Although the current metal 3D printing industry is developing very fast, there are still many problems to be solved, one of which is the removal of support structures. This can be achieved by machining or using soluble materials. Machining methods are cumbersome and may cause damage to the print. The types of soluble materials have their limitations, generally only suitable for plastic materials, for metal supports, this method will not work
Overview of Electrochemical Removal of Metal Supports
Researchers from Arizona State University (ASU) have proposed an electrochemical method to remove metal supports, and this method has been applied for a foreign patent. The principle is shown in the following figure:
Electrochemical removal of metal supports
The part entity actually required in the gray part in the figure, in order to ensure successful printing, it is usually necessary to add support to the part entity part, which is the black part in the figure. The electrochemical removal of support is essential to "the cathodic protection method of sacrificial anode". This method is usually used to prevent metal corrosion, that is, the metal with strong reducibility is used as a protective electrode, and it is connected with the protected metal to form a primary battery, which has strong reducibility. The metal will be consumed by oxidation reaction as the negative electrode, and the protected metal will be used as the positive electrode to avoid corrosion. Whether it is two contacting metals in the same electrolyte or material in different parts of the electrolyte (such as the part of the hull above and below the water). The material that undergoes anodization has a more negative reduction potential, which is oxidized in preference to the part material. The support material is equivalent to the anode material with strong reducibility and is removed.
The first electrochemical metal support solution
The researchers 3D printed the part material is 304 stainless steel, and the support material used is carbon steel. The specific principle is to remove the carbon steel support through electrochemical etching technology using nitric acid and oxygen. This method has been shown to be feasible and, more importantly, it does not have any impact on stainless steel prints. The figure below shows the researchers fabricated a stainless steel bridge using carbon steel as a support, and then removed the carbon steel electrochemically (41 wt.% nitric acid solution and oxygen). At first, the researchers did not use oxygen, and they found that the corrosion rate was too slow in this case, removing the 1.4mm carbon steel in 10 hours. Later, they introduced oxygen to speed up the reaction, and it took only 6 hours to completely remove the remaining 7mm of carbon steel.
The removal of metal supports by electrochemical etching is feasible in principle (it has been proved by experiments), but there is very little research on this technology at home and abroad, and there is no mature industrial application at present, mainly in the following aspects of the problem:
●The sacrificial anode material must be metallurgically compatible with the printing material, which means it must have a similar crystal structure, thermal conductivity, and coefficient of thermal expansion, and must not generate harmful intermetallic compounds. This ensures that the contact between the sacrificial anode and the printed part has sufficient mechanical strength to withstand the stresses caused by extreme thermal cycling during metal 3D printing.
●A highly corrosive electrolyte must be used. This electrolyte must be able to dissolve the sacrificial anode with high selectivity (>100:1) compared to the printed material.
●The process needs to be controllable, and it is difficult to guarantee accuracy. The chemical reaction rate and time are difficult to control, and sometimes the printing material is also removed in a small amount or the support material is not completely removed.