TIG welding a closed structure issues

[Forced_Firebird]

I noticed when I close up a void such as sealing the end of a tube (pipe etc), just as the weld closes, it bubbles out as if the inside of the structure was pressurized. Here is a pic of the most recent one...






It's a simple business card holder I made for my sister in law. When I got to the end of the last weld to seal up the pipe, it was almost as if the inside of became pressurized - as if there was moisture inside that steamed up and was enough to blow the metal out of the weld pool. Then of course since the air was rushing out, the weld oxidized.

It's mild steel, so I didn't bother to back purge, but maybe that would help. Anyone else experience this and/or have a solution?

Thanks,

John

[ryanjones2150]

What's worked for me in the past is to almost seal it up, let it cool completely, then get back in there and zip it up. <br />
<br />
Gas expands when it's heated, that includes air. <br />
<br />
Another option would be to drill a small hole somewhere on the bottom to let the pressure out.

[dave powelson]

What's worked for me in the past is to almost seal it up, let it cool completely, then get back in there and zip it up. <br />
<br />
Gas expands when it's heated, that includes air. <br />
<br />
Another option would be to drill a small hole somewhere on the bottom to let the pressure out.

Ditto the above.
Cleaning, swabbing the tube ID's prior to welding to eliminate oil inside, is a kinda, must do, additionally.
Takes a minute or two to do, which is significantly less than dealing with oil created pressure/porosity problems after the fact.

[Jayare123]

Yep. Let it cool before the last little bit. Or weld it cold and fast. Cold and fast works well for me. I make a lot of restaurant equipment gotta cap 1/16th wall tube all the time. I never have blowout problems but other guys in the shop do.

[Sriffe]

Provided your puddle isn't blowing up or out..try to remove your filler rod and cool the heat from your cup.. while still manipulating the puddle till it cool hope it helps

[Oldgrandad]

Yup the whole thing is hot and you get a blow out. Easiest fix for me is drill a small hole where it won"t be seen. Second choice is let it cool before that last dab, to close it up. Never thought about it til just now. I've never had a blow out when I'm purging a part.

[Ltbadd]

Never thought about it til just now. I've never had a blow out when I'm purging a part.

I would say if you're purging then it's not a sealed part (otherwise how could you purge?) so this wouldn't be an issue, or am I missing something?

To the OP, would like to see a photo when this is finished.

[ryanjones2150]

You would most likely still have openings of some sort if you were back purging.

[Oldgrandad]

Brain Fart, your right vent for purge to leave. My bad. lol.

[Forced_Firebird]

My thought on the purge is that the argon would be left inside. Could purge till the last bit of weld. That way if the air inside is expanding, it would push the argon out, not oxygen. Good idea on letting it cool, guys. I had a similar issue once when building an aluminum tank. Not thinking, I welded up the tank before drilling the holes for the threaded fittings - had the same issue. Ended up cutting out some of the welded after making the holes, and re-welded it up.

Ltbadd That is the finished product. Rubbed it down with 300 grit paper before welding, also cleaned/sanded near the seems (inside and out), then washed them down with acetone. After the welding was finished, it got a clear coat of satin.