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Abstract

This study is part of a research project whose goal is to develop a process model for the production of aluminum alloy automotive bumpers. The problem originates from the industrial bumper manufacturing practice which uses aluminum alloys, AA7030.60 and AA7108.72 in particular. In the industrial production schedule, a forming (pre-deformation) process occurs after the solution heat treatment and water quenching, followed by a T6 ageing process. Due to the specific shape of the bumper product, the forming process introduces a heterogeneous distribution of deformation in the bumper. A systematic investigation of pre-deformation on the ageing behaviour of these alloys has been carried out. A series of pre-deformations (pre-strain from 0 to 1.2) was imposed on the materials, whose subsequent ageing responses were recorded in terms of the mechanical properties (yield stress, tensile strength, etc.) and electrical resistivity. Some transmission electron microscopy measurements were also conducted by other members of the research group. With a combination of these results the evolution of the material's microstructure and mechanical properties can be traced during the ageing process. Some of the significant results from the present study include the observation that the natural ageing response is reduced and the artificial ageing kinetics are accelerated for the pre-deformed alloys. Lower peak ageing strength is also observed for the pre-deformed alloys. These changes in the material behaviour are related to the changes in the microstructure when pre-deformation is present in the materials. At a microscopic level, dislocations are observed to arrange themselves into a loose cell structure. Pre-deformed alloys were observed to have larger average precipitate sizes. In addition, they also have different precipitate size distribution. For pre-deformed samples two peaks can be recognized in the precipitate size distribution curves, corresponding to the precipitates in the bulk and the precipitates at dislocation sites. Based on this study, it has been observed that deformation prior to artificial ageing leads to inferior final properties. A modified bumper manufacturing sequence is proposed in an attempt to eliminate this effect. However, further studies are needed to test the feasibility of the proposal.