Machining of Composite Materials R. Teti University of Naples Federico l, Italy Abstract Machining of composite materials is difficult to carry out due to the anisotropic and non-homogeneous structure of composites and to the high abrasiveness of their reinforcing constituents. This typically results in damage eing introduced into the workpiece and in very rapid wear development in the cutting tool ventional achining processes such as turning, drilling or milling can be applied to composite materials, provided propel ol design and operating conditions are adopted verview of the various issues involved in the conventional achining of the main types of composite materials is presented in this paper Machining, Composite Materials, Conventional Cutting Processes ACKNOWLEDGEMENTS such as glass, graphite, boron, alumina and silicon Acknowledgements are due for papers, contributions and carbide, are highly abrasive and hard (sometimes as hard correspondence received from Messrs ("CIRP members) as or even harder than the tool material), conventiona Aspinwal, D K, University of Birmingham UK;"Balazinski machining is considered for composites because their M, Ecole Polytechnique de Montreal, Canada: *Byrne, G reinforcements are brittle and material separation is University College Dublin, Ireland; "Brinksmeier, E accomplished G deformation ahead of the tool. However, the cutting tool Naples Federico Il, Italy; *Chandrasekaran, H, Swedish materials must be attentively chosen to minimize wear due nstitute for metals Research. stockhol en: Chen to the hard abrasive constituents of the reinforcing phase J. Rotors Business Center. USA: 'Dornfeld. D in the composite representing the work material University of California, Berkeley, USA: *Geiger Machining of a composite depends on the properties and University of Erlangen-Nurnberg, Germany: Inasaki elative content of the reinforcement and Kentucky, USA; Klocke, F, Technical University of process. In addition, the choice of the spec Aachen, Germany: Komanduri, R, Oklahoma State depends upon the following factors type of machining peration, part geometry and size, finish and accul Japa-rsity, USA, *Narutaki, N, Hiroshima University, Japan:"Poll mann, W, Daimler Crysler AG, Stuttgart requirements, number of parts, diversity of parts(including Germany: Spur, G, Technical University Berlin the material of the parts), availability of appropria machine and cutting tools, availability of in-house Germany: Tomizuka, M, University of California, Berkeley technol USA: *Uhlmann, F,, Technical University Berlin, Germany ufacturing schedule, capital requirements and justification for new equipment, environmental and safety considerations, and Weig, E, HSC-Manufact. Engineering, Austria; "Weinert overall costs K, University of Dortmund, Germany; *Wertheim, R ISCAR Ltd. Israel 2 COMPOSITE MATERIALS PhD student Doriana D'Addona, University of Naples Federico I, Italy is gratefully thanked for her help and Composite materials are formed from two more support in the preparation of the text materials producing properties that could not be obtained from any one material. One of the constituent materials acts as the matrix and at least one other constituent material acts as the reinforcement in the composite Composite materials are used extensively as their higher of th specific properties(properties per unit weight) of strength to protect the reinforcement materials and stiffness, when compared to metals, offer interesting to distribute the stress to the reinforcement material(s) opportunities for new product design. However, being non to provide for the final shape of the composite part homogeneous, anisotropic and reinforced by very abrasive The role of the reinforcement material(s)is the following components, these materials are difficult to machine to provide the composite high mechanical properties Significant damage to the workpiece may be introduced to reinforce the matrix in preferential directions and high wear rates of the cutting tools are experienced The properties of a composite material depend on the Conventional machining practices, such as turning, drilling nature of the reinforcement and the matrix. the form of the and milling are widely applied to the machining of reinforcement(particles, fibres) and the relative content of composite materials in view of the availability of equipment reinforcement and matrix expressed as volume fraction and experience in conventional machining. Although some Vt=(reinforcement volume)composite volume)and vm of the materials used as reinforcement in composites (matrix volume) /(composite volume), where Vr+ Vm
Machining of Composite Materials R. Teti University of Naples Federico II, Italy Abstract Machining of composite materials is difficult to carry out due to the anisotropic and non-homogeneous structure of composites and to the high abrasiveness of their reinforcing constituents. This typically results in damage being introduced into the workpiece and in very rapid wear development in the cutting tool. Conventional machining processes such as turning, drilling or milling can be applied to composite materials, provided proper tool design and operating conditions are adopted. An overview of the various issues involved in the conventional machining of the main types of composite materials is presented in this paper. Keywords: Machining, Composite Materials, Conventional Cutting Processes ACKNOWLEDGEMENTS Acknowledgements are due for papers, contributions and correspondence received from Messrs (*CIRP members): Aspinwal, D.K., University of Birmingham UK; *Balazinski, M., Ecole Polytechnique de Montreal, Canada; *Byrne, G., University College Dublin, Ireland; *Brinksmeier, E., University of Bremen, Germany; Caprino, G., University of Naples Federico II, Italy; *Chandrasekaran, H., Swedish Institute for Metals Research, Stockholm, Sweden; Chen, L.J., Rotors Business Center, USA; *Dornfeld, D., University of California, Berkeley, USA; *Geiger, M., University of Erlangen-Nurnberg, Germany; *Inasaki, I., Keio University, Japan; *Jawahir, I.S., University of Kentucky, USA; *Klocke, F., Technical University of Aachen, Germany; *Komanduri, R., Oklahoma State University, USA; *Narutaki, N., Hiroshima University, Japan; *Pollmann, W., DaimlerCrysler AG, Stuttgart, Germany; *Spur, G., Technical University Berlin, Germany; Tomizuka, M., University of California, Berkeley, USA; *Uhlmann, F., Technical University Berlin, Germany; *Venkatesh, V.C., University of Technology Malaysia; *Weigl, E., HSC-Manufact. Engineering, Austria; *Weinert, K., University of Dortmund, Germany; *Wertheim, R., ISCAR Ltd., Israel. PhD student Doriana D'Addona, University of Naples Federico II, Italy, is gratefully thanked for her help and support in the preparation of the text. 1 INTRODUCTION Composite materials are used extensively as their higher specific properties (properties per unit weight) of strength and stiffness, when compared to metals, offer interesting opportunities for new product design. However, being nonhomogeneous, anisotropic and reinforced by very abrasive components, these materials are difficult to machine. Significant damage to the workpiece may be introduced and high wear rates of the cutting tools are experienced. Conventional machining practices, such as turning, drilling and milling, are widely applied to the machining of composite materials in view of the availability of equipment and experience in conventional machining. Although some of the materials used as reinforcement in composites, such as glass, graphite, boron, alumina and silicon carbide, are highly abrasive and hard (sometimes as hard as or even harder than the tool material), conventional machining is considered for composites because their reinforcements are brittle and material separation is accomplished by brittle fracture rather than plastic deformation ahead of the tool. However, the cutting tool materials must be attentively chosen to minimize wear due to the hard abrasive constituents of the reinforcing phase in the composite representing the work material. Machining of a composite depends on the properties and relative content of the reinforcement and the matrix materials as well as on its response to the machining process. In addition, the choice of the specific process depends upon the following factors: type of machining operation, part geometry and size, finish and accuracy requirements, number of parts, diversity of parts (including the material of the parts), availability of appropriate machine and cutting tools, availability of in-house technology, current machining practice, manufacturing schedule, capital requirements and justification for new equipment, environmental and safety considerations, and overall costs. 2 COMPOSITE MATERIALS Composite materials are formed from two or more materials producing properties that could not be obtained from any one material. One of the constituent materials acts as the matrix and at least one other constituent material acts as the reinforcement in the composite. The role of the matrix material comprises the following: - to protect the reinforcement materials; - to distribute the stress to the reinforcement material(s); - to provide for the final shape of the composite part. The role of the reinforcement material(s) is the following: - to provide the composite high mechanical properties; - to reinforce the matrix in preferential directions. The properties of a composite material depend on the nature of the reinforcement and the matrix, the form of the reinforcement (particles, fibres) and the relative content of reinforcement and matrix expressed as volume fraction: Vf = (reinforcement volume)/(composite volume) and Vm = (matrix volume)/(composite volume), where Vf+ Vm = 1
Composite materials can be classified on the basis of the Among the thermoset resins, polyester resins are lower in matrix material used for their fabrication polymer matrix composites(PMc) matrix composites are used in the fabrication of boat hulls metal matrix composites(MMC) structural panels and parts for automobiles and aircrafts ceramic matrix composites(CMc) building panels and beams, electrical appliances, water Theoretically, a multitude of materials can come under tanks, pressure vessels, etc. Epoxy resins, in addition these categories. In the following a brief description of have a lower shrinkage after cure allowing for highe some of the PMc, MMc and CMc composites most fabrication accuracy. Epoxy matrix is used commonly in ly used for industrial applications is reported CFRP and AFRP composites for aerospace applications. 2.1 Polymer matrix composites(PMc) military equipments, satellite antennae, sports The most common types of reinforcement used for PMC equipments, medical prostheses, etc Thermoplastic polymers consists of flexible line A are placed before the acronym FrP to specify the strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used nature of the reinforcing fibres glass, carbon or aramid fibres. The fibres can be long (continuous)or short as matrix materials in FRP composites for applications in (discontinuous). Long fibres can be unidirectional(al the aerospace industry due to their superior mechanical properties and high glass transition temperature fibres parallel to each other) or woven into a fabric or Maximum service temperatures for FRP composites are cloth.Unidirectional fibres provide for the highest relatively low, as the matrix material is prone to softening. mechanical properties in a composite Glass fibre reinforced plastics(GFRP)are by far the most chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to commonly used materials in view of their high specifi mechanical properties and low cost. Carbon fibre the machining of FRP composites Table 1 reports the main mechanical properties of some einforced plastics( CFRP)and aramid fibre reinforced FRP composite materials lastics(AFRP)provide higher specific strength, higher specific stiffness and ligher weight. They are, however 2.2 Metal matrix composites(MMc) expensive and are used only for those applications where MMC are used for applications requiring higher operating performance and not cost is the major consideration emperatures than are possible with PMc materials AFRP is used instead of CFRP where strength, lightness Most of these composites are developed for the and toughness are major considerations, and stiffness and aerospace industry, but new applications are found in the high temperature performance are not automotive industry such as in automobile engine parts The common matrix materials for FRP composites ar making use of continuous fibre, discontinuous fibre or thermoset polymers(e.g. polyester, epoxy) particle reinforced MMC. Continuous fibres provide for the thermoplastic polymers(e.g polyamide, peek highest stiffness and strength properties obtainable in Thermoset polymers remain rigid when heated and consist MMc materials of a highly cross-linked three-dimensional network; they Boron-aluminium composites are one of the earliest are quite strong and stiff and have poor ductility layers of boron fibres between aluminium foils, so that the TensileElastic foils deform around the fibres and bond to each other [1] FRP odulus By reinforcing with boron, the tensile strength can be material ou(MPa)E(MPa) failure, d(g/ Increas ed by a factor of three to five while the elastic modulus can be tripled GFRP Further reinforcing materials for MMc are silicon carbide alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and itanium alloys are the most common matrix materials coth*10030010000200 used in MMc materials Table 2 reports the main mechanical properties of some 50-2006000-12000 13-2.1 MMC materials hort fibres Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional m materials. It can be seen that composites, in general, have compound* 1.3-19 higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys and mMc have properties superior to PMc compos Applications of continuous fibre reinforced use of B-Al for the fuselage of the space shuttle (V=60%) 145000 0.9 1.6 SiC-Al for the vertical tail section of advanced e orbiter Discontinuous fibre and particle reinforced MMc are low (V=60%) 0.3 1.6 cost MMC that provide higher strength and stiffness and 220000 better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement(V 20%)moderately increase the base alloy strength and stiffness. They also increase the wear resistance and Unidir 75000 1.6 contribute toward the difficulty in machining these materials. These MMc are used for sport equipments Table 1: Mechanical properties of FRP composite automobile engine parts(pistons cylinder liners, brake "For these materials: v+= 20%-50% drums), missile guidance parts, etc
Composite materials can be classified on the basis of the matrix material used for their fabrication: - polymer matrix composites (PMC); - metal matrix composites (MMC); - ceramic matrix composites (CMC). Theoretically, a multitude of materials can come under these categories. In the following, a brief description of some of the PMC, MMC and CMC composites most commonly used for industrial applications is reported. 2.1 Polymer matrix composites (PMC) The most common types of reinforcement used for PMC are strong and brittle fibres incorporated into a soft and ductile polymeric matrix. In this case, PMC are referred to as fibre reinforced plastics (FRP). Capital letters G, C and A are placed before the acronym FRP to specify the nature of the reinforcing fibres: glass, carbon or aramid fibres. The fibres can be long (continuous) or short (discontinuous). Long fibres can be unidirectional (all fibres parallel to each other) or woven into a fabric or cloth. Unidirectional fibres provide for the highest mechanical properties in a composite. Glass fibre reinforced plastics (GFRP) are by far the most commonly used materials in view of their high specific mechanical properties and low cost. Carbon fibre reinforced plastics (CFRP) and aramid fibre reinforced plastics (AFRP) provide higher specific strength, higher specific stiffness and ligher weight. They are, however, expensive and are used only for those applications where performance and not cost is the major consideration. AFRP is used instead of CFRP where strength, lightness and toughness are major considerations, and stiffness and high temperature performance are not. The common matrix materials for FRP composites are: - thermoset polymers (e.g. polyester, epoxy) - thermoplastic polymers (e.g. polyamide, peek). Thermoset polymers remain rigid when heated and consist of a highly cross-linked three-dimensional network; they are quite strong and stiff and have poor ductility. FRP material Strain to Density failure, d (g/cmS; Tensile Elastic strength, modulus, ail (MPa) E (MPa) Eu (%) GFRP I Unidirectional (Vr = 60 %) Woven cloth* Chopped looo 45000 2.3 2.1 100-300 10000-20000 - 1.5-2.1 .. roving* (short fibres) Sheet 50-200 6000-12000 - 1.3-2.1 modulus AFRP molding compound* (short fibres) CFRP Unidirectional (Vr = 60 %) High strength Unidirectional High (Vr = 60 %) 10-20 500-2000 1.3-1.9 1200 145000 0.9 1.6 800 220000 0.3 1.6 Table 1: Mechanical properties of FRP composite *For these materials: Vf = 20% - 50%. Unidirectional (Vr = 60 %) Among the thermoset resins, polyester resins are lower in cost and are not as strong as epoxy resins. Polyester matrix composites are used in the fabrication of boat hulls, structural panels and parts for automobiles and aircrafts, building panels and beams, electrical appliances, water tanks, pressure vessels, etc. Epoxy resins, in addition, have a lower shrinkage after cure allowing for higher fabrication accuracy. Epoxy matrix is used commonly in CFRP and AFRP composites for aerospace applications, military equipments, satellite antennae, sports equipments, medical prostheses, etc. Thermoplastic polymers consists of flexible linear molecular chains that are tangled together and, as the name indicates, soften when heated; they have lower strength and modulus but quite high ductility. Among the thermoplastic resins, polyamide and peek resins are used as matrix materials in FRP composites for applications in the aerospace industry due to their superior mechanical properties and high glass transition temperature. Maximum service temperatures for FRP composites are relatively low, as the matrix material is prone to softening, chemical decomposition or degradation at moderate temperatures. The same temperature limitations apply to the machining of FRP composites. Table 1 reports the main mechanical properties of some FRP composite materials. 2.2 Metal matrix composites (MMC) MMC are used for applications requiring higher operating temperatures than are possible with PMC materials. Most of these composites are developed for the aerospace industry, but new applications are found in the automotive industry, such as in automobile engine parts, making use of continuous fibre, discontinuous fibre, or particle reinforced MMC. Continuous fibres provide for the highest stiffness and strength properties obtainable in MMC materials. Boron-aluminium composites are one of the earliest developed MMC material types. It is made by hot pressing layers of boron fibres between aluminium foils, so that the foils deform around the fibres and bond to each other [I]. By reinforcing with boron, the tensile strength can be increased by a factor of three to five while the elastic modulus can be tripled. Further reinforcing materials for MMC are silicon carbide, alumina and graphite in the form of particles, short fibres (whiskers) or long fibres. Aluminium, magnesium and titanium alloys are the most common matrix materials used in MMC materials. Table 2 reports the main mechanical properties of some MMC materials. Figure 1 is a plot of specific strength versus specific stiffness for various composites and conventional metal materials. It can be seen that composites, in general, have higher specific strength and specific modulus over conventional steel, Al, Ti, and Mg alloys, and MMC have properties superior to PMC composites. Applications of continuous fibre reinforced MMC include use of B-AI for the fuselage of the space shuttle orbiter, SIC-AI for the vertical tail section of advanced fighter planes, SIC-TiAI for hypersonic aircraft, etc. Discontinuous fibre and particle reinforced MMC are low cost MMC that provide higher strength and stiffness and better dimensional stability over the corresponding unreinforced alloys. Small additions of reinforcement (Vr = 20%) moderately increase the base alloy strength and stiffness. They also increase the wear resistance and contribute toward the difficulty in machining these materials. These MMC are used for sport equipments, automobile engine parts (pistons, cylinder liners, brake drums), missile guidance parts, etc. looo 75000 1.6 1.4
Matrix Sic whisker Flexural MMC material volume fraction, strength, toughness ou(MPa)E(MPa)Eu(%) SiaN4 4060 A2124T6(45%B) 45022000081 Al2O3 A6061-76(51%B) 000 5-65 A6061T6(45%Sc)14602000 45-55 Discontinuous-fibre mmc Table 3: Mechanical properties of Sic whisker reinforced A21246(20%sc)65012500240 CMC materials at room temperature A6061T6(20%SiC) 480 120000 5.00 3 MACHINING APPLICATIONS Particle MMC Machining of composite materials differs significantly in many aspects from machining of conventional metals and A2124T6(20%ScC) 55010500700 their alloys [3-5]. In the machining of composites, the A6061-T6(20%SC) 50010500550 material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement No reinforcement and matrix properties, and the volume fraction of matrix A2124F 450700000900 and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to A6061-F machining can be entirely different. Thus, machining of composite materials imposes special demands on the Table 2: Mechanical properties of MMC materials eometry and wear resistance of the cutting tools Accordingly, tool wear mechanisms and developme must be attentively considered to establish correct cutting ool selectio In the following, applications of machining processes to 0. 34 Gr/Mg composite materials are reviewed with reference to FR 04 materials and MMC materials 0.37Gr/A As regards the machining of CMC materials, the very Conventiona small number of contributions received and the scarcity of s Steel,At,Ti,Mg information available in the open literature on this topic did not allow for the preparation of a dedicated section d02 0.60 Gr/Epox 3. 1 Machining of fibre reinforced plastic composites Orthogonal machining of FRP FRP composites with different fibre orientations allowed or the clarification of the cutting mechanisms taking place in FRP(Figure 2). When machining is conducted at an angle of o to the fibre orientation the laminate is Figure 1: Specific strength vs specific stiffness for varie subjected to stresses parallel to the fibres. In addition, the MMC materials. Number in front of the composite is the surface below the cutting edge is compressed. The reinforcement volume fraction [2 material failure occurring in front of the cutting edge is due to delamination. matrix fracture or fibre-matrix interface 2.3 Ceramic matrix com posites(CMC) failure, which is recognizable from the crack in the CMC materials are being developed mainly to improve the composite laminate ahead of the cutting edge. Individual fracture toughness of unreinforced ceramics which already fractures occurring in the fibres and in the matrix below the possess higher specific modulus and mechanical cutting edge are also visible and remain in the machined properties at high temperature superior to those of metals surface. As the angle between cutting direction and fibre Continuous fibres, discontinuous fibres (whiskers)or orientation increases, fibres are compressed and bent in particles can be utilised as reinforcing constituents in the direction opposite to the fibre orientation, ending up in CMC fibre breakage as a result of bending and pressure load The common reinforcement materials used in CMc are This can result in fibre-matrix interface failure which alumina and silicon carbide, a volume fraction V:= 20% of SiC whiskers added to alumina can increase the fracture directions. which are the least favourable for FRP toughness from 25 to 50 MPa. Such an increase in composites particularly at angles between 30 and 60 to toughness of a ceramic cutting tool will enable it to take the fibre direction, is reflected in a poor surface quality In heavy cuts or to perform without fracture in interrupted a composite machined at 90 to the fibre direction,the g. Conventional hot isostatic pressing techniques fibr subjected ng e used to consolidate CMC composites contrast to laminates with 0 fibres. each fibre has to be Other CMc include carbon/carbon composites in which cut separately. The compressive strain normal to the fibres high strength carbon fibres are embedded in a graphite creates problems as interfacial fractures extend into the matrix. The low density of carbon in combination with the unmachined surface, More favourable conditions develop very high strength of carbon fibres offers potential for the for fibre orientation 135. Fibres are subjected to bending development of ultra high specific strength materials and tensile stress and break in bundles problems arise Table 3 reports the main mechanical properties of some however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrix
Tensile strength, u,, (MPa) MMC material IContinuous-fibre MMC I I Elastic Strain to modulus, failure, E (MPa) E,, (%) IAI2124-T6(45% B) I 1450 I220000 I 0.81 I Matrix material SiqNd IAl6061-T6(51%B) I 1410 I230000 I 0.74 I Sic whisker Flexural Fracture volume fraction, strength, toughness, Vf (%) uf (MPa) k (MPa) 0 400-650 30-45 IAl 6061-T6 (45% Sic) I 1460 I 200000 I 0.89 I Discontinuous-fibre MMC Al 2124-T6 (2O%SiC) Al 6061-T6 (20% Sic) 650 125000 2.40 480 120000 5.00 lparticle MMC I I No reinforcement Al 2124-F Al 6061-F IAl2124-T6(20%SiC) I 550 I 105000 I 7.00 I 450 700000 9.00 310 70000 12.00 IAl 6061-T6 (20% Sic) I 500 I 105000 I 5.50 I Table 2: Mechanical properties of MMC materials 0.6 h E z D 0.4 a 3j I- 3 5 m C u) 0 E 0 8 a v) 0.2 c 0 0.45 B/Al 0 0.34 Gr/Mg SlCfr 0. 0.37 Gr/AI Be 0 Conventional 0 Steel, Al, Ti, Mg 0 0.37 Gr/AI 1 NO.giCw/Al 0.60 IsI Gr/Epoxy 0.50 Gr/Epoxy 0 50 100 150 Specific stiffness (10 Nrn/Kg) Figure 1: Specific strength vs. specific stiffness for various MMC materials. Number in front of the composite is the reinforcement volume fraction [2]. 2.3 Ceramic matrix composites (CMC) CMC materials are being developed mainly to improve the fracture toughness of unreinforced ceramics which already possess higher specific modulus and mechanical properties at high temperature superior to those of metals. Continuous fibres, discontinuous fibres (whiskers) or particles can be utilised as reinforcing constituents in CMC. The common reinforcement materials used in CMC are alumina and silicon carbide. A volume fraction Vf = 20% of Sic whiskers added to alumina can increase the fracture toughness from 25 to 50 MPa. Such an increase in toughness of a ceramic cutting tool will enable it to take heavy cuts or to perform without fracture in interrupted cutting. Conventional hot isostatic pressing techniques can be used to consolidate CMC composites. Other CMC include carbonkarbon composites in which high strength carbon fibres are embedded in a graphite matrix. The low density of carbon in combination with the very high strength of carbon fibres offers potential for the development of ultra high specific strength materials. Table 3 reports the main mechanical properties of some CMC materials. 400-550 40-60 350-500 45-65 400-500 45 20 500-800 45-55 Table 3: Mechanical properties of Sic whisker reinforced CMC materials at room temperature. 3 MACHINING APPLICATIONS Machining of composite materials differs significantly in many aspects from machining of conventional metals and their alloys [3-51. In the machining of composites, the material behaviour is not only non-homogeneous and anisotropic, but it also depends on diverse reinforcement and matrix properties, and the volume fraction of matrix and reinforcement. The tool encounters alternatively matrix and reinforcement materials, whose response to machining can be entirely different. Thus, machining of composite materials imposes special demands on the geometry and wear resistance of the cutting tools. Accordingly, tool wear mechanisms and development must be attentively considered to establish correct cutting tool selection. In the following, applications of machining processes to composite materials are reviewed with reference to FRP materials and MMC materials. As regards the machining of CMC materials, the very small number of contributions received and the scarcity of information available in the open literature on this topic did not allow for the preparation of a dedicated section. 3.1 Orthogonal machining of FRP Investigations carried out in [6] by orthogonal cutting of FRP composites with different fibre orientations allowed for the clarification of the cutting mechanisms taking place in FRP (Figure 2). When machining is conducted at an angle of 0" to the fibre orientation, the laminate is subjected to stresses parallel to the fibres. In addition, the surface below the cutting edge is compressed. The material failure occurring in front of the cutting edge is due to delamination, matrix fracture or fibre-matrix interface failure, which is recognizable from the crack in the composite laminate ahead of the cutting edge. Individual fractures occurring in the fibres and in the matrix below the cutting edge are also visible and remain in the machined surface. As the angle between cutting direction and fibre orientation increases, fibres are compressed and bent in the direction opposite to the fibre orientation, ending up in fibre breakage as a result of bending and pressure load. This can result in fibre-matrix interface failure which extends into the unmachined surface. These load directions, which are the least favourable for FRP composites particularly at angles between 30" and 60" to the fibre direction, is reflected in a poor surface quality. In a composite machined at 90" to the fibre direction, the fibres are subjected to bending and are sheared off. In contrast to laminates with 0" fibres, each fibre has to be cut separately. The compressive strain normal to the fibres creates problems as interfacial fractures extend into the unmachined surface. More favourable conditions develop for fibre orientation 135". Fibres are subjected to bending and tensile stress and break in bundles. Problems arise, however, from the fact that individual fibres can be pulled out due to insufficient adhesion to the matrix. Machining of fibre reinforced plastic composites
-orientation fibre-/matrix fibre matrix fracture and fracture interfacial fracture ackage crack- re- matrix fracture and fracture/ interfacial fracture fibre-matrix crack-turn round lue to fibre. tension Figure 3: Examples of turned FRP parts [15] fibre matrⅸx Although the cutting of FRP composite parts is rarely Figure 2: Cutting mechanisms for FRP composites [6] desired, it can be seldom avoided for the production of the final geometry, surface quality, and form accuracy The machinability of CFRP and GFrP was deeply conventionally produced parts. Turning is applied to investigated in [7]. A model for cutting force prediction in rotation-symmetric parts such as drag links, bearings orthogonal cutting operations was presented. Three spindles, axles, rolls, or steering columns, etc. Figure 3 initially varied during orthogonal cutting shows some typical FRP parts produced by turning sing HSS tools: tool rake angle, relief angle and depth of Particular attention was given by several authors to the cut. Their effects on cutting forces were investigated and aspects of tool wear mechanisms and development in an optimal tool geometry was found The effect of tool turning of FRP composites with the aim of establishing wear on cutting forces in the machining of unidirectional correct cutting tool selection criteria. Among the possible GFRP was investigated too [8]. In [9], cutting forces were found to increase with increasing depth of cut during tribo-oxidation and surface damage, only abrasion, surface orthogonal machining of unidirectional CFRP, The effect of damage and sometimes adhesion are of significance for bre orientation on cutting forces and cutting quality in FRP machining. Wear mechanisms are primarily related to orthogonal machining of unidirectional CFRP was treated the physical and mechanical characteristics of the different in [10]. In [11], the attention was focused on the fibre- matrix systems. Glass and carbon fibres show a mechanisms of chip generation. Because of the inferior strongly abrasive behaviour because they are extremel surface quality of unidirectional CFRP after orthogonal abrasive by nature. Aramid fibres, on the other hand machining for some fibre orientations, in [12the mpair the tool due to their low heat conductivity and development of a new tool geometry to reduce work ductile behaviour. Adhesive wear occurs when carbonised material surface damage was investigated or molten matrix depositions settle on the tool surfaces In [13, 14], tool wear development was studied and In [16], an analysis of tool wear during turning of GFRP monitored using acoustic emission(AE) signal detection and cfrP with diamond coated tools was carried out. the and analysis during orthogonal cutting of different types of dominating wear mechanisms during cylindrical turning GFRP and CFRP and sheet such as cutting edge blunting elimination of the coating moulding compound(SMC). Decision making on tool wear layer, retreat of the cutting edge, and crater wear formation were characterised and their development was explained material type: tool wear discrimination was reliably glass, epoxy-glass, polyamide-carbon) with carbide achieved for gfrP but not for cfrP and SMc diamond coated, PCD and cBn tools was presented Turning of FRP The machinability of GFRP in precision turning by means of tools made of various materials and geometries was A significant amount of research work has been carried investigated experimentally in [17]. It was found that, by out in applying turning processes to the various FRP proper selection of the tool material and geometry composites with different cutting tools excellent machining of the workpiece is achieved and the Turning, together with drilling, milling and sawing, belongs surface quality relates closely to the feed rate and the tool to the most important cutting technologies for the Flank wear as well as retreat and rounding of the cutting machining of FRP [15]. Turning differs from milling and edge are the most frequently observed wear effects during sawing mainly because an almost constant engagement of cutting of FRP [18-20]. Hereby, the wear speed is mainly the tool exists. Apart from fluctuations in stress caused by elated to the fibre content [18]. Furthermore, crater we the different cutting behaviour of the fibres and the matrix occurs only to a minor extent [19]. The cause for this wear a quasi-continuous cut exists during turning of FRP behaviour results from the discontinuous chip formation The machinability of FRP is primarily determined by the during cutting of FRP. Hence, fracture on the face occurs physical properties of the fibres and the matrix as well as only to a minor extent, whereas fracture on the flank is the tation and volume fraction whi main reason for the examined wear types [19, 22, 23].The and carbon fibres break in a brittle manner under bending selection of clearance angle is therefore stresses, aramid fibres undergo shearing fracture under ecommended to improve the tool life. However, it must be high deformation bending and tear under tensile loading noted that this causes a weakening of the tool that may composites is much easier than that of unidirectional FRP. clearance angle has to be determined for every to stimum Moreover, the machining of short fibre reinforced omote cutting edge chipping. Thus, an opt
Figure 3: Examples of turned FRP parts [15]. Figure 2: Cutting mechanisms for FRP composites [6] The machinability of CFRP and GFRP was deeply investigated in [7]. A model for cutting force prediction in orthogonal cutting operations was presented. Three parameters were initially varied during orthogonal cutting using HSS tools: tool rake angle, relief angle and depth of cut. Their effects on cutting forces were investigated and an optimal tool geometry was found. The effect of tool wear on cutting forces in the machining of unidirectional GFRP was investigated too [8]. In [9], cutting forces were found to increase with increasing depth of cut during orthogonal machining of unidirectional CFRP. The effect of fibre orientation on cutting forces and cutting quality in orthogonal machining of unidirectional CFRP was treated in [lo]. In [Ill, the attention was focused on the mechanisms of chip generation. Because of the inferior surface quality of unidirectional CFRP after orthogonal machining for some fibre orientations, in [I21 the development of a new tool geometry to reduce work material surface damage was investigated. In [13, 141, tool wear development was studied and monitored using acoustic emission (AE) signal detection and analysis during orthogonal cutting of different types of composites: unidirectional GFRP and CFRP, and sheet moulding compound (SMC). Decision making on tool wear state was performed through graphical examination and neural network computation of AE spectrum features. Different results were obtained according to the composite material type: tool wear discrimination was reliably achieved for GFRP but not for CFRP and SMC. Turning of FRP A significant amount of research work has been carried out in applying turning processes to the various FRP composites with different cutting tools. Turning, together with drilling, milling and sawing, belongs to the most important cutting technologies for the machining of FRP [15]. Turning differs from milling and sawing mainly because an almost constant engagement of the tool exists. Apart from fluctuations in stress caused by the different cutting behaviour of the fibres and the matrix, a quasi-continuous cut exists during turning of FRP. The machinability of FRP is primarily determined by the physical properties of the fibres and the matrix as well as by the fibre orientation and volume fraction. While glass and carbon fibres break in a brittle manner under bending stresses, aramid fibres undergo shearing fracture under high deformation bending and tear under tensile loading. Moreover, the machining of short fibre reinforced composites is much easier than that of unidirectional FRP. Although the cutting of FRP composite parts is rarely desired, it can be seldom avoided for the production of the final geometry, surface quality, and form accuracy of conventionally produced parts. Turning is applied to rotation-symmetric parts such as drag links, bearings, spindles, axles, rolls, or steering columns, etc. Figure 3 shows some typical FRP parts produced by turning. Particular attention was given by several authors to the aspects of tool wear mechanisms and development in turning of FRP composites with the aim of establishing correct cutting tool selection criteria. Among the possible wear mechanisms, which include abrasion, adhesion, tribo-oxidation and surface damage, only abrasion, surface damage and sometimes adhesion are of significance for FRP machining. Wear mechanisms are primarily related to the physical and mechanical characteristics of the different fibre-matrix systems. Glass and carbon fibres show a strongly abrasive behaviour because they are extremely abrasive by nature. Aramid fibres, on the other hand, impair the tool due to their low heat conductivity and ductile behaviour. Adhesive wear occurs when carbonised or molten matrix depositions settle on the tool surfaces. In [16], an analysis of tool wear during turning of GFRP and CFRP with diamond coated tools was carried out. The dominating wear mechanisms during cylindrical turning such as cutting edge blunting, elimination of the coating layer, retreat of the cutting edge, and crater wear formation were characterised and their development was explained. In [15], a survey on the possibilities and variants of application in turning of different types of FRP (polyesterglass, epoxy-glass, polyamide-carbon) with carbide, diamond coated, PCD and CBN tools was presented. The machinability of GFRP in precision turning by means of tools made of various materials and geometries was investigated experimentally in [17]. It was found that, by proper selection of the tool material and geometry, excellent machining of the workpiece is achieved and the surface quality relates closely to the feed rate and the tool. Flank wear as well as retreat and rounding of the cutting edge are the most frequently observed wear effects during cutting of FRP [18-201. Hereby, the wear speed is mainly related to the fibre content [18]. Furthermore, crater wear occurs only to a minor extent [19]. The cause for this wear behaviour results from the discontinuous chip formation during cutting of FRP. Hence, fracture on the face occurs only to a minor extent, whereas fracture on the flank is the main reason for the examined wear types [19, 22, 231. The selection of a large clearance angle is therefore recommended to improve the tool life. However, it must be noted that this causes a weakening of the tool that may promote cutting edge chipping. Thus, an optimum clearance angle has to be determined for every tool
In [20, 21], investigations on cutting of CFRP showed that the hardness and microstructure of the cutting edge for 4 various PCD tools exert a significant influence on the effectiveness of FRP machining. Coarse-grained PCD PCD tools, in particular, reveal higher resistance to wear than E103 medium- and fine- grained Pcd types. The wear appears in the form of cutting edge rounding, chipping and crack formation on the different PCD types. Carbide tools also display a flank wear yet more irregularly and with a tool life significantly shorter in comparison. The wear is haracterised by scratches and chippings. A longer tool life is achieved by PCd and Tic or Tac free carbide types 10 due to their higher thermal conductivity pressure strength and wear resistance. Interrupted cutting during turning of CFRP causes a higher wear than continuous cutting with carbide tools under equal cutting conditions In [15], tool wear was studied during turning of GFRP obtaining good results with carbide and PCD tools. Yet, the wear for these two tool materials is considerably different. Carbide tools exhibit mostly flank wear and unding of the cutting edge. Crater wear does not occur deposited on the tooray but carbonised chip material is Figure 5: Tool life for carbide, diamond-coated and PCD also show flank wear. However, the rate of wea development is clearly slower in comparison with that for tools vs cutting speed in turning of GFRP (Vi= 35%)[15] carbide tools. Furthermore the cutting speeds attainable th PCd tools are much higher than those possible with A comparison in tool life of uncoated and diamond-coated carbide too carbide tools shows the protective effect of the diamond Figure 4 shows the influence of cutting speed on tool life layer on the carbide substrate, granting protection against luring turning of different GFRP composites: unidirectional the diamond-coated tool life is surpassed by PCD tools lass cloth/epoxy with fibre volume fraction V, =55% The degradation of adhesion of the diamond layer to the EPRU 5), bidirectional glass roving fabric/epoxy with Ve carbide substrate is the main reason for this behaviour 45%(EPR 8), glass mat/polyester with Vt= 35%(UPM Tool wear in CFRP machining is significantly different from tool wear in GFRP machining. As Figures 6 and 7 performance was verified with increasing cutting speed illustrate, a minor relation of tool life to cutting speed exists he EPRu 5 composite displays the worst tool life, the EPR 8 composite an intermediate tool life, and the UPM for CFRP machining in comparison with GFRP machining 72 composite the best tool life behaviour. This can be igure 4 and 5). The flatter trend of tool life versus cutting peed for CFRP is related to the lower temperature explained by the fact that lower glass fibre volume development during machining due to the much higher fractions result in lower thermomechanical stresses of the heat conductivity of carbon fibres. Thus, higher cutting cutting edge and, consequently, higher tool life values speeds can be utilised during CFRP turning shown in Figure 5 during turning of the UPM 72 thermoset matrix CFRP(Figure 6)indicates that much composite. The superiority of diamond-based cutting materials, PCd and diamond-coated carbide, over higher cutting speeds can be used with PCD tools. It must monolithic carbide tools is clearly seen. While the tool life trend is decreasing for carbide tools, a linear development the two tool materials. While during machining with carbide tools a vb 0.2 mm was used as tool life is evident for diamond-coated tools and pcd tools criterion the vB for Pcd tools was reduced to 0. 1 mm. If a standard VB =0.2 mm were taken as a basis, the total volume of material removed for PCD would surpass the total volume of material removed for carbide by 250 times Moreover, the tool life for both cutting tools shows that with increasing cutting speed the temperature influence on the UPM 72 tool life increases. The steeper rise in tool life for PCD tools demonstrates the extended range in cutting speed g [EPR Figure 7 shows a comparison between uncoated and diamond-coated carbide tools in turning of thermoplastic 10 matrix CFRP. The tool wear during turning of polyamides matrix CFRP with diamond-coated tools is characterized by small chippings of the coating. The degree of chipping increases with the engagement time of the cutting edge up to little beyond the contact area between chip and rake face. The base carbide becomes smooth and the cutting edge rounded. The sharp-edged transition between carbide and coating layer or tool face is also subject m/min 10 to the abrasive action of the carbon fibres and is removed he direction of the chip flow. Independently of the cutting parameters, thermoplastic matrix deposits form on Figure 4: Tool life of diamond-coated carbide tools vs the tool face and flank but are periodically removed during cutting speed in turning of different GFRP composites [15] turnIng
In [20, 211, investigations on cutting of CFRP showed that the hardness and microstructure of the cutting edge for various PCD tools exert a significant influence on the effectiveness of FRP machining. Coarse-grained PCD tools, in particular, reveal higher resistance to wear than medium- and fine-grained PCD types. The wear appears in the form of cutting edge rounding, chipping and crack formation on the different PCD types. Carbide tools also display a flank wear, yet more irregularly and with a tool life significantly shorter in comparison. The wear is characterised by scratches and chippings. A longer tool life is achieved by PCD and TIC or TaC free carbide types due to their higher thermal conductivity, pressure strength and wear resistance. Interrupted cutting during turning of CFRP causes a higher wear than continuous cutting with carbide tools under equal cutting conditions. In [15], tool wear was studied during turning of GFRP obtaining good results with carbide and PCD tools. Yet, the wear for these two tool materials is considerably different. Carbide tools exhibit mostly flank wear and rounding of the cutting edge. Crater wear does not occur in any significant way but carbonised chip material is deposited on the tool rake face during cutting. PCD tools also show flank wear. However, the rate of wear development is clearly slower in comparison with that for carbide tools. Furthermore, the cutting speeds attainable with PCD tools are much higher than those possible with carbide tools. Figure 4 shows the influence of cutting speed on tool life during turning of different GFRP composites: unidirectional glass cloth/epoxy with fibre volume fraction Vf = 55% (EPRU 5), bidirectional glass roving fabridepoxy with Vf = 45% (EPR 8), glass matlpolyester with Vf = 35% (UPM 72). For all GFRP materials, a decrease in tool performance was verified with increasing cutting speed. The EPRU 5 composite displays the worst tool life, the EPR 8 composite an intermediate tool life, and the UPM 72 composite the best tool life behaviour. This can be explained by the fact that lower glass fibre volume fractions result in lower thermomechanical stresses of the cutting edge and, consequently, higher tool life values. The influence of various cutting tool materials on tool wear is shown in Figure 5 during turning of the UPM 72 composite. The superiority of diamond-based cutting materials, PCD and diamond-coated carbide, over monolithic carbide tools is clearly seen. While the tool life trend is decreasing for carbide tools, a linear development is evident for diamond-coated tools and PCD tools. Figure 4: Tool life of diamond-coated carbide tools vs. cutting speed in turning of different GFRP composites [15]. Figure 5: Tool life for carbide, diamond-coated and PCD tools vs. cutting speed in turning of GFRP (Vf = 35%) [15]. A comparison in tool life of uncoated and diamond-coated carbide tools shows the protective effect of the diamond layer on the carbide substrate, granting protection against abrasive wear and thermal wear. At high cutting speed, the diamond-coated tool life is surpassed by PCD tools. The degradation of adhesion of the diamond layer to the carbide substrate is the main reason for this behaviour. Tool wear in CFRP machining is significantly different from tool wear in GFRP machining. As Figures 6 and 7 illustrate, a minor relation of tool life to cutting speed exists for CFRP machining in comparison with GFRP machining (Figure 4 and 5). The flatter trend of tool life versus cutting speed for CFRP is related to the lower temperature development during machining due to the much higher heat conductivity of carbon fibres. Thus, higher cutting speeds can be utilised during CFRP turning. A comparison between carbide and PCD tools in turning of thermoset matrix CFRP (Figure 6) indicates that much higher cutting speeds can be used with PCD tools. It must be noted that different criteria of tool life were adopted for the two tool materials. While during machining with carbide tools a VB = 0.2 mm was used as tool life criterion, the VB for PCD tools was reduced to 0.1 mm. If a standard VB = 0.2 mm were taken as a basis, the total volume of material removed for PCD would surpass the total volume of material removed for carbide by 250 times. Moreover, the tool life for both cutting tools shows that with increasing cutting speed the temperature influence on the tool life increases. The steeper rise in tool life for PCD tools demonstrates the extended range in cutting speed compared with that for carbide tools. Figure 7 shows a comparison between uncoated and diamond-coated carbide tools in turning of thermoplastic matrix CFRP. The tool wear during turning of polyamides matrix CFRP with diamond-coated tools is characterized by small chippings of the coating. The degree of chipping increases with the engagement time of the cutting edge up to little beyond the contact area between chip and rake face. The base carbide becomes smooth and the cutting edge rounded. The sharp-edged transition between carbide and coating layer on the tool face is also subject to the abrasive action of the carbon fibres and is removed in the direction of the chip flow. Independently of the cutting parameters, thermoplastic matrix deposits form on the tool face and flank, but are periodically removed during turning