A Study of Hole Drilling on Stainless Steel AISI 431 by EDM Using Brass Tube Electrode

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1 2011 International Transaction Journal of Engineering, Management, & Applied Sciences & Technologies. International Transaction Journal of Engineering, Management, & Applied Sciences & Technologies A Study of Hole Drilling on Stainless Steel AISI 431 by EDM Using Brass Tube Electrode Pichai Janmanee a, and Apiwat Muttamara a* a Department of Industrial Engineering Faculty of Engineering, Thammasat University, THAILAND A R T I C L E I N F O Article history: Received 23 August 2011 Received in revised form 23 September 2011 Accepted 27 September 2011 Available online 27 September 2011 Keywords: EDM, AISI 431, Electrode, Brass, Taper A B S T R A C T When a depth hole is drilled by EDM, taper is occurred which is not desired in the process. This research was focused on influence of EDM parameters on material removal rate (MRR), electrode wear rate (EWR) and tapered hole of martensitic stainless steel AISI 431. The considered factors consist of electrical current, on-time, duty factor, water pressure and servo rate. The experimental results reveal that MRR increases when increasing of servo rate. The taper of hole increases with increasing of electrical current and servo rate. However, it is reverse proportion to water pressure and duty factor International Transaction Journal of Engineering, Management, & Applied Sciences & Technologies. 1 Introduction Nowadays, stainless steel AISI 431 is a popular material used in many industries such as water pump, kitchenware, and fittings manufacturing industries(jeong and Min, 2007). Because of its durability to environment. However, as drilling a hole on stainless steel is difficult due to its properties. Therefore, various meso or micro machining methods have been introduced, such as machining, ultrasonic, electrochemical, laser, electrical discharge machining. Especially an electrical discharge machining (EDM) is one of non-contact thermal process, which has advantages such as high-precision machining of conductive materials and 471

2 insulating materials regardless of material hardness, (Muttamara et al, 2009&2010). However, the important type of defect from hole drilling during EDM process is hole taper and hole accuracy (Ali et al.2009). Therefore, in order to improve hole quality, Sharma et al.(2002) and Bilgi et al.(2007) investigated optimum values of the process parameters experimentally such as voltage, feed rate, pulse on time, duty cycle, and the length of un insulate tool. Figure 1: Characteristic of taper on EDM In the EDM sparking hole drilling process, the viscous resistance in the narrow discharge gap causes difficulty in the removal of debris and bubbles from the working area, abnormal discharges are occurred and resulting in extensive electrode wear (Yu et al. 2009). Moreover, the debris moved by pressure flow of dielectric fluid make taper on the workpiece. The hole taper of workpiece from EDM process, which as shows in Figure 1, can be calculated by equation (1). tan (1) Where D is hole entrances, d is hole exits, and L is hole length. This research focuses on influence of machining variables affecting material removal rate (MRR), electrode wear rate (EWR), and taper on martensitic stainless steel AISI Experimental Procedure The experiments were carried out on 35 mm of hole depth of martensitic stainless steel AISI 431with ZNC-EDM model JM325DZ manufactured by Joemars company, as shown in Figure2(a). The EDM was carried out on meso-scale machining with a brass tube electrode as 472 Pichai Janmanee, and Apiwat Muttamara

outside diameter of 1 mm, inner hole diameter of 0.

Principle of tube electrode drilling is shown in Figure 2(c).

2004). The chemical compositions and essential physical properties are shown in Table 1 and Table 2, respectively.

3 outside diameter of 1 mm, inner hole diameter of 0.3 mm for flushing by dielectric fluid to remove wear debris and heat ventilation size as shown in Figure 2(b). Principle of tube electrode drilling is shown in Figure 2(c). The experiments were carried out in the de-ionised, because of the advantage of material removal rate (Diver et al. 2004). The chemical compositions and essential physical properties are shown in Table 1 and Table 2, respectively. Table 3 shows the physical properties of an electrode. The experiment conditions are contained in Table 4. (a) EDM super drill model JM325DZ (b) Brass (Cu-Zn40) tube electrode (c) ZNC-EDM drilling process Figure 2: Experimental EDM. 473

4 Table 1: Chemical compositions of stainless steel AISI 431. Element C Si Mn Cr Ni S P Weight (%) Table 2: Essential physical properties of stainless steel AISI 431. Properties Value Condition Temp. ( o C) Density(x1000 Kg/m 3 ) Elastic modulus(gpa) Tensile strength (MPa) Thermal conductivity(w/m K) Electrical resistivity(10-9 Ω m ) Table 3: Physical properties of brass tube electrode. Properties Cu-Zn (60-40) Melting point ( o C) 910 Density (g/cm 3 ) g / cm 3 Electrical conductivity(i.a.c.s% at 20 o C) 78~85 Hardness(HRB) 93 Elastic modulus (GPa) 648 Table 4: Experimental conditions. Parameters Values Polarity (Electrode) + Current (A) 1-9 On-time (μs) 10,30,50,90 Duty factor (%) Servo rate (mm/sec) 2-10 Water pressure (Kg/cm 2 ) Spindle speed (rpm) 100 Dielectric fluid De-ionised water 3 Results and Discussion As for the experiments were conducted under five process factors such as ; electrical current, on-time, duty factor, water pressure and servo rate, the data were analyzed as follows: 474 Pichai Janmanee, and Apiwat Muttamara

5 3.1 Analysis of Water Pressure The experiments were conducted by varying the water pressure 20, 30, 40, 60 kg/cm 2 and fixing parameter of current at 9 A, on-time at 10 μs, duty factor at 75 %, and servo rate at 2 mm/ sec. High water pressure has influence to cutting process makes gain of the drilling speed. It increases electrode wearing ratio but reduces taper of drilling wall as shown in Figure3(a-c). Because high water pressure make debris dispose out from clearance space between a workpiece and electrode surfaces. Consequently, complete spark often occurred and thus the material removal rate increased (Diver et al. 2004). (a) Effect of water pressure to MRR (b) Effect of water pressure to EWR (c) Effect of water pressure to taper Figure 3: Variations of water pressure with meso-hole results. 3.2 Analysis of current In this experiment, we consider of each parameter by varying current 1, 2, 3, 5, 7, 9A and fixing of parameter of on-time at 10μs, duty factor at 75%, servo rate at 2 mm/sec, and water pressure at 40kg/cm 2. As the results in Figure 4(a), it indicates that the increase of material removal rate resulted from the raise in current. High current increases material removal rate ratio and electrode wear ratio as shown in Figure 4(b) (Han et al. 2004). From Figure 4(c), 475

6 taper of drilled hole is increased because the discharges produce thermal energy according to an electrical current (Fukuzawa et al. 2004). As estimation of the current increasing while the cross section is still constant, it makes gain of the density of current per cross section area (Sen and Shan, 2005), (Guitrau,1997). (a) Effect of current to MRR (b)effect of current to EWR (c) Effect of current to taper Figure 4: Variations of current with meso-hole results. 3.3 Analysis of servo rate In this experiment, we consider factor of servo rate 2, 4, 6, 8, 10 mm/sec and fix parameter of current 9 A, on-time at 10 μs, duty factor at 75 %, and water pressure at 40 kg/cm 2. As shown in Figure 5(a), it was found that the increasing of servo rate lead to decreasing of MRR because the spark requires an appropriate gap. If servo rate is set up over the required value, narrow gap will be occurred thus the spark is incomplete or short circuit (Choi and Kim, 1998), (Han et al. 2004). When servo rate increases, electrode wear and taper increases as shown in Figure 5(b-c). It can be explained that EDM sparking on servo rate in the narrow discharge gap changes the contact angle of hole s wall and the edge of electrode, observed in Figure 5(c). 476 Pichai Janmanee, and Apiwat Muttamara

7 Figure 5(d) shows the phenomena of EDM drilling on shallow and deep holes (Yu et al. 2009). 3.4 Analysis of Duty Factor and On Time The duty factor, which means the ratio between pulse duration and pulse cycle time exerts an important role on the performance of EDM as shown in Figure6 (a). In this experiment, we consideration of each parameters by varying duty factor 10, 15, 20, 35, 45, 60, 75, 90 %, on time 10, 30, 50, 90 μs, and fixing parameters of current at 9 A, servo rate at 2 mm/sec, and water pressure at 40 kg/cm 2, respectively. As can be seen in Figure6 (b-c), the results were found that the each on-time was suitable for only a specific duty factor. MRR increases with increasing of duty factor of each on-time. However, increasing of duty factor leads to decreasing of EWR and taper. Moreover, if increasing of on-time less than 30 µs, it still affects to MRR increases. At 75 % of duty factor for on-time 30 and 50 µs, they give same values of MRR. However, these parameters results in low EWR and taper. (a) Effect of servo rate with MRR (b) Effect of servo rate with EWR (c) Effect of servo rate with taper (d) Edge wear influence to high servo rate Figure 5: Variations of servo rate with meso-hole results. 477

8 (a) Relations of duty factor and cycle time (b) Effect of duty factor to MRR (c) Effect of duty factor to EWR (d) Effect of duty factor to taper Figure 6: Variations of duty factor with meso-hole results. 478 Pichai Janmanee, and Apiwat Muttamara

Figure 7 shows impressions of the result from the experiment test with current at 9A, on-time at 10μs, duty factor at 75%, servo rate at 2 mm/sec,

08 mm and 0.97 mm as shown in Figure 7(a). Figure 7(b) shows result meso-hole machined exit.

(a) Hole entrance (b) Hole exit Figure 7: Characteristic of meso-hole and electrode results. Figure 8: SEM image of section meso-hole.

9 Figure 7 shows impressions of the result from the experiment test with current at 9A, on-time at 10μs, duty factor at 75%, servo rate at 2 mm/sec, and water pressure at 40 kg/cm 2, respectively. It shows the feature of taper could be achieved. Diameter at electrode entry was between 1.08 mm and 0.97 mm as shown in Figure 7(a). Figure 7(b) shows result meso-hole machined exit. Figure 8 shows an image of a section meso-hole machined using the same condition. (a) Hole entrance (b) Hole exit Figure 7: Characteristic of meso-hole and electrode results. Figure 8: SEM image of section meso-hole. 4 Conclusion As the results of the material removal rate, electrode wear, taper in the drill hole meso-scale by brass tube electrode, it can conclude that: 1. The MRR is lower when increasing of servo rate. 2. The taper of hole increases with increasing of electrical current and servo rate. 479

10 However, it is reverse proportion to water pressure and duty factor. 5 Acknowledgement This work was supported by the National Research University Project of Thailand Office of Higher Education Commission and the National Research Council of Thailand (NRCT). 6 References Ali S., Hinduja S., Atkinson J., Pandya M. (2009). Shaped tube electrochemical drilling of good quality holes. CIRP Annals - Manufacturing Technology, 58, Bilgi D.S., Jain V.K., Shekhar R., Kulkarni A. (2007). Hole quality and inter-electrode gap dynamics during pulse current electrochemical deep hole drilling. International Journal of Advanced Manufacturing Technology, 34, Choi I-H.,Kim J-D. (1998). A study of the characteristics of the electrochemical deburring of a governor-shaft cross hole. Journal of Material Processing Technology, 75, Diver C., Atkinson J., Helml H.J., Li L. (2004). Micro-EDM drilling of tapered hole for industrial applications. Journal of Material Processing Technology, 149, Fukuzawa Y., Mohri N., Tani T. and Muttamara A. (2004). Electrical discharge machining properties of noble crystals. Journal of Materials Processing Technology, 149(1-3), Guitrau E.B. (1997). The EDM Handbook. Handser Gardner publication Cincinnati, Han F., Wachi S., Kunieda M. (2004). Improvement of machining characteristics of micro- EDM using transistor type isopulse generator and servo feed control. Precision Engineering, 28, Jeong Y. H., Min B. K. (2007). Geometry prediction of EDM-drilled holes and tool electrode shapes of micro-edm process using simulation. International Journal of Machine Tool and Manufacture, 47, Muttamara A., Fukuzawa Y., Mohri N., and Tani T. (2009). Effect of electrode Materials on EDM of Alumina. Journal of Materials Processing Technology, 209, Muttamara A., Janmanee P., and Fukuzawa Y.(2010). A Study of Micro EDM on Silicon Nitride Using Electrode Materials. International Transaction Journal of Engineering, Management, & Applied Sciences & Technologies, 1(1), 1-7. Rajasekhar A., Madhusudhan Reddy G., Mohandas T. (2009). V.S.R. Murti. Influence of austenitizing temperature on microstructure and mechanical properties of AISI 431 martensitic stainless steel electron beam welds. Materials and Design, 30, Pichai Janmanee, and Apiwat Muttamara

11 Sen M., Shan H.S.(2005). A review of electrochemical macro- to micro-hole drilling processes. International Journal of Machine Tool and Manufacture,45, Sharma S., Jain V.K., Shekhar R. (2002). Electrochemical drilling of inconel super alloy with acidified NaCl electrolyte. International Journal of Advanced Manufacturing Technology, 19, Yu Z.Y., Zhang Y., Li J., Luan J., Zhao F., Guo D. (2009). High aspect ratio micro-hole drilling aided with ultrasonic vibration and planetary movement of electrode by micro-edm. CIRP Annals - Manufacturing Technology, 58, Pichai Janmanee is an Assistant Professor in the Department of Industrial Engineering at Rajamangala University of Technology Krungthep (RMUTK), Bangkok, Thailand. He received a BE in Industrial Engineering from Rajamangala University of Technology and ME in Production Engineering from King s Mongkut University of Technology North Bangkok(KMUT NB). His areas of research include machining and electrical discharge machining of hard materials.9 Dr.Apiwat Muttamara is an Assistant Professor of Department of Industrial Engineering at Thammasat University. He received his B.Eng. from Kasetsart University and the D.Eng. in Materials Science from Nagaoka University of Technology, Japan. His areas of research include machining and electrical discharge machining of hard materials. Peer Review: This article has been internationally peer-reviewed and accepted for publication according to the guidelines given at the journal s website. 481

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