Fretting and plain fatigue behavior of Al 7075-T651 at elevated temperatures

Fretting fatigue behavior of Al 7075-T651 is investigated at temperatures of 20?, 150?, 250? and 350? and at different stress levels. The results show that: (a) fatigue life increases from 15% to 160% for low stresses and reduces from -20% to -40% for higher stresses and (b) fretting fatigue life increases from 155% to 290% for low stress levels and reduces from -12% to -65% for high stress levels. All life changes have been measured with respect to the lives at ambient temperature. It is believed that under low stresses, the effect of glaze oxide layer created through surface oxygen absorption prevails and gives rise to improvement of plain and fretting fatigue of material at elevated temperatures. Precipitation and solid solution hardening and good lubrication of the oxide layer are the main reasons for this improvement. For the higher stresses, 180, 200 and 280MPa, the oxide layer breaks down and the degradation due to reduction in ultimate strength dominates the fatigue behavior of material and diminishes the fatigue lives. Fractography of specimens reveals brittle fracture mode in the fracture surface regardless of the test temperature. Optical microscopy of specimens shows more germination of precipitates at lower temperatures and shorter aging durations and more growth of precipitates for higher temperatures and longer aging durations.