Thermionic electrons in electron beam powder bed fusion process: An experimental investigation

Abstract

The melting conditions in Electron beam Powder Bed Fusion can be easily adjusted with real-time monitoring. Thermionic electrons (THER) have great potential for monitoring, as they are emitted by the hottest spots close to the depression zone, where the primary e-beam (PEB) interacts with the target (powder). This paper investigates THER characteristics for different PEB currents. The THER signal was collected and filtered by a Retarding Field Analyzer using two probes: the heat shield (large area) and a tiny probe, which has a surface one thousand times smaller. The analysis focuses on THER's recorded intensity, phase, spatial, and energy distributions. The origin and characteristics of the signal change from the thermal welding regime to the keyhole formation. Applying Richardson-Dushman’s equation, the emitted THER surface was inferred, having ∼310 µm of diameter for a high PEB, which is quite close to the value measured for the PEB spot diameter (∼250 µm), responsible for carrying 95% of the current.