An Arizona State University (ASU) researcher has develop what could be the Holy Grail solution to speeding up the end-to-end process of metal 3D printing.
Stratasys, one of the power-house 3D printing companies for thermoplastic applications, developed a dissolvable support methodology some years ago for their Fused Deposition Modeling systems. A similar system for metals has been elusive for almost 30 years.
Owen Hildreth, ASU assistant professor of 3D Nanofabrication, realized that any metal could be dissolved if you could control the local chemical environment — except for the oxide surface layer. The goal was to produce controlled degradation that would literally eat away the supports but leave the actual part virtually intact.
Because multi-material 3D printing systems are not yet widely available, Hildreth investigated ways to selectively remove the supports of powder-bed-type metal AM parts. Starting with a simple design for demonstration — a small 17-4 stainless steel cylinder 3D-printed with a single row of 100-micron-diameter needle-like supports — he tested two possible approaches.
In the first one, termed direct dissolution, the part was heat-treated (annealed) while packed with sodium ferrocyanide; this step precipitated out much of the protective chromium carbide, rendering the no-longer-stainless steel susceptible to chemical etching. The latter process was successful, but the part itself experienced significant etching, which continued the longer the part was allowed to sit in the solution.
To make the process less time-sensitive (self-terminating), Hildreth introduced a sensitizing agent, sodium hexacyanoferrate, during the post-print annealing step. At high temperatures, this chemical decomposes to give off carbon and nitrogen that diffuses into the stainless steel part to effectively turn the top 100 to 200 microns into carbon steel instead of stainless steel. Since the supports are only around 125 microns thick, they are completely “sensitized” while only a thin shell of the actual component is transformed. By applying the right bias voltage and chemistry, the supports become subject to anodic corrosion while the part itself takes on the role of a cathode and remains protected.
>> Read more by Pamela J. Waterman, RapidReady, April 19, 2017