By Mark Norfolk on Tuesday, February 9th, 2016
Fabrisoinc has been 3D printing aluminum and copper heat exchangers for years. Because UAM is a hybrid process, all of the flow paths printed into our parts are CNC milled allowing for high accuracy and great surface finish. Recently we finished up a large program for NASA that sought to improve the state of the art with embedded channels.
The primary concern when using UAM to build heat exchangers is unsupported material. The UAM process uses several thousand Newtons of force in order to create a bond. For structures such as channel ceilings, no normal force exists to push back and enable bonding. Directly above the cavity the lack of support can cause welding discontinuities (as seen below). Although hermetecity and burst pressure tests have shown that the resulting structure can far exceed possible mission requirements, the discontinuities are a concern simply because they exist.
Prior work with embedded channels, has mostly focused on using straight ball nose end mills to machine channels. While this uses standard off the shelf tooling, the resulting cross section is at its maximum width at the top of the channel thereby maximizing the crucial unsupported region. Custom end mills are available on the open market that could be used to create sufficient cross section while shrinking the unsupported width. Under the NASA contract, Fabrisonic developed techniques for using so-called ‘undercut’ endmills for creating channels. The resulting channels had 50% less unsupported area per the same unit volume. These new channels were able to be sealed with completely clean cross sections. This will allow higher quality UAM builds for thermal management devices.