Online publication date: 08 December 2010

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1 This article was downloaded by: [Tarek, Mabrouki] On: 8 December 2010 Access details: Access Details: [subscription number ] Publisher Taylor & Francis Informa Ltd Registered in England and Wales Registered Number: Registered office: Mortimer House, Mortimer Street, London W1T 3JH, UK Machining Science and Technology Publication details, including instructions for authors and subscription information: EXPERIMENTAL INVESTIGATION AND PERFORMANCE ANALYSES OF CBN INSERT IN HARD TURNING OF COLD WORK TOOL STEEL (D3) H. Bouchelaghem a ; M. A. Yallese a ; A. Amirat b ; T. Mabrouki c ; J. F. Rigal b a Université du 8 Mai 1945, Laboratoire Mécanique et Structures (LMS), BP, Guelma, Algérie b Université Mokhtar, Laboratoire de Recherche Mécanique des Matériaux et Maintenance Industrielle (LR3MI), BP, Annaba, Algérie c Université de Lyon, CNRS, INSA-Lyon, LaMCoS, UMR, France Online publication date: 08 December 2010 To cite this Article Bouchelaghem, H., Yallese, M. A., Amirat, A., Mabrouki, T. and Rigal, J. F.(2010) 'EXPERIMENTAL INVESTIGATION AND PERFORMANCE ANALYSES OF CBN INSERT IN HARD TURNING OF COLD WORK TOOL STEEL (D3)', Machining Science and Technology, 14: 4, To link to this Article: DOI: / URL: PLEASE SCROLL DOWN FOR ARTICLE Full terms and conditions of use: This article may be used for research, teaching and private study purposes. Any substantial or systematic reproduction, re-distribution, re-selling, loan or sub-licensing, systematic supply or distribution in any form to anyone is expressly forbidden. The publisher does not give any warranty express or implied or make any representation that the contents will be complete or accurate or up to date. The accuracy of any instructions, formulae and drug doses should be independently verified with primary sources. The publisher shall not be liable for any loss, actions, claims, proceedings, demand or costs or damages whatsoever or howsoever caused arising directly or indirectly in connection with or arising out of the use of this material.

2 Machining Science and Technology, 14: Copyright # 2010 Taylor & Francis Group, LLC ISSN: print= online DOI: / EXPERIMENTAL INVESTIGATION AND PERFORMANCE ANALYSES OF CBN INSERT IN HARD TURNING OF COLD WORK TOOL STEEL (D3) H. Bouchelaghem 1, M. A. Yallese 1, A. Amirat 2, T. Mabrouki 3, and J. F. Rigal 2 1 Université du 8 Mai 1945, Laboratoire Mécanique et Structures (LMS), BP, Guelma, Algérie 2 Université Mokhtar, Laboratoire de Recherche Mécanique des Matériaux et Maintenance Industrielle (LR3MI), BP, Annaba, Algérie 3 Université de Lyon, CNRS, INSA-Lyon, LaMCoS, UMR, France & Long-term wear tests on the CBN tool behaviour during hard turning of AISI D3 (60 HRC) have been investigated. The evolution of surface roughness, cutting forces and temperature has been studied according to the cutting parameters and tool wear. Results show that CBN is resistant to wear despite of aggressiveness of AISI D3 steel. A great part of heat generated is mainly dissipated throughout the chip; at a cutting speed of 320 m=min the chip temperature is found to be 14 times higher than that recorded in the workpiece. The cutting speed range of 90 to 240 m=min has been found suitable to cut this material. The former speed range is appropriate to respect the cutting constraints, whereas beyond 240 m=min a drastic increase in tool wear occurs, causing a remarkable drop in tool life. Roughness measurements reveal a dependence on CBN tool wear. However, although the wear rises up to the allowable flank wear of value 0.3 mm, roughness R a remains around 1.0 lm. The feed rate remains the most affecting factor on the roughness values. The proposed statistical models are based on the response surface methodology correlating the cutting parameters together with roughness, cutting forces and tool life. Keywords CBN insert, cutting force, hard turning, insert wear, response surface methodology, roughness INTRODUCTION Technological advances in industries such as automotive, railways, aeronautic, nuclear, aerospace have led to using more and more performing materials, i.e., high strength steels and cast iron, titanium alloys, Address correspondence to Dr. T. Mabrouki, Université de Lyon, CNRS, INSA-Lyon, LaMCoS, UMR5259, Bat. J. Jacguard, 20, Avenue Albert Einstein, 69621, Villeurbanne, France, F Tarek.Mabrouki@insa-lyon.fr

3 472 H. Bouchelaghem et al. nickel- and cobalt-based refractory alloys, silicon content light alloys and metallic matrix composites materials. These materials are known as abrasive materials with high mechanical resistance, and are classified among materials difficult to machine (Poulachon, 2004). In turning, the machining operation is achieved by using purposely developed cutting tool materials, such as ceramics and cubic boron nitride (CBN). The latter presents a unique property combination between hardness, good resistance at high temperature and thermo-chemical stability. These properties are strongly required in cutting tool materials when machining hard ferrous workpieces (Luo et al., 1999). Moreover, the reliability improvement of the ceramics and CBN cutting tools generated the hard turning; a machining process with significant importance in manufacture. This process is being employed in many cases of manufacture but it is still not entirely controlled (Remadna and Rigal, 2006) and needs more attention. Today, although it is well applied in machining shafts, bearings, pinions, cams and other mechanical components there is need of research to improve performance of ceramics and CBN tools by taking into account the choice of the machining systems. A great number of researchers have been interested in the field. Hard turning is, therefore, of a great interest to both the manufacturing industry and research community, as mentioned by Huang et al. (2007), who have detailed a survey on research progresses on CBN tool wear research in hard turning. For instance, Thiele and Melkote (1999) presented results of an experimental study dealing with the effect of cutting edge geometry and workpiece hardness on residual stresses in finish hard turning of 100Cr6 steel case. They concluded that both factors are significant for surface integrity of finish hard turned components. Chen (2000) studied the cutting force and surface finish during machining of medium hardened steel (45 55 HRC) using CBN tool and showed that thrust force was the largest among the three cutting force components. El-Wardany et al. (2000) looked at quality and integrity of the surface produced and the effects of cutting parameters and tool wear on chip morphology during high-speed turning of AISI D2 cold work tool steel in its hardened state (60 62 HRC). The metallographic analysis of the produced surface illustrates the damaged surface region that contains geometrical defects and changes in the sub-surface metallurgical structure. The types of surface damage are dependent on the cutting parameters, the tool geometry and the growth of the wear zones. Barry and Byrne (2001) investigated the mechanism of Al 2 O 3 =TiC cutting tool wear in the finish turning of hardened steels with particular knowledge of the work material inclusion content. The rate of tool wear appears to be determined by the hard inclusion content or alloy carbide content of the work material. A new mechanism is proposed to consider the superior wear resistance of CBN=TiC composites in comparison to

4 Hard Turning of Cold Work Tool Steel (D3) Using CBN 473 high-content CBN tools in the finish machining of AISI 4340 steel at 52 HRC. Nabahani (2001) found that PCBN tool reduces tool flank wear and delivers a good surface compared to the various carbide tools. Liu et al. (2002) investigated the wear performances of cutting tools, such as cubic boron nitride (CBN), ceramic, coated carbide and fine grained carbide, in high-speed face milling. Poulachon et al. (2003) underlined that the microstructure of hardened steels possesses a critical influence on the tool wear mechanisms involved, particularly the presence of carbides. Due to the variation in the hardness of the carbide particles, the wear of PCBN inserts may take place at different rates. Chou et al. (2003) experimentally investigated the performance and wear behaviour of different CBN tools in finish turning of AISI steel hardened at 62 HRC. It was established that low CBN content tools generate better surface finish and show lower flank wear than high CBN ones. Moreover, in order to model CBN insert wear, Huang and Liang (2004a) have proposed an analytical model, calibrated with experimental data, which permits to correlate the CBN tool flank wear rate with the tool=workpiece material properties and cutting parameters. The proposed model concerns the 3-dimensional finish turning of hardened bearing steel with a CBN insert (Huang and Liang, 2004b). The main observed insert wear mechanism over common cutting conditions was adhesion. Nevertheless, the chemical diffusion can gain dominance over extended periods of machining time under aggressive cutting conditions. Potentially, the authors work could help the design of CBN tool geometry and optimize cutting parameters in finish hard turning (Huang and Dawson, 2005). Aslan (2004) explored the performances and wear behaviour of different cutting tools in end milling of X210Cr12 cold work tool steel hardened to 62 HRC. The purpose of the experiments was to investigate the wear of TiCN coated tungsten carbide, TiCN þ TiAIN coated tungsten carbide, TiAIN coated cermet, mixed ceramic with Al 2 O 3 þ TiCN and CBN tools. The results indicated the CBN tool exhibited the best cutting performance in terms of both flank wear and surface finish. The highest volume of metal removal was obtained with CBN tool. Noordin et al. (2004) explored the performances of a multilayer tungsten carbide tool using response surface methodology (RSM) when turning AISI 1045 steel. The experimental results indicated that the feed rate is the most significant factor that influences the surface roughness and the tangential force. However, there are other factors that provide secondary contributions to the performance indicators. Other authors have investigated the cutting tool performance in the high speed end milling of AISI D3 cold work tool steel hardened to 35 HRC according to the work of Camuşcu and Aslan (2005). The results were

5 474 H. Bouchelaghem et al. also discussed in terms of tool cost. The best cutting performance was obtained with CBN tool. TiCN mixed Al 2 O 3 ceramic tools proved to be suitable for high speed end milling of AISI D3 with 35 HRC hardness. Unfortunately, coated carbide and coated cermet tools did not exhibit good performance. They should be used at relatively lower cutting speeds. Lima et al. (2005) studied the machinability of hardened steels at different levels of hardness and using various cutting tool materials. Their study was focused on the machining of hardened AISI 4340 (high strength low alloy steel) and AISI D2 (cold work tool steel). The results indicated that when turning AISI 4340 steel the surface roughness of the machined parts was improved at high cutting speed and deteriorated with feed rate increase whereas depth of cut presented little effect on the surface roughness improvement. Turning AISI D2 steel with mixed alumina inserts allowed a surface finish as well as that produced by cylindrical grinding. Özel et al. (2005) used a four-factor, two-level factorial design (2 4 ) with 16 replications to determine the effects of cutting tool edge geometry, workpiece hardness, feed rate and cutting speed on surface roughness and resultant forces in finish hard turning of AISI H13 steel using CBN tools. The results have indicated that the effect of cutting edge geometry on the surface roughness is remarkably significant. The cutting forces are influenced not only cutting by conditions but also by the cutting edge geometry and workpiece surface hardness. Orhan et al. (2007) put foreword the relationship between workpiece vibration and tool wear during end milling of AISI D3 cold work tool steel hardened to 35 HRC. They showed the vibration amplitude is not considerably increased until a flank wear value of 160 mm was reached, above which the vibration amplitude increased significantly. Davim and Figueira et al. (2007) investigated the machinability of cold work tool D2 with ceramic tools using statistical techniques in hard turning. The results indicate that with appropriate cutting parameters it is possible to obtain a surface roughness R a < 0.8 mm that allows the elimination of cylindrical grinding operations. Lahiff et al. (2007) concluded that different theories exist regarding involved tool wear mechanism; however, there is a general agreement among researchers that PCBN tool wear is complex and no a single mechanism provides a complete explanation. These authors underlined also that the flank wear affects surface finish, integrity and dimensional accuracy of the machined workpiece, while crater wear influences process reliability. Nevertheless, the abrasion mechanism, due to carbide particles and martensite in the workpiece and CBN grains, is the most dominant cutting tool wear mechanism. This result was also demonstrated by More et al. (2006) when cutting CBN TiN and PCBN inserts. They have also shown that the crater wear of the CBN TiN coated inserts is smaller than that in

6 Hard Turning of Cold Work Tool Steel (D3) Using CBN 475 PCBN ones because of the lubricity of TiN capping layer on the CBN TiN coating. For CBN insert wear, Thamizhmnaii et al. (2008) have shown that when turning a SS 440 C stainless steel, the abrasion, adhesion and diffusion are the main wear mechanisms in flank wear formation. In addition, due to high temperature during the turning process some diffusion of material on crater wear can take place when cutting with CBN and PCBN inserts. This caused a built up layer formation over a wider area as it was mentioned by Thamizhmnaii and Hasan (2009). By carrying out experimentation Yallese et al. (2009) have underlined that CBN tool offers a good wear resistance during hard turning of 100Cr6 tempered steel (60 HRC) despite the aggressiveness of the machined material hardness. Surface quality obtained with CBN tool is significantly improved compared to grinding operation despite an increase in the feed rate by a factor of 2.5. The present work lays out experimental results on the cubic boron nitride (CBN) wear behaviour when machining hardened cold work tool steel (D3). In addition, surface quality degradation, cutting forces and temperature evolution are examined as a function of cutting conditions (V c, f, a p ) and tool wear. Finally, statistical models have been developed using response surface methodology (RSM) correlating the cutting parameters together with workpiece roughness, cutting force and tool life. EXPERIMENTAL PROCEDURE To evaluate the CBN tool life long term tests, according to the ISO 3685 (1977) standard, have been applied. Straight turning operations have been carried out on 80 mm diameter and 400 mm length bars made of heat treated AISI D3 steel with the chemical specification given in Table 1. Due to its wear resistance, this material is usually employed for the manufacture of punches, blanking, stamping, drawing dies, rollers profilers, wood tools, etc., (Yallese et al., 2006). The test specimens have been hardened to the required hardness of 60 HRC through quenching at 940 C and tempering at 280 C. Machining operations have been achieved on a 6.6 KW power TOS TRENCIN model SN40 lathe. A TiN layer covered cutting insert (57% CBN), of the standard designation SNGA T01020 (tool nose radius: 0.8 mm, chamfered TABLE 1 Chemical Composition of AISI D3 Steel in Weight % C Mn Si P S Cr Ni

7 476 H. Bouchelaghem et al. insert: (0.1 mm 20 )) is mounted on PSBNR2525K12 toolholder. The geometry of the tool-in-holder is characterised by the following angles: v r ¼ 75 ; a ¼ 6 ; c ¼ 6 ; k ¼ 6. The tool wear has been measured using an optical microscope (HUND W-AD) equipped with a CCD camera. All roughness measurements have been obtained directly on the tool machine and without disassembling the workpiece by means of Roughness meter (Surftest 301 Mututoyo). The tool holder was mounted on a three-component piezoelectric dynamometer (Kistler 9257B). The measurement chain also included a charge amplifier (Kistler 5019B130), data acquisition hardware (A=D 2855A3) and graphical programming environment (DYNOWARE 2825A1 1) for data analysis and visualization. The whole measurement chain has been statically and dynamically calibrated. The static calibration of the dynamometer was made in each force direction. The outputs are voltage signals which are averaged for each direction. The loads up to 3000 N by 25 N intervals were applied and the deflection values were recorded for each load intervals. Thus calibration curves were obtained to convert the output readings into cutting force values. In order to verify the consistency, the measurements were repeated three times. The effect of loading in a given dynamometer direction on the other force components was also examined and minor fluctuations were observed. These effects were small enough to be ignored. The cross-sensitivity was estimated in the range of 0.14% 0.81%. For the dynamic calibration, it can be said that natural frequencies of the dynamometer in the three directions are approximately near 3 khz. The recorded forces are not influenced by the dynamic response of the dynamometer because these natural frequencies are higher than the frequency of exciting vibration. So, this dynamometer can be assumed as a reliable device for the measurement of cutting forces. For the temperature measurements in the chip, the part, and the cutting tool, a distance IR pyrometer, Rayner model 3I ( 30 to 1200 C), was used with a remote data acquisition system. The calibration sources for this instrument were certified by a national calibration laboratory and are traceable to primary standards. The certificate describes the equipment used for calibration. In addition, the certificate lists test accuracy data and the next calibration date. The used Pyrometer was calibrated systematically before each cutting test. Wear has been investigated without lubrication under the following cutting parameters: feed rate (f) of 0.08 and 0.16 mm=rev, cut depth (a p )of 0.5 mm and a cutting speed (V c ) range of 85 to 310 m=min. An allowable wear value [VB] of 0.3 mm is adopted for the CBN tool life. However, wear values have been obtained beyond this limit in order to observe the global CBN tool behaviour under extreme condition.

8 Hard Turning of Cold Work Tool Steel (D3) Using CBN 477 RESULTS AND DISCUSSION Analysis of CBN Tool Wear Long-term wear tests have been carried out through straight turning to evaluate CBN tool life for various cutting conditions. These tests provide reliable results closer to industrial reality (Bodart, 1990). Results on CBN tool wear evolution (flank and crater) as a function of various cutting conditions are illustrated in Figures 1 and 2. It can be clearly seen that the wear trend obeys the universal wear law of any mechanical workpiece (initial wearing, normal wear, accelerated wear). Nevertheless, the cutting speed has a great effect on the tool wear behaviour as a rapid increase of the wear is started from a cutting speed of 115 m=min. Consequently the extent of the normal wear zone is reduced considerably; this is justified when testing at 85, 150 and 310 m=min. At 85 m=min, the normal wear zone extends during 35 min, at 150 m=min it requires only 15 min, whereas at 310 m=min the normal wear zone is almost non-existent. Moreover, Figure 2 reveals that the crater trend of wear KT is relatively weak comparing to wear VB. Therefore the principal criterion of tool life is the VB wear. FIGURE 1 Flank wear (VB) evolution as a function of cutting time at various cutting conditions.

9 478 H. Bouchelaghem et al. FIGURE 2 Crater wear (KT) evolution as a function of cutting time at various cutting conditions. During the machining of steel AISI D3 with CBN tool and for all conditions tested, grooves on tool flank surface were observed. These grooves are oriented along the cutting speed direction. The latter seems to be the results of high abrasive wear. Grooves usually appear at the beginning of the machining and never disappear (Figure 3), and this is in good agreement with what is reported in literature by Luo et al. (1999) and Poulachon and Moisan (2003). Figure 4 shows the rake crater and flank chipping wears of a CBN tool observed under a scanning electron microscope (SEM) during machining of AISI D3 at working parameters: V c ¼ 115 m=min, f ¼ 0.08 mm=rev and a p ¼ 0.5 mm, after 32 min of machining. The flank insert wear presents grooves which characterise abrasive wear mechanism and material adhesion. The latter is confirmed through energy dispersive X-ray spectroscopy (EDS) test (Figure 5) showing the presence of Fe and Cr in the material deposited on the flank of the tool. In the same way Gaitonde et al. (2009) has confirmed the presence of both Fe and Cr on ceramic insert surfaces (Al 2 O 3 þ TiC) with wiper geometry during the machining of AISI D2 steel. Moreover, Luo et al. (1999) affirm that the origin of grooves is attributed to the tool binder damage caused by hard carbide particles of the workpiece leading to

10 Hard Turning of Cold Work Tool Steel (D3) Using CBN 479 FIGURE 3 Microphotos of CBN insert wear both on flank face and rake face (Zf: flank face zone, Zr: rake face zone, Zn: insert nose zone). CBN tool grains falling out when machining hardened steel (55 HRC) with a CBN tool. Also, results on the cutting of AISI D2 (55 HRC) steel carried out by Poulachon et al. (2003) have indicated that the origin of grooves is attributed the presence of carbide in the cut material microstructure. The major FIGURE 4 SEM photo of the flank and crater wears of CBN cutting tool.

11 480 H. Bouchelaghem et al. FIGURE 5 Example of EDS analysis on the flank face of a CBN cutting tool. groove size is close to the size of M7C3 carbide clusters. However, deep studies of the worn areas show that the wear phenomenon due to diffusion generated with great cutting speeds takes part in wear process of the rake surface of CBN tool (Nabahani, 2001; Zimmerman et al., 1997; Chou et al., 2003). When combined, these wear mechanisms (abrasion, adhesion and diffusion) lead to acceleration of the chipping process of tool surfaces and degradation of the cutting edge. Huang et al. (2007) have reported that regarding the wear mechanisms involved in CBN hard turning, abrasion, adhesion, and diffusion phenomena can be generally considered the main tool wear mechanisms. Moreover, the contribution from each mechanism depends on mechanical and thermal loading during machining, CBN content, binder phase and chemical stability of CBN tools, and composition of the workpiece materials. Good physical understanding of CBN tool wear mechanisms is the indispensable step toward a viable hard-turning technology. Analysis of CBN Tool Life and Productivity Graphic analysis of the flank wear results (VB) (Figure 1) as a function of cutting time in the investigated cutting speeds allows the determination of different CBN tool lives. It should be noted that the tool life (T) has been obtained for the flank wear as the latter is much higher than the crater wear, especially when speed is increased. On the basis of two allowable wear criteria [VB] ¼ 0.3 mm and [VB] ¼ 0.4 mm, the obtained values of tool life are summarized in Figure 6. The decision to raise the acceptable value of [VB] to 0.4 mm was taken when observing that the machined surface presents an acceptable surface state defined by R a < 1 mm. This is very interesting since the dimensional accuracy and surface finish is maintained.

12 Hard Turning of Cold Work Tool Steel (D3) Using CBN 481 FIGURE 6 Cutting speed effect on CBN tool life. As a consequence, the cutting tool lasts longer, allowing a saving in production cost (see Figure 7). For a variation of the cutting speeds from 85 to 115 m=min, the tool life falls to 45%. Increasing the cutting speed range [115 to 150] m=min, for a speed ratio of 1.3, a 23% tool life fall is observed. FIGURE 7 Gain in tool life when [VB] ¼ 0.4 mm.

13 482 H. Bouchelaghem et al. For three speed ranges [150 to 200], [200 to 240] and [240 to 310] m=min, the corresponding tool life reduction has been found to be 36%, 19%, and 67%, respectively. So, there is obviously a great sensitivity of lifetime to cutting speed variation. This can be explained by the presence of high temperatures generated by the cutting process around the tool nose, supporting various wear mechanisms (abrasion and diffusion) and consequently the cutting tool capacity decreases. Even more, when working at high cutting speeds, the mechanical and thermal effects increase on the cutting edge. As a consequence, friction and high pressures that are applied on the chip-tool and tool-workpiece interfaces generate wear (Yallese et al., 2009). Figure 8 illustrates the chip volume produced during the machining of AISI D3 steel at various cutting speeds for the two wear criteria [VB] ¼ 0.3 mm and 0.4 mm. Mathematically, this volume is defined as the multiplication of cutting speed by feed rate, cutting depth and cutting time, simultaneously. Usually in machining industry this volume of metal removal is considered as productivity indicator. From the results, it can be clearly seen that for a cutting speed of 85 m= min there have been better productivity performance as volume V corresponding to the two wear criteria is equal to 136 cm 3 and 153 cm 3, respectively. The smallest chip volume (37.2 and 49.6) cm 3 corresponding to wear allowed values are obtained for a cutting speed of 310 m=min. The optimized selection of the cutting parameters shall be carried out balancing the tool life versus the productivity. In general terms, cutting conditions that improve the productivity lead to a decrease in the tool life, as reported by de Siqueira Galoppi et al. (2006). FIGURE 8 Effect of cutting speed on the generated chip volume.

14 Hard Turning of Cold Work Tool Steel (D3) Using CBN 483 Analysis of Surface Roughness Figure 9 illustrates results of the effect of the cutting conditions (V c, f and a p ) on the surface roughness. Machined surface roughness decreases FIGURE 9 Roughness evolution as a function of cutting conditions: (a) Speed effect on roughness at f ¼ 0.08 mm=rev; a p ¼ 0.1 mm. (b) Feed rate effect on roughness at V c ¼ 110 m=min; a p ¼ 0.1 mm. (c) Depth of cut effect on roughness at V c ¼ 110 m=min; f ¼ 0.08 mm=rev.

15 484 H. Bouchelaghem et al. when V c increases up to 140 m=min, then it stabilizes around V c ¼ 280 m= min. This surface state improvement is due to high cutting speeds characterized by the absence of built-up edge. So, there is no surface state degradation due to the adhesion of particles or fragmented chips on the machined surface. Beyond 280 m=min the surface roughness increases because of vibrations in the machining system and the rapid tool wear (Bouchelaghem et al., 2006) (Figure 9a). Figures 9b and 9c show that when increasing f and a p surface roughness increases, however noticeable effect is observed for f. In practice the consequences of the influence of f on the surface roughness could be summarized as follows: when f increases from 0.08 to 0.16 mm=rev, surface roughness increases 2.11 times. Nevertheless, despite the increase of f, the values of surface roughness are still acceptable and even can be compared to those obtained by grinding process. The roughness evolution as a function of wear VB is illustrated in Figure 10(a, b), for V c ¼ 115 and 160 m=min cutting speeds. There is neither stable nor uniform roughness value for criteria (R a, R z, R t ) during the cutting, but roughness is subjected to sudden increase with tool wear resistance regarding machining time. Therefore, superficial damage obtained on tool faces and cutting edges leads to the degradation of the machined surface. The curves also show that CBN ensures a good surface quality during a large spare machining time. At V c ¼ 115 m=min and after 32.5 min of cutting, roughness R a has not exceeded 1 mm; similarly at higher speed V c ¼ 160 m=min, R a remains lower than 1 mm during 20 min of machining. The change in the roughness behaviour as a function of VB wear can be explained through microscopic observations of the tool nose. Figure 10b demonstrates increasing wear micrographics describing the transition from low roughness value at VB ¼ 0.14 mm, to high value at VB ¼ 0.31 mm and the zone with higher roughness value at VB ¼ 0.43 mm, for a cutting speed of 160 m=min. 4 min machining generates a weak wear bandwidth that is appreciable with a constant total roughness R t ¼ 2.75 mm. 8 min machiningreveals an increase in the width of the wear band with the formation of small grooves on the flank face (abrasion wear). After 12 min of machining, the thermo-mechanical effects increase on the cutting edge, causing different forms of the CBN tool wear, resulting in a total roughness value R t of 3.44 mm and a VB of 0.25 mm. When machining ended after 20 min, the tool underwent a rapid wear degradation (VB ¼ 0.43 mm), where a small collapse on the tool CBN nose was observed, leading to a machined surface quality drop R t ¼ 5.01 mm. Finally, at V c ¼ 160 m=min, the VB wear increased from 0.14 to 0.43 mm and led to roughness values increase (R a, R z, R t ) by %, % and 82.18%, respectively.

16 Hard Turning of Cold Work Tool Steel (D3) Using CBN 485 FIGURE 10 Flank wear effect on roughness at: (a) V c ¼ 115 m=min; f ¼ 0.08 mm=rev; a p ¼ 0.2 mm. (b) V c ¼ 160 m=min; f ¼ 0.08 mm=rev; a p ¼ 0.2 mm. Kishawy and Elbestawi (2001) have shown during machining of AISI D2 (62 HRC) with CBN tool that the machined surface roughness is deteriorated as the tool wear increases. A similar effect is observed by Bruni et al. (2008) during the finish turning of hardened steel (58 HRC). The direct relationship between the surface finish and CBN tool flank wear is correlated by an increase in roughness R a with VB. A comparative analysis between roughness values gathered from the literature review Torbaty (1999) and those obtained experimentally by hard turning AISI D3 steel using CBN insert have been made (Figure 11). It can be clearly seen that most values of R a obtained in hard turning are within the range of those obtained by grinding process. Meanwhile, there has been an

17 486 H. Bouchelaghem et al. FIGURE 11 Comparison of roughness criteria in grinding (Torbaty, 1999) and hard turning. overlapping in the values of R z and R t for both processes, in the order of 1.47 mm for R z and 5.74 mm for R t. According to the values of roughness obtained in the case of hard turning, it can be said that the latter process can replace or be complementary to grinding for the manufacture of precision. It should be also noted that even when completely machining a heat treated workpiece using hard turning, it is still not useful to implement general substitution of the grinding (Coelho et al., 2007; Yallese et al., 2004). Analysis of Cutting Forces The studied force components characterizing the interaction of (CBN=AISI D3) are shown in Figure 12. The evolution of the cutting forces as a function of the machining parameters (V c, f, a p ) is given in Figure 13. The cutting force decreases when V c increases (Figure 13a). This can be explained through the improvement of machined metal deformation capacity and the friction conditions at chip-tool-workpiece interfaces. Indeed, when V c increases the temperature in the cutting zone increases making the material more ductile and leading to a decrease of the cutting face. Regarding the curve trends, the cutting forces decreased as V c increased to 80 m=min; beyond this value they stabilized. The decrease in the cutting forces is more expressed at low speeds. In fact, an increase of V c from 30 to 80 m=min cause a drop of the three components (F y, F x, F z ) at 46.9%, 35.1% and 37.5%, respectively, while an increase of V c from 80 to 420 m=min caused a drop of 21.9%, 33.1% and 48%, respectively.

18 Hard Turning of Cold Work Tool Steel (D3) Using CBN 487 FIGURE 12 Schematic representation of force components during cutting operations. It is also worth noting that for a p ¼ 0.1 mm, the radial force F y is more important than F x and F z. This can be explained by the fact that the cutting tool works exclusively with its nose. The latter has a radius equals to 0.8 mm, greater than the a p value, and possessing a negative rake angle. Figures 13b and 13c show that as the feed rate and the depth of cut increase the cutting efforts increase because of the sheared chip cross section that is getting larger together with the volume of the deformed metal. Therefore, the workpiece material becomes more resistant to shearing and there should be much effort applied to remove the chip. The radial force is most dominating, followed by tangential and axial force, in all testing feed rates. Throughout this investigation; the axial effort is less sensitive to f. For a p ¼ 1 mm, the cutting occurs beyond the limit of the tool-nose radius and the workpiece presents better resistance to penetration in the direction of axial force because the cutting edge length in contact with the workpiece increases. This means that the working zone is out of the tool-nose radius. Hence the axial force is more dominant than the tangential one. Figure 14 shows that cutting forces increased as a function of tool wear, and hence as a function of cutting time. This is due to wear evolution on the rake and clearance surfaces of the tool. As a consequence the workpiece-tool contact surface increased together with the friction forces, generating higher cutting forces. The first stage lasted 6.5 min and

19 488 H. Bouchelaghem et al. FIGURE 13 Cutting forces evolution as a function of: a) Cutting speed at a p ¼ 0.1 mm, f ¼ 0.08 mm=rev, b) Feed rate at V c ¼ 110 m=min, a p ¼ 0.1 mm, c) Depth of cut at V c ¼ 110 m=min, f ¼ 0.08 mm=rev. corresponded to a VB wear of 0.15 mm. The cutting forces (F y, F x and F z ) recorded were , 61.5 and N, respectively. The second stage was varied from 6.5 to 26 min of machining, where the measured wear

20 Hard Turning of Cold Work Tool Steel (D3) Using CBN 489 FIGURE 14 Cutting force evolution as a function of flank wear at V c ¼ 115 m=min; f ¼ 0.08 mm=rev; a p ¼ 0.2 mm. VB increased from 0.15 to 0.31 mm, causing an increase in the cutting forces (F y, F x and F z ) of 87.96, and 76.07%, respectively. In this stage, the evolution of the cutting forces was more accentuated than during the first stage. The third stage was characterized by a fast evolution of wear VB value passing from 0.31 to 0.42 mm during a machining spare time of 26 to 32.5 minutes. At this stage the cutting forces reached maximum values, with an increase of 20.47, and 4.84%. When comparing the evolution of the cutting forces (F y, F x and F z ) from beginning to end of machining, there has been respectively , and 84.60%, increase. Analysis of Cutting Temperature Figure 15 presents the evolution of maximum temperature in the cutting zone as a function of the cutting parameters (V c, f and a p ). Results show that there is a clear increase of the temperature when one of working parameters, increases. This is attributed to friction and deformations generated in the vicinity of the CBN tool nose. In practice, consequences of the effects of cutting parameters using CBN tools, on the cutting temperature distribution can be summarized as follows. Increase in feed rate and depth of cut from 0.08 to 0.22 mm=rev, respectively, and 0.05 to 0.8 mm, generated an increase in cutting temperature 1.17 to 2.85 times, whereas when V c passes from 60 to 310 m=min, cutting temperature was increased by 2.37 times. Figure 16 illustrates the evolution of cutting temperature versus wear VB. The longer the cutting time, the larger the wear; therefore, the temperature generated in the cutting zone increased, significantly. Within 4

21 490 H. Bouchelaghem et al. FIGURE 15 Maximum temperature evolution as a function of cutting condition at: a) a p ¼ 0.2 mm; f ¼ 0.08 mm=rev., b) V c ¼ 110 m=min; a p ¼ 0.2 mm., c) V c ¼ 110 m=min; f ¼ 0.08 mm=rev. minutes, temperature reached 548 C while the flank wear was weak, VB ¼ 0.14 mm. As the contact surface between the workpiece and tool increased in 12 minutes, because of increases in wear VB ¼ 0.25 mm, there was a rise in

22 Hard Turning of Cold Work Tool Steel (D3) Using CBN 491 FIGURE 16 Temperature measured in the cutting area as a function of flank wear at V c ¼ 160 m=min; f ¼ 0.08 mm=rev; a p ¼ 0.2 mm. friction and heating, allowing the deterioration of cutting edges by physicochemical phenomena. The acceptable wear limit was reached after 16 min of machining, when a small collapse of the CBN tool nose occurred and a maximum temperature of 597 C was recorded. Later, after 20 min in the advanced stage, a catastrophic collapse of the nose was observed when VB ¼ 0.43 mm. Hence, a mechanism of abrasion and diffusion (Luo et al., 1999; Nabahani, 2001) occurred, due to the amount of heating and higher values of cutting forces. At this stage, a maximum temperature of 615 C was recorded, and the tool lost its cutting qualities by thermal fatigue. Finally, it can be said that the three parameters (friction, temperature and wear) are closely related and mutually affected each other. Consequently, the higher the cutting friction, the higher the cutting temperature and heat diffusion in the tool. So, the tool life is affected by the decrease in tool mechanical characteristics due to accelerated wear phenomena. Thus, valuable information concerning heat exchange between tool and workpiece provides more physical comprehension of tool wear (O Sullivan and Cotterell, 2001; da Silva and Wallbank, 1999). SURFACE ROUGHNESS, CUTTING FORCES AND TOOL LIFE MODELS Roughness and Cutting Force Models To develop models of surface roughness and cutting force components with machining parameters a regression procedure has been used. It can be

23 492 H. Bouchelaghem et al. TABLE 2 Roughness and Cutting Force as a Function of Cutting Parameters (V c, f, a p, r e ) According to the Experimental Plan 2 4 Cutting parameters Experiment Number f (mm=rev) a p (mm) V c (m=min) r e (mm) Roughness criterion R a (mm) Cutting forces component F z (N) expressed as follows: Y ¼X 0 þ X 1 ðaþþx 2 ðbþþx 3 ðcþþx 4 ðdþþx 5 ðabþþ X 6 ðacþþx 7 ðadþþx 8 ðbcþþx 9 ðbdþþx 10 ðcdþ where X 0 ¼ constant; A, B, C, D are variables representing different machining parameters; and, the X i are regression coefficients that depend on the average effects and interactions. Experimental results from Table 2 allowed computing the coefficients for roughness and cutting force models. With respect to units given in the nomenclature, these computed results are given Equations 2 and 3: ð1þ R a ¼ 0:22208 þ 7:93750f þ 0:36667a p 0:00250V c 0:11042r e þ 4:37500fa p þ 0:00263fV c 2:50000fr e 0:00105a p V c 0:33333a p r e þ 0:00145V c r e ðr 2 ¼ 0:940Þ ð2þ F z ¼ 85:81 þ 807:19f þ 132:75a p 1:18V c 17:62r e þ 1865:63fa p þ 0:48fV c 716:41fr e þ 0:94a p V c 84:37a p r e þ 0:89V c r e ðr 2 ¼ 0:930Þ The coefficient of determination, R 2, is calculated using the above expression and is higher than 93% for all the above models developed, ð3þ

24 Hard Turning of Cold Work Tool Steel (D3) Using CBN 493 FIGURE 17 Effect of the main cutting parameters Vc, f, a p, r e on a) cutting force (F z ), and b) surface roughness (R a ). which indicates the high degree of correlation existing between the experimental and predicted values. Figure 17 represents plots of main factor effects on surface roughness and cutting force. This plot is used to visualize the relation between factors and the output response. Both Figures 17a and 17b show surface roughness value of R a and cutting force value of F z as a function of cutting parameters (V c, f, a p and r e ), respectively. The surface roughness increased considerably with increasing feed rate. The cutting force also increased with increasing depth of cut. Figure 18 shows the estimated response of surface roughness and the contour for the parameters namely cutting speed, depth of cut, feed rate and tool nose radius. Figure 18 (a, b, c) shows the estimated response of surface roughness for the corresponding feed rate and three cutting parameters (V c, a p, r e ). It is established that feed rate has the highest impact on surface roughness. Figures 18d and 18e show the estimated response of surface roughness for the corresponding r e and V c, a p variations. From the graph, it is seen that tool nose radius has a less effect on surface roughness and its variation is too little when compared to other parameters. Figure 19 shows the estimated response of cutting force and the contour for the cutting parameters (V c, f, r e ). Figure 19 (a, b, c) shows that the F z increases with the increase of depth of cut for different (V c, f and a p ). From Figures 19d and 19e, it can be noted that the cutting force parameter F z increases with a small torsion when varying (V c, r e ) at different f.

25 494 H. Bouchelaghem et al. FIGURE 18 The response surface of surface roughness (R a ) according to change of cutting speed (V c ); feed rate (f); depth of cut (a p ) and tool nose radius (r e ).

26 Hard Turning of Cold Work Tool Steel (D3) Using CBN 495 FIGURE 19 The response surface of cutting force (Fv) according to variation of cutting speed (Vc); feed rate (f); depth of cut (a p ) and tool nose radius (r e ).

27 496 H. Bouchelaghem et al. TABLE 3 Tool Life as a Function of Cutting Parameters (V c, f) Cutting parameters Tool life (min) V c (m=min) f (mm=rev) VB ¼ 0.3 mm VB ¼ 0.4 mm Tool Life Model The treatment of the results presented in Table 3 allows the determination of tool life models according to Equations 4 and 5. T ¼ 75:439 0:323V c 129:825f þ 0:351V c f ; ðr 2 ¼ 0:998Þ for VB ¼ 0:3mm ð4þ T ¼ 73:7544 0:2912V c 35:0877f 0:1754V c f ; ðr 2 ¼ 0:997Þ for VB ¼ 0:4mm ð5þ The coefficient of determination is calculated using the above expression and is more than 99% for all the above models developed. Figure 20 (a, b) shows tool life T as a function of cutting parameters (V c, f). From figure, it is confirmed that cutting speed has the most influence on tool life for VB ¼ 0.3 mm and VB ¼ 0.4 mm. Figure 21 shows the estimated response of tool life for the corresponding cutting speed and feed rate at VB ¼ 0.3 mm and 0.4 mm. From the FIGURE 20 Effect of main cutting parameters such as V c, f, a p on tool life (T), a) VB ¼ 0.3 mm, b) VB ¼ 0.4 mm.

28 Hard Turning of Cold Work Tool Steel (D3) Using CBN 497 FIGURE 21 Response surface of tool life (T) according to variation of feed rate (f) and cutting speed (V c ). graph, it is confirmed that feed rate has the low significance on tool life and cutting speed is more dominating over the tool life. CONCLUDING REMARKS The main results that can be deduced from the present investigation concerning the machinability of 60 HRC AISI D3 steel using CBN tools are summarized here: 1. Wear phenomenon is dominantly by abrasion process. It is generated through scratches behaviour on the tool flank surface. Abrasion wear is due to the removal of CBN tool particles by hard small grains of the material being machined. The presence of Fe and Cr on the tool rake face implies that adhesive wear can also participate to the wear mechanisms of CBN tools. CBN tool wear appears also through craters on the flank surface. With time, VB and KT increase until the tool nose collapses. At higher speeds ( ) m=min the tool nose collapse occurs

29 498 H. Bouchelaghem et al. in the early stages within few minutes. This needs much attention as it greatly affects the surface finish and the dimensional precision of the workpiece. 2. Cutting speeds above 240 m=min imply very short tool life, thus they should be avoided. Meanwhile, for industrial application, cutting speeds varying between 80 and 240 m=min can be considered as an interesting cutting speed range for CBN use towards AISI D3 heat treated to 60 HRC. 3. Flank wear is an important factor to consider. Its evolution damages the surface finish of the workpiece. Even so when [VB] is 0.3 mm, R a values still lie within acceptable values of 1 mm. 4. When VB of CBN tool increases, cutting forces increase. Machining at 115 m=min for 32.5 min led to a VB of 0.42 mm and an increase of the cutting forces to 126% for F y,66% for F x and 84% for F z. The most dominating force component was the radial force, followed by the axial force; the tangential force was less sensitive to wear evolution. 5. The analysis of machining parameters using RSM technique has the advantage of investigating the influence of each machining parameter on machinability evaluation. 6. The analysis of cutting parameters (V c, f, a p, r e ) was carried out using three-dimensional surface plots. The cutting force increases with the increase of depth of cut. The surface roughness increases with the increase of feed rate and almost decreases with the increase of cutting speed. ACKNOWLEDGMENTS This work was achieved in the laboratories LMS (Guelma University, Algeria) and LR3MI (Annaba University, Algeria) in collaboration with LaMCoS (CNRS-UMR5259, INSA-Lyon, France). The authors would like to thank the Algerian Ministry of Higher Education and Scientific Research (MESRS) and the Delegated Ministry for Scientific Research (MDRS) for granting financial support for CNEPRU Research Project, CODE: (Guelma University). NOMENCLATURE A, B, C, D Variables representing different machining parameters a p Depth of cut [mm] CBN Cubic boron nitride f Feed rate [mm=rev]

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