Copper is an incredibly active metal, so you need to go to great lengths to prevent any atmospheric contamination, as well as using only oxygen-free pure copper filler rod. The electrical wire is pretty pure copper, but generally not O2-free.

I'd suggest getting some proper TIG filler rod, then setting up a box or cabinet you can fill with argon purge gas to do the welding in. If that isn't possible, get a "gas lens" for your tig torch and run the argon at about half again what you would for regular welding. With a regular gas cup, the velocity of the argon stream can entrain atmospheric oxygen and nitrogen and that gets absorbed into the weld area causing pinholes.

Of course, when welding copper you need a lot of amperage at the start, or a clean pre-heat, in order to get enough heat into the weldment area. Once you get the puddle established, you back off on the amps to just the minimum amount required to maintain the puddle and no more, otherwise you risk boiling the copper in the puddle, which will also cause pinholes.

There you have the theory of it - putting it into practice is more difficult, unfortunately. Still, it can be done so I'm sure you'll work it out. Heavy copper isn't a lot of fun to weld if you need X-ray quality welds. If you have much of it to deo you'll definitely appreciate having a water-cooled TIG torch, that's for sure.

Best of luck with it,

Rich

In my experience with any metal that means .......too hot in one way or another. Am I supposed to be psychic and understand you better?

Not really a fix, but a sprayed copper on the area then polished back to bright. I have done this fon a gate (copper on steel) and it seemed to work great. I have a metal sprayer machine ($18,000) new but it sure can do some amazing stuff!!!!!

Pinhole porosity can be a problem with gold castings. Usually, the first remedy I'll try is to simply hammer the surface to tighten it up and close any voids. Even if I have to to fuse additional metal onto the area I'll often hammer it a bit before finishing -- just in case. I have no experience TIG welding copper, so there may be fundamental differences I don't see. If the problem is just appearances and the "pinholes" are clean, this may help.

For those who are following this thread, I just received an email from Ken (metal222) who reported that he had tried a 75%He/25%Ar shield mix and it eliminated the pinhole problem completely. Something to keep in mind for the future.

Rich