They used steel sealing washers in this location for two reasons.....to prevent galvonic corrosion between the aluminum head, steel fitting, and the ethylene glycol, and because the steel was able to hold tight the fitting to the head throughout the massive temperature changes and vibrations from an engine running 4,000-6,000 RPM. The copper will obviously work ok, but can vibrate loose or corrode to the point of leaking. Only, if you want to use copper then try to change the copper washer every two years when you change your coolant. Follow Wallace's advice about sealing. Years ago I tried copper washers, too, and it leaked. I ended up having to put sealant on the threads and on the copper sealing washer. I thought maybe my mating surfaces weren't true, or that my threads were worn, but that wasn't the case.