Ageing and strengthening of cold-rolled Al-Mg(-Cu)-Si-Mn alloys: experimental analysis and modelling

Abstract

Application prospects in the automotive industry have led to increasing studies on Al-Mg-Cu-Si alloys. In this thesis, nine Al-(1-3)Mg-(0-0.4)Cu-0.15Si-0.25Mn (in wt%) alloys with potential applications in packaging and automotive industries have been investigated. By means of mechanical testing, differential scanning calorimetry (DSC) and transmission electron microscopy (TEM), several mechanisms was identified that influence the final strength of cold rolled alloys during ageing: solid solution, work hardening, recovery and precipitation. Microstructure analyses revealed the formation of undissolved particles consuming the small Si addition, which influences age hardening behaviour of the alloys. Tensile testing was performed to evaluate the strength and work hardening. The integrated experimental results showed that for cold worked samples, b²(Mg2Si) contributes to age hardening of Cu-free alloys, whilst both b² and S (Al2CuMg) contribute to that of Cu-containing alloys. According to the experimental findings, a yield strength model has been developed to elucidate the relation between processing and the final strength. It consists of three main components: i) dissolution of intermetallic phase Mg₂Si; ii) precipitation of two strengthening phases b² and S; iii) strengthening contributions from solution strengthening, dislocation strengthening and precipitation hardening due to the strengthening phases. The model was calibrated and tested using separate tensile data and was applied to predict the yield strength evolution of cold worked samples during ageing. An accuracy of 8.6 MPa (about 4% of the total range of strengths) has been achieved. Based on the analysis of the relation of work hardening with cold work, composition and ageing time, three primary findings were obtained: i) cold worked samples usually have the lowest work hardening rate (WHR); ii) WHR increases after 30-minute ageing due to recovery and iii) WHR increases with decreasing level of cold work and increasing Mg and Cu contents. Work hardening models based on the Kocks-Mecking (KM) model and the Kocks-Mecking-Estrin (KME) model have been utilized to explain the main trends.The modelling results showed that the KM model is able to predict the work hardening behaviour of cold worked samples reasonably well. However, the KME model is insufficient to fully describe that of cold-worked-and-aged samples.

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