Quench sensitivity of Al–Mg–Si alloys: A model for linear cooling and strengthening

• New accurate model for continuous cooling quench sensitivity of 6xxx Al alloys.
• The model considers two consecutive quench induced phases: Mg2Si and B′.
• Model contains composition dependency and is successfully tested on five alloys.
• Model is verified by extensive DSC, SEM, TEM and XRD experiments.
• Model predicts enthalpy changes, volume fractions, T6-strength and -hardness.

This work studies the quench-induced precipitation during continuous cooling of five Al–Mg–Si alloys over a wide range of cooling rates of 0.05–2 × 104 K/min using Differential Scanning Calorimetry (DSC), X-ray diffraction, optical- (OM), transmission electron- (TEM) and scanning electron microscopy (SEM) plus hardness testing. The DSC data shows that the cooling reactions are dominated by a high temperature reaction (typically 500 °C down to 380 °C) and a lower temperature reaction (380 °C down to 250 °C), and the microstructural analysis shows they are β-Mg2Si phase formation and B′ phase precipitation, respectively. A new, physically-based model is designed to model the precipitation during the quenching as well as the strength after cooling and after subsequent age hardening. After fitting of parameters, the highly efficient model allows to predict accurately the measured quench sensitivity, the volume fractions of quench induced precipitates, enthalpy changes in the quenched sample and hardness values. Thereby the model can be used to optimise alloy and/or process design by exploiting the full age hardening potential of the alloys choosing the appropriate alloy composition and/or cooling process. Moreover, the model can be implemented in FEM tools to predict the mechanical properties of complex parts after cooling.

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