Influence of short-term heat treatment on the microstructure and mechanical properties of EN AW-6060 T4 extrusion profiles—Part B

In the automotive industry aluminium and its corresponding semi-finished products contribute an essential part to the aim of weight reduction in car body structures. Aluminium alloys of the 6000 series with Mg and Si contents are preferred because of the possibility to increase strength by ageing processes. However, the cold formability in comparison to other materials like mild steels is quite low and due to this, complex parts are only producible at higher temperatures. Therefore, the so called Tailor Heat Treatment was developed to improve the cold formability of aluminium alloys. In this approach, a short-term heat treatment is conducted to achieve a local softening of the material due to dissolution of Mg and Si clusters (retrogression). This effect is used to improve the material flow, relief critical forming zones and enhance the overall formability of the material. Afterwards, strength can be increased again by ageing processes. However, up till now a holistic process understanding, taking into account all process parameters as well as a microstructural explanation is missing. Therefore, the focus of the fundamental investigations lies on connections between the mechanical properties and short-term heat treatment with industry-relevant heating rates as well as natural and artificial ageing process. Conclusively, the evolution of the mechanical properties with regard to the natural ageing process is compared with findings of DSC analysis, which were discussed in Part A. Based on these results, a process window is derived for the subsequent forming process and the final mechanical properties of the final part in dependency of the forming history as well as the artificial ageing process, are identified.