Effect of thermal treatment on Sn-alloyed Al-Mg-Si alloysWednesday (06.11.2019) 15:15 - 15:35 Part of:
Al-Mg-Si alloys are widely used in the automotive industry as car body sheets due to an advantageous combination of strength and formability. As this unique feature is based on age hardenability, a significant strength increase upon bake hardening is inevitable. Both, micro-alloying as well as pre-aging have proven suitable to meet and further improve these demands. Sn-alloying leads to a mitigation of the undesired natural aging behavior of 6xxx alloys by vacancy trapping and pre-aging leads to the formation of nuclei for precipitates forming upon the paint bake process. Here, we aim to combine these two approaches and discuss the thermal treatments route for Sn-alloyed Al-Mg-Si alloys. Our results indicate that alloys modified with 100 at.-ppm Sn require an altered heat treatment. Concerning solution aging and quenching, it is shown that the required cooling rate depends on the type of alloys. It has to be adapted regarding the amount of intermetallic particles to guarantee a sufficient high level of Sn atoms in solid solution. The rather high amount of intermetallic phases in EN-AW-6061 requires fast quenching, while the comparable low amount of precipitate forming elements in an EN AW 6016 alloy is not as sensitive to quenching variations. It is shown that Sn also reduces pre-aging kinetics. Consequently, the optimal pre-aging temperature and time was found to increase when Sn is added to an EN-AW-6016 alloy. Moreover, we study the effect of an additional thermal spike added to a usual long-term pre-aging at different positions within the processing route. These results are discussed based on a simulation of the vacancy evolution in the alloy upon such treatments.