The use of additive manufacturing for highly stressed engine components has been an area of considerable research in recent years. The flexibility of the technology facilitates design features that would be impossible to achieve using traditional manufacturing techniques. However, ensuring the material properties of a finished part are sufficiently robust has proved to be a stumbling block, not least in ensuring consistency from batch to batch, which is highly dependent on a plethora of parameters during the manufacturing process.
Many non-functional pistons have been displayed at trade shows over the years, and both Mercedes and Ferrari’s Formula 1 teams have suggested they are using the technology in race engines. Now they are finally a public reality, with Mahle announcing the production of functional, additive manufactured aluminum pistons, which have been bench tested in a turbocharged, Porsche 911 GT2 RS engine.
Working in conjunction with both Porsche and additive manufacturing specialist Trumpf, Mahle produced the prototypes using a laser metal fusion process. This technique enabled complex features – the result of extensive topological optimization on Mahle’s part – to be incorporated into the design. This, says Mahle, gives the potential for a 20% weight reduction compared with a traditional forged and machined component. It also means that features such as a geometrically complex cooling gallery could be incorporated into the piston near the rings, reducing the thermal load near the top land.
The alloy used for the pistons was based on a casting material developed by Mahle, which was then processed into a fine powder suitable for laser melting. Each piston required 1,200 layers of material to be melted, which took around 12 hours to produce (though multiple parts can be made on a machine’s powder bed in a single run).
Key to ensuring the successful running of the pistons was an extensive quality control process, which involved both FEA analysis of the piston design (with particular attention paid to the piston skirt and pin bore) as well as inspection of the final parts using CT scanning and microscopy. Here, proper characterization of the deposited material is vital to ensure that results from FEA correlate with the real-world performance. The final pistons were bench tested for a total of 200 hours, covering around 6,000km (3,700 miles) of running, at an average representative vehicle speed of 250km/h (155mph).
Pistons are not the only area where Mahle has looked to harness additive manufacturing. Again, working with Porsche and Trumpf, it developed an in-line charge air cooler, which sits between the 911’s original charge cooler and the turbo. The design freedom provided by the manufacturing process enabled careful sculpting of the cooler passageways to maximize heat transfer.
According to the company, following this proof of concept project, it will look to further harness additive manufacturing for short run and prototype work, as well as investigating its potential for use on other engine components.