There are only three groups of wrought aluminum alloys that can be successfully thermally strengthened using heat-treatment procedures. Those alloy groups are:

• 2xxx (copper as the main alloying element)
• 6xxx (magnesium and silicon as the main alloying elements)
• 7xxx (zinc is the main alloying element but is often with copper and magnesium)

These particular wrought aluminum groups are very broadly selected and used where high strengths and high mechanical properties are necessary from the aluminum alloy. They are particularly common in the aerospace and defense industry.

The focus of this post will be on the heat-treatable wrought aluminum alloys rather than the wrought non-heat-treatable groups. The strengthening method of the heat-treatable aluminum alloys is simple and straightforward.

The strengthening heat-treatment procedures are usually focused on the “T” temper group. The first letter T designates the type of treatment (temper) and the numeral designates what has been done to it.

We will commence with the T3 treatment, and the other solution-treatment designations follow.

T3 Temper
The T3 temper requires the aluminum alloy to be solution treated from the appropriate solutionize temperature according to the alloy and then naturally aged to a stable condition. This treatment is given to aluminum products that are cold worked after solution treating to improve their strength.

T4 Temper
The T4 temper necessitates a solutionize treatment only (no artificial age procedure to follow) from an appropriately selected solutionizing temperature (according to the alloy in question). The T4 solutionize temperature has a very specific transfer time from the moment the furnace door is opened to the time that the part is completely immersed in the quench medium.

The aging procedure is a natural age process and not artificial by the application of heat. Typically, this procedure is applied to products that are not cold worked after solutionizing. They can, however, cold work the alloy after solutionization providing the cold work does not alter the mechanical properties. The specification calls for a natural age procedure up to 96 hours maximum at room temperature.

T5 Temper
This procedure is carried out by cooling the alloy from a high-temperature shaping process followed by an artificial age process. The part should not be worked after the cooling process as this will affect the mechanical properties.

The remainder of the “T” tempers will be covered in the next blog, but the following are five very critical aspects to the successful heat treatment of these aluminum alloys:

• Very careful solutionize temperature selection
• Uniform temperature distribution in the solutionize furnace
• Very careful quench-medium selection for the solutionize procedure
• Very careful artificial age (precipitation) temperature
• Uniform temperature distribution in the artificial age furnace

These are the principle conditions necessary for accomplishing good uniform metallurgical and mechanical properties for the heat-treatable aluminum alloys.