[morzh]

Nothing is going to hold it the way it is; I also doubt it could be soldered. It is polished hardened steel that does not lend itself to soldering very well.

If you want to increase the adhesion, you should attempt (and I am not sure how to do this with objects this small) roughen the surface of the ball, and, desirably, the surface of the convex.
Perhaps an adhesive exists that would do it without roughening, but I do not know adhesives that well.