“We’ve been working with SLM for several months printing on the SLM 280, to identify heat-exchanger applications,” Coyne explains. “We’ve also built these parts on our binder-jetting machine, but believe that the SLM technology makes more sense when it comes to making these parts in production, due to part complexity.”
Why the focus on heat exchangers? That traces back to 2012-2013 when GE first launched its Evolution Series of locomotives, designed to meet U.S. EPA emission regulations that eventually became standard and went into effect January 1, 2015. Compared to Tier Three locomotive engines, dating back to 2005, these newer, cleaner engines yielded a 70-percent drop in nitrous oxide and particulate matter emissions—good for the environment but challenging to suppliers like GE Transportation and Wabtec.
“At the time we became pioneers in developing EGR (exhaust-gas recirculation) engines of this size,” Coyne recalls, “to attain the EPA-required levels. Most significant were challenges presented by recirculating extremely hot exhaust gas through the engine.”
AM’s Role in Developing EGR Locomotive Engines
The solution for removing much of this heat: adding a cooler, or heat exchanger, inline with the gas stream, a meter-long welded assembly of fins and tubes that, in service, lacked sufficient durability to satisfy customers. Enter metal AM.
“We see AM as the perfect solution to the reliability challenge,” says Wabtec lead mechanical engineer Brett Heher, “allowing us to take the hundreds of parts in a heat exchanger of this size and combine them into one AM part, thus eliminating all of those potential failure points.”
What Wabtec’s AM team now is producing—in prototype development at its lab in Grove City, PA—is a 3D-printed precooler that it will install inline and just ahead of the cooler. The precooler will remove 30 percent of the total heat from the recirculating exhaust gas before it enters the main cooler. Wabtec expects to begin field testing the new solution later in 2021, prior to retrofitting its more than 1000 Tier Four locomotives currently in service, and then make the AM precooler standard on all new locomotive engines.
“Key to the design of the precooler,” Heher explains, “are unique features, enabled by AM, that allow it to handle the soot and contaminants in the exhaust gas. We developed a trifurcating geometry for the core, printed from Inconel 718, with larger passages to allow the contaminants to pass through. And we’re limited in available space on the locomotives for the retrofits; the trifurcating geometry allows us to design the shape of the precooler to fit neatly into the space envelope.”
Heher also stresses that his team has gone through a lot of optimization of the build parameters to make the material as thin and structurally rigid as possible, minimizing the need for support structures. He’s using Siemens NX software for 3D modeling, and Ansys software for build simulation and to analyze part performance.
Also on Track: Printing Aluminum Braking Systems
Moving forward, the SLM 280 will remain dedicated to printing Inconel and stainless-steel parts, explain Coyne and Heher, including the heat-exchanger parts. The plan is to focus on aluminum printing with the SLM 800, turning their focus to the transit side of the business where weight savings is emphasized. The Wabtec team has identified several parts, for serial production, where it expects to see significant benefits printing from aluminum at the Neighborhood 91 site. Among them: pneumatic-brake panels, fraught with complex parts that lend themselves to AM.
“Lightweighting is important on the transit side, and we’ve been actively redesigning the brake panels to optimize weight and space while also improving air flow through the system,” says Heher. Printing parts from AlSi10Mg has resulted in a 75-percent weight reduction compared to the conventional fabrication process—stacking and milling aluminum plates.
“We’re making the stack in one AM part on the SLM, complete with internal channels optimized for the mounted pneumatic components,” Heher explains. “And, the new design allows us mount the panel inside the vehicle oriented so that we save 200 mm of space.”
“It’s important to note that these are safety-critical parts,” adds Coyne, “and so there’s a lot of scrutiny by customers. While the part and process designs are mature and validated, we’ve just started to make the parts available to customers for testing. The initial reaction has been good; of course, they really appreciate the weight and space savings.”
Coyne and Heher are quick to praise the build consistency from the SLM machines, of particular importance when working on safety-critical applications. As explained to 3D Metal Printing by SLM engineer Shawn Kelly, director of application engineering and technology, a critical variable here is maintaining seamless overlap between the laser overlap zones, ensuring that the material microstructure and properties remain homogeneous throughout.
“Also critical,” shares Kelly, “is consistent gas flow across the build platform from the top to the bottom of the chamber, helping to ensure a stable and repeatable process with little to no variability.” 3DMP
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