Bake-hardening of Aluminum Alloys for Automotive Applications Influence of Temperature, Time and Pre-Strain Parameters

An experimental evaluation of the effect of bake hardening on aluminum alloys for automotive lightweight applications is developed through tensile tests and dent tests. All the alloys belong to the 6xxx series and include high formability alloys, improved hemming types and improved bake response alloys. An experimental matrix which contains different time-temperature baking conditions is built, and two pre-strain conditions are considered: no pre-strain and 2% pre-strain. After a data analysis phase performed by means of MATLAB software, the results of the different alloys are compared in terms of mechanical resistance and bake response to identify the materials that are potentially more suitable for certain applications. Furthermore, the outcomes are discussed in correlation to the alloys’ chemical content as well. The improved hemming alloy from North America turned out to be the best in terms of mechanical resistance (both tensile strength and resistance to indentation), while the improved bake response alloy from North America gave the best strength gain after the baking phase. Last, the high formability alloy, as expected, stood out as the alloy with the best ductility. Baking at lower temperatures (120°C and 130°C) was not particularly effective for durations of 10 and 20 minutes, as similar final tensile strength was obtained by just aging specimens at ambient temperature. Furthermore, an increase of baking time by 10 minutes caused the final strength to be higher than what was obtained by raising the baking time by 10°C. A numerical modeling approach was also attempted by means of experimental data fitting by adapting a model for strain aging originally developed for steels. This was done to show what should be the step that immediately follows the data gathering phase, which is building a predictive model for the 6xxx alloys based on the aforementioned data that can be implemented into CAE softwares. Unfortunately, the obtained results turned out to be not satisfying, as the modeled bake-hardening values were constant when varying baking temperature and time. However, the adopted approach can be further developed in future studies.