Effect of retrogression and reaging treatment on properties of 7010 Aluminium alloy forgings

AA7010 Al Alloy is a material in 7xxx series, used in aircraft and automobile industries have high strength to weight ratio. Long term application of the alloy is susceptible to exfoliation corrosion, corrosion fatigue, and stress corrosion cracking. The alloy in T6 condition presents a high strength but susceptible to stress corrosion cracking. To overcome this problem, a multistage heat treatment process was introduced known as Retrogression and Re-aging in which, the alloy is subjected to T6 condition followed by a two stage thermal cycle. The first stage called as retrogression, involves heating to a high temperature for a short period of time followed by quenching. In the second stage, the alloy is re-aged at the same temperature equal to that of T6 condition. The two-stage process thus provides good tensile strength as well as comparable SCC resistance. In the present work, AA7010 alloy forgings were subjected to T6 condition and then retrogressed in a salt bath furnace at temperatures of 200 and 210oC for time periods of 30, 40, 50, 60 and 90 minutes, followed by re-aging. Retrogression temperature and time period of 200oC and 40 minutes respectively were found to be the optimum schedule which gives the best combination of strength and SCC resistance.