Powder bed charging during electron-beam additive manufacturing

Abstract

a b s t r a c tElectrons injected into the build envelope during powder bed electron-beam additive manufacturing can accumulate on the irradiated particles and cause them to repel each other. Under certain conditions, these electrostatic forces can grow so large that they drive the particles out of the build envelope in a process known as "smoking". In the present work, we investigate the causes of powder bed charging and smoking during electron-beam additive manufacturing. In the first part of the paper, we characterize the surface chemistry of a common feedstock materialdgas-atomized Ti-6Al-4V powderdand find that a thick, electrically insulating oxide overlayer encapsulates the particles. Based on these experimental results, we then formulate an analytical model of powder bed charging in which each particle is approximated as a capacitor, where the particle and its substrate are the electrodes and the oxide overlayer is the dielectric. Using this model, we estimate the charge distribution in the powder bed, the electrostatic forces acting on the particles, and the conditions under which the powder bed will smoke. It is found that the electrical resistivity of the oxide overlayer strongly influences the charging behavior of the powder bed and that a high resistivity promotes charge accumulation and consequent smoking. This analysis suggests new quality control and process design measures that can help suppress smoking.

Thermal debinding mass transfer mechanism and dynamics of copper green parts fabricated by an innovative 3D printing method

“…6 However, this very innovative tool still cannot completely solve the problems of the limited range of printable materials and extremely expensive equipment that have existed for a long time. 7 To avoid these problems, a low-cost 3D printing technique via metal paste injection 3D printing was developed. It involves the use of the material deposition principle for forming a 3D shape layer-by-layer, similar to the fused deposition modeling (FDM).…”

Effect of Powder Recycling on Defect Formation in Electron Beam Melted Alloy 718

“…This may have a negative effect on the sintering behavior which may lead to an increased risk of ''powder smoking'' (ejection of powder from the powder bed), with the formation of LOFDs in the subsequent layer as a result. [1,31] As shown in Figure 10, the oxygen level in the powder is consistently higher than that in the bulk material of the solid samples, which means that there is a net release of oxide from the bulk material during the EBM build process. The reason for this may be twofold.…”

Processing of Alnico Magnets by Additive Manufacturing

“…The combination of parameters in the preheat step (e.g., line order, line offset, beam focus and beam speed) are tailored in such a way to add sufficient heat to the system to raise the temperature of the surface of the powder bed, while also allowing sufficient time for charge to dissipate between line scans until the powder is sintered. Only recently has work by Cordero et al modeled these charge and heating/sintering effects for a mono layer of spherical metallic powder with an oxide layer during preheating [31]. However, the local variations in particle size distribution, morphology oxide thickness in typical AM powders significantly complicate the prediction of the onset of scattering and require further investigation.…”

“…The somewhat high observed sintering temperature of the alnico powders (~1100 • C) is relatively close to the melting temperature of the material (~1257 • C), leading to difficulty in maintaining a consistent heat flux through the build chamber without over-sintering the powders during preheating. Surface temperatures below~900 • C were observed to be susceptible to charge induced powder scattering events (i.e., "smoking" [29][30][31]), one cause of premature failure of these runs. On the other hand, over-sintering would cause the entire layer to curl and separate from the powder bed, and the resulting vacuum gap breaks the conductive pathway and exacerbates the effect.…”