Selection of a welding method depends on various factors. If you are trying to do a very general fabrication weld of a component, then just simple GTAW process with IN718 weld filler wire would just work fine. I am assuming the material is in solution HT condition. But be aware that cast IN718 could have laves phases that might cause some issues. If the cast IN718 has been properly heat treated to dissolve laves phases, and the material is generally "metallurgically clean" then you should have no problems.