I've pondered quite a bit on the advisability of doing that and there's a recent thread on the forum - I think the chap lives in Spain or maybe Portugal? He had it welded up twice and both times it failed again. Being a twist beam design I would guess that, at least part of the axle spanning the width of the vehicle, is not going to be mild steel but some sort of high strength and therefore high carbon steel? If this is so then welded repairs will be prone to failing because unless welded under very specific and controlled conditions high strength steels become brittle when welded. There's quite a lot about this problem appertaining to crash repairs where serious structural repairs are required due to the use of high strength steels in modern monocoque construction.
There's also the possibility of problems with your insurance company if an accident happens which they can attribute to this sort of repair. Personally, given that the replacement axles are now available at such a reasonable price, I don't think I'd risk doing anything more than welding up a wee hole in a spring pan.