Study on high speed milling of steam turbine blade materials - Differences in cutting characteristics of titanium alloy and stainless steel

Titanium alloy and stainless steel are used as steam turbine blade materials. However, their machining efficiency is low because they are difficult-to-cut materials. In particular, the high cutting point temperature and short tool life are major problems. Highspeed milling can reduce the cutting point temperature and tool wear. In this study, highspeed milling of a titanium alloy and stainless steel was investigated for the high-efficiency cutting of a steam turbine blade. In the experiment, workpieces were made of titanium alloy Ti-6Al-4V and stainless steel 13Cr. The experiment was conducted at cutting speeds from 100 m/min to 600 m/min. The flank wear increased rapidly with increase in the cutting speed. The loss of the coating on the flank of the end mill was confirmed via energy-dispersive Xray spectroscopy analysis. It was demonstrated that the cutting point temperature was higher than the heat resistance temperature of the coating. The cutting point temperature was analyzed using AdvantEdge FEM. It was found that the cutting point temperature at a cutting speed of 350 m/min or more was higher than the heat resistance temperature of the coating. On the other hand, in the case of the stainless steel 13Cr, the flank wear increased in proportion to the cutting speed, and the loss of the coating on the flank of the end mill was also confirmed. However, the loss of the coating was less than that in the case of the titanium alloy. It was found that the high-speed milling of the stainless steel did not reach the heat resistance temperature of the coating. The cutting characteristics of the high-speed milling of the titanium alloy and stainless steel differed, which was mainly attributed to the difference in the thermal conductivity. In the high-speed milling of the titanium alloy Ti-6Al-4V and stainless steel 13Cr, it was not possible to determine the factors that result in a low cutting point temperature. If the cutting point temperature is lower than the heat resistance temperature of the coating, high-speed milling may be possible. Therefore, the ways in which the cutting point temperature can be lowered will be examined in the future.