Page 1 of 2

Bonding Tungsten and/or Iridium?

Posted: Fri Aug 04, 2017 1:52 am
by ian_krase
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?

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 04, 2017 3:54 am
by John Futter
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

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 04, 2017 4:17 am
by ian_krase
And this works for both tungsten and iridium to structural metals?

Is brass OK for pincher jaws?

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 04, 2017 7:08 am
by Sarvesh Sadana
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

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 16, 2017 3:53 am
by ian_krase
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.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 16, 2017 5:56 am
by Jerry Biehler
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.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 16, 2017 11:40 am
by Dan Knapp
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.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 16, 2017 3:00 pm
by John Futter
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

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 16, 2017 4:00 pm
by ian_krase
Yeah, I'm going to set up better cable and connections when I can.

I also will do more tests with short cycles.

Re: Bonding Tungsten and/or Iridium?

Posted: Thu Aug 17, 2017 12:26 am
by Richard Hull
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

Re: Bonding Tungsten and/or Iridium?

Posted: Thu Aug 17, 2017 12:48 am
by Jerry Biehler
Short cycle is not going to cut it, you need more current. I am making a wild ass guess you are using a microwave oven transformer to build one. You only want about one or two turns around the core and that wire needs to be HEAVY, nothing at a model shop is going to do it. You really need something in the 4 to 2 gauge range. Then it is kept as short as possible to the tongs for spot welding.

A proper weld will leave material on the substrate when you do a pull test.

This guy made a pretty nice one: http://www.5bears.com/welder.htm

Re: Bonding Tungsten and/or Iridium?

Posted: Thu Aug 17, 2017 2:56 am
by ian_krase
The transformer is from a 1950s-era... machine (which also provided the housing for my variac) called a "Microbrazer". (some kind of hybrid between resistance welding and silver brazing?) It had a vacuum tube *and* a large relay.

The transformer has a lot more than 2 turns, and it looks like along the lines of double 12 gauge *solid* wire. I suspect it is indeed better suited to i.e. driving evaporation filaments than welding.

Re: Bonding Tungsten and/or Iridium?

Posted: Thu Aug 17, 2017 9:57 pm
by Jerry Biehler
Ok. Hats a resistance soldering unit. It will not work for spit welding. Too high of voltage and too low current. It's even a little small for evap filaments.

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 18, 2017 2:11 am
by ian_krase
I was successfully able to heat a (small) filament from "red hot" to "white hot" to "light bulb" to "bright light bulb" to "AAAAHHHH MY EYES" and even to "OH YIKES" with that thing, so that's what I am sticking to.

Does that mean I need to go down to Halted for some huge capacitors?

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 18, 2017 10:05 am
by Dennis P Brown
Not totally clear what you want but capacitors do not generally like being shorted; a spot welder is a massive short. Not sure it makes economic sense to add a cap bank to make up for low current in a x-former. I'd think buying a better x-former is the easier option. If it is just a power supply, then maybe that would work for a pulse system but not for continuous or long term supply for a filament.

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 18, 2017 4:20 pm
by John Futter
I know some spot welders use caps
stud welders being an example where they need quite a few thousand amps to to weld a high tensile 1" dia stud to a RSJ or UB.
All the good scientific welders i have used over the last 40 years have been low voltage timed spot welders
ie timer to xfmer primary a selector switch for a couple of output winding choices (current choice Hi/lo)
The nice little Beckman unit I have was made for welding mass spec and SEM filaments no caps in sight
I use this unit to make K type thermocouples almost on a daily basis

Re: Bonding Tungsten and/or Iridium?

Posted: Fri Aug 18, 2017 10:04 pm
by ian_krase
Yeah, i was looking at the potential for capacitative discharge but failed to find the requisite farad of electrolyts at a low price.

Re: Bonding Tungsten and/or Iridium?

Posted: Sun Aug 20, 2017 6:56 am
by Jerry Biehler
These resistance soldering units are only intended for a short duty cycle, run them for long at any major portion of their max current out and you will fry something. Most of smaller filaments/boats I have take about 150amps minimum. Trust me on this, I used to work on welding equipment for a living including spot welders.

You can also use batteries to make a spot welder. Hackaday has had several posts about homemade units: https://hackaday.com/?s=spot+welder

Re: Bonding Tungsten and/or Iridium?

Posted: Mon Aug 21, 2017 2:06 pm
by Richard Hull
We have been through this spot welding bit before in these forums.

Professional spot welders of the sort we need all use capacitors but they fire into a special transformer and not the welding lines.

Typical would be 500 volt capacitors of 20-200ufd (paper or plastic film, oil filled.... electrolytics are out) these are charged and a thyratron or SCR is used to place their voltage on a 120 volt transformer primary for a number of micro-seconds. The secondary, (usually a 2.5 volt 50 amp), are the weld lines.

I have used a normal 120 volt transformer with a 2.5 volt 50 amp secondary as noted above to successfully do all manner of spot welds in the early days of my fusor efforts until I obtained a formal Raytheon, vacuum tube, spot welder that has a joule rated selector switch.

Richard Hull

Re: Bonding Tungsten and/or Iridium?

Posted: Tue Aug 29, 2017 1:02 am
by ian_krase
I replaced my feed wires with 8 AWG (the largest that can fit into my terminal blocks) and directly soldered them to the jaws. I also silver-brazed copper jaw tips to the jaws.

I think it discernably works better.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 30, 2017 4:59 am
by Jerry Biehler
Based off of what? When you pull the wire off does the wire break and leave material behind? Is it slightly embedded in the base material?

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 30, 2017 6:33 am
by ian_krase
Yes and yes.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Aug 30, 2017 4:44 pm
by Bruce Meagher
Ian,

As Jerry and others have mentioned, ideally you want a transformer whose secondary has one or two turns to make proper spot welds. You’re after gobs of current for a very short interval (with pressure). Check out the attached two documents for a great primer on the subject. Of course you can weld two pieces together by pumping power in like you are doing, but a proper spot weld requires lots of current for a short duration to create an ideal nugget. Dissimilar metals and different thickness material require special attention to the electrode design to make the nugget form properly.

Last year I was struggling with spot welding a tungsten filament to an NPS support for a vacuum tube. George clued me in to the proper surface preparation requirements for the material being welded so don't forgot about that aspect too.

Bruce
Resistance welding.pdf
(1.1 MiB) Downloaded 1786 times
Fundamentals of Resistance Welding.pdf
(1.38 MiB) Downloaded 2083 times

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Sep 27, 2017 5:03 pm
by Justin Fozzard
Here are some photos of the innards of a Hughes 250 Watt-Seconds impulse welder that I use for electrode welding.
The capacitors are charged to a maximum of about 450V and are discharged into the output transformer using surprisingly small thyristors.

Front Panel:
Hughes Welder 1.jpg
The supply and output transformers:
Hughes Welder 2.jpg
The 450V capacitor bank and thyristor pcb. The large green wirewound resistor at bottom right is in series between the capacitor bank and thyristors.
Hughes Welder 3.jpg
Underside of thyristor pcb with the thyristors at top left.
Hughes Welder 4.jpg
The output transformer and primary winding selector switch for altering the pulse shape.
The five turn secondary winding is made from several layers of thin copper tape about 1 inch wide:
Hughes Welder 5.jpg
Justin Fozzard.

Re: Bonding Tungsten and/or Iridium?

Posted: Wed Sep 27, 2017 7:10 pm
by Richard Hull
My 1950's Raytheon vacuum tube spot welder, (got it for free), also charges its oil capacitor bank to about 500 volts and is an adjustable 40 to 200 joule unit. Like your system, it has a monster transformer with 1/8" thick strap copper output windings. and the switching is done with large vacuum tube thyratrons. It gives a nice "thump" sound as the weld is made.

Small, capacitive discharge, spot welding systems give a consistent power delivery to what is welded each time.

Richard Hull