Understanding quench sensitivity is essential for controlling the precipitation behavior and age-hardening response of Al-Zn-Mg alloys. In this study, we investigate the formation of quench-induced precipitates and their impact on subsequent hardening by applying differential scanning calorimetry (DSC) across a wide range of cooling rates. Two characteristic exothermic reactions were identified: a high-temperature precipitation event and a distinct low-temperature reaction beginning near 110 °C. The latter corresponds to the formation of GP(I) zones and solute clusters that develop during slow cooling.

Slow linear cooling was found to significantly enhance the low-temperature reaction, indicating the preferential formation of large GP(I) zones. However, nonlinear cooling—accelerating the cooling rate in the high-temperature region while slowing it in the low-temperature region—proved more effective in suppressing high-temperature precipitation of coarse η′/η phases while promoting GP(I) formation. DSC measurements confirmed that nonlinear cooling increases the low-temperature exothermic peak without enhancing undesired high-temperature precipitation.

STEM observations revealed abundant GP(I) zones after slow and nonlinear cooling, along with only small amounts of short η′ precipitates. These nanoscale structures directly reflect the thermal signatures observed in DSC curves and demonstrate that appropriately controlled cooling paths can tailor the early-stage precipitation process.

In addition, the influence of trace alloying elements such as Zr and Cr will be discussed, particularly regarding their role in stabilizing the microstructure and modifying precipitation behavior during slow and nonlinear cooling. These findings provide a deeper understanding of the mechanisms governing quench sensitivity and support the development of robust heat-treatment strategies for high-strength aluminium alloys.