Titanium (Ti-6Al-4V).
Thermal conductivity challenges, work hardening, and rigid milling parameters.
The Heat Problem
Titanium has an exceptionally low thermal conductivity (roughly 1/4 that of steel). In aluminum or steel machining, 70-80% of the heat generated by the cut is evacuated in the chip. In titanium, the chip cannot absorb the heat fast enough. Instead, the heat is forced directly into the cutting tool.
If surface footage (SFM) exceeds roughly 150-200, the extreme localized heat causes the carbide binder to break down, resulting in rapid crater wear and catastrophic failure. Machining titanium is an exercise in heat management.
Rigidity & Work Hardening
Titanium is "springy" (low modulus of elasticity). As the tool engages, the material tends to push away and rub rather than cut. This rubbing generates heat and causes the surface to work-harden. If a subsequent pass attempts to cut this work-hardened layer, tool life drops to zero.
To prevent this: ensure maximum rigidity in the setup, use climb milling exclusively, and ensure the tool is constantly feeding (never dwell or let the tool rub in a corner).