7: Solution treated and over-aged. T8: Solution treated + cold worked + artificial aged. T9: Solution treated + artificial aged + cold worked. T10: Cooled from a high-temperature forming operation + cold worked + artificial aged. Non-heat treatable grades. There are some alloys that have a very limited content of alloying elements in their chemistry, and therefore will produce none to very little precipitates as a response to a given heat treatment. As a consequence, those alloys are classified as non- heat treatable.

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FIA MAGAZINE | MAY 2021