Machining of New Materials

Keynote Papers

Machining of New Materials W. Konig, IPT AachenIFRG; L. Cronjager, U Dortmund/FRG; G. Spur, TU BerlinIFRG; H. K. Tonshoff, U HannoverIFRG; M. Vigneau, SNECMAIF; W. J. Zdeblick, METCUTIUSA

Complex requirements regarding the combination of geometric functions and mechanical properties cannot be met by conventional materials. Low wear and high strength in aggressive environments, at high temperatures or high strength combined with low weight, are some characteristics of those materials. Full exploitation of the outstanding properties requires machining processes which base on the respective material features and preserve the advantages of the material as well as cost efflcicncy. Hlgh performance ceramics, high temperature alloys, metal-matrix composites and fibre reinforced plastlcs are examples of materials which have to be machined with adapted conventional machining techniques. On the other hand, they initiate the development of new machining processes. Especially the high flexibility of laser-assisted turning opens up new applications for machining metal-matrix composites or high temperature alloys. Key words : high performance ceramics, high temperature alloys, metal-matrix composites, fibre reinforced plastics, machinabllity, workpiece quality, stress condition, laser-assisted machining. Acknowledgements : D. Biermann, U Dortmund; A. EBmcke, ZAPPlFRC; C . Byrne, T U Berlin; J. Friedrich, MTUIFRC; K. Gerschwiler, T H AachenlFRC; K.C. Cirnter, SIEMENSIFRC; C.A. v.Luttervelt, T U DelftlNL; CI. khmitz-Justen, IPT Aachen.1.H. Tlo, T U Berlin; E. Uhlmann, T U Berlln; B. Wedding, U Hannover; written by R. Kleinevoss, IPT Aachen 1. Introduction The function and reliability of highly-developed technical systems can, in many cases be provided only if the properties of the components employed meet the highest requirements. The more complex a component's function and, consequently, the requirements on the material properties are, the closer the correlation between those properties and the machining technologies in machining this component (Piaure 1)Condensing component functions is not only achieved through restricted dimensional and shape tolerances or improved surface quality but is perfected with those materials featuring a combination of the required material properties. Due to the improvement of already established materials and the development of new ones - summarized under the notion of "materials-design" the designer has materials available which meet the required property profile. These materials, however, can clearly be told apart from their conventional predecessors in terms of composition, structure and properties. Thus, machining has to be adapted especially to the specific characteristics of the material.

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The machining target therefore has to aim at providing high and reproducible product quality. This requirement is also an inevitable result of the aspects of product liability and competitiveness /1,2,3,4/. Another requirement on production is cost efficiency, meaning that both operator and machining equipment have to be employed as economically as possible.

machining is illustrated using example of grinding high-performance ceramics / 7 4 / . This method has been chosen because it is most widely spread among the machining processes employed in this group of materials /5/. Materials in the group of composites are based on resin matrices, such as fibre reinforced thermosets, thermo-plastics and elastomers, as well as materials based on metal or ceramic matrices. Thus, machining must aim at cutting the individual components without damaging the whole compound. To that end, it is necessary to adapt both tool parameters and process of the parameters to the different properties individual components on the one hand. On the other hand, the orientation and relative proportion of the components within the compound have to be taken into account.

A paper on "Machining of New Materials" cannot Only deal with the transfer and adaptation of conventional methods, but must also cover the development and application potentials of innovative machining methods. Heat-assisted machining of high-strength or very abrasive materials, is a machining technology which is still being developed. Two examples of plasma-assisted and laser-assisted machining will be illustrated and discussed under this subject. 2. Material-adaDted machinina of brittle materiala

Brittle materials feature low ductility and fracture toughness at room temperature and standard pressure so that fracture will occur once atomic linkage forces are exceeded. As far as brittle materials are concerned /8,9,28,10/, ductile grinding of optical glass is presently considered to be the most perfect

Figure 1: Full exploitation of material properties requires material-adapted machining It is a matter of fact that these rules are valid for the adaption of present machining techniques as well as for the development and application of new machining techniques. This correlation becomes obvious when machining brittle materials. Here, the focus is on high performance ceramics which are often used because of their hard-ness, resistance to wear and corrosion, strength at high temperatures and capacity of heat insulation. These advantages, however, are opposed by low ductility, a marked tendency to crack and, last but not least, high machining costs. Thus, machining tech-nologies which guarantee high and reproducible Component strengths and permit the highest exploitation of the tools at the same time are necessary. In the following chapters material-adapted

Annals of the ClRP Vol. 39/2/1990

Figure 2: Crackless material removal via ductile grinding / 6 , 7 , 8 / adaptation of a machining method to the material ( F~auJ..e 2 ) . Although glass is a solidified fluid, the condition of which is described by specifying its viscosity dependent on temperature, it behaves like a solid. Its high brittleness stems from the irregular arrangement of the atoms. Compared to crystal, in

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which the atoms have a fixed arrangement and regularity, described by Miller's indication and grating constants, the network structure of glass does not show any orientation (left of figure) /6/. Missing sliding planes and strong, directed covalent bonds of the Si02-tetrahedra, such as in fused silica, limit fracture toughness to values of less than 1 MPafm. Never-theless, glass can be machined in a ductile process if cutting depths are below 10 MI and are, thus by a order of magnitude, lower than that in machining silicon carbide /7/ (upper diagram). The ground surface machined in a purely ductile manner shows a material removal without cracks, featuring a peak-to-valley roughness of 14 nm (SEM-photography). If high shaping accuracy is achieved at the same time, such optical elements can be applied without being polished.

concentrations as illustrated in in a sealing disc (top left of initiate crack propagation. Here, stress which leads to spontaneous

the example of holes figure 4) /21/, can the critical tensile crack propagation is

Due to high investment costs for an ultra-precision grinding machine and comparatively low material removal rates this kind of machining is limited to highly precise optical components. However, this example clearly shows that the generation and propagation of cracks can be suppressed or at least considerably restricted, even in extremely brittle materials, by means of adapted process parameters. The fundamental mechanisms which permit this kind of material removal are the same with glass and highperformance ceramics. They were already described at the end of the 19th century and in the 1920s. The background to Hertz's /in 24/ and Boussinesq's /in 24/ observations shows complete mathematical solutions describing the spatial indentation stress field under a spherical (Hertz) and a sharp (Boussinesq) indenter. Griffith demanded the existence of a hydrostatic state of stress in order to restrict crack propagation when atoms leave their state of equilibrum /13/. Today, these insights are exploited in material-adapted machining of high performance ceramics, particularly since instruments, machine tools and tools have been developed to such an extent that applying theory to practise is now possible. According to Griffith, fracture under tensile load of a solid made of brittle material is always initiated by a crack. With chemically polished glass fibres, it can be proven, that tensile strength rises with the increasing depth of the layer etched but reaches a maximum value at least one order of magnitude below the theoretical strength /14/. These results show that, despite the micro inhomogeneities in the structure, surface damage crucially affects strength. This is also true of ceramic materials which, unlike glass, have a polycrystalline structure. Tensile strength is not only determined by contamination in preprocessing stages, inhomogeneities or residual stress after sintering, but above all by cracks in the surface zone /18,44/. Cracka present after grinding especially diminish the tensile strength of a component /5,15/ (Piaure 3). The stronger a ground sample made of hot-pressed silicon nitride is, the more the damaged zone is removed by means of precision lapping. At the same fai ure probability, fracture strength is about 200 N/mm' higher after removing an allowance of 50 p . The lower bending fracture strength at a removed depth of 200 pm is seen as the result of the inhomogeneous distribution of the material strength. r a t e r i a l : msr i A l grinam PTlndlW *heel: 0126 K-DIUS 888 JV C50 cwlmt ewlsion 4,s z direction of wlnalw: t r m v e r s a l

Figure 4: Causes for failure of ceramic components not only determined by the crack form but mainly by fracture toughness. Fracture toughness also called is a material constant with stress intensity factor a linear character. Approximate calculation of the critical tensile stress dependent on fracture toughness clearly shows that this strength value is about a order of magnitude lower with ceramics than with high-strength steels and titanium alloys /19,20,22/ (right diagram), although the cracking lengths are the same. The type of production process determines the strength and shows a wider spread than with metal materials. Cracks which occur or enlarge during machining explain for the behaviour of ceramic components which deviate from the layout data. Key focus of machining, therefore, is to avoid cracks altogether or to restrict crack propagation to a magnitude which increases the fracture strength and at the same time increases Weibull's modulus as an indicator for reproducibility /23/.

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Material behaviour during the penetration of a hard indenter, i.e. during the grinding process, can be observed through indentation tests and scratch tests /67,68/. They are the basis for describing materialspecific mechanisms of fracture and removal ( P i a u r e

5 )*

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Figure 5: Edge load and chip thickness determine removal behaviour The illustrated diamond vertical penetration of material.

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Tu Berlin

Figure 3: Determination of the depth of damage by means of lapping the damaged surface /5/ The theory of notch tension /16,17/ gives reasons for strength diminishing by the effects of surface zone cracks. It proves that sharp-edged roundings show tensions which are several factors higher than the nominal stress upon the component (Fiaure 4). Stress

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indentations simulate a cutting edge into

the the

At a load of 3 N, the indentation depth amounts to about 1,6 pm (top left of figure). The corners of the rectangle show cracks. Here, the critical tensile stresses are exceeded and spontaneous crack propagation is initiated. These cracks cross the surface but extend mainly into the bulk material /11,12/. As they almost completely close on recess of the load they can only be identified by methods of destructive testing at present. At reduced forces (right part of figure), the diamond penetrates into the ceramics to a depth of only 0,4 p , thus generating plastic deformations which can be seen as bulges at the edges of the indentation.

The results of scratch tests which adapt the diamond's depth of cut to the penetration depths clearly show the dependence of the removal mechanism on the scratch depth. There is brittle fracture material removal at a scratch depth of 1,2 p . Only if the scratch depth is reduced to half, the diamond forms a track by plastic deformation (bottom right figure]. For a grinding process adapted to ceramic material:

ceramic material is a large cutting edge radius. This can be carried out with a block diamond grit which does not break or split but flattens equally during machining. The application of such grit in connection with harder bonds offers yet another potential for the material-adapted process layout. How can these technology?

results

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to

process

During surface grinding with peripheral wheels, the material removal rate is the product of infeed and feed

Figure 6: Suppressing cracks with self-induced compressive stress

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the force per individual cutting edge has to be low the scratch depth must not exceed a magnitude specified for the material.

The identification of other relevant characteristic values for the material-oriented machining has to take the shape of the cutting edge into account; since the tensile stress field induced by the cutting edge in the material is significant for the kind of material removal - brittle fracture or ductile deformation. It is known that crack propagation can be restricted in a hydrostatic compressive stress field. Wherever marble becomes ductile at a combination of sufficient compressive stress and shear stress, it can be deformed without cracks /13/. Stress distribution of materials penetrated by sharp-edged and spherical elements can be described mathematically and is (Fiaure 6). If illustrated in diagrams / 2 4 , 2 5 , 2 6 / Boussinesq's and Hertz's cases of load are contrasted, the first kind of stress is not advantageous with regard to the machining task of "crack avoidance". A sharp-edged element limits the compressive stress field to an extremely small area immediately under the tip of the cutting edge (upper series of figures). In radial direction, there are exclusively tensile stresses which may lead to the formation of cracks. The corresponding scratch track, generated with a cutting edge radius of 5 pn features a rough topography which results almost exclusively from brittle fracture. spherical element, as used for Hertz's case of load, causes a stress field which consists mainly of compressive stress (lower series of figures). This stress condition can be considered a precondition for the sliding of the crystallites caused by dislocations, although the mechanisms for ceramics have not yet been explained sufficiently nor in detail. The scratch track of a diamond with a cutting edge radius of 60 pm is generated at the same depth of scratch by plastic deformation. This was also observed by Shore /27/. He flattened the diamond (grittruncating) in order to obtain negative rake angles and large cutting edge radii where the cutting edge and material have contact. Bending test samples that have been ground in a ductile way with this tool are twice as strong as conventionally machined workpieces. Hydrostatic stress also supports activation of gliding systems also with materials of very low fracture toughness (SiSiC) and restricts the formation of cracks to a large extent. A grinding process carried out under these conditions opens up the possibility of improving material strength by inducing compressive residual stresses. Spur and Ti0 / 2 9 / , Tonshoff /30/ as well as a cooperative work of CIRP /71/ proved compressive residual stresses in ground A1203: and Si3N4-'eramics. Evans, Marshal et a1 /31/ investigated also Si-,N4 and observed that residual stresses occur even at a depth of 100 m. A

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Figure 7: Ceramic-adapted parameters in creep feed grinding speed (Fiaure 7 ) . At a constant material removal rate, engagement conditions, e.g. contact length, vary greatly (see hatched parts of upper figures) due to the combination of these two values. This allows differentiation of two process types: alternating grinding and creep feed grinding. To realize the above mentioned material-adapted characteristic cutting values, creep feed grinding (right illustration) is better for the distribution of material removal on a much larger number of cutting edges than alternating grinding /32,15/. At a constant material removal rate, contact length increases with decreasing feed speed and rising infeed. As the number of cutting edges in engagement rises proportionally, the individual chip thickness decreases. Diamonds are loaded only in the area of the cutting edge tip. They slowly become blunt and generate a grinding wheel topography, the characteristics of which are a large number of kinematic cutting edges. At cutting speed increases, graphitising processes are initiated which seem to support the formation of large cutting edge radii. simultaneous engagement of a large number of edges and the large proportion of energetically necessary friction lead to the required low forces at the individual cutting edges; however, they total in the rise of the specific grinding normal force.

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0 15 30 45 60 N/nm 90 mrml winding force per unit of actlve arlndlw *heel wldth Fh,nax

Figure 8 : High component quality and double cutting performance via ceramic-adapted process parameters

Thus, the third step for a grinding process adapted to

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Therefore, machine parameters adjusted to ceramic materials can only be achieved on grinding machines with high rigidity and sufficient spindle drive power. Applying material-adapted grinding methods to highperformance ceramics improves application behaviour, illustrated here as an example of characteristic fracture stress and Weibull's modulus, it also diminishes the peak-to-valley height (Piaure 8). Oxide ceramics (A1 0 ) nitride ceramics (SSN) and carbide ceramics (Sibs,' do not only bear higher fracture stresses but also prove to be more reliable: a higher Weibull's modulus is synonymous with a narrower distribution of the strength values measured. This is especially important with respect to reproducible component manufacture / 1 5 / . The reduction of the peakto-valley roughness is large, especially in the case of the SiSiC-ceramics so that, depending on the application of the component, a finish-machining can be avoided. These results are particularly interesting when comparing material removal rates: with improval in workpiece quality, optimized machining shortens the machining time by about 50 p.c.1 However, as grinding normal forces rise to about six times the previous amount (SiSiC), machining can only be carried out with machines especially designed for such conditions.

3.

Material-adauted machinina with aeometrically defined cuttina edae

Generating ductility is essential for material-adapted machining of high-performance ceramics. This condition can easily be realized in the grinding process via the low depths of cut of the cutting edges. When grains impact the material, it is assumed that heat concentration occurs in the immediate contact area of the edge due to low heat conduction within the ceramics /69/. Together with appearance of high compressive stresses, this heat is sufficient to cause local ductility and generate plastification. Local ductility is also required for the cutting of metallic materials, especially if these materials, so far only machined in grinding processes, have to be cut with a geometrically defined cutting edge / 7 2 / . A n example of this is the turning of hardened steel which can be machined with adapted process parameters in a ductile manner, although its hardness is more than 60 HRC. Here too, chip formation is generated under extreme compressive stress induced into the area of engagement by a tool with an extremely negative rake angle / 3 3 / ( P i a u r e 2). If forces exceed the local material strength, a shearing crack will occur in front of the cutting edge (see diagram). The crack reduces the induced energy and serves as a sliding plane for the material segment being pushed onto it. This crack formation at the immediate contact area of "cutting edge - chip bottom surface" can be clearly seen in the cross section of the chip root (figure 9, lower photo). Compressive stresses and heat generated by friction at the cutting surface of the tool improve the local ductility in the shear zone to such an extent that a chip similar to a saw tooth energes. It is held together by means of the initially plastified and then newly-hardened material (upper photo).

conductivity of the cutting material is of particular importance here. Poor heat conductors, such as oxide ceramics or mixed ceramics, impede heat transfer through the tool so that the induced amount of heat is sufficient for the plastification of the material aid a so-called "self-induced hot-cutting process" will occur. The development of cutting materials made of polycrystalline cubic boron nitride (PCBN) takes this into account: PCBN-cutting materials of low heat conductivity are now successfully employed in hard machining of steel /34/. The low conductivity of the PCBN-cutting materials is achieved by a ceramic bonding phase with a reduced proportion of CBN-grains. Hard machining of metal alloys with defined cutting edges has advantages over grinding processes: higher removal rate and geometrical flexibility as well as the lack of dressing processes. However, tool wear and especially effects on the boundary zone close to the surface have to be taken into account. Otherwise, this could cause non-acceptable component behaviour. In tcrning with PCBN, hard coatings made of cobaltalloys show not only plastic deformation in the machined zone of the workpiece but also material strengthening and destruction of the network consisting of chromium carbides in a metal matrix /35/. The temperature at the cutting point is considered to be the relevant parameter. Thus, by means of external heat input during a simultaneous variation of the cutting parameters, the process temperature has to be set at a point where both the effects on the surface and the wear of the cutting material are minimized. This is particularly interesting in the case of alloys containing a higher amount of hard material and which presently allow only a limited application of cutting processes with defined cutting edges. During the first cutting tests, the relief face wear was cut back by 40 % with a plasma burner. Investigations into the surface zone still need to be made /36/. This example only partly illustrates the possibilities of applying external heat in a cutting process with defined cutting edge, as far as the range of materials to be machined is concerned. If the removal mechanisms are known and if the predominant physical principle can be transferred to corresponding process parameters as well as adjusted to the machining task by means of technical characteristics, the hot machining process does not have be limited to metal materials /66/. In the case of machining ceramics, the significant physical parameters are both compression and temperature. The scratch depth is the characteristic technical efficient parameter. When using tools with defined cutting edges, machining must take place under the same conditions as in grinding: local heat treatment to soften the material is necessary in the primary shear zone. Since the chip cross sections are several orders of magnitude larger than those of the grinding process, the energy induced by the tool is, of course, not sufficient for local softening in the shear zone because of the high material strength and the poor heat conductivity. Therefore, external heat input has to balance the energy deficit. However, requirements on this energy source can only be fulfilled with a laser. Only a laser beam offers the required density of energy and can realize the necessary speed for the heating. Moreover, owing to its beam characteristics, it generates an extreme tempezatcre gradient in the material which .ezkes it possi?'= - c focus the heat onto a very l i d t e d area which, r
F i g u r e 9 : Formation of chips in turning of case

hardened steel / 3 3 / The better the induced heat generated via plastification during the cutting process is concentrated in the secondary shear zone, the more developed the newly hardened zone will be. The heat

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In order to generate higher material removal rates and to enhance the flexibility of component design, it has been tested for external diameter turning to what extent the advantages of the defined cutting process of ductile materials can be transferred to the machining process of HPSN /37/. Since the strength of

this material depends on temperature, a materialadapted machining technology is also possible for tools with defined cutting edges (Fiaure 10, top left diagram). Since the grain boundaries of the mostly crystalline structure of silicon nitride show amorphous surfaces, a partial ductility of the vitreous phase, important for chip formation, can be expected at temperatures of more than 1000°C. HPSN is of particular interest for hot ma-chining due to its comparatively high fracture strength combined with relatively low thermal expansion and, consequently, its low thermal shock sensitivity.

F i g u r e 10: Laser-assisted turning of hot-pressed

silicon nitride Test results corroborate the following hypotheses: At temperatures of less than ll0O'C strength is not sufficiently lowered and still breaks in a brittle mode. Chipping piece and the cutting edge lead to quality and rapid cutting edge wear.

the material the material on the workpoor surface

A temperature rise of 1200 to 1450°C, however, results in the required plastic chip formation. The removed material can be distinguished as a continuous chip, separated in a ductile mode (SEW-photo on the right side). The produced surface on the workpiece shows grinding quality. Temperatures of more than 1450°C lead to oxidations which cannot be tolerated on the workpiece surface. If the temperature rises further, the material dissociates. The development of laser-assisted machining with geome-trically defined cutting edges for highperformance ceramics has just been launched. Further investigations have to be carried out: performance tests of the machined components, process optimization by means of changing the location of the laser focus so that the shear zone faces even a higher thermal concentration. Moreover, laser-assisted machining opens up new and interesting perspectives, in particular with cutting metallic high-temperature materials generally difficult to cut. Hightemperature strength leads to short tool lives and low cutting performance. Increasing removal rates realized in turning nickelbased-alloys by employing cutting ceramics and polycrystalline cubic boron nitride (PCBN) /38,39,50/ cannot be transferred to other members of this group such as titanium and titanium-aluminium alloys, monocrystals and poly-crystals as well as ODSmaterials /46-48/. Here, laser application offers advantages as its heat energy can be exactly controlled and the heat-affected zone can be closely limited. Por instance, a defined cutting of ODSferrite8 at temperatures of 600 to 800°C with a laser beam as a heat source is feasible. Especially jet engine manufacturers show great interest in this technology. By the year 2006, civil aviation will need about 3500 new jet aircrafts /43/. More emphasis will be placed in the efficiency of turbine engines' performance. Therefore, the use of high-resistance materials will increase be used and the demand for economic machining technologies rise correspondingly. 4. Machinina of comwsites In a composite, matrix and reinforcing fibres are combined in such a way that the new material incorporates the advantages of both original components. The concept of "Material design" is most

evident here since as the properties of the material can be predetermined by computer simulation incorporating the type and composition of the matrix, the proportion of the reinforcing fibres, and the process of integration. Consequently, there are a variety of fibre-matrix-combinations, although only a few are presented here. In the aerospace industry, materials are required which feature low specific weight in addition to high strength or high temperature resistance. Examples of a high-temperature resistant composite are the ceramic matrix composites (CMC), in which a multiple layer of silicon-carbide fibres is surrounded by a siliconcarbide matrix via a CVD-process. These materials, used as heat-shields in turbine engines, withstand temperatures of about 1000°C /40/. Machining is carried out by means of grinding, abrasive water-Jet appli-cation, laser treatment or, partly, by ultrasonic machining /39,40/. Often, machining with electro-plated diamond grinding wheels is used for contouring, drilling or cutting. Here, too, machining is mainly done in a creep feed grinding process to avoid damage of the composite and to achieve excellent surface quality. Another group consists of the metal matrix composites (MMC), composites based on a metal matrix, usually aluminium, the reinforcement of which is realized by ceramic fibres, particles or whiskers of boron carbide, aluminium oxide or silicon carbide. They are either machined with electroplated diamond grinding wheels or with carbide or PCD cutting tools. Due to the highly abrasive tool wear which increases with rising fibre volume proportion, PCD-cutting edges are more efficient than carbides /39,40,51,52,70/. During drilling, the application of lubricant enhances wear as the formation of wear-diminishing built-up edges is reduced / 5 2 / . Furthermore, lower cutting temperatures occuring in wet machining allow the complete maintenance of the strength and support of the abrasive fibres by the aluminium matrix. Among the composites, the group of fibre reinforced plastics (frp) could gain much importance due to their wide applicability, i.e. in aeronautics or high speed machine components with low moment of inertia /53-55/. Frp matrices can be subdivided thermoplastics and elastomers / 5 6 / .

in

thermosets,

Elastomers, reinforced by short fibres, can be found in multi-rib-vee-belts. A multi-rib-vee-belt consists of force-transmitting fibres and fibre-reinforced elastomers in which the vee-belt profile can be machined by grinding (Piaure 11 upper diagram). A characteristic feature of elastomers is their capacity to nearly complete return to their initial form after deformation has occured. This high reversibility must be taken into account in the machining process. Process layout has to be designed to keep machining forces at a low level so that no thermal damage of the composite may occur. Tests with grinding wheels (having a flat profile) of corundum, CBN or diamond with a grain sizeof D126 or 8126 have shown that especially the diamond wheel permits a low relative grinding force at a cutting speed of more than 80 m/s (lower diagram). If rough machining is followed by a spark-out-time,

F i g u r e 11: Grinding of short-fibre reinforced elastomers / 5 6 /

the relative diameter error which arises because of material resilience can be reduced to about 15 %. Thermosets and thermoplastics,

finally, are two more

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groups of frp's. Since cutting of thermosets has already been presented in a key-note paper / 5 7 / , this paper focuses on the ultrasonic cutting of prepregs and the routing of thermoplastics. The near-net-shape manufacture via pressing, injection moulding or coiling, is typical for the processing of frps /53-55/. However, in these machining steps, complete form accuracy is very costly. Thus, cutting processes are necessary which, apart from deburring and trimming operations, also include the generation of functional surfaces featuring high accuracy of shape and dimension as well as surface quality /So/. Whereas both fibre and matrix are simultaneously cut in finish-machining, only fibre fabrics or fibre mats have to be cut to a size and shape needed for subsequent operations. Beside the widely spread glass fibres the comparatively expensive carbon fibres and aramide fibres are employed as fibre materials for structure components. Structure, properties and machining behaviour of these fibres are described in

processing techniques, thermal curing is better than chemical curing of the thermosets. Complex part geometries can be reversibly formed and manufactured by spinning, deep-drawing or rolling, the material is weldable and remnants can be partially reutilized /53,61/. Fibre reinforced thermoplastics are widely used because of their impact toughness, fracture strain and, in particular, nondeformability at temperatures of up to 250°C. Due to the development of high temperature resistant thermoplastics, such as polyetherimid, polyetheretherketone, polyethersulfone or polyamidimid. The process for cutting this group of materials differs greatly from the cutting of metals, engineering ceramics or glass /63/. This is due to the inhomo-geneous structure of components with extremely different physical properties. Whereas the latter group's necessary ductility is achieved by means of high compressive stresses or high temperatures during the cutting process, it is the extreme ductility of the thermoplastic matrix which is a problem specific

/57,59/.

The machining of fibre-reinforced components starts by cutting of fabric layers, which are increasingly impregnated with resin. Impregnation of the fibres and the mixture ratio of the resin component is of utmost importance for the final component properties because the impregnation process should be disincorporated from the machining operation. Thus, prepegs are used for many processes. Prepegs are fibre structures preimpregnated with reaction resin (usually epoxy resin or phenolic resin) (Fiaure 1 2 ) . They consist mostly of a number of unidirectionally or multidirectionally arranged rovings. These rovings are bundles of parallel glass fibres, carbon fibres or aramid fibres (SEM-photo, center of figure). In the mainly manual process of cutting and trimming these mats with conventional tools (knives and scissors), machining times are very long. Additional problems arise through resin and fibre particles adhering to the blade /60/ and to the deviation and deformation of the fabric during the cutting operation, as the highstrength rovings are not separated immediately but are pushed away within the fabric and only tear on exceeding fracture tension. Here, it is advantageous to cut prepregs with an ultrasonic knife which separates the individual fibres in the rovings. When applied to brittle fibres (carbon, glass), this oscillation, supported by feed force, initiates separation at a low degree of deviation by inducing locally limited fracture systems (lower series of pictures). In contrast to machining of glass and ceramics, this process specifically exploits the brittleness and low fracture toughness of the material in order to achieve the machining result on a low-force level. Heat created by the friction of the knife at the ductile aramid fibre leads to material softening (bottom right SEM photo). A small cutting edge radius and feed motion generate the shear and tensile stresses which facilitate the separation process.

Figure 12: Ultrasonic cutting of prepregs After trimming, the prepregs are put into a mould and pressed at high temperature. A wide range of forming processes is available especially for the increasing use of prepregs with thermoplastic matrix. In terms of

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Figure 13: Tools, cut surfaces and chips for routing of long-fibre reinforced thermoplastics to this group of materials (Fiaure 13). The routing of unidirectionnally glass-fibre reinforced plytherimid shows / 6 2 / that high process temperatures combined with the poor thermal conductivity of the material soften the polymers in the area of engagement (right part of figure). The molten material adheres to the relief face of the tool, thus intensifying friction. As many composites tend to absorb liquids, coolant is not used for milling and drilling operations. The only way to achieve a permanent reduction of friction, therefore, is to use wear-resistant tools with cutting edge radii of 10 to 15 pm and surface qualities at rake and relief faces of less than 0,s p . This requirement is met by cutting edges of plycrystalline diamond. However, the process turns out to be economic only, if a CNC-machine is employed which permits sufficient cutting speed (vc>400 m/min) and a feed per tooth of 0 , l to 0 , 2 mm to be maintained even at small tool diameters. Like the machining of thermosets, the surface quality depends on the fibre orientation. In cuts at O D against the fibre orientation, the cutting forces separate the laminate at the boundary of fibre and matrix (lower photo), and tear individual fibres out of the matrix. Thus, peaks of roughness are generated in the otherwise smooth surface. Cuts at 4 5 O perform the poorest surface qualities. Due to the combined compressive and bending load, the fibres kink and break or are pulled out of the composite. At a 90' fibre orientation the fibre can be shorn. Fibre ends appear to be smooth and the plastified matrix has a partially smoothened surface (top left photo). Unlike the cutting of fibre reinforced thermosets there is no dust generation in the routing of thermoplastics. Chip formation occurs especially at a 9 0 ° fibre orientation (bottom left photo). Due to the brittle fibres, there are no continuous chips, but shapes which can be compared to lamellar chips. According to the state-of-the-art, the formation of chips is caused by the considerably high impact toughness and fracture strain of thermoplastic matrices. These properties manifest themselves in a plastic deformation of the matrix before this is cut. Carbon fibre and glass fibre reinforcements are only a little different here. This kind of material removal best regarding the protection of the operator and machine tool. Since the chips are about 15 times larger than those of thermosets, there is no hazard of pulmonary affection, and the enclosure of both working area and machine parts is less problematic.

5 . Summarv "Machining of New Materials" stands for machining operations that maintain the material's special features while being as cost efficient as possible. Demand for machining materials has always existed. However, conditions are new /64/. In the future, many new materials will be used. Moreover, introduction periods into industrial application will be shorter and machining will have to be more cost efficient and precise than today. Maintaining the outstanding properties largely depends on the circumstances under which the new materials are machined. Anyone who wants to do machining has to understand and consider the material-specific removal mechanisms. At the same time, he must be capable of transferring his knowledge into practise in order to provide a maximum amount of process reliability and reproducibility. Only then will he be able to exploit the potentials of the machining processes so that he can produce components featuring a high degree of integration in geometrical and mechanical functions. The effect of a material-adapted process layout spreads into all business areas. It thus directly determines product quality and competitiveness. ( p i a r e 1 4) /2/. Preconditions for the realiziation of a materialadapted machining technology are machines and tools designed according to the required machining operation. For instance, machining high-performance ceramics is hardly possible with conventional grinding machines. Machining forces require machines which feature high rigidity, precision drives and sufficient spindle drive power. Furthermore, they must also offer balancing and dressing processes which can be automatized. The manufacturing of components featuring high accuracy and surface quality will be a market of increasing growth in the next few years. With the development and application of ultra-precision machines, it is possible to machine components with geometrically defined cutting edges to tolerances formerly the domain of the conventional process sequence of grinding, lapping and polishing, a machining process which was very costly. With respect to material-adapted and component-adapted processes, these parts open the gates to a product range, which

twls to achleve hl(h-m.lalltY DrOdllCtE

rutwe market reaulrments

production also entails changes in the plant organization. The operator has to be released from the responsibility of a direct process control by an external calculation of cutting data and tool selection. Material-adapted machining demands comparatively high investment costs for machines, tools and data processing equipment. On the whole, it will remain cost efficient and it can open up the possibility of innovative product design to help keep the enterprise competitive in the future. Pursuing this train of thought is urgent as conventional technologies will fail with the engineering materials of tomorrow.

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adapted

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(-3

F i g u r e 14: Competitiveness and increased productivity

due to material-adapted process layout in the future, applications.

will

not

be

restricted

to

optical

The utilization of adapted machine tools also shows the way for machining of fibre-reinforced plastics. Short and economic machining cycles can only be achieved by means of high feed rates. Cutting processes require high cutting speeds, which in combination with the application of small tool diameters demand high spindle speeds at a minimum of 50000 l/min /65,73/. The complex component shapes require the use of five CNC-axes combined with a control which features particularly short cycle times. Automatic tool change and flexible workpiece clamping are two additional features for reducing machining time. AB far as the protection of the operator is concerned, an enclosure is necessary, since dust emissions occur which, especially in the machining of cfr-duromers, are classified as hazardous to health. Integrating a material-adapted process layout into

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