Abstract
Since the 1920ies the age-hardening behaviour of Al-Mg-Si alloys has been studied and numerous papers have been presented on the crystallographic structure of the hardening precipitates in this alloy system. Especially over the last 10-15 years the results from extensive studies by many groups and the utilisation of new experimental techniques has established a fundamental understanding of the structure of many of the precipitates that are formed during artificial ageing of Al-Mg-Si alloys. However, the mechanical response and hardening effect of the various precipitates are less studied. In commercial alloys the precipitate structure is often a mixture of different precipitates. In order to optimise the hardening structure for different utilisations of these alloys the knowledge of the mechanical effect of each type of precipitate may be a significant contribution to further optimisation of these alloys. In this work five alloys are selected with a chemical composition optimised for the formation of β’’ and each of the post-β’’ precipitates, i.e., U1, U2, B’ and β’. The total amount of solute is the same for all alloys. The alloys are cast, homogenised and extruded under identical condition and will be subject to solid solution heat treatment and artificial heat treatment specially designed for precipitation of each category of precipitates. The heat treatment procedure is selected with focus on a resulting precipitate structure with similar number density and size distribution. The characterisation of the material will involve microstructural studies using several methods (e.g. optical microscopy, SEM and TEM) to evaluate the hardening state of each alloy. The mechanical properties will be characterized by various methods (e.g. Vickers hardness and tensile testing). In the present work we will report on the hardening effect of the post-β’’ precipitates, which will form a basis for further optimisation of the Al-Mg-Si alloys for specific purposes.
