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Completed TRL 1 (started at 1, targeting 2)
The proposed project will demonstrate self-terminating, dissolvable powder removal for GRCop Copper-alloy components fabricated using Selective Laser Melting (SLM) Additive Manufacturing (AM) techniques. This project is specifically designed to address the fact that removing trapped powder from interior passages is the longest lead process in fabricating of GRCop-84 copper-alloy components (see example in Figure 1). To overcome this issue, Dr. Hildreth at Colorado School of Mines (CSM) recently demonstrated a process to dissolve residual powder and support structures in a self-terminating manner that works independent of component geometry. The process is capable of removing powder and supports from extremely high-aspect ratio channels and integrates seamlessly with existing SLM printing technologies and processes. This project will adapt Dr. Hildreth's technology to work with GRCop-84 copper alloys to eliminate the residual powder issues and reduce component lead-time by weeks or months.
This project will support the development of self-terminating etching processes for GRCop-84 components fabricated using SLM AM techniques for applications in removing trapped powder. After printing, the specimens will be submerged in a solution containing either magnesium salts or elemental sulfur. Next, the component will be heated to high temperatures between 300 ˚C and 500 ˚C to drive the sensitizing agent into the top 20 µm to 100 µm of the component's surface. Since the typical PBF powder diameters range from 20 µm to 50 µm, any trapped powder exposed to the sensitizing agent should be completely sensitized. Next, the sensitized region will be dissolved using electrochemical processes designed selectively remove the sensitized regions while keeping the base component material cathodically protected (Figure 2a). Since only sensitized region are dissolved, the component loses only a small amount of material even over excessively long etch times (Figure 2c). Preliminary tensile data shows no difference when compared to mechanically processed samples.
The proposed project will: 1) generate an initial set of processes and chemistries to selectively dissolve trapped powder and support structures for GRCop-84 copper-alloys; and 2) generate data assessing impacts mechanical properties, including tensile strength, and surface finish. These data will be used to identity if and how processing parameters need to be modified along with how much additional research is needed before this technology could be integrated into SLM processing guidelines and used by NASA to improve component performance.
To date, CSM has focused on developing proof-of-principle sensitization and etching processes. Initially, CSM conducted a literature review on the sulfidation of copper alloys in both liquid and vapor-phase sulfur. These studies revealed that two copper sulfides are likely to form depending on temperature, with CuS2 forming at temperatures below 350 ˚C and CuS formed at temperatures above 400 ˚C. Next, CSM has started testing initial experiments to sulfurize copper samples wet jet cut from a sheet of copper. We tested sulfurization at 150 ˚C to 200 ˚C in liquid sulfur to sulfidize between 660 µm and 1,990 µm of copper. These experiments verified that copper sulfurizes extremely quickly. This rapid sulfurization of copper makes process control difficult at the temperatures necessary to form the smoother CuS films. Instead, CSM is currently testing a two-step sulfurization process where an initial CuS2 film is formed at lower temperatures (below 200 ˚C) to better control the sulfur dose followed a high temperature annealing in an inert environment to transform the CuS2 to CuS. This should allow CSM to control both the sulfurization depth and produce a reasonably smooth surface (Ra ≤ 5 µm initial target).
In summary, CSM has started proof-of-principle sulfurization processes. These initial experiments help bound the processing conditions and informed future process development. Specifically, the high sulfurization rates of copper alloys CSM observed confirms that directly forming a CuS film is not feasible from a process control standpoint. CSM is currently testing a two-step sulfurization process to provide increased control and smooth surfaces. CSM will continue developing the sulfurization process and then start developing an etching process to selectively etch stop at the GRCop-84/copper sulfide interface.
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