Welding Copper Flashing. The steel is used for strength the copper is used for conductivity and the aluminum is used for its combination of cost and conductivity. A weld wont stick to copper.
I have also used copper electric wire as filler with good results. The attractive look of copper roofing materials is also not to be overlooked. In this photo the jaw has been formed and tack welded in place.
You can use silicon bronze filler wire and a TIG welder to weld steel to copper most easily while maintaining as strong of a bond as possible between the two metals.
As compared to welding it is performed at a lower temperature. The aluminum industry uses flash welding to join aluminum steel and copper in various current-carrying conductors called busbars. A weld wont stick to copper. In building construction today some lead flashing is welded but soldered copper flashing is much more common in America.