it has been pretty common for racks to be made with the ends of tubes to be pressed together with a hole drilled for the bolt. I was thinking about doing this for a decaleur, which doesn't take much load. With a little practice, you could crush the ends of a tube with a vise or even just a hammer. I feel like the manufacturers don't use heat, I wouldn't

For racks, I braze in a flat piece of metal for the attachment points.