In this thesis the effect of heat treatment parameters on hardening of AlZn5.5MgCu alloy (AA 7075) was followed. The alloy was melted and casted in steel mold and then heat treated. Heat treatment included homogenization annealing on 460 °C, solution heat treatment on 480 °C, quenching in water and aging on room temperature and on temperatures of 100, 200 and 300 °C. Two-stage aging was also performed with the first aging at 100 °C and second at 200 °C. The effect of homogenization annealing time on the microstructure and the effect of the temperature and aging time on hardness were investigated. Microstructure analysis was made by scanning electron microscope (SEM), for predictions of microstructural constituents and solidification process Thermo-Calc was used.
It was discovered that in the microstructure of casted alloy, in addition to the dendrites of primary phase α, the phases Al12Cu4Mg8Zn, Al7Cu2Fe and Mg2Si are present, which are distributed on the grain boundaries of phase α. Due to the non-equilibrium solidification the micro segregations and non-equilibrium phases were formed. Thermo-Calc determined that the liquidus temperature of alloy was 643,5 °C and solidus 461 °C. By homogenization annealing the microsegregations were eliminated after 4 hours. With heat treating the highest hardness was reached at specimen that was aging naturally. The higher hardness as hardness of specimen in quenched condition was also found at aging at 100 °C til aging time of 24 hours, at aging at 200 °C, where hardness was increasing till 15 minutes and at two-stage aging till 1 hour. The aging temperature at 300 °C was too high, which was apparent in and immediate decrease in hardness. In the microstructure of the alloy that was aged at 300 °C, fine homogenously distributed precipitates of MgZn2 phase were easily visible by a scanning electron microscope.
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