Copper.... I used to work for a Company that made Manual and Automated Resistance Welders. I dealt with the assembly end - often had to fabricate Buss Bars made of half-hard copper then apply the silver paste coating. Cutting, bending, and machining the copper flat bars was bad enough - try tapping it. Not easy. The drilled holes often were bell mouthed as the copper "grows" from the heat generated from drilling. We had to use plenty of a fast-evaporating tapping fluid to keep the material cool; other wise the heat makes the copper grab the tap, it either taps oversize or snaps the tap... This shop was in Massachusetts; I learned how to "Cuss like a Kennedy"!!