Glue for attaching PLA pieces to titanium
I've been a fan of epoxies for unusual adhesion problems. I found on Amazon a product with titanium in the name, but there's a caution regarding polypropylene plastics. 



PLA is not of that family of plastic, which gives it a good chance of success. Epoxy is typically more viscous than cyanoacrylates, giving you a bit more control of the application, but also creating the need for care with "ooze-out."

The big glue company, Gorilla, also makes an epoxy that includes plastic and metal in the adhesion listing.



As PLA is somewhat sensitive to heat, one would consider that fast-cure epoxies generate more heat than slow-cure epoxy, but the amounts you'll be using are not likely to create enough for concern.