In this study, the effect of different heat treatment processes applied to AA7075 alloys on the fatigue behavior was examined. The processes applied to AA7075 aluminum included annealing (O), high temperature pre-precipitating (HTPP), artificial aging (T6), retrogression and re-aging (RRA). The annealing heat treatment was performed for 2 hours at 500°C and samples were cooled in the furnace. In the artificial aging (T6) process, after the samples were solution treated for 2 hours at 500°C, they were quenched at room temperature and aged for 24 hours at 120°C. In the retrogression and re-aging process, samples were solution treated for 1 hour at 220°C after the T6 process and then re-aged for 24 hours at 120°C. In the high temperature pre-precipitating, pre-precipitates were formed for 30 minutes at 450°C and then, it was aged for 24 hours at 120°C. All samples were characterized through the scanning electron microscope (SEM + EDS), hardness measurements and X-ray difraction (XRD) techniques. At the end of experimental studies, SEM and EDS examinations XRD results revealed that η (MgZn 2 ) phase formed in the microstructure following the HTTP, RRA and T6 heat treatment processes. As a result of the fatigue tests, the highest fatigue strength was measured in samples treated with artificial aging (T6), the lowest fatigue strength was measured in the annealed (O) samples.

In this study, 7075 aluminum alloy was subjected to triple-aging of retrogression and re-aging (RRA) treatments. The alloy was retrogressed at 220 °C for 60 minutes following the T6 heat treatment and later re-aged at temperatures between 100 °C and 140 °C for 24 hours and at 120 °C for various durations in the range of 15–35 hours. The effects of temperature and duration of re-aging on hardness and the fatigue behaviors of RRA tempered 7075 aluminum alloys were investigated. The results show that temperature and duration of re-aging have an influence on both hardness and fatigue resistances. An increment of the hardness values depends on increasing of re-aging temperature and times until 120 °C and 24 hours and reaches to the maximum value. However, those values decrease with higher re-aging temperatures and longer duration than 120 °C and 24 hours, respectively. On the other hand, in the fatigue tests, the highest fatigue resistance was observed for the sample re-aged at 120 °C for 24 hours, while the sample re-aged at 140 °C for 24 hours showed the longest fatigue life under lower fatigue stress.