Copper-Silicon Cast Alloy

You don't weld copper-silicon casting alloys like normal copper. The silicon makes the melt more fluid, which is good for casting, but can lead to unclean welds when welding. The material tends to crack and the weld becomes brittle if you get too hot. So you have to keep the heat input low and let the weld cool slowly after welding, otherwise you'll get stresses. Clean preparation and a suitable filler material are the be-all and end-all, then the part will hold.