Date of Award

2022

Publication Type

Thesis

Degree Name

M.A.Sc.

Department

Mechanical, Automotive, and Materials Engineering

Keywords

Aluminum alloys, Bake-hardening, Experimental analysis, Materials, Painting, Pre-Strain

Supervisor

D.Green

Supervisor

J.Johendt

Rights

info:eu-repo/semantics/openAccess

Creative Commons License

Creative Commons Attribution 4.0 International License
This work is licensed under a Creative Commons Attribution 4.0 International License.

Abstract

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. A suitable testing phase is designed to be as much representative as possible of the actual manufacturing process of an automotive panel. 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 also correlated to the alloys’ chemical content. 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. Lastly, the high formability alloy, as expected, stood out as the alloy with the best ductility. Similar bake-hardening values were obtained by either just aging specimens at ambient temperature or baking at 120°C-130°C for 10 or 20 minutes. However, if the material is 2% pre-strained, the difference in terms of both final yield strength and dent resistance between high and low baking temperature is such that 120°C, 130°C and 150°C conditions are still able to fulfil eventual mechanical requirements. 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 build a predictive model for 6xxx alloys, based on the experimental data, that can be implemented into CAE software. Unfortunately, the results obtained turned out to be not satisfying, as the modeled bake-hardening values were constant regardless of baking temperature and time. However, the adopted approach can be further developed in future studies.

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