What is the role of chemical polishing in metal 3D printing?

一, The main idea behind chemical polishing is to selectively dissolve and rebuild the surface.
The oxidation-reduction interaction between chemical reagents (such acidic or alkaline solutions) and the metal surface makes the surface layer dissolve in a regulated way during chemical polishing. Its main parts are:
Microscopic protrusions dissolve more easily: When the surface is uneven (as when weld pool overlap marks or unmelted powder particles are present), the "protrusion preferentially dissolves" phenomenon happens because there is more exposed area and a faster chemical reaction rate.
Leveling via surface tension: When metal ions dissolve, they go toward concave places because of surface tension. This creates a smooth, even surface once it hardens. For instance, chemical polishing can lower the surface roughness (Ra) of 3D printed titanium alloy from 6–12 μm to 0.2–1 μm, which is within the biocompatibility criteria needed for medical implants.
Removal of the fault layer: Chemical polishing can precisely remove unmelted powder particles that are stuck to the surface (approximately 70 μ m thick), which prevents subsurface damage that can happen with standard mechanical polishing. Singaporean researchers have created a new way to polish 316L stainless steel that uses high-voltage pulses and traditional electrochemical polishing to make the surface less rough. The new method keeps the energy-absorbing properties of the original lattice structure.
二, The main purpose of chemical polishing is to improve the surface and make it more useful.
1. Improving the quality of the surface: getting around geometric limits and polishing complex structures
When it comes to polishing, metal 3D printing's complicated geometric shapes, such lattices, internal channels, and porous structures, can be hard to work with. Chemical polishing, which doesn't require contact, is now the best way to handle these kinds of structures:
Polishing of internal channels: Chemical reagents can go into internal channels that are only a few micrometers wide to get rid of burrs created by the melt pool overlapping. For instance, following chemical polishing, the internal flow channel of the fuel nozzle in an airplane engine cuts flow resistance by 15% and makes the engine use less fuel.
Optimizing the porous structure: Chemical polishing can get rid of loose spherical layers on the pore surface of medical implants like hip cups and interbody fusion devices. This makes it easier for bone cells to stick to the implants. Studies indicate that the surface roughness of polished porous titanium alloy is diminished by 90%, while the bone integration rate is enhanced by 40%.
Chemical polishing may work on any shape of part, therefore it can work on free-form surfaces as well. For instance, chemical polishing made the complicated aerodynamic surface of 3D printed turbine blades smoother by 10 μ m to 1 μ m and boosted their aerodynamic performance by 8%.
2. Improving mechanical performance: get rid of flaws and make the fatigue life longer
Fatigue cracks in metal 3D printed objects start with surface flaws. Chemical polishing improves mechanical characteristics in the following ways:
reduce of residual stress: Chemical polishing can partially reduce the residual stress that is caused by printing by dissolving tiny regions. For instance, chemical polishing can extend the low cycle fatigue life of 3D printed nickel-based high-temperature alloys from 5000 cycles to 12000 cycles.
Eliminating the cause of cracks: The overlapping marks of unmelted powder particles and the molten pool are common places where cracks start. Chemical polishing can fix these problems and raise the threshold for fatigue crack growth by 30%.
Surface densification: The surface layer becomes denser after chemical polishing because the process of dissolving and then solidifying again. For instance, following chemical polishing, the porosity of a 3D-printed cobalt chromium alloy drops from 0.8% to 0.02%, and its resistance to corrosion goes up by five times.
3. Biocompatibility optimization: fulfills strict medical standards
Medical implants need to have a very smooth surface and be biocompatible. Chemical polishing serves the needs of the medical field in the following ways:
Rough surfaces can readily become hiding spots for bacteria, so make sure to keep them clean. Chemical polishing can make surfaces smoother down to the sub-micron level, which greatly lowers the risk of infection. After chemical polishing, the attachment of Staphylococcus aureus to knee implants made of 3D-printed titanium alloy went down by 90%, for example.
Control ion release: When you polish anything mechanically, it can add contaminants that cause dangerous ions (like nickel ions) to be released too much. Chemical polishing cleans the surface by using only chemical reactions, which makes sure that ion release fulfills the ISO 10993 biocompatibility requirement.
Promoting tissue regeneration: Smooth surfaces can help cells stick together and lower immunological reactions. Studies indicate that the growth rate of osteoblasts on the surface of chemically polished 3D printed porous titanium alloy is doubled.
三, Uses of chemical polishing in industry: from testing prototypes to making final products
1. Aerospace: Making things more reliable in harsh conditions
Aircraft engine blades, combustion chambers, and other parts have to be able to handle extreme temperatures, pressures, and corrosive conditions. Chemical polishing makes parts last longer by improving the condition of the surface.
Turbine blades: Chemical polishing can get rid of the marks left by the molten pool on the blades' surface and slow down the process of high-temperature oxidation. After chemical polishing, the turbine blades of GE Aviation's LEAP engine, for example, can handle temperatures 50 °C higher and last 20% longer.
Chemical polishing can get rid of microcracks on the inside of the combustion chamber liner, which makes it better able to handle thermal stress. A 1000 °C thermal cycling test showed that polished nickel-based alloy liners' crack propagation rate is 60% lower.
2. Medical implants: making them unique and useful
Chemical polishing is the most important part of medical 3D printing post-processing. It supports the whole process, from customizing designs to making them work:
Orthopedic implants, such as hip cups and interbody fusion devices, are made to fit the patient's body and need chemical polishing to get the right balance between smoothness and bone integration performance. Johnson&Johnson's DePuy Synthes 3D printed titanium alloy interbody fusion device, for instance, has a surface roughness of 0.8 μ m after chemical polishing and a clinical success rate of more than 95%.
Cardiovascular stent: Chemical polishing can get rid of burrs on the stent's surface and lower the chance of thrombosis. After chemical polishing, the surface roughness of Boston Scientific's 3D printed nickel titanium alloy stent went down from 5 μm to 0.5 μm, and the rate of endothelialization went up by 30%.
3. Industrial equipment: cut down on maintenance costs and make things work better
Chemical polishing improves the condition of surfaces, which lowers the friction and energy use of industrial equipment.
Making molds: After chemical polishing, the surface roughness of 3D printed injection molds goes from 8 μm to 0.5 μm, the force needed to remove the mold goes down by 40%, and the mold lasts three times longer.
Chemical polishing can get rid of burrs in the flow channel of the valve body and make it easier for fluids to flow through. For instance, after chemical polishing, Caterpillar's 3D-printed hydraulic valve body cuts down on pressure loss by 15% and boosts system efficiency by 8%.