Bonding Tungsten and/or Iridium?

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Jerry Biehler
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Re: Bonding Tungsten and/or Iridium?

Post 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
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post 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.
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Re: Bonding Tungsten and/or Iridium?

Post 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.
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post 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?
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Dennis P Brown
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Re: Bonding Tungsten and/or Iridium?

Post 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.
John Futter
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Re: Bonding Tungsten and/or Iridium?

Post 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
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post 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.
Jerry Biehler
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Re: Bonding Tungsten and/or Iridium?

Post 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
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Richard Hull
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Re: Bonding Tungsten and/or Iridium?

Post 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
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post 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.
Jerry Biehler
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Re: Bonding Tungsten and/or Iridium?

Post 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?
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post by ian_krase »

Yes and yes.
Bruce Meagher
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Re: Bonding Tungsten and/or Iridium?

Post 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
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Fundamentals of Resistance Welding.pdf
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Justin Fozzard
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Re: Bonding Tungsten and/or Iridium?

Post 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.
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Richard Hull
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Re: Bonding Tungsten and/or Iridium?

Post 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
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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
Jerry Biehler
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Re: Bonding Tungsten and/or Iridium?

Post by Jerry Biehler »

Mine is an old hughes, I think max 40J. It uses a mercury contactor to dump the cap into the transformer.
ian_krase
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Re: Bonding Tungsten and/or Iridium?

Post by ian_krase »

While my inadequate resistance brazing transformer was working pretty well for soldering tungsten filaments to SS conductors, I decided I wanted something more. .

But... The whole machine was rather dangerous and clumsy (exposed line current wiring attached to variac by screw terminals, disconnect only on the hot line, no momentary switch, etc), ugly, and the pincers I made were never very good to start with and didn't get much better after I set them on fire trying to solder the power cables to them. To make things worse, the cables were very stiff when cold, but the insulation would quickly become melty and soft after a few seconds of welding.

So... I built this using the same transformer.

Box contains transformer along with a momentary DPDT toggle switch that switches the primary and a fuse holder for safety. Timing is by hand, in typical messy-and-simple Krase Laboratories style. Hold-pressure is also by hand -- for a while I thought this thing wasn't working (was barely getting workpieces red hot) but turns out I was using far too much pressure.

The tongs are very simple and a vast improvement. Two beefy terminal blocks (called "distribution blocks" are used to bond the hefty but flexible silicone insulated #6 (?) cables to the 1/4 inch copper tongs. Each has a brass section brazed to the end to accommodate the set screw to attach the pincer tips which are made of 8 AWG copper electrical wire. Note that brazing will make the copper DEAD SOFT and you need to hammer it a little to re-harden it up enough for further machining.

The terminal blocks also serve as mechanical structure; with a few extra holes drilled they are both epoxied and screwed to a leaf spring "hinge" made of thin stainless. I think a real hinge might be a good upgrade.

I broke both a drill and a tap trying to tap the copper, hence the brass.

Last picture is closeup of 19 gauge SS wire welded with this spot welder. The upper right weld shows molten squeeze-out, and it's easily possible to melt the wire in half with this system (usually when applying too little hand pressure)
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Richard Hull
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Re: Bonding Tungsten and/or Iridium?

Post by Richard Hull »

Resistance welding is made very easy if capacitive discharge is used, as you just dial a voltage to get a rather precise joule energy in every weld.
Professional machines for the vacuum tube industry usually had a selector switch to choose joule energy.

My first resistance welder was a 10 volt 60 amp transformer feeding professional spot welder electrodes (welder supply). The electrodes helped, (no sticking), but it was still an art to judge pressure and on-time.

The Raytheon tube welder made small spot welds a snap.

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
The more complex the idea put forward by the poor amateur, the more likely it will never see embodiment
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