Deep Drawability Optimization And The Related Mechanism Of Advanced Al-Mg-Si-Cu Alloys For Automotive Application

With the progress in light weight in automobile, Al-Mg-Si (6xxx series) alloys have played an important role in car body outer panels. However, there is a need for further improvement in properties, such as, deep drawability, bake-hardening response and bendability. Deep drawability is mainly dependent on microstructure and crystallographic texture, which are strongly influenced by composition and thermomechanical processing. Accordingly, the evolution of microstructure and texture during thermomechanical processing for commercial AA6111 alloy was investigated, and the thermomechanical processing was also optimized. Based on the effect of thermomechanical processing on microstructure and properties of AA6111 alloy, new chemistry of 6xxx series alloys with variation in Cu, Zn and Fe were designed. Moreover, the evolution of their microstructure, texture and mechanical properties during thermomechanical processing, and the high formability mechanism were studied systematically. The following conclusions were drawn:The microstructure and texture of commercial AA6111 alloy during thermomechanical processing using current laboratory facilities are non-uniform. The microstructure, texture and mechanical properties change significantly with the change of rolling process (especially reduction per pass), intermediate annealing and heating rate of solution treatment. The rolling geometry in the range of 1～5, the intermediate annealing at 400℃ for 1h and rapid solution heating rate are all beneficial to control the microstructure and texture, and finally improve the deep drawability.The microstructure, texture and mechanical properties of the new 6xxx series alloys are strongly influenced by the quantities of Fe-rich phase and Zn addition. With increasing Fe-rich phase particles, the solution treated or pre-aged alloys lead to finer recrystallization grains, weaker texture and higher r value resulted from the effect of coarse particles and fine particles. In addition, the effect of solution time on recrystallization microstructure, texture and mechanical properties are relatively weaker. Zn addition does not significant affect the recrystallization grain size and texture. Based on the relationship between composition, thermomechanical processing, microstructure, texture and mechanical properties, a new alloy possessing high strength and excellent deep drawability was developed:Al-0.8Mg-1.2Si-0.5Cu-0.3Mn-0.5Fe-0.5Zn(wt%).The microstructure and texture of the new 6xxx series alloys possessing a low Fe-rich phase can also be effectively controlled by adjusting the cold rolling reduction after intermediate annealing, which attributes to the optimized cold rolling texture and distribution of particles with different sizes formed during intermediate annealing. The optimized thermomechanical processing, including hot rolling, first cold rolling with 50% reduction, intermediate annealing at 400℃ for 1h, final cold rolling with 50% reduction, solution treatment and pre-aging treatment, is beneficial to the deep drawability of the alloys, and can also improve the average plasticity strain ratio r values above 0.6.For both the commercial AA6111 alloy and new 6xxx series alloys, in order to improve the deep drawabilities, it is very important and necessary to control the particles with different sizes distributed in the alloy matrix due to their active effects on the evolution of microstructure and texture during thermomechanical processing. According to the predicted r value by VPSC model and measured r value, it can be found that the deep drawability of the alloys mainly dependent on the texture.