Differential scanning calorimetry of age-hardenable aluminium alloys: effects of sample preparation, experimental conditions, and baseline correction

Precipitation processes in age hardenable aluminium alloys are often investigated by differential scanning calorimetry (DSC). The endothermic and exothermic peaks of the DSC signal correspond to the dissolution and formation of phases, respectively. However, parasitic effects can lead to an unintended curvature of the DSC signal. Although a baseline correction can be used, some imperfections typically remain. Additionally, sample preparation and experimental conditions can influence the precipitation sequence itself and, therefore, the DSC curve. In this study, we investigate the influence of sample preparation by milling and punching on DSC curves for three different aluminium alloys: EN AW-2024, EN AW-6082, and EN AW-7075. Additionally, the influence of quenching and natural ageing was investigated for EN AW-6082. We found that deformation introduced by punching the DSC samples with a piercing die after heat treatment leads to a change in the precipitation kinetics in 2xxx, 6xxx, and 7xxx series alloys. The influence was strongest for punching the samples after solution heat treatment and less significant for punching after artificial ageing. The influence of sample preparation can be avoided by punching the samples before solution heat treatment. If this is not practicable, milling of the samples is a good alternative. The choice of quenching medium and short storage at room temperature before measurement (5 min) had only small effects on the DSC curves. Moreover, the start temperature of the measurement was found to be crucial. For observing phases forming below 100 degrees C and for low-bias baseline correction, the measurement should start at low-temperatures (i.e. similar to \documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$$\sim$$\end{document} - 50 degrees C).