Modulated laser thermal interrogation (MLTI): A novel in situ metal powder evaluation technique for laser powder bed fusion

Abstract

Assessment of metal powders in powder bed additive manufacturing is crucial, as the quality of the powders significantly impacts the final printed parts. This study introduces a novel technique to characterize metal powders by analyzing changes in their thermal properties, specifically heat capacity and thermal conductivity. The Modulated Laser Thermal Interrogation (MLTI) method utilizes frequency domain responses of temperature to facilitate this characterization. To validate the performance of MLTI, a benchtop setup was made, which identified distinct thermal responses related to various material features, including core material detection, age, oxygen content, and particle size distribution. The powder was heated by a 7 W laser (445 nm) that was modulated at frequencies between 100 Hz and 2 kHz. By capturing the IR emission of the surface with the photodetector and sending the signals to the lock-in amplifier, demodulated amplitude and phase could be extracted which represent the characteristics of the metal powder. We tested common metal powders used in powder bed fusion, such as Cu, AlSi10Mg, SS316L, IN718, and Ti-6Al-4V G5 and G23, to demonstrate the capabilities of the MLTI method. The frequency-domain measurements provided by MLTI offer reduced noise compared to traditional methods. By leveraging machine learning, we could accurately characterize the powder, identify the core material of the powder, determine whether the powder is fresh or reused, assess interstitial oxygen content, verify the powder deposition layer thickness, and analyze particle size distribution. This enhances quality control and process monitoring in powder bed additive manufacturing.