Hi Artgirl,
I'm not sure about the silicon bronze as I haven't done that before, but your post has me wondering about a few things.
Is this sculpture going to be placed outside? If so, the wind catching on the roofing panels may put a lot of stress on the welds, depending of course on your design. I wonder if the welds would be strong enough to compensate.
My first alternative thought was mechanical rivits for holding the copper and panels together.
Also, you could weld some steel half threaded couplings to the panels and screw in the copper, or bolt a copper end cap to the panel and solder the pipe.
Anyway, I'm sure you have something specific in mind, so I'll quit rambling on.
Good Luck,
Tim