How to determine if metal 3D printed parts have been completely de powdered?

一,The powder left over from 3D printed metal pieces has hidden and cumulative properties. In complicated flow channel constructions, powder can build up in "powder locking zones" in places where you can't see them, like the osseointegration interface of titanium alloy acetabular cups. If the leftover powder is thicker than 0.1mm, it will make it much harder for the bone to integrate. More dangerously, aluminium alloy powder can split when it comes into contact with hydrogen in high-temperature conditions. In the combustion chamber of a specific aviation engine, leftover powder can cause local overheating during hot testing, which can lead to structural deformation. These examples show that finding powder residue needs to be a part of the whole design, printing, and post-processing process.
二, A system for multi-dimensional detecting technologies
1. Ways to find things on a large scale
Visual inspection: good for open structural sections, watching changes in surface glossiness when exposed to intense light. For instance, if there is powder on the surface of a stainless steel interbody fusion device that hasn't been cleaned, it will look foggy.
Test of pressure: Use a flow meter to keep an eye on the leakage rate while applying 0.5MPa compressed air to the cooling channel. For a given type of turbine blade, the standard for detection is a leakage rate of ≤ 0.1L/min.
2. Technology for detecting things at the microscopic level
CT scanning for industry: It can take three-dimensional pictures with a resolution of 0.01mm. A certain aircraft engine fuel injector was checked, and it was found that CT scanning may find powder agglomerates that are 0.05mm thick that X-ray fluoroscopy can't see.
Scanning Electron Microscopy (SEM), when used with Energy Dispersive Spectroscopy (EDS), may find powder particles that are only a few microns in size and look at their chemical makeup. SEM discovered that the oxygen content of leftover powder was 300% greater than that of the basic material while evaluating cobalt chromium alloy joint prosthesis.
3. Testing for physical performance
Thermal conductivity test: The powder left behind will lower the material's thermal conductivity. The test results for an aluminium alloy radiator shows that the thermal conductivity goes down by 2.3% for every 1% rise in residual rate.
Testing with ultrasound: This method offers a 92% sensitivity in finding powder agglomeration in titanium alloy bracket testing by employing the sound velocity attenuation coefficient to find internal faults.
三, Improving processes and stopping problems before they happen
1. Prevention during the design phase
Improving the Powder Discharge Path: Using fluid dynamics simulation design to make a tapered flow channel that lets the powder flow out naturally under the force of gravity. The optimisation instance of a certain aviation engine combustion chamber illustrates that changing the angle of the flow channel from 60 ° to 45 ° can make cleaning the powder 40% more effective.
Design for clearable powder: In important places, keep powder cleaning pores with a diameter of more than 0.8 mm. The design change to a certain orthopaedic implant has cut the time it takes to manually clean the powder from 120 minutes to 15 minutes.
2. Control of printing parameters
Optimising the thickness of the layers: Changing the layer thickness from 50 μm to 30 μm can help stop powder bridging. A test of printing a certain model of rocket engine nozzle showed that thin-layer printing cuts down on the amount of powder left over by 65%.
Better Support Structure: Using a dot matrix support instead of a solid one cuts the support volume of a given turbine blade by 70% and makes it 50% easier to clean the powder.
3. New ideas in post-processing technologies
Bi-directional inert gas impact: Changing the direction of gas flow at a pressure of 0.6 MPa, the powder cleaning test of a certain aerospace valve body indicated that this method cut the residual rate from 3.2% to 0.5%.
Help with ultrasonic vibrations: Ultrasonic vibration at 20kHz can weaken the binding between powder and substrate by 60% and cut the time it takes to clean powder to one-fifth of what it used to be.
四, Standards for the industry and quality control
1. The ASTM F3303 international standard system There should be no more than 0.5mg/cm² of leftover powder in medical implants. CT scanning and solvent extraction are two ways to find this out.
ISO/ASTM 52921: For aerospace parts, the powder cleaning qualification rate must be 99.99%, and this is checked using the probability sampling method.
2. Quality control at the enterprise level
Managing a database: A certain aviation manufacturing company built a library of 2000 sets of data on powder cleaning processes and used machine learning models to figure out the best powder cleaning settings for different materials.
System for digital traceability: RFID tags keep track of the cleaning process for each part, making it possible to trace the whole process from printing to detection.