Investigation on the quench sensitivity during isothermal treatment of a high Mg-containing Al–Mg–Zn–Si alloy

In this study, the quench sensitivity of a novel high Mg-containing Al–Mg–Zn–Si alloy was investigated using an isothermal treatment process. The electrical conductivity in the quenched state and the hardness in the aged state of the alloy were measured, complemented by Time-Temperature-Property (TTP), Avrami kinetics, Time-Temperature-Transformation (TTT), and Continuous Cooling Transformation (CCT) analyses. The results indicated that the nose temperature of this novel alloy was approximately 340 °C with an incubation period of 0.96 s. Isothermal treatment process conducted near the nose temperature revealed the fastest growth rate of the T-phase, which initiated from small sizes and exhibited subsequent decreased growth or nucleation rates. When the T-phase transformation reached 10 %, the nose temperature was 338 °C with an incubation period of 18.91 s. CCT calculations demonstrated that the alloy could fully retain its supersaturated solid solution state with a cooling rate exceeding 50 °C/s. Transmission electron microscopy (TEM) results showed that after isothermal holding at 330 °C, compared to samples without isothermal holding and those exposed to non-sensitive temperature ranges, precipitation and desorption phenomena were the most severe within grains and at grain boundaries, maintaining a broader precipitate-free zone (PFZ). To reduce the quench sensitivity of this alloy, it is recommended that quenching within the sensitive temperature ranges should be avoided during production, or alternatively, the cooling rate should be increased. Conversely, when the alloy was quenched within other temperature ranges, the cooling rate should be appropriately reduced to minimize residual stresses.