Experimental investigations into micro-drilling using air ... - IIT Guwahati

5th International & 26th All India Manufacturing Technology, Design and Research Conference (AIMTDR 2014) December 12th–14th, 2014, IIT Guwahati, Assam, India

Experimental investigations into micro-drilling using air assisted jet electrochemical machining Harsha Goel1, Pulak M Pandey2*, 1 Research Scholar, IIT Delhi, 110016, E-Mail:[email protected] 2* Associate Professor, IIT Delhi, 110016, E-mail:[email protected] Abstract The work describes the development of the process to drill the micro-hole with jet-electrochemical machining (JetECM) method assisted with air. The experimental set up has been fabricated and the effects of air pressure along with the other process parameters has been investigated experimentally on the process responses namely MRR and hole taper. Use of air along with the jet-improves the machining conditions during Jet-ECM. It has been observed that the flow of air pushes away the accumulated electrolyte from the area of impingement of the jet over the workpiece. Due to this, thin film flow of electrolyte in the machining area is observed which is necessary for jetelectrochemical machining. This reduces the chances of sparking occurring at the electrolyte jet and work-piece interface due to accumulation of electrolyte on work-piece surface. The coaxial air flow tends to localize the workpiece dissolution which results into more accurate holescloser to the nozzle or tool diameter. No change in MRR is observed with the assisted air supply but hole accuracy improves. Keywords: Jet-ECM, Assisted Air, MRR, Taper.

1 Introduction The growing trends have led to developments of new materials with high strength, modified microstructure, light weight, toughness and other such properties. Machining of difficult to machine materials and cutting of complex contours on such materials have been almost impossible with the existing traditional machining methods [4]. In aerospace, electronics and computer industries there is need of micro-hole drilling on the parts like turbine blades, nozzles, microcircuits, semiconductors [1]. For such applications jetelectrochemical machining (Jet-ECM) technique is gaining more preference over the other processes for the reasons that there is no heat affected zone near the work area, no cutting forces act on the tool and the workpiece, no significant tool wear, the work-piece is free from mechanical impact and residual stresses do not develop in the work-piece. The machined surface with the process is free from burrs and cracks [6]. The process is independent of material properties like strength, hardness, ductility of the material. The only requirement is that work-piece should be electrically conducting [7].The presented work focuses on microdrilling with one of the variant of ECM process namely air assisted jet-electrochemical machining. From the review of available literature of Jet-ECM it is clear that process can be used for various applications like drilling of holes of small sizes in hard and conducting materials [2]. Various attempts to study the jet-electrochemical machining process and few attempts to model the process mathematically have been

made. Schubert et al. [5] performed Jet-ECM experiments to determine the effects ofapplied voltage and machining gap on the machining accuracy and machining depth.They concluded that maximum width of the cavity was obtained with smallest working gap and higher voltages resulted in deeper cavities.Kai et al. [6]developed the method to cut the metal sheets and incorporated the coaxial flow of assist gas through the nozzle of electrolyte flow. They found out that coaxial gas flow reduced the momentum in the flow of electrolyte and shifted the hydraulic jump away from jet impingement area. Theyalso observed that the required electrolyte film flow for drilling can be obtained at lower flow rate of the electrolyte with the assist gas flow.

2 Principle of Jet-ECM The Jet-ECM process works on the principle of conventional ECM. In Jet-ECM acidic electrolyte of concentration 20-30% by weight is forced to flow through the nozzle of small diameter with the pressure (0.3-1.0 N/mm2) to form a jet and is then impinged on work piece [3].The material is removed from the work piece by localized anodic dissolution in the form of metal ions.The material removed is washed away with the flowing electrolyte. Figure 1(a) shows machining principle of Jet-ECM. As the electrolyte jet strikes the work piece sudden change in the radial electrolyte flow occurs in area away from the nozzle,surrounding the jet 222-1

Experimental investigations into micro-drilling using air assisted jet electrochemical machining

area,due to hydraulic jump phenomenonas shown in fig. 1 (a).The current density gets concentrated under the area of electrolyte jet due to the hydraulic jump [8].

Figure 1: (a) Jet-ECM without air-assistance (b) Thin film flow of electrolyte due to air assistance in Jet-ECM.

(a)

3 Experimental set up The schematic diagram of the proposed experimental work is shown in figure 3. The experimental set up fabricated for air assisted Jet-ECM process consists of high voltage DC power source, electrolyte supply system, work-piece and work-piece holding fixtures, nozzle, and air supply chamber. The high voltage DC power supply 0-600 V is used to apply potential between tool and work-piece. The electrolyte supply system includes the air compressor with pressure gauge to pressurize the electrolyte in the electrolyte tank. The electrolyte flows with required pressure though the pipe to the filter to remove any impurity to avoid clogging of the nozzle, another pressure gauge is mounted near the nozzle to minimize pressure losses in tubes and pipes and filter.

(b)

Figure 2: (a) Jet-ECM without air assistance hole diameter=180.091µm. (b) Jet-ECM with air assistance hole diameter=162.361µm. (215X).[V=100 V, C=0.75 M, Pa=0.6 kg/cm2, Pe=2.0 kg/cm2,, IEG=2.5mm] High current density concentrated at small jet area give better drilling accuracy when it strikes the workpiece area. This results in selective machining of the work-piece under the jet. In the present work the authors have tried to supply the air co-axially with the electrolyte jet as shown in fig. 1 (b).It can be observed that the co-axial air flow effectively pushes away the accumulated electrolyte on the work-piece resulting in the thin electrolyte flow required for jet-electrochemical machining so as to localize the effect of current density in the machining area. Also to find out the efficacy of the developed process the preliminary experimentation was carried out which proved that the air assisted jet electrochemical drilling resulted in the holes with smaller diameters which are closer to the jetdiameter than the holes without air assistance and the accumulated electrolyte is prevented back to contact the nozzle which minimizes the chances of sparking between the nozzle tip and work piece thus improving the machining performance of the process. Figure 2 shows the effect of assisted air on actual hole diameters.

Figure 3: Schematic diagram of air assisted Jetelectrochemical machining experimental set-up The electrolyte then flows into the nozzle and emerges out as jet through the nozzle. The nozzle through which the electrolyte jet impinges is connected to the negative terminal of the DC power supply.Work-piece is made positive [8]. The nozzle has a special attachment of air supply chamber to pass the air coaxially with the electrolyte jet onto the work-piece. The air compressor is used to supply air in the air supply chamber. Another pressure gauge with control knob is mounted near the air supply chamber to monitor the air pressure. The inter-electrode gap (IEG) between the negatively charged nozzle tip and positively charged work-piece can be adjusted with the help of Vernier scale attached with the nozzle and air supply chamber assembly. Suitable fixture is used to hold the work-piece under the jet at proper position.The machining chamber, the workpiece holding fixture, and nozzle and air supply chamber assembly as well the tubes, pipes and connectors for electrolyte and air supply are all made of 222-2


acid resistant materials to avoid corrosion due to acidic environment.

4 Selection of process parameters and process responses Based on the available literature applied voltage (V) across tool and work-piece, inter-electrode gap i .e gap between work-piece and tool (IEG), electrolyte concentration (C), electrolyte pressure (Pe) and air pressure (Pa) were selected as process parameters to predict determine the behavior of the Jet-ECM. To determine the process performance the drilled hole dimensions like entry diameter of the hole and exit diameter of the hole, time required to drill the hole, initial and final weights of the work pieces, machining current were measured and hole taper and MRR were calculated as process responses. Following formulas were used to calculate MRR and hole taper.

2

(1)

Applied Voltage (V) Electrolyte conc. (C) in M solution Electrolyte pressure (Pe) in kg/cm2 Inter electrode gap (IEG) in mm Air pressure (Pa) in kg/cm2

125

150

175

200

0.25

0.5

0.75

1

1.4

1.6

1.8

2.0

1.75

2

2.25

2.5

1

0.8

0.6

0.4

Value of MRR was then obtained from the equation 1 given above. To calculate the hole taper the entry diameter and exit diameters images were captured and the diameters were measured using Dino lite microscope camera at 215 X. The value of the taper was obtained using the equation 2.

6 Analysis of the experimental data (2)

Where, is entry diameter of drilled, is exit diameter of drilled hole. is thickness of the workpiece, is initial weight of the workpiece, is final weight of the workpiece after drilling , is drilling time, is material removal rate.

5 Design of experiments Suitable method of design of experiments has to be used to minimize the number of experiments for selected experimental conditions. In the present work for five process parameters as mentioned in the above section four levels were selected.Taguchi’s method of design the experiments have been used to plan the experiments. Table 1 gives the different parameters considered with their levels for the present work. Copper and SS-304 plates of 350 um thickness were selected as work-piece materials. Stainless steel nozzle of diameter 140 µm is selected. Diluted sulfuric acid has been selected as electrolyte.To calculate the MRR the machining time for drilling a through hole in the copper or SS-304 sheet was measured using. The weights of the work-pieces before drilling and after drilling were measured to calculate the weight loss.

Experimental data has been analyzed using analysis of variance (ANOVA). The objective if ANOVA is to find out the effects of variations of each parameter over the overall variations in the response. ANOVA was performed to analyze the obtained data. Statistical models for the process responses namely MRR and hole are obtained by regression analysis of the data. In order to determine the relationships among various process parameters and to predict responses regression analysis has been performed. The obtained models to predict MRR and taper for SS 304 and Cu are given below. The regression equations for SS 304 are, MRRcu = 0.089 + 0.00194 V - 0.122 Pe - 0.0125 Pa 0.0620 IEG + 0.218 C (3) Tapercu = 0.190 + 0.00377 V + 0.101 Pe + 0.104 Pa 0.185 IEG - 0.103 C (4) The regression equations for Cu are, MRRss 304 = 2.04 + 0.0320 V - 1.72 Pe + 0.321 Pa - 1.98 IEG + 2.69 C (5) Taperss 304= - 0.039 + 0.00296 V + 0.345 PE + 0.280 PA - 0.256 IEG - 0.344 C (6)

Table 1 Process parameters and their levels. Description of parameter

1

2

Levels 3

4

The Percentage contributions of the various factors affecting the MRR and taper in both the work-piece materials are shown in figure 4(a) and (b). From figure 222-3

Experimental investigations into micro-drilling drilling using air assisted jet electrochemical machining

% Contributions

4 difference erence in the percentage contributions of the various factors for the two different materials can be observed.

5

C

1.0

1.0

1.0

1.0

7 Results and discussions

50

7.1 Main effects plots for MRR and hole taper 0 V

Pe

Pa

IEG

C

R. Er. Cu

Parameters

SS 304

To investigate the effects of various process parameters on the process responses the experimental data is analyzed using Taguchi’s design analysis to obtain the main effects plotss of means for MRR and hole taper. Figure 5 shows the main effects plots for both the work- piece materials for MRR and hole taper.

% Contributions

(a)

100

0 V

Cu SS 304

Pe Pa IEG C R.Er Parameters

(a)

(b) Figure 4: Percentage contributions of process parameters on (a) MRR and (b) hole taper taper. The optimum set of data for maximum MRR and minimum hole taper are obtained from the main effects plots of MRR and hole taper respectively as the regression models for both the cases are linear. linear.Table 2 shows the optimum values of process parameters for maximizing imizing MRR and minimizing hole taper for Cu and SS-304 304 obtained from the main effects plots.

(b)

Table 2 Optimum process parameter value obtained for minimum taper and maximum MRR for Cu and SS-304 from main effects plot. Sr.

Paramete

No

rs

Values MRR

Hole taper

Cu

SS-304

Cu

SS-304

1

V

200

200

125

125

2

Pe

1.4

1.4

1.4

1.8

3

Pa

1.0

0.6

0.4

0.6

4

IEG

1.75

2.0

2.5

2.5

(c)

222-4


SS 304 Cu

2.5

MRR (gms/min)

2.0

(d)

1.5

1.0

0.5

0.0

Figure 5: Main effects plots for hole taper in (a) Cu (b) SS 304 and for MRR in (c) Cu (d) SS 304

1.4

1.5

1.6

1.8

1.9

2.0

2

(b)

7.2 Effects of process parameters SS 304 Cu

2.2 2.0 1.8 1.6

MRR (gms/min)

Figure 6and 7show the graphs of effects of various parameters on the MRR and hole taper which are plotted using the obtained regression equations. From figure 6,it can be seen that the MRR increases with increase in the applied voltage as well as electrolyte pressure for both the work-piece materials and it reduces with increase in the inter-electrode gap and electrolyte concentrations. As the voltage increases the current density under the jet area increases which increases the rate of metal ion removal from the anode work-piece. The decrease in MRR with increase in electrolyte pressure may be due less time available for electrolyte jet and work-piece electrochemical reactions although at higher pressures the removal of dissolved material from the machining area is better.The increase in concentration of the electrolyte increases the number of ions in the electrolyte solution which improves the rate of electrochemical reactions and hence increases MRR. The increase in the inter-electrode gap reduces the machining current between the nozzle and workpiece due to which the MRR decreases.

1.4 1.2 1.0 0.8 0.6 0.4 0.2 0.4

0.5

0.6

0.7

0.8

0.9

1.0

2

Assisted air pressure (kg/cm )

(c) SS 304 Cu

2.5

2.0

3.5

MRR (gms/min)

SS 304 Cu

3.0

2.5

MRR (gms/min)

1.7

Electrolyte pressure (kg/cm )

2.0

1.5

1.0

1.5

0.5 1.0

0.5

0.0 1.7

0.0 120

130

140

150

160

170

180

190

200

210

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

Inter electrode gap (mm)

Voltage (V)

(a)

(d)

222-5

Experimental investigations into micro-drilling using air assisted jet electrochemical machining

3.5

0.80

SS 304 Cu

3.0

0.70

Hole taper (degrees)

2.5

MRR (gms/min)

SS 304 Cu

0.75

2.0

1.5

1.0

0.5

0.65 0.60 0.55 0.50 0.45 0.40

0.0 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

120

n

Electrolyte concentration (M sol )

130

140

150

(e)

170

180

190

200

210

(a)

Figure 6: Effects of (a) Voltage (b) Electrolyte concentration (c) Assisted air pressure (d) Interelectrode gap (e) Electrolyte concentration on MRR for SS-304 and Cu.

0.65

SS 304 Cu


0.60

0.55

0.50

0.45

0.40 0.4

0.5

0.6

0.7

0.8

0.9

1.0

2

Assited air pressure (kg/cm )

(b) 0.65

SS 304 Cu 0.60


Figure 7 shows the effects of various parameters on the hole taper for SS-304 and Cu work material. From figure 7,it can be seen that hole taper increases with increase in applied voltage, electrolyte pressure and air pressure. Hole taper is found to decrease with increase in inter-electrode gap and electrolyte concentration. The increase in applied voltage increases the current density with which the rate of electrochemical reactions increases and the electrolyte in contact at the upper thin micro layer of the work-piece remain for longer time which results in the larger entry diameters of the hole than the exit diameters. The machining time for SS-304 is more as compared to Cu resulting in longer contact time of jet with work-piece in case of SS-304, so taper in SS-304 is greater than Cu. With the increase in pressure of the electrolyte jet it has been observed that jet reboundsback from the machining area, which causes the stray cutting of the hole during the drilling process. Due to this entry diameters of the drilled holesare larger than the exit diameters thus increasing the hole taper. Increase in inter-electrode gap reduces the machining current which reduces the overall rate of electrochemical reactions reducing the taper and MRR. The increase in electrolyte concentration is found to reduce the taper; the reason for this may be accounted as with the increase in concentration of ions the rate of electrochemical reactions increases which reduces the machining time as well as contact time of the electrochemical jet with the work-piece thus resulting in reduced taper.

160

Voltage (V)

0.55

0.50

0.45

0.40

1.4

1.5

1.6

1.7

1.8

1.9

2.0

2

Electrolyte pressure (kg/cm )

(c)

222-6


Acknowledgements

0.66

The authors gratefully acknowledge the financial support providedby Department of Science and Technology, (DST), New Delhi,India for carrying out this work.

SS 304 Cu

0.64 0.62 0.60


0.58 0.56 0.54 0.52

References 1. H. Zhang, J.W. Xu and J.S. Zhao, (2010), Modeling

0.50 0.48 0.46 0.44 0.42 0.40 0.38 0.36 1.7

1.8

1.9

2.0

2.1

2.2

2.3

2.4

2.5

2.6

2.

Inter electrode gap (mm)

3.

(d) 0.65

4. Hole taper (degrees)

0.60

0.55

5. 0.50

0.45

0.40

SS 304 Cu

0.35

6. 0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

1.1

n

Electrolyte Concentration (M sol )

(e)

7. 8.

Figure 7: Effects of (a) Voltage (b) Electrolyte concentration (c) Assisted air pressure (d) Interelectrode gap (e) Electrolyte concentration on hole taper for SS 304 and Cu.

and experimental investigation of jet electrolytic drilling, Key Engineering Materials,vol. 458, pp.277–282. J.W. Mcgeouh, (1988), Micromachining of engineering materials, Chapman and Hall. J. Kozak, K. P. Rajurkar, R. Balkrishna(1996), Study of electrochemical jet, Transactions of the ASME, vol.118, pp.490–498. P.C. Pandey and H.S. Shan, (1995) Modern machining processes, TMH Publishing Company Ltd., New Delhi. Schubert Andreas,Hackert-Oschätzchen, M.Meichsner, Gunnar,Zinecker, Mike,Martin, A. (2011), Evaluation of the influence of the electric potential in jet electrochemical machining, 7th International Symposium on Electrochemical Machining Technology, Vol. 7, pp 47-54. Shoya Kai, Haruo Saia, Masanori Kunieda, Heikan Izumi(2012), Study on electrolyte jet cutting, Procedia CIRP, vol. 1, pp. 644 –649 V.K. Jain, (2004), Advanced machining processes, Allied Publisher Pvt. Ltd., New Delhi. W. Natsu, S. Ooshiro, M. Kunieda. (2008), Research on generation of three-dimensional surface with micro-electrolyte jet machining, CIRP Journal of Manufacturing Science and Technology, vol.1, pp. 27–34.

8. Conclusions In the present work, experimental set up for air assisted Jet-ECM has been developed to investigate the effect of coaxial air flow along the electrolyte jet. It is concluded that there is significant improvement in the dimensional accuracy of the drilled micro-holes in terms of closeness of drilled hole diameter to the tool diameter.Effects of process parameters namely applied voltage, electrolyte pressure; electrolyte concentration and inter-electrode gap along with air pressure have been investigated on MRR and hole taper.From the analysis of experimental data it has been found that applied voltage and electrolyte concentration are the most significant parametersfor MRR and taper of the holes. For SS-304 the voltage has 53% contribution in affecting the taper whereas for Cu the voltage has 36% contribution affecting the MRR of the process.

222-7