What level can polishing elevate the surface of metal 3D printing to?

1. Surface roughness: a jump from micrometers to nanometers
Surface roughness is a key measure of the quality of a part's surface. It has a direct effect on how well it resists corrosion, how well it works with friction, and how well it works with light. The process settings, the type of material, and the direction of printing all have a big effect on how rough the metal 3D printed parts are at first. For instance, the surface roughness of titanium alloy parts made with the laser powder bed melting (LPBF) method can be as high as Ra15–20 μm when measured across the printing direction. When measured along the printing direction, however, the roughness can be as low as Ra8–12 μm because the layers are more tightly overlapped. Polishing can make the surface much smoother:
Mechanical polishing: Using automated polishing machines and diamond abrasives, the surface roughness of aluminum alloy parts made by the BJ method (adhesive spraying) can be lowered from Ra2.4 μ m to Ra0.8 μ m or less, which is good enough for most mechanical assembly jobs. For high-precision needs, like supports for optical mirrors, multi-stage polishing (coarse polishing → fine polishing → ultra fine polishing) can lower the surface roughness to Ra0.05 μ m, which is close to the level of typical mirror grinding.
Chemical polishing: This method uses acidic or alkaline solutions to selectively dissolve surface bumps. It works well on intricate internal cavity structures. For instance, the surface roughness of 316L stainless steel cardiovascular stents went from Ra6 μm to Ra0.2 μm after chemical polishing. This got rid of the microsphere-like attachments that formed during printing, which lowered the risk of thrombosis.
Laser polishing: Using powerful laser beams to melt surface materials in a small area and then letting the surface tension of the liquid metal do the work of smoothing it out. Studies indicate that following five laser scans of 316L stainless steel components produced using the SLM method, the surface roughness diminished from Sa21 μm to Sa1 μm, achieving a reduction rate of 96%, without the formation of a subsurface damage layer attributable to mechanical polishing.
2. Microstructure: Improving from flaws to densification
Polishing not only makes the surface look better, but it also makes the material stronger by getting rid of little flaws:
Crack closure: Microcracks that form when metal 3D printing cools too quickly can be partially closed by mechanical polishing pressure. For instance, following vibration polishing, the surface fracture density of a certain aviation engine turbine blade dropped by 40%, and the high cycle fatigue life went up by 25%.
Release of residual stress: Chemical polishing relieves residual tensile stress by breaking down the surface layer, which prevents stress corrosion cracking. The pickling treatment of TC4 titanium alloy parts showed that the residual tension on the surface went down from -150MPa to -50MPa, and the rate of corrosion from salt spray went down by 60%.
Laser polishing can cause surface remelting, which can make the grain size more uniform. Studies on the high-temperature alloy Inconel 718 indicate that laser polishing refines the surface grain size from 50 μm to 10 μm, enhances hardness by 15%, and decreases the oxidation weight gain rate at 650 °C by 30%.
3. Functional performance: going from basic to high-end
The enhancement in surface quality post-polishing directly correlates with the optimization of functional performance:
Improved wear resistance: Smoother surfaces can make it less likely for tiny convex entities to touch each other on the contact surface. The friction test on GCr15 bearing steel components indicated that polishing the surface from Ra1.6 μm to Ra0.2 μm made the friction coefficient go down from 0.15 to 0.08 and the wear quantity go down by 70%.
Better resistance to corrosion: A smooth surface makes it harder for corrosive substances to stick, and the passivation film that forms during chemical polishing adds even more protection. After electrochemical polishing, the corrosion current density of 304 stainless steel parts in a 3.5% NaCl solution went down from 1.2 × 10 ⁻⁵ A/cm ² to 2.5 × 10 ⁻⁶ A/cm ², and the resistance to pitting corrosion went up by 5 times.
Improved optical performance: Research on polishing AlSi10Mg aluminum alloy mirrors demonstrates that when the surface roughness is lowered from Ra3.2 μm to Ra0.05 μm, the visible light reflectance goes up from 85% to 92%, which is what laser communication systems need.
4. Industry Application: Moving from the lab to the factory
Polishing technique has been extensively utilized in domains necessitating stringent surface quality standards:
Aerospace: Laser polishing is used on a certain type of rocket engine nozzle to make the surface less rough, from Ra12 μ m to Ra0.8 μ m. The thermal cycle life in a simulated space environment (temperature range: -180 °C to 300 °C) has been raised from 50 to 200 times.
Medical implants: After chemical polishing, the surface roughness of titanium alloy hip joint prosthesis went from Ra8 μ m to Ra0.5 μ m. This made cells stick to the implants 40% better and made bones integrate with them 30% faster.
Precision mold: After polishing with shape adaptive grinding (SAG), the surface roughness of the automotive injection mold core goes down to Ra0.02 μ m, the mold life goes up from 100,000 times to 500,000 times, and the product's surface glossiness goes up by 2 levels.