Re: Bonding Tungsten and/or Iridium?
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?
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.
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.
The 450V capacitor bank and thyristor pcb. The large green wirewound resistor at bottom right is in series between the capacitor bank and thyristors.
Underside of thyristor pcb with the thyristors at top left.
The output transformer and primary winding selector switch for altering the pulse shape.
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.
Mine is an old hughes, I think max 40J. It uses a mercury contactor to dump the cap into the transformer.
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.
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.