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German Scientists Develop Method for Dissolvable Supports in Metal Powder Bed Fusion

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In a study published on Volume 4 of 3D Printing and Additive Manufacturing, a group of scientists demonstrated, for the first time, two approaches for dissolvable supports in powder bed fusion of metal parts. Expanding on recent innovations with dissolvable supports in multimaterial direct energy deposition (DED) printing, this work focused on adding dissolvable support capabilities to additive manufacturing systems that are limited to single-material builds.

Support scaffolding and structures are an inconvenient necessity in 3D printing. These structures play an integral role in most 3D printing technologies; they mechanically anchor components to the build platform, reduce thermal stress distortions, support large overhangs, and provide a thermal pathway for heat removal.

Unfortunately, removing support structures can be difficult and expensive. This is especially true for 3D-printed metals, where support removal operations can negate the benefits of 3D printing. Inspired by the polymer 3D printing community’s use of soluble support structures, the scientists initially sought to introduce dissolvable metal supports for multimaterial-directed energy deposition (DED) 3D printed metals.

Powder bed fusion (PBF) 3D printers are currently limited to single-material builds and cannot use the same multi- material ‘‘trick’’ used in DED dissolvable supports. The novelty of the work in this article lies in two new approaches that bring dissolvable support capabilities to PBF printers and other single-material printing systems. These approaches were designed to work seamlessly with existing PBF printing technologies and do not require any changes to the PBF process parameters or the metallic powder feedstock used for printing.

The fundamental observation driving this work is that most support structures in PBF connect to the component over a very small region, typically 100–200 lm wide. With this in mind, only a small amount of material needs to be dissolved to separate the component from the supports. If necessary, this small amount of material loss could be accounted for using simple scaling operations applied to the computer model before printing.

The first approach implemented by the team of researchers consisted in “direct dissolution”: here the supports are dissolved electrochemically. The study found that while this process works, it is not self-terminating and thus maintaining dimensional accuracy for complex geometries is difficult.

The second approach incorporated a sensitizing agent during the normal stress, relieving thermal annealing step to decrease the chemical stability of the top 100–200 lm of the component’s surface. The component was then cut down under etching conditions with a high selectivity toward the ‘‘sensitized’’ surface over the base component material.

This created an etching process that self-terminated once the sensitized layer was removed. Both these processes were first demonstrated using 17-4 PH stainless steel. Direct dissolution was conducted under anodic bias in a solution of HNO3/ KCl/HCl; 120 lm of material were removed from the component’s surface.

For the self-terminating dissolution process, surface sensitization was obtained by exposing the sample to sodium hexacyanoferrate at 800°C for 6 hour in order to carburize the top 100–200 lm of the sample. This carburization process captured the protective chromium in chromium carbide precipitates and rendered the surface sensitive to chemical and electrochemical dissolution.

The self-terminating etching reaction was demonstrated under anodic bias in a solution of HNO3/KCl. Open circuit potentials were measured at 10 s and 1h, and polarization curves were used to identify the corrosion potential. Self-termination was verified by monitoring component diameter over time, and 120 lm of material were removed from the sensitized surface of the component.

To further test the self-terminating sensitized approach, a set of 316 stainless steel interlocking rings were fabricated to demonstrate that this approach scales to large components with complex geometry. For these parts, this approach replaced 4–5 days of machining operations with a 32.5 h electrochemical bath.

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