Tool Condition Monitoring in Machining Superalloys 79 FIGURE 2.2 An aircraft turbine engine blade under machining. In machining Inconel material as well as other nickel-based alloys,high tool wear rates occur due to very high time-varying mechanical and thermal loads exerted on the cutting edges of the cutter.As tool wear deteriorates accuracy and surface finish of the parts being machined,the maxi- mum length that can be machined is thus very short,and tools have to be replaced sooner.If cut- ting tools are not replaced in time,tool breakage may occur.The workpiece being machined may become a scrap then.In addition,tool breakage may also cause severe damage on the machine tool itself.Repairing a machine tool or removing the causes for process interruption costs production time and money. 2.1.1 MICROSTRUCTURE The most important characteristics of nickel as an alloy base are the high phase stability of the face- centered cubic (fcc)nickel matrix,and the capability to be strengthened by different approaches. The surface stability of nickel is readily improved by alloying with chromium and/or aluminum [2]. The atomic number of nickel is 28,with an atomic weight of 58.71.Its crystal structure is fcc, as shown in Figure 2.3,from ambient conditions to the melting point,1455C,which represents an absolute limit for the temperature capability of the nickel-based superalloys.The density under ambient conditions is 8907 kg/m3 [6]. FIGURE 2.3 Face-centered cubic lattice.Tool Condition Monitoring in Machining Superalloys 79 In machining Inconel material as well as other nickel-based alloys, high tool wear rates occur due to very high time-varying mechanical and thermal loads exerted on the cutting edges of the cutter. As tool wear deteriorates accuracy and surface finish of the parts being machined, the maxi￾mum length that can be machined is thus very short, and tools have to be replaced sooner. If cut￾ting tools are not replaced in time, tool breakage may occur. The workpiece being machined may become a scrap then. In addition, tool breakage may also cause severe damage on the machine tool itself. Repairing a machine tool or removing the causes for process interruption costs production time and money. 2.1.1  Microstructure The most important characteristics of nickel as an alloy base are the high phase stability of the face￾centered cubic (fcc) nickel matrix, and the capability to be strengthened by different approaches. The surface stability of nickel is readily improved by alloying with chromium and/or aluminum [2]. The atomic number of nickel is 28, with an atomic weight of 58.71. Its crystal structure is fcc, as shown in Figure 2.3, from ambient conditions to the melting point, 1455°C, which represents an absolute limit for the temperature capability of the nickel-based superalloys. The density under ambient conditions is 8907 kg/m3 [6]. FIGURE 2.2  An aircraft turbine engine blade under machining. FIGURE 2.3  Face-centered cubic lattice