Tool Wear Performance of CVD-Insert during Machining of Ti-6%Al-4%V ELI at High Cutting Speed

Transcription

1 Key Engineering Materials Vol. 443 (2010) pp (2010) Trans Tech Publications, Switzerland doi: / Tool Wear Performance of CVD-Insert during Machining of Ti-6%Al-4%V ELI at High Cutting Speed Gusri Akhyar Ibrahim a, Che Hassan Che Haron b and Jaharah Abd. Ghani c Mechanical and Material Engineering Department, Universiti Kebangsaan Malaysia, Bangi 43600, Malaysia a sati1771@gmail.com, b chase@vlsi.eng.ukm.my, c jaharah@vlsi.eng.ukm.my Keywords: Tool life; wear progression; wear mechanism; surface quality; CVD tool; Ti-6Al-4V; dry machining Abstract. Machining of titanium alloys as aerospace material that has extremely strength to weight ratio and resistant to corrosion at high-elevated temperature, become more interested topic. However, titanium alloys have low thermal conductivity, relative low modulus elasticity and high chemical reactivity with many cutting tool materials. The turning parameters evaluated are cutting speed (55, 75, 95 m/min), feed rate (0.15, 0.25, 0.35 mm/rev), depth of cut (0.10, 0.15, 0.20 mm) and tool grade of CVD carbide tool. The results that pattern of tool life progression is rapidly increase at the initial stage. It was due to small contact area between the cutting tool and the workpiece. At the first step of machining, the chip welded at the cutting edge but some chip removed away from the cutting edge. Wear mechanism produced are abrasive wear, adhesive, flaking, chipping at the cutting edge and coating delamination. Introduction Titanium and its alloys are utilized in aero-engine and airframe structure is due to their ability to maintain their high strength at high temperature of engine, exceptional resistance to corrosion in high temperature, which savings on protective like panting [1,2]. Some typical characteristics of aero-engine super alloys are austenitic matrix which promote rapid work hardening, retention of high strength levels at elevated temperature, reactivity with cutting tool materials under atmospheric conditions, tendency to Build-Up-Edge and well onto cutting tools, presence of abrasive carbide in their microstructures [3,2]. The great advancement in the development of cutting tool has taken place in recent few decades, this has had only limited success in improving the machinability of titanium alloy. Almost every cutting tool developed so far, including diamond, ceramics and cubic boron nitride. The straight tungsten carbide (WC/Co) grade has proven its superiority in the machining of titanium alloys, and still remains the first choice in turning [4]. There have several attempts to improve the machinability of titanium alloys when dry machining condition using advanced and conventional cutting tool materials. The dry machining method or green machining was selected in order to avoided from using degradable coolants, which are harmful for human, degrade the environment and increase cost operation [5,6]. Coated tungsten carbide tools are suitable for finishing machining titanium alloys at speed of 75 m/min, feed rate of 0.25 mm/rev. and depth of cut 0.25 mm. Satisfactory tool life and surface finish have been reported when machining titanium alloys, hard coating layer on cutting tool could reduce the friction between cutting tool and work piece material [7]. Multilayers of hard coating (liketin-al2o3-ticn-tin using chemical vapor deposition) on carbide tools in dry machining of titanium alloys have reported significant increase in productivity relative to single or without layer [8,9]. The aim of this study is to investigate the tool wears performance and machined quality during machining of Ti-6Al-4V ELI alloy under dry machining. All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of the publisher: Trans Tech Publications Ltd, Switzerland, (ID: /04/10,10:24:39)

2 372 Advances in Materials Processing IX Experimental Method The workpiece material used in the machining trials was a titanium alloy alpha-beta Ti-6Al-4V Extra Low Interstitial (Ti-6Al-4V ELI), which is lamella phase and surrounded by in the grain boundary. The chemical composition and physical properties of workpiece material are given in Table 1 and 2, respectively. At least 3 mm of material at the top surface of workpiece was removed in order to eliminate any surface defects and residual stress that can adversely affect the machining result [10]. Table 1 Factors and levels used in the experiment Factors Levels A- Cutting speed (m/min) B- Feed rate (mm/rev) C- Depth of cut (mm) The machining trials under dry machining condition and high cutting speed were carried out using the Colchester T CNC lathe machine. Tools and tool holders were selected based on the recommendation of the tool supplier (Kennametal, 2006). Chemical Vapor Deposition (CVD) inserts with designation KC9225 (CCMT LF, ISO designation) were used to turn the titanium alloy Ti-6Al-4V ELI under dry cutting condition. Four layers of coating materials for each insert, which consist of TiN-Al2O3-TiCN-TiN, were selected. The parameters for turning operation are as shown in Table 3. Table 2 Chemical composition of the workpiece material Ti- 6Al-4V ELI (% wt) HEAT C Si Fe Ti Al N V S O H Y C < Bal < < Results and Discussions Tool Wear. The progress of flank wear land against tool life for turning Ti-6Al-4V ELI using CVD carbide tools at cutting speed of 55, 75 and 95 m/min, at various feed rate and depth of cut are shown in Fig. 1. It is clearly seen that a typical three-stages pattern of tool wear was obtained, while was similar with the pattern reported by Jawaid et al. [8] when machining titanium alloy with coated and uncoated carbide tools. The wear occurred rapidly at the initial stage, gradually increased at the second stage and extremely increased at the final stage. Rapidly increased at the initial stage was due to small contact area between the cutting tool and the workpiece, which caused temperature increased at the cutting edge, and some material easily removed from the cutting tool [3,11] found that the burn mark appeared on the coating layers of cutting tool when machining titanium alloy with coated carbide tool under dry machining due to high temperature concentration on the cutting edge. Sharp tool condition also caused rapidly increases in the cutting tools predominantly at the cutting edge and nose radius. The tool wear progression at all combination of feed rate and depth of cut shows that at the initial cutting, the wear increases dramatically until the VB reached 0.08 mm, mainly at the depth of cut of 0.10 and 0.20 mm (Fig. 1a). The combination at high feed rate and high depth of cut more significantly effect on the tool life. The high depth of cut directly influenced the cutting force due to the large contact area between the cutting tool and workpiece. Meanwhile, machining at high cutting generate more heat at the cutting edge, it can cause severe wear at the cutting edge. Due to low conductivity of titanium alloys cause the heat generated concentrate at the cutting edge. Jawaid [8] found that the feed rate also contributed to increase in temperature during machining titanium

3 Flank wear (mm) Flank wear (mm) Key Engineering Materials Vol alloy. Further machining at high cutting parameters cause severe wears at the cutting edge compare to at low cutting parameters. The wear progression patterns at feed rate of 0.25 mm/rev and depth of cut of 0.20 mm are similar to that at feed rate of 0.35 mm/rev and depth of cut of 0.15 mm from beginning until end of the tool life (Fig. 1b). At the initial stage of machining, the wear progression increases rapidly until VB reached 0.14 mm, then followed by gradual increases until VB reached 0.22 mm. Therefore, the feed rate and depth of cut give similar contribution for the wear. On the other hand, machining at low feed rate (0.15 mm/rev) and low the depth of cut (0.10 mm) produced low wear progression. Machining titanium at low depth of cut produced low cutting force, which also contributed to the wear progression of titanium alloys [12,1,2]. They also found that operating at low depth of cut and low cutting speed, can caused a chip stick on the cutting edge and wear occurred at nose radius, predominantly (a) (b) 0.15 mm/rev, 0.10 mm 0.25 mm/rev, 0.20 mm 0.35 mm/rev, 0.15 mm mm/rev, 0.15 mm 0.25 mm/rev, 0.10 mm 0.35 mm/rev, 0.20 mm Cutting time (min) Cutting time (min) Fig. 1 The flank wears progression of cemented carbide tools at cutting speed of (a) 55 m/min (b) 75 m/min Progression Wear. Fig. 2 shows a flank wear progression stage at the initial machining when cutting speed of 55 m/min, feed rate of 0.15 mm/rev and depth of cut of 0.15 mm. At the first stage, the wear increases rapidly to the VB = mm. When the VB reached mm or the cutting time of 1 min 8.10 sec., there is a chip that welded at the cutting edge of the tool. Next the chip removed away but some material of titanium alloy still remained at the cutting edge. Later more titanium alloy adhered on the cutting edge was much more, which looked like a build-up-edge. When the flank wears of the cutting tool reached mm or the cutting time of 3 min sec, the maximum titanium alloy that welded at the cutting edge are reached. Further machining, the welded titanium alloy was removed away from the cutting edge. With further machining severe wear was observed after machining more than 6 min when VB reached mm. This condition was found at VB of mm or the cutting time of 6 min sec. The welded titanium occurred at the cutting edge of tool because of the cutting process operated at low cutting speed (55 m/min). Similar to previous researcher found that operating at low cutting speed and low depth of cut, can caused a chip stick on the cutting edge [1]. Operating at low cutting speed generates a low temperature between the cutting tool and chip, so that this temperature is not high enough to release chip from the cutting edge, as shown in Figure 2 at stage 2. Che Haron et al. [6] reports that there was a strong bonding between the chip and tool material. When generated temperature was high, this condition was conducive for adhesive wear, therefore rough machined surface will produced. The accumulated adhered or welded titanium on the rake face of the cutting tool and the cutting edge replaces the chip from direct contact with tool. Most commonly it occurs at intermediate cutting speed and extremely low feed rate [12]. At shown in Fig. 2 at stage 3, the welded titanium removed from the tool after 5 min sec of cutting.

4 374 Advances in Materials Processing IX Some material of tool also was probably removed which can lead to the initiation of chipping at the cutting edge Fig. 2 The flank wear progression steps at the initial machining when cutting speed of 55 m/min, feed rate of 0.15 mm/rev and depth of cut of 0.15 mm. (a) Crater wear (b) Adhesive wear Flaking Flank wear (c) (d) Chipping Welded Fig. 3 Wear mechanism occurred when machining at cutting speed of 75 m/min, feedrate of 0.25 mm/rev, depth of cut of 0.20 mm, and tool life of min. Wear Mechanism. Machining of titanium alloy using Chemical Vapor Deposition (CVD) coated carbide tool at cutting speed of 75 m/min, feed rate of 0.15 mm/rev and depth of cut of 0.10 mm produced various wear mechanism such as abrasive wear and adhesive wear at the flank face and flaking at the rake face as shown in Fig. 3. The wear rate of coated tool at this condition was very slow. It can be seen when the cutting time was min, the flank wear or VB only reached 0.13 mm. It was due to low depth of cut and low feed rate. As stated by Gusri et al. [13] that the depth cut significantly contributed to the cutting time when machining titanium alloy at high cutting speed. They also mentioned that machining at high cutting speed and low depth of cut and low feed rate caused wear at nose radius. As shown in Fig. 3(a) that coating delamination at the flank race occurred.

5 Key Engineering Materials Vol The wear on the flank was severe after the hard layer coating remove out from the surface. Delamination on the cutting tools would be attributed to greater adhesion of the CVD coating to the substrate [8]. However, using hard coated layers (TiN-Al2O3-TiCN-TiN) on the cutting tools have given contribution in reducing the friction, not only between the cutting tool and work piece material but also between the cutting tool and chip material. Flaking at the rake face and welded material at the flank face were also observed, even the chipping of tool material at the cutting edge. The chipping at the cutting edge was probably caused by operating at bigger dept of cut selected coupled with small nose radius of insert. As stated by Che Haron [3] that increasing in depth of cut would increase the contact area between the cutting tool and work piece. In this way caused increase in the cutting force. Summary 1. Tool life Rapidly increased at the initial stage was due to small contact area between the cutting tool and the workpiece (tool is still sharp), which caused high temperature at the cutting edge, addition to the increase at final stage was due to more wear on the tools. 2. Machining at low cutting speed, at the first step, the chip welded at the cutting edge. Some chip removed away from the cutting edge but more chip still welded at the cutting edge until almost the end of tool life. Operating at low cutting speed generates a low temperature between the cutting tool and chip, so that this temperature is not high enough to release chip from the cutting edge (strong bounded). 3. Machining of Ti-6Al-4V ELI with CVD carbide tool produced abrasive wear and adhesive at the flank wear, flaking at the rake face, flank wear more at the nose radius, chipping at the cutting edge, coating delamination and chip welded at the cutting tool. The chipping at the cutting edge was caused by operating at bigger dept of cut coupled with small nose radius of insert. References [1] E.O. Ezugwu, J. Booney, and Y. Yamane: Journal of Material Processing and Technology Vol. 134 (2003), p. 233 [2] E.O. Ezugwu: International Journal of Machine Tools & Manufacture Vol. 45 (2005), p [3] C.H. Che Haron: Journal of Material Processing and Technology vol. 118 (2001), p. 231 [4] A. Jawaid, S. Sharif, and S. Koksal: Journal of Material Processing and Technology Vol. 99 (2000), p. 266 [5] A. Ginting: Universiti Kebangsaan Malaysia (2003), Thesis Doktor Falsafah. [6] C. H. Che Haron, A. Ginting, and H. Arshad: Journal of Material Processing and Technology Vol. 185 (2007), p. 77 [7] Kennametal, Kennametal Lathe Tooling Catalogue, Kennametal Company (2006). [8] A. Jawaid, C.H. Che Haron and A. Abdullah: Journal Material Processing and Technology Vol. 92 (1999), p. 329 [9] E.O. Ezugwu, R.B. Da Silva, J. Bonney and A.R. Machado: Journal of Machine Tools and Manufacture vol. 45 (2005), p [10] S. Kalpakjian and S.R. Rchmid: Manufacturing Engineering and Technology Fourth Edition (USA, Prentice Hall 2001). [11] G.A. Ibrahim, C.H. Che Haron, j.a. Ghani, Y. Burhanuddin, A., Yasir and N. El-Maghribi: EMARC Universiti Kebangsaan Malaysia (2007). [12] E.M. Trent: Metal Cutting Thirth Edition (Butterworth-Heinemann, Oxford 1995). [13] A.I. Gusri, C.H. Che Hassan, J.A. Jaharah, B. Yanuar, A. Yasir and E. Nagi: Proceedings of Seminar on Engineering Mathematics Universiti Kebangsaan Malaysia (2008), p. 57