Thermodynamic insights into the laser powder bed fusion process in additive manufacturing are critical to ensuring the production of quality parts. For example, if too much of the material vaporizes, or the melt pool becomes unstable during the manufacturing processes, manufacturers could end up with entirely different material properties than intended and/or defects that render the part unusable. Accurate temperature measurements of the melt pool can also be used to predict and identify keyholing. This defect signature can lead to cracking and porosity which can result in unsuitable parts. Thermal infrared (IR) cameras — as well as high-speed visible spectrum cameras and photodetectors — have previously been used to measure the melt pool temperature. However, according to a team of researchers at Carnegie Mellon University, most thermal cameras do not provide theframe rate, exposure time, and resolution required to capture melt pool level temperature transients. “Although the use of thermal cameras provides temperature measurements, these approximations require the assumption of a single emissivity, potentially resulting in large temperature errors,” the researchers said. To resolve these drawbacks, the Carnegie Mellon researchers developed a single-camera method to measure the melt pool temperature during laser powder bed fusion. The method can be applied to any color camera to deliver information of the physics occurring in the melt pool during additive manufacturing. To sense visible colors, the researchers used a commercial color camera, which 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 color, the researchers acquired unique measurements for each pixel. Using a technique called demosaicing, the researchers reconstructed a full color image and measured the ratio between each of its colors to calculate the temperature. This novel ratiometric approach avoids complications related to surface properties, as well as view factors that challenge the application of conventional infrared imaging to additive manufacturing processes. The method, the researchers said, offers the additional advantage of negating the need for preexisting knowledge of melt pool emissivity, and/or plume transmissivity. Carnegie Mellon researchers developed a single-camera method to measure the melt pool temperature during laser powder bed fusion. The method can be applied to any color camera to deliver information of the physics occurring in the melt pool during additive manufacturing. The technique surpasses the capabilities of using conventional thermal imaging for melt pool temperature measurements. Courtesy of Carnegie Mellon College of Engineering. The researchers determined previously unknown parameters in a computational fluid dynamics model. They validated the camera’s ability to accurately measure temperature with a blackbody source and tungsten filament lamp between temperatures of 1600 K and 2800 K. To demonstrate the technique, the researchers used an off- color camera operating at 22,500 fps, capturing a 2.8 mm x 2.8 mm area on the build plate. The researchers imaged both no-powder and powder single beads on a commercial laser powder bed fusion machine. “Without analysis, the temperatures of these liquid metals are interesting but don’t directly explain the physics in the melt pool,” said researcher Alex Myers, a Ph.D. candidate. Specifically, Myers said, peak temperatures in the melt pool help researchers understand material vaporization during production. The gradient toward the tail of the melt pool helps them to understand the microstructure of the final part. The researchers plan to use the technique to understand different processes, such as wire arc additive manufacturing and directed energy deposition. The research was published in Additive Manufacturing (www.doi.org/10.1016/j.addma.2023.103663).
