Retrogression and Re-aging for Aluminum Alloys (part 2)
Retrogression heat treatment and re-aging (RRA) is used for retaining optimal strength in aluminum alloys while avoiding stress corrosion cracking (SCC). Last time, we learned about the SCC phenomenon and its effects on aluminum alloys. Part 2 discusses more of the history and process, RRA forming and tools for processing the alloys with RRA. Let’s learn more.
Retrogression and Re-aging (RRA)
In 1974, Baruch M. Cina, a metallurgist from Israel, first proposed and then patented the RRA concept for AA7075-T6 as a way of reducing SCC susceptibility while still maintaining peak-aged strength.
RRA is a multi-step heat treatment consisting of two distinct steps: retrogression and re-aging. The retrogression is a short-duration, high-temperature heat treatment followed by a rapid cooling. During the retrogression, it is thought that the strengthening microstructural precipitates within the aluminum matrix partially dissolve, or retrogress, back into solution. This step reduces part strength.
Re-aging is a low-temperature heat treatment performed after the retrogression. It is performed at the optimal temperature for precipitation and regrowth of the strengthening precipitates. Re-aging returns the part strength to its initial value by reforming partially dissolved precipitates or precipitating new strengthening precipitates.
AA7075-T7 Temper Achieved by RRA
The T7 temper, also known as the “over-aged” condition, has a lower overall strength than the T6 temper. Over-aged sheet material is traditionally obtained by aging for 6-8 hours at 107°C (225°F) and then aging for 24-30 hours at 163°C (325°F).
It should be noted that the re-aging is usually performed at the same temperature as the peak aging treatment. Cina first characterized the retrogression times and temperatures useful for RRA treatments of AA7075. According to Cina, retrogression can be successfully performed from 160-260°C (320-500°F) for AA7075. The recommended retrogression times for 160° and 260°C are 30 minutes and 7 seconds, respectively.
Compared to the T6 temper, the T7 microstructure contains far fewer η′ and η precipitates, with more η precipitates forming than η′ precipitates. The η precipitates are generally larger in the T7 condition than they are in the T6 temper. After aging at 100°C (212°F) for 8 hours and 165°C (329°F) for 24 hours, the resulting microstructure consists of large η precipitates along the grain boundaries and fewer smaller η precipitates lightly dispersed throughout the aluminum matrix, which reduce the alloy’s susceptibility to SCC. An example of the process cycle can be seen in Figure 2.
Another benefit of RRA is using it when forming is required. RRA forming is a new multi-step heat-treatment and forming concept designed to improve formability and maintain the peak-aged strength of high-strength aluminum alloys during automotive manufacturing. The new concept integrates hot forming and the paint baking cycles into an RRA heat treatment.
For example, a sheet material is aged to the T6 condition and then preheated for a controlled time before stamping. The heated sheet is then stamped in either warm or cold dies. The hot stamping operation acts as a retrogression treatment. The material is quickly cooled to room temperature in the dies either during or at the end of the forming procedure.
After the forming/retrogression operation, the sheet must be re-aged back to peak strength. The painting and baking cycle of the sheet/part is used as the re-aging treatment in lieu of the optimal re-aging treatment. An example of the RRA forming process cycle for AA7075-T6 can be seen in Figure 3.
Another option can be used for extrusion, mill product or even castings as follows. Localized heating of age-hardenable aluminum alloys – including F, W, T4, T5, T6, and even T7, T8 and T9 tempers – to a peak temperature that extends into the solvus region of the hardening precipitate or zone and quenching achieves the most ductile/formable condition that the alloy allows while providing for recovery of strength after forming. Heating can be done by induction utilizing a variety of induction coils suited to the shape of aluminum extrusions (Fig. 4).
RRA is a unique precipitation process applicable to the aluminum 7xxx alloys of all forms (plate, extrusion, etc.). It was originally designed and patented to increase the 7075 alloy’s resistance to SCC by a way of performing a retrogression of the 7xxx-T6 material at an intermediate temperature between the aging temperature and the solutioning temperature and re-aging of the retrogressed alloy at a lower temperature.
The concept was later embraced in the forming industry for improving formability of aluminum by rapidly heat treating 6xxx and 7xxx aluminum-alloy extrusions in various tempers, either locally or throughout their length, in order to return to or approach their highly ductile state in their solutionized and freshly quenched state.
Editor’s note: We have some news to share about The Heat Treat Doctor and his monthly column. As of Jan. 1, 2020, Dan Herring decided to officially retire. Dan’s columns will continue in 2020 with a combination of new material and best-of columns until we find another technical expert. We know that, like us, you wish Dan the best for his retirement, and we appreciate the great content he has provided to our readers during his time with Industrial Heating.
- U.S. Patent No. 3,856,584 “Reducing the Susceptibility of Alloys, Particularly Aluminum Alloys, to Stress Corrosion Cracking”
- Benedyk, Joseph C., “Retrogression Heat Treatment as a Means of Improving Formability of Aluminum Extrusions,” Light Metal Age, 2008