Porsche is earning a reputation as an early adopter of 3D printing technology, with about 20 different parts for classic models that have been made available using this manufacturing technology and a custom-formed 3D-printed seat available for its 718 and 911 sports cars.
Now, Porsche is looking to exploit this technology right in the very heart of its most powerful model, the 911 GT2 RS. As reciprocating mass in the engine, pistons need to be as light as possible. This lets the engine rev faster and make more power.
However, there are gigantic loads on the pistons as they reverse direction twice every revolution of the crankshaft at these higher revs. So, they also have to be very strong. Typically, pistons are cast from aluminum. In high-stress applications, they might be forged.
Using 3D printing to optimize piston mass and strength, Porsche has been able to shave off 10 percent of the pistons’ weight in the GT2 RS engine, while increasing their rigidity. The company estimates that such pistons will permit the twin-turbocharged, 700-horsepower flat six to add another 30 horses to its stable.
Additionally, an oil cooling passage integrated into each of the printed pistons reduces the temperature of the pistons’ ring land by 20 degrees C. This kind of internal passage would not be possible using conventional casting or forging techniques, according to Porsche.
Porsche partnered with industry specialists on this project, turning to piston supplier Mahle, additive manufacturing specialist Trumpf GmbH and to precision test company Carl Zeiss AG for their help.
“Additive manufacturing is very important to Mahle,” noted Volker Schall, Mahle’s head of product design. “It allows us to supply components to our customers more quickly and on a flexible and agile basis, which means we can reduce development time.”
Porsche is a high-profile customer, and the 911 GT2 RS is the top of the company’s flagship model line. “This project involved multiple challenges,” he said. “From the design of the piston through the specification of the material and the development of the appropriate printing parameters, we had to make many fine adjustments to achieve the optimum result.”
Indeed, the array of challenges look daunting, even in retrospect. “This project involved multiple challenges,” noted Schall. “From the design of the piston through the specification of the material and the development of the appropriate printing parameters, we had to make many fine adjustments to achieve the optimum result,” he said. “We have now not only mastered the technical side of things, but can also assess how the method can be embedded into existing manufacturing processes.”
“We have simulated the piston virtually and entered the loads,” said Frank Ickinger, from the advanced engineering powertrain department at Porsche AG. “The topological optimization finds the load paths and tells me where materials should be added or reduced in order for the part to ultimately remain intact. This generates a bionic structure that could not be manufactured using conventional casting or forging methods.”
Trumpf printed these optimized “bionic” parts using a proprietary aluminum powder in a 12-hour process requiring 1,200 layers of powder to be burned into solid parts using a laser metal fusion printer.
This process requires finesse to achieve the intended result. “A series of tests have been conducted in order to calibrate various parameters, such as scan speed and laser power,” explained Bastian Leutenecker-Twilsiek, head of AM-Consulting at Trumpf. “This is a repetitive process, ensuring the stability and the load capabilities of the pistons.”
Once the rough piston blanks came out of the printer, they have to be tested for quality and then they undergo all of the usual preparation of a conventionally produced piston.
“The part is analyzed for hidden defects such as pores and fractures using computer tomography,” explained Bernhard Wiedemann, director additive manufacturing process and control at Carl Zeiss AG. “This helps ensure that the final product is of the required quality.”
The pistons are also tested in a pulsation test and a tear-off test. “The pulsation test is used to test the thickness of the skirt to ensure no cracks develop,” said Ulrich Kunzmann, Mahle’s head of prototype manufacturing. “The tear-off test makes sure the [wrist] pin bore will not shear.”
Porsche tested the finished products in its own way, running the engine through a punishing 200-hour test that included 135 hours at full load. The additively manufactured pistons passed this test, potentially clearing the way to their eventual use in production models.
“Thanks to the close cooperation of everyone involved, we were able to demonstrate the potential of additive manufacturing in our top-of-the-line high-performance sports car, the Porsche 911 GT2 RS, thus clearing the way for its use in future drives,” Ickinger observed. “In terms of technology, this is the start of a new chapter for us, which opens up completely new possibilities in design and production.”
That’s the hope at Trumpf too. “The project illustrates how 3D printing can be used to further improve components whose performance potential has already been exhausted by decades of development,” said Steffen Rübling, project manager at Trumpf. “This will benefit many other industries, such as aerospace and energy.”