Design Optimization of the Formation of Boss or Ridge and Passivation of Electrolyte Flow in ECM Pro

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Design Optimization of the Formation of Boss or Ridge and Passivation of Electrolyte Flow in ECM Process Using CFD Simulation Krishna Mohan Singh and R. N. Mall ABSTRACT: The main objective of the paper is to investigate the flow pattern of electrolyte in the flow path for the initial work shape conforms to the tool shape, the machining process itself causes the formation of boss or ridge in the work piece; this helps in a proper distribution of the electrolyte flow. A tool with an electrolyte supply slot is simple to manufacture, but such a slot leaves small ridges on the work. And slot design to avoid development of passive area KEYWORDS: ECM, Electrolyte, CFD CODE, Boss and Ridge Formation, Boss and Ridge Formation.

1 INTRODUCTION Electrochemical machining (ECM) has seen a resurgence of industrial interest within the last couple of decades due to its many advantages such as no tool wear, stress free and smooth surfaces of machined product and ability to machine complex shape in electrically conductive materials, regardless of their hardness. It has been applied in diverse industries such as aerospace, automotive and electronics, to manufacture airfoils and turbine blades, die and mold, artillery projectiles, surgical implants and prostheses, etc. Moreover with recent advances in machining accuracy and precision, based on the development of advanced electrochemical metal-removal processes, demonstrate that the ECM can be effectively used for micromachining components in the electronics and precision industries.

Fig.1: Formation of boss and ridge on machined surface 1.2 Experimental Set-Up and Principle of ECM

1.1 Scope of the Work 1. To simulate the electrolyte flow from Boss and ridge formation and discuss the results 2. To simulate the electrolyte flow for different velocity range and result discussion based on CFD results 3. To investigate the pressure distribution (contours), temperature distribution (contours), velocity vectors etc. during the flow path of electrolyte Krishna Mohan Singh and R. N. Mall Department of Mechanical Engineering M.M.M. Engineering College Gorakhpur 273010 (UP) India Email: kmssh1988@gmail.com and mall_rn@radiffmail.com

ECM works on the principle of Faradays law i.e. when two conductive electrodes are placed in an electrolyte maintained at low potential difference then there is discharge of electrons

Š ijbstr.org 21

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013 taking place. When the current is switched on, the electrolyte (NaCl+H2O) gets ionised according to the following relationshipNaCl Na++ ClH2O H++ OHAs hydrogen ions reach the cathode (tool), they combine with free electrons resulting into evolution of H2 gas.

2H++ 2e- H2 Assume that pure iron is being machined by this process, reactions that would occur are 2Na++ 2OH- = 2NaOH & Fe2++2Cl--= FeCl2 2NaOH + FeCl2 = 2NaCl + Fe(OH)2 Thus in ECM of iron, using NaCl as the electrolyte, iron is removed as Fe(OH)2 and sodium chloride is recovered back.

The iron hydroxide produced during the process must be removed continuously from the electrolyte by filtration before it is re-circulated.

ISSN 2320 – 6020 2. It carries the current between the tool and the work-piece.

3. It removes the products of machining from the cutting region.

For ECM process the electrolytes must have high electrical conductivity, low toxicity and low corrosiveness. The electrolyte is pumped at about 14 Kg/cm2 and at speed ranging from 5-50 m/s.

1.3.2 SIZE, SHAPE AND MATERIAL OF THE TOOL: The tool must match the required shape of the work piece depending on the material and the profile to be produced. Tool materials used in ECM must have good thermal and electrical conductivity; corrosion resistance must be highly machinable and should be stiff enough to withstand the electrolytic pressure without vibrating.         

SERVO SYSTEM TOOL FEED RATE: TEMPERATURE CONTROL: MATERIAL REMOVAL RATE: TOOL DESIGN: SURFACE FINISH: PUMPS: FILTRATION AND STORAGE TANKS: VALVES AND PIPING

Fig 2: Before machining

Fig 3: After machining It is to be noted that in ECM process, the machining gap size increases as the metal is removed. The larger gap leads to a decrease in the metal removal rate. Therefore to maintain a constant gap between the tool and the work piece, the cathode (tool) should be advanced towards the anode (work) at the same rate at which the metal is removed.

1.3 ECM MACHINE PARAMETERS

Fig.4 Schematic diagram to show the electrolyte flow

1.3.1 ELECTROLYTE The electrolyte is very essential parameter for the electrolytic process to work. In ECM process the electrolyte is used to perform three main functions:1. It dissipates heat produced in the operation.

ECM process is quite similar in concept to electrical discharge machining with a high current passed across its inter-electrode gap through the electrolyte, a material removal process having a negatively charged electrode (cathode-tool), conductive fluid (electrolyte), and a conductive work-piece (anode), however in ECM there is no tool wear. The ECM cutting tool is guided

© ijbstr.org 22

IJBSTR RESEARCH PAPER VOL 1 [ISSUE ] JULY 2013

ISSN 2320 – 6020

along the desired path very close to the work but it does not touch the piece. Unlike EDM however, no sparks are created.

creation and manipulation of highly complex geometries and mesh generation.

1.4 SPECIFICATIONS OF ECM

2.1 Selection of Solver

TABLE 1 ECM specifications

Pressure based or segregated solver is selected for solving incompressible flow through the electrolyte path with implicit scheme with cell based approach. 2D approach is used.2.2 Boundary Conditions For solving any problem using numerical technique, it is first step to apply appropriate and exact boundary conditions, which defines the problem, in order to get the solution of that specific problem. A solution is always sensitive to the inlet boundary conditions; a great care is needed to be taken while imposing the boundary condition.

After meshing the domain, it is imported to the solver “FLUENT” version 6.3.26.The dimensions of the domain properly scaled (conversion from cm to meter). The pressure based or segregated solver is used for the current problem.

Primarily the standard k-ε model is used for viscous modelling, after a converged result it is switched over to other model to find out the appropriate viscous model suitable for the present case.

2.3 Boundary Conditions: the inlet of the tool

VELOCITY_INLET- is given at

Pressure outlet – is given at the outlet of the electrolyte flow WALL – is selected for all other sections FLUID- is specified as continuum type boundary condition.

Technical Data  Tool area - 30 mm2.  Cross head stroke - 150 mm.  Job holder - 100 mm opening X 50 mm depth X 100 mm width.  Tool feed motor - DC Servo type.

CFD CODE

2.4 Properties of Electrolyte used: Density of electrolyte (ρ) = 1050 kg/m3 Dynamic viscosity of water (μ) = 0.001 kg/m-sec.

2.5 Calculations: Electrolyte flow in the ECM flow path can be recognized by calculating Reynold's number, which is given as (using equation 4.1)

There are several commercial CFD codes such as CFX, FLOWTRAN, FLUENT, STAR CD, PHOENICS etc. which are available for analyzing the complex flows.

But in the present investigation the commercial CFD code “FLUENT” version 6.2.26 is to be used and for modelling and Meshing “GAMBIT” version 2.3.16 is to be used.

RN = Reynolds’s number

2.0 Geometric Modelling of Flow Domain

V = Mean Velocity of flow

The geometry and flow domain of the electrolyte flow path is modelled by using GAMBIT 2.3.16 software. The geometry modelling and meshing tool of GAMBIT allows us for

D = diameter of tubular flow

© ijbstr.org 23

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Îź = Kinematics Viscosity (Dynamic viscosity / Density) Ď = Fluid density When the Reynolds No. (RN) is less than 2000 fluid flow results in laminar flow and if the Reynolds No. (RN) is more than 2000 turbulent flow occurs.

Calculation for inlet velocity=5m/s For Electrolyte Density of fluid (Ď ) = 1050 kg/m3 Dynamic viscosity of water (Îź) = 0.001 kg/m-sec.

Inlet diameter of electrolyte=2mm=0.002m Inlet velocity=5 m/s Substituting above values in equation 1.1 RN=

1050 đ?‘‹.002 đ?‘‹5 0.001

Figure 4.166: Contours of static pressure for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k  ď Ľ model with standard wall functions]

=10500 (turbulent flow)

Discussion on Boss and Ridge Formation Fig.4.166 to fig. 4.169 shows the pressure contours, velocity contours, velocity vectors and path lines for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s using k  ď Ľ model with standard wall functions.

It can be observed that boss formation produces negative pressure inside flow path and vortex formation near bottom corners if corner radius is not selected proper.

Similarly Fig.4.170 to fig. 4.173 shows the pressure contours, velocity contours, velocity vectors and path lines for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s using k  ď Ľ model with standard wall functions. It can be observed that for Ridge formation there is no negative pressure and vortex formation inside flow path, hence allows smooth flow of electrolyte.

Discussion on passivation due to flow interruption Fig. 4.174 and Fig. 4.175 shows that if the flow geometry is like the figure shown in fig. 4.174 then electrolyte cannot approach the entire area of the geometry hence creating passive area. To avoid or reduce this passive area formation the geometry should be designed like shown in fig. 4.176 and fig. 4.177

Figure 4.167: Contours of velocity magnitude for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k  ď Ľ model with standard wall functions]

Š ijbstr.org 24

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Figure 4.168: velocity vectors for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.170: Contours of static pressure for electrolyte flow over ridge formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.169: path lines for electrolyte flow over boss formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.171: Contours of velocity magnitude for electrolyte flow over ridge formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

© ijbstr.org 25

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020

Figure 4.172: velocity vectors for electrolyte flow over ridge formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.174: Contours of velocity magnitude for passivation due to flow interruption at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.173: path lines for electrolyte flow over ridge formation with corner radius of 0.7mm at inlet velocity, V=5m/s [ k   model with standard wall functions]

Figure 4.175: velocity vectors for passivation due to flow interruption at inlet velocity, V=5m/s [ k   model with standard wall functions]

© ijbstr.org 26

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013

ISSN 2320 – 6020 3.1: CONCLUSIONS For the present study simulations have been done by making 2-D geometric models of electrolyte flow path.

Literature suggest that cavitations, stagnation and vortex formation in the flow path should be avoided since these lead to a bad surface finish, hence for better surface finish cavitations, stagnation and vortex formation should be completely eliminated from the electrolyte flow path.

Simulation has also been done for boss and ridge formation and results shows that these formations help in a proper distribution of the electrolyte flow. Due to these formations no stagnation of electrolyte occurs in the flow path which leads to better surface finish.

From simulation results it is obvious that ridge formation is more advantageous than boss formation.

Simulation has also been done for passivation due to flow interruptions and simulation results suggest that proper tool design is necessary to avoid or reduce the passive area formation so that the electrolyte can reach to every portion of the surface of work.

Figure 4.176: Contours of velocity magnitude for slot design to reduce or avoid development of passive area at inlet velocity, V=5m/s [ k   model with standard wall functions]

3.2: SUGGESTIONS FOR FUTURE WORK There is always a future scope for further investigation. The following suggestions may be useful for future work related to simulation work 1. Same work can be extended for electrolyte flow path by making axis symmetric and 3d models and results can be compared 2. This work can be extended for other electrolytes and by using user defined functions.

3. Work can also be extended by using other parameters like moving tool or wall functions, varying gap between tool and work etc.

REFERENCES

Figure 4.177: velocity vectors for slot design to reduce or avoid development of passive area at inlet velocity, V=5m/s [ k   model with standard wall functions]

1.

Ghosh and Mallik, Manufacturing Science, 2nd edition, EWP press, 2010

2.

Dr. D. S. Bilgi and Mr. P. V. Jadhav, Enhancement of surface finish of Pulse Electrochemically Machined (PECM) surface using rotating electrode.

International Journal of Computer Communication and Information System (IJCCIS) – Vol2. No1.

ISSN: 0976–1349 July – Dec 2010

3.

J. Kozak, Mathematical model for computer simulation of electrochemical machining processes, Journal of Material Processing Technology 76(1998) 170-175.

© ijbstr.org 27

IJBSTR RESEARCH PAPER VOL 1 [ISSUE 7] JULY 2013 4.

5.

6.

7.

8.

ISSN 2320 – 6020

Jerzy kozak, Antoni F. Budzynski, PiotrDomanowski, Computer simulation electrochemical shaping (ECM-CNC) using a universal tool electrode. Journal of Material Technology 76(1998) 162-164. www.sciencedirect.com www.nitrkl.ac.in Computational fluid dynamics by H.Choi, H.G.Choi and J.Y.YOO (EDS).

Text book of Production technology HMT.

© ijbstr.org 28