Is there a difference in the powder removal method for metal printed parts made of different materials?

一, The underlying logic of the particle removal process is based on the qualities of the material.
1. Titanium alloy: high activity needs protection from an inert environment
When titanium alloys (such Ti6Al4V) get hot, they can react with oxygen and nitrogen to generate a hard, brittle oxide coating. The particle size of its powder is normally kept between 15 and 53 μm, however if there is a lot of tiny powder (<20 μm), it can clump together and leave behind more residue. When taking out powder, you should use an inert gas protection system, such the Tuobo TCB-100 explosion-proof powder cleaning equipment. This system replaces the air within the box with nitrogen or argon gas to keep the oxygen level below 50ppm and stop the powder from oxidising and catching fire on its own. To clean porous structure implants like intervertebral fusion devices, you should use 0.5–0.6MPa compressed air to wrap the same type of metal particles. This will ensure that the rate of powder removal inside the pores is at least 99.5%.
2. Stainless steel: It doesn't rust, therefore it can be cleaned with chemicals.
316L stainless steel powder includes chromium in it, which makes it resistant to chemical corrosion and allows it to be cleaned in several steps. For instance, this is how one aircraft manufacturer cleans the blades of their engines: high-pressure water jet (15 MPa) to get rid of surface powder first, then an alkaline cleaning solution (pH=11) steeped for 2 hours to dissolve organic matter, then ultrasonic cleaning (40kHz, 10 minutes) to shake out any remaining pores, then pure water rinsing, and finally hot air drying (80 °C). To avoid pitting corrosion, the cleaning solution must have a chloride ion level of less than 25ppm for precision parts.
3. Aluminium alloy: The low melting point makes it hard to clean mechanically.
AlSi10Mg aluminium alloy melts at only 577 °C, and typical vibration cleaning can quickly change the shape of parts. The EOS company has come up with a "soft scraper + low-temperature blowing" process. During the powder laying stage, a silicone scraper (temperature resistance ≤ 80 °C) is used to reduce splashing. After printing is done, CO₂ snow jet cleaning (temperature -78.5 °C) is used to remove powder through solid particle impact while avoiding thermal stress concentration. To keep thin-walled constructions from falling apart, such radiator fins, the blowing pressure should be kept below 0.2 MPa.
二, The type of structure drives the different types of particle removal technology.
1. Porous structure: spraying in one direction and hoover adsorption working together
The porous layer on orthopaedic implants, including acetabular cups, is usually less than 3 mm thick, therefore they need to be cleaned from multiple angles. Spraying in six different orientations (0°, 60°, 120°, 180°, 240°, and 300°) on buildings that can be seen through will lower the amount of powder left behind from 8.2mg/cm² to 0.3mg/cm². Vacuum adsorption technology should be used in conjunction with structures that are not permeable. A medical company, for instance, utilises a negative pressure powder suction device (with a vacuum degree of -80kPa) and a fixture that can rotate 360 degrees to get a powder recovery rate of 98.7%.
2. Thin-walled structure: cleaning with low stress and support for the structure
Aircraft engine blades are generally less than 1mm thick, and when removing powder, the mechanical stress must be kept in check. The "electrostatic adsorption+airflow guidance" technology used by GE works like this: a conductive coating is put on the printing platform, and the powder sticks to the coating surface through an electrostatic field. Then, laminar airflow (flow rate of 0.5m/s) is used to blow along the blade surface to keep it from vibrating. For complicated internal cavity structures, it is necessary to create unique fixtures. For instance, a certain type of turbine disc has a core shaft that can be removed. This helps the powder come out by using the centrifugal force that the core shaft creates while it spins.
3. Physical Structure: Good for recycling and keeping dust down
When removing powder from big solid items like car engine cylinders, you need to find a balance between being efficient and protecting the environment. The Tuobo ultra-large powder cleaning machine (load 1000kg) uses a combination of "vibration screening+airflow classification" to clean the powder. First, a vibration motor (frequency 25Hz) loosens the powder, and then a cyclone separator (cutting particle size 5 μm) collects usable powder. Finally, a bag filter (filtration efficiency 99.99%) catches ultrafine dust. This method meets PM2.5 emission regulations and recovers 95% of the powder.
三, The risk of the process promotes the upgrade of particle removal technology.
1. Risk of powder oxidation: a closed-loop recycling system
After using titanium alloy powder 15 times, the oxygen concentration will rise from 0.08% to 0.25%. This will make the parts 40% weaker in terms of fatigue strength. Siemens has created a closed-loop powder management system that uses inert gas to keep the conveying pipeline safe. → Using the laser diffraction method to find the size of powder particles online (precision 0.1 μm) → air flow grinding and peeling treatment for powders with an oxide layer thickness of more than 2 μm → re-screening (150 μm sieve) and reuse. This technology raises the powder utilisation rate to 92% and keeps the oxygen concentration of the parts to ≤ 0.13%.
2. Risk of residual stress: heat treatment synergy for powder removal
After printing, the residual tension in the nickel-based alloy Inconel 718 can be as high as 300 MPa. If you don't load the vibration properly when removing the powder, you could easily have cracks. A certain aerospace company uses the "low-temperature powder removal + ageing treatment" process. First, the parts are cooled in liquid nitrogen at -196 °C to release 50% of the residual stress. Then, low-pressure blowing at 0.3MPa is used to remove the powder. Finally, ageing treatment at 720 °C for 8 hours is done to lower the stress even more, to less than 50MPa. The qualification percentage for parts has gone up from 78% to 95% thanks to this approach.
3. Biosafety risks: regulations for sanitising medical equipment
Orthopaedic implants must meet the ISO 10993 biocompatibility criteria, and when the powder is gone, they must be able to:
Powder left over: 0.1mg per piece
Endotoxin from bacteria <0.25EU/mL
Level 1 or lower for cytotoxic reaction
A certain medical company uses a three-step cleaning process:
Rinse with clean water (conductivity ≤ 1.3 μ S/cm)
Soak in 75% ethanol for 30 minutes at 60 °C.
Sterilisation with ethylene oxide (800 mg/L, 6 hours)
After testing, this technique can guarantee that all products are safe for biosafety.