I have to "fix" some 50 micron Tungsten wire to Copper/Brass. Has anyone soldered this material? If so, what solder was used and what advice can you give? Note that this is on a scale that is too small for a mechanical connection to be used eg screws, clamps etc
I would first look at welding under a microscope using a tweezer handpiece.
If you don't have access to such a welder with fine low current control, contact the makers and they should be able to help you. The machine should be under $10K for a good one.
You could also consider using a platinum plated tungsten wire, which would be more resistant to corrosion and easier to deal with. They are used in some kinds of instrumentation.
Assuming you need a Tungsten wire for high temperature heating, then :
Use a tiny copper pipe, hose and crimp to the filament ends. Screw or solder the copper terminals to PCB or whatever you need. In such way you will have a good contact which won't excessively heat when the filament will be turned on. In place of copper pipe you can use also a wrapped copper foil, but firmly crimping the copper will give best performance.