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Using Additive Manufacturing for the Shift in Car Drive Technology towards Electromobility
The shift in passenger car drive technology towards electric mobility is known to pose a number of challenges for the die casting industry. These include the need to produce components that include integrated cooling channels, such as housings for electric motors.
A frequently used approach in this respect is the two-shell design of such housings: This involves creating two die cast components that have to be pushed into each other, sealed, and joined together. A one-piece structure would be desirable to reduce the effort involved in mold production and assembly. But if the high pressure die casting (HPDC) manufacturing process has to be retained, an approach based on lost cores often fails due to a lack of suitable materials for such filigree cores, which remain stable under high pressure die casting conditions.
One alternative is to integrate tubes or hollow sections in general via a compound casting approach. However, if aluminum alloys are chosen for the hollow inserts due to their thermal properties, these have to be stabilized by means of a filling that has to be removed after casting [1,2]. The necessity for this kind of filling can be avoided, however, if the corresponding cooling channels are stabilized by internal structures.This is hardly feasible by means of conventional extruded profiles, especially if the cooling channels in question are curved, as is inevitably the case with an electric motor housing, for example.
This is where additive manufacturing comes into play: It not only provides the greatest possible freedom in the geometry of the media-carrying channel, up to and including massive cross-sectional changes, but also permits parallel optimization in terms of flow resistance, heat transfer, and structural strength.
The principal feasibility of this approach has already been demonstrated in stub tests [3]. In a recently started research project ("BioniCast - Bionically optimized, additively manufactured cooling channels for die casting of single-shell housings for the electrical drive train," AiF-IGF- Project No. 22357 N), Fraunhofer IFAM and Fraunhofer IAPT are jointly investigating this issue in more detail. A key aspect is the ability to design the casting and the structures to be cast in a process-safe manner: For a future series application, it must be possible to dimension the inserts in such a way that they can safely withstand the boundary conditions of the casting process. This must take into account that, depending on the configuration, the additively manufactured components can locally reach temperatures above 450°C while simultaneously being exposed to the intensification pressure of the HPDC process.
A basic prerequisite for the design in terms of structural stability is therefore detailed knowledge of the temperature-dependent mechanical properties of additively manufactured materials. To this end, Fraunhofer IFAM and Fraunhofer IAPT, together with BDG-Service GmbH, where the high-temperature tensile tests were carried out, have established an initial database for the alloy AlSi10Mg in the stress-relieved annealed condition. The results were recently summarized in an open access publication [4].
The publication is available at https://www.mdpi.com/1996-1944/15/20/7386. Further investigations into the behavior of the alloy in the T5 state, which ensures an increased tensile strength of about 100 MPa at room temperature, are currently under evaluation. As the project progresses, there are also plans to extend the studies to include special high-temperature aluminum alloys.
Literature:
[1] Rupp, S.; Heppes, F. Combicore - Giesskerne für den Druckguss. Giesserei-Erfahrungsaustausch 2013, 3/4, 6-9.
[2] Lehmhus, D.; Pille, C.; Borheck, D.; Bumbu, F.; Schwegler, T.; Lee, J.; Yoo, J.; Lutze, P.; Vomhof, R.; Weiß, K. Lösungen für die Elektromobilität: Leckagefreie Kühlkanäle für die nächste Generation von Druckguss-Gehäusekomponenten. Giesserei 2021, 108, 40-49.
[3] Lehmhus, D.; Pille, C.; Rahn, T.; Struss, A.; Gromzig, P.; Seibel, A.; Wischeropp, T.; Becker, H.; Diefenthal, F. Druckgießen und Additive Fertigung: Durch strategische Kombination das Beste aus zwei Welten nutzen. Giesserei 2021, 108, 36-43.
[4] Lehmhus, D.; Rahn, T.; Struss, A.; Gromzig, P.; Wischeropp, T.; Becker, H. High-Temperature Mechanical Properties of Stress-Relieved AlSi10Mg Produced via Laser Powder Bed Fusion Additive Manufacturing. Materials 2022, 15, 7386.
