Sebuah Kajian Pustaka: JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 23 Forces Perspective of Drillability of Titanium Alloy 6Al-2Sn-4Zr-6Mo Mahros Darsina , Tim Pasangb, Zhan Chenc a Department of Mechanical Engineering, University of Jember Jalan Kalimantan 37 Jember 68121, Indonesia +62 331 410243 e-mail: mahros.teknik@unej.ac.id b,c Department of Mechanical Engineering, Auckland University of Technology 55 Wellesley Street, Auckland 1010, New Zealand +64 9 921 9999 e-mail: timotius.pasang@aut.ac.nz & zhan.chen@aut.ac.nz Abstract This paper concerns on drillability of Ti-6Al-2Sn-4Zr-6Mo (Ti-6246) from the point view of thrust force (Fz) & torque (Mz) using a TiAlN CVD coated carbide tool. The condition of the material was varied with three different heat treatments. Whereas, the machining parameters were varied in cutting speed, feed rate and cooling application method. Taguchi method L-18 was employed to design the experiments. Both type of forces, thrust force and torque, were measured using a Kistler dynamometer, and the data were analyzed using a Minitab 17 software. The thrust force was influenced by the cutting speed 24%, depth of drilling 21%, heat treatment 13%, and feed rate 11%. The torque was influenced predominantly by feed rate up to 94%. Coolant application has no effect on reducing both thrust force as well as torque. Keywords: Ti-6246; drillability; Taguchi method; force; torque 1. INTRODUCTION Drillability term is derived from machinability, which means how easy the material be drilled with a drill bit. This paper discusses drillability of titanium alloy 6Al-2Sn-4Zr-6Mo when being drilled with TiAlN-coated carbide from forces point of view. Cutting forces is a measure of machinability. Normally, it is desired the lower cutting forces. In drilling, an elevated cutting force can arouse the vibration of the spindle axis, consequently resulting in low quality of drilled surface. It may also cause premature devastation of drills and lessen the tool life. An elevated temperature at the interface of tool-workpiece may be produced when the torque was increased due to friction between tool and workpiece [1]. There is a close connection between forces that work during drilling with the surface quality [2]. Therefore, it is interesting to study drillability from the forces point of view. In drilling, there are two different motions: cutting speed and feed rate. Cutting speed makes the tool cut the workpiece only once of full rotation and feed rate provides the continuity of drilling process. Torque is the force that make the drill able to rotate along vertical axis; it relates to cutting speed. While, thrust force is the force that make the drill move along vertical axis (Z-axis) and it relates to feed rate. Titanium alloy 6Al-2Sn-4Zr-6Mo (Ti-6246) is among alpha + beta titanium alloys. It has excellent corrosion ratio than the most famous titanium alloy, Ti-6Al-4V, therefore, it is potentially applied for sea water medium and high chemical influent working area also for deep & sour-well applications. It is heat treatable and designed to combine the strength properties at long term elevated temperature of Ti-6Al-2Sn-4Zr-2Mo-0.08 Si with the high developed short term strength properties of fully hardened alpha + beta alloy. Hence, it possibly for forging parts which receive withstand high at intermediate temperature such as turbine blades, compressor disks and airframe components. mailto:mahros.teknik@unej.ac.id mailto:timotius.pasang@aut.ac.nz mailto:zhan.chen@aut.ac.nz JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 24 Some previous researchers have observed relation of forces that works during machining titanium alloys and the machining parameters. Cutting force (Fc) and feed force (Fk) have been discussed on machining three kind of titanium alloys Ti-6Al4V, Ti-54M and Ti-10.2.3 with variation in machining parameters. They concluded that feed rate was the most influential factor which affects the forces [3]. Laser assisted machining (LAM) has reduced the forces up to maximum 15% in compare to conventional machining of Ti-6Cr- 5Mo-5V-4Al [4]. There is also some published paper on drilling titanium which focus on cutting forces. The main forces that work drilling (thrust force and torque) were greatly affected by the type of coolant used [5,6]. Thrust force decreased as cutting speed increased but a lower torque values were obtained at the higher cutting speed applied [7]. Other researchers concerned on effect of drilling technique on forces as reviewed on Sharif et al. [6]. Literatures studied show that there is no published paper discussing drilliablity of titanium alloy 6Al-2Sn-4Zr-6Mo especially from forces point view. Therefore, this study worth to value. 2. METHODS The material used were titanium alloy 6246 in form of 56 mm - rod with the nominal chemical composition in compare to the result of OES (optical emission spectroscopy) is presented in Table 2.1. Table 2.1 Taguchi method L-18 design experiments, the forces and S/R ratio Work Material Ti- 6Al-2Sn-4Zr-6Mo Alloying elements, wt.% Impurity limits, wt.% max Al Sn Zr Mo N C H Fe O From literature [8] 8 6 2 4 6 0.04 0.04 0.0125 0.15 0.15 OES Test Result 6.69 2.18 4.09 5.85 0.012 0.062 In advance of drilling, the workpiece was machined to shape rectangular blocks according to the depth of the proposed drilling and to fit with the fixture, width x length = 25 x 25 mm; the heights were varied as 15, 35 and 50 mm according to the proposed depth of drilling 10, 30 and 45 mm respectively. The workpiece was then fastened in a fixture. The fixture itself was mounted on a Kistler piezoelectric dynamometer to measure the forces that worked during drilling. The recorded forces were displayed and recorded in a PC outside the CNC. Four forces were recorded, i.e. Fx, Fy, Fz and Mz. Five parameters were varied to get the optimum value of forces. Three parameters from drilling ones: cutting force, feed rate, depth of drilling. One variation made from the block being drilled: heat treatments. Another variation came from the environment, i.e. whether drilling with or without coolant. Each variation of parameters has 3 levels except for the coolant application method, only two levels. The experiments were carried out according to Taguchi method L-18 to reduce the number of experiments [9,10] 9,10. The recorded forces then would be analysed with Minitab 17 and analysis of variance (ANOVA) for checking the significance of each parameters. The variations or level of each parameters is presented in Table 2.2. The initial of AR in heat treatment row means that the material was drilled as-received condition. While HT1 represent heat treatment at 870oC for 3 hours following by furnace cooling and HT2 denotes heat treatment at 870oC for 3 hours followed by water quenching. Both heat treatments were chosen based on the preliminary research which result in decreasing the hardness compare to as-received. The value of the hardness of AR, HT1 and HT2 was 318, 311 and 289 HV respectively. Lessening the hardness hopefully result in easier to machine. The coolant (coolant β€˜on’) is a synthetic coolant to water ratio 1:10 of HOCUT 795B made by Houghton Australia with flood method at flow rate of 0.02 l/s through a nozzle. The low and high level of both cutting speed and feed rate were chosen according to the specification of the drill manufacturer for drilling titanium. Hence, the variation of depth of drilling was made because the typical application of this material is for thick parts. The drill insert was denoted as IC908 Sumocham of TiAlN PVD coated carbide. JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 25 Table 2.2 Variation of drilling parameters and their level Machining Parameters Level Low Medium High Coolant Off - On Heat Treatment AR HT1 HT2 Depth of drilling (mm) 10 30 45 Cutting speed (m/min) 27 35 50 Feed rate (mm/rev) 0.08 0.11 0.15 3. RESULTS AND DISCUSSION 3.1 Results A photo of a moment after drilling showing of the drill and the block as well as the dynamometer is presented in Fig.3.1. The recorded forces (Fx, Fy, Fz and Mz) then being plotted as a graph using Microsoft Excel program. The average of Fx and Fy were around zero value, therefore both were abandoned in further analysis and only Fz (thrust force) and Mz (torque around the vertical axis) were considered. For analysis in Minitab 17, the forces were sorted out from only at the steady state then take the average as illustrated in Fig. 3.2. The steady state indicating that the tool was fully engage in drilling process [11] 11. The increase of forces in the graph may relate to tool deterioration. Fig. 3.1 Images showing (a) a moment after a complete drilling, (b) appearance of forces measurement in a PC monitor in the CCW direction from the upper right: Fy, Fx, Fz, and Mz JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 26 The completed average of thrust force (Fz) and torque (Mz) is presented in Table 3.1 along the calculated signal to noise ratio (S/N ratio). The calculation of S/N ratio was based on minimization because the smaller forces are preferable, as the following formula3: [ 𝑆 𝑁 ] 𝐿𝐡 = βˆ’10 π‘™π‘œπ‘” ( 1 𝑛 βˆ‘ 𝑦𝑖 2𝑛 𝑛=1 ) (1) Fig. 3.2. Illustration how the average thrust force and torque were calculated from the steady state condition Table 3.1 Taguchi method L-18 design experiments, the forces and S/R ratio Exp Control variables Average of responses S/N ratio (dB) Coo- lant HT h (mm) Vc (m/min) Fr (mm/rev) Fz (N) Mz (N.cm) Fz Mz 1 No AR 10 27 0.08 2638 147 -68.43 -43.35 2 No AR 30 35 0.11 5762 165 -75.21 -44.33 3 No AR 45 50 0.15 3567 227 -71.05 -47.10 4 No HT1 10 27 0.11 2679 171 -68.56 -44.66 5 No HT1 30 35 0.15 6701 215 -76.52 -46.64 6 No HT1 45 50 0.08 2234 135 -66.98 -42.63 7 No HT2 10 35 0.08 2245 139 -67.02 -42.86 8 No HT2 30 50 0.11 6761 164 -76.60 -44.31 9 No HT2 45 27 0.15 2744 200 -68.77 -46.01 10 Yes AR 10 50 0.15 3895 235 -71.81 -47.42 11 Yes AR 30 27 0.08 5256 143 -74.41 -43.12 12 Yes AR 45 35 0.11 7231 171 -77.18 -44.67 13 Yes HT1 10 35 0.15 3064 210 -69.73 -46.44 14 Yes HT1 30 50 0.08 2260 137 -67.08 -42.72 15 Yes HT1 45 27 0.11 2735 167 -68.74 -44.43 16 Yes HT2 10 50 0.11 2761 180 -68.82 -45.11 17 Yes HT2 30 27 0.15 2037 207 -66.18 -46.32 18 Yes HT2 45 35 0.08 5225 128 -74.36 -42.12 3.2 Discussion ANOVA analyses was used to detect which factors affecting the forces. A confidence level 95% (or significance level of Ξ± = 0.05) was used to carry out the critical analysis. The ANOVA of thrust force and torque were presented in Table 3.2 & Table 3.3. The factor with the P-values less than 0.05 means statistically significant at 95% confidence level and vice JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 27 versa [3]. Whereas, the larger the F-value for certain parameter the bigger the effect on the characteristic of performance due to change in that process parameter [3]. Table 3.2 Analysis of variance for thrust force Source DF Adj SS Adj MS F-Value P-Value Contribution, % Coolant 1 41762 41762 0.02 0.892 0 HT 2 6829219 3414609 1.62 0.257 13 h 2 11065125 5532562 2.62 0.133 21 Vc 2 13077118 6538559 3.10 0.101 24 Fr 2 5823134 2911567 1.38 0.306 11 Error 8 16896812 2112102 31 Total 17 53733170 Table 3.3 Analysis of variance for torque Source DF Adj SS Adj MS F-Value P-Value Contribution, % Coolant 1 13.3 13.28 0.46 0.518 0 HT 2 446.8 223.38 7.69 0.014 2 h 2 315.5 157.75 5.43 0.032 2 Vc 2 249.4 124.69 4.29 0.054 1 Fr 2 18164.6 9082.29 312.82 0 94 Error 8 232.3 29.03 1 Total 17 19421.7 From Table 3.2 it is clear that each factor contributed in affecting the thrust force in order are 24% by cutting speed, 21% by depth of drilling, 13% by heat treatment and 11% by feed rate. In contrast, torque was predominantly affected feed rate up to 94% (Table 3.3). While other machining parameters influence cumulatively about 6% toward the torque. The result is in accordance with what was found by Khanna in Davim [12] that feed rate contribute 97.2% on cutting force. Some previous researchers in drilling Al7075 using Response Surface Methodology (RSM) found that increase cutting speed did not result in increase of Fz and Mz (Kyratsis et al. in Davim [12]), while increase feed rate and tool diameter would increase both forces in drilling. The difference result may due to difference material used. Another research on drilling on titanium using RSM design experiment shown that cutting force and feed rate both were significantly affecting thrust force and torque [13]. It is also evidence that cutting fluid does not play a role in affecting both forces. It may due to the method of applying coolant in this experiment – an external coolant supply - was not effective. The coolant could not reach the tool-chips interface therefore there was no different in forces whether drilling with or without coolant application. A compressive flood coolant application might help to reduce the forces during drilling as claimed by Rahim & Sasahara [5]. There was a difference up to 1000 N of thrust force between MQL synthetic ester and flood coolant while torque difference up to 11 N.m. An important note in interpreting of experimental analysis, if the percent contribution due to error (unknown and uncontrolled factors) is low, 15% or less, then it is assumed that no important factors were omitted from the experiments. If it is high value, 50% or more, then some important factors were definitely omitted, conditions were not precisely controlled, or measurement error was excessive [14]. In case of ANOVA result of thrust force, the error is 31%, it means some factors that may influenced the thrust force are omitted from the experiments. However, the error is less than 50% or it is still acceptable. JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 28 The next step is analysis to find the optimum forces that may works by varying the machining parameters. The S/N ratio of Table 3.3 and 3.4 of both thrust and torque then being plotted as shown in Fig 3.3. Signal to noise ratio indicates how the controlled parameters (signal) affecting the measured result in compare to disturbance (noise or uncontrolled parameters). Therefore, the higher S/N ratio is preferable. From Fig. 3.4 we can detect that the optimum thrust force would be achieved by choosing machining with coolant and the material being HT1 treated on drilling depth of 10 mm, cutting speed of 27 m/min and feed rate 0.08 mmm/rev. While, minimum torque would be achieved when drilling without coolant, material as HT2, depth of drilling 45 mm, cutting speed at 35 m/min and feed rate of 0.08 mm/rev. Fig. 3.3. Contribution of each factors to the thrust force (a) and to the torque (b) Fig. 3.4. Means of forces and S/N effect for each control factor; (a) thrust force, (b) torque. JEMMME, Vol.3, No. 1, May 2018 ISSN 2541-6332 e-ISSN 2548-4281 JEMMME | Journal of Energy, Mechanical, Material, and Manufacturing Engineering 29 Both forces require different level of parameters in order to achieve their minimum values. Therefore, we should smartly decide which one we should choose. As mentioned previously that application of coolant would not change significantly to thrust and torque, together with environment consideration, the drilling without coolant may be chosen. Furthermore, as feed rate predominantly affecting the torque we may abandon the level of three other factors and follow the ones which result the minimum thrust force. Thus, optimum thrust force and torque may be achieved by applying Vc of 27m/min, Fr of 0.08 mm/rev on depth of 10 mm on material at HT1 without coolant. 4. CONCLUSION Following the result and discussion, we may come to conclusion regarding drillability of Ti- 6Al-2Sn-4Zr-6Mo from the forces point of view: a. Among five parameters that varied: cutting speed, depth of drilling, heat treatment and feed rate influenced the thrust force by order in percentage as 24, 21, 13, and 11 respectively. While torque was greatly influenced by feed rate up to 94%. Applying of coolant did not contribute in reducing the drilling forces. b. The optimum drilling forces condition would be achieved when drilling with cutting speed of 27 m/min, feed rate of 0.08 m/rev on depth of only 10 mm without coolant while material should be HT1 treated. The further reduction in forces may be gained either by applying high pressure coolant or using the through coolant tool design. ACKNOWLEDGMENT We would like to thank Ministry of Research, Technology and Higher Education of Republic Indonesia through the DIKTI scholarship that financially supports the main author to pursue his PhD degree at the Auckland University of Technology. We would like also appreciate the Auckland University of Technology that support for the funding after third year passed. REFERENCES 1. Zhang, P. F.; Churi, N. J.; Pei, Z. J.; and Treadwell, C. (2008). Mechanical drilling processes for titanium alloys: a literature review. Machining Science Technology, 12(4):417-444. 2. Pirtini, M.; and Lazoglu, I. (2005). Forces and hole quality in drilling. International Journal Machine Tools & Manufacture, 45(11), 1271-1281. 3. Khanna, N; and Davim, J. P. (2015). Design-of-experiments application in machining titanium alloys for aerospace structural components. Measurement, 61, 280-290. 4. Rashid, R. A.; Sun, S.; Wang, G.; and Dargusch, M. S. (2012). 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