Work-hardening behaviour in age-hardenable Al-Zn- Mg(-Cu) alloys

Abstract

This work is mainly an experimental study of work-hardening in age-hardenable Al-Zn-Mg(-Cu) alloys. The main objective has been to characterize the microstructure and mechanical properties and evaluate the data with respect to already existing work-hardening models.

The thesis is organized into a synopsis, providing information about the background for the study, the objectives, the experimental techniques and a summary of the work. The synopsis is followed by four papers and a report.

The first two papers consider a commercial AA7108 aluminium alloy in the as-cast and homogenized state, heat treated to different tempers and deformed in uniaxial tension and compression, respectively. An extended Voce equation has been useful for analyzing the work-hardening behaviour in tension and in compression up to larger strains. In both papers two tempers have been selected for further investigations of the precipitate size and distribution, and the influence of the microstructure on the work-hardening behaviour has been discussed.

The third paper is not directly considering work-hardening behaviour. Instead the effect of ageing, anisotropy and alignment of particles on the performance of plate material in three-point bending is considered. This is a comprehensive experimental study where an AA7108 aluminium alloy has been tested in two different tempers and with three different microstructures. The microstructures include as-cast and homogenized material, extruded and fibrous material, and rolled and recrystallized material. The influence of crystallographic texture, alignment of constituent particles and grain morphology on the fracture behaviour has been discussed.

The fourth paper is dealing with work-hardening behaviour and the transition from shearable to non-shearable precipitates in four different alloys. The composition of these alloys varies more or less systematically and, hence, the influence of the different alloying elements may be discussed.

The last part in this work is presented as a report. Here a first step has been taken to further develop an already existing work-hardening model to include the 7xxx (Al- Zn-Mg) aluminium series. The model parameters are based on experimental investigations of the mechanical properties and the microstructure of a commercial AA7108 alloy in the as-cast and homogenized state. The work-hardening model is first described in details before the experimental procedures, results and data processing are presented. Finally the model is calibrated and implemented based on the experimental data.