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Effect of HM substrates’ cutting edge roundness manufactured by laser machining and micro-blasting on the coated tools’ cutting performance
G Model CIRPJ 412 No. of Pages 10
CIRP Journal of Manufacturing Science and Technology xxx (2016) xxx–xxx
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CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Effect of HM substrates’ cutting edge roundness manufactured by laser machining and micro-blasting on the coated tools’ cutting performance K.-D. Bouzakisa,* , P. Charalampousa , T. Kotsanisa , G. Skordarisa , E. Bouzakisb , B. Denkenac, B. Breidensteinc, J.C. Aurichd , M. Zimmermannd, T. Herrmanne, R. M’saoubif a
Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Department, Aristotle University of Thessaloniki, Greece German University of Technology in Oman (GUtech), Department of Engineering, Oman Institute of Production Engineering and Machine Tools, Leibniz Universität Hannover, Germany d Institute for Manufacturing Technology and Production Systems, University of Kaiserslautern, Germany e Photonik-Zentrum Kaiserslautern e.V., Germany f Seco Tools AB, Fagersta, Sweden b c
A R T I C L E I N F O
Article history: Available online xxx
Keywords: Cutting edge rounding Laser machining Milling
A B S T R A C T
The effective application of laser machining for manufacturing the cutting edge roundness of cemented carbide inserts was investigated in a cooperative work of the Scientific Technical Committee (STC) “Cutting” of the International Academy for Production Engineering (CIRP). Machining with diverse laser beam pulsing duration and feed velocity as well as micro blasting were employed prior to the coating deposition for shaping the cutting tool’s edges. The wear behaviour of the coated inserts was tested in milling TiAl6V4 and Inconel 718. Considering the inserts’ surface micro-structure and mechanical properties developed after the laser treatment, the coated tools’ cutting performance deterioration or improvement compared to untreated substrates was explained. © 2017 CIRP.
Introduction The cutting edge geometry and the applied tool wedge rounding methods affect significantly the wear behaviour of coated tools [1–7]. In this context, appropriately large cutting edge radii in combination with optimized chamfer’s geometries reduce the tool mechanical stresses and may contribute to a significant improvement of the coated tool cutting performance [1–3]. Hereupon, by means of the K-factor method, a more precise cutting edge geometry description and optimization is enabled [2]. Furthermore, a key issue for attaining an augmentation of the coated cemented carbide tools’ life via increasing the cutting edge roundness is the applied manufacturing method. A thorough study of the efficiency of various cutting edge rounding methods as well as the effects of these treatments on the wear behaviour of coated tools in milling various metallic materials is introduced in Reference [1]. According to this study, the application of laser machining for manufacturing the cutting edge roundness of cemented carbide inserts could have a beneficial effect on the wear behaviour of coated tools. The main advantage of
* Corresponding author. E-mail address: [email protected] (K. -D. Bouzakis).
using laser machining for manufacturing the cutting edge radii is the achievement of a reproducible micro-geometry [4,8]. However, tools treated by laser machining may possess a reduced film adhesion leading to a cutting performance deterioration [1]. Hereupon, due to the melting of the Co-binder in micro areas of the cutting edges of cemented carbide substrates, WC grains can be covered by cobalt and a thermal affected zone with deteriorated material properties formed. Moreover, depended on the applied conditions during the laser machining, WC carbides may be transformed to comparably softer ones [9]. In this way, a worsening of the coating-substrate interface mechanical properties and film adhesion may occur. The aim of the conducted cooperative work was to investigate the effect of laser machining parameters such as of duration and feed velocity on the film adhesion and cutting performance of coated tools. The laser machining parameters affect variously the cemented carbide’s surface structure and properties and thus the coated tools wear behaviour. The explanation of the relevant mechanisms renders possible, among others, a targeted optimization of the laser machining parameters. Finally, the possibility to improve the cutting performance of coated tools’ life with lasertreated cutting edges via their additional micro-blasting is presented. The investigations related to the previous tasks were undertaken by the individual participants exhibited in Table 1. Seco
http://dx.doi.org/10.1016/j.cirpj.2017.02.003 1755-5817/© 2017 CIRP.
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Table 1 Tasks undertaken by the individual participants in the described cooperative work of the STC “Cutting” of CIRP. Participant
Task HM inserts preparation
Seco Tools Aristotle University of Thessaloniki Leibniz University of Hannover Technical University of Kaiserslautern
Laser machining of cutting edges
SEM and XRD investigations
Micro blasting
HM inserts Substrate and coatings PVD coating characterization and FEM modeling
Cutting tests
Tools provided the cemented carbide inserts and coated them after their laser machining and in some cases the additional microblasting. The participated Universities performed the indicated in Table 1 procedures, since they possess a wide experience in these fields documented in numerous scientific publications. The project coordination was carried out by the Aristotle University of Thessaloniki. Experimental and calculations’ details Cemented carbide inserts of HW-K05 SEAN1203AFTN ISO specifications were used. These inserts possess a cutting edge radius of around 8 mm with a chamfer inclination angle and length of 20 and 80 mm respectively (see Fig. 1a). The laser treatment was performed at the highlighted regions of the rake and flanks of the insert’s cutting edges, as illustrated in Fig. 1a. The same laser treatment was also conducted on the orthogonal region of the tool rake indicated in Fig. 1a. In the latter region, nanoindentations were carried out to determine the superficial mechanical properties after the laser machining. Moreover, in this region inclined impact tests were carried out after the coating deposition, for characterizing the film adhesion. The main data of the used laser beams as well as the applied laser machining conditions are demonstrated in Fig. 1b. Two different laser beams with nano- or pico-second pulsing duration were employed. All cemented carbide inserts were coated with a bilayer TiN/TiAlN coating with layer's thicknesses of 0.4 mm and 2.6 mm respectively (see Fig. 1c). The coating and the untreated substrate mechanical properties were determined as described in Reference [10] and they are exhibited in Fig. 1c. The roundness along the cutting edge was defined with the aid of confocal measurements. The average cutting edge roundness of the untreated and uncoated cemented carbide inserts amounts to roughly 8 mm (see Fig. 1a). The corresponding value after the laser treatment of the uncoated cemented carbide inserts is displayed in the left diagram of Fig. 2. In all cases, the conduct of laser machining at various conditions led to approximately the same radius growth. Furthermore, the average cutting radii of the coated tools with treated or untreated substrates are exhibited in the right diagram of Fig. 2. The increase of the cutting radius is roughly equal to the coating thickness since the coating approximately followed the curvature of the substrate during the deposition process. Because of the cutting edge radius growth after the laser machining, the coatings are less stressed compared to those on untreated substrates. This is verified by appropriate FEMcalculations of the developed equivalent stress fields in the cutting edge region, as for instance in milling Inconel 718 (see Fig. 3). In these calculations, the relevant mechanical properties of the tool’s material and coating were taken into account, whereas the film adhesion was considered as ideal. An increase of the cutting edge radius reduces the maximum stress developed in the coating and in the substrate. The higher the cutting edge radius, the lower the maximum equivalent stress is. Furthermore, at cutting edge radii
larger than approximately 12 mm the developed stresses in the coating and substrate are less than the corresponding yield ones. In this way, the risk of an early coated tool failure during milling Inconel 718 due to mechanical overstressing is diminished. Effect of the applied laser machining parameters on the cemented carbide surface structure For clarifying the effect of the laser machining parameters on the developed superficial material structure of the employed
Fig. 1. (a) The employed laser machining parameters and cutting edge geometry. (b) The applied TiN/TiAlN coating’s structure and strength properties of the coating and of the substrate.
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Fig. 3. Effect of the cutting edge radius on the maximum stress developed in the coating and in the substrate. Fig. 2. Obtained cutting edge radii by laser machining at various conditions.
cemented carbide inserts, SEM and XRD investigations were conducted. Characteristic SEM micro-graphs of cutting edges treated by nano-second pulsing laser at various velocities are illustrated in Fig. 4. The cutting edge in some positions along its length has an irregular shape. This irregularity develops due to cutting edge micro-breakages during grinding prior to laser treatment. A Thermal Affected Zone (TAZ) with superficial melted and solidified cobalt phases in both velocity cases can be observed. Moreover, in the magnified micro-graphs in the upper figure part, the presence of numerous cracks in the TAZ are visible. These cracks occur, because the thermal expansion coefficient of the formed superficial melted and solidified Co-zone is comparably larger than that of the cemented carbide substrate. In this way, tensile stresses are developed in this region during cooling after the laser treatment leading to the cracks' formation. Corresponding SEM-supported investigations were carried out in the case of pico-second pulsing laser, as exhibited in Fig. 5. The cutting edge micro breakage which can be observed in this figure also resulted during its previous grinding, as already mentioned. In
the present case, due to the restricted thermal energy provided by the laser beam onto the cemented carbide surface compared to nano-second laser machining, no obvious signs of a TAZ appear, especially at the larger laser feed velocity of 400 mm/s. In this way, it can be assumed that the formed TAZ is thinner in the case of pico-second laser treatment compared to the occurring TAZ thickness if nano-second laser machining is applied. This assumption is experimentally-analytically verified, as described in the next section. Further investigations were performed via XRD-analysis for detecting WC carbides of additional phases in the TAZ. Hereupon, an additional phase has been created, which can be identified on the basis of its peak positions as the cubic tungsten sub-carbide WC1-x. The parameter x indicates the carbon deficit in this additional carbide phase, as described in the Reference [11]. This phase is an intermediate product on the way to the hexagonal tungsten sub-carbide W2C. The sub-carbides possess different physical properties than the tungsten carbide WC [11–14]. A main difference is that the material after the phase transition needs a
Fig. 4. Characteristic SEM micro-graphs of cutting edges treated by nano-second pulsing laser at various velocities.
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Fig. 5. Characteristic SEM micro-graphs of cutting edges treated by pico-second pulsing laser at various velocities.
smaller volume. In this way, a residual stress shift into tensile direction develops, which deteriorates the superficial substrate hardness and may have a negative effect on the coated tool wear behaviour. The XRD-analysis of cemented carbide surfaces treated by nano- or pico-seconds pulsing laser are shown in Figs. 6 and 7 respectively. According to the obtained results, when the duration of the laser treatment increases, as in the case of nano-second laser (see Fig. 6), prismatic facets are generated by WC grains and exist simultaneously with the basal facets. The WC grains with a plane close to the prismatic facets possess lower hardness compared to those ones with basal facets [12]. Hence, the mechanical properties in the developed TAZ, are expected to be lower compared to the corresponding ones of the untreated substrates. Due to this fact, the cutting performance of cemented carbide inserts subjected to laser treatment for nano-seconds is expected to be negatively influenced. This will be experimentally ascertained, as presented in a following section.
The developed surface structures after laser pulsing at various durations are schematically depicted in Fig. 8a. Melted and solidified cobalt phases form a TAZ with deteriorated mechanical strength properties. The TAZ thickness in the case of a pico-second pulsing laser is comparably thinner since smaller energy amounts are transferred onto the treated insert surfaces, as already mentioned (see Fig. 8a). The melted and solidified cobalt phases cover the carbides, thus decreasing the adhesion of the afterwards deposited film. Moreover, when applying nano-second pulsing laser treatment numerous cracks in the TAZ of the cemented carbide inserts are created, as indicated in Fig. 8a and explained in Fig. 4. In order to reduce the TAZ thickness for improving the substrate superficial strength, micro-blasting can be employed (see Fig. 8b). Via micro-blasting, the carbides are released from the cobalt binding phase thus enhancing the coating adhesion and the coated inserts cutting performance as well. The latter statement will be experimentally verified, as described in the next sections.
Fig. 6. XRD analysis of cemented carbide surfaces treated by nano-second laser at various manufacturing conditions.
Fig. 7. XRD analysis of cemented carbide surfaces treated by pico-second laser at various manufacturing conditions.
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Fig. 8. Schematic representation of the developed surfaces after (a) laser pulsing at various durations and (b) micro-basting.
Effect of the applied laser machining parameters on the substrate and coating mechanical properties For determining the superficial strength properties of the employed cemented carbide inserts before and after their laser machining, nanoindentations were conducted at a maximum load of 15 mN using a Berkovich indenter. Related characteristic results are exibited in Fig. 9a. The measurements were carried out on the indicated orthogonal area of the tool rake. The latter was lasertreated applying the same process parameters as in the cutting edge roundness regions. For excluding the specimen roughness effect on the nanoindentation results accuracy, 40 measurements per nanoindentation were conducted for stabilizing the moving average of the indentation depth versus the indentation force [10]. The maximum indentation depth in the case of an untreated cemented carbide insert amounts to roughly 180 nm. In all lasertreated cases, when the laser beam velocity decreases, the maximum indentation depths grow and remain larger in relation to that of the untreated substrate (see Fig. 9b). This augmentation is higher in the case of nano- laser pulsing compared to the corresponding one of pico-second laser machining, since the developed TAZ thickness is larger and with lower mechanical strength, as already explained. Based on the previously introduced nanoindentation results, the superficial stress-strain curves of the untreated and lasertreated cemented carbide inserts were calculated [10]. In the case of the laser-treated inserts, the two extreme cases associated with the lowest and the highest nanoindentation depth were considered (see Fig. 10). These cases correspond to the pico-second laser pulsing with the highest velocity and the nano-second laser with the lowest velocity respectively. According to the obtained results, the film elasticity modulus, the yield and rupture stress of the laser-treated tools due to the development of the described TAZ are
Fig. 9. (a) Nanoindentation diagrams on untreated and variously laser treated cemented carbide inserts for shaping their cutting edge roundness. (b) Attained maximum indentation depths at a load of 15 mN on cemented carbide surfaces treaded by laser machining at various conditions.
significantly affected by the applied conditions. The mechanical properties of the nano-second laser treated surface at the beam velocity of 80 mm/s correspond practically to these ones of pure Co. In the case of the treated inserts by pico-second laser machining, the superficial strength properties are comparably larger due to the restricted thermal energy provided by the laser beam, as already described. To estimate an effective thickness of the developed TAZs in the investigated laser machining cases, the following analytical– experimental procedures were performed. Nanoindentations were carried out at increased indentation loads of 35 and 30 mN in the case of inserts treated by nano- or pico-second laser pulsing respectively (see Fig. 11). Hereupon, it was intended that nonthermal affected regions possessing the pristine mechanical properties of the cemented carbide inserts also deform plastically. Via the FEM model simulating the nanoindentation, as described in the references [10,15], the course of the penetration depth versus the indentation force, as well as the developed equivalent stress
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Fig. 10. Calculated stress–strain curves and further data of the cemented carbide inserts treated by laser machining at various conditions.
fields at diverse loads were calculated. In the applied FEM model, the structure of the cemented carbide material was considered by two layers. The thickness of the first layer corresponds to the developed TAZ with mechanical properties dependent on the
applied laser machining parameters, as exhibited in Fig. 10. The second layer simulates the untreated cemented carbide material. Comparisons between the measured and FEM indentation depths versus the indentation loads at diverse TAZ-thicknesses in the indicated two laser machining cases are presented in the upper part of Fig. 11. The courses of the FEM calculated indentation depths versus the indentation load converge sufficiently with the measured ones at TAZ-thicknesses of 3 mm and 2 mm in the cases of nano- and pico- second laser machining respectively. In this way, it can be concluded that the effective TAZ thickness of the nano- or picosecond laser treated inserts amount to roughly 3 mm and 2 mm correspondingly. By means of these effective TAZ thicknesses, the elastic–plastic deformation of the compound coating-TAZ-substrate are sufficiently described. The cemented carbide material under the developed TAZs start to deform plastically at indentation loads larger than approximately 32 mN and 17 mN in the cases of nano- or picosecond laser pulsing respectively. This can be observed in the related graphs at the bottom of Fig. 11, where at the indentation loads of 35 mN and 20 mN in the previously mentioned laser machining cases, the plastically deformed non thermal affected substrate material regions are visible (Seqv >3.3 GPa). Furthermore, the effect of the substrate’s treatment on the mechanical properties of the used coating was also investigated. Fig. 12a presents the measured nanoindentation depths at an indentation load of 15 mN into the PVD TiN/TiAlN films deposited on cemented carbides inserts, which were laser-machined at various conditions. It can be observed that the maximum indentation depth into the coating remains practically stable in all substrate cases. At the applied indentation load, the stress field
Fig. 11. Calculated and measured nanoindentation curves and stress fields developed in the cemented carbide inserts treated by laser machining at various conditions.
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Fig. 12. (a) Attained nanoindentation depths on coating deposited on cemented carbide inserts laser-treated at diverse conditions. (b) Calculated stress-strain laws of the coatings deposited on substrates variously laser-machined.
is developed only within the coating material. In this way, the TAZ and the substrate mechanical properties do not affect the elastic– plastic coating deformation during the nanoindentation. In Fig. 12b, the calculated coating stress strain laws are demonstrated. Considering these results, it can be concluded that the various laser substrate treatments practically did not affect the film mechanical properties. However, due to the development of a TAZ with thickness and mechanical properties dependent on the applied laser machining conditions, the coatings are variously stressed during cutting, as explained in a following section. Effect of the applied laser machining parameters on the coating adhesion To evaluate qualitatively the effect of the applied laser machining parameters on the film adhesion, Rockwell HRC indentations were conducted on the untreated or laser-treated and afterwards coated cemented carbide inserts. Characteristic Rockwell imprints scanned by confocal microscopy are shown in Fig. 13. Even in the case of the at most thermal-affected substrate surface via nano-pulsing laser at the feed velocity of 80 mm/s, no cracks or detachments appear in the imprints vicinity of the coated inserts. According to these results, the adhesion may be characterized as good, in all coating cases. Since these test results may lead to wrong conclusions concerning the film adhesion, the inclined impact test was
Fig. 13. Rockwell C indentation imprints on the coated cemented carbide inserts pre-treated by different laser machining parameters (indentation load F = 1471N).
employed [16]. The inclined impact tests were performed at an impact load of 300 N and inclination angle of 15 , as shown in Fig. 14. At the start of a new test, an unused region of the carbide ball surface was employed. The films’ fatigue fracture was quantified by the coating failure depth (CFD). CFD is determined considering the remaining imprint depths on the inserts’ surface after 104 (RID4) impacts and 10x impacts (RIDx). After 104 impacts no failures on the film surface were detected in all cases. The measured imprint depth RID4 develops due to the remaining substrate plastic deformation at the applied impact load. The curves shown in Fig. 14 are determined by subtracting RID4 from the remaining imprint depth RIDx at the same impact force after a certain number of impacts. In case of an untreated substrate, the coated inserts withstand more effectively the repetitive oblique impact loads compared to the corresponding ones, subjected to laser-treatment. The deteriorated mechanical properties of the developed TAZ dependent on the laser machining conditions increase the coating shear stresses during the inclined impact test, thus accelerating the film failure. Hence, the development of a TAZ corresponds to an effective film adhesion deterioration. This deterioration is more intensive in the case of cemented carbide substrates treated by nano-second laser because of the softer carbides formation, as already described. Furthermore, since micro-blasting releases the carbides from the cobalt binding
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Fig. 14. Coating failure depth CFD in the inclined impact test versus the number of impacts of coated cemented carbide inserts pre-treated at different laser machining parameters or additionally micro-blasted.
phase it leads to an effective adhesion improvement. The relevant curves in Fig. 14 ascertain this stipulation. Cutting performance of the coated cemented carbide inserts with laser-treated substrates The cutting performance of the coated untreated and lasertreated cemented carbide inserts was investigated in milling TiAl6V4 and Incocel 718. Characteristic results describing the flank wear evolution versus the number of cuts in milling Inconel 718 are shown in Fig. 15. These results were attained by the University of Hannover which participated in this cooperative work. The cutting edge radius enlargement after the conduct of laser treatment at the shown conditions was not beneficial for increasing the coated tool cutting performance. Since it was not possible that all participants
Fig. 15. Flank wear development vs. the number of cuts in milling Inconel 718 of coated cemented carbide inserts pre-treated by different laser machining parameters.
Fig. 16. Cutting performance of cemented carbide inserts pre-treated by different laser machining parameters in milling TiAl6V4 and Inconel 718.
in this cooperative work have the same material supplier, a cutting test with a coated insert of untreated substrate was conducted by all participants. For comparison reasons, the achieved number of cuts in this test up to a flank wear width of 0.15 mm was considered as reference (100%). An overview concerning the effect of the applied lasertreatment parameters on the coated tool life in milling Ti6Al4V and Inconel 718 is illustrated in Fig. 16. The reference (100%) for each participant corresponds to the attained number of cuts up to a flank wear width of 0.15 mm when an untreated substrate was used, as previously described. According to these results, only in the case of the shorter laser pulsing duration at the higher feed velocity, an increase of the coated tool life was obtained. In the rest laser machining cases a reduction of the tool performance resulted, despite the enlargement of the cutting edge radius. To explain the described tool wear results, a FEM simulation of the coated cutting edge and of the developed von Mises stresses during the material removal process was conducted. The calculated equivalent stress fields in the coated tool wedge, in the case of a substrate subjected to nano-second pulsing laser at the low feed velocity of 80 mm/s are demonstrated in the left graph of Fig. 17a. Moreover, in the right diagram of this figure, the developed von Mises stresses versus the distance x from the at most external area of the tool flank is displayed. In these FEM calculations, the mechanical properties of the cemented carbide substrate and its TAZ with mechanical strength properties dependent on the laser treatment conditions was considered. On the one hand, as it can be observed in both graphs of Fig. 17a, in the case of a treated substrate by nano-second laser the material of the TAZ is loaded over its yield stress during milling. In this way, a pre-mature material fracture in this region and a consequent failure of the coated tool cannot be avoided. On the other hand, in the case of a substrate machined by pico-second laser at a beam velocity of 400 mm/s, this area is not overloaded during the material removal. This happens since the developed TAZ possesses sufficient strength properties (see Fig. 10). In this way, due to the reduction of the coating stress at the larger cutting edge radius of 18 mm compared to 11 mm of the untreated cutting edge (see
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Fig. 17. Developed stress fields in the cutting edge region of coated cemented carbide inserts pre-treated by laser machining at nano- and pico-second pulsing duration.
Fig. 2), an augmentation of the tool life can be achieved. This stipulation was experimentally ascertained considering the results presented in Fig. 16. Cutting performance of the coated cemented carbide inserts with laser-treated and additionally micro-blasted substrates For releasing the carbides in the TAZ from the cobalt binding phase micro-blasting was conducted. More specifically, wet microblasting with Al2O3 grains was performed on the cemented carbide inserts after the laser machining. For ensuring negligible substrate material removal, thus not affecting the cutting edge geometry attained by the laser treatment, the micro-blasting process lasted only 4 s at a pressure of 0.3 MPa [17]. The wear evolution of coated tools with laser pre-treated and micro-blasted substrates are presented in Fig. 18. When inserts subjected to nano-second pulsing laser are used, the coated tool’s life is associated with approximately 150,000 cuts up to a flank wear width of 0.15 mm (NC0.15) The application of micro-blasting on the previousmentioned cemented carbide inserts enhances NC0.15 up to approximately 200,000 cuts. A comparable coated tool life increase is also attained after micro-blasting in the case of cemented carbide substrates subjected to pico-second pulsing laser at a beam velocity of 400 mm/s. The described improvements were attained mainly due to the effective adhesion enhancement of the coating after micro-blasting (see Fig. 14).
Fig. 18. Flank wear development vs. the number of cuts in milling TiAl6V4 of coated cemented carbide inserts subjected to laser-treatment or additionally to microblasting after the laser machining.
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Conclusions In the framework of a CIRP cooperative project, the feasibility of using laser machining for manufacturing the cutting edge roundness of cemented carbide inserts was investigated. In this context, diverse laser machining parameters such as of the laserpulsing duration and –beam feed velocity were applied for shaping the cutting edge roundness. The laser-treated and afterwards coated cemented carbide tools were used in milling Inconel718 and TiAl6V4. The attained results were explained with the aid of appropriate SEM and XRD measurements, inclined impact tests, as well as FEM supported calculations. Hereupon, shorter laser pulsing durations in the order of pico-seconds and larger feed velocities eliminate the negative effects of a TAZ associated with its thickness, material properties and adhesion on the coated tool life. Moreover, the conduct of micro-blasting on the already lasertreated cemented carbide substrates increases the effective coating adhesion, thus leading to a cutting performance improvement of the coated tools. In this way, the laser machining can be effectively employed as a reliable manufacturing method for shaping the cutting edge roundness. References [1] Bouzakis, K.-D., Bouzakis, E., Kombogiannis, S., Makrimallakis, S., Skordaris, G., Michailidis, N., Charalampous, P., Paraskevopoulou, R., M'Saoubi, R., Aurich, J. C., Barthelmä, F., Biermann, D., Denkena, B., Dimitrov, D., Engin, S., Karpuschewski, B., Klocke, F., Özel, T., Poulachon, G., Rech, J., Schulze, V., Settineri, L., Srivastava, A., Wegener, K., Uhlmann, E., Zeman, P., 2014, Effect of Cutting Edge Preparation of Coated Tools on Their Performance in Milling Various Materials. CIRP Journal of Manufacturing Science and Technology, 7/3: 264–273. [2] Denkena, B., Biermann, D., 2014, Cutting Edge Geometries. CIRP Annals – Manufacturing Technology, 63:631–653. [3] Bouzakis, K.-D., Michailidis, N., Skordaris, G., Kombogiannis, S., Hadjiyiannis, S., Efstathiou, K., Pavlidou, E., Erkens, G., Rambadt, S., Wirth, I., 2003, Optimisation of the Cutting Edge Roundness and its Manufacturing Procedures of Cemented Carbide Inserts, to Improve their Milling Performance after a PVD Coating Deposition. Surface and Coatings Technology, 163–164:625–630.
[4] Denkena, B., Kramer, N., Siegel, F., Kästner, J., 2007, Methoden zur Präparation von Zerspanwerkzeugen: Leistungsoptimierung an der Schneidkante. Spezialwerkzeuge, II:24–26. [5] Aurich, J.C., Zimmermann, M., Leitz, L., 2011, The Preparation of Cutting Edges Using a Marking Laser. Production Engineering –Research and Development, 5/ 1: 17–24. [6] Rech, J., 2006, Influence of Cutting Edge Preparation on the Wear Resistance in High Speed Dry Gear Hobbing. Wear, 261/5–6: 505–512. [7] Rech, J., Yen, Y.-C., Schaff, M.J., Hamdi, H., Altan, T., Bouzakis, K.-D., 2005, Influence of Cutting Edge Radius on the Wear Resistance of PM-HSS Milling Inserts. Wear, 259/7–12: 1168–1176. [8] Breidenstein, B., Gey, C., Denkena, B., 2013, Surface Integrity of Laser Machined Carbide Cutting Tools Within the PVD-coating Process Chain. Materialwissenschaft und Werkstofftechnik, 44/8: 704–709. [9] Breidenstein, B., Denkena, B., 2013, Significance of Residual Stress in PVD coated Carbide Cutting Tools. CIRP Annals –Manufacturing Technology, 62/1: 67–70. [10] Bouzakis, K.-D., Michailidis, N., Hadjiyiannis, S., Skordaris, G., Erkens, G., 2002, The effect of specimen roughness and indenter tip geometry on the determination accuracy of thinhard coatings stress-strain laws by nanoindentation. Materials Characterization, 49/2: 149–156. [11] Krainer, E., Robitsch, J., 1967, Röntgenographischer Nachweis des kubischen Wolframkarbides in funkenerosiv bearbeiteten Hartmetallen und in reinen Wolframschmelzkarbiden. Planseeeberichte für Pulvermetallurgie, 15. 46-46. [12] Duszová, A., Halgaš, R., Blanda, M., Hvizdoš, P., Lofaj, F., Dusza, J., Morgiel, J., 2013, Nanoindentation of WCCo Hardmetals. Journal of the European Ceramic Society, 33:2227–2232. [13] Cuadrado, N., Casellas D., Llanes L., Gonzalez I., Caro J., 2011. Effect of crystal anisotropy on the mechanical properties of WC embedded in WC-Co cemented carbides. Euro PM2011–Hard Materials, CD. [14] Roebuck, B., Klose, P., Mingard, K.P., 2012, Hardness of Hexagonal Tungsten Carbide Crystals as a Function of Orientation. Acta Materialia, 60:6131–6143. [15] Skordaris, G., Bouzakis, K.-D., Charalampous, P., 2015, A Dynamic FEM Simulation of the Nano-impact Test on Mono- or Multi-layered PVD Coatings Considering Their Graded Strength Properties Determined by Experimentalanalytical Procedures. Surface and Coatings Technology, 265:53–61. [16] Bouzakis, K.-D., Asimakopoulos, A., Skordaris, G., Pavlidou, E., Erkens, G., 2007, The Inclined Impact Test: A Novel Method for the Quantification of the Adhesion Properties of PVD Films. Wear, 262/11-12: 1471–1478. [17] Bouzakis, K.-D., Bouzakis, E., Skordaris, G., Makrimallakis, S., Tsouknidas, A., Katirtzoglou, G., Gerardis, S., 2011, Effect of PVD Films Wet Micro-blasting by Various Al2o3 Grain Sizes on the Wear Behaviour of Coated Tools. Surface and Coatings Technology, 205:S128–S132.
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CIRP Journal of Manufacturing Science and Technology xxx (2016) xxx–xxx
Contents lists available at ScienceDirect
CIRP Journal of Manufacturing Science and Technology journal homepage: www.elsevier.com/locate/cirpj
Effect of HM substrates’ cutting edge roundness manufactured by laser machining and micro-blasting on the coated tools’ cutting performance K.-D. Bouzakisa,* , P. Charalampousa , T. Kotsanisa , G. Skordarisa , E. Bouzakisb , B. Denkenac, B. Breidensteinc, J.C. Aurichd , M. Zimmermannd, T. Herrmanne, R. M’saoubif a
Laboratory for Machine Tools and Manufacturing Engineering, Mechanical Engineering Department, Aristotle University of Thessaloniki, Greece German University of Technology in Oman (GUtech), Department of Engineering, Oman Institute of Production Engineering and Machine Tools, Leibniz Universität Hannover, Germany d Institute for Manufacturing Technology and Production Systems, University of Kaiserslautern, Germany e Photonik-Zentrum Kaiserslautern e.V., Germany f Seco Tools AB, Fagersta, Sweden b c
A R T I C L E I N F O
Article history: Available online xxx
Keywords: Cutting edge rounding Laser machining Milling
A B S T R A C T
The effective application of laser machining for manufacturing the cutting edge roundness of cemented carbide inserts was investigated in a cooperative work of the Scientific Technical Committee (STC) “Cutting” of the International Academy for Production Engineering (CIRP). Machining with diverse laser beam pulsing duration and feed velocity as well as micro blasting were employed prior to the coating deposition for shaping the cutting tool’s edges. The wear behaviour of the coated inserts was tested in milling TiAl6V4 and Inconel 718. Considering the inserts’ surface micro-structure and mechanical properties developed after the laser treatment, the coated tools’ cutting performance deterioration or improvement compared to untreated substrates was explained. © 2017 CIRP.
Introduction The cutting edge geometry and the applied tool wedge rounding methods affect significantly the wear behaviour of coated tools [1–7]. In this context, appropriately large cutting edge radii in combination with optimized chamfer’s geometries reduce the tool mechanical stresses and may contribute to a significant improvement of the coated tool cutting performance [1–3]. Hereupon, by means of the K-factor method, a more precise cutting edge geometry description and optimization is enabled [2]. Furthermore, a key issue for attaining an augmentation of the coated cemented carbide tools’ life via increasing the cutting edge roundness is the applied manufacturing method. A thorough study of the efficiency of various cutting edge rounding methods as well as the effects of these treatments on the wear behaviour of coated tools in milling various metallic materials is introduced in Reference [1]. According to this study, the application of laser machining for manufacturing the cutting edge roundness of cemented carbide inserts could have a beneficial effect on the wear behaviour of coated tools. The main advantage of
* Corresponding author. E-mail address: [email protected] (K. -D. Bouzakis).
using laser machining for manufacturing the cutting edge radii is the achievement of a reproducible micro-geometry [4,8]. However, tools treated by laser machining may possess a reduced film adhesion leading to a cutting performance deterioration [1]. Hereupon, due to the melting of the Co-binder in micro areas of the cutting edges of cemented carbide substrates, WC grains can be covered by cobalt and a thermal affected zone with deteriorated material properties formed. Moreover, depended on the applied conditions during the laser machining, WC carbides may be transformed to comparably softer ones [9]. In this way, a worsening of the coating-substrate interface mechanical properties and film adhesion may occur. The aim of the conducted cooperative work was to investigate the effect of laser machining parameters such as of duration and feed velocity on the film adhesion and cutting performance of coated tools. The laser machining parameters affect variously the cemented carbide’s surface structure and properties and thus the coated tools wear behaviour. The explanation of the relevant mechanisms renders possible, among others, a targeted optimization of the laser machining parameters. Finally, the possibility to improve the cutting performance of coated tools’ life with lasertreated cutting edges via their additional micro-blasting is presented. The investigations related to the previous tasks were undertaken by the individual participants exhibited in Table 1. Seco
http://dx.doi.org/10.1016/j.cirpj.2017.02.003 1755-5817/© 2017 CIRP.
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Table 1 Tasks undertaken by the individual participants in the described cooperative work of the STC “Cutting” of CIRP. Participant
Task HM inserts preparation
Seco Tools Aristotle University of Thessaloniki Leibniz University of Hannover Technical University of Kaiserslautern
Laser machining of cutting edges
SEM and XRD investigations
Micro blasting
HM inserts Substrate and coatings PVD coating characterization and FEM modeling
Cutting tests
Tools provided the cemented carbide inserts and coated them after their laser machining and in some cases the additional microblasting. The participated Universities performed the indicated in Table 1 procedures, since they possess a wide experience in these fields documented in numerous scientific publications. The project coordination was carried out by the Aristotle University of Thessaloniki. Experimental and calculations’ details Cemented carbide inserts of HW-K05 SEAN1203AFTN ISO specifications were used. These inserts possess a cutting edge radius of around 8 mm with a chamfer inclination angle and length of 20 and 80 mm respectively (see Fig. 1a). The laser treatment was performed at the highlighted regions of the rake and flanks of the insert’s cutting edges, as illustrated in Fig. 1a. The same laser treatment was also conducted on the orthogonal region of the tool rake indicated in Fig. 1a. In the latter region, nanoindentations were carried out to determine the superficial mechanical properties after the laser machining. Moreover, in this region inclined impact tests were carried out after the coating deposition, for characterizing the film adhesion. The main data of the used laser beams as well as the applied laser machining conditions are demonstrated in Fig. 1b. Two different laser beams with nano- or pico-second pulsing duration were employed. All cemented carbide inserts were coated with a bilayer TiN/TiAlN coating with layer's thicknesses of 0.4 mm and 2.6 mm respectively (see Fig. 1c). The coating and the untreated substrate mechanical properties were determined as described in Reference [10] and they are exhibited in Fig. 1c. The roundness along the cutting edge was defined with the aid of confocal measurements. The average cutting edge roundness of the untreated and uncoated cemented carbide inserts amounts to roughly 8 mm (see Fig. 1a). The corresponding value after the laser treatment of the uncoated cemented carbide inserts is displayed in the left diagram of Fig. 2. In all cases, the conduct of laser machining at various conditions led to approximately the same radius growth. Furthermore, the average cutting radii of the coated tools with treated or untreated substrates are exhibited in the right diagram of Fig. 2. The increase of the cutting radius is roughly equal to the coating thickness since the coating approximately followed the curvature of the substrate during the deposition process. Because of the cutting edge radius growth after the laser machining, the coatings are less stressed compared to those on untreated substrates. This is verified by appropriate FEMcalculations of the developed equivalent stress fields in the cutting edge region, as for instance in milling Inconel 718 (see Fig. 3). In these calculations, the relevant mechanical properties of the tool’s material and coating were taken into account, whereas the film adhesion was considered as ideal. An increase of the cutting edge radius reduces the maximum stress developed in the coating and in the substrate. The higher the cutting edge radius, the lower the maximum equivalent stress is. Furthermore, at cutting edge radii
larger than approximately 12 mm the developed stresses in the coating and substrate are less than the corresponding yield ones. In this way, the risk of an early coated tool failure during milling Inconel 718 due to mechanical overstressing is diminished. Effect of the applied laser machining parameters on the cemented carbide surface structure For clarifying the effect of the laser machining parameters on the developed superficial material structure of the employed
Fig. 1. (a) The employed laser machining parameters and cutting edge geometry. (b) The applied TiN/TiAlN coating’s structure and strength properties of the coating and of the substrate.
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Fig. 3. Effect of the cutting edge radius on the maximum stress developed in the coating and in the substrate. Fig. 2. Obtained cutting edge radii by laser machining at various conditions.
cemented carbide inserts, SEM and XRD investigations were conducted. Characteristic SEM micro-graphs of cutting edges treated by nano-second pulsing laser at various velocities are illustrated in Fig. 4. The cutting edge in some positions along its length has an irregular shape. This irregularity develops due to cutting edge micro-breakages during grinding prior to laser treatment. A Thermal Affected Zone (TAZ) with superficial melted and solidified cobalt phases in both velocity cases can be observed. Moreover, in the magnified micro-graphs in the upper figure part, the presence of numerous cracks in the TAZ are visible. These cracks occur, because the thermal expansion coefficient of the formed superficial melted and solidified Co-zone is comparably larger than that of the cemented carbide substrate. In this way, tensile stresses are developed in this region during cooling after the laser treatment leading to the cracks' formation. Corresponding SEM-supported investigations were carried out in the case of pico-second pulsing laser, as exhibited in Fig. 5. The cutting edge micro breakage which can be observed in this figure also resulted during its previous grinding, as already mentioned. In
the present case, due to the restricted thermal energy provided by the laser beam onto the cemented carbide surface compared to nano-second laser machining, no obvious signs of a TAZ appear, especially at the larger laser feed velocity of 400 mm/s. In this way, it can be assumed that the formed TAZ is thinner in the case of pico-second laser treatment compared to the occurring TAZ thickness if nano-second laser machining is applied. This assumption is experimentally-analytically verified, as described in the next section. Further investigations were performed via XRD-analysis for detecting WC carbides of additional phases in the TAZ. Hereupon, an additional phase has been created, which can be identified on the basis of its peak positions as the cubic tungsten sub-carbide WC1-x. The parameter x indicates the carbon deficit in this additional carbide phase, as described in the Reference [11]. This phase is an intermediate product on the way to the hexagonal tungsten sub-carbide W2C. The sub-carbides possess different physical properties than the tungsten carbide WC [11–14]. A main difference is that the material after the phase transition needs a
Fig. 4. Characteristic SEM micro-graphs of cutting edges treated by nano-second pulsing laser at various velocities.
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Fig. 5. Characteristic SEM micro-graphs of cutting edges treated by pico-second pulsing laser at various velocities.
smaller volume. In this way, a residual stress shift into tensile direction develops, which deteriorates the superficial substrate hardness and may have a negative effect on the coated tool wear behaviour. The XRD-analysis of cemented carbide surfaces treated by nano- or pico-seconds pulsing laser are shown in Figs. 6 and 7 respectively. According to the obtained results, when the duration of the laser treatment increases, as in the case of nano-second laser (see Fig. 6), prismatic facets are generated by WC grains and exist simultaneously with the basal facets. The WC grains with a plane close to the prismatic facets possess lower hardness compared to those ones with basal facets [12]. Hence, the mechanical properties in the developed TAZ, are expected to be lower compared to the corresponding ones of the untreated substrates. Due to this fact, the cutting performance of cemented carbide inserts subjected to laser treatment for nano-seconds is expected to be negatively influenced. This will be experimentally ascertained, as presented in a following section.
The developed surface structures after laser pulsing at various durations are schematically depicted in Fig. 8a. Melted and solidified cobalt phases form a TAZ with deteriorated mechanical strength properties. The TAZ thickness in the case of a pico-second pulsing laser is comparably thinner since smaller energy amounts are transferred onto the treated insert surfaces, as already mentioned (see Fig. 8a). The melted and solidified cobalt phases cover the carbides, thus decreasing the adhesion of the afterwards deposited film. Moreover, when applying nano-second pulsing laser treatment numerous cracks in the TAZ of the cemented carbide inserts are created, as indicated in Fig. 8a and explained in Fig. 4. In order to reduce the TAZ thickness for improving the substrate superficial strength, micro-blasting can be employed (see Fig. 8b). Via micro-blasting, the carbides are released from the cobalt binding phase thus enhancing the coating adhesion and the coated inserts cutting performance as well. The latter statement will be experimentally verified, as described in the next sections.
Fig. 6. XRD analysis of cemented carbide surfaces treated by nano-second laser at various manufacturing conditions.
Fig. 7. XRD analysis of cemented carbide surfaces treated by pico-second laser at various manufacturing conditions.
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Fig. 8. Schematic representation of the developed surfaces after (a) laser pulsing at various durations and (b) micro-basting.
Effect of the applied laser machining parameters on the substrate and coating mechanical properties For determining the superficial strength properties of the employed cemented carbide inserts before and after their laser machining, nanoindentations were conducted at a maximum load of 15 mN using a Berkovich indenter. Related characteristic results are exibited in Fig. 9a. The measurements were carried out on the indicated orthogonal area of the tool rake. The latter was lasertreated applying the same process parameters as in the cutting edge roundness regions. For excluding the specimen roughness effect on the nanoindentation results accuracy, 40 measurements per nanoindentation were conducted for stabilizing the moving average of the indentation depth versus the indentation force [10]. The maximum indentation depth in the case of an untreated cemented carbide insert amounts to roughly 180 nm. In all lasertreated cases, when the laser beam velocity decreases, the maximum indentation depths grow and remain larger in relation to that of the untreated substrate (see Fig. 9b). This augmentation is higher in the case of nano- laser pulsing compared to the corresponding one of pico-second laser machining, since the developed TAZ thickness is larger and with lower mechanical strength, as already explained. Based on the previously introduced nanoindentation results, the superficial stress-strain curves of the untreated and lasertreated cemented carbide inserts were calculated [10]. In the case of the laser-treated inserts, the two extreme cases associated with the lowest and the highest nanoindentation depth were considered (see Fig. 10). These cases correspond to the pico-second laser pulsing with the highest velocity and the nano-second laser with the lowest velocity respectively. According to the obtained results, the film elasticity modulus, the yield and rupture stress of the laser-treated tools due to the development of the described TAZ are
Fig. 9. (a) Nanoindentation diagrams on untreated and variously laser treated cemented carbide inserts for shaping their cutting edge roundness. (b) Attained maximum indentation depths at a load of 15 mN on cemented carbide surfaces treaded by laser machining at various conditions.
significantly affected by the applied conditions. The mechanical properties of the nano-second laser treated surface at the beam velocity of 80 mm/s correspond practically to these ones of pure Co. In the case of the treated inserts by pico-second laser machining, the superficial strength properties are comparably larger due to the restricted thermal energy provided by the laser beam, as already described. To estimate an effective thickness of the developed TAZs in the investigated laser machining cases, the following analytical– experimental procedures were performed. Nanoindentations were carried out at increased indentation loads of 35 and 30 mN in the case of inserts treated by nano- or pico-second laser pulsing respectively (see Fig. 11). Hereupon, it was intended that nonthermal affected regions possessing the pristine mechanical properties of the cemented carbide inserts also deform plastically. Via the FEM model simulating the nanoindentation, as described in the references [10,15], the course of the penetration depth versus the indentation force, as well as the developed equivalent stress
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Fig. 10. Calculated stress–strain curves and further data of the cemented carbide inserts treated by laser machining at various conditions.
fields at diverse loads were calculated. In the applied FEM model, the structure of the cemented carbide material was considered by two layers. The thickness of the first layer corresponds to the developed TAZ with mechanical properties dependent on the
applied laser machining parameters, as exhibited in Fig. 10. The second layer simulates the untreated cemented carbide material. Comparisons between the measured and FEM indentation depths versus the indentation loads at diverse TAZ-thicknesses in the indicated two laser machining cases are presented in the upper part of Fig. 11. The courses of the FEM calculated indentation depths versus the indentation load converge sufficiently with the measured ones at TAZ-thicknesses of 3 mm and 2 mm in the cases of nano- and pico- second laser machining respectively. In this way, it can be concluded that the effective TAZ thickness of the nano- or picosecond laser treated inserts amount to roughly 3 mm and 2 mm correspondingly. By means of these effective TAZ thicknesses, the elastic–plastic deformation of the compound coating-TAZ-substrate are sufficiently described. The cemented carbide material under the developed TAZs start to deform plastically at indentation loads larger than approximately 32 mN and 17 mN in the cases of nano- or picosecond laser pulsing respectively. This can be observed in the related graphs at the bottom of Fig. 11, where at the indentation loads of 35 mN and 20 mN in the previously mentioned laser machining cases, the plastically deformed non thermal affected substrate material regions are visible (Seqv >3.3 GPa). Furthermore, the effect of the substrate’s treatment on the mechanical properties of the used coating was also investigated. Fig. 12a presents the measured nanoindentation depths at an indentation load of 15 mN into the PVD TiN/TiAlN films deposited on cemented carbides inserts, which were laser-machined at various conditions. It can be observed that the maximum indentation depth into the coating remains practically stable in all substrate cases. At the applied indentation load, the stress field
Fig. 11. Calculated and measured nanoindentation curves and stress fields developed in the cemented carbide inserts treated by laser machining at various conditions.
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Fig. 12. (a) Attained nanoindentation depths on coating deposited on cemented carbide inserts laser-treated at diverse conditions. (b) Calculated stress-strain laws of the coatings deposited on substrates variously laser-machined.
is developed only within the coating material. In this way, the TAZ and the substrate mechanical properties do not affect the elastic– plastic coating deformation during the nanoindentation. In Fig. 12b, the calculated coating stress strain laws are demonstrated. Considering these results, it can be concluded that the various laser substrate treatments practically did not affect the film mechanical properties. However, due to the development of a TAZ with thickness and mechanical properties dependent on the applied laser machining conditions, the coatings are variously stressed during cutting, as explained in a following section. Effect of the applied laser machining parameters on the coating adhesion To evaluate qualitatively the effect of the applied laser machining parameters on the film adhesion, Rockwell HRC indentations were conducted on the untreated or laser-treated and afterwards coated cemented carbide inserts. Characteristic Rockwell imprints scanned by confocal microscopy are shown in Fig. 13. Even in the case of the at most thermal-affected substrate surface via nano-pulsing laser at the feed velocity of 80 mm/s, no cracks or detachments appear in the imprints vicinity of the coated inserts. According to these results, the adhesion may be characterized as good, in all coating cases. Since these test results may lead to wrong conclusions concerning the film adhesion, the inclined impact test was
Fig. 13. Rockwell C indentation imprints on the coated cemented carbide inserts pre-treated by different laser machining parameters (indentation load F = 1471N).
employed [16]. The inclined impact tests were performed at an impact load of 300 N and inclination angle of 15 , as shown in Fig. 14. At the start of a new test, an unused region of the carbide ball surface was employed. The films’ fatigue fracture was quantified by the coating failure depth (CFD). CFD is determined considering the remaining imprint depths on the inserts’ surface after 104 (RID4) impacts and 10x impacts (RIDx). After 104 impacts no failures on the film surface were detected in all cases. The measured imprint depth RID4 develops due to the remaining substrate plastic deformation at the applied impact load. The curves shown in Fig. 14 are determined by subtracting RID4 from the remaining imprint depth RIDx at the same impact force after a certain number of impacts. In case of an untreated substrate, the coated inserts withstand more effectively the repetitive oblique impact loads compared to the corresponding ones, subjected to laser-treatment. The deteriorated mechanical properties of the developed TAZ dependent on the laser machining conditions increase the coating shear stresses during the inclined impact test, thus accelerating the film failure. Hence, the development of a TAZ corresponds to an effective film adhesion deterioration. This deterioration is more intensive in the case of cemented carbide substrates treated by nano-second laser because of the softer carbides formation, as already described. Furthermore, since micro-blasting releases the carbides from the cobalt binding
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Fig. 14. Coating failure depth CFD in the inclined impact test versus the number of impacts of coated cemented carbide inserts pre-treated at different laser machining parameters or additionally micro-blasted.
phase it leads to an effective adhesion improvement. The relevant curves in Fig. 14 ascertain this stipulation. Cutting performance of the coated cemented carbide inserts with laser-treated substrates The cutting performance of the coated untreated and lasertreated cemented carbide inserts was investigated in milling TiAl6V4 and Incocel 718. Characteristic results describing the flank wear evolution versus the number of cuts in milling Inconel 718 are shown in Fig. 15. These results were attained by the University of Hannover which participated in this cooperative work. The cutting edge radius enlargement after the conduct of laser treatment at the shown conditions was not beneficial for increasing the coated tool cutting performance. Since it was not possible that all participants
Fig. 15. Flank wear development vs. the number of cuts in milling Inconel 718 of coated cemented carbide inserts pre-treated by different laser machining parameters.
Fig. 16. Cutting performance of cemented carbide inserts pre-treated by different laser machining parameters in milling TiAl6V4 and Inconel 718.
in this cooperative work have the same material supplier, a cutting test with a coated insert of untreated substrate was conducted by all participants. For comparison reasons, the achieved number of cuts in this test up to a flank wear width of 0.15 mm was considered as reference (100%). An overview concerning the effect of the applied lasertreatment parameters on the coated tool life in milling Ti6Al4V and Inconel 718 is illustrated in Fig. 16. The reference (100%) for each participant corresponds to the attained number of cuts up to a flank wear width of 0.15 mm when an untreated substrate was used, as previously described. According to these results, only in the case of the shorter laser pulsing duration at the higher feed velocity, an increase of the coated tool life was obtained. In the rest laser machining cases a reduction of the tool performance resulted, despite the enlargement of the cutting edge radius. To explain the described tool wear results, a FEM simulation of the coated cutting edge and of the developed von Mises stresses during the material removal process was conducted. The calculated equivalent stress fields in the coated tool wedge, in the case of a substrate subjected to nano-second pulsing laser at the low feed velocity of 80 mm/s are demonstrated in the left graph of Fig. 17a. Moreover, in the right diagram of this figure, the developed von Mises stresses versus the distance x from the at most external area of the tool flank is displayed. In these FEM calculations, the mechanical properties of the cemented carbide substrate and its TAZ with mechanical strength properties dependent on the laser treatment conditions was considered. On the one hand, as it can be observed in both graphs of Fig. 17a, in the case of a treated substrate by nano-second laser the material of the TAZ is loaded over its yield stress during milling. In this way, a pre-mature material fracture in this region and a consequent failure of the coated tool cannot be avoided. On the other hand, in the case of a substrate machined by pico-second laser at a beam velocity of 400 mm/s, this area is not overloaded during the material removal. This happens since the developed TAZ possesses sufficient strength properties (see Fig. 10). In this way, due to the reduction of the coating stress at the larger cutting edge radius of 18 mm compared to 11 mm of the untreated cutting edge (see
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Fig. 17. Developed stress fields in the cutting edge region of coated cemented carbide inserts pre-treated by laser machining at nano- and pico-second pulsing duration.
Fig. 2), an augmentation of the tool life can be achieved. This stipulation was experimentally ascertained considering the results presented in Fig. 16. Cutting performance of the coated cemented carbide inserts with laser-treated and additionally micro-blasted substrates For releasing the carbides in the TAZ from the cobalt binding phase micro-blasting was conducted. More specifically, wet microblasting with Al2O3 grains was performed on the cemented carbide inserts after the laser machining. For ensuring negligible substrate material removal, thus not affecting the cutting edge geometry attained by the laser treatment, the micro-blasting process lasted only 4 s at a pressure of 0.3 MPa [17]. The wear evolution of coated tools with laser pre-treated and micro-blasted substrates are presented in Fig. 18. When inserts subjected to nano-second pulsing laser are used, the coated tool’s life is associated with approximately 150,000 cuts up to a flank wear width of 0.15 mm (NC0.15) The application of micro-blasting on the previousmentioned cemented carbide inserts enhances NC0.15 up to approximately 200,000 cuts. A comparable coated tool life increase is also attained after micro-blasting in the case of cemented carbide substrates subjected to pico-second pulsing laser at a beam velocity of 400 mm/s. The described improvements were attained mainly due to the effective adhesion enhancement of the coating after micro-blasting (see Fig. 14).
Fig. 18. Flank wear development vs. the number of cuts in milling TiAl6V4 of coated cemented carbide inserts subjected to laser-treatment or additionally to microblasting after the laser machining.
Please cite this article in press as: K.-D. Bouzakis, et al., Effect of HM substrates’ cutting edge roundness manufactured by laser machining and micro-blasting on the coated tools’ cutting performance, NULL (2017), http://dx.doi.org/10.1016/j.cirpj.2017.02.003
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Conclusions In the framework of a CIRP cooperative project, the feasibility of using laser machining for manufacturing the cutting edge roundness of cemented carbide inserts was investigated. In this context, diverse laser machining parameters such as of the laserpulsing duration and –beam feed velocity were applied for shaping the cutting edge roundness. The laser-treated and afterwards coated cemented carbide tools were used in milling Inconel718 and TiAl6V4. The attained results were explained with the aid of appropriate SEM and XRD measurements, inclined impact tests, as well as FEM supported calculations. Hereupon, shorter laser pulsing durations in the order of pico-seconds and larger feed velocities eliminate the negative effects of a TAZ associated with its thickness, material properties and adhesion on the coated tool life. Moreover, the conduct of micro-blasting on the already lasertreated cemented carbide substrates increases the effective coating adhesion, thus leading to a cutting performance improvement of the coated tools. In this way, the laser machining can be effectively employed as a reliable manufacturing method for shaping the cutting edge roundness. References [1] Bouzakis, K.-D., Bouzakis, E., Kombogiannis, S., Makrimallakis, S., Skordaris, G., Michailidis, N., Charalampous, P., Paraskevopoulou, R., M'Saoubi, R., Aurich, J. C., Barthelmä, F., Biermann, D., Denkena, B., Dimitrov, D., Engin, S., Karpuschewski, B., Klocke, F., Özel, T., Poulachon, G., Rech, J., Schulze, V., Settineri, L., Srivastava, A., Wegener, K., Uhlmann, E., Zeman, P., 2014, Effect of Cutting Edge Preparation of Coated Tools on Their Performance in Milling Various Materials. CIRP Journal of Manufacturing Science and Technology, 7/3: 264–273. [2] Denkena, B., Biermann, D., 2014, Cutting Edge Geometries. CIRP Annals – Manufacturing Technology, 63:631–653. [3] Bouzakis, K.-D., Michailidis, N., Skordaris, G., Kombogiannis, S., Hadjiyiannis, S., Efstathiou, K., Pavlidou, E., Erkens, G., Rambadt, S., Wirth, I., 2003, Optimisation of the Cutting Edge Roundness and its Manufacturing Procedures of Cemented Carbide Inserts, to Improve their Milling Performance after a PVD Coating Deposition. Surface and Coatings Technology, 163–164:625–630.
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Please cite this article in press as: K.-D. Bouzakis, et al., Effect of HM substrates’ cutting edge roundness manufactured by laser machining and micro-blasting on the coated tools’ cutting performance, NULL (2017), http://dx.doi.org/10.1016/j.cirpj.2017.02.003