A02 - Graded, mechanically joined aluminium castings

The realisation of multi-material design in modern automotive construction requires the combination of different materials in body construction. However, this involves the joining of sheet metal as well as extruded profiles with castings. Aluminium is often used as casted material due to its low density plus its excellent mechanical properties. Furthermore, aluminium castings allow a high level of design flexibility combined with a geometry that is suitable for the adapted load as well as cost-effectiveness. Castings are usually produced using the die casting/chill casting process or sand casting. To achieve the desired weight reduction on the one hand and to be able to guarantee the high strength for safety-relevant components, on the other hand, the cast components consist of 85 % – 90 % of a hardenable aluminium alloy of the aluminium-silicon system. The AlSiMg system is especially suitable for a lightweight application in the vehicle body. Since the weldability of such alloys is limited, mechanical joining is an adequate option. For this application, the challenge is to produce a crack-free joint, since the less ductile cast aluminium alloys lead to cracks during the joining process. This issue is being investigated in sub-project A02.

This sub-project aims to produce graded, high-strength and joinable aluminium castings, thereby enabling a crack-free joint in a versatile process chain. Grading the joint locally increases the ductility of the casting, whereby cracking at the joint of cast aluminium components can be reduced or even avoided. As joining methods, clinching and self-piercing riveting are employed, among others. The cast aluminium components are manufactured using the three casting processes “sand casting” and “permanent-mould castings” as well as “twin-roll casting”. On the one hand, the local grading of the castings can be realised via the solidification conditions and, on the other hand, via heat treatment.

The different casting processes exhibit a significant influence on the solidification of the castings for the solidification rate varies between 1 K/s and up to 200 K/s. High solidification rates lead to a finer microstructure, which enhances the ductility and reduces the crack susceptibility. Such a solidification gradation is realised using a temperature-controlled permanent-mould casting tool. Moreover, it is possible to achieve additional grading of the material through local heat treatment.