Performance evaluation of cemented carbide tools in turning AISI 1010 steel

Performance evaluation of cemented carbide tools in turning AISI 1010 steel

Journal of Materials Processing Technology 116 (2001) 16±21 Performance evaluation of cemented carbide tools in turning AISI 1010 steel M.Y. Noordina...

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Journal of Materials Processing Technology 116 (2001) 16±21

Performance evaluation of cemented carbide tools in turning AISI 1010 steel M.Y. Noordina,*, V.C. Venkatesha, C.L. Chana, A. Abdullahb a

Department of Manufacturing and Industrial Engineering, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor, Malaysia b Kulliyyah of Engineering, International Islamic University Malaysia, Jalan Gombak, 53100 Kuala Lumpur, Malaysia

Abstract In this paper, the performance of three cemented carbide cutting tools, two coated tungsten based cemented carbide Ð one with Al2O3 (black) outer layer and the other with TiN (golden), and an uncoated titanium based (silver grey) cemented carbide tool, with 808 Ð diamond insert shape were investigated during ®nish turning of AISI 1010 steel. The PCBNR tool holder, giving a side cutting edge angle of ‡158, was used. Cutting tests were performed with constant depth of cut and at various cutting speeds and feed rates to investigate the performance of the tools under dry cutting conditions. Cutting forces and surface roughness were measured. The chips produced by the three type of tools during experimental trials were examined to determine the secondary shear zone, chip thickness and the angle of maximum crystal elongation. The tool which had the CVD with TiCN/TiC and PVD with TiN coating layer sequence performs best under the conditions tested, as lower forces with little variation were encountered, very good surface ®nish could be obtained and chips with minimum SSZ thickness could be produced, that contributed to low chip strain and therefore to low residual stresses on the workpiece surface. # 2001 Elsevier Science B.V. All rights reserved. Keywords: Turning; Cutting forces; Surface roughness; Secondary shear zone thickness; Chip strain

1. Introduction A century has passed since the development of the ®rst cutting tool material, carbon steel, suitable for use in metal cutting [1]. Since then, numerous other cutting tool materials have been developed continuously. The industry is the driving force behind the development of cutting tool materials as the need to increase productivity, to machine more dif®cult materials, to move to `unmanned' machining operations and to improve quality in high volume are always there [2]. Cemented carbides, ®rst introduced around 1926, are probably the most popular and most common high production tool materials available today [3]. Chemical vapour deposition (CVD) coated cemented carbides had a runaway success since their introduction in the late 1960s [4]. Physical vapour deposition (PVD) is the other widely used technique for coating very sharp edged cemented carbide cutting tools. Its usage is relatively low compared to the CVD technique, but it is growing [5]. Seventy

*

Corresponding author.

percentage of the cemented carbide tools used in the industry are coated [6]. Cutting tool performance can be evaluated by considering factors such as cutting forces, cutting temperature, surface ®nish, microstructure and tool life. In this investigation, the performance of three cemented carbide cutting tools, two coated tungsten based cemented carbide Ð one with TiN (golden) outer layer and the other with Al2O3 (black), and an uncoated titanium based (silver grey) cemented carbide tool, with 808 Ð diamond insert shape were investigated during ®nish turning of AISI 1010 steel. The performance was judged on the basis of cutting forces encountered during turning, the surface ®nish of the turned part, and the microstructure studies performed on the chips produced. Cutting forces can be regarded as one of the machinability indices [7]. In oblique cutting, the forces generated can be resolved into three major components acting on the cutting tool. The main force is the tangential force which acts on the rake face of the tool. The two other forces, which are numerically smaller, are the feed force which resists the feed of the tool and the radial force which tends to push the tool away from the work in the direction of the Z-axis. Good surface integrity qualities on the workpiece surface necessitate a ®ne surface ®nish and low residual stresses.

0924-0136/01/$ ± see front matter # 2001 Elsevier Science B.V. All rights reserved. PII: S 0 9 2 4 - 0 1 3 6 ( 0 1 ) 0 0 8 3 8 - X

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2. Experimental details 2.1. Cutting conditions Cutting tests were carried out on a 9.2 kW Harrison M500 lathe machine under dry cutting conditions. The tools were tested at the cutting speeds (V) of 140, 200 and 280 m/min and high feed rates (f) of 0.23 and 0.28 mm/rev were used for high productivity. A low depth of cut (d) of 1 mm was used for near net shape manufacturing and was kept constant throughout the tests. 2.2. Cutting inserts Two types of commercially available coated tungsten based cemented carbide inserts and an uncoated titanium based cemented carbide insert were tested. The grade of the tools are Kennametal black KC 935 (CVD with TiCN/TiC/ Al2O3 coating layer sequence) Ð insert 1, Kennametal golden KC 792 (CVD with TiCN/TiC and PVD with TiN coating layer sequence) Ð insert 2 and Mitsubishi silver grey G 5560 A5 (uncoated) Ð insert 3, respectively. All the inserts have identical geometry designated by ISO as CNMG 120408 (808 Ð diamond insert shape with 0.8 mm nose radius). The inserts were rigidly mounted on a right-hand style tool holder designated by ISO as PCBNR 2525 M12 with a side cutting edge angle (SCEA) of ‡158, back rake angle of 58 and side rake angle of 58. The choice of this SCEA was to get low radial forces for stability at high feed rates.

ary shear zone thickness (z), chip thickness (t2) and angle of maximum crystal elongation (c) were measured from the images obtained. 3. Results and discussion 3.1. Cutting forces The tangential, feed and radial forces versus cutting speed relationships for the various experimental trials were shown in Figs. 1±3, respectively. The ®gures also showed the effect when different inserts and feeds were used. For insert 1, the tangential force increases when the cutting speed was increased from low to medium. The force, however, decreased when the cutting speed was at the maximum. The feed and radial forces were about twice lower than the tangential force. This is mainly due to the

2.3. Workpiece materials The cutting performance tests were performed on automotive AISI 1010 steel bars. Based on the AISI-SAE standard carbon steel table, it is a non-resulphurised grade steel and its composition is 0.08±0.13%C, 0.3±0.6%Mn, maximum of 0.04%P and maximum of 0.05%S. The hardness of the bar was measured and found to be 131 Hv. The workpiece material used has a dimension of 300 mm in length and 100 mm in diameter.

Fig. 1. Tangential force and cutting speed relationship.

2.4. Experimental techniques The cutting performance tests involved 18 trials. The response variables measured were the cutting forces (tangential force, Fc, feed force, Ff and the radial force, Fr) and the surface roughness. The cutting forces were measured using a 3-component dynamometer (Kistler, Type 9265 B), a multi-channel charge ampli®er (Kistler, Type 5019A) and a data acquisition system. The surface roughness of the turned surface was measured using a portable surface roughness tester (Mitutoyo, Surftest 301). The chips produced during each trial were collected, mounted onto specimen holder, ground, polished and etched. These were then observed using a polarised light microscope (Nikon) and the second-

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Fig. 2. Feed force and cutting speed relationship.

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the tangential force and the cutting speed were generally found to be similar when the feed rate was increased. The same can also be said for the other two forces. Lower forces with little variation were encountered when insert 2 was used. In particular, at 0.23 mm/rev, the tangential force was in the range of 723.32±880.28 N, while the feed and radial forces were in the range of 450±520 N. 3.2. Surface roughness The surface roughness measurements obtained from the various experimental trials were shown in Fig. 4. Classical surface roughness related equations [8]: h

Fig. 3. Radial force and cutting speed relationship.

positive SCEA used. Both the other forces, however, decreased with increasing cutting speed. For insert 2, the tangential force remained almost constant when the cutting speed was increased from low to medium and decreased when the cutting speed was at the maximum. Again, the feed and radial forces were found to be lower. The feed force decreased when the cutting speed was increased from low to medium. It, however, reverted back to almost its initial value when the cutting speed was at the maximum. The radial force, however, increased when the cutting speed was increased from low to medium and remained almost constant when the cutting speed was at the maximum. These variations, however, occurred in the range of about 400±650 N. For insert 3, the tangential force gradually decreased when the cutting speed was increased from low to medium. The force decreased more sharply when the cutting speed was increased to the maximum. Yet again, the feed and radial forces were found to be lower. The feed force gradually decreased when the cutting speed was increased from low to medium. It, however, increased when the cutting speed was at the maximum. On the other hand, the radial force increased when the cutting speed was increased from low to medium. It, however, decreased when the cutting speed was at the maximum. These variations, however, occurred in the range of about 400±650 N. At a cutting speed of 200 m/min, the radial force at the lower feed rate was higher compared to when the feed rate was high. This was the only instance whereby the above occurred. Overall, it can be seen that the tangential force was much higher than the other forces at any particular experimental trial. This is consistent with the fact that the tangential force is the main force acting on the tool. All three forces increased when the feed rate was increased from 0.23 to 0.28 mm/rev. An exception to this occurred only once. For any particular insert used, the relationship curves between

f2 ; 8R

hCLA 

f2 p 18 3R

where h is the peak-to-valley height, hCLA the center-lineaverage roughness, f the feed and R the nose radius, show that surface roughness is primarily dependent on feed rate and the tool nose radius. These equations give ideal surface finish values which can only occur when satisfactory cutting conditions are achieved. It is evident from the ®gure that the surface roughness is dependent on the feed rate whereby the use of lower feed rate produced better surface ®nish. The ®gure also seemed to indicate that the surface roughness measurements obtained increased when the cutting speed was increased contrary to established theory where Chisholm had shown that the surface ®nish decreases as the cutting speed increases, until the surface ®nish equals the ideal ®nish [8]. This deviation, though small in value, can be attributed to various reasons such as inconsistencies related to the measuring process, workpiece surface damage due to the chips curling back into the work [9] and chatter [10]. Considering the results obtained, very good surface ®nish was obtained by using insert 2.

Fig. 4. Surface roughness measurements for the 18 experimental trials.

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Table 1 Measured data from the micrographs Insert

f (mm/rev)

V (m/min)

t2 (mm)

z (mm)

c (8)

rc

e

1 (KC 935)

0.23

140 200 280 140 200 280 140 200 280 140 200 280 140 200 280 140 200 280

0.32 0.35 0.34 0.30 0.32 0.28 0.31 0.29 0.32 0.30 0.31 0.29 0.28 0.26 0.30 0.28 0.31 0.29

10.3 5.2 5.0 9.3 4.1 2.9 8.6 2.4 2.4 8.1 2.3 2.0 12.1 12.1 5.9 9.6 7.2 3.1

23 24 18 28 32 27 25 23 26 26 22 23 25 28 25 29 24 26

0.719 0.657 0.676 0.933 0.875 1.000 0.742 0.793 0.719 0.933 0.903 0.966 0.821 0.885 0.767 1.000 0.903 0.966

2.34 2.41 2.38 2.23 2.24 2.23 2.32 2.28 2.34 2.23 2.24 2.23 2.26 2.24 2.30 2.23 2.24 2.23

0.28 2 (KC 792)

0.23 0.28

3 (G 5560)

0.23 0.28

3.3. Microstructure studies Microstructure studies were conducted on the chips collected from the experimental trials. Micrographs of the chips collected and prepared onto specimen holders for all the trials were taken. The chip thickness (t2), the secondary shear zone thickness (z) and the angle of crystal elongation (c) were extracted from the micrographs. The results from the microstructure studies were given in Table 1. The chip ratio (rc) and strain (e) values were also calculated using theoretical formulae and given in the same table. Secondary shear zone (SSZ) in machining is caused by the frictional resistance to sliding afforded by the tool as the chip pass over it. The strain (e) in the SSZ is generally higher than those observed in the primary shear zone. Additionally, the contact condition in between the tool and the chip may generate high temperature at the tool chip interface. The friction between the tool and the chip may also in¯uence the primary formation, built up edge formation and tool wear. Therefore minimum value of SSZ thickness is encouraged in machining process. The SSZ thickness measured values for the various experimental trials were shown graphically in Fig. 5. For any particular insert used, the relationship between the SSZ thickness and the cutting speed were generally found to be similar when the feed rate was increased. For inserts 1 and 3, the SSZ thickness decreased when the feed rate was increased, whereas for insert 2 there was no appreciable difference in SSZ thickness when the feed rate was increased. The use of higher cutting speeds generally resulted in the formation of smaller SSZ. Minimum SSZ thickness could be obtained by using insert 2. This can be attributed to the PVD (TiN) outer layer. The PVD process has the ability to produce a uniform, smooth and ®ne coating

compared to the CVD process and this coating gives rise to minimum friction between the tool face and the chips thereby producing minimum SSZ thickness. This result is in agreement with the work done by Venkatesh et al. [4] which amongst others concluded that the use of CVD coated carbides resulted in the formation of a fairly thick SSZ thereby indicating high temperatures while the ¯ow zone is not thick when PVD coated carbides are used. On the other hand insert 3, with no coating, gives rise to maximum friction between the tool face and the chips thereby producing maximum SSZ thickness. Fig. 6a±c are examples of the micrographs taken. Thus, it can be concluded that the use of insert 2 is most desirable as it produces chips with minimum SSZ thickness.

Fig. 5. SSZ thickness for the 18 experimental trials.

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taper ampli®cation could also take place. The thinnest SSZ occurs with insert 2 at a higher feed rate of 0.28 mm/rev and the highest speed of 280 m/min. This cutting condition combines high productivity with low chip ratio and low SSZ. Venkatesh and Chandrasekaran [11] have measured bulk strain in the chip using the hot microscopy technique and observed it to be 2. Venkatesh and Chandrasekaran [11] also measured shear strain in the SSZ by using the diffraction ring technique and found it to be 28. A combination of high strain in the chip and in the SSZ can contribute to high surface residual stress on the workpiece surface and hence high chip ratios and low SSZ or even zero SSZ values are highly desirable. This has been achieved in this study with insert 2 under high productivity conditions. 4. Conclusions The following conclusions could be made on the basis of the investigation carried out:

Fig. 6. Micrographs (1000) of chip roots taken from (a) insert 1, (b) insert 2, (c) insert 3, when V ˆ 280 m=min and f ˆ 0:28 mm=rev.

It may be observed from Table 1 that the chip ratio is high even equalling unity, which is impossible for conventional machining. This could be due to the distorted chips which when moulded for micrographic work could present a smaller area than is actually the case. On the other hand,

1. A remarkable achievement in this study is the reduction in bulk chip strain (high chip ratio) and low strain in SSZ (thickness of only 2 mm), obtained by the tool which has the CVD with TiCN/TiC and PVD with TiN coating layer sequence (insert 2), under high productivity conditions (V ˆ 280 m=min and f ˆ 0:28 mm=rev). 2. Having compared the performance of three cemented carbide cutting tools, insert 2 performs best under the conditions tested as lower forces with little variation were encountered, very good surface finish could be obtained and chips with minimum SSZ thickness could be produced. 3. The tangential force was much higher than the other forces at any particular experimental trial. This is consistent with the fact that the tangential force is the main force acting on the tool. Generally all three forces increased when the feed rate was increased from 0.23 to 0.28 mm/rev. 4. For any particular insert used, the relationship curves between the forces (tangential, feed and radial) and the cutting speed were generally found to be similar when the feed rate was increased. The same can also be said for the surface roughness and the SSZ thickness measurements. 5. The surface roughness measurements obtained increases when the cutting speed was increased contrary to established theory where Chisholm had shown that the surface finish decreases as the cutting speed increases, until the surface finish equals the ideal finish. Acknowledgements This work has been supported by the Ministry of Science, Technology and Environment, Malaysia through the IRPA

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funding Vote No. 72196. Tooling support from Kennametal and Mitsubishi is also gratefully acknowledged. References [1] G.T. Smith, Advanced Machining: The Handbook of Cutting Technology, IFS Publications, 1989. [2] J. Wallbank, Development in tool materials, advanced machining for quality and productivity, in: Proceedings of the Second International Conference on Behaviour of Materials in Machining, York, UK, November 14±15, 1991. [3] Metals Handbook, Machining, Vol. 7, 9th Edition, ASM, USA, 1980, pp. 773±783. [4] V.C. Venkatesh, C.T. Ye, D.T. Quinto, D.E.P. Hoy, Performance studies of uncoated, CVD coated and PVD coated carbides in turning and milling, Ann. CIRP 40 (1) (1991) 545±551.

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[5] J.R. Koelsch, Beyond TiN: New Tool Coatings Pick Up where TiN Left Off, Manufacturing Engineering, October 1992, pp. 27± 32. [6] A. Abdullah, Machining of aluminium based metal matrix composite (MMC), Ph.D. Thesis, University of Warwick, Warwick, UK, 1996. [7] K. Nakayama, M.C. Shaw, R.C. Brewer, Relationship between cutting forces, temperatures, built-up edge and surface finish, Ann. CIRP 16 (1966) 211±223. [8] E.J.A. Armarego, R.H. Brown, The Machining of Metals, PrenticeHall, Englewood Cliffs, NJ, 1969, pp. 172. [9] M.P. Groover, Fundamental of Modern Manufacturing, Prentice-Hall, Englewood Cliffs, NJ, 1996. [10] D.H. Jack, Ceramic tool materials, in: M. Schwartz Mel (Ed.), Engineering Applications of Ceramic Materials, American Society for Metals, USA, 1985, pp 192±199. [11] V.C. Venkatesh, H. Chandrasekaran, Experimental Techniques in Metal Cutting, Prentice-Hall, India, 1987, pp. 115±120, 169±179.