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SiCp metal matrix composites
Journal of Materials Processing Technology 209 (2009) 4704–4710
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Analysis of chip formation mechanism in machining of Al/SiCp metal matrix composites Uday A. Dabade, Suhas S. Joshi ∗ Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India
a r t i c l e
i n f o
Article history: Accepted 6 October 2008 Keywords: Chip formation Size and volume fraction of reinforcement MMCs Taguchi method Effective shear angle
a b s t r a c t Al/SiCp composites are known to cause a significant wear of cutting tools. But, with the use of PCD/CBN tools, machining can be continued over longer time duration. Nevertheless, the problem associated with the quality of machined surfaces such as pit marks and particle pull-out still persists. The quality of surface generated during machining can be easily related to the types of chips formed during machining of Al/SiCp composites as a function of processing conditions and composition of constituents in composites. It is observed from the Taguchi method-based experimentation using L27 (313 ) orthogonal array, that in machining of coarser reinforcement composites complete gross fracture takes place causing smaller segments of chips and higher shear plane angle. However, in finer reinforcement composites, secondary crack formation is evident at inner surfaces of the chips formed. Conceptual models illustrating these effects have been arrived at. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Applications of Al/SiCp metal matrix composites (AMCs) in a variety of engineering fields have undergone a substantial increase because of their tailor-made properties by varying its composition. During the product development stage, the raw composites are processed using a number of machining operations while the rapid wear of cutting tools can be avoided with the use of diamond or CBN-based tools. However, the problems related to the machined surface quality still persist. The machined surfaces do show presence of various defects such as feed marks, pits, particle pull-out, etc. eventhough machining was done using PCD/CBN tools. It is understood that the variation in the dimensional accuracy and finish of machined surfaces on Al/SiCp composites are often a function of changes in the processing conditions and compositions of its constituents. These variations are clearly reflected in the types of chips found during machining besides other response variables like cutting forces, surface roughness, residual stresses micro-hardness variation, etc. Therefore, it is envisaged that a fundamental study of chips and mechanism of their formation could be very useful in analysing the machining process (Monaghan, 1994; Joshi et al., 1999). Though, the extensive investigations on mechanism of chip formation in homogeneous material have been carried out, but
∗ Corresponding author. Tel.: +91 22 25767527; fax: +91 22 25726875. E-mail address: [email protected] (S.S. Joshi). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.10.057
there are very few studies on MMCs (Monaghan, 1994; Iuliano et al., 1998; Lin et al., 1998; Joshi et al., 1999, 2001; Ozcatalbas, 2003). Monaghan (1994) reported that the chip formation is due to combined fracture/rupture/crumbling process with higher shear plane angle. Whereas, partial tearing and partial shearing of the chips along the shear plane has been reported by Joshi et al. (1999). The chip segmentation is based on the size and volume fraction of reinforcement in composites. An increase in volume fraction of reinforcement in composites reduces the number of chip curls due to reduction in strain during machining, Joshi et al. (2001). A semi-continuous type chips were observed during machining of Al/SiCp composites by Lin et al. (1998) and was attributed to reduction in ductility of work material due to addition of SiC reinforcement in composite material. Whereas, elemental and curl type chips were found and was attributed to increase in hardness due to addition of higher volume fraction of Al4 C3 reinforcement (Ozcatalbas, 2003). Iuliano et al. (1998) has shown that the reinforcement particles pile up along shear planes, which divide the deformed chip into layers while high-speed turning of MMCs. Thus, it is observed that the chip formation mechanism changes with a change in composition of composite as well as processing conditions. These effects are not adequately understood from the above studies by varying the size of reinforcement and processing conditions using different tool geometries. Therefore, in this paper a fundamental study of chips and mechanism of their formation during machining of Al/SiCp composites of different composition has been carried out, which reflects on the quality of machined surfaces on these composites.
U.A. Dabade, S.S. Joshi / Journal of Materials Processing Technology 209 (2009) 4704–4710
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Table 1 Details of the composition of composite material. Matrix
Reinforcement
Vol. fraction (%)
Reinforcement size (mesh size)
Shape and size
Al-2124
SiCp
20 30
220, 600 220, 600
Cylindrical rod diameter 15 mm and length 70 mm
2. Experimental design and procedure The main objective of the present research paper is to investigate the effect of change in size and volume fraction of reinforcement on mechanism of chip formation by changing processing conditions and tool geometries. Accordingly, four types of composites are used in present experimental work, refer Table 1. 2.1. Design of experiment Taguchi method-based design of experiment (Phadke, 1989), the L27 (313 ) orthogonal array, was performed independently on four types of composites presented in Table 1. Five independent variables such as tool nose radius, insert geometry, feed rate, cutting speed and depth of cut are used in this work. Based on the preliminary experiments performed on machining of Al/SiCp composites, three levels of each independent variable has been selected, see values of independent variables in Table 2. In all, 108 experimental runs (27 each) on four types of composites were performed.
ing of Al/SiCp composites; see Figs. 1(a–l)–3(a–l). At lower cutting speed (40 m min−1 ) chips generated are of segmented type. The length and thickness of chip segment reduces with a decrease in feed rate and generates ½ curled segmented to flake type chips, see Figs. 1(a–d) for higher, 2(a–d) for medium and 3(a–d) for lower feed rate. During machining at lower cutting speed, composite material behaves in a brittle manner with little influence of either temperature or strain rate. Moreover, at lower cutting speed, the displacement of reinforcement particles is less, which generates voids by either displacement or fragmentation of reinforcement particles hence brittle failure becomes more prominent and segmented chips are formed. This effect is more significant in case of coarser reinforcement composites, compare column 1 with column 3 in Figs. 1 and 2. These chip form change to 11⁄4 curled, washer type and smaller radii C-type chips with a decrease in feed rate at medium cutting speed (80 m min−1 ), see Figs. 1(e–h), 2(e–h) and 3(e–h). Further, increase in cutting speed to 120 m min−1 generates tubular helix and spring type chips at higher feed rates (0.2 mm rev−1 ). However, reduction in feed rate (0.1 and
2.2. Experimental setup and procedure The turning experiments on the MMCs were performed on CNC turning machine (EMCO/PCTURN Model 345-II) using CBN inserts with conventional (non-wiper, NW) and wiper (W1 and W2) types insert geometry. Each experimental run was continued for a length of 7 mm. A large number of chips were collected during the machining operation. The chip thickness was measured using Nikon measurescope MM-22 at 50X magnification. Typical chip samples were examined using SEM (Make: FEI Quanta 200 HV) and Olympus microscope. The machined surface roughness was measured using a portable surface roughness instrument (Mahr Perthometer, Model M2) having 2 m diamond-stylus tip radii. The cut-off and sampling length were taken as 0.8 mm and 5.6 mm, respectively. 3. Experimental results and discussions Depending on the composition of composite and machining conditions, various types of chips are formed such as, thin flakes, needle type, segmented, continuous, spring and helix type. See typical photographs of chips in Figs. 1(a–l)–3(a–l). 3.1. Effect of cutting speed, feed rate and composition of composites The effect of change in cutting speed and feed rate is more dominant on physical form of the chips generated during machinTable 2 Process parameters and their levels. Parameters
Tool nose radius (mm) Insert geometry Feed rate (mm rev−1 ) Cutting speed (m min−1 ) Depth of cut (mm)
Levels 1
2
3
0.4 W1 0.05 40 0.2
0.8 W2 0.1 80 0.6
0.8a NW 0.2 120 1.0
a Dummy level which fulfils the requirement of L27 orthogonal array and provides precise information about 0.8 mm TNR.
Fig. 1. [a–l] Effect of cutting speed on chip form (feed = 0.2 mm rev−1 and depth of cut = 1 mm).
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as a chip breaker and produce both segmented and small curled chips. As compared to the coarser reinforcement, the chips of finer reinforcement composites are semi-continuous and spring type; compare Fig. 2(i–j) and (k–l). An increase in volume (from 20 to 30%) and size of reinforcement (from 15 to 65 m) in the composites reduces the number of circles through which the chips curl before breaking. The number of chip circles varies form 1/4 to 2, depending on the size and volume fraction of reinforcement; see Fig. 1(a–d). The variation in the number of chip curls can be attributed to the variation in mechanical and physical properties of composite materials with change in composition. At higher volume fraction and a given size of reinforcement, the number of chip curl is lower; compare Fig. 1(e and f) and Fig. 1(g and h). Secondly, the radii of chip curls observed with finer (15 m) reinforcement composites is larger than the coarser (65 m) reinforcement composites for the same volume fraction of reinforcement, compare Fig. 1(i and k) and Fig. 1(j and l). This could be attributed to the higher ductility and lesser hardness of finer reinforcement composites (Whitehouse and Clyne, 1993). At the same time, the finer reinforcement composites will have a greater tendency to stick to the tool face for longer duration. This may force the chips to take a longer path by curling through a larger diameter circle. 3.1.1. Effect of chip formation on surface roughness After visual inspection of chips shown in Figs. 1(a–l)–3(a–l), the chips formed at processing conditions (feed rate 0.2 mm rev−1 ; 1 mm depth of cut; and speed at 40, 80 and 120 m min−1 ) are of
Fig. 2. [a–l] Effect of cutting speed on chip form (feed = 0.1 mm rev−1 and depth of cut = 1 mm).
0.05 mm rev−1 ) at 120 m min−1 cutting speed generates the chips of continuous and semi-continuous type. This trend is more prominent especially in finer reinforcement composites; see Fig. 1(i–j). But, in coarser reinforcement composites, the length and number of chip curls are lower than finer reinforcement composites at same machining conditions, compare Fig. 1(i–j) with Fig. 1(k–l). The change in form of chips from partially C-type to washer Ctype and spring type chips as the cutting speed increases (from 40 to 120 m min−1 ) is due to increased ductility of the work material because of the high machining temperature at higher cutting speed, see column #1 of Fig. 2. At higher feed rate, 0.2 mm rev−1 , the number of chip curls observed is more than lower feed rate and is attributed to the increased deformation volume and tool–chip contact length. It increases the machining temperature, which improves the ductility (especially in finer reinforcement) and increases the number of chip curls, compare Figs. 1(a–l) with 2(a–l). However, at lower feed rate, 0.05 mm rev−1 , the chip cross-sectional area is very small due to which flake, needle type segmented, small radii curled and 1/4–1/2 C-type chips are generated, refer Fig. 3(a–l). The effect of a change in size and volume fraction of reinforcement is also evident on chips at lower as well as at higher feed rate. At lower feed rate (0.05 and 0.1 mm rev−1 ), i.e. during finish turning operation, the coarser reinforcement particles themselves act
Fig. 3. [a–l] Effect of cutting speed on chip form (feed = 0.05 mm rev−1 and depth of cut = 1 mm).
U.A. Dabade, S.S. Joshi / Journal of Materials Processing Technology 209 (2009) 4704–4710
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Table 3 Effect of chip forms on surface roughness. Chip condition
Exp no.
Parameter
Surface roughness (mRa)
FR
CS
DOC
Al/SiC20p/220
Al/SiC30p/220
Al/SiC20p/600
Al/SiC30p/600
Favorable chips
22 10 7
0.05 0.05 0.05
40 80 120
1 1 1
0.85 0.87 1.13
0.7 0.67 1.14
0.34 0.24 0.68
0.18 0.14 0.35
Non-favo. chips
27 15 3
0.2 0.2 0.2
40 80 120
1 1 1
1.84 1.82 1.91
2.06 1.65 1.99
1.41 1.1 1.75
1.29 0.78 1.29
F.R., feed rate; C.S., cutting speed; DOC, depth of cut
continuous, spring type and tubular helix type, refer Fig. 1(a–l). Such types of chips deteriorate the quality of machined surfaces. Whereas, the chips formed at (feed rate 0.05 mm rev−1 ; 1 mm depth of cut; and cutting speed of 40, 80 and 120 m min−1 ), have different forms like thin flakes, needle type, and segmented chips, refer Fig. 3(a–l), which are desirable to reduce the surface roughness on the machined surfaces. A comparison of surface roughness obtained at the processing conditions mentioned above, are investigated as a part of present research work and presented in Table 3. 3.2. Modeling of chip formation Knowing the factors influencing chip segmentation, in the following section, attempt has been made to investigate how these parameters influence the deformation along the shear plane and consequently formation of a single chip segment. This analysis is based on optical as well as SE microscopy of the chip longitudinal cross-section.
3.2.1. Optical and SEM analysis of chips A scanning electron microscopy of the chips produced during machining of coarser reinforcement composites shows that the chips are highly strained leading to separation of chip segments at outer (free) surfaces, see SE micrograph of chip in Fig. 4(a) and cross-section of chip in Fig. 4(b). It shows that the segmentation is not complete over an entire chip thickness and has clear saw-toothprofile (STP) as shown in Fig. 4(b). These chips are formed while machining of coarser reinforcement composites (Al/SiC/20p/220). Thus, in the case of coarser reinforcement composites, the chips are highly strained and the segmentation is more or less complete over its thickness. However, the observation of inner side of the chips shows that the chip segments are joined at bottoms see chip inner side in Fig. 4(b). On the other hand, in the case of finer reinforcement composites, the chips appear to be highly strained too, but the segmentation is restricted to the outer (free) surface of the chip, see optical micrograph of chip cross-section in Fig. 5. This is attributed to an increase in ductility of these composites. Thus, low hardness and high ductility, increase the sticky period at tool-chip interface causing secondary crack formation at inner surface of chips, refer Fig. 6.
Fig. 5. Optical micrograph of chip: Al/SiC/30p/600.
Fig. 4. SEM photograph of chip: Al/SiC/20p/220.
Fig. 6. Secondary crack formations in finer reinforcement composites.
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Table 4 ANOVA (P-values): effective shear angle. Parameter
DOF
TNR IG FR (mm rev−1 ) CS (m min−1 ) DOC (mm) TNR*IG TNR*FR IG*FR
1 2 2 2 2 2 2 4
P-values Al/SiC 20p/220
Al/SiC 20p/600
Al/SiC 30p/220
Al/SiC 30p/600
0.615 0.710 0.013 0.236 0.000 0.584 0.689 0.728
0.804 0.894 0.001 0.078 0.000 0.674 0.604 0.432
0.687 0.423 0.102 0.548 0.000 0.104 0.881 0.302
0.506 0.752 0.008 0.686 0.000 0.769 0.976 0.320
Bold value indicates significance at 95% confidence level
Fig. 7. [a–e] Effect of process parameters on effective shear angle.
U.A. Dabade, S.S. Joshi / Journal of Materials Processing Technology 209 (2009) 4704–4710
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Fig. 8. [a–b] Conceptual model of chip formation.
3.2.2. Effective shear angle analysis In addition to the qualitative analysis of the chip forms, their quantitative analysis was done by measuring the thickness and estimation of effective shear angle ‘e ’ given by Shaw (1984): sin e =
cos cos ˛ s e cos cos ˛n
sin n
(1)
The ANOVA and AOM were performed to identify the parameters statistically influencing the effective shear angle, see Table 4 for ANOVA. The effect of process parameters on effective shear angle is shown by AOM plots in Fig. 7(a–e). The P-values (<0.05) of ANOVA in Table 4 indicate that feed rate and depth of cut are the significant parameters, which statistically influence the effective shear angle. An increase in feed rate reduces the effective shear angle; see Fig. 7(c). Whereas, an increase in depth of cut increases the magnitude of effective shear angle, see Fig. 7(e). The AOM plots in Fig. 7(a–e) indicate that a change in tool nose radius, insert geometry and cutting speed does not influence the effective shear angle. Secondly, the influence of change in composition of composites is evident on the effective shear angle. It is observed that the effective shear angle is higher in case of composites having coarser and higher volume fraction of reinforcement (Al/SiC/30p/220) and lower with finer and lower volume fraction of reinforcement (Al/SiC/20p/600), refer AOM plots in Fig. 7(a–e). 3.2.3. Conceptual model of chip formation Based on the results from optical and SE micrographs of the chip cross-sections as well as statistical analysis of effective shear angle, a conceptual model of chip formation has been developed and shown in Fig. 8(a and b). It is observed that the chip formation mechanism is significantly influenced by the size of reinforcement in composites.
• At lower cutting speed (40 m min−1 ) thin flakes, needle type as well as segmented chips are formed, whereas at higher cutting speed (120 m min−1 ) generally, semi-continuous, continuous, scrambled ribbon, and tubular helix chips are formed. • The length of chip and the number of chip curls increases with an increase in feed rate at given cutting speed and depth of cut. • The size and volume fraction of reinforcement significantly influences the chip formation mechanism. In case of finer reinforcement composites, the chip segments are longer in length and gross fracture occurs at outer surface of the chips only. Whereas in coarser reinforcement composites, complete gross fracture causes formation of smaller chip segments. • Secondary crack formation is evident at inner surface of the chips in case of finer reinforcement composites due to its higher ductility. • The effective shear angle is higher in case of coarser reinforcement than finer reinforcement composites. • During finish turning operation, the coarser reinforcement particles themselves act as a chip breaker and produce both segmented and small curled chips and improve the machined surface roughness. Acknowledgements The authors wish to acknowledge the DST-FIST Precision Prototyping Cell sponsored by Department of Science and Technology, Government of India, at Machine Tools Laboratory, Indian Institute of Technology, Bombay, for providing facilities to carry out this experimental work. The author Uday A. Dabade wish to acknowledge the support of management of Walchand College of Engineering, Sangli (India), for giving him an opportunity to carry out his Ph.D. research work at IIT Bombay, Mumbai under QIP scheme. References
4. Conclusions Based on the experimental investigations on chip formation mechanism in machining of Al/SiCp composites following conclusions can be drawn.
Iuliano, L., Settineri, L., Gatto, A., 1998. High speed turning experiments on metal matrix composites. Compos. Part A: Appl. Sci. Manuf. 29 A, 1501–1509. Joshi, S.S., Ramakrishnan, N., Ramakrishnan, P., 1999. Analysis of chip breaking during orthogonal machining of Al/SiCp composites. J. Mater. Process. Technol. 88, 90–96.
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Joshi, S.S., Ramakrishnan, N., Ramakrishnan, P., 2001. Micro-structural analysis of chip formation during orthogonal machining of Al/SiCp composites. Trans. ASME, J. Eng. Mater. Technol. 123 (3), 315–321. Lin, J.T., Bhattacharyya, D., Ferguson, W.G., 1998. Chip formation in the machining of SiC-particle reinforced aluminium–matrix composites. Compos. Sci. Technol. 58, 285–291. Monaghan, J.M., 1994. The use of a quick stop test to study the chip formation of SiC/Al metal matrix composite material and its matrix alloy. Process. Adv. Mater. 9, 170–179.
Phadke, M.L., 1989. Quality Engineering Using Robust Design. Prentice Hall Publication, New Jersey. Shaw, M.C., 1984. Metal Cutting Principles. CBS Publishers and Distributors, New Delhi, India. Whitehouse, A.F., Clyne, T.W., 1993. Effects of reinforcement content and shape on cavitation and failure in metal–matrix composites. Composites 24 (3), 256–261. Ozcatalbas, Y., 2003. Chip and built-up edge formation in the machining of in situ Al4 C3 –Al composite. Mater. Des. 24, 215–221.
Contents lists available at ScienceDirect
Journal of Materials Processing Technology journal homepage: www.elsevier.com/locate/jmatprotec
Analysis of chip formation mechanism in machining of Al/SiCp metal matrix composites Uday A. Dabade, Suhas S. Joshi ∗ Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India
a r t i c l e
i n f o
Article history: Accepted 6 October 2008 Keywords: Chip formation Size and volume fraction of reinforcement MMCs Taguchi method Effective shear angle
a b s t r a c t Al/SiCp composites are known to cause a significant wear of cutting tools. But, with the use of PCD/CBN tools, machining can be continued over longer time duration. Nevertheless, the problem associated with the quality of machined surfaces such as pit marks and particle pull-out still persists. The quality of surface generated during machining can be easily related to the types of chips formed during machining of Al/SiCp composites as a function of processing conditions and composition of constituents in composites. It is observed from the Taguchi method-based experimentation using L27 (313 ) orthogonal array, that in machining of coarser reinforcement composites complete gross fracture takes place causing smaller segments of chips and higher shear plane angle. However, in finer reinforcement composites, secondary crack formation is evident at inner surfaces of the chips formed. Conceptual models illustrating these effects have been arrived at. © 2008 Elsevier B.V. All rights reserved.
1. Introduction Applications of Al/SiCp metal matrix composites (AMCs) in a variety of engineering fields have undergone a substantial increase because of their tailor-made properties by varying its composition. During the product development stage, the raw composites are processed using a number of machining operations while the rapid wear of cutting tools can be avoided with the use of diamond or CBN-based tools. However, the problems related to the machined surface quality still persist. The machined surfaces do show presence of various defects such as feed marks, pits, particle pull-out, etc. eventhough machining was done using PCD/CBN tools. It is understood that the variation in the dimensional accuracy and finish of machined surfaces on Al/SiCp composites are often a function of changes in the processing conditions and compositions of its constituents. These variations are clearly reflected in the types of chips found during machining besides other response variables like cutting forces, surface roughness, residual stresses micro-hardness variation, etc. Therefore, it is envisaged that a fundamental study of chips and mechanism of their formation could be very useful in analysing the machining process (Monaghan, 1994; Joshi et al., 1999). Though, the extensive investigations on mechanism of chip formation in homogeneous material have been carried out, but
∗ Corresponding author. Tel.: +91 22 25767527; fax: +91 22 25726875. E-mail address: [email protected] (S.S. Joshi). 0924-0136/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.jmatprotec.2008.10.057
there are very few studies on MMCs (Monaghan, 1994; Iuliano et al., 1998; Lin et al., 1998; Joshi et al., 1999, 2001; Ozcatalbas, 2003). Monaghan (1994) reported that the chip formation is due to combined fracture/rupture/crumbling process with higher shear plane angle. Whereas, partial tearing and partial shearing of the chips along the shear plane has been reported by Joshi et al. (1999). The chip segmentation is based on the size and volume fraction of reinforcement in composites. An increase in volume fraction of reinforcement in composites reduces the number of chip curls due to reduction in strain during machining, Joshi et al. (2001). A semi-continuous type chips were observed during machining of Al/SiCp composites by Lin et al. (1998) and was attributed to reduction in ductility of work material due to addition of SiC reinforcement in composite material. Whereas, elemental and curl type chips were found and was attributed to increase in hardness due to addition of higher volume fraction of Al4 C3 reinforcement (Ozcatalbas, 2003). Iuliano et al. (1998) has shown that the reinforcement particles pile up along shear planes, which divide the deformed chip into layers while high-speed turning of MMCs. Thus, it is observed that the chip formation mechanism changes with a change in composition of composite as well as processing conditions. These effects are not adequately understood from the above studies by varying the size of reinforcement and processing conditions using different tool geometries. Therefore, in this paper a fundamental study of chips and mechanism of their formation during machining of Al/SiCp composites of different composition has been carried out, which reflects on the quality of machined surfaces on these composites.
U.A. Dabade, S.S. Joshi / Journal of Materials Processing Technology 209 (2009) 4704–4710
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Table 1 Details of the composition of composite material. Matrix
Reinforcement
Vol. fraction (%)
Reinforcement size (mesh size)
Shape and size
Al-2124
SiCp
20 30
220, 600 220, 600
Cylindrical rod diameter 15 mm and length 70 mm
2. Experimental design and procedure The main objective of the present research paper is to investigate the effect of change in size and volume fraction of reinforcement on mechanism of chip formation by changing processing conditions and tool geometries. Accordingly, four types of composites are used in present experimental work, refer Table 1. 2.1. Design of experiment Taguchi method-based design of experiment (Phadke, 1989), the L27 (313 ) orthogonal array, was performed independently on four types of composites presented in Table 1. Five independent variables such as tool nose radius, insert geometry, feed rate, cutting speed and depth of cut are used in this work. Based on the preliminary experiments performed on machining of Al/SiCp composites, three levels of each independent variable has been selected, see values of independent variables in Table 2. In all, 108 experimental runs (27 each) on four types of composites were performed.
ing of Al/SiCp composites; see Figs. 1(a–l)–3(a–l). At lower cutting speed (40 m min−1 ) chips generated are of segmented type. The length and thickness of chip segment reduces with a decrease in feed rate and generates ½ curled segmented to flake type chips, see Figs. 1(a–d) for higher, 2(a–d) for medium and 3(a–d) for lower feed rate. During machining at lower cutting speed, composite material behaves in a brittle manner with little influence of either temperature or strain rate. Moreover, at lower cutting speed, the displacement of reinforcement particles is less, which generates voids by either displacement or fragmentation of reinforcement particles hence brittle failure becomes more prominent and segmented chips are formed. This effect is more significant in case of coarser reinforcement composites, compare column 1 with column 3 in Figs. 1 and 2. These chip form change to 11⁄4 curled, washer type and smaller radii C-type chips with a decrease in feed rate at medium cutting speed (80 m min−1 ), see Figs. 1(e–h), 2(e–h) and 3(e–h). Further, increase in cutting speed to 120 m min−1 generates tubular helix and spring type chips at higher feed rates (0.2 mm rev−1 ). However, reduction in feed rate (0.1 and
2.2. Experimental setup and procedure The turning experiments on the MMCs were performed on CNC turning machine (EMCO/PCTURN Model 345-II) using CBN inserts with conventional (non-wiper, NW) and wiper (W1 and W2) types insert geometry. Each experimental run was continued for a length of 7 mm. A large number of chips were collected during the machining operation. The chip thickness was measured using Nikon measurescope MM-22 at 50X magnification. Typical chip samples were examined using SEM (Make: FEI Quanta 200 HV) and Olympus microscope. The machined surface roughness was measured using a portable surface roughness instrument (Mahr Perthometer, Model M2) having 2 m diamond-stylus tip radii. The cut-off and sampling length were taken as 0.8 mm and 5.6 mm, respectively. 3. Experimental results and discussions Depending on the composition of composite and machining conditions, various types of chips are formed such as, thin flakes, needle type, segmented, continuous, spring and helix type. See typical photographs of chips in Figs. 1(a–l)–3(a–l). 3.1. Effect of cutting speed, feed rate and composition of composites The effect of change in cutting speed and feed rate is more dominant on physical form of the chips generated during machinTable 2 Process parameters and their levels. Parameters
Tool nose radius (mm) Insert geometry Feed rate (mm rev−1 ) Cutting speed (m min−1 ) Depth of cut (mm)
Levels 1
2
3
0.4 W1 0.05 40 0.2
0.8 W2 0.1 80 0.6
0.8a NW 0.2 120 1.0
a Dummy level which fulfils the requirement of L27 orthogonal array and provides precise information about 0.8 mm TNR.
Fig. 1. [a–l] Effect of cutting speed on chip form (feed = 0.2 mm rev−1 and depth of cut = 1 mm).
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as a chip breaker and produce both segmented and small curled chips. As compared to the coarser reinforcement, the chips of finer reinforcement composites are semi-continuous and spring type; compare Fig. 2(i–j) and (k–l). An increase in volume (from 20 to 30%) and size of reinforcement (from 15 to 65 m) in the composites reduces the number of circles through which the chips curl before breaking. The number of chip circles varies form 1/4 to 2, depending on the size and volume fraction of reinforcement; see Fig. 1(a–d). The variation in the number of chip curls can be attributed to the variation in mechanical and physical properties of composite materials with change in composition. At higher volume fraction and a given size of reinforcement, the number of chip curl is lower; compare Fig. 1(e and f) and Fig. 1(g and h). Secondly, the radii of chip curls observed with finer (15 m) reinforcement composites is larger than the coarser (65 m) reinforcement composites for the same volume fraction of reinforcement, compare Fig. 1(i and k) and Fig. 1(j and l). This could be attributed to the higher ductility and lesser hardness of finer reinforcement composites (Whitehouse and Clyne, 1993). At the same time, the finer reinforcement composites will have a greater tendency to stick to the tool face for longer duration. This may force the chips to take a longer path by curling through a larger diameter circle. 3.1.1. Effect of chip formation on surface roughness After visual inspection of chips shown in Figs. 1(a–l)–3(a–l), the chips formed at processing conditions (feed rate 0.2 mm rev−1 ; 1 mm depth of cut; and speed at 40, 80 and 120 m min−1 ) are of
Fig. 2. [a–l] Effect of cutting speed on chip form (feed = 0.1 mm rev−1 and depth of cut = 1 mm).
0.05 mm rev−1 ) at 120 m min−1 cutting speed generates the chips of continuous and semi-continuous type. This trend is more prominent especially in finer reinforcement composites; see Fig. 1(i–j). But, in coarser reinforcement composites, the length and number of chip curls are lower than finer reinforcement composites at same machining conditions, compare Fig. 1(i–j) with Fig. 1(k–l). The change in form of chips from partially C-type to washer Ctype and spring type chips as the cutting speed increases (from 40 to 120 m min−1 ) is due to increased ductility of the work material because of the high machining temperature at higher cutting speed, see column #1 of Fig. 2. At higher feed rate, 0.2 mm rev−1 , the number of chip curls observed is more than lower feed rate and is attributed to the increased deformation volume and tool–chip contact length. It increases the machining temperature, which improves the ductility (especially in finer reinforcement) and increases the number of chip curls, compare Figs. 1(a–l) with 2(a–l). However, at lower feed rate, 0.05 mm rev−1 , the chip cross-sectional area is very small due to which flake, needle type segmented, small radii curled and 1/4–1/2 C-type chips are generated, refer Fig. 3(a–l). The effect of a change in size and volume fraction of reinforcement is also evident on chips at lower as well as at higher feed rate. At lower feed rate (0.05 and 0.1 mm rev−1 ), i.e. during finish turning operation, the coarser reinforcement particles themselves act
Fig. 3. [a–l] Effect of cutting speed on chip form (feed = 0.05 mm rev−1 and depth of cut = 1 mm).
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Table 3 Effect of chip forms on surface roughness. Chip condition
Exp no.
Parameter
Surface roughness (mRa)
FR
CS
DOC
Al/SiC20p/220
Al/SiC30p/220
Al/SiC20p/600
Al/SiC30p/600
Favorable chips
22 10 7
0.05 0.05 0.05
40 80 120
1 1 1
0.85 0.87 1.13
0.7 0.67 1.14
0.34 0.24 0.68
0.18 0.14 0.35
Non-favo. chips
27 15 3
0.2 0.2 0.2
40 80 120
1 1 1
1.84 1.82 1.91
2.06 1.65 1.99
1.41 1.1 1.75
1.29 0.78 1.29
F.R., feed rate; C.S., cutting speed; DOC, depth of cut
continuous, spring type and tubular helix type, refer Fig. 1(a–l). Such types of chips deteriorate the quality of machined surfaces. Whereas, the chips formed at (feed rate 0.05 mm rev−1 ; 1 mm depth of cut; and cutting speed of 40, 80 and 120 m min−1 ), have different forms like thin flakes, needle type, and segmented chips, refer Fig. 3(a–l), which are desirable to reduce the surface roughness on the machined surfaces. A comparison of surface roughness obtained at the processing conditions mentioned above, are investigated as a part of present research work and presented in Table 3. 3.2. Modeling of chip formation Knowing the factors influencing chip segmentation, in the following section, attempt has been made to investigate how these parameters influence the deformation along the shear plane and consequently formation of a single chip segment. This analysis is based on optical as well as SE microscopy of the chip longitudinal cross-section.
3.2.1. Optical and SEM analysis of chips A scanning electron microscopy of the chips produced during machining of coarser reinforcement composites shows that the chips are highly strained leading to separation of chip segments at outer (free) surfaces, see SE micrograph of chip in Fig. 4(a) and cross-section of chip in Fig. 4(b). It shows that the segmentation is not complete over an entire chip thickness and has clear saw-toothprofile (STP) as shown in Fig. 4(b). These chips are formed while machining of coarser reinforcement composites (Al/SiC/20p/220). Thus, in the case of coarser reinforcement composites, the chips are highly strained and the segmentation is more or less complete over its thickness. However, the observation of inner side of the chips shows that the chip segments are joined at bottoms see chip inner side in Fig. 4(b). On the other hand, in the case of finer reinforcement composites, the chips appear to be highly strained too, but the segmentation is restricted to the outer (free) surface of the chip, see optical micrograph of chip cross-section in Fig. 5. This is attributed to an increase in ductility of these composites. Thus, low hardness and high ductility, increase the sticky period at tool-chip interface causing secondary crack formation at inner surface of chips, refer Fig. 6.
Fig. 5. Optical micrograph of chip: Al/SiC/30p/600.
Fig. 4. SEM photograph of chip: Al/SiC/20p/220.
Fig. 6. Secondary crack formations in finer reinforcement composites.
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Table 4 ANOVA (P-values): effective shear angle. Parameter
DOF
TNR IG FR (mm rev−1 ) CS (m min−1 ) DOC (mm) TNR*IG TNR*FR IG*FR
1 2 2 2 2 2 2 4
P-values Al/SiC 20p/220
Al/SiC 20p/600
Al/SiC 30p/220
Al/SiC 30p/600
0.615 0.710 0.013 0.236 0.000 0.584 0.689 0.728
0.804 0.894 0.001 0.078 0.000 0.674 0.604 0.432
0.687 0.423 0.102 0.548 0.000 0.104 0.881 0.302
0.506 0.752 0.008 0.686 0.000 0.769 0.976 0.320
Bold value indicates significance at 95% confidence level
Fig. 7. [a–e] Effect of process parameters on effective shear angle.
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Fig. 8. [a–b] Conceptual model of chip formation.
3.2.2. Effective shear angle analysis In addition to the qualitative analysis of the chip forms, their quantitative analysis was done by measuring the thickness and estimation of effective shear angle ‘e ’ given by Shaw (1984): sin e =
cos cos ˛ s e cos cos ˛n
sin n
(1)
The ANOVA and AOM were performed to identify the parameters statistically influencing the effective shear angle, see Table 4 for ANOVA. The effect of process parameters on effective shear angle is shown by AOM plots in Fig. 7(a–e). The P-values (<0.05) of ANOVA in Table 4 indicate that feed rate and depth of cut are the significant parameters, which statistically influence the effective shear angle. An increase in feed rate reduces the effective shear angle; see Fig. 7(c). Whereas, an increase in depth of cut increases the magnitude of effective shear angle, see Fig. 7(e). The AOM plots in Fig. 7(a–e) indicate that a change in tool nose radius, insert geometry and cutting speed does not influence the effective shear angle. Secondly, the influence of change in composition of composites is evident on the effective shear angle. It is observed that the effective shear angle is higher in case of composites having coarser and higher volume fraction of reinforcement (Al/SiC/30p/220) and lower with finer and lower volume fraction of reinforcement (Al/SiC/20p/600), refer AOM plots in Fig. 7(a–e). 3.2.3. Conceptual model of chip formation Based on the results from optical and SE micrographs of the chip cross-sections as well as statistical analysis of effective shear angle, a conceptual model of chip formation has been developed and shown in Fig. 8(a and b). It is observed that the chip formation mechanism is significantly influenced by the size of reinforcement in composites.
• At lower cutting speed (40 m min−1 ) thin flakes, needle type as well as segmented chips are formed, whereas at higher cutting speed (120 m min−1 ) generally, semi-continuous, continuous, scrambled ribbon, and tubular helix chips are formed. • The length of chip and the number of chip curls increases with an increase in feed rate at given cutting speed and depth of cut. • The size and volume fraction of reinforcement significantly influences the chip formation mechanism. In case of finer reinforcement composites, the chip segments are longer in length and gross fracture occurs at outer surface of the chips only. Whereas in coarser reinforcement composites, complete gross fracture causes formation of smaller chip segments. • Secondary crack formation is evident at inner surface of the chips in case of finer reinforcement composites due to its higher ductility. • The effective shear angle is higher in case of coarser reinforcement than finer reinforcement composites. • During finish turning operation, the coarser reinforcement particles themselves act as a chip breaker and produce both segmented and small curled chips and improve the machined surface roughness. Acknowledgements The authors wish to acknowledge the DST-FIST Precision Prototyping Cell sponsored by Department of Science and Technology, Government of India, at Machine Tools Laboratory, Indian Institute of Technology, Bombay, for providing facilities to carry out this experimental work. The author Uday A. Dabade wish to acknowledge the support of management of Walchand College of Engineering, Sangli (India), for giving him an opportunity to carry out his Ph.D. research work at IIT Bombay, Mumbai under QIP scheme. References
4. Conclusions Based on the experimental investigations on chip formation mechanism in machining of Al/SiCp composites following conclusions can be drawn.
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