Electron Beam welding of Titanium

[Jianglin Huang]

Hello everyone. My name is Jianglin Huang, I am studying my PhD in University of Birmingham, UK. My research focus on electron beam welding of titanium, aiming to understand why porosity ouccurs in titanium weld. The welding is performed in high vacuum condition, about 10 minus 7 torr, and the base metal have beem cleaned very well.

Right now, the mechanism of porosity generation is still not clear, if anyone have idea of that, please give me suggestion and help. I really apprecaite your help. My email is "hxj783@bham.ac.uk" Thank you

[smithboy]

Hey Jianglin,
Good to have you here. We have a few folks that use electron beam welding, so maybe they can chime in here and give you some ideas.

[pulser]

First, have you got any typical cross section photos showing the weld shape and the voids?

Porosity in EBW could be from hydrocarbon contamination, outgassing from the base metal itself, or keyhole cold shutting.

It may be very helpful to try to characterize the typical size, shape, and location of the voids through radiography and macro sectioning. Generally, gas porosity in welds is spherical, although I am guessing this may not be entirely true of some very narrow and rapidly solidified EB welds. But it may be very obvious from looking at the voids that they result from a cold shut rather than from gas evolution.

It seems that contamination could be eliminated as a source.

Don't know enough about Titanium, but I would guess that outgassing from the base metal is unlikely. Are there any inclusions or precipitates like nitrides or oxides associated with the Ti alloy?

It is faily common to produce cold shut voids, areas of the keyhole that do not fully backfill because of factors such as too narrow/sharp keyhole, excessive travel speed, excessive keyhole depth, and fluid flow dynamics possibly associated with Marangoni flow.

I had a void problem in a 0.250" deep EBW in 300 series SS. In a 25" long weld, there were several hundred assorted, rather large voids, not very spherical. I suspected it was a cold shut type problem. I did a very careful comparison of the four possible combinations of high and low travel speed, and small and large beam size, and ajusted the beam current in each case to produce a consistent depth of 0.025". I found that a large beam diameter and slow travel speed reduced void occurance to less than 5 or 10, as compared to serveral hundred for small beam size/fast travel welds. Whether the problem was soley irregular back filling of the joint, possibly associated with Marnagoni flow, or if it was also related to gas evolution issures, there is logic as to why the larger keyhole and slower speed would help in both cases.

[Jianglin Huang]

Thank you for your information.

Right now, I am considering the hydrogen segregation as the mechanism of porosity generation. Hydrogen contamination also exist because the using of grease and oil in the welding.

The welding process is performed at high vacuum condition about 10 minus 7 Torr. Base metal is IMI834 (Ti-5.8%Al-4%Sn-3.5%Zr-0.7%No-0.5%Mo-0.3%Si-0.06%C). There is no low boiling point element in the base metal. The width of the welding bead is about 1mm, and the thickness is about 5mm. So I do not think the instability of the keyhole can play as a significant role in the forming of porosity.

From the optical microscopy of a weld bead, the porosities have round shape, about 100-200 micrometers, distributing in all of the weld bead. I have the photoe, but I do not know how to upload right now

Problem at present is that the mechanism of hydrogen segregation in Titanium is unknown. In literately, it is widely believed that solubility is much lower in solid state than liquid state, Hydrogen atoms then leave their position during solidification and by combining together, from hydrogen molecules by rejecting of the advancing solid/liquid interface. But the problem is how much hydrogen is needed to form bubble?

From Jianglin Huang

Originally Posted by pulser

First, have you got any typical cross section photos showing the weld shape and the voids?

Porosity in EBW could be from hydrocarbon contamination, outgassing from the base metal itself, or keyhole cold shutting.

It may be very helpful to try to characterize the typical size, shape, and location of the voids through radiography and macro sectioning. Generally, gas porosity in welds is spherical, although I am guessing this may not be entirely true of some very narrow and rapidly solidified EB welds. But it may be very obvious from looking at the voids that they result from a cold shut rather than from gas evolution.

It seems that contamination could be eliminated as a source.

Don't know enough about Titanium, but I would guess that outgassing from the base metal is unlikely. Are there any inclusions or precipitates like nitrides or oxides associated with the Ti alloy?

It is faily common to produce cold shut voids, areas of the keyhole that do not fully backfill because of factors such as too narrow/sharp keyhole, excessive travel speed, excessive keyhole depth, and fluid flow dynamics possibly associated with Marangoni flow.

I had a void problem in a 0.250" deep EBW in 300 series SS. In a 25" long weld, there were several hundred assorted, rather large voids, not very spherical. I suspected it was a cold shut type problem. I did a very careful comparison of the four possible combinations of high and low travel speed, and small and large beam size, and ajusted the beam current in each case to produce a consistent depth of 0.025". I found that a large beam diameter and slow travel speed reduced void occurance to less than 5 or 10, as compared to serveral hundred for small beam size/fast travel welds. Whether the problem was soley irregular back filling of the joint, possibly associated with Marnagoni flow, or if it was also related to gas evolution issures, there is logic as to why the larger keyhole and slower speed would help in both cases.

[aczeller]

wow... this stuff is a lot more technical than i ever thought. i knew it would be interesting ( i really like the "rare" welding types... especially friction-stir welding... it jsut looks so darn cool), but not this in-depth. i had to read that last post about 3 or 4 times just to get a general idea of what you guys are talking about. hehehehe just out of curiosity, what is the difference between EBW and Arc welding? just wondering.

thanks,

Later,
Andy