Experimental Investigation of Powder Electrical Characteristics and Smoke Behavior in Electron Beam Additive Manufacturing Using AlSi10Mg and 64Ti Powder

Powder bed fusion using an electron beam (PBF-EB) is expected to be applied for medical equipment and aerospace since PBF-EB process can produce object with little thermal strain and contaminant because of high temperature vacuum process.<br/>However, when powders are charged up due to electron beam scanning on powder bed, powders are often spread out because of electrostatic force between powders. This phenomenon is called as smoke phenomenon and is problem in PBF-EB process.<br/>The physical origin of smoke phenomenon is empirically considered to be charge-up, but it is not known in detail what kinds of factor affect occurrence of smoke phenomenon.<br/>In order to clarify the mechanism of smoke phenomenon, we have investigated powder properties and conducted experimental smoke test using AlSi10Mg and 64Ti powder.<br/><br/>In the confirmation of the physical properties of the powder, the electrical characteristics of the powder were investigated using the electrochemical impedance measurement that has been conducted in the previous studies, and the time constant of the powder was analyzed by waveform analysis. The Coulomb repulsion between powders was calculated using the time constant, and it was assumed that smoke is generated when the Coulomb repulsion is larger than the weight of one powder particle, and smoke is not generated when the Coulomb repulsion is smaller than the weight of one powder particle. We predicted the possibility of smoke generation during the building process on this assumption. By comparing the prediction by calculation with the presence or absence of smoke generation (smoke test) by experiment, the mechanism of smoke phenomenon was considered.<br/>In 64Ti, there was a good match between the prediction of smoke generation and the results of the smoke test, proving that the smoke mechanism is due to the Coulomb repulsion. However, in AlSi10Mg, the prediction of smoke generation and the result of the smoke test did not match, suggesting that factors other than the Coulomb repulsion are involved as the smoke mechanism. As factors other than the Coulomb repulsion, the permeability of the electron beam and the temperature rise of the powder due to the electron beam were considered, but none of them can completely explain the mechanism of smoke generation. It was found that detailed discussion on the mechanism is needed in the further study.<br/>In conclusion, it is necessary to consider not only the electrical factor due to the Coulomb repulsion but also other complex factors in the mechanism of smoke generation.