Welding in an enclosure (bubble)

[dieselofdog]

The workshop has a bubble which has been collecting dust. There is a Ti job on the books so I thought it would be good idea to get it out and set it up.

After filling with bubble with 100% Ar, the purge meter reading oxygen 0.5%, I ran a bead with GTAW on some carbon steel which produced porosity. Should I still be using the normal shielding gas supply to my torch while in the bubble?

 

[EdStainless]

As long as your gas is pure Ar, yes.
0.5% is still fairly high, you should be able to do better than that.

Take some scraps of Ti and run a bead on them. What color do you get?

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Perhaps you know this, but you can't just fill the bubble (glovebox) with argon- you need to do a sweep purge with an amount of gas equal to at least four volumes of the enclosure to remove the oxygen, and harder still, all the nitrogen, if you're welding Ti! If you're at 0.5% oxygen, that means you might be as high as 2.5% nitrogen and that's WAY too much! N embrittles Ti too, not just oxygen. In fact, four volumes is just a rule of thumb- you will probably need way more than that.

You'll need to do that purge-out every time you bring material into or out of the glovebox also- at very least, in the antechamber of the glovebox assuming it has one.

Do you need shield gas while welding in a glovebox? I suspect so, but you should not need either backing gas or a trailing shield, nor to be paranoid about cutting off the end of your filler metal that you've just heated when you take it away from the purge gas stream before it cools down (which is second nature to most welders and a very hard habit to break). If these things weren't true, there'd be no point in having the "bubble"...

 

[EdStainless]

If you can purge enough to get less than 20ppm (less than 5ppm is ideal) O2 then you can weld without shield gas in the box. We also used to check dew point, we would get to about -50F. Our Ar was about -100F.
We used to warm all of our parts (to help drive off moisture) and then put them in the afternoon.
Then with a low flow purge they would sit overnight.
If you get more discoloration than just a light straw color then you are not clean enough.

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P.E. Metallurgy, Plymouth Tube

 

[tbuelna]

Argon is denser than air, so when purging the glovebox after placing a part inside you need to make sure the flow of argon is adequate to also purge all air within the internal volume of the part.

 

[kingnero]

... and you purge from below and outgoing flow is near the top.

 

[dieselofdog]

Thanks for the info.

I have had another go. I was able to get oxygen down to 0.03% (just purge ran the purge for longer) I was then able to weld without suppling the torch with shielding gas and get a nice shinny finish on the Ti.

 

[EdStainless]

We always kept scraps in the box, before any real welds were done the operator had to run a bead on a scrap and show that there was no color.
Winter is better, dryer. Getting good welds in Aug is difficult.

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P.E. Metallurgy, Plymouth Tube

 

[moltenmetal]

Starting with liquid argon helps with the dryness...

Welding on scrap to demonstrate that you're good to go is a very good idea.

Sounds like you've got things sorted. Good luck- titanium is a wonderful material, but it certainly can be a pain in the @ss to weld!

 

[EdStainless]

Yes, we always worked from Ar lq, even if it was GP45 cylinders.
We had a welder that would use a plasma torch to freehand weld together sheets of 0.008" Ti sheet. The torch was about the size of crayon.

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P.E. Metallurgy, Plymouth Tube