I have cast Titanium in place in 14K using a vacuum caster,
nothing out of the ordinary. I haven’t tried Niobium, maybe
their heat treatment characterisics are different? Titanium is
heat-colorable. After the piece has been cast the Titanium has
been heat colored a greyish color evenly over the whole surface,
which could have a more subltle use than the bright colors you
can get with heat coloring by a small torch. The integrity of
the Titanium seemed to be fine.
As you said there is no way to fuse the metal to it, so you need
to mechanically lock the Titanium / Niobium piece in place in
the wax before it is cast, in such a way that it will stay there
after it is cast, ie: undercut/dovetail, drill holes in insert
with wax coming through, etc…
I have not tried stone setting in Titanium, but it is really
hard to move, Niobium is a lot easier to work.
How do you secure the niobium to the 18K in your rings?