Influence of Milling Conditions on the Surface Quality in High-Speed Milling of Titanium Alloy

Transcription

1 Influence of Milling Conditions on the Surface Quality in High-Speed Milling of Titanium Alloy Xiaolong Shen, Laixi Zhang, and Chenggao Ren Hunan Industry Polytechnic, Changsha, , China Abstract. The study was focused on the machined surface quality of titanium alloy under different milling conditions, including milling speed, tool rake angle and cooling method. The surface quality was investigated in terms of residual stress and surface roughness. The results show that the compressive residual stresses are generated on the machined surface under all milling conditions. The compressive residual stresses in both directions decreased and the surface roughness increased with the milling speed increasing. The compressive residual stresses increased with the tool rake angle increasing. The lowest surface roughness was obtained when the rake angle was 8Ü. Under the condition of dry milling, the highest compressive residual stresses were obtained, approximately 350 MPa. The highest surface roughness was obtained when the oil mist coolant was used. Water cooling was the best cooling method with uncoated cemented carbide tool in high-speed milling of titanium alloy. Keywords: High-speed milling, Titanium alloy, Milling condition, Surface roughness, Residual stress. 1 Introduction Titanium alloys have been widely used in aerospace and other industrial applications because of their elevated mechanical resistance having a low density and their excellent corrosion resistance, even at high temperatures. However, titanium alloys are difficult to machine due to their high temperature strength, relatively low modulus of elasticity, low thermal conductivity and high chemical reactivity [1-2].Conventional milling speeds range from 30 to 100 m min -1 when sintered carbide tools were used in the machining of titanium alloys, resulting in low productivity [3]. High speed machining is widely appreciated in industry for its high material removal rate, low milling force, high machining accuracy and high surface quality. All these advantages have led to the application of high-speed machining technology in the machining of titanium alloys [4]. Many researchers have researched on surface quality of milling of titanium alloy. Che-Haron [5] carried on the research on surface quality of rough turning of titanium alloy Ti 6Al 4V with uncoated carbide. The results showed that the machined surface experienced microstructure alteration and increment in microhardness on the top white layer of the surface. Machined surfaces have shown severe plastic deformation and hardening after prolonged machining time with worn tools, especially when machining S. Lin and X. Huang (Eds.): CSEE 2011, Part I, CCIS 214, pp , Springer-Verlag Berlin Heidelberg 2011

2 424 X. Shen, L. Zhang, and C. Ren under dry milling condition. A metallurgical analysis on chip obtained from high speed machining of titanium alloy Ti 6Al 4V was performed by Puerta Velásquez [6].The titanium β phase was observed in all chips for any tested milling speeds. No evidence of phase transformation was found in the shear bands. Ezugwu [7] investigated the effect of machining parameters on residual stress of titanium alloy IMI-834 by milling. The residual stresses were found to be compressive in nature at the milling speed of 11~56m min -1 and a linear relationship could not explain the variation of the residual stresses with respect to the milling parameters. Ge [8] reported experimental evidence that the surface roughness was less than Ra 0.44μm when milling gamma titanium aluminide alloy at speeds of 60~240m min -1. Workpiece surface has a maximum microhardness of approximately 600HV (0.100), and the depth of maximum hardened layer was confined to 180μm below the surface. Che-Haron s study showed that surface roughness values recorded were typically less than 1.5µm Ra when milling gamma titanium aluminide alloy at high speed. Microhardness evaluation of the subsurface indicated a hardened layer to a depth of 300µm. Initial residual stresses analysis showed that the surface contained compressive stresses more than 500MPa. Tool flank wear and milling speed have the greatest effect on residual stress. Ezugwu investigated the chip formation mechanism and surface quality as well as milling process characteristics of titanium alloy in high speed milling. The high milling speed with much more milling tooth will be beneficial to reduction of milling forces, enlarge machining stability region, depression of temperature increment, auti-fatigability as well as surface roughness. The aim of this paper is to investigate the influence of milling speed, tool rake angle and cooling method on surface quality in high-speed milling of titanium alloy. The paper will provide experimental and theoretical basis for optimizing high-speed milling parameters and controlling surface quality of titanium alloy. Consequently, it will lead to the generation of counteractive machining procedures to improve component fatigue life and machinability. 2 Experimental Procedure 2.1 Experimental Material The workpiece material used in all the experiments was an alpha-beta titanium alloy TC11. The nominal chemical composition of the workpiece material confirms to the following specification (wt.%): 6.42 Al; 3.29 Mo; 1.79 Zr; 0.23 Si; C; O; H; Fe; N; allowance Ti. The mechanical properties of tested material at room temperature and high temperature are shown in Table 1. The workpiece dimension was 80mm 50mm 30 mm. Table 1. Mechanical properties of TC11 titanium alloy Room temperature mechanical properties High temperature mechanical properties Tensile strength σ b /MPa 1128 Test temperature/ 500 Yield strength σ 0.2 /Mpa 1030 Tensile strength σ b /MPa 765 Elongation δ/% 12 Rupture strength σ 100 /MPa 667 Shrinkage ψ/% 35 Impact value а k /J cm

3 Influence of Milling Conditions on the Surface Quality in High-Speed Milling Milling Conditions All the machining experiments were carried out on a three-axis Mikron HSM 800 high speed milling center with an ITNC 530 controller. The milling tools used were K44 uncoated cemented carbide milling cutter with four teeth. The diameter of the cutter was 10 mm, the helix angle and relief angle were 30, 10, respectively. The milling mode was down milling. Three different milling speeds were selected as m/min, 471 m/min and m/min, and feed per tooth, milling depth and milling width were maintained constant at 0.05 mm/tooth, 0.2 mm and 10 mm, respectively. Meanwhile the tool rake angle used was 14 º. The water coolant was used. The three tool rake angles used were 4º, 8 º and 14 º to investigate the influences of rake angle on surface quality. Meanwhile the milling speed, feed per tooth, milling depth and milling width were maintained constant at 251 m/min, 0.05 mm/tooth, 0.2 mm and 10 mm, respectively. The coolant used was oil mist. In addition, three different cooling methods including dry, water and oil mist were selected. 2.3 Measurement of Surface Quality After each processing step, the surface residual stress was measured by X-ray diffraction. The residual stress was measured at three locations both in the feed and stepover directions, and then the average value is calculated. The surface roughness of the machined surface after each test was measured using a contact type profilometer instrument (Taylor Hobson Form Talysurf 120). The evaluation length was set at 5.6 mm and the sampling length was fixed at 0.8 mm. The instrument was calibrated using a standard calibration block prior to the measurements. The surface roughness was measured by positioning the stylus in the feed direction. The surface roughness was taken at five locations and repeated twice at each point on the face of the machined surface, and then the average value is gained. 3 Results and Discussion 3.1 Influence of Milling Speed on Surface Quality Fig. 1 shows the influence of milling speed on surface residual stresses. It is observed that all surface residual stresses are compressive, and increasing the milling speed tends to decrease the compressive residual stresses on the surface in both directions. The results show the trend of the residual stresses is from compression to tensile with the milling speed increasing. This trend is expected because as milling speed increases, machining becomes more adiabatic, so the temperature rise softens the metal and thus reducing the milling forces. Fig. 2 shows the influence of milling speed on surface roughness. It can be seen that an increase of milling speed causes an increase of surface roughness. This was probably due to rapid tool wear at the milling edge closer to the nose.

4 426 X. Shen, L. Zhang, and C. Ren Milling speed v c /m min Residual stress/mpa σ x σ y -400 Fig. 1. The influence of milling speed on residual stress Surface roughness Ra/μm Milling speed v c /m min -1 Fig. 2. The influence of milling speed on surface roughness 3.2 Influence of Rake Angle on Surface Quality Fig. 3 shows the influence of rake angle on surface residual stresses. It is observed that all the surface residual stresses are compressive, and the magnitude of the compressive stresses increased on the surface in both directions with the rake angle increasing. Fig. 4 shows the influence of rake angle on surface roughness. It can be seen that the surface roughness decreases slightly when the rake angle is changed from 4 º to 8 º. However, the surface roughness increases remarkably when the rake angle is increased from 8 º to 14 º.

5 Influence of Milling Conditions on the Surface Quality in High-Speed Milling Rake angle γ /degree σ x σ y Residual stress/mpa Fig. 3. The influence of rake angle on residual stress 0.8 Surface roughness Ra/μm Rake angle γ/degree Fig. 4. The influence of rake angle on surface roughness 3.3 Influence of Cooling Method on Surface Quality Fig. 5 shows the influence of cooling methods on surface residual stresses. It is observed that all the surface residual stresses are compressive, and the highest compressive stresses on the surface in both directions appeared under dry milling condition. The magnitude of the highest compressive stress is about 350 MPa in feed direction. The lowest compressive stresses on the surface appeared in both directions under water cooling condition. Fig. 6 shows the influence of cooling methods on surface roughness. It can be seen that the highest surface roughness appeared under oil mist cooling condition. The lowest surface roughness appeared under water cooling condition, and the value of that surface roughness is only about 0.271μm. By comprehensive consideration of residual stresses and surface roughness, the best cooling method is water cooling.

6 428 X. Shen, L. Zhang, and C. Ren 0 Dry Water Oil mist Residual stress/mpa σ x -350 σ y Fig. 5. The influence of cooling method on residual stress Surface roughness Ra/μm Dry Water Oil mist Fig. 6. The influence of cooling method on surface roughness 4 Conclusion Experimental investigation of the influence of milling conditions, involving milling speed, tool rake angle and cooling methods, on the surface quality in terms of residual stress and surface roughness in high-speed milling titanium alloy with uncoated cemented carbide tool, were carried out. The results show the milling speed, rake angle and cooling method have important influence on residual stress and surface roughness. Compressive residual stresses are higher in the feed direction than in the stepover direction. The best surface quality can be obtained when the rake angle is 8º. Water cooling is the best cooling method. Acknowledgements. This project is supported by Scientific Research Fund of Hunan Provincial Education Department (No. 10C0113) and 2010 College Research Project of Hunan Industry Polytechnic Grant (No. GYKYZ201004).

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