Abstract
The effect of different retrogression and re-aging (RRA) treatment on the microstructure, strength and stress corrosion cracking (SCC) resistance of a high Zn content Al alloy was investigated. The peak microhardness values after retrogression and re-aging at 180 °C/30 min and 200 °C/10 min were 214 HV and 215 HV, respectively, which are higher than that after T6 and T7 aging. The retrogression re-aged samples have large numbers of η′ precipitate (MgZn2), resulting in high mechanical properties of the alloy. The SCC resistance order of the alloys was found to be: T6 < RRA (120 °C/24 h + 180 °C/30 min + 120 °C/24 h) < RRA (120 °C/24 h + 200 °C/10 min + 120 °C/24 h) < T7, and the optimal retrogression re-aging process is 120 °C/24 h + 180 °C/30 min + quenching + 120 °C/24 h. The improvement in the stress corrosion cracking resistance of the retrogression re-aged samples can be attributed to the increased interspaces of grain boundary precipitates and uniform distribution of the elements (Cu and Mg) in the grain boundary precipitates.
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