Welding Aluminum for high Vacuum
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  1. #1
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    Default Welding Aluminum for high Vacuum

    Having some trouble with this one. I weld stainless for hi-Vac all the time without much issue. Now, i have Al tubing 1/2" OD x .120" wall thickness that snakes back-n-forth across a large shroud. 2 fittings were snapped off while tightening (form factor is called VCR), and then brought to me with " please help, this thing costs as much as a good used car to replace".

    I have attached a pic of the old weld where it broke off, it is almost certainly a socket weld connection, fitting socketed over tube and then the joint lap-welded from outside (vacuum is from the outside, inside is for flowing some kind of cryogenic capable oil) . The fitting I'm trying to weld is "explosion welded" stainless to AL fitting, about $160. a piece.

    First time around there was significant porosity in both welds, both times at the point where i stopped. Soapy water test reveled bubbles across a 1/4" circle sized area. I remove that metal with a clean new coarse file and re-weld. One fitting is now fine, passes Helium vacuum leak-check. The other is kicking my ass.

    I get a tiny pin-hole , so small that the tubing will hold 50psi pressure under Argon for hours, but Helium check reveals a leak. Soapy water will show the tiniest of bubbles form. Some internet searches on vacuum sites reveals not much useful info
    1)Use Helium instead of Argon purge for sections thicker than Ľ” = not applicable
    2)Clean weld and filler rod with abrasive like Scotch-brite before welding. This is to reduce thickness of oxicde layer (which gets thicker over time). I sanded the old tubing inside and out , but My filler rods are ancient, and while they look immaculate, could have thick oxide layer. The Oxide causes porosity. Will try this next
    3)One thing I’m not sure of is which Alloy my filler actually is. All I know is that it’s “Rockmount Nassau Neptune TIG” and that it’s very expensive. I can't get an ER number for it though, not even from Rockmount.
    If anyone knows the best filler please chime in, any other info appreciated
    4) Welder is Miller Dynasty 350 with AC setting at 75% EN and 25% EP, 120 hz, 100 amps

    pictures are of the old factory weld, my tube prep, and the fitting (which appears to have Copper in the join between AL and stainless)
    Attached Thumbnails Attached Thumbnails img_3956.jpg   img_3957.jpg   img_3961.jpg   img_3953.jpg  

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  3. #2
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    Do you know what the alloy the tubing and fitting are made from? This will tell you what filler to use.
    Also I am surprised you are having success welding at 100 amps, I would start at 150 amps.
    I would also try using a brand new stainless wire brush instead of scotch brite to reduce the chance of contamination from the scotch brite particles

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    100amps is what i settled on because there is a mismatch in the tubing wall thickness of .120" and the fitting socket-wall thickness of .045" . Even at 100, i have to back-off the pedal once the puddle forms. at 150 amps, you will blow right thru .045 wall AL. Looking back, i should have used a hand-drill and ripped out some material from the tubing ID
    The fitting is 6061T6
    Quote Originally Posted by draganm
    Specifications: Atlas aluminum ATCR™ face-seal fittings are fully compatible with Swagelok VCR® and Parker-Hannifin VacuSeal® fittings.

    Materials: Body/Weld Interface: Aluminum 6061-T6
    Seal-Face: Stainless Steel 316L, Optional Titanium (commercially pure)
    Rotatable Nut: Aluminum 6061-T6, Optional Stainless Steel 304
    The tubing = no idea but i know the standard AL tubing is 6061,it's alloyed with both Magnesium and Silicon added, and the only ones that comes in .120" wall thickness.
    The other 2 tubing options are either 3003 "easy bend" AL tubing or possibly 5052 Corrosion resistant but neither comes in .120 wall, at least not from Mcmaster.

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    after searching high and low, no idea what " Neptune TIG" is actually made of, they won't release any info. It's tensile strength of 34Kpsi falls somewhere in between ER4043 =27kpsi and ER5356= 39Kpsi, and one of these (the 5356) would be completely wrong for my application.
    I think I'm going to try and rip out as much of this filler as possible and go back in with some 4043 rod.

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    Oxygen doesnt cause porosity with aluminium, hydrogen does. Aluminium is like 100X times as soluble to hydrogen when molten vs solid, when it freezes it pukes out the extra hydrogen as pin holes. In some cases ( likely the realm you're into) even the oils from your skin are enough to cause pinholes. Chase down and eliminate ANY source of hydrogen. Rubbing alcohol and acetone are your friends. Things like the parts being machined with a hydrocarbon based coolant can lead to tiny trapped pockets of hydrogen, more than enough to fuck up the kinda specs you're after.

    Avoid any abrasive that is aluminium oxide based. Human powered stainless brushes and things made of silicone carbide are good. Power tools have a chance of being "too much" and driving crap into the base metal.
    Smooth and steady, clean the living piss out of everything you touch and maybe up the EP range/time for more cleaning.
    Dip more filler, tighten up the ripples and make sure the toes of the weld are tying in nice and tight. Add a little circle movement to the torch to make sure everything's tied in properly.

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    Is brazing an option? I'm no welder but it seems like it wouldn't have as many issues given how common brazed fittings are

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    Call Miller tech support and discuss it with them. Good luck.

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    Quote Originally Posted by PlasmaOnTheBrain View Post
    Oxygen doesnt cause porosity with aluminium, hydrogen does. Aluminium is like 100X times as soluble to hydrogen when molten vs solid, when it freezes it pukes out the extra hydrogen as pin holes. In some cases ( likely the realm you're into) even the oils from your skin are enough to cause pinholes. Chase down and eliminate ANY source of hydrogen. Rubbing alcohol and acetone are your friends. Things like the parts being machined with a hydrocarbon based coolant can lead to tiny trapped pockets of hydrogen, more than enough to fuck up the kinda specs you're after.

    Avoid any abrasive that is aluminium oxide based. Human powered stainless brushes and things made of silicone carbide are good. Power tools have a chance of being "too much" and driving crap into the base metal.
    Smooth and steady, clean the living piss out of everything you touch and maybe up the EP range/time for more cleaning.
    Dip more filler, tighten up the ripples and make sure the toes of the weld are tying in nice and tight. Add a little circle movement to the torch to make sure everything's tied in properly.
    thanks, all that makes a helluva a lot of sense and good to know it's Hydrogen, i never heard that before. I did switch to stainless brushes, wound up dry re-machining as many parts as i could with a skim-cut, and increased time in EP to 30%. My first fitting wound up a failure, so i cut it off. wound up re-using an old fititng , skim-cutting the AL on the lathe, and then re-welding. Passed Helium leak down this morning. Ughh, this one pushed me up against a wall. Just goes to show even trusted sources like vacuum parts distributors don't know it all, they just said " use clean sand-paper". Now i know it's dry-machine, file, or SS wire brush only. I will come back and read this thread if anything like his comes back, but i told the Engineer, please DON'T over-tighten the fittings and break any more off.
    Quote Originally Posted by cscott92 View Post
    Is brazing an option? I'm no welder but it seems like it wouldn't have as many issues given how common brazed fittings are
    i have heard rumors of vacuum compatible Braze fillers, but they are very high silver content, very hard to flow out properly, and not easy to do. Any soot from the torch would also contaminate the enclosure, + danger of melting all the electrical wires i had to protect with that Fiberglass gloves in the pics. At best, perfectly Brazed and cleaned joints might do high-vacuum, but almost certainly not Ultra hi-Vac, UHV is a Bear, your literally trying to pump out individual gas molecules with a turbo-pump that spins at 30K RPM's.

    I did not check with Miller, and i haven't heard of them talk about vacuum before. Should of probably called their tech line.

    Just glad it's behind me

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    I would try 60-65% EN and manually pulse with the foot peddle. I run my machine cranked so I can run way more heat at first and then taper it back as the part warms up. Also you might try running around it a time or two without filler like you are welding cast to cook the crap out. Tig welding Aluminum is like stick welding steel where you can burn crap out to make the weld clean. Keep going over the weld zone not worrying about if the two pieces fuse together (its better if they don't) without filler until you have good clean aluminum, then you can start actually welding it.

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    Quote Originally Posted by draganm View Post
    i have heard rumors of vacuum compatible Braze fillers, but they are very high silver content, very hard to flow out properly, and not easy to do. Any soot from the torch would also contaminate the enclosure, + danger of melting all the electrical wires i had to protect with that Fiberglass gloves in the pics. At best, perfectly Brazed and cleaned joints might do high-vacuum, but almost certainly not Ultra hi-Vac, UHV is a Bear, your literally trying to pump out individual gas molecules with a turbo-pump that spins at 30K RPM's.

    I did not check with Miller, and i haven't heard of them talk about vacuum before. Should of probably called their tech line.

    Just glad it's behind me

    glad you got through it

    If brazing had worked, I would suggest TIG brazing. We don't call it a torch fer nuthin.

    Wouldn't it be funny if that 'as seen on TV' aluminum brazing stick was vacuum compatible.

    I temped at a place that built high vac semiconductor equipment years ago, can still remember the sound of all the vacuum pumps running, and the techs explaining how a fingerprint would outgas for a week............

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    Quote Originally Posted by Hardplates View Post
    I would try 60-65% EN and manually pulse with the foot peddle. I run my machine cranked so I can run way more heat at first and then taper it back as the part warms up. Also you might try running around it a time or two without filler like you are welding cast to cook the crap out. Tig welding Aluminum is like stick welding steel where you can burn crap out to make the weld clean. Keep going over the weld zone not worrying about if the two pieces fuse together (its better if they don't) without filler until you have good clean aluminum, then you can start actually welding it.
    thanks, i think part of the problem is we started with socket weld fittings, which looking back now was a huge mistake as the socket can trap all kinds of contaminants. From now on, butt-weld and tapered edges only

    Quote Originally Posted by gustafson View Post
    glad you got through it

    If brazing had worked, I would suggest TIG brazing. We don't call it a torch fer nuthin.

    Wouldn't it be funny if that 'as seen on TV' aluminum brazing stick was vacuum compatible.
    my brother bought pack of that stuff, if it's same stuff they advertise on TV, to fix a broken AL grill on a Plymouth. It has Cadmium in it, i told him he was nuts if he played with it. That stuff is so toxic, the EPA closed a shop that did Cadminum plating for us in the 90's


    Quote Originally Posted by gustafson View Post
    I temped at a place that built high vac semiconductor equipment years ago, can still remember the sound of all the vacuum pumps running, and the techs explaining how a fingerprint would outgas for a week............
    yeah, that's the world I'm in. Really tired of it, if i could retire tomorrow i would
    Thanks to good folks on this forum, i get thru it one day at a time. Not just welding, but the whole machine shop to worry about and constantly evolving technology

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    Not sure of the content of all of them but the bernzomatic version sds sheet says zinc aluminum and copper


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