This is one of my favourite ways to tackle seized fixings. The extreme heat of the welding process heats up the fixing which then contracts on cooling which all helps to loosen it too.Would the weld have to be professional 9r could I just tack it at the sides were top nut sits
"professional"? well, you do need to be good enough to get full penetration with your weld so It's probably not a job to attempt if you're not reasonably proficient. Doesn't need to be to North Sea Pipe Welder's standards though. So, assuming you're talking about the type of wheel fixing on our Fiats (not sure what your car is?) So I mean a bolt which goes through the wheel and screws into the hub? (as compared to a threaded stud sticking out of the hub which you "hang" the wheel on before screwing a nut onto it) So, assuming we are talking about the first type, you need a nut which has a big enough bore to allow you to feed either your welding rod, or wire if you're MIG welding, through it so the arc can be struck between the end of the rod/wire and the bolt itself. If the nut is too small the arc will tend to strike between the nut itself and the rod which won't allow you to get a good melt into the wheel bolt. I like to use my MIG welder, set to quite a high heat so I can be sure the initial arc gets a good chance of melting the end of the bolt. Then it's just a matter of being sure to initiate the arc to the end of the siezed wheel bolt and not the nut, then filling up the middle of the nut with weld. Let it cool and undo. If you try to undo it too soon the weld will still be soft and the nut will just shear off. I do it this way because access to the sides of wheel bolts is often very restricted on alloy wheels, although much better on steel wheels. Also welding down the middle of the nut tends to stop so much spatter getting on the alloy of the wheel. Having said all that I've never had to remove a wheel nut this way as our boss provided a very good set of hammer on wheel nut remover tools. All you had to do was sign them out of the stores. However I have removed quite a number of broken manifold and other studs like this and this method works very well.
The technique if you have studs and nuts is a bit different, so won't waste time discussing here.