Green Machining of Titanium Alloy Using Uncoated and Multilayer PVD-Coated Carbide Tools: Wear Mode, Wear Mechanism and Performance

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1 Hamidah Harahap / Jurnal Teknologi Proses 5(2) Juli 2006: Jurnal Teknologi Proses Media Publikasi Karya Ilmiah Teknik Kimia 5(2) Juli 2006: ISSN Green Machining of Titanium Alloy Using Uncoated and Multilayer PVD-Coated Carbide Tools: Wear Mode, Wear Mechanism and Performance Armansyah Ginting Department of Mechanical Engineering, Faculty of Engineering, University of Sumatera Utara, Medan Abstract The preliminary result on looking at performance of the off-center ball end mill carbide tools: alloyed uncoated (W-Ti/Ta/Nb)C-Co (tool A) and multilayer PVD-coated (WC-Co)(TiCN/TiAlN/TiN) (tool B) with nominal diameter of 16 mm in end milling of titanium alloy Ti-6%Al-2%Sn-4%Zr-2%Mo-0.08%Si (Ti-6242S) under green machining is reported in this paper. Results of machining trial show that localised flank wear (VB 3 ) is a dominant failure mode for both tools due to a concentrated of the coupled thermo-mechanical load at the tool leading edge. After machining for approximately 20 minutes, tools A and B start to reach VB mm when used at cutting speed of 100 m/min and feed of 0.15 mm/tooth. Further examination using SEM/EDAX shows that flank wear, rake wear, plastic deformation and brittle fracture are all failure modes suffered by tools because of adhesion (attrition) and dissolution-diffusion wear mechanisms. In conclusion, tools A and B can be recommended for green milling of Ti-6242S and the best compromise between cutting speed and tool life is given by end milling at cutting speeds of m/min. Keywords: Ti-6242S, end milling, flank wear, brittle fracture, plastic deformation, attrition, dissolutiondiffusion. Introduction Dry cutting is becoming increasingly more popular as a means of reducing overhead costs while protecting the environment. It is ecologically desirable and considered as a necessity for manufacturing industries in the near future. From environmental perspective, dry cutting can be characterised as green machining (Sreejith & Ngoi, 2000). Reducing or when possible eliminating the use of cutting fluid in green machining means less waste dumped in landfills, less mist in the factory atmosphere, no residue on the chip, reducing disposal and cleaning cost, cleaner workshop floor, and fewer dermatological for operator. So far, the switch to green machining has been reported successfully done for machining of cast iron, carbon and alloy steels (Klocke & Eisenblätter, 1997; Sreejith & Ngoi, 2000; Graham, 2000; Che Haron et al., 2001). For the aerospace materials, however, green machining is not widely reported. The only report was the paper of Klocke and Eisenblätter (1997) where they presented the result of green machining of nickel alloy and threw an idea that high speed milling might be a method for green machining of aerospace materials including titanium alloy. Unfortunately, when refer to the results of the DARPA Advanced Machining Research Program (Komanduri et

2 85 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: al., 1985), high speed machining as an alternative method for green machining of titanium alloy is controversial. Komanduri et al. (1985) had placed their emphasis on high speed turning and milling at cutting speed ranging from ,500 m/min of aluminum-, nickel-based-, titanium- and ferrous alloys. They concluded that there was no indication at high speed machining the temperature at the tool-chip interface would decrease and allow an efficient material removal in term of reduced tool wear. They obtained the cutting temperature increased with speed and it was approaching the melting point of the workpiece material. Consistent with this result, Shaw (1984) stated that the very high tool temperature experienced when machining of titanium and its alloys were due to the very low value of the multiplication between thermal conductivity (k) and volume specific heat (C) of these materials and thus, they must be machined at low cutting speed. Objective of the present study is to evaluate the performance of carbide cutting tools in green machining of titanium alloy Ti-6%Al-2%Sn-4%Zr-2%Mo-0.08%Si (Ti- 6242S). Based on the discussion in the latter paragraph, green machining in this study is carried out by milling at relatively low cutting speed. It is expected that the reasonable tool life can be obtained at cutting speed higher than the common speed in machining of titanium alloy using carbide (60 m/min). Materials and Method The forging stock of α β titanium alloy, Ti-6242S was taken as the workpiece material in this study (see Figure 1, Tables 1 and 2). In aerospace applications, this material is primarily used for elevated temperature up to 540 o C like in a gas turbine engine for making the rotating components such as blades, discs and rotors. In the past 7 years, Ti-6242S was beginning to be used as the airframe components in areas such as engine mounts, exhaust system, and exhaust impingement (Boyer, 1995). FIGURE 1: The microstructure of Ti-6242S used in this study. TABLE 1: Chemical composition of titanium alloy Ti-6242S Element Al Zr Mo Sn Fe O 2 Si C N 2 H 2 Y Others each total min max Ti Rem. Tensile strength (MPa) Yield strength (MPa) TABLE 2: Physical properties of titanium alloy Ti-6242S Creep stress (MPa) Hardness (HRc) Density (kg/m 3 ) Linear thermal expansion (10-6 / o C) Thermal conductivity (W/mK) ~ Tool code TABLE 3: Chemical composition and physical properties of tools A and B Chemical compositions Element Amount (%) Grain size (m) Hardness (HV10) Physical properties Coating Material Total coating thickness (m) A WC 69.8 Ti/Ta/NbC Co 9.50 B WC 87 < layers of 3 4

3 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: Co 13 TiCN/TiAlN/TiN Prior to the actual machining trials, the workpiece materials were trued and cleaned by removing the outer surface. The premachining area was made in every pass of the trials, which was aimed to avoid a premature tool failure at the initial entry of the tool. The alloyed uncoated carbide (W- Ti/Ta/Nb)C-Co (tool A) and multilayer PVD-coated (WC-Co)(TiCN/TiAlN/TiN) (tool B) were selected as the tool materials throughout the trials (Table 3). Both were insert types and round shape with diameter of 12 mm. During machining trials, each tool was rigidly mounted on a tool holder to provide an off-center ball end milling tool with nominal diameter of 16 mm (Figure 2). leading edge (point A), machining parameters are calculated based on the effective diameter (D e ): 2 2 (1) D D i i D = 2 a + d e a 2 2 Vc N = π D e = 2π V Di 2 f = f z 2 V Di a 2 N + d (2) (3) where: V c = cutting speed (m/min), V f = feed rate (mm/min) N = spindle speed (rpm), f z = feed (mm/tooth) c a 2 FIGURE 2: The off-center ball end mill. (a a =axial depth-of-cut; D e =effective diameter; D n =nominal diameter; D i =insert diameter; d=offcenter radius) FIGURE 3: View of the experimental set-up. Machining trials were carried out on a 3- axis CNC vertical machining center with a 9 kw motor driver and variable spindle speed ranging from 60 to 10,000 rpm (Figure 3). The cutting conditions used for the trials were cutting speeds of 60 to 150 m/min and feeds of 0.1 and 0.15 mm/tooth, while axial and radial depth-of-cut were kept constant at 2 mm and 8.8 mm, respectively. These are the typical cutting conditions used for roughing as suggested by the tools manufacturer. As shown in Figure 2, to obtain the constant cutting speed at the The tool rejection criteria stipulated for the study were uniform flank wear (VB 1 ) 0.2 mm, localized flank wear (VB 3 ) 0.3 mm and excessive chipping, flaking and/or fracturing of the cutting edge (ISO8688-2, 1989). Machining trial was terminated if none of those criterion attained after machining for ~20 minutes. Results

4 87 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: Tool wear and tool life All tool lives data recorded and tool failure modes observed from the machining trials are presented in Table 4. From the table, it can be seen that localised flank wear (VB mm) is a dominant tool failure mode for tools A and B. As shown in Figures 4 and 5, the VB 3 occurs at tool s leading edge or at point A (Figure 2). This point is equal to the value of the axial depth-of-cut. Beyond this point (a a < 2 mm), both tools experienced uniform flank wear VB mm. The other failure modes were also observed such as plastic deformation and brittle fracture (Figures 4, 5 and 6a-c). TABLE 4: Tool lives recorded and tool failure modes observed from machining trials V c (m/min) f z (mm/toot h) Tool life of tool A (min) VB (mm) Tool failure modes Tool life of tool B (min) VB (mm) Tool failure modes VB VB VB VB VB VB VB VB VB VB a 0.30 BF, VB BF, VB a 0.30 BF, VB a 0.30 BF, VB a 0.30 BF, VB a 0.30 BF, VB a BF, VB a,b 0.30 BF, VB a,b 0.30 BF, VB a,b 0.30 BF, VB 3 a Interpolation results. b Repetition to avoid error data collection due to premature tool failure. VB 1 uniform flank wear, VB 3 localised flank wear, BF brittle fracture (flaking, chipping, cracking and fracturing) FIGURE 4: Typical wear on tool A flank face. (100 m/min, 0.15 mm/tooth, 20.4 min., VB mm) FIGURE 5: Typical wear on tool B flank face. (100 m/min, 0.15 mm/tooth, min., VB mm)

5 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: (a) increased to 125 and 150 m/min. The greatest was recorded at cutting speed of 150 m/min and feed of 0.10 mm/tooth where tool life of tool A almost 5 times longer than tool B. However, when feed increased to 0.15 mm/tooth, tool life of tool A was dramatically decreasing almost 5 times from down to ( ) minutes. Wear Mechanism (b) The results of microanalysis using SEM/EDAX indicate that adhesion (attrition) and dissolution-diffusion are the wear mechanisms of both tools in green milling of Ti-6242S. (c) FIGURE 7: Typical condition when chip is adhered on tool (tool A, 100 m/min, 0.15 mm/tooth, 20.4 min., VB mm). FIGURE 6: Typical brittle fracture in tools A and B: (a) tool A (150 m/min, 0.15 mm/tooth, 3.15 min.), (b) tool B, (c) cross-section of tool B (150 m/min, 0.15 mm/tooth, 1.70 min.). (Note: adherent chip on worn flank face, plastic deformation on rake face, a part of tool has been chipped and or fractured, dashed line is the tool s original shape) Both tools have the same cutting condition to attain VB mm that is at cutting speed of 100 m/min and feed of 0.15 mm/tooth. Their tool lives are almost the same at this cutting condition, i.e minutes for tool A and minutes for tool B. However, tool life gap between both tools becomes greater when cutting speed is The evidence of adhesion was easy to observe where at all cutting conditions, chip was found adhere on tool, both at flank and rake faces (Figures 4, 5, 6c and 7). The high magnification in Figure 7 shows that chip is strongly adhered with no gap at all between tool-chip interfaces. This intimate area is very narrow at the vicinity of tool. In Figure 7, it is also observed that a small part of tool material (rectangle area) migrates from its origin position to the adherent chip. The migration part was analysed by EDAX (the rectangle area). The result in Figure 8 shows that positively, a small amount of tool material has migrated and it is shown by the high peak of W.

6 89 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: FIGURE 8: Result of EDAX analysis on the rectangle area in Figure 7 (see Table 3 for the content of tool A). Discussions From tool lives data in Table 4, the performance of tools A and B in term of tool wear and tool life can be summarised as follows: 1. At the cutting conditions up to cutting speed of 100 m/min and feed of 0.10 mm/tooth, tool B is better than tool A because tool B is tougher than tool A in overcoming cutting load due to finer grain size of substrate material and the presence of hard metal coating materials. Moreover, it can be stated that cutting load in these cutting conditions is mainly dominated by mechanical load that contributed by compressive, tensile and shear stresses because wear on tool flank face is mainly observed. 2. At the cutting condition where cutting speed of 100 m/min and feed of 0.15 mm/tooth, both tools A and B have a similar performance and experience the same failure modes. In this case, it is believed that cutting load in this cutting condition is a combination between mechanical and thermal load (coupled thermo-mechanical load) since brittle fracture is also observed besides localised flank wear at the tool s leading edge. 3. At the cutting conditions ranging from 100 m/min and 0.15 mm/tooth to 150 m/min and 0.15 mm/tooth, a coupled thermo-mechanical load that together with chemical reaction between titanium and tool materials at high temperature produce localised flank wear, plastic deformation and brittle fracture on tools A and B. In these cutting conditions, the influenced of thermal load is believed greatly significant due to higher cutting temperature that produced by higher cutting speed. 4. Although low cutting speeds of m/min provide longer tool lives; however, the best compromise between tool life and cutting speed is given by machining at cutting speeds of m/min where at VB mm, tool life ranging from 5.82 to 21.2 minutes. High thermal load in titanium machining is primarily due to the poor thermal conductivity of this material. When it is coupled with the fact that chip in titanium machining is thin, they will produce too much higher surface heating on the tool-chip contact area (Maranchik Jr. & Snider, 1968; König, 1979; Komanduri, 1982; López de Lacalle et al., 2000). This condition is deleterious to tool materials because heat engenders and excites adverse chemical reactions; thus, titanium is strongly bonded with tool materials (Siekmann, 1955; Stashko, 1982; Komanduri, 1982; López de Lacalle et al., 2000). The strong bonding between chip and tool material at high temperature as those shown in Figures 4 to 7 will produce such wear mechanism called adhesive wear. The best evidence of the adhesive wear is shown in Figure 6a where rough region and smooth region are clearly observed. Since grain size of both tools A and B is fine enough (see Table 3, tool A < 1 mm and tool B 1-2 mm), the adhesive wear for the rough region is called attrition wear (Shaw 1984). In the rough region, some parts of the worn surface are still covered by molten chip and as the proposal of Dearnley and Grearson (1986), the irregular attrition wear occurs in this region. In this study, the irregular attrition wear is due to the intermittent adhesion during interrupted cutting which makes a periodic attachment and detachment of the work material on the tool surface. Therefore, when the seizure between workpiece to tool is broken, the small fragments of tool

7 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: material is plucked and brought away by the chip (Figure 7). In line with the proposal of Dearnley and Grearson (1986), smooth region is due to the dissolution-diffusion wear mechanism. In this case, the action of heat dissolves the tool materials and the dissolved matters diffuse to the higher temperature gradient or at the toolchip interface. The continuous action of heat in dissolving and diffusing the tool materials and coupled with the nature of interrupted cutting during milling process will end with brittle fracture failure mode. Cobalt as the bond material in tools substrate is also dissolved and diffused by heat. Once cobalt absents from the carbide grain interface, the intermittent impact of milling as the mechanical load splits the grains and finally, cracking, flaking, chipping and fracturing are resulted (pointed by arrow in Figures. 4 to 7). In particular, shown in Figure 7 that the carbide grains in rake face region are not as denser as the inner region. It is believed that cobalt in that region has been diffused into the adherent chip due to the high cutting temperature in the seizure tool-chip interface; thus, this condition will end with flaking, chipping and fracturing. Chemical reaction between workpiece and tool material at high cutting temperature is believed occurred mainly when machining at cutting speeds of 150 m/min because tool lives for both tools are dramatically decreased. For tool A, chemical reaction between Ti and C to form TiC is also combined with the dissolution of TiC/TaC/NbC (Freeman, 1974; Dearnley & Grearson, 1986); thus, protruding, deformation and disintegration of cutting edge as shown in Figure 7 are occurred. The result of EDAX shown in Figure 8 also supports the fact that chemical reaction occurs at the tool-chip interface since all chemical elements of tool A are detected at the tool-chip interface. It is thought that they have dissolved and diffused into the interface. In case of tool B, chemical reaction is not only due to the formation of TiC but also believed due to the reaction among coating (TiCN/TiAlN/TiN) and workpiece materials. It is believed that chemical reaction also has a close relationship with smooth wear. The formation of TiC from the diffusion of dissolved C at the tool-chip interface strengthens the chip surface and when it flows on tool surface, the grinding like process producing the smooth surface. Conclusions The following conclusions are based on the preliminary result on looking at performance of the off-center ball end mill carbide tools: alloyed uncoated (W- Ti/Ta/Nb)C-Co and multilayer PVD-coated (WC-Co)(TiCN/TiAlN/TiN) in end milling of titanium alloy Ti-6242S under green machining: 1. The best compromise between cutting speed and tool life is given by end milling at cutting speeds of m/min where for VB mm where tool life ranging from 5.82 to 21.2 minutes. 2. Localised flank wear (VB 3 ) is a dominant failure mode for both tools due to a concentrated of the coupled thermomechanical load at the tool leading edge. 3. Plastic deformation and brittle fracture such as rake face flaking, cracking, chipping and fracturing are also observed as the other failure modes of both tools. 4. Adhesion (attrition) and dissolutiondiffusion are observed as the wear mechanisms to form the tool failure modes. 5. The adhesive wear rough region is produced by irregular attrition wear and it is due to the intermittent adhesion during interrupted cutting which makes a periodic attachment and detachment of the work material on the tool surface. 6. The adhesive wear smooth region is due to dissolution-diffusion wear mechanism where the action of heat dissolves the tool materials and the dissolved matters diffuse to the higher temperature gradient or at the tool-chip interface. 7. Chemical reaction also has a close relationship with smooth wear where the formation of TiC from the diffusion of dissolved C at the tool-chip interface strengthens the chip surface and when it flows on tool surface, the grinding like process producing the smooth surface.

8 91 Armansyah Ginting / Jurnal Teknologi Proses 5(2) Juli 2006: Acknowledgements The author wishes to thank Prof. Dr. Che Hassan Che Haron for his warm supervision. The continuation of collaboration in research and publication is appreciated. Thank is also addressed to Prof. Dr. M. Nouari from Lamefip Ensam CER Bordeaux, France for the continuous collaboration and the visiting professor program in 2004, 2006 and the next. References Boyer, R.R Titanium for Aerospace: Rationale and Applications. Advance Performance Materials. 2: Che Haron C.H., Ginting A. & Goh J.H Wear of Coated and Uncoated Carbides in Turning Tool Steel. J. Mater. Proc. Tech. 116: Dearnley, P.A. & Grearson, A.N Evaluation of Principal Wear Mechanism of Cemented Carbides and Ceramics Used for Machining Titanium Alloys IMI 318. Mat. Sci. and Tech. 2: Freeman, R.M The Machining of Titanium and Some of Its Alloys. PhD Thesis. Univ. Birmingham. Graham D Going Dry. Manuf. Eng. SME. 1: ISO Tool Life Testing in Milling Part 2: End Milling. ISO. Switzerland. pp Klocke, F. & Eisenblätter, G Dry Cutting. Annals of the CIRP. 46(2): Komanduri, R Some Clarifications on the Mechanics of Chip Formation when Machining Titanium Alloys. Wear. 76: Komanduri, R., Flom, D.G. & Lee, M Highlights of the DARPA Advanced Machining Research Program. J. of Engg. Ind. 107: König, W Applied Research on the Machinability of Titanium and Its Alloys. AGARD Conf. Proc. No. 256 Adv. Fabrication Processes. pp López de Lacalle, L.N., Perez, J., Llorente, J.I. & Sanchez, J.A. Advanced Cutting Conditions for the Milling of Aeronautical Alloys. J. Mat. Proc. Tech. 100: Maranchik Jr. & Snider, R.E Machining of Titanium Alloys. Engineering Conf. ASTME. Technical Paper MR pp Shaw, M.C Metal Cutting Principles. Oxford. London. Siekmann, H.J How to Machine Titanium. The Tool Engineer. pp Sreejith P.S. & Ngoi B.K.A Dry Machining: Machining of the Future. J. Mater. Proc. Tech. 101: Stashko, D.R Let s Take the Myths Out of Milling Titanium. Tooling & Production. pp