mig (ARGON) welding Alu pipework

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derekm
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mig (ARGON) welding Alu pipework

Post by derekm »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

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 »

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
derekm
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Re: mig (ARGON) welding Alu pipework

Post by derekm »

John Futter wrote:
> 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
I will
btw a circle a doesnt have a start or a finish.- hence the turntable and if there are cracks or porosity there's always Technoweld
Linda Haile
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Re: mig (ARGON) welding Alu pipework

Post by Linda Haile »

As I stated in my first post in this thread, heat the parts to be welded first. This will considerably reduce the risk of cracks.

A hot air gun is ideal for this, and can heat to 500 degrees C.

You then need a lot less power for welding, so you can also use a slower wire feed rate.

This also drives out any moisture which reduces porosity as well.

(Melting point of ally is six to eight hundred degrees C. The hotter it is before welding the less the distortion and cracking due to localized heating.)
Jerry Biehler
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Re: mig (ARGON) welding Alu pipework

Post by Jerry Biehler »

On the outside of something yes you can mig it, but the ID of a 16mm hole like a vacuum fitting is supposed to be welded, good luck.

Problem with aluminum mig is I have seen weeping even on fluid tanks. I cant imagine what it would be like on vacuum.

But its still worth a try. Nothing to loose but a little aluminum.
derekm
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Re: mig (ARGON) welding Alu pipework

Post by derekm »

Had some lessons from a professional welder on both TIG and Mig with Al (about 15 hours). he showed its quite possible to produce good welds with cheap MIG welders with Al. And then he got me to do it. The first thing he did was to modify the shroud so that the tip protruded 4mm from the shroud. then a few runs to get the power and feed right. On my set that was quite low power and a high feed rate. Then the crucial part was tilting the torch to ensure the shield gas stayed ahead of the arc. Get that right and it was then quite easy.
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