mig (ARGON) welding Alu pipework

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derekm
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mig (ARGON) welding Alu pipework

Post by derekm » Wed Aug 11, 2010 12:34 pm

I have:
Pumps (E2M18, Diffstak 63/150)
SS chamber (old turbo pump casing) & Glass Bell jar
valves
HT Psu
support metalwork, & base plate.

Now I need the pipework, my plan is to use thickwall Ally tubing (1/8") with flanges turned from blanks cut from 1/4" ally flat, to create NW16, NW25 and NW40 Spools.

Has anyone here used MIG to weld Ally to create the Vacuum pipework? .. Any experiences you can pass on.

Linda Haile
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Re: mig (ARGON) welding Alu pipework

Post by Linda Haile » Wed Aug 11, 2010 1:08 pm

Derek, porosity and distortion will be the main issues MIG welding the pipework.

Heat up the parts to be welded first, a hot air gun is good for this. This will help to minimize distortion. Obviously only weld the inner faces (you can tack weld the outer faces first if necessary.)

derekm
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Re: mig (ARGON) welding Alu pipework

Post by derekm » Wed Aug 11, 2010 3:56 pm

Since these are NW flanges I'm probably going to weld the outside. almost definitely onthe NW16. I'm setting up a motor driven turn table to turn the pipe at a constant speed. I hadnt got as far as using shielding gas from other than the gun.

Linda Haile
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Re: mig (ARGON) welding Alu pipework

Post by Linda Haile » Wed Aug 11, 2010 6:43 pm

Derek, welding around the outside will result in cavities on the inside which will be hard to evacuate once the gas is in 'molecular flow'. Much better to have the weld on the inside. All professional equipment is manufactured this way. You are likely to end up with 'virtual leaks' which will likely lead to problems later.

I can elucidate further if required.

myID
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Re: mig (ARGON) welding Alu pipework

Post by myID » Wed Aug 11, 2010 7:36 pm

Hi-

I think you will get problems welding on the outside- if you have not done so far you should search for "virtual leaks"...

Greets
Roman

derekm
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Re: mig (ARGON) welding Alu pipework

Post by derekm » Wed Aug 11, 2010 7:58 pm

there are physical difficulties in welding the inside of 16mm dia pipes, which may cause more virtual and real leaks than the outside.... I have plenty of pipe, wire and Argon I'll try to do both and test them.

Linda Haile
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Re: mig (ARGON) welding Alu pipework

Post by Linda Haile » Wed Aug 11, 2010 9:05 pm

Derek, the reason the pros only weld on the inside (apart from tack welding on the outside) is that if you have a leak on the inside weld it will be impossible to find and will appear as a virtual leak if the outside is fully welded.

Roman and I aren't saying don't do it, we are just pointing out the likely pitfalls.

I would recommend three tack welds on the outside and full weld on the inside. I assume you are familiar with chamfering and weld preparation conventions.

Jerry Biehler
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Re: mig (ARGON) welding Alu pipework

Post by Jerry Biehler » Thu Aug 12, 2010 2:13 am

Mig is just not going to work unless you have pulse and a spool gun. And even with that, which I have, I can guarantee you will have a mess. Tig is the only way to go on this. I have KF-16 fittings that are aluminum and tig welded so it is not too terrible.

FWIW, I am a certified Miller Tech.

derekm
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Re: mig (ARGON) welding Alu pipework

Post by derekm » Thu Aug 12, 2010 7:28 am

Jerry Biehler wrote:
> Mig is just not going to work unless you have pulse and a spool gun. And even with that, which I have, I can guarantee you will have a mess. Tig is the only way to go on this. I have KF-16 fittings that are aluminum and tig welded so it is not too terrible.
>
> FWIW, I am a certified Miller Tech.

That's strange since I have had success mig welding Aluminium without either a spool gun or "pulse" . Its not a walk in the park, and the speed of travel is high which takes quite a bit of practice, hence the preparation in having a driven turn table. Care in pushing the shield gas ahead of the torch is also needed. My teflon lined gun doesn't appear to have issues that some have.
if I weld the outside, it will only be the outside, if the inside, only the inside. I will obviate tacking by the joint positively locating part together prior to welding (see the Lesker KF weld flanges)

John Futter
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Re: mig (ARGON) welding Alu pipework

Post by John Futter » Thu Aug 12, 2010 9:34 am

Derek
In my opinion you should listen to Jerry
From my limited knowledge moving fast on Al with mig on small parts tends to leave microcracks especially at the start and finish of the weld -fine if you are building a boat but no good for high vac work.
All the people I have seen doing this type of small work including myself use TIG and even then it takes a bit of trial and error to get the DC offset on the AC setting right.
have a go anyway and report back

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