Tool wear mechanisms in machining of titanium alloys

Transcription

1 Pravin Pawar, Shashikant Joshi, Asim Tewari, Suhas Joshi, Evaluation of tool wear s in machining of three different types of titanium alloys, Page no , 4th International & 25 th AIMTDR Conference 2012, Jadhavpur University, Kolkata Tool wear s in machining of titanium alloys 1 Pravin Pawar, 2 Shashikant Joshi, 3 Asim Tewari, 4 Suhas Joshi 1 Pravin Pawar; Dept. of Mech. Engg.; Indian Institute of Technology Bombay; India pravinpawar9818@gmail.com 2 Shashikant Joshi; Dept. of Mech. Engg.; Indian Institute of Technology Bombay; India snjoshi53@rediffmail.com 3 Asim Tewari; Dept. of Mech. Engg.; Indian Institute of Technology Bombay; India asim.tewari@iitb.ac.in 4 Suhas Joshi; Dept. of Mech. Engg.; Indian Institute of Technology Bombay; India ssjoshi@iitb.ac.in Abstract: - With the change in phase fraction, mechanical and micro structural properties of titanium alloy change. Therefore, machining of these alloys gives rise to different s of tool-wear. To evaluate the change in dominant tool wear s, orthogonal turning experiments were carried out on three titanium alloys viz. α, α+, and rich. The turning experiments were performed on CNC lathe with coated carbide inserts. The cutting speed employed in the range of 31.4 m/min to 157 m/min at a fixed feed rate of 0.11 mm/rev. SEM images of worn out edge of the cutting tool inserts were analysed to identify tool wear s. The abrasion on rake face was the dominant tool wear in machining of α titanium alloy. Abrasion due to built-upedge formed was the major tool wear for α+ alloy. Plastic deformation of cutting edge prompted tool wear in the machining of titanium alloy. Keywords:tool wear, orthogonal Turning,Ti-alloy. 1. INTRODUCTION Titanium alloys have two-phase (α- HCP and β- BCC) microstructure at room temperature. A wide range of mechanical properties and microstructures are obtained by changing the phase fraction in titanium alloy. Also, this changes the shear-band forming tendency and the corresponding segmentation frequency during machining. Therefore, the of tool wear differs widely in machining of titanium alloys for the same tool geometry and material. An investigation on dominant tool wear would help in design cutting tools, lubrication systems for machining of titanium alloys. Several researchers have worked to identify wear s in machining titanium alloys with different cutting tool material. Wear s in machining of titanium alloy with HSS and cemented carbide tools were identified as excessive cratering and deformation at the nose. They change to diffusion-dissolution with the use of composite tool wbn-cbn, which has high fracture toughness and hardness [1]. Maximum flank face wear and excessive chipping at flank edge was identified as major tool wear in machining of Ti with cemented carbide inserts. Also, it was observed that the cemented carbide inserts with fine grain size and honed cutting edge show longer tool life in machining of titanium alloys [2]. Face milling of Ti6Al4V shows chipping on rake face as a dominant tool wear. Further, CVD coated tungsten carbide inserts outperformed PVD coated inserts due to their strong adhesion property [3]. Machining temperature above 700C shows excessive diffusion wear in machining with PCBN tools [4]. Cemented carbide tools have longer tool life as compared to the CBN tools under high pressure coolant [5]. Slot milling with BCBN tools show adhesion of work material and attrition as a significant tool wear [6]. Tool wear

2 s in machining using uncoated carbides were identified as diffusion and superficial plastic deformation. In case of PCD inserts, diffusion, attrition and notching were identified the dominant tool wear s [7]. Above studies shows that the tool wear changes with the cutting tool material, machining processes and processing conditions. It is also noted that very few studies have concentrated on influence of material properties on the tool wear. The titanium alloys with a change in phase fraction, differ widely in microstructure and mechanical properties. Therefore, the tool is subjected to different magnitude of cutting forces and its fluctuation. Also, microstructure of titanium alloy change from coarse equi-axed to fine lamellar. Therefore, identifying dominant tool wear in each of the titanium alloy will help in selecting tool material, method of lubrication, tool geometry, which in turn would help minimize tool wear rate. Therefore, this work presents a detailed analysis of cutting tool wear in machining of three different types of titanium alloy microstructures. The analysis is done using scanning electron microscopy and EDAX of the cutting tool worn surfaces. 2. THEME OF EXPERIMENT The objective of the present study is to identify the dominant tool wear in machining three different titanium alloys. The titanium alloys are: α, α+, rich alloys with increasing phase fraction were considered for the study. These alloys differ widely in mechanical properties, microstructure and phase fraction. The detailed theme of experiment is shown in Fig. 1. Material α -alloy α + -alloy -rich alloy Controlling parameter Mechanical properties Microstructure Phase fraction Operation Orthogonal Machining Tool wear Identifying tool wear Figure 1: Theme of experiment Result Dominant tool wear The orthogonal turning on CNC lathe was performed to obtain the tool wear patterns on carbide inserts. Various tool wear s operating during machining are adhesion, abrasion, diffusion, plastic deformation, etc. With the SEM and EDAX analysis, dominant tool wear in machining each titanium alloy have been investigated, which eventually will help selection of material, geometry and lubrication in machining. 2.1 Experimental Specifications Experimental specifications which include alloy composition, machine and cutting tools, lubrication and processing parameters and response variables under study are presented in Table 1. SR NO. Table 1 Experimental specification Parameters 1 Material Specifications 2 Machine CNC lathe 3 Tool 4 Lubrication Dry 5 6 Processing parameter Response variables 2.2 Experiment procedure α alloy Ti5.26Al3.03Sn α+β alloy Ti6Al4V Β rich alloy Ti4.16Al3.26V3.89Mo Carbide Insert CNMG M5-TP-3500 Tool Holder PTGNR16X16 Cutting Speed (m/min) :- 28.2, 78.2, Feed (mm/rev) : Depth of cut:- 1mm Wear, dimensions of worn out edges, diffused alloying elements on insert The orthogonal turning experiments were carried out on CNC lathe. For machining each titanium alloy, new carbide insert edge was used. The machining experiments were carried out at the identical cutting speed and feed rate. Same volume of material was removed during machining for each of the three titanium alloys. The cutting forces were measured with the Kistler dynamometer. The machined chips were collected after each experiment. The worn out inserts were examined with SEM. EDAX at the cutting edge was carried to identify the change in the alloying composition at worn out edge. The predominant tool wear for each of the titanium alloy was identified. 3. RESULTS AND DISCUSSION In this section, results of experiments described in the previous section are presented. The SEM images of worn out edges of tools were analysed to identify various tool wear s operating

3 during machining of the titanium alloys. The results of EDAX are presented to show the diffusion of various alloying element from titanium alloys to the insert edges. 3.1 Tool Wear in α Alloys The worn out cutting edge of tool shows the wear in machining of α- titanium alloys, see Fig. 2 a-c. A magnified image of edge shows that material is removed from the cutting edge; see Fig. 2 b-c CoK 0.71 CaK MoL Tool Wear in α+β Alloys The worn out edge of insert shows the adherence of chip on the cutting edge, see Fig. 3 a- c. The chips appear to weld the cutting edge, see Fig. 3 c. A change in colour of the worn out part of insert shows that coating de-lamination takes place. However, de-lamination process occurs slowly by moving chips on the rake face, which confirms the utility of the tool coating in machining Ti6AL4V. However, the worn out edge appears uniform with no dimples or troughs on rake face. This shows uniform contact of chips on the rake face. No abrading of chips on the rake face is observed which gives diffusion-dissolution type of wear. EDAX analysis shows presence of vanadium on the worn out edge, see Table 2 and Fig. 3 d. Vanadium is significant alloying element of α+ alloys. Due to high pressure at the tool edge, built-up-edge gets formed at the tool edge. The built-up-edge affects both rake face and flank face. No plastic deformation or chipping of the tool edges is observed. The thickness of the worn out cutting edge is about 200 m. Figure 2a-c. SEM image of worn out cutting edge used to machine α alloys. d. EDAX analysis The material removal is observed nonuniformly from the surface which points to abrasive wear on the rake face. The rough rake surface indicates complete removal of coating due to rubbing action of chip on the rake face. Therefore, the tool coating appears less effective in minimizing tool wear in case of α alloy. The rough surface shows that chip abrades the rake face while moving over from it. However, no damage appears on the flank face of the tool. Also, no plastic deformation of the cutting edge is observed. EDAX analysis shows that the percentage of Sn on the worn edge. It confirms the diffusion of Sn from the work piece on to the tool; see Table 2 and Fig. 2 d. The width of worn out edge on the rake face measured is 200 m. Table 2Element analysis of worn out edge α alloy α+β alloy β rich alloy Element Wt. % Element Wt. % Element Wt. % CK CK CK AlK 6.23 AlK 4.89 AlK 4.22 WM 1.71 WL WL 6.74 SnL SiK 2.77 TiK TiK TiK VK 0.0 VK 1.17 VK 0.0 Figure 3 a-c. SEM image of worn out cutting edge used to machine α+β alloys. d. EDAX analysis 3.3 Tool Wear in β rich Alloys The worn out cutting edge shows damage to the cutting edge, see Fig. 4 a-c. Due to high temperature, softening of the edge takes place, see Fig 4 c. A small contact of the chip over the cutting edge gives rise to high pressure and temperature. It softens the cutting edge of material. Further, it affects the integrity of edge due to deformation of insert material. Simultaneously, no adherence of work piece material on the tool edge was observed.

4 A colour change at the width of worn out edge, shows removal of coating again. However, the uniform surface with smooth surface indicates slow rate of coating de-lamination, which again underlines the effectiveness of coating in machining of rich alloys. As the abrasion type of wear on the rake face is not present, it promotes dissolutiondiffusion type of wear. However, there is no sign of built up edge formation at the cutting edge. Due to deformation of the cutting edge, both rake and flank faces of the tool get affected. EDAX analysis shows presence of Mo on the worn out edge, see Table 2 and Fig. 4 d. This confirms the diffusion of Mo, which again is a significant alloying element of the work material. The measured width of worn out insert edge is about 210 m. machining of titanium alloys. Diffusion of Mo to the insert edge is confirmed using EDAX analysis. The width of worn out edge measured from SEM images was almost identical for α and α+ alloys (200 m). However, an increase in width of worn out edge is observed for rich alloy. In general, diffusion of major alloying elements to the worn out cutting tool is observed for α, α+, and rich titanium alloys. ACKNOLEDGEMENT The authors gratefully acknowledge the partial support provided for this work by National Centre for Aerospace Innovation and Research, IIT- Bombay, a Dept. of Science and Technology- Government of India, The Boeing Company and IIT Bombay Collaboration and Bharat Forge Ltd. Pune. REFERENCES Figure 4a-c SEM image of worn out cutting edge used to machine rich alloys. e. EDAX analysis 4. CONCLUSIONS The above discussions lead to the following conclusions: Machining of α alloy shows the dominance of abrasive tool wear during the machining operation. Due to abrasion of material from the insert, earlier coating delamination occurs. Thus, coating on tool is less effective in machining α alloys. Machining α+β alloy shows adherence of material on the cutting edge. Thus, built-upedge formed encourages abrasion and diffusion during machining operations. Slow and uniform coating de-lamination underlines the effectiveness of tool coating in machining of α+ titanium alloys. Machining rich alloy shows the deformation of the tool cutting edge. The damage to cutting edge encourages both face and flank wear during machining operation. The coating delaminates at slow rate. Therefore, tool coating will help improve the tool performance in 1. Bhaumik S. K., Divakar C. and Singh A. K., Machining Ti-6AI-4V alloy with a wbn-cbn composite tool, Materials & Design, Vol. 16, pp , Jawaid A., Che-Haron C.H., Abdullah A., Tool wear characteristics in turning of titanium alloy Ti-6246, Journal of Materials Processing Technology, Vol. 92/93, pp , Jawaid A., Sharif S., Koksal S., Evaluation of wear s of coated carbide tools when face milling titanium alloy, Journal of Materials Processing Technology, Vol. 99, pp , Zoya Z. A., Krishnamurthy R., The performance of CBN tools in the machining of titanium alloys, Journal of Materials Processing Technology, Vol. 100, pp , Ezugwu E. O., Da Silva R. B., Bonneya J., Machado A. R., Evaluation of the performance of CBN tools when turning Ti 6Al 4V alloy with high pressure coolant supplies, International Journal of Machine Tools & Manufacture, Vol. 45, pp , Wang Z. G., Wong Y. S., Rahman M., High-speed milling of titanium alloys using binderless CBN tools, International Journal of Machine Tools & Manufacture, Vol. 45, pp , Nurul Amin A. K. M., Ismail A. F., Effectiveness of uncoated WC Co and PCD inserts in end milling of titanium alloy Ti 6Al 4V, Journal of Materials Processing Technology, Vol , pp , 2007.

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