84 Aerospace Materials Handbook a hardening effect that slows further machining and may also cause warping in small parts.In addition,work hardening is severe during the cutting process. To minimize work hardening in cutting superalloys,sharp cutting edges should be used,with positive rake angles,adequate clearance angles.Dwelling.the process of stalling the tool feed while the tool is still contacting the workpiece,can result in severe work hardening and cause damage to both the workpiece and the tool.Therefore,it should be avoided.Machines and setups should have sufficient power and rigidity to keep vibration to a minimum.Feed rate and cutting depth should be set so that in following passes cutting is done below the previously work-hardened layer.Vibration can be reduced by using the largest possible tools and holders,and by limiting overhang [17]. The most common chip-making processes used for superalloys are turning,grinding,milling, broaching,drilling,boring,and so on.The conventional chip-making processes provide much higher metal removal rates than processes such as electrochemical machining(ECM). 2.3.1 CUTTING TOOL MATERIALS In machining nickel-based superalloys,tooling-related technologies are treated seriously.The appropriate consideration of these technologies can lead to optimum production output,consistency of machined product,and value-added activities.In order to produce good quality and economical parts,a cutting tool must have enough hardness and strength,toughness,and wear resistance.The characteristics of common cutting tool materials are summarized below [16,19]. 2.3.1.1 High-Speed Steel The most commonly used alloying elements for HSS are:manganese,chromium,tungsten, vanadium,molybdenum,cobalt,and niobium (columbium).The M and T series HSS materials are the most frequently used in cutting tools.The M series represents tool steels of the molybdenum type and the T series represents those of the tungsten type.Although there seems to be a great deal of similarity among these HSS,each one serves a specific purpose and offers significant benefits in its special application.Some of the HSS are now available in a powdered metal (PM)form. Many surface treatments have been developed in an attempt to extend tool life,to reduce power consumption,and to control other factors which affect operating conditions and costs.One of the more recent developments in coatings for HSS is titanium nitride by the physical vapor deposition (PVD)method. 2.3.1.2 Cemented Tungsten Carbide The term "tungsten carbide"describes a comprehensive family of hard carbide compositions used for metal cutting tools,dies of various types,and wear parts.In general,these materials are composed of the carbides of tungsten,titanium,tantalum,or some combination of these,sintered or cemented in a matrix binder,usually cobalt.The hardness of the carbide is greater than that of most other tool materials at room temperature and it has the ability to retain its hardness at elevated temperatures to a greater degree,so that greater speeds can be adequately supported. It is important to choose and use the correct carbide grade for each job application.The difference between the right and wrong carbide for the job is crucial for success or failure. It is advisable to consider coated carbides for most applications.Numerous types of coating materials are used,each for a specific application.The most common coating materials are:titanium carbide;titanium nitride;ceramic coating;diamond coating;and titanium carbo-nitride.In addition, multilayered combinations of these coating materials are used.Coated carbides are more resistant to abrasive wear,cratering,and heat.They are more resistant to work material build-up at lower cutting speeds.Therefore,tool life is extended,reducing tool replacement costs.Coated carbides permit operation at higher speeds,reducing production costs.84 Aerospace Materials Handbook a hardening effect that slows further machining and may also cause warping in small parts. In addition, work hardening is severe during the cutting process. To minimize work hardening in cutting superalloys, sharp cutting edges should be used, with positive rake angles, adequate clearance angles. Dwelling, the process of stalling the tool feed while the tool is still contacting the workpiece, can result in severe work hardening and cause damage to both the workpiece and the tool. Therefore, it should be avoided. Machines and setups should have sufficient power and rigidity to keep vibration to a minimum. Feed rate and cutting depth should be set so that in following passes cutting is done below the previously work-hardened layer. Vibration can be reduced by using the largest possible tools and holders, and by limiting overhang [17]. The most common chip-making processes used for superalloys are turning, grinding, milling, broaching, drilling, boring, and so on. The conventional chip-making processes provide much higher metal removal rates than processes such as electrochemical machining (ECM). 2.3.1  Cutting Tool Materials In machining nickel-based superalloys, tooling-related technologies are treated seriously. The appropriate consideration of these technologies can lead to optimum production output, consistency of machined product, and value-added activities. In order to produce good quality and economical parts, a cutting tool must have enough hardness and strength, toughness, and wear resistance. The characteristics of common cutting tool materials are summarized below [16,19]. 2.3.1.1  High-Speed Steel The most commonly used alloying elements for HSS are: manganese, chromium, tungsten, vanadium, molybdenum, cobalt, and niobium (columbium). The M and T series HSS materials are the most frequently used in cutting tools. The M series represents tool steels of the molybdenum type and the T series represents those of the tungsten type. Although there seems to be a great deal of similarity among these HSS, each one serves a specific purpose and offers significant benefits in its special application. Some of the HSS are now available in a powdered metal (PM) form. Many surface treatments have been developed in an attempt to extend tool life, to reduce power consumption, and to control other factors which affect operating conditions and costs. One of the more recent developments in coatings for HSS is titanium nitride by the physical vapor deposition (PVD) method. 2.3.1.2  Cemented Tungsten Carbide The term “tungsten carbide” describes a comprehensive family of hard carbide compositions used for metal cutting tools, dies of various types, and wear parts. In general, these materials are composed of the carbides of tungsten, titanium, tantalum, or some combination of these, sintered or cemented in a matrix binder, usually cobalt. The hardness of the carbide is greater than that of most other tool materials at room temperature and it has the ability to retain its hardness at elevated temperatures to a greater degree, so that greater speeds can be adequately supported. It is important to choose and use the correct carbide grade for each job application. The difference between the right and wrong carbide for the job is crucial for success or failure. It is advisable to consider coated carbides for most applications. Numerous types of coating materials are used, each for a specific application. The most common coating materials are: titanium carbide; titanium nitride; ceramic coating; diamond coating; and titanium carbo-nitride. In addition, multilayered combinations of these coating materials are used. Coated carbides are more resistant to abrasive wear, cratering, and heat. They are more resistant to work material build-up at lower cutting speeds. Therefore, tool life is extended, reducing tool replacement costs. Coated carbides permit operation at higher speeds, reducing production costs