converting 220 to 3 phase?

You need a single to 3 phase convertor. That can be either a solid state device that does it using electronics or a rotating machine - i.e. a single phase motor driving a 3 phase generator.
You really need to check how many kVA your 3 phase welder requires. If its 3 phase its likely to be a big unit. Your domestic (assumed) single phase supply can only deliver a limited kVA and if the new welder needs more than that, converting your single phase to 3 phase will be a waste of time anyway and you will need to apply for a proper 3 phase supply.

J
May 2009