In the root part of the electron beam, power density must be lowered by vapor in the keyhole. Observations are opposite, however. There we can see a decrease of weld cross-section and of heat-affected zone width. So, could the sharp shape of electron beam welding be explained with beam self-focusing?
Dear Mr. Mladenov,
the shape of the welding seam depends on different factors: high voltage, focus level regarding surface and focus distance etc. If you use a low voltage level (maybe 60kV) the cross section at the root part is wider and has a round shape compared to a higher voltage level (maybe 150kV) which is spiky and thin (at the same power!).
Therfore I don´t see that big influece of vapour on the electron beam, the welding depth is stabilized in that moment, when the energy of the beam is as high as the energy needed to establish the keyhole.
The decrease of heat affected zone can be explained by the different heat conduction (2d to 3d) and the reduced power (heat) at the root pass.
This question is very important for electron beam powder bed fusion, as the powder is a very poor conductor, and results in large amounts of vapor generation.
To me, this question could be answered if you could electrical bias your weld sample during keyhole EBW. The electrical bias could either drive your positive charges away or not.
Also, another way to look at this problem, is with everything kept constant, how does the addition of small amounts of low melting point alloys affect the keyhole. Ti, Ti-6Al-4V and Ti-48Al-2Cr-2Nb would be interesting.
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