Capacitive Discharge welding of thermocouples to 3/8" steel plate.

[30yearLabtech]

I have been using a HOTSPOT capacitive discharge welder to mainly weld the tips of type K thermocouples. We needed to expand the use of capacitive discharge welding because we have a project that requires me to weld thirty 26-gauge type K thermocouple wires to a 3/8-inch thick steel plate for heat dissipation testing. We purchased a new HOTSPOT II welder to do the job because it has a greater range of operating power. I had no problem, well as much you run into trying to capacitive weld thirty 26-gauge thermocouples to a 3/8-inch steel plate. Since that time I have heated the steel plate and thermocouples up to about 600°F several times. Two of the thermocouples I am using have broken free and I cannot get the welder to reattach them to the surface of the steel plate. Though I can get either of the HOTSPOT welders to weld new thermocouple wires, I cannot get them to weld the ones that broke free from the steel plate back onto the steel plate. I also can't get the 26-gauge thermocouples to weld the two wires together. I have cleaned the steel plate and thermocouple wires mechanically using a file and wire brush. I can get a new thermocouple wire to weld but not the ones that have been heated. I can also get either welder to weld other small metal objects. Has the heating of the steel plate and thermocouples changed there structure somehow that will not allow the discharge welder to operate properly? I do not want to either get new 26 gauge type K thermocouple wire or a new steel plate to try to get the setup testing again. Can anyone give me any idea what might be happening between the thermocouple wires and that steel plate that has caused the capacitive discharge welder to not operate as intended?

 

[berkshire]

Most Capacitive welders work on the principle of sending a short duration high current through the parts to be welded. For thermocouples and wires this requires a small area at the initiation of the weld which rapidly melts and the pressure of the welder consolidates the weld. If you have (Cleaned off) a part and attempt to re use it. It may be too flat. i.e. there may be too much surface area at the point of contact to enable proper melting.
B.E.

You are judged not by what you know, but by what you can do.

 

[EdStainless]

When you try to re-weld a TC bead how far back did you cut the wire?
You cannot reweld the same bead, too much oxidation from the first try.
Same reason why you can't just reweld to the same spot on the plate.
A wire brush will not remove oxidation, it just makes it look shiny.


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P.E. Metallurgy, consulting work welcomed

 

[btrueblood]

As Ed says, you need clean, unoxidized surfaces to make decent welds. We used to use an argon purge when tacking thermocouples to stainless and other alloys, to help mitigate oxide formation and improve the weld adhesion.

 

[EdStainless]

And Berk is correct, shape matters. The units don't deliver enough current to weld flat items together, they rely on the very limited contact area for local heating.

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P.E. Metallurgy, consulting work welcomed

 

[berkshire]

To add to what I said earlier, Take a Look at a brand new stud for a stud welder. It has a pin like structure in the center of the stud. when the stud welder is activated, that pin instantly vaporizes and the pressure of the stud gun completes the weld. Capacitive discharge welders for wires tend to put out more current than stud welders because the wires have more surface area than a stud.
B.E.

You are judged not by what you know, but by what you can do.