“A lot of the replacement parts for these planes are in short supply and are very expensive,” Murray tells 3D Metal Printing. “There just aren’t many of these planes or spare parts around. To display the role that metal AM can play in terms of aircraft operational readiness, we looked for a part on Rob’s plane that was visible, showed a lot of wear and tear, but was not necessarily flight-critical, and settled on the exhaust manifold of the plane’s Merlin V12 1500-hp engine.”
Scanning to Create the Data Cloud
Late in 2016, Chu’s team at PADT blue-light scanned and digitized one of the plane’s exhaust stacks, using a Zeiss Comet 6 high-resolution scanner. “We captured some 2 million datapoints,” Chu says, “scanning at a resolution of 0.0005 in., and then cleaned the data to create the surface mesh. That mesh then was used as the basis to define geometric features and dimensions to create a 3D CAD model, eventually converted into an STL file for 3D printing.”
Chu and his team also performed some simulation work on the CAD model, using Ansys simulation software to study how the resulting 3D-printed exhaust stack would perform in service. They ran heat-transfer and fluid-flow analyses, “the idea being that we could, if we wanted to, modernize and optimize the design of a part to improve its performance over an original design,” Chu says.
Chu then sent two STL files to Concept Laser for printing—the resulting model and a second mirror-image model. In all, Concept Laser printed four stacks, building two at a time on one of its M2 casing laser-powder-bed machines.
“The original stacks were made from rolled sheets of Type 316 stainless steel,” Murray notes, “and were welded along the backside, as well as along the base flange (a racetrack shape) where they mount to the engine. These stacks, while functional, were somewhat lacking in dimensional accuracy, as a result of the fabrication techniques used at the time and the rush of wartime production.”
3D-printing one pair of stacks took two days, for a total build volume of 125,542 mm3 and a weight of 1.98 kg. For the stacks to build successfully without internal support structures, they had to be rotated approximately 25 deg. off parallel with the build plate. This resulted in a relatively large support structure under the mounting flange, which had to be added manually (using Materialise Magics data-preparation and STL-editor software).
“The placement and design of the support structure proved critical to the success of the build,” Murray says. “Additionally, one of the two pieces built had an additional support structure under its upper flange. However, there were no internal support structures required—with the angle of the build the structure was self-supporting, and we refined the process parameters to create exceptionally smooth internal surfaces.”
The supports on the lower flange for both pieces then were removed via CNC milling; hand tools were used to remove the extra support added to one of the upper flanges. Both exhaust stacks were then bead-blasted to enhance surface finish.
“This was a great exercise that’s suitable for numerous military applications and very relevant to the future use of 3D metal printing to maintain fleets in the field,” Chu says. “Maintaining spare-parts inventory has become a significant challenge, for example, to the Air Force. Additive manufacturing could be the solution.” 3DMP
View Glossary of 3D Metal Printing Terms
See also: Concept Laser Inc.
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