An experimental method for accurately measuring melt pool temperature during laser powder bed fusion has been developed using a single commercial colour camera.
The method, described in Additive Manufacturing, can be used to predict and identify defect signatures such as keyholing in real time, which can lead to cracking and porosity that can result in unsuitable parts.
According to the researchers at Carnegie Mellon University, the new method can be applied to any colour camera to achieve ‘entirely unique’ measurements of melt pools across a wide range of additive manufacturing (AM) processes.
The colour camera used has a built-in Bayer filter on the sensor with two green pixel filters for every red and blue pixel filter. Because each pixel senses light from only one colour, the team acquires unique measurements for each pixel. Using a technique called demosaicing, they reconstruct a full colour image and measure the ratio between each of its colours to calculate the temperature.
The researchers say that this novel ratiometric approach avoids complications related to surface properties and view factors that challenge the application of conventional IR imaging to AM processes.
“Without analysis, the temperatures of these liquid metals are interesting but don’t directly explain the physics in the melt pool,” said Alex Myers, a PhD candidate involved in the research. “We determined unknown parameters in a computational fluid dynamics model using the experimental results to tell us more about what happens at the microscale of the melt pool.”
Peak temperatures in the melt pool helps the researchers understand material vaporisation during production while the gradient toward the tail of the melt pool helps them to understand the microstructure of the final part. Understanding the physics in the melt pool is critical to ensuring quality parts, because if too much of the material vaporises or the melt pool becomes unstable, manufacturers could end up with entirely different material properties and defects that render the part unusable.
Supplemental materials are available with the Additive Manufacturing paper to enable researchers at any company or university to recreate this system on their own machines.
Moving forward, researchers plan to use this technique to understand different processes such as wire arc additive manufacturing and directed energy deposition.
“These measurements and analogous simulations are opening the door to a more complete and fundamental understanding of how process parameters affect melt pool physics, which is a central issue in AM process development,” said Jack Beuth, a professor of mechanical engineering at the university.