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

Abstract

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•10 4 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 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.

Influence of Solution-Annealing Parameters on the Continuous Cooling Precipitation of Aluminum Alloy 6082

“…As seen from Figure 9C,E, as well as Figure 10C,E, Mg 2 Si is already present when the HTRs are nearly finished, as detected by DSC. This is in perfect agreement with previous studies that report the HTRs in AlMgSi alloys were predominantly caused by the precipitation of β-Mg 2 Si-phase particles [5]. These Mg 2 Si particles are found almost unchanged if there is continuous cooling down to room temperature.…”

“…EDX showed an enrichment of Mg and Si for these particles. Based on both our experience and the published literature, these are β-Mg 2 Si-phase particles [4,5]. Nucleation of Mg 2 Si is reported to occur on primary Fe, Mn, and Si rich intermetallic particles.…”

“…Both the heating and cooling curves are shown here. In terms of the specific heat capacity, the endothermic area is shown upwards in the heating experiments [15,19], whereas the exothermic area is shown upwards in the cooling experiments [5]. Any figures presented in the results and discussion section below that consist of DSC curves from different scanning rates are arranged and shifted in order of the scanning rate used, starting with the lowest rate on top.…”

“…The solution-annealing temperature was chosen as 540 • C in several previous investigations of the quench sensitivity of AlMgSi alloys [4,5,12]. However, the DSC heating experiments on the particular alloy batch and the initial T651 condition used here (shown in Figure 2), even for slow heating at 0.01 K/s, indicate that dissolution of the secondary precipitates of interest is just completed at about 560 • C. Furthermore, Figure 2 shows that the temperature at which the dissolution of the secondary phases is completed strongly depends on the heating rate.…”

“…: The three applied solution temperatures are indicated in the quasi-binary phase diagram Al-Mg 2 Si and shown in Figure 6, as well as the estimated maximum amount of Mg 2 Si for the particular batch of EN AW-6082 that was used in the experiments. While the comparison of EN AW-6082 with this phase diagram is a simplification, the precipitation of the β-Mg 2 Si phase from the α-Al solid solution is dominating the HTRs during cooling from the solution treatment of AlMgSi alloys [5], including two batches of EN AW-6082. It is thus inferred that the most likely phase to be considered in terms of complete solution is β-Mg 2 Si.…”