Smoking effect in Electron Beam Melting

In Electron-beams systems, the back-scatter of electrons at the layer surface produces local electrical charge build-up and electrostatic ejection of particles from the powder bed: ’smoking’.

During Electron Beam Melting, electrons interact with the material to transfer energy and electrical charges. If repulsive electrostatic forces are greater than the forces holding particles to the powder bed, particles may be ejected from the layer [1]. This effect is known as ‘smoking’ and can cause the bulk displacement of loose powder [2, 3].

‘smoking’ caused by electrostatic repulsion: a: distributed powder bed; b: applied beam and c: ‘smoking’ = cloud of charged powder particles [1]

Smoking can be minimised by pre-sintering the surface layer with the rapid scan of a low-energy beam prior to actual processing/melting.

Small quantities of helium gas can also be injected during melting to dissipate charge from the melt surface.

Pre-sintering in Selective Laser Melting systems is not necessary, as photons do not cause charge build-up.

References
[1] T. R. Mahale: ‘Electron beam melting of advanced materials and structures’, PhD thesis, North Carolina State University, 2009.
[2] C. Eschey, S. Lutzmann and M. F. Zaeh: ‘Examination of the powder spreading effect in electron beam melting (EBM)’, in ‘Solid
freeform fabrication symposium’, Austin, TX, 2009, 308–319.
[3] M. Kahnert, S. Lutzmann and M. F. Zaeh: ‘Layer formations in electron beam sintering’, in ‘Solid freeform fabrication symposium’, Austin, TX, 2007