- Email: [email protected]
CBN cutting tool wear during machining of particulate reinforced MMCs
Wear 257 (2004) 1041–1046
CBN cutting tool wear during machining of particulate reinforced MMCs Ibrahim Ciftcia , Mehmet Turkerb,∗ , Ulvi Sekerb b
a ZKU, Karabuk Technical Education Faculty, 78050 100. Yil, Karabuk, Turkey Gazi University Technical Education Faculty, Gazi University, 06500 Besevler, Ankara, Turkey
Received 12 January 2004; received in revised form 16 June 2004; accepted 2 July 2004
Abstract In this experimental study, three SiC/Al metal matrix composites (MMCs) with SiC particles of 30, 45 and 110 m in mean sizes were produced using a melt stirring-squeeze casting route. Machining tests were carried out on the MMCs using cubic boron nitride (CBN) cutting tools at various cutting speeds under a constant feed rate and depth of cut. The effect of reinforcement particulate sizes and cutting speeds on tool wear was investigated. Furthermore, surface roughness measurements were also carried out on the machined surfaces. The results showed that tool wear was mainly dominated by flank wear and strongly influenced by reinforcement particulate size. The MMC containing SiC particles of 110 m proved to be unsuitable for the machining operation using CBN cutting tools due to the heavy fracture of the cutting edge and nose. In the composites reinforced with SiC particles of 30 and 45 m, 150 m/min cutting speeds led to the lowest tool flank wear values while 100 and 200 m/min cutting speeds resulted in higher tool flank wear values. © 2004 Elsevier B.V. All rights reserved. Keywords: Tool wear; CBN; Machining; Metal matrix composites (MMCs)
1. Introduction Considerable research in the material science has been directed toward the development of new lightweight engineering materials possessing high specific strength and stiffness at elevated temperatures and good creep, fatigue and wear resistance. That is because advanced automotive and aerospace technology requires these materials to improve performance. At present these properties seem to be not achievable with lightweight monolithic titanium, aluminium, and magnesium alloys. In contrast, metal matrix composites (MMCs) have demonstrated their potential for providing a major jump in performance [1,2]. Particulate reinforced MMCs have received considerable attention due to their low cost when compared to long fibre reinforced MMCs and due to their better properties than those ∗ Corresponding author. Tel.: +90 312 212 39 93; fax: +90 312 212 00 59. E-mail address: [email protected] (M. Turker).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.07.005
of monolithic alloys [1–3]. These materials may have a wide application, especially for components which are exposed to friction [4]. However, cemented carbide cutting tools, widely used in metal cutting industry, wear rapidly when machining these materials due to the presence of hard reinforcement particles like SiC or Al2 O3 in particulate reinforced MMCs [5–7]. That is not surprising as the reinforcement particles are harder than cemented carbide tools. Therefore, cutting tools harder than reinforcement particles present in the MMCs are required to machine these materials. Although some ceramic cutting tools are harder than these reinforcement materials, they wear more rapidly than cemented carbide ones [8–10]. Polycrystalline diamond (PCD) tools are recommended for machining particulate reinforced MMCs, especially for finish machining [6,7]. Work to date has shown that the majority of the papers dealing with the machinability of MMCs mainly concentrates on PCD cutting tools. In these papers, optimum cutting parameters, tool wear modes and mechanisms were tried to be identified [6,11–13]. On the other hand, cubic boron nitride (CBN), close to diamond in hardness, was
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rarely used to machine MMCs [10–14]. Work carried out by Looney et al. [10] and Hung et al. [14] showed that CBN cutting tools can also be an alternative to PCD tools for machining particulate MMCs. As these cutting tools are very expensive and cutting conditions significantly affect the tool wear, correct selection of cutting parameters is essential. It is clear from the available literature that reinforcement particle size in the composites significantly affect tool wear [15,16], but as yet only few works related to the influence of reinforcement particle size on tool life have been reported. In this study, particulate reinforced three MMC grades, each containing 16 wt.% SiC particles of 30, 45 and 110 m in mean sizes were machined using CBN cutting tools. The influences of the particulate size and the cutting speed on tool wear and the surface roughness were investigated.
furnace power was put on. Initially, the furnace temperature was raised to 725 ◦ C and held at that temperature until the matrix material was melted completely. Then, it was cooled to 675 ◦ C to enable the particulate incorporation. Prior to the particulate addition, 0.5 wt.% Mg was added to the matrix to improve the wetting. At 675 ◦ C, graphite impeller was started turning and SiC particles, which were preheated at 1000 ◦ C for 2 h to make their surfaces oxidised, were added at a uniform rate. During this stage, the temperature was gradually increased up to 725 ◦ C in order to improve fluidity of the mixed slurry. After particulate addition was completed, the stirring continued for 5 min more. When the stirring was completed, the mixed slurry was poured in a preheated steel die. The die was placed on the bed of a hydraulic press and the slurry was squeezed until complete solidification occurred under the pressure of 40 MPa. The microstructures of the MMCs with different SiC sizes are shown in Fig. 1.
2. Experimental procedure 2.2. Machinability tests Three MMCs, each containing 16 wt.% SiC particles of 30, 45 and 110 m in mean sizes, were fabricated using a melt stirring-squeeze casting route. The matrix material was a 2014 Al alloy. The chemical composition of the matrix and the list of MMC grades fabricated are given in Tables 1 and 2, respectively. 2.1. Preparation of MMCs In all cases melting was carried out in a graphite crucible in a resistance furnace under argon atmosphere. A metered amount of matrix alloy was charged into the crucible and the Table 1 Chemical composition of matrix % Si Fe Cu Mn Mg Zn Ni Cr Pb Sn Ti Sb Al
0.66 0.504 4.49 0.62 0.6 0.11 0.01 0.03 0.03 0.005 0.025 0.04 92.9
Table 2 MMCs prepared for machinability tests Material of particles
Code
Weight percentage
2014 Al + 16SiC (30 m) 2014 Al + 16SiC (45 m) 2014 Al + 16SiC (110 m)
Al-SiC/30 Al-SiC/45 Al-SiC/110
16 16 16
The machining tests were performed by single point continuous turning of the MMC specimens in cylindrical form on a CNC DYNA lathe. The workpiece specimens were 120 mm long and 28 mm in diameter. Coolant was not used during the tests. The cutting tools used were commercial grade CBN inserts and produced by Mitsubishi Carbide with the geometry of CCGW 09T308G2. The cutting edges of these inserts are chamfered. The inserts were clamped mechanically on a rigid tool holder. Its code was SCGCR-1616-H09 according to ISO 5608. This resulted in a cutting geometry with side rake angle = 0◦ , back rake angle = 0◦ and side cutting edge angle = 90◦ . Cutting speeds used were ranged from 50 to 200 m/min. The cutting speed was increased in steps of 50 m/min. Feed rate and depth of cut were kept constant at 0.12 mm/rev and 1 mm, respectively. Tool flank wear was measured using a toolmaker’s microscope and the surface roughness was measured using a Mitutoyo Surftest 211 instrument. Worn cutting tools were also examined under scanning electron microscope (SEM). All measurements were made after the same volume of metal removed (2500 mm3 ).
3. Results and discussion 3.1. Tool wear Initially, the machining tests for all grades of the MMCs were conducted at 100 m/min cutting speed. It was seen that the cutting tools used to machine Al-SiC/110 MMC encountered very high wear and therefore the cutting speed was lowered for machining this MMC grade. Extensive fracture of the cutting edge and the nose as shown in Fig. 2 was observed after machining Al-SiC/110 MMC. On the other hand, for both Al-SiC/30 and Al-SiC/45 MMCs, flank wear was found to be the dominant wear mode for CBN cutting tools as shown in
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Fig. 1. Microstructures of the MMC specimens with SiC particles of (a) 30 m, (b) 45 m and (c) 110 m in size.
Fig. 3. Variations of cutting tool flank wear with cutting speed and SiC particulate size is shown in Fig. 4. It should be noted that the flank wear/cutting speed curves given in this paper show the magnitude of the wear sustained after removing a fixed volume of workpiece material at various cutting speeds
and at a constant feed rate and depth of cut. Therefore, these are not flank wear curves in the usual sense, in that flank wear is not plotted as a function of cutting time at a fixed cutting speed. However, these curves give a clear indication of the behaviour of CBN cutting tools when tested over a range of
Fig. 2. SEM image of the worn CBN cutting tool used for machining AlSiC/110 MMC at 100 m/min.
Fig. 3. SEM image showing the CBN cutting tool flank wear for cutting Al-SiC/45 MMC at 100 m/min.
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Fig. 4. Tool flank wear for equal material removal varying against the cutting speed for cutting the three MMC grades at the feed rate of 0.12 mm/rev and depth of cut of 1 mm.
cutting speeds [10]. It is seen from Fig. 4 that the cutting tools used to machine Al-SiC/110 MMC encountered very high wear when compared to the others. Therefore, it can be concluded that CBN is unsuitable tool material to machine MMCs with larger SiC particles. As can be seen from Fig. 4, the lowest tool flank wear values were recorded at 150 m/min cutting speed during the machining of Al-SiC/30 and Al-SiC/45 MMCs even though the lowest cutting speed used for the both MMC grades was 100 m/min. Although tool wear generally increases with increasing cutting speed in machining, in this work a reduction in tool wear was observed for the both MMC grades when the cutting speed was increased from 100 to 150 m/min. The worn parts of the CBN cutting tools used to machine Al-SiC/30 and Al-SiC/45 MMCs were investigated using the SEM. Fig. 5 shows the SEM images of the worn cutting edges. It is seen from these images that the shapes of the BUE, especially at the depth of cut lines of the cutting tools, considerably differ depending on the cutting speed applied for the
Fig. 5. SEM images showing the BUEs on the cutting tools after machining (a) Al-SiC/30 at 100 m/min, (b) Al-SiC/30 at 150 m/min, (c) Al-SiC/45 at 100 m/min and (d) Al-SiC/45 at 150 m/min.
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both MMC grades. At 100 m/min cutting speed, the BUEs (Fig. 5a and c) were formed on a smaller area when compared to those (Fig. 5b and d) formed at 150 m/min cutting speed. In machining, as speed increases, temperature also increases [17] and this, in turn, softens the BUE on the rake face and the adhered workpiece material on the flank face. The images in Fig. 5b and d are the evidence for softening of the BUE on the rake face, especially at the depth of cut lines. When these images are examined carefully, it is seen that the BUEs in the both images are expanded on a large area (Fig. 5b and d) at 150 m/min cutting speed. It is likely that the cutting temperature generated at 150 m/min cutting speed was higher than that at 100 m/min cutting speed. Due to this higher cutting temperature, the BUE and any adhered workpiece material on the flank face were softened and less adhesive force was exerted on the cutting tool. That is because, when there is BUE and adhered workpeice material on the cutting tool, it is likely that the particles of the cutting tool wear away through workpiece seizure and pull-out process during cutting. Therefore, the lower flank wear value at 150 m/min cutting speed can be attributed to less adhesive wear at this cutting speed. Andrewes et al. [6] reported that when machining MMCs using PCD cutting tools, the initial flank wear starts with the abrasive effect of the hard particles and the workpiece material adheres to these abrasive grooves strongly due to the high pressure generated at the tertiary cutting zone as machining progresses. Each time, a workpiece film adheres and breaks off as a result of the hard abrasive particles present in the workpiece, small diamond particles are also removed from the PCD cutting tool surface. This cyclic process leads to progressive loss of the PCD tool material. Unlike Andrewes et al. [6], CBN cutting tools were used in this study but it was assumed that similar mechanisms controlled the tool wear during the initial stages. That is because, CBN is close to diamond in hardness and harder than the SiC particles. In this study, it is believed that the wearing away off the CBN particles through workpiece seizure and pull-out process was reduced because of less adhesive force generated at 150 m/min cutting speed.
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Fig. 6. SEM images showing the wear marks on the flank face.
machine Al-SiC/45 MMC at 200 m/min. The bottom SEM image in Fig. 6 indicates that that abrasion and adhesion are the mechanisms dominating the tool wear. 3.3. Workpiece surface roughness
3.2. Tool wear mechanisms In order to examine the tool wear in detail, the worn cutting tools were investigated using the SEM. The cutting tools used to machine Al-SiC/110 MMC exhibited significant amount of fracture as shown in Fig. 2. For the tools used to machine Al-SiC/30 and Al-SiC/45 MMCs, the dominant wear mechanisms could not be determined clearly as the workpiece material adhered to the cutting edges at 100 and 150 m/min cutting speed. As explained in Section 3.1, both abrasive and adhesive wear mechanisms seem to be dominant in the tool wear. When the cutting speed was increased to 200 m/min, the mechanisms dominating the tool wear became clear as shown in Fig. 6. Fig. 6 shows the SEM images of the tool used to
Workpiece surface roughness values (Ra ) measured after the machining tests are given in Fig. 7. These values are the averages of three readings. As can be seen from this figure, Al-SiC/110 MMC produced the poorest surface finish at both cutting speeds in terms of Ra values. It was considered that relatively big particulate size and the fracture of the cutting edges (Fig. 2) caused these significantly high Ra values. In addition, particulate fracture and voids present in this MMC [18] can be another reasons for the increase of Ra parameter. The machined surfaces of the Al-SiC/30 and Al-SiC/45 MMCs exhibited lower Ra values generally irrespective of the cutting speed and reinforcement particulate size. It was considered that as the difference between these two particulate sizes were relatively small, the resulting Ra values did
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of the BUE and the adhering workpiece material with an increase in the cutting speed.
References
Fig. 7. Variation of surface roughness with cutting speed and SiC particulate size.
not vary significantly. In addition, this can also be attributed to the some inconsistencies in the microstructures of these MMCs [19] and the BUE formed on the cutting tool.
4. Conclusion Three Al/SiC MMCs reinforced with 16 wt.% of SiC particles of different mean sizes and produced by a melt stirringsqueeze casting route were subjected to machining test. The influences of reinforcement particulate size and cutting speed on tool wear and surface roughness were investigated. Based on the results obtained, the following conclusions can be drawn: - In the machining of the MMCs containing SiC particles of 30 and 45 m in sizes, flank wear was found to be the dominant wear mode of CBN cutting tool while abrasion and adhesion were observed as the predominant wear mechanisms. On the other hand, the cutting tool used to machine the MMC with SiC particles of 110 m in sizes encountered both the cutting edge and nose fractures. - The MMC with SiC particles of 110 m in mean size caused very high tool wear. When compared to the others, this MMC proved to be unsuitable for the machining operation using CBN cutting tools due to the heavy fractures of both the cutting edge and nose. - In the machining of the MMCs containing SiC particles of 30 and 45 m in mean sizes, 150 m/min cutting speed led to the lowest tool flank wear while 100 and 200 m/min cutting speeds caused higher tool flank wear after the same volume of metal removed. The decrease in tool flank wear with increasing cutting speed can be attributed to the softening
[1] B. Inem, The development of the structures and properties of magnesium matrix silicon carbide reinforced composites, Ph.D. Thesis, School of Materials, University of Leeds, England, 1993, pp. 4–7. [2] A. Jawaid, S. Barnes, S.R. Ghadimzadeh, Drilling of particulate aluminium silicon carbide metal matrix composites, in: Proceedings of the Machining of Composite Materials Symposium, ASM Materials Week, Chicago, Illinois, 1992, pp. 35–47. [3] G.S. Cole, A.M. Sherman, Lightweight materials for automotive applications, Mater. Charact. 35 (1995) 3–9. [4] N. Tomac, K. Tonnessen, Machinability of particulate aluminium matrix composites, Ann. CIRP 41 (1992) 55–58. [5] R.T. Coelho, S. Yamada, T. le Roux, D.K. Aspinwall, M.L.H. Wise, Conventional machining of an aluminium based SiC reinforced metal matrix composite (MMC) alloy, in: Proceedings of 30th MATADOR, 1993, pp. 125–133. [6] C.J.E. Andrewes, H.Y. Feng, W.M. Lau, Machining of an aluminum/SiC composite using diamond inserts, J. Mater. Process. Technol. 102 (2000) 25–29. [7] A.R. Chambers, S.E. Stephens, Machining of Al-5Mg reinforced with 5 vol.% Saffil and 15 vol.% SiC, Mater. Sci. Eng. A 135 (1991) 287–290. [8] I. Ciftci, M. Turker, U. Seker, Evaluation of tool wear when machining SiCp-reinforced Al-2014 alloy matrix composites, Mater. Des. 25 (2004) 251–255. [9] O. Quigley, J. Monaghan, P. O’Reilly, Factors affecting the machinability of an Al/SiC metal-matrix composite, J. Mater. Process. Technol. 43 (1994) 21–36. [10] L.A. Looney, J.M. Monaghan, P. O’Reilly, D.M.R. Taplin, The turning of an Al/SiC metal-matrix composite, J. Mater. Process. Technol. 33 (1992) 453–468. [11] M.K. Brun, M. Lee, F. Gorsler, Wear characteristics of various hard materials for machining SiC-reinforced aluminum alloy, Wear 104 (1985) 21–29. [12] M. El-Gallab, M. Sklad, Machining of Al/SiC particulate metalmatrix composites, Part I: tool performance, J. Mater. Process. Technol. 83 (1998) 151–158. [13] J.P. Davim, A.M. Baptista, Relationship between cutting force and PCD cutting tool wear in machining silicon carbide reinforced aluminium, J. Mater. Process. Technol. 103 (2000) 417–423. [14] N.P. Hung, F.Y.C. Boey, K.A. Khor, Y.S. Phua, H.F. Lee, Machinability of aluminium alloys reinforced with silicon carbide particulates, J. Mater. Process. Technol. 56 (1996) 966–977. [15] X. Li, W.K.H. Seah, Tool wear acceleration in relation to workpiece reinforcement percentage in cutting of metal matrix composites, Wear 247 (2001) 161–167. [16] Q. Yanming, Z. Zehua, Tool wear and its mechanism for cutting SiC particle-reinforced aluminium matrix composites, J. Mater. Process. Technol. 100 (2000) 194–199. [17] S. Kalpakjian, Manufacturing Engineering and Technology, third ed., Addision-Wesley, Massachusette, 1995, p. 613. [18] I. Ciftci, Investigation of reinforcement weight fraction and size on mechanical properties and machinability of aluminium based composites, Ph.D. Thesis, Gazi University, Institute of Science and Technology, Ankara, Turkey, October 2003, p. 102. [19] A.R. Chambers, The machinability of light alloy MMCs, Composites, Part A, 27A (1996) 143–147.
CBN cutting tool wear during machining of particulate reinforced MMCs Ibrahim Ciftcia , Mehmet Turkerb,∗ , Ulvi Sekerb b
a ZKU, Karabuk Technical Education Faculty, 78050 100. Yil, Karabuk, Turkey Gazi University Technical Education Faculty, Gazi University, 06500 Besevler, Ankara, Turkey
Received 12 January 2004; received in revised form 16 June 2004; accepted 2 July 2004
Abstract In this experimental study, three SiC/Al metal matrix composites (MMCs) with SiC particles of 30, 45 and 110 m in mean sizes were produced using a melt stirring-squeeze casting route. Machining tests were carried out on the MMCs using cubic boron nitride (CBN) cutting tools at various cutting speeds under a constant feed rate and depth of cut. The effect of reinforcement particulate sizes and cutting speeds on tool wear was investigated. Furthermore, surface roughness measurements were also carried out on the machined surfaces. The results showed that tool wear was mainly dominated by flank wear and strongly influenced by reinforcement particulate size. The MMC containing SiC particles of 110 m proved to be unsuitable for the machining operation using CBN cutting tools due to the heavy fracture of the cutting edge and nose. In the composites reinforced with SiC particles of 30 and 45 m, 150 m/min cutting speeds led to the lowest tool flank wear values while 100 and 200 m/min cutting speeds resulted in higher tool flank wear values. © 2004 Elsevier B.V. All rights reserved. Keywords: Tool wear; CBN; Machining; Metal matrix composites (MMCs)
1. Introduction Considerable research in the material science has been directed toward the development of new lightweight engineering materials possessing high specific strength and stiffness at elevated temperatures and good creep, fatigue and wear resistance. That is because advanced automotive and aerospace technology requires these materials to improve performance. At present these properties seem to be not achievable with lightweight monolithic titanium, aluminium, and magnesium alloys. In contrast, metal matrix composites (MMCs) have demonstrated their potential for providing a major jump in performance [1,2]. Particulate reinforced MMCs have received considerable attention due to their low cost when compared to long fibre reinforced MMCs and due to their better properties than those ∗ Corresponding author. Tel.: +90 312 212 39 93; fax: +90 312 212 00 59. E-mail address: [email protected] (M. Turker).
0043-1648/$ – see front matter © 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.wear.2004.07.005
of monolithic alloys [1–3]. These materials may have a wide application, especially for components which are exposed to friction [4]. However, cemented carbide cutting tools, widely used in metal cutting industry, wear rapidly when machining these materials due to the presence of hard reinforcement particles like SiC or Al2 O3 in particulate reinforced MMCs [5–7]. That is not surprising as the reinforcement particles are harder than cemented carbide tools. Therefore, cutting tools harder than reinforcement particles present in the MMCs are required to machine these materials. Although some ceramic cutting tools are harder than these reinforcement materials, they wear more rapidly than cemented carbide ones [8–10]. Polycrystalline diamond (PCD) tools are recommended for machining particulate reinforced MMCs, especially for finish machining [6,7]. Work to date has shown that the majority of the papers dealing with the machinability of MMCs mainly concentrates on PCD cutting tools. In these papers, optimum cutting parameters, tool wear modes and mechanisms were tried to be identified [6,11–13]. On the other hand, cubic boron nitride (CBN), close to diamond in hardness, was
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rarely used to machine MMCs [10–14]. Work carried out by Looney et al. [10] and Hung et al. [14] showed that CBN cutting tools can also be an alternative to PCD tools for machining particulate MMCs. As these cutting tools are very expensive and cutting conditions significantly affect the tool wear, correct selection of cutting parameters is essential. It is clear from the available literature that reinforcement particle size in the composites significantly affect tool wear [15,16], but as yet only few works related to the influence of reinforcement particle size on tool life have been reported. In this study, particulate reinforced three MMC grades, each containing 16 wt.% SiC particles of 30, 45 and 110 m in mean sizes were machined using CBN cutting tools. The influences of the particulate size and the cutting speed on tool wear and the surface roughness were investigated.
furnace power was put on. Initially, the furnace temperature was raised to 725 ◦ C and held at that temperature until the matrix material was melted completely. Then, it was cooled to 675 ◦ C to enable the particulate incorporation. Prior to the particulate addition, 0.5 wt.% Mg was added to the matrix to improve the wetting. At 675 ◦ C, graphite impeller was started turning and SiC particles, which were preheated at 1000 ◦ C for 2 h to make their surfaces oxidised, were added at a uniform rate. During this stage, the temperature was gradually increased up to 725 ◦ C in order to improve fluidity of the mixed slurry. After particulate addition was completed, the stirring continued for 5 min more. When the stirring was completed, the mixed slurry was poured in a preheated steel die. The die was placed on the bed of a hydraulic press and the slurry was squeezed until complete solidification occurred under the pressure of 40 MPa. The microstructures of the MMCs with different SiC sizes are shown in Fig. 1.
2. Experimental procedure 2.2. Machinability tests Three MMCs, each containing 16 wt.% SiC particles of 30, 45 and 110 m in mean sizes, were fabricated using a melt stirring-squeeze casting route. The matrix material was a 2014 Al alloy. The chemical composition of the matrix and the list of MMC grades fabricated are given in Tables 1 and 2, respectively. 2.1. Preparation of MMCs In all cases melting was carried out in a graphite crucible in a resistance furnace under argon atmosphere. A metered amount of matrix alloy was charged into the crucible and the Table 1 Chemical composition of matrix % Si Fe Cu Mn Mg Zn Ni Cr Pb Sn Ti Sb Al
0.66 0.504 4.49 0.62 0.6 0.11 0.01 0.03 0.03 0.005 0.025 0.04 92.9
Table 2 MMCs prepared for machinability tests Material of particles
Code
Weight percentage
2014 Al + 16SiC (30 m) 2014 Al + 16SiC (45 m) 2014 Al + 16SiC (110 m)
Al-SiC/30 Al-SiC/45 Al-SiC/110
16 16 16
The machining tests were performed by single point continuous turning of the MMC specimens in cylindrical form on a CNC DYNA lathe. The workpiece specimens were 120 mm long and 28 mm in diameter. Coolant was not used during the tests. The cutting tools used were commercial grade CBN inserts and produced by Mitsubishi Carbide with the geometry of CCGW 09T308G2. The cutting edges of these inserts are chamfered. The inserts were clamped mechanically on a rigid tool holder. Its code was SCGCR-1616-H09 according to ISO 5608. This resulted in a cutting geometry with side rake angle = 0◦ , back rake angle = 0◦ and side cutting edge angle = 90◦ . Cutting speeds used were ranged from 50 to 200 m/min. The cutting speed was increased in steps of 50 m/min. Feed rate and depth of cut were kept constant at 0.12 mm/rev and 1 mm, respectively. Tool flank wear was measured using a toolmaker’s microscope and the surface roughness was measured using a Mitutoyo Surftest 211 instrument. Worn cutting tools were also examined under scanning electron microscope (SEM). All measurements were made after the same volume of metal removed (2500 mm3 ).
3. Results and discussion 3.1. Tool wear Initially, the machining tests for all grades of the MMCs were conducted at 100 m/min cutting speed. It was seen that the cutting tools used to machine Al-SiC/110 MMC encountered very high wear and therefore the cutting speed was lowered for machining this MMC grade. Extensive fracture of the cutting edge and the nose as shown in Fig. 2 was observed after machining Al-SiC/110 MMC. On the other hand, for both Al-SiC/30 and Al-SiC/45 MMCs, flank wear was found to be the dominant wear mode for CBN cutting tools as shown in
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Fig. 1. Microstructures of the MMC specimens with SiC particles of (a) 30 m, (b) 45 m and (c) 110 m in size.
Fig. 3. Variations of cutting tool flank wear with cutting speed and SiC particulate size is shown in Fig. 4. It should be noted that the flank wear/cutting speed curves given in this paper show the magnitude of the wear sustained after removing a fixed volume of workpiece material at various cutting speeds
and at a constant feed rate and depth of cut. Therefore, these are not flank wear curves in the usual sense, in that flank wear is not plotted as a function of cutting time at a fixed cutting speed. However, these curves give a clear indication of the behaviour of CBN cutting tools when tested over a range of
Fig. 2. SEM image of the worn CBN cutting tool used for machining AlSiC/110 MMC at 100 m/min.
Fig. 3. SEM image showing the CBN cutting tool flank wear for cutting Al-SiC/45 MMC at 100 m/min.
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Fig. 4. Tool flank wear for equal material removal varying against the cutting speed for cutting the three MMC grades at the feed rate of 0.12 mm/rev and depth of cut of 1 mm.
cutting speeds [10]. It is seen from Fig. 4 that the cutting tools used to machine Al-SiC/110 MMC encountered very high wear when compared to the others. Therefore, it can be concluded that CBN is unsuitable tool material to machine MMCs with larger SiC particles. As can be seen from Fig. 4, the lowest tool flank wear values were recorded at 150 m/min cutting speed during the machining of Al-SiC/30 and Al-SiC/45 MMCs even though the lowest cutting speed used for the both MMC grades was 100 m/min. Although tool wear generally increases with increasing cutting speed in machining, in this work a reduction in tool wear was observed for the both MMC grades when the cutting speed was increased from 100 to 150 m/min. The worn parts of the CBN cutting tools used to machine Al-SiC/30 and Al-SiC/45 MMCs were investigated using the SEM. Fig. 5 shows the SEM images of the worn cutting edges. It is seen from these images that the shapes of the BUE, especially at the depth of cut lines of the cutting tools, considerably differ depending on the cutting speed applied for the
Fig. 5. SEM images showing the BUEs on the cutting tools after machining (a) Al-SiC/30 at 100 m/min, (b) Al-SiC/30 at 150 m/min, (c) Al-SiC/45 at 100 m/min and (d) Al-SiC/45 at 150 m/min.
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both MMC grades. At 100 m/min cutting speed, the BUEs (Fig. 5a and c) were formed on a smaller area when compared to those (Fig. 5b and d) formed at 150 m/min cutting speed. In machining, as speed increases, temperature also increases [17] and this, in turn, softens the BUE on the rake face and the adhered workpiece material on the flank face. The images in Fig. 5b and d are the evidence for softening of the BUE on the rake face, especially at the depth of cut lines. When these images are examined carefully, it is seen that the BUEs in the both images are expanded on a large area (Fig. 5b and d) at 150 m/min cutting speed. It is likely that the cutting temperature generated at 150 m/min cutting speed was higher than that at 100 m/min cutting speed. Due to this higher cutting temperature, the BUE and any adhered workpiece material on the flank face were softened and less adhesive force was exerted on the cutting tool. That is because, when there is BUE and adhered workpeice material on the cutting tool, it is likely that the particles of the cutting tool wear away through workpiece seizure and pull-out process during cutting. Therefore, the lower flank wear value at 150 m/min cutting speed can be attributed to less adhesive wear at this cutting speed. Andrewes et al. [6] reported that when machining MMCs using PCD cutting tools, the initial flank wear starts with the abrasive effect of the hard particles and the workpiece material adheres to these abrasive grooves strongly due to the high pressure generated at the tertiary cutting zone as machining progresses. Each time, a workpiece film adheres and breaks off as a result of the hard abrasive particles present in the workpiece, small diamond particles are also removed from the PCD cutting tool surface. This cyclic process leads to progressive loss of the PCD tool material. Unlike Andrewes et al. [6], CBN cutting tools were used in this study but it was assumed that similar mechanisms controlled the tool wear during the initial stages. That is because, CBN is close to diamond in hardness and harder than the SiC particles. In this study, it is believed that the wearing away off the CBN particles through workpiece seizure and pull-out process was reduced because of less adhesive force generated at 150 m/min cutting speed.
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Fig. 6. SEM images showing the wear marks on the flank face.
machine Al-SiC/45 MMC at 200 m/min. The bottom SEM image in Fig. 6 indicates that that abrasion and adhesion are the mechanisms dominating the tool wear. 3.3. Workpiece surface roughness
3.2. Tool wear mechanisms In order to examine the tool wear in detail, the worn cutting tools were investigated using the SEM. The cutting tools used to machine Al-SiC/110 MMC exhibited significant amount of fracture as shown in Fig. 2. For the tools used to machine Al-SiC/30 and Al-SiC/45 MMCs, the dominant wear mechanisms could not be determined clearly as the workpiece material adhered to the cutting edges at 100 and 150 m/min cutting speed. As explained in Section 3.1, both abrasive and adhesive wear mechanisms seem to be dominant in the tool wear. When the cutting speed was increased to 200 m/min, the mechanisms dominating the tool wear became clear as shown in Fig. 6. Fig. 6 shows the SEM images of the tool used to
Workpiece surface roughness values (Ra ) measured after the machining tests are given in Fig. 7. These values are the averages of three readings. As can be seen from this figure, Al-SiC/110 MMC produced the poorest surface finish at both cutting speeds in terms of Ra values. It was considered that relatively big particulate size and the fracture of the cutting edges (Fig. 2) caused these significantly high Ra values. In addition, particulate fracture and voids present in this MMC [18] can be another reasons for the increase of Ra parameter. The machined surfaces of the Al-SiC/30 and Al-SiC/45 MMCs exhibited lower Ra values generally irrespective of the cutting speed and reinforcement particulate size. It was considered that as the difference between these two particulate sizes were relatively small, the resulting Ra values did
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of the BUE and the adhering workpiece material with an increase in the cutting speed.
References
Fig. 7. Variation of surface roughness with cutting speed and SiC particulate size.
not vary significantly. In addition, this can also be attributed to the some inconsistencies in the microstructures of these MMCs [19] and the BUE formed on the cutting tool.
4. Conclusion Three Al/SiC MMCs reinforced with 16 wt.% of SiC particles of different mean sizes and produced by a melt stirringsqueeze casting route were subjected to machining test. The influences of reinforcement particulate size and cutting speed on tool wear and surface roughness were investigated. Based on the results obtained, the following conclusions can be drawn: - In the machining of the MMCs containing SiC particles of 30 and 45 m in sizes, flank wear was found to be the dominant wear mode of CBN cutting tool while abrasion and adhesion were observed as the predominant wear mechanisms. On the other hand, the cutting tool used to machine the MMC with SiC particles of 110 m in sizes encountered both the cutting edge and nose fractures. - The MMC with SiC particles of 110 m in mean size caused very high tool wear. When compared to the others, this MMC proved to be unsuitable for the machining operation using CBN cutting tools due to the heavy fractures of both the cutting edge and nose. - In the machining of the MMCs containing SiC particles of 30 and 45 m in mean sizes, 150 m/min cutting speed led to the lowest tool flank wear while 100 and 200 m/min cutting speeds caused higher tool flank wear after the same volume of metal removed. The decrease in tool flank wear with increasing cutting speed can be attributed to the softening
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