Some of the factors that can make CNC machining titanium difficult are:
Poor Thermal Conductivity - cutting tools end up taking the burden of heat generation.
“Sticky” or “Gummy” Properties - can lead to problems with long “stringy” chips as well as creating built-up edge.
Low Modulus of Elasticity - leads to spring back and chatter while machining.
High Work Hardening Tendency - further increases difficulty of machining.