In general, the welder is qualified to weld with all the F6 filler metals that are applicable to the process and they are included in the applicable WPS(s).

For example, if the process is GTAW, the welder can weld carbon steel, stainless steel as well as nickel alloys if they can be joined with a F6 filler metal.

Even if there is a change in the A number due to the combinations of different base metals the welder is still qualified because the A number isn't an essential variable for welder qualification.

If the welder switches to a different process, FCAW for example, the F6 filler metal used with FCAW isn't compatable with the GTAW process, so he would not be able to utilize the filler metal used with FCAW. However, because the F6 filler metals used for GMAW are the same as those used for GTAW, it is perfectly acceptable to use GMAW filler metals as rods with the GTAW process.

In all cases the welder must follow an approved WPS listing the F6 filler metal if he is welding to a code such as AWS or ASME.

Best regards - Al