| Additive manufacturing


Left: First trial group of additively manufacturerd AORB impellers: a) solid CAD model, b) in Velo3D printer during manufacture, c) complete build plate removed from AM system


Above: A 3D-printing trial build, with a number of AORB impeller designs


the superalloy. “Our tests demonstrated that LPBF 3D-printed Inconel 718 had mechanical properties, like yield stress and creep tolerance, that were higher than those of cast material,” Lea says. “This was more than adequate for high-stress centrifugal blower and compressor applications within the operational temperature range.”


Iteration made easy


As their impeller work progressed, Mohawk’s engineers collaborated with Velo3D experts on design iterations, modifications and printing strategies. “It was really interesting because we didn’t have to make any major design changes to the original impeller we were working with — with Velo3D’s Sapphire system we could just print what we wanted,” says Cordova. “We did do some process adjustments and tweaking in


Unit cost comparisons (US $) for Inconel 718 impeller, 700°C inlet gas AORB: conventional manufacturing v 3D printing


Quantity Method Number of blades Unit cost


1 or 2 1 or 2 5 to 10 2 12 5 axis


machining 5 axis


machining


Investment casting


LPBF 3D printing


LPBF 3D printing


19 15 15 or 19 $18 950 $15 845


$3800 to $1900


15 or 19 $1005 15 or 19 $614


terms of support-structure considerations and surface-finish modifications.”


Of course tweaking is just another day in the office for design engineers. As the impeller project progressed, AM provided much faster turnaround times than casting or milling would have allowed, since parts could be printed, evaluated, iterated and printed again quickly. In subsequent 3D printing runs, multiple examples of old and new impeller designs could be simultaneously made on the same build plate to compare results.


The relatively small size of the impellers (60 mm in diameter) necessitated the team’s development of a “sacrificial shroud” — a temporary printed enclosure that held the blades true during manufacturing. “What was really interesting about this approach is that shrouded impellers are, for most current additive technology, basically untouchable because of all the traditional support structures they require,” says Velo3D’s Mohawk-project leader Matt Karesh. “We used a, not support-free, but reduced-support approach. Mohawk was saying, ‘we don’t need the shroud in the end, but the shroud makes our part better, so we’ll attach this thing that’s typically extremely hard to print—and just cut it off after.’ Using Velo3d’s technology, they were able to build that disposable shroud onto their impeller, get the airfoil and flow-path shapes they wanted, and then it was a very simple machining operation to remove the shroud.” Surface finish was another focus. “The surface was a bit rough in our early iterations,” says Mohawk engineer Rochelle Wooding. “What was interesting about the sacrificial


shroud was that it gave us a flow path through the blades that we could use to correct for roughness using extrusion honing; it took some further iteration to determine how much material to add to the blades to achieve the required blade thickness that we wanted. The final surface finish we achieved is comparable to that of a cast part, and suits our purposes aerodynamically.” What’s more, all critical design dimensions enabling proper impeller operation were within tolerances.


Next steps


Next steps are retrofitting AORBs with the new impellers and testing them in field conditions. “We expect that successful execution of these two tasks will fully demonstrate that 3D-printed Inconel parts delivered by LPBF technology are a viable and reliable alternative for manufacturing turbomachinery components,” says Cordova. Work is already underway using AM for other blower parts like housings and volutes.


Above: Solid oxide fuel cell (photo courtesy FCE) www.modernpowersystems.com | November/December 2022 | 37


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