My understanding of tubing and the end strength after joining is that both welding and brazing create a HAZ but that the welded one is closer to the joint then the brazed one. So a tube specifically butted for welding will have a longer thin section, similar transitions and a shorter thick portion. The reason to butt, as I was told years ago, was to lighten up the frame, as the center section of the tubes are not stressed much and have had no heating they can be thinner and still strong enough. Andy.