Dissolvable metal support enables Additive Manufacturing of complex metallic structures
July 13, 2016
Researchers in America have fabricated dissolvable carbon steel structures using 3D printing technology that can provide temporary support for components of larger stainless steel structures made by Additive Manufacturing. Published in the journal 3D Printing and Additive Manufacturing, the article “Dissolvable Metal Supports for 3D Direct Metal Printing” demonstrates an application of this novel approach, in which the researchers printed and later dissolved a metal structure to support a 90° overhang.
Co-authors Owen Hildreth, Arizona State University (Tempe), Abdalla Nassar and Timothy Simpson, Pennsylvania State University (State College, PA), and Kevin Chasse, Naval Surface Warfare Center (W. Bethesda, MD), propose that this technique could dramatically reduce the amount of post-processing needed for metal AM components to remove support structures.
Images of the sample taken before, during, and after etching. (a) Before etching, the red square roughly outlines where the carbon steel support layer was printed. (b) After 10 h of etching 1.4 mm of carbon steel was removed from each side. (c) After etching showing that the carbon steel was completely etched from the sample. (d) Images of sidewall and bottom with lines etched into a chromium-depleted layer of the component and stainless steel baseplate, where the carbon steel leeched the chromium during printing
Unlike polymer AM processes, soluble sacrificial support materials have not been identified and characterised for metallic materials and, as a result, support structures in metal Additive Manufacturing must be removed using additional machining operations. In this study, the authors demonstrated that sacrificial metal supports can be fabricated by taking advantage of differences in the chemical and electrochemical stability between different metals.
A stainless steel bridged structure with a 90° overhang was fabricated using a carbon steel sacrificial support that was later removed through electrochemical etching in 41 wt.% nitric acid with bubbling oxygen. Open circuit potentials and potentiodynamic polarisation curves were gathered to verify etch selectivity. No machining, grinding, or finishing operations were required to remove the metallic supports using this approach.
Fig. 2 shows the sample before, during, and after etching. In Fig. 2a, a red square roughly outlines the carbon steel section. Figure 2b shows the component after 1.4mm of carbon steel was removed with 6 h of etching with no oxygen bubbling. Adding oxygen bubbles increased the etch rate dramatically, and the rest of the carbon steel was dissolved in 6 h until the stainless steel bridge was the only structure remaining (Fig. 2c).
The inset in Fig. 2d shows the impact of using two materials with dramatically different chromium content. The authors stated that it is well known that stainless steel welds on carbon steel (and vice versa) offering reduced corrosion resistance near the carbon/stainless steel interface. This local reduction in resistance is attributed to the formation of a depleted chromium layer within the stainless steel as chromium at the interface diffuses into the chromium-deficient carbon steel.
The grooves etched into the stainless steel component sidewalls and into the stainless base verify that this phenomenon occurs in bimetallic DED printed components. It is important to account for this additional etching because, even though no observable etching of the overall stainless steel component was observed, some tens of microns of stainless steel will be removed at the stainless/carbon steel interface that authors added.
This novel approach introduces new capabilities to directed energy deposition (DED) AM and could drastically reduce the post-processing needed for these types of parts. For example, a carbon steel layer could be printed, deposited, or developed onto the stainless steel base and then used as a sacrificial support layer for the entire component so that the component can be removed from the build plate without any machining operations. The authors expect this process to be applicable to a wide range of metals and even oxides through selective chemical dissolution.
