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Thread: A tubing question
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27th Apr 2016, 10:22 AM #1Golden Member
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A tubing question
I'm doing my usual tube frame something-or-other at the moment and I'm running into something that occasionally occurs but it's been happening more than usual. The whole frames I'm building are sealed up, mitred corners at various angles. When I'm finishing off sections sometimes, I get a reverse pop and a big crater appears. It's like the opposite of a zinc pop but inwards instead of outwards. I can then fill it in but have to be careful to really feed filler or it can turn into a hole. Sometimes I can even see it happening in slow motion, where the puddle disappears into a crater but when I stop it's filled in.
The pops are quiet. Not like a scary zinc or other contaminant pop, that sometimes makes me jump. It just sounds like an offgassing sound and just disappears inwards.
Pertinent details:
TIG, 1.6mm x 25mm x 25mm duragal tube cleaned on the outside only, 1.6mm er70s6 (probably), painted to nearby the weld, it's not a gas problem, 1.6mm 2% lanthanated tungsten, mostly happens on tight fillets (or maybe just because that's where I do my last welds around the tube)
I kind of thought it's due to the trapped gasses inside the tube expanding/contracting with the heat and having nowhere to go. Maybe if I drilled a relief hole somewhere it might help. Other theory is the galv on the inside pops inwards, leaving behind a concave hole behind it.
It's non-structural application and not a big deal but annoying and I'd rather eliminate the problem rather than have to manage the consequences afterwards.
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27th Apr 2016, 11:19 AM #2
Presumably this is happening only when welding the last part of what becomes a sealed member, in which case it's almost certainly trapped air pressure. Drill a relief hole (1/16"-1/8" is usually enough) to let the pressure escape. If the member has to be completely sealed then the relief hole can then be filled at low amps such that there's not enough heat build up to repeat the problem.
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28th Apr 2016, 08:26 PM #3Golden Member
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Totally agree. Alternatively you try and put a bit of heat on another part of your tubing network, as it cools, the air inside would contract to counteract air expansion at your weld site - maybe. I've always just let it cool and zip up the last bit quickly.
Cheers
- Mick
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29th Apr 2016, 10:18 AM #4
As thin as it is, Duragal will have a gassing effect inside the weld pool. It may not be always visible but is there.
I do agree with Gavin that that the sealing of the tube can exacerbate this gassing.
Grahame
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29th Apr 2016, 10:26 AM #5Golden Member
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OK thanks guys. I wish I'd drilled holes first. Much easier on the press and I could have prepainted them like I did most of the rest of the members rather than have raw steel holes. I'll remember next time.
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