Retrogression and Reaging of AA7075 and AA6013 Aluminum Alloys

Retrogression and reaging (RRA) is of interest to the automotive industry for manufacturing components of high-strength aluminum alloys. RRA heat treatments are investigated for AA7075-T6 and AA6013-T6 materials. Retrogression is demonstrated to be a thermally activated process reasonably characterized with a single activation energy. Activation energies for retrogression are measured as 97 +/- 7 and 160 +/- 30 kJ/mol for AA7075-T6 and AA6013-T6, respectively. Critical retrogression times, t(R)* and tR max, are defined and measured across a range of retrogression temperatures. These data are used with the concept of reduced time to predict combinations of temperature and time that produce successful retrogression heat treatments. Recommended retrogression heat treatments are 200 degrees C for 3 to 12 minutes for AA7075-T6 and 240 degrees C for 7 minutes for AA6013-T6. Data from reaging heat treatments confirm a significant RRA response in AA6013. Recommended reaging heat treatments are 120 degrees C for 24 hours for retrogressed AA7075 and 190 degrees C for 1 hour for retrogressed AA6013. A reaging heat treatment that simulates the automotive paint-bake cycle, 185 degrees C for 25 minutes, is almost as effective as the recommended reaging heat treatment for AA6013 but is significantly less effective than the recommended reaging heat treatment for AA7075.