The Effect of Ageing on the Mechanical and Tribological Properties of Al-Zn-Mg Alloy

The aim of this study was to evaluate the effect of heat treatment, including solutionising and ageing in the temperature range of 20-250 °C, on the microstructural, mechanical, and tribological properties of the Al 7075 alloy. Microscopic analysis revealed that in the as-received condition and after natural ageing, the microstructure is characterised by the presence of elongated grains and a banded distribution of precipitates, whereas higher ageing temperatures lead to their coarsening and the initiation of recrystallisation processes. The highest hardness (189 HV) was obtained after ageing at 100 °C for 48 h, while further increases in temperature caused a systematic decrease in hardness-down to 85 HV at 250 °C for 4 h. Impact tests showed that in the as-received condition, the material reached a value of 7 J/cm², after natural ageing 15 J/cm², and the maximum (26 J/cm²) was achieved for samples aged at 250 °C for 4 h. Tribological tests conducted using the T-07 method confirmed the dependence of wear resistance on heat treatment parameters-the lowest relative abrasive wear resistance coefficient was observed after natural ageing (<i>k_b</i> = 0.860), and the highest after ageing at 250 °C for 4 h (<i>k_b</i> = 1.216). The results obtained indicate that moderate ageing conditions (100-150 °C) favour increased hardness, whereas higher temperatures (200-250 °C) lead to an improvement in impact strength and tribological resistance, which showed an inversely proportional relationship with hardness, contrary to Archard's law.