As for the Titanium: D1.9- Structural Welding Code- Titanium.

And yes, Clause 3 deals with WPS's and Welder Qualification. 

Other alloys, it all depends upon their usage and applicable codes.  If they are being used within D1.1 then a PQR would be in order. 

Have a Great Day,  Brent

Hi

The first question is the application of the product. Once you have that, the relevant "code" will become more apparent e.g:

- Pressure vessel = ASME VIII = ASME IX welding
- Piping =  ASME B31.3 = ASME IX welding
- Structural = AWS D1.9

Obviously there will be others depending on the relevant product and industry and country you are operating in.

Regards
Niekie

In addition to the various industry codes there are many proprietary company or project specific standards for welding titanium.
 
I recently worked on an Exxon project and we used a special spec created by Exxon that was dedicated only to titanium. 

Titanium is especially sensitive to oxidation, so that would be the main difference between different Ti welding specs.  Some customers want ultra-sensitive PPM oxygen monitors and even chamber only welding.
 
There are sometimes different color standards which control the level of oxidation for the completed weldment.  It really depends on the application how strict the controls are.  It would not be wise to recommend one catch all spec for this material since each project will have its own unique requirements.