Pre-Ageing to Improve the Microstructure and Tensile Properties of Al-0.72Mg-0.42Si-0.1Cu Artificially Aged Alloy

Abstract

This paper describes a low temperature pre-ageing method that can improve the strength of artificially aged Al-Mg-Si-Cu alloys. The precipitation behaviour of the alloy samples was analyzed by use of differential scanning calorimetry (DSC) and the results were confirmed by microstructural observations using high resolution transmission electron microscopy (HRTEM). Tensile testing was performed to determine the mechanical properties of the samples. The results indicate that GP zones (I) formed in the alloy matrix with either natural ageing or pre-ageing at 50 °C and the formation of GP zones (I) retarded precipitation of the β'' phase during artificial ageing: it can be concluded that increasing the holding period would significantly decrease the strength after artificial ageing at 170 °C. Pre-ageing either at 70 or 90 °C, however, immediately after quenching of nuclei for precipitation of the β'' phase formed in the alloy matrix, showed that the precipitate particles formed in the pre-ageing condition grow in the subsequent artificial ageing process. It was concluded that in this case, increasing the pre-ageing time would increase the strength after artificial ageing at 170 °C.

This paper describes a low temperature pre-ageing method that can improve the strength of artificially aged Al-Mg-Si-Cu alloys. The precipitation behaviour of the alloy samples was analyzed by use of differential scanning calorimetry (DSC) and the results were confirmed by microstructural observations using high resolution transmission electron microscopy (HRTEM). Tensile testing was performed to determine the mechanical properties of the samples. The results indicate that GP zones (I) formed in the alloy matrix with either natural ageing or pre-ageing at 50 °C and the formation of GP zones (I) retarded precipitation of the β'' phase during artificial ageing: it can be concluded that increasing the holding period would significantly decrease the strength after artificial ageing at 170 °C. Pre-ageing either at 70 or 90 °C, however, immediately after quenching of nuclei for precipitation of the β'' phase formed in the alloy matrix, showed that the precipitate particles formed in the pre-ageing condition grow in the subsequent artificial ageing process. It was concluded that in this case, increasing the pre-ageing time would increase the strength after artificial ageing at 170 °C.

Cet article décrit une méthode de pré-vieillissement à basse température qui peut améliorer la résistance des alliages Al-Mg-Si-Cu vieillis artificiellement. On a analysé le comportement de précipitation des échantillons de l'alliage en utilisant la calorimétrie à balayage différentiel (DSC) et l'on a confirmé les résultats par des observations de la microstructure en utilisant la microscopie électronique en transmission à haute résolution (HRTEM). On a effectué des essais de résistance afin de déterminer les propriétés mécaniques des échantillons. Les résultats indiquent que des zones GP (I) se formaient dans la matrice de l'alliage, soit avec un vieillissement naturel ou par pré-vieillissement à 50 °C, et que la formation des zones GP (I) retardait la précipitation de la phase β'' lors du vieillissement artificiel. On peut conclure que l'augmentation de la période de rétention diminuerait significativement la résistance après le vieillissement artificiel à 170 °C. Le pré-vieillissement à 70 ou 90 °C, cependant, immédiatement après la trempe des noyaux pour la précipitation de la phase β'' formée dans la matrice de l'alliage, montrait que les particules de précipité formées en condition de pré-vieillissement croissent lors du procédé subséquent de vieillissement artificiel. On a conclu que, dans ce cas, l'augmentation du temps de pré-vieillissement augmenterait la résistance après le vieillissement artificiel à 170 °C.