Effects of Heating Rates on Age-Hardening Behavior of an Al-Zn-Mg Alloy with Different Quenching Rates

Generally, age-hardenable aluminum alloys are quenched in water to obtain the required strength. However, recent studies have revealed that furnace cooling (FC) of an Al-Zn-Mg alloy can provide strength comparable to that of water quenched (WQed) material. The reason why higher strength can be obtained by FC has not been clarified. Furthermore, since hardness after artificial aging is also affected by the heating rate up to the aging temperature, in this study, electrical conductivity and TEM microstructure investigations were conducted to elucidate the effects of quenching rate and heating rate up to the aging temperature on changes in hardness. As a result, it was confirmed that the hardness of an Al-6Zn-0.8Mg (mass%) alloy could be obtained by FC to be equivalent to that by WQ. This may be due to the formation of solute clusters and GP zones during FC. Furthermore, in the effect of heating rate up to the aging temperature treatment, the hardness of WQed material decreased when the heating rate was rapid, especially after aging treatment at high temperature such as 473 K. At slower heating rates, higher hardness was obtained in WQed material. On the other hand, high hardness was obtained for FCed material regardless of the heating rate, suggesting that rapid heating of WQed material resulted in rapid decomposition of GP(I), which made the formation of GP(II) difficult and caused the precipitation only of the coarse η phase, resulting in a significant decrease in hardness. In FCed material, GP(I) was partially decomposed during rapid heating, but the clusters and GP zones formed during FC remained even at temperatures above 160°C, which is considered to have resulted in high hardness even after aging.