First thing is you dont call it welding, it is soft solder are hard solder. Silver solder is about $75.00 a # I think that is what I heard. We use silfus solder thats a hard solder and you can use a bronge rod. What do you want to use the copper for, then we can decide what solder to use.
Some definitions: –Soldering is the joining of two metals (alike or dissimilar) by the coalescence to their surfaces of a molten binder metal called “solder.” In this joining method only the solder is melted. The metals to be joined are merely heated below their melting point(s) but above the melting point of the solder.
–Brazing is similar in concept to soldering, except that it uses a binder metal (brazing metal) that melts above 840 degrees Fahrenheit. As with soldering, the metals to be joined are not themselves melted.
–Welding involved joining metals by heating them to a molten state and allowing them to unite.
Most of the heat joinery of plumbing pipes involves soldering (“sweating”). Some brazing is used in industrial applications and steamfitting, and some really big high pressure pipes are actally welded.
Technically, the heat and chemical processes that are used in plastic pipe joinery are a form of “welding.”