Welded a few different rear arms/stays, but whether or not it will be successful will depend on many factors. The material/type of alloy, the location of the crack, the weld prep, the welding itself, and the post weld treatment.
Yours looks quite severe. If I was doing it, I'd want to replace the entire tube, which is time consuming and not always cost effective. An alternative is to sleeve the break internally. Again not always possible if the tube tapers or access to the end of the tube is limited.
More pictures would help, and knowing what the bike is....?
