Tool Condition Monitoring in Machining Superalloys 83 Cutting force/power consumption:Measured by the power required to remove a unit volume of material under specified machining conditions,or the forces acting on the tool. Surface finish.The surface finish achieved under specified cutting conditions. Chip-form/chip breakability. Commercially pure nickel has poor machinability on the basis of almost all the criteria.Tool life tends to be short and the maximum permissible rate of metal removal is low.The tools fail by rapid flank wear plus deformation of the cutting edge,at relatively low cutting speeds.With high-speed steel (HSS)tools,a recommended turning speed is 50 m/min (150 ft/min)at a feed rate of 0.4 mm (0.015 in)per revolution.Tool forces are higher than when cutting commercially pure iron.The contact area on the rake face is very large,with a small shear plane angle and very thick chips [18]. When cutting superalloys at low cutting speeds,a built-up edge is formed.As the speed is raised,the built-up edge disappears but very high temperatures are generated even at relatively low speeds in the flow-zone at the tool/work interface.The temperatures are often high enough to take into solution the dispersed second phase in the nickel alloy,and may be well over 1000C [18]. Since superalloys are metallurgically designed to retain high strength at elevated temperatures, the stresses in the flow zone are very high.The result is a destruction of the cutting edge under the action of shear and compressive stresses acting at high temperature. In terms of the cutting tool materials,because of the high strength and work hardening of nickel alloys,for many operations such as drilling and tapping small holes,broaching and most milling operations involving interrupted cuts,HSS tools are chosen based on the toughness.These tools must be run at quite low speeds,but they are often the only choice available with small tooling. When the operation allows for larger tools,carbide (usually WC-Co alloys of medium to fine grain size)provides a good first choice for turning and some milling and drilling operations because of the higher speeds and longer tool life.The selection of a reasonably strong carbide grade used in conjunction with a positive rake angle will usually give good results in turning, provided machine horsepower and setup rigidity are adequate.Relatively large diameter short holes can be handled with carbide drills on rigid machining centers and numerically controlled (NC)lathes [17]. On the other hand,it is rare to find carbide tools operating at a speed as high as 60 m/min.The steel-cutting grades of carbide are usually worn more rapidly than the WC-Co grades.Coated carbides have been found to offer limited advantages.When cutting the most advanced aerospace superalloys,however,the inadequacy of cemented carbide tools becomes apparent [18]. The cost of machining the nickel-based aerospace alloys is very high.Metal removal rates are limited by the ability of conventional tool materials to withstand the temperatures and stresses gen- erated.Much effort is now being put into employing ceramic tools to increase the efficiency of these operations.Using both sialon and A12O/SiC whisker ceramics,cutting speeds up to 250 m/min are now employed for the machining of nickel-based gas turbine disks.Much effort has to be put into the machine tools,tooling,and the details of the operation to achieve success [18]. 2.3 MACHINING TECHNIQUES FOR SUPERALLOYS Machining plays a very important role in superalloy part manufacture.However,the machinability of superalloys is poor,and the allowable cutting speeds for superalloys are only 5-10%of those used for steel,which contribute to much of the high machining cost in machining superalloys. Compared to other materials,the most significant characteristic of nickel-based superalloys is that they are usually much stronger at metal cutting temperatures.Superalloys possess poor thermal diffusivity,which leads to high cutting tool tip temperatures.While superalloys maintain good strength at high temperatures,common tool steels begin to soften. Superalloys contain hard carbides in their microstructure,which makes superalloys abrasive. Superalloys work harden rapidly,and the high pressures produced during machining causeTool Condition Monitoring in Machining Superalloys 83 • Cutting force/power consumption: Measured by the power required to remove a unit volume of material under specified machining conditions, or the forces acting on the tool. • Surface finish. The surface finish achieved under specified cutting conditions. • Chip-form/chip breakability. Commercially pure nickel has poor machinability on the basis of almost all the criteria. Tool life tends to be short and the maximum permissible rate of metal removal is low. The tools fail by rapid flank wear plus deformation of the cutting edge, at relatively low cutting speeds. With high-speed steel (HSS) tools, a recommended turning speed is 50 m/min (150 ft/min) at a feed rate of 0.4 mm (0.015 in) per revolution. Tool forces are higher than when cutting commercially pure iron. The contact area on the rake face is very large, with a small shear plane angle and very thick chips [18]. When cutting superalloys at low cutting speeds, a built-up edge is formed. As the speed is raised, the built-up edge disappears but very high temperatures are generated even at relatively low speeds in the flow-zone at the tool/work interface. The temperatures are often high enough to take into solution the dispersed second phase in the nickel alloy, and may be well over 1000°C [18]. Since superalloys are metallurgically designed to retain high strength at elevated temperatures, the stresses in the flow zone are very high. The result is a destruction of the cutting edge under the action of shear and compressive stresses acting at high temperature. In terms of the cutting tool materials, because of the high strength and work hardening of nickel alloys, for many operations such as drilling and tapping small holes, broaching and most milling operations involving interrupted cuts, HSS tools are chosen based on the toughness. These tools must be run at quite low speeds, but they are often the only choice available with small tooling. When the operation allows for larger tools, carbide (usually WC–Co alloys of medium to fine grain size) provides a good first choice for turning and some milling and drilling operations because of the higher speeds and longer tool life. The selection of a reasonably strong carbide grade used in conjunction with a positive rake angle will usually give good results in turning, provided machine horsepower and setup rigidity are adequate. Relatively large diameter short holes can be handled with carbide drills on rigid machining centers and numerically controlled (NC) lathes [17]. On the other hand, it is rare to find carbide tools operating at a speed as high as 60 m/min. The steel-cutting grades of carbide are usually worn more rapidly than the WC–Co grades. Coated carbides have been found to offer limited advantages. When cutting the most advanced aerospace superalloys, however, the inadequacy of cemented carbide tools becomes apparent [18]. The cost of machining the nickel-based aerospace alloys is very high. Metal removal rates are limited by the ability of conventional tool materials to withstand the temperatures and stresses gen￾erated. Much effort is now being put into employing ceramic tools to increase the efficiency of these operations. Using both sialon and A12O3/SiC whisker ceramics, cutting speeds up to 250 m/min are now employed for the machining of nickel-based gas turbine disks. Much effort has to be put into the machine tools, tooling, and the details of the operation to achieve success [18]. 2.3  MACHINING TECHNIQUES FOR SUPERALLOYS Machining plays a very important role in superalloy part manufacture. However, the machinability of superalloys is poor, and the allowable cutting speeds for superalloys are only 5–10% of those used for steel, which contribute to much of the high machining cost in machining superalloys. Compared to other materials, the most significant characteristic of nickel-based superalloys is that they are usually much stronger at metal cutting temperatures. Superalloys possess poor thermal diffusivity, which leads to high cutting tool tip temperatures. While superalloys maintain good strength at high temperatures, common tool steels begin to soften. Superalloys contain hard carbides in their microstructure, which makes superalloys abrasive. Superalloys work harden rapidly, and the high pressures produced during machining cause