Jul 14, 2016 | By Alec
While metal 3D printing beats regular FDM 3D printing on just about every front, plastics still emerge victorious from two battles: the costs involved and the support structure removal possibilities. For while metal support structures need to be removed through extensive machining (which possibly damages the part), numerous researchers and makers have been remarkably successful in safeguarding the accuracy of plastic prints by relying on soluble supports – such as PLA supports for ABS parts, which are removed with an isopropyl alcohol and potassium hydroxide mixture that leaves the ABS intact.
But even in this respect, metal 3D printing could take over. For researchers have just completed a proof-of-concept with dissolvable carbon steel structures, that act as support for 3D printed stainless steel. In this first-of-its-kind soluble metal support solution, the carbon steel is removed using an electrochemical etching technique with nitric acid and bubbling oxygen – without affecting the stainless steel.
This remarkable breakthrough has been detailed in a paper entitled Dissolvable Metal Supports for 3D Direct Metal Printing, in which a 3D printed metal structure with a 90 degree overhang is showcased. According to authors Owen Hildreth (Arizona State University), Abdalla Nassar and Timothy Simpson, (Pennsylvania State University), and Kevin Chasse (Naval Surface Warfare Center), this breakthrough could pave the way for significant metal 3D printing innovations. In particular, they believe it will dramatically reduce the amount of post-processing that is needed – which could greatly enhance the technology’s ability to produce very complex structures.
As the researchers explain, the real manufacturing breakthroughs can be found in overhanging surfaces, which require support structures to minimize thermally induced distortion. Unfortunately, these require extensive post-process machining to remove. But as they reveal, there is a precedent for soluble metal support: sacrificial anodes. “Sacrificial anodes are often used to protect important parts from galvanic corrosion whenever two different metals are in contact with an electrolyte or if the same material is exposed to different electrolytic environments (e.g., above and below the water line of a ship’s hull),” they explain. “A sacrificial anode is a material with a more negative reduction potential than the part material that will be preferentially oxidized over the part material.”
Taking that concept to metal 3D printing, they have combined a more chemically resistant structural material (stainless steel, AISI type 431) with a sacrificial metal with a lower chemical resistance: the Metco 91 (mild steel) carbon steel. Instead of relying on the salty waters of the ocean, this mild steel is removed using a solution of nitric acid. “Stainless steel has excellent resistance to nitric acid, whereas carbon steel is rapidly chemically dissolved by nitric acid with or without an external bias,” they argue.
To illustrate this remarkable process, they have realized a proof-of-concept bridge 3D printed on an Optomec Laser Engineered Net Shaping [LENS] MR-7 3D printer with two powder feeders, which uses directed energy disposition (or DED) 3D printing. “In DED AM processes, metal powder(s) are blown or a wire is fed into a melt pool formed by a laser or electron beam. This added material increases the melt pool, and parts are built up in a layer-by-layer manner by moving the substrate relative to the energy/material deposition head,” they explain.
Specifically, they 3D printed a solid block of steel, of which the middle third of the component was made from the milder carbon steel, surround on three sides by deposited stainless steel (including the top). After 3D printing, this block was exposed to nitric acid through an electrochemical etching – initially without oxygen. “Immediately before electrochemical analysis/dissolution, the samples were sonicated in acetone, then isopropyl alcohol, and finally dried using N2 gas,” they add.
But the initial results were not as impressive as you might expect, as the acid didn’t exactly eat its way through the carbon steel. “The etch rate is too slow for practical applications, as it took 10 h to remove 1.4 mm of carbon steel from each end,” they say. This can be seen in image 4b (above). “We increased the etch rate significantly by bubbling O2 onto the carbon steel section of the part. The O2 helps break down passivated carbon steel, and an anodic current develops. This increases the etch rate dramatically, and the remaining 7 mm of the carbon steel section was removed in 6 hours,” they reveal.
With that small alteration, soluble support structures suddenly become viable. “This first-of-its kind approach introduces new capabilities to DED AM and could drastically reduce the post processing needed for these types of parts,” its developers say. “These sacrificial materials will enable DED and other metal deposition systems, such as wire-feed and PBF, to use dissolvable supports to help 3D print complex structures with arbitrarily large overhangs and geometries.”
In fact, they believe that this same principle can be applied to a far wider range of metals and even oxides, through selective chemical dissolution. But there are obviously a few conditions that must be met. “Specifically, the sacrificial anode must be metallurgically compatible with the part material: it must have similar crystal structures, similar thermal conductivities, similar coefficient of thermal expansions, and should avoid forming unwanted intermetallics. This compatibility ensures that the interface between the sacrificial anode and the component will be mechanically strong enough to handle the stresses caused by the extreme thermal cycling that occurs in DED of metals,” they say. “Additionally, a corrosive electrolyte must be identified that will dissolve the sacrificial anode with a reasonably high selectivity (>100:1 preferred) compared with the component material.”
But if those circumstances are met, this breakthrough could certainly pave the way for a lot more metal 3D printing opportunities. MIT’s Skylar Tibbits was also very impressed. “This innovative new approach using Directed Energy Deposition for 3D printing of dissolvable metallic components, without the need for machining operations to remove the sacrificial support materials, creates opportunities for new types of applications,” he said. “I'm excited to see what effects this research has on the future of metal printing.”
Posted in 3D Printing Technology
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Great to learn about this "why didn't I think of that?" insight into new support structures. Also, it's good to know oxygen helps the etch rate. But if they're going to the trouble of using ultrasound to clean the final product, why not seek the optimum frequency & amplitude of sound to apply to the acid phase? Many chemical reactions are sped up by sound and heat.
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