Effects of Quench Rate on Properties and Microstructures of a novel Al-Mg-Zn-Si Alloy with high Mg content

For next-generation high-performance aluminum alloys, the Al-Mg-Zn-Si alloy has excellent potential for development. In this study, the quenching sensitivity of Al-Mg-Zn-Si alloy with high Mg content was systematically investigated using the Jominy end-quench test, complemented by electrical conductivity and hardness measurements as well as microstructural observations. The results indicate that the cooling rate decreases as the end-quench distance increases, which leads to an increase in electrical conductivity of the alloy at the quenched state and a decrease in hardness at the aged state. When the cooling rate drops to 1.65 °C/s, the hardness reaches 90% of the maximum hardness with a depth of 80 ~ 85 mm. The scanning electron microscopy and transmission electron microscopy observations show that the quench-induced T phases nucleate preferentially at Al-Mn phases, AL 3 Zr phases, and grain boundaries. During artificial aging, the T phases coarsen and absorb surrounding solute atoms, resulting in the formation of precipitation-free zones and a reduction in the volume fraction of strengthening T’ phases. Furthermore, due to the decreased concentration of solute atoms and vacancies near the grain boundaries during slow cooling, the distribution of grain boundary precipitates becomes discontinuous with expanded PFZs.