Why is it difficult to machine titanium alloy?

      Titanium alloy’s machinability is 20-40% of 45# steel’s. If hardness is less than HB300, adhesion of cutters will occur; if hardness is more than HB370, titanium alloy is difficult to machine. Titanium alloy with hardness between HB300-370 is easiest to machine. Main reasons that influence its machinability are as follows. 1) Gas impurities (such as oxygen, nitrogen and hydrogen) have a great influence on titanium alloy’s machinability, because titanium has high level of chemical activity and it is easy to combine with gas impurities. When the temperature is over 600℃, titanium is oxidized and forms the embrittled layer, i.e. α structure layer; hydrogen embrittlement is produced when it combines with hydrogen; hard and embrittled TiN forms when it combines with nitrogen. 2) Titanium alloy has low plasticity, which significantly influences plastic deformation during cutting.Titanium alloy’s deformation coefficient is only 1, even less than 1, while normal carbon steel’s deformation coefficient is around 3. During cutting, there is a small contact area between cuttings and the cutter’s front face, which increases pressure and local temperature in the contact area and makes the cutter wear quickly. 3) Serious hardening will occur when titanium alloy is machined. 4) When C>0.2%, hard carbides will form, making the cutter wear due to abrasive particles and decreasing the machinability.