Hard material does not automatically mean hard to cut
The remarkable strength-to-weight ratio and highly corrosion-resistant properties of titanium have resulted in the ever-growing use of this important engineering material in many demanding sectors. Current developments make the cutting of the material more effective.
The production of critical structural parts from titanium ensures their required performance and reliability whilst significantly reducing components mass. Although relevant to all users of titanium, the enhanced strength and reductions in weight that the material delivers are of particular importance to the aerospace industry, as these advantages improve the aircraft's' performance and improve fuel economy.
The negative trade-offs produced by the use of titanium are the many problems uncounted for when machining this difficult-to-cut material. When used in metalworking industries, the word “titanium” normally relates not only to pure titanium but also to titanium alloys. In accordance with metallurgical properties, depending on the present elements, there are several groups of titanium: commercially pure titanium (unalloyed), α-, β-, α-β- and other alloys. It is sometimes stated that titanium machinability is similar to that of austenitic stainless steel. This proposition is more or less true if it relates to commercially pure titanium, although it is totally wrong with respect to treated α-β- and especially β-titanium alloys.
Machinability rating depends heavily on the type of titanium and its treatment. The machinability of the widely used annealed titanium TiAl6V4 is approximately 35-40% less than annealed stainless steel AISI 304. However, if we take the machinability of the aforementioned titanium grade as 100%, the so-called “triple 5”, titanium 5-5-5-3, a major manufacturing headache for many machine shops, features machinability characteristics that are twice as difficult .
Modern machine tools allow operators to apply advanced machining strategies and to employ one-hit production methods. However, the typically low cutting speeds used in the machining of titanium severely limits machine tools’ efficiency potential and results in the cutting tool becoming the weakest element of the whole technical production system. In short, the cutting tool determines the productivity boundaries when machining titanium, and such has become a major factor in the quest for a radical improvement of this situation.
Due to the low thermal conductivity of titanium, the main problem in cutting this material is the generation of heat. Poor heat transfer leads to considerable thermal loads being directly transferred to the tools' cutting edge. Also, less of a problem when machining steel, titanium’s modulus of elasticity contributes to vibration during cutting; as a result, surface finish and accuracy problems can be encountered.