Bonding Tungsten and/or Iridium?

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ian_krase
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Bonding Tungsten and/or Iridium?

Post by ian_krase » Fri Aug 04, 2017 5:52 am

As far as I can tell, tungsten thermionic emission filaments are bonded to passthrough electrodes (made of copper, SS, etc) via resistance welding. Conveniently, I have a high current transformer appropriate for spot welding of this sort, and just need to cobble together some pincers.

Unless... can I do this with continuous high current welding, or do I need capacitor discharge? And do I need a shield gas like argon?


Meanwhile, thoriated iridium (or possibly bariated or yttriated tungsten) is much better at emitting and more durable to air. Can I weld iridium to structural metals the same way? Is there any chance I can get useful filaments out of burned ionization gauges?

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Re: Bonding Tungsten and/or Iridium?

Post by John Futter » Fri Aug 04, 2017 7:54 am

Just bog standard transformer with variac on primary a good push to make switch on the primary of the transformer to pulse it on
I suggest 3-6 volt secondary
you need good sharp clean copper pincers to hold /weld

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Re: Bonding Tungsten and/or Iridium?

Post by ian_krase » Fri Aug 04, 2017 8:17 am

And this works for both tungsten and iridium to structural metals?

Is brass OK for pincher jaws?

Sarvesh Sadana
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Re: Bonding Tungsten and/or Iridium?

Post by Sarvesh Sadana » Fri Aug 04, 2017 11:08 am

It depends on the thickness of the tungsten. For thinner pieces (<.75 mm), continuous high current welding is fine. A simple hobby welder would do fine. Argon may be necessary, as it tends to oxidize. For thicker pieces, capacitive discharge welding is a must. In my experience, between 100-200 joules will give an excellent weld, no argon needed.

I'm not sure about brass. Perhaps a brass pincer with copper electrodes may work.

Sarvesh

ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post by ian_krase » Wed Aug 16, 2017 7:53 am

It seems to work. I had some fairly small W wire, and some stainless steel nuts, so I built a pair of brass pincers and successfully welded the tungsten to the nut. I basically stopped once the nut was glowing bright red around the tungsten. The large box is a variac, but I mostly just turned it to near the max. Jaw life is not what I would consider good (deep "scratches" under where the tungsten was); I may try brazing or otherwise attaching copper face plates to the jaws.

The welds are reasonably strong as long as one applies tension/shear only, any kind of peeling moment cracks them right off. I don't have a vacuum setup appropriate for testing this kind of filament (soon!) so that will have to wait. I also need to get some higher current and higher temperature (teflon? silicone?) power feed wires as I kind of toasted the ones that I have. The PVC insulation hardens up OK when they cool down... I have some hefty, high-flexibility cables meant for large R/C quadcopters that I might try.

Interestingly, the tungsten seems to be able to be snapped off the nut without leaving much of a mark at all so I might try just welding filaments to my feedthroughs without any connectors.


I'd be very glad of any advice on spotwelding tungsten to alumel, spotwelding iridium to anything, and whether I can anneal alumel.
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Jerry Biehler
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Re: Bonding Tungsten and/or Iridium?

Post by Jerry Biehler » Wed Aug 16, 2017 9:56 am

If anything is getting red you are using too much dwell time, resistance welding happens very quickly. And if it is coming off the nut without leaving anything then is its not welding properly. You need more current.

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Re: Bonding Tungsten and/or Iridium?

Post by Dan Knapp » Wed Aug 16, 2017 3:40 pm

I routinely spot weld tungsten and rhenium/tungsten filaments to 304 and 316 Stainless steel posts using a small spot welder made for welding dental appliances. The clamping posts on this welder are copper.

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Re: Bonding Tungsten and/or Iridium?

Post by John Futter » Wed Aug 16, 2017 7:00 pm

Ian
use copper for the jaws get rid of those alligator clips and solder your wires to the copper jaws or super tight bolted connection every milliohm in the way needs eliminating
time to weld should be sub second

ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post by ian_krase » Wed Aug 16, 2017 8:00 pm

Yeah, I'm going to set up better cable and connections when I can.

I also will do more tests with short cycles.

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Re: Bonding Tungsten and/or Iridium?

Post by Richard Hull » Thu Aug 17, 2017 4:26 am

Of course real commercial copper tips are very high in phosphorus content. My local welding shop got out of resistance welding and sold me about 35 lbs of electrodes for the copper scrap price.
They are rather big but usable and machinable to usable sizes. I have some modified ones on my Raytheon vacuum tube spot welder.

Richard Hull
Progress may have been a good thing once, but it just went on too long. - Yogi Berra
Fusion is the energy of the future....and it always will be
Retired now...Doing only what I want and not what I should...every day is a saturday.

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