2 Tool Condition Monitoring in Machining Superalloys H.Z.Li and X.Q.Chen CONTENTS 2.1 Introduction97 2.1.1 Microstructure… .79 2.1.2 Classifications. 81 2.13 Strengthening Mechanisms..... 8 2.2 Machinability of Superalloys...... .82 2.3 Machining Techniques for Superalloys............................ .83 2.3.1 Cutting Tool Materials. .84 2.3.1.1 High-Speed Steel........... .84 2.3.1.2 Cemented Tungsten Carbide.84 2.3.1.3 Ceramic Cutting Tools............. .85 2.3.14 Cermet Cutting Tools..85 2.3.1.5 Cubic Boron Nitride.............. 85 2.3.1.6 Whisker-Reinforced Materials. .85 2.3.2 Turning… .86 2.3.3 Milling .87 2.3.4 Drilling… 87 2.3.5 Grinding. .88 2.3.6 Nontraditional Machining........... .88 2.4 Case Study:Process and Tool Wear in Cutting Inconel 718............................................89 2.4.1 Tool Wear Monitoring.............. 89 2.4.2 Experimental Study................ .91 2.4.3 Development of a Tool Wear Model...... .98 2.4.4 Adaptive Machining Using Outer Feedback Loop Control..........104 2.1 INTRODUCTION Superalloys are important enabling materials for modern aircraft.Although there have been different definitions over the years,superalloys are generally regarded as a class of high-temperature,high- strength alloys based on Group VIIIA-base elements of the periodic table,which achieve high strength and corrosion resistance for elevated-temperature service [1].According to Ref.[2].super- alloys are heat-resisting alloys based on nickel,nickel-iron,or cobalt.They exhibit a combination of mechanical strength and resistance to surface degradation,including high strength at elevated temperature;resistance to environmental attack;excellent creep resistance,stress-rupture strength, toughness,and metallurgical stability;and resistance to thermal fatigue and corrosion.Superalloys 77
77 Tool Condition Monitoring in Machining Superalloys H. Z. Li and X. Q. Chen 2.1 INTRODUCTION Superalloys are important enabling materials for modern aircraft. Although there have been different definitions over the years, superalloys are generally regarded as a class of high-temperature, highstrength alloys based on Group VIIIA-base elements of the periodic table, which achieve high strength and corrosion resistance for elevated-temperature service [1]. According to Ref. [2], superalloys are heat-resisting alloys based on nickel, nickel-iron, or cobalt. They exhibit a combination of mechanical strength and resistance to surface degradation, including high strength at elevated temperature; resistance to environmental attack; excellent creep resistance, stress-rupture strength, toughness, and metallurgical stability; and resistance to thermal fatigue and corrosion. Superalloys 2 CONTENTS 2.1 Introduction ............................................................................................................................77 2.1.1 Microstructure ............................................................................................................79 2.1.2 Classifications............................................................................................................. 81 2.1.3 Strengthening Mechanisms ........................................................................................ 81 2.2 Machinability of Superalloys..................................................................................................82 2.3 Machining Techniques for Superalloys..................................................................................83 2.3.1 Cutting Tool Materials................................................................................................84 2.3.1.1 High-Speed Steel .........................................................................................84 2.3.1.2 Cemented Tungsten Carbide ........................................................................84 2.3.1.3 Ceramic Cutting Tools.................................................................................85 2.3.1.4 Cermet Cutting Tools...................................................................................85 2.3.1.5 Cubic Boron Nitride.....................................................................................85 2.3.1.6 Whisker-Reinforced Materials.....................................................................85 2.3.2 Turning........................................................................................................................86 2.3.3 Milling ........................................................................................................................87 2.3.4 Drilling .......................................................................................................................87 2.3.5 Grinding......................................................................................................................88 2.3.6 Nontraditional Machining ..........................................................................................88 2.4 Case Study: Process and Tool Wear in Cutting Inconel 718 ..................................................89 2.4.1 Tool Wear Monitoring ................................................................................................89 2.4.2 Experimental Study .................................................................................................... 91 2.4.3 Development of a Tool Wear Model...........................................................................98 2.4.4 Adaptive Machining Using Outer Feedback Loop Control...................................... 104 2.5 Concluding Remarks ............................................................................................................ 105 References...................................................................................................................................... 106
78 Aerospace Materials Handbook are primarily used in gas turbines,coal conversion plants,and chemical process industries,and for other specialized applications requiring heat and/or corrosion resistance. There are a great variety of superalloys.Nickel-based superalloys,which are based on Ni,Cr,Al, Ti,Mo,and C.are precipitation hardened by precipitation of a Ni Al-like y'phase that is rich in Al and Ti [3].Most of these alloys contain substantial chromium for oxidation resistance;refractory metals for solid-solution strengthening;small amounts of grain-boundary-strengthening elements, such as carbon,boron,hafnium,and/or zirconium;and aluminum and titanium for strengthening by precipitation of an Ni(Al,Ti)compound known as gamma prime during age-hardening.A great variety of cast and wrought alloys are available.Among the well-known wrought alloys are D-979; GMR-235-D:N102:Inconel625,700,706,718,722,X750,and751;MAR-M200and412:Rene 41,95,and 100;Udimet 500 and 700;and Waspaloy.Cast alloys include B-1900;GMR-235-D;IN 100,162,738,and792:M252:MAR-M200,246,and42l;Nicrotung:Rene41,77,80,and100: and Udimet 500 and 700.Some wrought alloys are also suitable for casting,primarily investment casting.In recent years,considerable attention has been focused on the use of powder metallurgy techniques as a means of attaining greater compositional uniformity and finer grain size [4]. Nickel-based superalloys,primarily developed for high-temperature structural applications,are widely used in aerospace industry,especially for the gas turbine aeroengine.To meet the require- ments of harsh and corrosive working environments in aircraft engines and gas turbines,new engineering materials such as nickel-based superalloys or titanium alloys have been increasingly introduced for high-strength and high-temperature components,such as turbine blades or turbine rotors [5]. The operating temperatures of gas turbine aeroengine have kept rising steadily ever since its invention,and the trend is set to continue.This is driven by the need to increase the thrust,decrease the fuel consumption,and reduce the emissions.The modern jet engine relies very heavily upon superalloys to withstand the significant loads and temperatures developed during operation.Nickel- based superalloys are the materials of choice in the engine with high operating temperatures.For example,the turbine rotor is a set of massive disks of nickel-based superalloy that is especially composed to survive that environment.It is noted that the turbine disks represent about 20%of its total weight and their cost accounts for about 10%of the engine's value upon entry into service. Aeroengine manufacturers are under increasing pressure to develop and adopt new machining technologies to reduce cost and increase profit margin.Machining plays a vital role in superalloy part manufacture.Figure 2.1 shows an example of machined aircraft engine blades and Figure 2.2 shows an aircraft turbine engine blade being machined.Nickel-based superalloys possess excellent creep-rupture strength up to very high temperatures of 700C as well as high resistance against cor- rosion and fatigue.Unfortunately,nickel-based alloys tend toward work-hardening and adhesion, and are very hard-to-machine compared with conventional steels.Hence,the machining costs are considerably high. FIGURE 2.1 Example for aircraft engine turbine blades
78 Aerospace Materials Handbook are primarily used in gas turbines, coal conversion plants, and chemical process industries, and for other specialized applications requiring heat and/or corrosion resistance. There are a great variety of superalloys. Nickel-based superalloys, which are based on Ni, Cr, Al, Ti, Mo, and C, are precipitation hardened by precipitation of a Ni3Al-like γ ′ phase that is rich in Al and Ti [3]. Most of these alloys contain substantial chromium for oxidation resistance; refractory metals for solid-solution strengthening; small amounts of grain-boundary-strengthening elements, such as carbon, boron, hafnium, and/or zirconium; and aluminum and titanium for strengthening by precipitation of an Ni3(Al,Ti) compound known as gamma prime during age-hardening. A great variety of cast and wrought alloys are available. Among the well-known wrought alloys are D-979; GMR-235-D; IN 102; Inconel 625, 700, 706, 718, 722, X750, and 751; MAR-M 200 and 412; Rene 41, 95, and 100; Udimet 500 and 700; and Waspaloy. Cast alloys include B-1900; GMR-235-D; IN 100, 162, 738, and 792; M252; MAR-M 200, 246, and 421; Nicrotung; Rene 41, 77, 80, and 100; and Udimet 500 and 700. Some wrought alloys are also suitable for casting, primarily investment casting. In recent years, considerable attention has been focused on the use of powder metallurgy techniques as a means of attaining greater compositional uniformity and finer grain size [4]. Nickel-based superalloys, primarily developed for high-temperature structural applications, are widely used in aerospace industry, especially for the gas turbine aeroengine. To meet the requirements of harsh and corrosive working environments in aircraft engines and gas turbines, new engineering materials such as nickel-based superalloys or titanium alloys have been increasingly introduced for high-strength and high-temperature components, such as turbine blades or turbine rotors [5]. The operating temperatures of gas turbine aeroengine have kept rising steadily ever since its invention, and the trend is set to continue. This is driven by the need to increase the thrust, decrease the fuel consumption, and reduce the emissions. The modern jet engine relies very heavily upon superalloys to withstand the significant loads and temperatures developed during operation. Nickelbased superalloys are the materials of choice in the engine with high operating temperatures. For example, the turbine rotor is a set of massive disks of nickel-based superalloy that is especially composed to survive that environment. It is noted that the turbine disks represent about 20% of its total weight and their cost accounts for about 10% of the engine’s value upon entry into service. Aeroengine manufacturers are under increasing pressure to develop and adopt new machining technologies to reduce cost and increase profit margin. Machining plays a vital role in superalloy part manufacture. Figure 2.1 shows an example of machined aircraft engine blades and Figure 2.2 shows an aircraft turbine engine blade being machined. Nickel-based superalloys possess excellent creep-rupture strength up to very high temperatures of 700°C as well as high resistance against corrosion and fatigue. Unfortunately, nickel-based alloys tend toward work-hardening and adhesion, and are very hard-to-machine compared with conventional steels. Hence, the machining costs are considerably high. FIGURE 2.1 Example for aircraft engine turbine blades
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 maximum length that can be machined is thus very short, and tools have to be replaced sooner. If cutting 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 facecentered 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
80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions:aluminum,titanium,and chromium reduce density,whereas tungsten,rhenium,and tantalum increase it.The corrosion resistance of superal- loys depends primarily on the alloying elements added,particularly chromium and aluminum,and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composi- tion and the processing conditions that control the state of microstructure.The microstructure of a typical superalloy consists of different phases.The major phases that may be present in most nickel- based superalloys are as follows [2,7-9]: Gamma matrix (Y).The continuous matrix is a fcc nickel-based nonmagnetic phase that usu- ally contains a high percentage of solid-solution elements such as cobalt,iron,chromium, molybdenum,and tungsten.All nickel-based alloys contain this phase as the matrix. Gamma prime (y).This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc y'[Ni(Al,Ti)]. which precipitates coherently with the austenitic gamma matrix.Other elements,particu- larly niobium,tantalum,and chromium,also enter y.This phase is required for high-tem- perature strength and creep resistance.Gamma prime phase has an ordered Ll2 structure, which coherently precipitates in the austenitic gamma phase.The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the y'to pre- cipitate homogeneously throughout the matrix and have long-time stability.It is interesting that the flow stress of y'increases with increasing temperature up to about 650C.Other intermetallics behave in a similar way;the flow stress increases with temperature.This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime(Y").It is a body-centered tetragonal (bct)phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum.In this phase, nickel and niobium combine in the presence of iron to form bct NiaNb,which is coherent with the gamma matrix,while including large mismatch strains of the order of 2.9%.This phase provides very high strength at low to intermediate temperatures,but is unstable at temperatures above about 650C.This precipitate is found in nickel-iron alloys. Carbides.Carbon is added in an amount of about 0.02-0.2 wt%;combining with reactive elements,such as titanium,tantalum,hafnium,and niobium to form metal carbides(MC). During heat treatment and service,these MC carbides tend to decompose and generate other carbides,such as M23C6 and/or M6C,which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fec crystal structure.It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries,though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases.These are generally undesirable,brittle phases that can form during heat treatment or service.The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as o,u,and Laves that may form for some compositions and under certain conditions.These cause lowered rupture strength and ductility.The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of yly'relationship,the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance,and the introduction of a range of novel processing techniques such as directional modification,single crystal technology,powder processing,mechanical alloying,and so on [9]
80 Aerospace Materials Handbook Superalloy density is influenced by alloying additions: aluminum, titanium, and chromium reduce density, whereas tungsten, rhenium, and tantalum increase it. The corrosion resistance of superalloys depends primarily on the alloying elements added, particularly chromium and aluminum, and the environment experienced [5]. The mechanical properties of nickel-based superalloys are determined by the chemical composition and the processing conditions that control the state of microstructure. The microstructure of a typical superalloy consists of different phases. The major phases that may be present in most nickelbased superalloys are as follows [2,7–9]: Gamma matrix (γ). The continuous matrix is a fcc nickel-based nonmagnetic phase that usually contains a high percentage of solid-solution elements such as cobalt, iron, chromium, molybdenum, and tungsten. All nickel-based alloys contain this phase as the matrix. Gamma prime (γ ′). This is the primary strengthening phase in nickel-based superalloys. Aluminum and titanium are added in amounts required to precipitate fcc γ ′ [Ni3(Al,Ti)], which precipitates coherently with the austenitic gamma matrix. Other elements, particularly niobium, tantalum, and chromium, also enter γ ′. This phase is required for high-temperature strength and creep resistance. Gamma prime phase has an ordered L12 structure, which coherently precipitates in the austenitic gamma phase. The close match in matrix/ precipitate lattice parameter combined with chemical compatibility allows the γ ′ to precipitate homogeneously throughout the matrix and have long-time stability. It is interesting that the flow stress of γ ′ increases with increasing temperature up to about 650°C. Other intermetallics behave in a similar way; the flow stress increases with temperature. This unique characteristic provides the basic ground for Ni-based superalloys. Gamma double prime (γ″). It is a body-centered tetragonal (bct) phase which is the primary strengthening phase in alloys containing niobium or niobium and tantalum. In this phase, nickel and niobium combine in the presence of iron to form bct Ni3Nb, which is coherent with the gamma matrix, while including large mismatch strains of the order of 2.9%. This phase provides very high strength at low to intermediate temperatures, but is unstable at temperatures above about 650°C. This precipitate is found in nickel–iron alloys. Carbides. Carbon is added in an amount of about 0.02–0.2 wt%; combining with reactive elements, such as titanium, tantalum, hafnium, and niobium to form metal carbides (MC). During heat treatment and service, these MC carbides tend to decompose and generate other carbides, such as M23C6 and/or M6C, which tend to form at grain boundaries. Carbides in nominally solid-solution alloys may form after extended service exposures. These common carbides all have an fcc crystal structure. It is believed that carbides are beneficial by increasing rupture strength at high temperature in superalloys with grain boundaries, though results vary on whether carbides are detrimental or advantageous to superalloy properties. Topologically close-packed (TCP)-type phases. These are generally undesirable, brittle phases that can form during heat treatment or service. The cell structure of these phases have close-packed atoms in layers separated by relatively large interatomic distances. TCPs are usually platelike or needle-like phases such as σ, μ, and Laves that may form for some compositions and under certain conditions. These cause lowered rupture strength and ductility. The likelihood of their presence increases as the solute segregation of the ingot increases. The development of viable superalloys has been achieved by a combination of compositional modifications that control aspects of γ/γ ′ relationship, the use of more conventional alloying approaches to solid solution strengthening and corrosion resistance, and the introduction of a range of novel processing techniques such as directional modification, single crystal technology, powder processing, mechanical alloying, and so on [9]
Tool Condition Monitoring in Machining Superalloys 81 2.1.2 CLASSIFICATIONS Superalloys can be classified into three types as nickel-iron-(or iron-nickel-),nickel-,and cobalt- based superalloys.They may be further subdivided into cast and wrought.The main characteristics of nickel as an alloy base are the high phase stability of the fcc nickel matrix and the capability to be strengthened by different means.Many nickel-based superalloys contain significant amounts of chromium,cobalt,aluminum,and titanium,and small amounts of boron,zirconium,hafnium, and carbon.There are also common additions like molybdenum,niobium,tantalum,rhenium,and tungsten which work as strengthening solutes and carbide formers.Certain superalloys,referred to as nickel-iron superalloys such as IN718 and IN706,contain significant proportions of iron [10,11]. Nickel-based superalloys typically consist of y'dispersed in a y matrix.The strength increases with increasing y'volume fraction.y'causes strengthening through the necessity to disorder the particles as they are shared,while y"strengthens by virtue of high coherency strain in the lattice.In Inconel 718,y"often precipitates together with y,but y"is the principal strengthening phase under such circumstances. In high-temperature service,the properties of the grain boundaries are very important.Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal car- bides;small additions of Zr and B improve the morphology and stability of these carbides.Optimum properties are developed by multistage heat treatment [12]. When nickel-based alloys are loaded under high temperature,plastic strain will accumulate over time by the process of creep.Creep strengthening in polycrystalline nickel alloys arises both from solid-solution strengthening due to the presence of solute atoms and from precipitation hardening due to phases such as y'[6]. Superalloys are available in cast or wrought forms,where wrought includes powder metallurgy processing.Wrought alloys generally are considered more ductile than cast alloys.On the other hand,castings are intrinsically stronger than forgings at elevated temperature.A principle for super- alloys selection is to choose wrought alloys for intermediate-temperature applications where homo- geneity and ductility is desired,and cast alloys for high-temperature applications.Intermediate temperatures imply a range from about 1000F up to about 1400F(540C up to 760C),while high temperature can be considered to be about 1500F(816C)and up to the melting point of an alloy [1,5].Recently the use of powder metallurgy techniques has attracted considerable attention as a means of attaining greater compositional uniformity and finer grain size. 2.1.3 STRENGTHENING MECHANISMS There are different types of strengthening in superalloys,which include solid-solution hardening (substituted atoms interfere with deformation),work hardening (energy is stored by deformation), precipitation hardening (precipitates interfere with deformation),and carbide production as well. Carbides or other ceramics act as dispersion strengthening or second phase strengthening. Typical solid-solution alloys include Hastelloy X;Inconel 600,601,604,617,615,625,783; RA333,and so on.Precipitates strengthen an alloy by impeding the deformation process that takes place under load.The precipitation-strengthened alloys are the most numerous.Inconel X-750, Inconel 718,and IN-100 are famous examples.Other precipitation-strengthened wrought alloys include Astroloy;D-979;IN 102;Inconel 706 and 751;M252;Nimonic 80A,90,95,100,105. 115,and 263;Rene 41,95,and 100;Udimet 500,520,630,700,and 710;Unitemp AF2-1DA; and Waspaloy.Other cast alloys,mainly investment-cast,include B-1900;IN-738;IN-792;Inconel 713C:M252:MAR-M200,246.247,and421:NX-188;Rene77,80,and100:Udimet500.700,and 710;Waspaloy;and WAZ-20 [4]. Generally,the creep-rupture strengths of the iron-nickel-based alloys and the nickel-based solid- solution strengthened alloys are considerably lower than those of the nickel-based precipitation strengthened and carbide-hardened cobalt-based alloys at temperatures above about 1200F(649C)
Tool Condition Monitoring in Machining Superalloys 81 2.1.2 Classifications Superalloys can be classified into three types as nickel–iron- (or iron–nickel-), nickel-, and cobaltbased superalloys. They may be further subdivided into cast and wrought. The main characteristics of nickel as an alloy base are the high phase stability of the fcc nickel matrix and the capability to be strengthened by different means. Many nickel-based superalloys contain significant amounts of chromium, cobalt, aluminum, and titanium, and small amounts of boron, zirconium, hafnium, and carbon. There are also common additions like molybdenum, niobium, tantalum, rhenium, and tungsten which work as strengthening solutes and carbide formers. Certain superalloys, referred to as nickel–iron superalloys such as IN718 and IN706, contain significant proportions of iron [10,11]. Nickel-based superalloys typically consist of γ ′ dispersed in a γ matrix. The strength increases with increasing γ ′ volume fraction. γ ′ causes strengthening through the necessity to disorder the particles as they are shared, while γ ′′ strengthens by virtue of high coherency strain in the lattice. In Inconel 718, γ ′′ often precipitates together with γ ′, but γ ′′ is the principal strengthening phase under such circumstances. In high-temperature service, the properties of the grain boundaries are very important. Grain boundary strengthening is produced mainly by precipitation of chromium and refractory metal carbides; small additions of Zr and B improve the morphology and stability of these carbides. Optimum properties are developed by multistage heat treatment [12]. When nickel-based alloys are loaded under high temperature, plastic strain will accumulate over time by the process of creep. Creep strengthening in polycrystalline nickel alloys arises both from solid-solution strengthening due to the presence of solute atoms and from precipitation hardening due to phases such as γ ′ [6]. Superalloys are available in cast or wrought forms, where wrought includes powder metallurgy processing. Wrought alloys generally are considered more ductile than cast alloys. On the other hand, castings are intrinsically stronger than forgings at elevated temperature. A principle for superalloys selection is to choose wrought alloys for intermediate-temperature applications where homogeneity and ductility is desired, and cast alloys for high-temperature applications. Intermediate temperatures imply a range from about 1000°F up to about 1400°F (540°C up to 760°C), while high temperature can be considered to be about 1500°F (816°C) and up to the melting point of an alloy [1,5]. Recently the use of powder metallurgy techniques has attracted considerable attention as a means of attaining greater compositional uniformity and finer grain size. 2.1.3 Strengthening Mechanisms There are different types of strengthening in superalloys, which include solid-solution hardening (substituted atoms interfere with deformation), work hardening (energy is stored by deformation), precipitation hardening (precipitates interfere with deformation), and carbide production as well. Carbides or other ceramics act as dispersion strengthening or second phase strengthening. Typical solid-solution alloys include Hastelloy X; Inconel 600, 601, 604, 617, 615, 625, 783; RA333, and so on. Precipitates strengthen an alloy by impeding the deformation process that takes place under load. The precipitation-strengthened alloys are the most numerous. Inconel X-750, Inconel 718, and IN-100 are famous examples. Other precipitation-strengthened wrought alloys include Astroloy; D-979; IN 102; Inconel 706 and 751; M252; Nimonic 80A, 90, 95, 100, 105, 115, and 263; René 41, 95, and 100; Udimet 500, 520, 630, 700, and 710; Unitemp AF2-1DA; and Waspaloy. Other cast alloys, mainly investment-cast, include B-1900; IN-738; IN-792; Inconel 713C; M252; MAR-M 200, 246, 247, and 421; NX-188; René 77, 80, and 100; Udimet 500, 700, and 710; Waspaloy; and WAZ-20 [4]. Generally, the creep-rupture strengths of the iron–nickel-based alloys and the nickel-based solidsolution strengthened alloys are considerably lower than those of the nickel-based precipitation strengthened and carbide-hardened cobalt-based alloys at temperatures above about 1200°F (649°C)
82 Aerospace Materials Handbook Although oxide-dispersion-strengthened (ODS)nickel-based superalloys generally are not as strong as precipitation-strengthened nickel-based superalloys,they have a much flatter rate of creep-rup- ture strength reduction with time than the precipitation-strengthened alloys [5]. The modern superalloys are made and used as single crystals(monocrystals).They are extra alloyed,especially with ruthenium,and can operate up to 1100C.The introduction of improved casting methods,such as processing by directional solidification,enabled significant improvements to be made.In directionally solidified castings,the grain runs only unidirectionally,as along the length of the turbine blades.Eliminating transverse grains improves stress-rupture properties and fatigue resistance.Single-crystal alloys,which are in fact grain boundary-free,have also been cast, further improving high-temperature creep resistance.The grain boundaries are completely removed such that monocrystalline (single-crystal)superalloys are produced.Single-crystal directionally solidified casting technology,pioneered by Pratt Whitney mainly for aircraft-engine turbine blades,has extended the useful temperature range of alloys.Single-crystal directionally solidified alloys are in wide use in aircraft gas turbines at the present time and are expected to see significant use in industrial gas turbines [4]. 2.2 MACHINABILITY OF SUPERALLOYS It is important to select cutting tools and cutting conditions for optimum economic and technological machining performance.Machinability is used to indicate the ease or difficulty with which a mate- rial can be machined to the size,shape,and desired surface finish relative to the cost [2].Although it has been used for many years,the term machinability is in fact an ambiguous one,which may have a variety of different meanings.There is no standard or universally accepted method of mea- suring machinability.In general,low energy consumption,short(broken)chips,smooth finish and long tool life are aspects of good machinability.Some of these aspects are directly related to the continuum mechanical and thermal conditions of the machining process [13-16]. The machinability of a material is affected by many factors,such as the composition,microstructure, and strength level;the feeds,speeds,and depth of cut;and the choice of cutting fluid and cutting tool material.These machining characteristics,in turn,affect the cost of producing superalloy parts, particularly when the cost of machining represents a major part of the cost of the finished part. During the machining operation,some factors can be used to evaluate the machinability,such as cut- ting force,tool wear,temperature,tool-workpiece vibration,and machined surface integrity. Superalloys have an austenitic structure which imparts properties of high ductility and work hardening.Work hardening occurs when the metal ahead of the cutting tool is plastically deformed. This hardened layer is very hard to penetrate in subsequent passes or following operations.The superalloys designed for high-temperature applications remain strong at the temperatures of chip formation during machining,and thermal conductivity is much less than that of steel and many other materials.The age hardening nickel alloys also contain abrasive titanium and aluminum particles. These factors make nickel alloys more difficult to machine than steel,and it is an understanding of the extent to which each nickel alloy is affected by these factors which is the key to their successful machining [17]. Machinability ratings can be measured based on cutting speed or metal removal rate.On the other hand,there are also ratings that permit estimations of machining costs and shop load for production scheduling which are more useful.The evaluation and judgments of machinability have been historically based on one or more of the following major machining performance criteria: Tool life:Measured by the amount of material that can be removed by a standard cutting tool under standard cutting conditions before tool performance becomes unacceptable or tool wear reaches a specified amount. Cutting speed:Measured by the maximum speed at which a standard tool under standard conditions can continue to provide satisfactory performance for a specified period
82 Aerospace Materials Handbook Although oxide-dispersion-strengthened (ODS) nickel-based superalloys generally are not as strong as precipitation-strengthened nickel-based superalloys, they have a much flatter rate of creep-rupture strength reduction with time than the precipitation-strengthened alloys [5]. The modern superalloys are made and used as single crystals (monocrystals). They are extra alloyed, especially with ruthenium, and can operate up to 1100°C. The introduction of improved casting methods, such as processing by directional solidification, enabled significant improvements to be made. In directionally solidified castings, the grain runs only unidirectionally, as along the length of the turbine blades. Eliminating transverse grains improves stress-rupture properties and fatigue resistance. Single-crystal alloys, which are in fact grain boundary-free, have also been cast, further improving high-temperature creep resistance. The grain boundaries are completely removed such that monocrystalline (single-crystal) superalloys are produced. Single-crystal directionally solidified casting technology, pioneered by Pratt & Whitney mainly for aircraft-engine turbine blades, has extended the useful temperature range of alloys. Single-crystal directionally solidified alloys are in wide use in aircraft gas turbines at the present time and are expected to see significant use in industrial gas turbines [4]. 2.2 MACHINABILITY OF SUPERALLOYS It is important to select cutting tools and cutting conditions for optimum economic and technological machining performance. Machinability is used to indicate the ease or difficulty with which a material can be machined to the size, shape, and desired surface finish relative to the cost [2]. Although it has been used for many years, the term machinability is in fact an ambiguous one, which may have a variety of different meanings. There is no standard or universally accepted method of measuring machinability. In general, low energy consumption, short (broken) chips, smooth finish and long tool life are aspects of good machinability. Some of these aspects are directly related to the continuum mechanical and thermal conditions of the machining process [13–16]. The machinability of a material is affected by many factors, such as the composition, microstructure, and strength level; the feeds, speeds, and depth of cut; and the choice of cutting fluid and cutting tool material. These machining characteristics, in turn, affect the cost of producing superalloy parts, particularly when the cost of machining represents a major part of the cost of the finished part. During the machining operation, some factors can be used to evaluate the machinability, such as cutting force, tool wear, temperature, tool-workpiece vibration, and machined surface integrity. Superalloys have an austenitic structure which imparts properties of high ductility and work hardening. Work hardening occurs when the metal ahead of the cutting tool is plastically deformed. This hardened layer is very hard to penetrate in subsequent passes or following operations. The superalloys designed for high-temperature applications remain strong at the temperatures of chip formation during machining, and thermal conductivity is much less than that of steel and many other materials. The age hardening nickel alloys also contain abrasive titanium and aluminum particles. These factors make nickel alloys more difficult to machine than steel, and it is an understanding of the extent to which each nickel alloy is affected by these factors which is the key to their successful machining [17]. Machinability ratings can be measured based on cutting speed or metal removal rate. On the other hand, there are also ratings that permit estimations of machining costs and shop load for production scheduling which are more useful. The evaluation and judgments of machinability have been historically based on one or more of the following major machining performance criteria: • Tool life: Measured by the amount of material that can be removed by a standard cutting tool under standard cutting conditions before tool performance becomes unacceptable or tool wear reaches a specified amount. • Cutting speed: Measured by the maximum speed at which a standard tool under standard conditions can continue to provide satisfactory performance for a specified period
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 cause
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 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 generated. 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
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
Tool Condition Monitoring in Machining Superalloys 85 2.3.1.3 Ceramic Cutting Tools Ceramics are nonmetallic materials.The application of ceramic cutting tools is limited because of their extreme brittleness.The transverse rupture strength (TRS)is very low.This means that they will fracture more easily when making heavy or interrupted cuts.However,the strength of ceramics under compression is much higher than HSS and carbide tools.Proper tool geometry and edge preparation play an important role in the application of ceramic tools and help to overcome their weakness.Some of the advantages of ceramic tools are:high strength for light cuts on very hard work materials;extremely high resistance to abrasive wear and cratering;capability of run- ning at speeds in excess of 2000 SFPM(surface feet per minute);extremely high hot hardness;and low thermal conductivity.To use ceramic tools successfully,insert shape,work material condition, machine tool capability,setup,and general machining conditions must all be correct. 2.3.1.4 Cermet Cutting Tools Cermets are basically a combination of ceramic and titanium carbide.The term"cermet"is derived from the words"ceramic"and"metal."The strength of cermets is greater than that of hot pressed ceramics.Therefore,cermets perform better on interrupted cuts.However,when compared to solid ceramics,the presence of 30%titanium carbide in cermets decreases the hot hardness and resis- tance to abrasive wear.The hot hardness and resistance to abrasive wear of cermets are high when compared to HSS and carbide tools.The greater strength of cermets allows them to be available in a significantly larger selection of geometries,and to be used in standard insert holders for a greater variety of applications.The geometries include many positive/negative,and chip breaker conhgurations. 2.3.1.5 Cubic Boron Nitride Cubic boron nitride (CBN),bonded to a carbide base,is similar to diamond in its polycrystalline structure.With the exception of titanium,or titanium alloyed materials,CBN will work effectively as a cutting tool on most common work materials.However,the use of CBN should be reserved for very hard and difficult-to-machine materials. CBN will run at lower speeds,and will take heavier cuts with higher lead angles than diamond. Due to the extreme hardness and brittleness,CBN should mainly be considered as a finishing tool material.Machine tool and setup rigidity for CBN as with diamond is critical. 2.3.1.6 Whisker-Reinforced Materials Whisker-reinforced composite cutting tool materials have been developed to machine new work materials and composites for improved cutting performance and wear resistance of cutting tools. Whisker-reinforced materials include silicon-nitride-based tools and aluminum-oxide-based tools, reinforced with silicon carbide(SiC)whiskers.Such tools are effective in machining composites and nonferrous materials,but are not suitable for machining irons and steels. In machining superalloys,the regularly used cutting tool materials include HSS,carbides,coated carbides,boron nitride,and ceramics.Carbide tools are the most common cutting tool material. High-speed cobalt tool steels are recommended for milling,drilling,tapping,and broaching of superalloys.Carbides are used for turning,planing,and face milling.The most commonly used carbide is the C-2 grade.Modification of tungsten carbide tools with the addition of 0.5-4%tan- talum carbide has been beneficial in improving abrasion resistance.Titanium carbide tools are not applicable for superalloys because of the high solubility of titanium carbide in nickel and cobalt [5]. Most nickel-based alloys should be machined using positive cutting geometries.Since these mate- rials are machined with carbide at 120 SFPM or less,positive rake angle geometries are required to minimize cutting forces and heat generation.In the machining of most materials.increased tem- perature enhances chip flow and reduces the physical force on the cutting edge.Adequate clearance angles must be utilized on these materials,since many of them are very ductile and prone to work hardening.When a tool is stopped and left to rub on the workpiece,hardening of the workpiece
Tool Condition Monitoring in Machining Superalloys 85 2.3.1.3 Ceramic Cutting Tools Ceramics are nonmetallic materials. The application of ceramic cutting tools is limited because of their extreme brittleness. The transverse rupture strength (TRS) is very low. This means that they will fracture more easily when making heavy or interrupted cuts. However, the strength of ceramics under compression is much higher than HSS and carbide tools. Proper tool geometry and edge preparation play an important role in the application of ceramic tools and help to overcome their weakness. Some of the advantages of ceramic tools are: high strength for light cuts on very hard work materials; extremely high resistance to abrasive wear and cratering; capability of running at speeds in excess of 2000 SFPM (surface feet per minute); extremely high hot hardness; and low thermal conductivity. To use ceramic tools successfully, insert shape, work material condition, machine tool capability, setup, and general machining conditions must all be correct. 2.3.1.4 Cermet Cutting Tools Cermets are basically a combination of ceramic and titanium carbide. The term “cermet” is derived from the words “ceramic” and “metal.” The strength of cermets is greater than that of hot pressed ceramics. Therefore, cermets perform better on interrupted cuts. However, when compared to solid ceramics, the presence of 30% titanium carbide in cermets decreases the hot hardness and resistance to abrasive wear. The hot hardness and resistance to abrasive wear of cermets are high when compared to HSS and carbide tools. The greater strength of cermets allows them to be available in a significantly larger selection of geometries, and to be used in standard insert holders for a greater variety of applications. The geometries include many positive/negative, and chip breaker configurations. 2.3.1.5 Cubic Boron Nitride Cubic boron nitride (CBN), bonded to a carbide base, is similar to diamond in its polycrystalline structure. With the exception of titanium, or titanium alloyed materials, CBN will work effectively as a cutting tool on most common work materials. However, the use of CBN should be reserved for very hard and difficult-to-machine materials. CBN will run at lower speeds, and will take heavier cuts with higher lead angles than diamond. Due to the extreme hardness and brittleness, CBN should mainly be considered as a finishing tool material. Machine tool and setup rigidity for CBN as with diamond is critical. 2.3.1.6 Whisker-Reinforced Materials Whisker-reinforced composite cutting tool materials have been developed to machine new work materials and composites for improved cutting performance and wear resistance of cutting tools. Whisker-reinforced materials include silicon-nitride-based tools and aluminum-oxide-based tools, reinforced with silicon carbide (SiC) whiskers. Such tools are effective in machining composites and nonferrous materials, but are not suitable for machining irons and steels. In machining superalloys, the regularly used cutting tool materials include HSS, carbides, coated carbides, boron nitride, and ceramics. Carbide tools are the most common cutting tool material. High-speed cobalt tool steels are recommended for milling, drilling, tapping, and broaching of superalloys. Carbides are used for turning, planing, and face milling. The most commonly used carbide is the C-2 grade. Modification of tungsten carbide tools with the addition of 0.5–4% tantalum carbide has been beneficial in improving abrasion resistance. Titanium carbide tools are not applicable for superalloys because of the high solubility of titanium carbide in nickel and cobalt [5]. Most nickel-based alloys should be machined using positive cutting geometries. Since these materials are machined with carbide at 120 SFPM or less, positive rake angle geometries are required to minimize cutting forces and heat generation. In the machining of most materials, increased temperature enhances chip flow and reduces the physical force on the cutting edge. Adequate clearance angles must be utilized on these materials, since many of them are very ductile and prone to work hardening. When a tool is stopped and left to rub on the workpiece, hardening of the workpiece
86 Aerospace Materials Handbook surface will often occur.To avoid this condition,care should be taken to insure that as long as the cutting edge and part are touching,the tool is always feeding. The common causes of tool failure in machining superalloys are excessive flank wear,excessive groove formation at chip edges,and the inability to meet surface finish and accuracy requirements. Cutting temperatures can reach 1400-1850F(760-1010C).These temperatures are so high that oxidation and diffusion become significant contributing factors to total tool wear even for the carbide tools [5]. The literature provides recommendations for most machining operations for various superalloys as well as tool geometries.All data,however,are only general guides.A change in casting pro- cess from conventional to columnar grain directional solidification in a nickel-based superalloy will change the distribution and size of the carbides,owing to the differing heat transfer situations in each process type.Moreover,alloy chemistries may be changed to accommodate a different casting process.Machining of superalloys is so difficult that careful study should be undertaken for any alloy to develop a set of machining parameters that result in reasonable tool life as well as an economic analysis that covers speeds,feeds,tool materials,and cutting tool reconditioning costs [5]. The most common machining operations carried out on nickel-based superalloys are turning, milling,drilling,and grinding.Turning is the predominant machining operation in the manufacture of disks for gas turbines.Milling is the major operation carried out in the manufacture of jet engine mounts and blades for the compressor of jet engines [20]. To date,machining of nickel-based superalloys in the industry is mostly carried out at low cut- ting speeds(20-30 m/min)and feeds(0.15 mm/tooth)using carbides.But a rather large axial depth of cut is used for roughing operations.This is due to the toughness and reliability of carbides coupled with lower tool costs.However,carbides have poor hot-hardness;thus they are unsuitable for machining nickel-based superalloys at speeds above 30 m/min [20]. There has been extensive research in the application of high-speed machining(HSM)to various machining processes like turning,milling,and so on of nickel-based superalloys.HSM,apart from increasing productivity,also offers the advantage of better surface finish,better chip disposal,sim- plified tooling,reduction in the damaged layer,reduced burr formation,and increased machining accuracy.Ceramic inserts are capable of achieving these high cutting speeds.Silicon carbide whis- ker reinforced alumina(SiC WRA),Sialon,and CBN are promising materials for HSM of nickel- based superalloys because of their greater mechanical and thermal integrity.However,machine shops are yet to implement wide adoption of ceramic tools in the machining of nickel-based super- alloys.Many users are comfortable with machining using carbides,and the innumerable grades of ceramic tools make tool selection a very complicated task [20]. 2.3.2 TURNING Turning is a metal cutting technology in which the cutting movement is carried out by the workpiece, whereas the tool performs the auxiliary motion of feed and infeed.In machining superalloys,more heat is generated in the shear zone since the superalloys retain most of their strength at cutting temperatures,and greater tool wear occurs for a given cutting speed than with most other metals.In addition,because the cutting of superalloys requires a larger force(about twice the force for cutting medium-carbon alloy steel in turning operations),tool geometry,tool strength,and/or rigidity of the toolholder are also important concerns. Carbide tools are frequently used in turning superalloys,although ceramic,coated carbide,CBN. and HSS tools are also used.A C-2 grade is often selected for roughing.A C-3 grade is used in finishing.Standard carbide inserts with positive or negative rakes are suitable for the roughing and finishing of superalloys [21,22]. It is important to use positive rake angles for the single-point turning tools in cutting nickel alloys so that the metal is cut instead of pushed.Negative rake angles should be avoided.Positive rake angles also help to guide the chip away from the finished surface.Another important tool geometric
86 Aerospace Materials Handbook surface will often occur. To avoid this condition, care should be taken to insure that as long as the cutting edge and part are touching, the tool is always feeding. The common causes of tool failure in machining superalloys are excessive flank wear, excessive groove formation at chip edges, and the inability to meet surface finish and accuracy requirements. Cutting temperatures can reach 1400–1850°F (760–1010°C). These temperatures are so high that oxidation and diffusion become significant contributing factors to total tool wear even for the carbide tools [5]. The literature provides recommendations for most machining operations for various superalloys as well as tool geometries. All data, however, are only general guides. A change in casting process from conventional to columnar grain directional solidification in a nickel-based superalloy will change the distribution and size of the carbides, owing to the differing heat transfer situations in each process type. Moreover, alloy chemistries may be changed to accommodate a different casting process. Machining of superalloys is so difficult that careful study should be undertaken for any alloy to develop a set of machining parameters that result in reasonable tool life as well as an economic analysis that covers speeds, feeds, tool materials, and cutting tool reconditioning costs [5]. The most common machining operations carried out on nickel-based superalloys are turning, milling, drilling, and grinding. Turning is the predominant machining operation in the manufacture of disks for gas turbines. Milling is the major operation carried out in the manufacture of jet engine mounts and blades for the compressor of jet engines [20]. To date, machining of nickel-based superalloys in the industry is mostly carried out at low cutting speeds (20–30 m/min) and feeds (0.15 mm/tooth) using carbides. But a rather large axial depth of cut is used for roughing operations. This is due to the toughness and reliability of carbides coupled with lower tool costs. However, carbides have poor hot-hardness; thus they are unsuitable for machining nickel-based superalloys at speeds above 30 m/min [20]. There has been extensive research in the application of high-speed machining (HSM) to various machining processes like turning, milling, and so on of nickel-based superalloys. HSM, apart from increasing productivity, also offers the advantage of better surface finish, better chip disposal, simplified tooling, reduction in the damaged layer, reduced burr formation, and increased machining accuracy. Ceramic inserts are capable of achieving these high cutting speeds. Silicon carbide whisker reinforced alumina (SiC WRA), Sialon, and CBN are promising materials for HSM of nickelbased superalloys because of their greater mechanical and thermal integrity. However, machine shops are yet to implement wide adoption of ceramic tools in the machining of nickel-based superalloys. Many users are comfortable with machining using carbides, and the innumerable grades of ceramic tools make tool selection a very complicated task [20]. 2.3.2 Turning Turning is a metal cutting technology in which the cutting movement is carried out by the workpiece, whereas the tool performs the auxiliary motion of feed and infeed. In machining superalloys, more heat is generated in the shear zone since the superalloys retain most of their strength at cutting temperatures, and greater tool wear occurs for a given cutting speed than with most other metals. In addition, because the cutting of superalloys requires a larger force (about twice the force for cutting medium-carbon alloy steel in turning operations), tool geometry, tool strength, and/or rigidity of the toolholder are also important concerns. Carbide tools are frequently used in turning superalloys, although ceramic, coated carbide, CBN, and HSS tools are also used. A C-2 grade is often selected for roughing. A C-3 grade is used in finishing. Standard carbide inserts with positive or negative rakes are suitable for the roughing and finishing of superalloys [21,22]. It is important to use positive rake angles for the single-point turning tools in cutting nickel alloys so that the metal is cut instead of pushed. Negative rake angles should be avoided. Positive rake angles also help to guide the chip away from the finished surface. Another important tool geometric
