Tool Condition Monitoring in Machining Superalloys 87 parameter is the side cutting edge angle.It must be large enough to provide clearance,but small enough to give adequate support to the cutting edge.The nose radius,which joins the end and side cutting edges,strengthens the tool nose and helps to dissipate the heat generated in cutting [22]. 2.3.3 MILLING Milling is an intermittent cutting process.In the milling process,each milling cutter tooth removes its share of the workpiece material in the form of small individual chips.The essential require- ments are accuracy and smooth finish.It is important to use sharp tools and rigid machines and fixtures.HSS cutters are used in most applications due to the interrupted cutting action involved in milling operations.However,carbide is frequently more economical than HSS when milling the more difficult-to-machine alloys,such as the highly alloyed precipitation-hardening grades [22,23]. In general,low cutting speeds and light chip loads are required.However,an excessively work- hardened layer in the superalloy workpiece can be caused by too light a feed,approximating rub- bing.Climb milling is preferred to conventional milling(up milling)since rubbing at the beginning of the cut is avoided.Also,the downward motion of the cut assists rigidity and suppresses chatter. Face milling is preferable to slab milling because it reduces the work hardening and chatter. For milling cutters,two important cutter design principles are needed:(1)tooth strength must be greater than that required for milling steel or cast iron,and(2)relief angles must be large enough to prevent rubbing and subsequent work hardening.The teeth of milling cutters should have positive rake and helix angles.Inserted blades are used on nearly all but the smallest cutters,because even under the most favorable machining conditions,the life of the cutting edges is short.Mechanical methods of securing the blades in the cutter body are preferred because replacement of chipped or broken blades is easier [22,23]. Sulfochlorinated oil introduced in copious amounts at the exhaust side of the cutter is the preferred condition for milling superalloys.Soluble oil emulsions are often used,and they provide better cooling for the tools and workpieces than straight oils.However,some sacrifice in surface finish and tool life occurs with the use of soluble oil emulsions compared to sulfochlorinated oils. The latter are often diluted with mineral oil (up to 50%)to obtain fluidity with no large sacrifice in the ability to promote cutting and achieve a good surface finish.Workpieces milled with sulfochlo- rinated or other chemically active oils must be thoroughly cleaned before being heated to elevated temperature [5,22]. 2.3.4 DRILLNG Drilling involves extrusion of metal by the chisel edge in the center of the drill and shear cutting by the lips of the tool.Due to high strength and work-hardening tendencies,drilling of nickel-based superalloys can be a difficult operation if good drilling practice is not used. In drilling nickel alloys,it is important that steady feed rates should be used.If the drill is allowed to dwell,excessive work hardening of the metal at the bottom of the hole will make it difficult to resume cutting and may result in breaking of the drill when it does take hold.The setup should be as rigid as possible.Stub drills are recommended.Drill jigs should be used whenever possible.Heavy lubricants and very rich mixtures of chemical solutions are needed in drilling nickel-based alloys. The proper feed for the job should be determined from the chip-breaking characteristics [22,23]. Conventional HSS drills are satisfactory for general-purpose drilling of group A and B alloys. These drills have a 118 point angle,a helix of about 30,a 12 lip relief angle,and a chisel-edge angle of125-135°[22,23]. Heavy-duty HSS drills with a heavy web are recommended for drilling group C and D alloys. Cobalt-bearing HSS drills give longer tool life.Cutting pressures are reduced and a positive effec- tive rake maintained if the web is thinned at the chisel point.Increasing the point angle to 135 is helpful [22,23]
Tool Condition Monitoring in Machining Superalloys 87 parameter is the side cutting edge angle. It must be large enough to provide clearance, but small enough to give adequate support to the cutting edge. The nose radius, which joins the end and side cutting edges, strengthens the tool nose and helps to dissipate the heat generated in cutting [22]. 2.3.3 Milling Milling is an intermittent cutting process. In the milling process, each milling cutter tooth removes its share of the workpiece material in the form of small individual chips. The essential requirements are accuracy and smooth finish. It is important to use sharp tools and rigid machines and fixtures. HSS cutters are used in most applications due to the interrupted cutting action involved in milling operations. However, carbide is frequently more economical than HSS when milling the more difficult-to-machine alloys, such as the highly alloyed precipitation-hardening grades [22,23]. In general, low cutting speeds and light chip loads are required. However, an excessively workhardened layer in the superalloy workpiece can be caused by too light a feed, approximating rubbing. Climb milling is preferred to conventional milling (up milling) since rubbing at the beginning of the cut is avoided. Also, the downward motion of the cut assists rigidity and suppresses chatter. Face milling is preferable to slab milling because it reduces the work hardening and chatter. For milling cutters, two important cutter design principles are needed: (1) tooth strength must be greater than that required for milling steel or cast iron, and (2) relief angles must be large enough to prevent rubbing and subsequent work hardening. The teeth of milling cutters should have positive rake and helix angles. Inserted blades are used on nearly all but the smallest cutters, because even under the most favorable machining conditions, the life of the cutting edges is short. Mechanical methods of securing the blades in the cutter body are preferred because replacement of chipped or broken blades is easier [22,23]. Sulfochlorinated oil introduced in copious amounts at the exhaust side of the cutter is the preferred condition for milling superalloys. Soluble oil emulsions are often used, and they provide better cooling for the tools and workpieces than straight oils. However, some sacrifice in surface finish and tool life occurs with the use of soluble oil emulsions compared to sulfochlorinated oils. The latter are often diluted with mineral oil (up to 50%) to obtain fluidity with no large sacrifice in the ability to promote cutting and achieve a good surface finish. Workpieces milled with sulfochlorinated or other chemically active oils must be thoroughly cleaned before being heated to elevated temperature [5,22]. 2.3.4 Drilling Drilling involves extrusion of metal by the chisel edge in the center of the drill and shear cutting by the lips of the tool. Due to high strength and work-hardening tendencies, drilling of nickel-based superalloys can be a difficult operation if good drilling practice is not used. In drilling nickel alloys, it is important that steady feed rates should be used. If the drill is allowed to dwell, excessive work hardening of the metal at the bottom of the hole will make it difficult to resume cutting and may result in breaking of the drill when it does take hold. The setup should be as rigid as possible. Stub drills are recommended. Drill jigs should be used whenever possible. Heavy lubricants and very rich mixtures of chemical solutions are needed in drilling nickel-based alloys. The proper feed for the job should be determined from the chip-breaking characteristics [22,23]. Conventional HSS drills are satisfactory for general-purpose drilling of group A and B alloys. These drills have a 118° point angle, a helix of about 30°, a 12° lip relief angle, and a chisel-edge angle of 125–135° [22,23]. Heavy-duty HSS drills with a heavy web are recommended for drilling group C and D alloys. Cobalt-bearing HSS drills give longer tool life. Cutting pressures are reduced and a positive effective rake maintained if the web is thinned at the chisel point. Increasing the point angle to 135° is helpful [22,23]
88 Aerospace Materials Handbook 2.3.5 GRINDING Grinding is frequently used to machine highly alloyed heat-treated cast turbine blades.When only a small amount of metal is to be removed,the finishing operation can be done on a grinding machine, using a rough and then a fine grind.When the machining operation calls for a significant amount of material removal,a rough grind followed by a finish grind can be used to produce a defect- free surface.Because high-temperature superalloys are sensitive to the level of energy used during grinding,metallurgical alterations and microcracking may occur at the surface.If an extremely accurate surface is required,the work should be allowed to cool to room temperature after the final roughing grind.This allows a redistribution of internal stresses,and the resulting distortion,if any, can be corrected in the final grinding operation [22,23]. For best results,the alloys should be ground wet.Due to the low thermal conductivity of superalloys, copious amounts of grinding fluid are important.Highly sulfurized water-based soluble oils provide the best heat removal.A good grinding oil is the best lubricant for crush form and thread grinding. Sodium chromate can be added to sal soda solutions to inhibit corrosion of the equipment [22,23]. Silicon carbide and aluminum oxide wheels work best for superalloys.On the other hand,CBN is also used for some precision grinding applications.Grinding pressures should be great enough to cause slight wheel breakdown.Reciprocating tables are preferred to rotary tables,because they have reduced wheel contact,generate less heat,and cause less distortion of the workpiece [22,23]. Creep feed grinding is frequently used for turbine components,such as cast single crystal blades. In creep feed grinding,the workpiece is traversed at a very low table speed with a large depth of material removed per pass.In addition,the entire width of the wheel is used in creep feed grinding. Since creep feed grinding uses a low workpiece speed and a large depth of cut,it requires a larger total force,and therefore more power,than conventional surface grinding.However,creep feed grind- ing allows much higher metal removal rates.A typical grinding cycle would be to take the majority of the stock in one or two roughing passes,dress the wheel,and then take a finishing pass [22,23]. 2.3.6 NONTRADITIONAL MACHINING There are several nontraditional machining methods that have been used for shaping superalloys [22,24].A significant potential application is to supplement conventional practice with the more difficult-to-machine alloys such as those of groups D-I and D-2. Electrochemical machining (ECM)involves electrolytic deplating of the workpiece.The"cutting tool"is a shaped cathode,usually of copper,and the workpiece is made anodic.Metal-removal rates are not affected by the mechanical properties of the workpiece but rather depend upon the gram equivalent weight of the alloy (Faraday's law)and the output of the equipment in terms of current density at the electrodes.Plating of the cathode is avoided by precipitation of the dissolved metal in the form of insoluble salts. For the nickel-based alloys,electrolytes are salt solutions (sodium or potassium nitrate or chloride) or dilute acid (sulfuric).The resistance of nickel alloys to chemical attack does not seem to retard electrochemical solution.Some differences occur in processing the various alloy families because of differences in their current-carrying capacities.The nickel and nickel-copper alloys,for instance, have fair-to-good electrical conductivities whereas the chromium-bearing materials are essentially resistance alloys.Compensations will have to be made for greater power loss,voltage drop,and elec- trolyte heating with the latter alloys.Since mechanical properties do not affect ECM,alloys can be shaped in the fully cold-worked and age-hardened condition as easily as in the annealed condition. This is particularly advantageous with the D-2 group of alloys and permits operations such as small- diameter deep hole drilling not considered feasible by conventional methods [22,24]. Ultrasonic machining is a form of abrasive machining in which the abrasive particles,suspended in a liquid medium,are propelled linearly into the workpiece by a vibrating tool.Vibration is achieved with a magnetorestrictive transducer,attached to the tool,which converts high-frequency
88 Aerospace Materials Handbook 2.3.5 Grinding Grinding is frequently used to machine highly alloyed heat-treated cast turbine blades. When only a small amount of metal is to be removed, the finishing operation can be done on a grinding machine, using a rough and then a fine grind. When the machining operation calls for a significant amount of material removal, a rough grind followed by a finish grind can be used to produce a defectfree surface. Because high-temperature superalloys are sensitive to the level of energy used during grinding, metallurgical alterations and microcracking may occur at the surface. If an extremely accurate surface is required, the work should be allowed to cool to room temperature after the final roughing grind. This allows a redistribution of internal stresses, and the resulting distortion, if any, can be corrected in the final grinding operation [22,23]. For best results, the alloys should be ground wet. Due to the low thermal conductivity of superalloys, copious amounts of grinding fluid are important. Highly sulfurized water-based soluble oils provide the best heat removal. A good grinding oil is the best lubricant for crush form and thread grinding. Sodium chromate can be added to sal soda solutions to inhibit corrosion of the equipment [22,23]. Silicon carbide and aluminum oxide wheels work best for superalloys. On the other hand, CBN is also used for some precision grinding applications. Grinding pressures should be great enough to cause slight wheel breakdown. Reciprocating tables are preferred to rotary tables, because they have reduced wheel contact, generate less heat, and cause less distortion of the workpiece [22,23]. Creep feed grinding is frequently used for turbine components, such as cast single crystal blades. In creep feed grinding, the workpiece is traversed at a very low table speed with a large depth of material removed per pass. In addition, the entire width of the wheel is used in creep feed grinding. Since creep feed grinding uses a low workpiece speed and a large depth of cut, it requires a larger total force, and therefore more power, than conventional surface grinding. However, creep feed grinding allows much higher metal removal rates. A typical grinding cycle would be to take the majority of the stock in one or two roughing passes, dress the wheel, and then take a finishing pass [22,23]. 2.3.6 Nontraditional Machining There are several nontraditional machining methods that have been used for shaping superalloys [22,24]. A significant potential application is to supplement conventional practice with the more difficult-to-machine alloys such as those of groups D-I and D-2. Electrochemical machining (ECM) involves electrolytic deplating of the workpiece. The “cutting tool” is a shaped cathode, usually of copper, and the workpiece is made anodic. Metal-removal rates are not affected by the mechanical properties of the workpiece but rather depend upon the gram equivalent weight of the alloy (Faraday’s law) and the output of the equipment in terms of current density at the electrodes. Plating of the cathode is avoided by precipitation of the dissolved metal in the form of insoluble salts. For the nickel-based alloys, electrolytes are salt solutions (sodium or potassium nitrate or chloride) or dilute acid (sulfuric). The resistance of nickel alloys to chemical attack does not seem to retard electrochemical solution. Some differences occur in processing the various alloy families because of differences in their current-carrying capacities. The nickel and nickel–copper alloys, for instance, have fair-to-good electrical conductivities whereas the chromium-bearing materials are essentially resistance alloys. Compensations will have to be made for greater power loss, voltage drop, and electrolyte heating with the latter alloys. Since mechanical properties do not affect ECM, alloys can be shaped in the fully cold-worked and age-hardened condition as easily as in the annealed condition. This is particularly advantageous with the D-2 group of alloys and permits operations such as smalldiameter deep hole drilling not considered feasible by conventional methods [22,24]. Ultrasonic machining is a form of abrasive machining in which the abrasive particles, suspended in a liquid medium, are propelled linearly into the workpiece by a vibrating tool. Vibration is achieved with a magnetorestrictive transducer, attached to the tool, which converts high-frequency