Effect of composition and processing on the microstructure and formability of aluminium automotive body sheet alloys

2017-02-23T04:36:08Z (GMT) by Zhong, Hao
Stretch forming is a common deformation mode during the stamping of 6xxx series Al-Mg-Si(-Cu) automotive body panels. Good formability in this mode requires high work hardening and strain rate hardening capabilities, which are controlled by the alloy composition and processing (particularly heat treatments) through their influence on the microstructure. The influence of composition and heat treatment on the formability of 6xxx alloys has been previously investigated, but systematic research is still needed in order to provide a deeper understanding of the underlying mechanisms for the purpose of maximising the formability of 6xxx alloys, whenever possible, by composition and heat treatment modifications. Therefore, in this PhD project, the effects of alloy composition (Si, Mg and Cu contents) and heat treatment (natural ageing and/or pre-ageing at 100 ºC for 2 h or at 200 ºC for 20 s) on the formability of eight 6xxx alloys were systematically studied. The microstructure of these alloys was investigated by scanning electron microscopy (SEM) and transmission electron microscopy (TEM). The macro-texture was characterised by X-ray diffraction. A surface profilometer was used to characterise the strain localisation. Tensile testing was used to determine the uniaxial tensile properties, and to study the work hardening and strain rate hardening behaviour of these alloys, which were used to correlate the microstructural features with the formability results. The use of Kocks-Mecking-Estrin model enabled a separation of the contributions of different microstructural features to the work hardening capability of the alloys studied. Furthermore, based on the knowledge from the experimental and constitutive modelling results, thermodynamic modelling was employed to predict the microstructure that could improve the formability of a high Mg content alloy. In this work it was found that solutes in solution are the most important factor that influences the stretch formability. It increases the work hardening capability by increasing the dislocation storage rate and impeding the dynamic recovery. For instance, Cu atoms were found to have a significant influence on the dislocation storage rate while having little influence on the dynamic recovery rate; conversely, Mg atoms have a substantial influence on the dynamic recovery rate while having little effect on the dislocation storage rate; and, finally, Si atoms have a slight influence on both the dynamic recovery rate and the dislocation storage rate. It should be noted that increasing Si content also leads to a high number density of dispersoids, although the contribution of dispersoids to work hardening is much smaller than that of solute atoms in solution. In terms of strain rate hardening behaviour, solute atoms in solution (particularly Mg atoms) reduce the strain rate sensitivity (SRS). However, if the addition of solutes promotes the natural ageing kinetics, then a high SRS is observed after extended natural ageing. This is the case in the high Si content alloys. By contrast, high Mg content alloys normally show a sluggish natural ageing kinetics, thus a larger negative contribution to SRS associated with dynamic strain ageing (DSA) can be observed. It was also found that Cu additions can retard the natural ageing kinetics regardless of the magnitude of the Mg/Si ratio in the alloys. The degree of retardation does depend on the Mg/Si ratio, though. This is particularly the case in the alloys with a high Mg/Si ratio where serrated yielding can still be observed in the alloy after one week of natural ageing. This is at variance with other alloys where serrated yielding disappeared within one day of natural ageing after quenching, which can be associated with the depletion of solutes and quenched-in vacancies. Vacancies were also found to influence the formability. This is particularly important in pre-aged samples. After one week of natural ageing, pre-aged samples showed a lower SRS than those without pre-ageing, which is probably due to a higher vacancy concentration in the pre-aged samples. The excess vacancies may exist as solute-vacancy complexes or be bound with solute clusters depending on pre-ageing time. A model was proposed to account for the decreased SRS in the pre-aged samples. Stretch formability is a property that is crucial in the selection of Al alloys for targeted applications. It was investigated in the course of the project in great detail. The overall effect of the alloy composition on the formability is that increasing Si content or decreasing Mg/Si ratio can improve the stretch formability due to the increased work hardening and strain rate hardening capabilities. The addition of Cu can significantly enhance the stretch formability of alloys with Mg/Si > 1, which is because the magnitude of the work hardening is more important than that of the strain rate hardening. An important property that influences acceptance of an alloy of the 6xxx series by industry is its bake-hardening response. This aspect of alloy development was also studied in the thesis. It was shown that although pre-ageing can improve the paint-bake response of the 6xxx alloys, pre-ageing was found to reduce their stretch formability due to the decreased work hardening and strain rate hardening capabilities. This effect is especially pronounced in the case when the samples are pre-aged at 100 ºC for 2 h, as pre-ageing reduces the strain rate hardening appreciably. Therefore, this work suggests that any attempts at improving the stretch formability of 6xxx alloys must also consider their effects on the paint-bake response. Finally, thermodynamic modelling results show that the stretch formability of the excess Mg alloy A2, which has a relatively poor stretch formability, can be improved by increasing the Si and Mn contents and/or decreasing the Mg content. Thus, a tangible outcome of the PhD project is a detailed set of reliable experimental results on the effect of the alloy composition and heat treatment of 6xxx alloys studied on their strength and ductility characteristics, stretch formability, and bake-hardening response. Moreover, practically, this PhD project not only explains why most of the commercial 6xxx alloys for automotive outer panels are Si-rich alloys, but also provides guidance for the development of Mg-rich 6xxx alloys for automotive panel applications.