Inconel 718 has a number of special controls both before and after welding.  A precipitation hardenable super alloy that has been around a long time.  

Don’t know the purpose or service conditions of your weldment, but this alloy wants strong engineering behind both design and fabrication.  Repair also requires special heat treatments before and after welding.

STT May be an option, Lincoln may even have a waveform in their bag of tricks that may be helpful.   Inco 718 doesn’t really behave exactly like stainless from the welders point of view with GTAW and certainly not with GMAW.

If welder skill is an issue, you need to rethink your approach to get that skill by training or hiring, or consider subcontracting the work.