What are the common surface treatment methods for metal 3D printing?

1, Mechanical precision machining: from traditional craftsmanship to intelligent upgrade
Mechanical precision machining levels surfaces by physically taking away materials. This is the main way to finish metal 3D printing. The main things it does are:
manual polishing
Using equipment like sandpaper and polishing paste to polish step by step can greatly reduce the roughness of the surface (the Ra value can go from 10–20 μm to less than 0.8 μm). This procedure, on the other hand, depends a lot on operating experience, is not very repeatable or efficient, and is only good for making small batches of high-value-added products like jewelry and art.
CNC numerical control grinding
Using CNC machine tools and diamond cutting tools together can make it possible to produce complicated surfaces with very high accuracy (± 0.01mm). But it's hard to work with intricate characteristics like internal flow channels and lattice structures since the tools are hard to get to. Electrical discharge machining (EDM) technique is needed to make cooling holes in the turbine blades of aircraft engines, for example.
System for polishing automatically
Zhejiang Tuobo and other companies have released a robot automatic polishing system that can remove supporting structures and polish surfaces at the same time using 3D visual positioning and force feedback control. This system can work with robots from different companies, like ABB and KUKA. It is 3–5 times faster than doing the same job by hand, and it keeps the surface inaccuracy under ± 0.05mm. It has been used a lot in areas like medical equipment and car parts.
2. Chemical and electrochemical treatment: controlling the microstructure and adding new functions
Chemical treatment changes the surface of a material by dissolving or depositing it. Its main operations are:
polishing with chemicals
Using acidic or alkaline solutions to selectively dissolve the surface can get rid of flaws like spheroidization and slag that happen during printing. For instance, chemical polishing can make the surface of titanium alloy implants less rough, going from 6–12 μm to 0.2–1 μm, and it can also create a passivation layer to make them more resistant to corrosion. This process has a significant effect on the treatment of hollow structures, but strict control of solution concentration and temperature is required to avoid excessive corrosion.
Electrochemical polishing (ECP)
Use direct current in the electrolyte to selectively dissolve the micro protrusions on the metal surface. This will make the surface as smooth as a mirror (Ra value can be 0.01 μ m or less). A lot of medical equipment use this method. For example, after ECP treatment, the surface roughness of cobalt chromium alloy joint prostheses is reduced by 90%, the wear resistance is increased by three times, and printing layer patterns can be eliminated, meeting the requirements of biocompatibility.
anodizing
Electrolytic processes can create dense oxide coatings (5–20 μm thick) on lightweight alloys like aluminum alloys. These films can greatly increase hardness (up to 500HV) and resistance to corrosion. For example, after hard anodizing treatment, aviation structural components can withstand corrosion for more than 5000 hours in a 3.5% NaCl salt spray environment. The microporous nature of the film layer can also soak up lubricants and lower the friction coefficient.
3. Coating and Plating Technology: Combining Functional Protection and Decoration
Coating technology creates a protective coating on the surface by depositing something physically or chemically. The main steps in this process are:
PVD stands for Physical Vapor Deposition.
Using high-energy ion bombardment to put hard coatings like TiN and CrN on the surface of the substrate. This process can significantly improve the wear resistance of mold steel (extending its lifespan by 3-5 times), and the coating thickness is only 1-5 μ m, without affecting the dimensional accuracy of the parts. For instance, one company used PVD to process 3D printed molds and raised the stamping frequency from 100,000 to 500,000 times.
Electroplating and Chemical Plating
Electroplating uses electrolytic reactions to deposit metal layers (like Ni and Cu) on a surface, which makes it less likely to corrode and more conductive. Chemical plating, on the other hand, uses self-catalytic reactions to make the surface even (like chemical nickel phosphorous alloy plating). For instance, one company employs electroless nickel plating to 3D print copper alloy heat sinks. This makes them resistant to salt spray for 1,000 hours instead of 48 hours, while still having a thermal conductivity of 200 W/(m · K) or more.
Spraying and covering with powder
Spray coating uses high-pressure airflow to stick powder or liquid coating to the surface, creating a protective layer that is 20–100 μm thick. Powder spraying, on the other hand, uses electrostatic adsorption to evenly distribute powder, which forms a thick coating when it cools down. This method works for outdoor tools, industrial machines, and other situations. For instance, one company utilizes powder coating to treat 3D-printed steel structural elements, which makes them resistant to neutral salt spray for more than 2000 hours.
4. New Technologies: Laser and Composite Processes Lead Innovation: Laser Polishing
Using high-energy laser beams to melt surface materials in a small area and then flow the molten pool to level the surface. This method can work on curved surfaces that are hard to reach and has a small heat impacted zone (≤ 0.1mm). For example, a certain enterprise uses laser polishing to 3D print nickel based high-temperature alloys, reducing the surface roughness from Ra 8 μ m to Ra 2 μ m while maintaining the material's mechanical properties unchanged.
Abrasive Flow Machining (AFM)
To polish complex features like cross holes and internal flow channels, viscoelastic abrasive material is flowed through the component's inner chamber. This procedure can work in places that are hard to get to. For example, one company utilizes AFM to process 3D-printed aviation fuel nozzles, which makes the inner surface less rough (from Ra 16 μ m to Ra 1.6 μ m) and improves flow uniformity by 20%.
Integration of composite processes
Using more than one processing method to work together to boost performance. For example, a certain enterprise adopts a combination process of "chemical polishing+anodizing+PVD coating" for 3D printing titanium alloy implants, which reduces the surface roughness to Ra 0.05 μ m, improves corrosion resistance by 5 times, and the bonding strength between the coating and the substrate reaches 40MPa, meeting the long-term service requirements of orthopedic implants.