Abstract
In situ monitoring is required to improve the understanding and increase the reliability of additive manufacturing methods such as laser powder bed fusion (LPBF). Current diagnostic methods for LPBF capture optical images, X-ray radiographs, or measure the emission of thermal or acoustic signals from the component. Herein, a methodology based on the thermal emission of electrons - thermionic emission - from the metal surface during LPBF is proposed which can resolve laser-material interaction dynamics. The high sensitivity of thermionic emission to surface temperature and surface morphology is revealed to enable precise determination of the transition between conduction and keyhole mode melting regimes. Increases in thermionic emission are correlated to laser scanning conditions that give rise to pore formation and regions where surface defects are pronounced. The information presented here is a critical step in furthering our understanding and validation of laser-based metal additive manufacturing.
Highlights
In situ monitoring is required to improve the understanding and increase the reliability of additive manufacturing methods such as laser powder bed fusion (LPBF)
We have uncovered the generation of thermionic emission during LPBF additive manufacturing and used the signal to identify dynamics caused by laser-metal interactions
The observation of thermionic emission reveals that the formation of plasma during LPBF additive manufacturing previously ascribed to ionization of vaporized metal by the laser beam, could be caused by liberation of electrons from the surface of the metal into the argon cover gas and subsequent interaction with the large electric field of the laser
Summary
In situ monitoring is required to improve the understanding and increase the reliability of additive manufacturing methods such as laser powder bed fusion (LPBF). For LPBF additive manufacturing where the lasermaterial interface is extremely dynamic and consisting of metal vapor, liquid, powder and bulk solid, assumptions that emissivity follow a well-known temperature development are inaccurate This challenge has limited precise optical temperature measurements to regions with approximately constant emissivity, such as areas behind the melt pool or the cooler solidified regions. Other in situ diagnostic methods used for the probing dynamics during LPBF additive manufacturing include X-ray radiography[9,10,11,12], high-speed optical imaging[13,14], optical emission spectroscopy[15], thermal imaging[6,16], scanning interferometry[17], and acoustic spectroscopy[18,19] These methods have proven effective in resolving laser-induced dynamics including melt pool flow, pore formation, surface morphology, vapor plume generation, and powder denudation. Correlation of the thermionic signal to surface defects and regions where material overheating leads to pore formation shows the potential for thermionic emission as an in-process monitoring diagnostic
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