170-175

International Conference on Advancements and Futuristic Trends in Mechanical and Materials Engineering (October 5-7, 2012)

Paper selected for publishing in International Journal of Surface engineering and Materials Technology

Punjab Technical University, Jalandhar-Kapurthala Highway, Kapurthala, Punjab-144601 (INDIA) 170

WEAR REDUCTION OF ALUMINIUM ELECTRODE BY CRYOGENIC

TREATMENT IN ELECTRICAL DISCHARGE MACHINING

Amoljit Singh Gill1, Sanjeev Kumar

2

1. Mechanical Engineering Dept., PEC University of Technology, Chandigarh-160012, India. 2. Mechanical Engineering Dept., PEC University of Technology, Chandigarh-160012, India

Email: [email protected]) (Corresponding author)

ABSTRACT Deep Cryogenic Treatment (DCT) is a process in which the material is subjected to very low temperature of the order of −185° C and below, to cause beneficial changes in the material properties. It makes the crystal more perfect, stronger, relieves residual stresses, and improves electrical properties. Electro discharge machining (EDM)

is a well known non-traditional machining process, which converts electrical energy to thermal energy and removes material by melting and evaporation from electrically conductive materials. In this process the dimensional stability and machined surface characteristics depends greatly on electrode wear. In this research work, the effect of cryogenic treatment on the wear of tool electrode is experimentally investigated using Taguchi design approach. The study has been carried out on hot die steel (AISI H11) using Aluminium as tool material. Experiments were conducted to study the effect of various process parameters. The results of study suggest that deep cryogenic treatment significantly reduces electrode wear.

Keywords: Electrical discharge machining; Aluminium tool electrode; Deep cryogenic treatment; Tool wear; AISI H11 hot die steel.

1. Introduction

Electrical discharge machining (EDM) is a thermo-

electric process which erodes material from the

workpiece by a series of discrete sparks between the

the electrodes submerged in a dielectric fluid [1]. The

sparks, occurring at high frequency, generate a narrow

plasma channel having a high energy density and high

temperature (8,000–12,000°C) that melts and

evaporates a small amount of workpiece [2]. Each

discharge results in a tiny crater both on the tool and

the workpiece surface. The size of the craters is related to the released discharge energy at the

discharge location. The dielectric evacuates the

resolidified debris from the gap and acts as a

deionising medium between two electrodes assuring

optimal conditions for spark generation. The thermal

nature of EDM allows machining any electrical

conductive material irrespective of its hardness and

strength [3]. Generally, EDM allows the shaping of

complex structures with high accuracy in the order of

several micrometres and achievable surface roughness

[4]. The surface texture created by the sparks has a matte appearance covered by shallow craters, debris

particles that are resolidified after the discharge, and

pockmarks formed by entrapped gases escaping from

the resolidifying material.

The most important performance measures in

EDM process are the surface roughness, material

removal rate (MRR), and electrode wear (EW) [5].

Efforts have been done on fast machining with better

surface finish and less tool wear. The quality of a

machined surface is an important factor in evaluating

the productivity of machine tools and machined parts.

Surface roughness is significant design factor that has

a considerable influence on properties such as fatigue

strength, corrosion and wear resistance. It is therefore

important to achieve a good surface finish [6]. In the

past few decades, many methods were studied to improve surface finish in EDM process. These include

making an electrode planetary movement at the lateral

gap allowing dielectrics to flow in from one side and

leave at the other side of workpiece [7]. Several

authors [8] utilised magnetic field to transport

magnetic debris through the gap. Guu and Hocheng

[9] improved the circulation of the dielectric fluid in

the spark gap by providing a workpiece rotary motion

that effects temperature distribution of the workpiece

yielding better MRR and SR. Similarly, the rotary

motion has been introduced to the electrode to improve the performance measures of the EDM

process. It serves as an effective gap flushing

technique, which significantly improves the MRR and

SR [10, 11].

Deep cryogenic treatment (DCT) is a onetime

permanent process. There is not a standard process for

cryogenic treatments but most of them are quite

similar. In conventional cryogenic treatments the

materials are slowly cooled down to a temperature

around -185ºC and maintained for a period of time

that lasts from eight hours to two or even more

days. After the soak, the materials are slowly heated up to ambient temperature [12].




The cryogenic treatments are performed in

chambers designed for this purpose. The material

is usually cooled using liquid nitrogen that is

introduced in the processor through solenoid valves

controlled by computer. Most of the modern

chambers have heaters that also allow controlling the

temperature during the heating phases of the process.

The DCT has a lot of benefits. It not only gives

dimensional stability to the material, but also

improves wear resistance, strength and hardness of the

materials [13, 14]. It is believed that cryogenic

processing makes the crystal more perfect and

therefore stronger. Besides, there is some amount of

grain size refinement and grain boundary realignment

occurring in the material. These two aspects lead to a

tremendous improvement in the electrical and thermal

conductivity of the material thus transporting the heat

generated during the operation of the tool away from the source and increasing its life.

2. Experimentation

Material employed in this study was AISI H11

hot die steel. It is a high alloy tool steel, and well known for high hardness and toughness. The chemical

composition of H11 is 0.36% C, 1% Si, 0.4% Mn, 5%

Cr, 0.4% V, 1% Mo. The specimens were then heat

treated to relieve the residual stresses. Experiments

were conducted on die-sinking Electrical Discharge

Machine Elektra EMS 5535, Electronica Machine

Tools, India. Cylindrical electrodes used in the

experimentation are deep cryogenic treated aluminium

and non cryogenic treated aluminium (electrolytic

copper of 99.9% purity) with a nominal diameter of

15 mm. Machining time for each cut was 12 minutes.

In this experiment, there were four controlled variables investigated including peak current (I), pulse

on-time (Ton), duty factor (τ) and gap voltage (V).

Three levels of each factor were selected for the

experimentation. To evaluate the effects of machining

parameters on performance characteristic (Tool Wear,

TW), and to identify the performance characteristics

under the optimal machining parameters, a specially

designed experimental procedure is required [15, 16].

Classical experimental design methods are too

complex and difficult to use. Additionally, large

number of experiments has to be carried out when number of machining parameters increases [17, 18].

In this study, Taguchi method, a powerful tool for

parameter design of performance characteristics, was

used to determine optimal machining parameters for

minimum TW. In Taguchi method, process

parameters which influence the products are separated

into two main groups: control factors and noise factors

[19]. The control factors are used to select the best

conditions for stability in design of manufacturing

process, whereas the noise factors denote all factors

that cause variation. Taguchi proposed to acquire the

characteristic data by using orthogonal arrays, and to

analyze the performance measure from the data to

decide the optimal process parameters [16, 19]. This

method uses a special design of orthogonal arrays to

study the entire parameter space with small number of

experiments only. In this study, four machining

parameters were used as control factors and each

parameter was designed to have three levels, denoted

L1, L2 and L3 (Table 1). According to the Taguchi

quality design concept, a L9 orthogonal arrays table

with 9 rows (corresponding to the number of

experiments) was chosen for the experiments (Table

2).

Table 1. Levels of input machining parameters

Parameter L1 L2 L3

I (Amps) 5 10 15

Ton (µSec) 100 150 200

τ (%) 64 80 96

V (V) 30 40 50

3. Results and Discussions

Experiments were conducted by fixing the parameters according to L9 OA using non-cryogenic

treated aluminium electrode and cryogenic treated

aluminium electrode. The standard procedure

suggested by Taguchi is employed. The mean or the

average values and S/N ratio of the response/quality

characteristic for each parameter at different levels

have been calculated from experimental data. For the

graphical representation of the change in value of

quality characteristic and that of S/N ratio with the

variation in process parameters, the response curves

have been plotted. These response curves have been used for examining the parametric effects on the

response characteristic.

Table 2. Orthogonal array L9 matrix

Experiment Current (Amps)

A

On

time

(µSec)

B

Duty factor

(%) C

Voltage (V)

D

1 5 100 96 40

2 5 150 64 50

3 5 200 80 30

4 10 100 64 30

5 10 150 80 40

6 10 200 96 50

7 15 100 80 50

8 15 150 96 30

9 15 200 64 40

The most favorable conditions (optimal settings) of

process parameters in terms of mean response of

characteristic have been established by analyzing

response curves. Tool Wear is required to be as low as

possible so S/N ratio was calculated for lower-the-

better type (LB) of response characteristic as:




(S/N)LB = – 10 log [

Where, yj = Observed value of the response

characteristic.

R = Number of repetitions

The next step of the Taguchi approach is to predict

and verify the enhancement of quality characteristics

using the optimal parametric combination. The

estimated S/N ratio using the optimal level of the

design parameters can be calculated as:

Where is the total mean S/N ratio, is the mean S/N ratio at optimum level and ‘o’ is the number of

main design parameter that affect quality

characteristic.

A. Results with non cryogenic aluminium electrode

The average values of TW and S/N ratio for each

parameter at level 1, 2 and 3 were calculated and

given in Table 3 and these values are plotted in Fig. 1.

It can be observed from Fig. 1 (a) that higher current

increases TW which is a well known fact, because of

high energy per discharge. Second level of current

(i.e. 10 Amps) would be optimum as S/N ratio is

maximum at this level. Similarly, Fig. 1 (b) shows

that TW is minimum at moderate values of pulse on-

time which corroborates earlier findings. Fig. 1 (c)

shows that a moderate value of duty factor gives

lower TW. TW varies slightly with increase in open

circuit voltage [Fig. 1 (d)]. With increase in open

circuit voltage Electric field strength increases which

results in more electrode wear. On the other hand,

lower open circuit voltage decreases working gap,

which leads to insufficient flushing and destabilized arc.

Table 3. Observed values of TW (non cryogenic aluminium

electrode)

Exp.

No.

TW (%) S/N Ratio

Run 1 Run 2 Run3

1 0.30192 0.38461 0.35774 9.12386

2 0.20438 0.25997 0.25367 12.37311

3 0.22457 0.20911 0.34341 11.50603

4 0.50198 0.5545 0.48342 5.777664

5 0.103429 0.079273 0.09239 20.70274

6 0.21748 0.28402 0.17962 12.72382

7 0.45073 0.53591 0.54294 5.821737

8 0.33674 0.39958 0.37311 8.619405

9 0.52193 0.55734 0.5143 5.489594

From the main effect plots, the best combination of

input process parameters for the lowest value of

surface roughness is found to be A2B2C2D2 and the

experimental value of Ra at this setting was

0.08897%.

(a)

(b)

(c)

(d)

Fig. 1. Effect of various parameters on Average values of TW and S/N ratio (Non cryogenic treated

aluminium electrode)

Based on the above equation, the estimated multi-

response signal to noise ratio can be obtained as:




= 10.23755 + (13.06807-10.23755) + (13.89842-

10.23755) + (12.67683-10.23755) + (11.77206-

10.23755)

= 20.70273

And the calculated value of TW =

= 0.0922 %, which shows only 3.5% error with

experimental result.

Scanning Electron Microscope (SEM) micrograph of

the workpiece surface after machining with non-

cryogenic treated aluminium electrode (Fig. 2) shows

some micro-cracks and small depositions of the

resolidified material which are the characteristic

features of EDM.

Fig. 2. SEM micrograph after machining with non-cryogenic treated Aluminium electrode at 1000x

(Peak current = 10 A, Pulse On-time = 150 µs, Duty Factor

= 80%, Gap Voltage = 40 V)

B. Results with cryogenic treated aluminium electrode

The average values of TW and S/N ratio for each

parameter at level 1, 2 and 3 were calculated from

Table 4 and these values are plotted in Fig. 3. It is

observed that cryogenic treatment has significantly

affected the response to each parameter. The most

important observation is seen for peak current where

TW reports a steady decline with decrease in peak

current. Fig. 3 (b) shows that the response of TW to

variation in pulse on-time is almost the same – there is

an initial sharp decrease and then it again increases.

Hence, the fact that lower pulse on-time reduces TW

holds good after cryogenic treatment also. There is a

sharp decrease in TW with increase in duty factor upto moderate level then it remains almost constant

[Fig. 3 (c)].

Table 4. Observed values of TW (cryogenic aluminium

electrode)

Exp.

No.

TW (%) S/N

Ratio Run 1 Run 2 Run3

1 0.14109 0.13367 0.13951 17.19444

2 0.12355 0.13371 0.12896 17.80122

3 0.22541 0.19895 0.14588 14.29178

4 0.64329 0.59255 0.60148 4.253069

5 0.099811 0.09321 0.09583 20.32556

6 0.10288 0.17712 0.22706 15.06254

7 0.38228 0.4465 0.38748 7.81954

8 0.41178 0.36196 0.40385 8.109098

9 0.33453 0.39733 0.40658 8.385434

(a)

(b)

(c)

(d)

Fig. 3. Effect of various parameters on Average values of TW and S/N ratio. (Cryogenic treated aluminium electrode).




From the main effect plots, the best combination of

input process parameters for the lowest value of TW

is found to be A1 B2 C2 D2. The experimental value

of Ra at this setting was 0.07029 % and the estimated

S/N ratio can be calculated as:

= 12.58252 + (16.42914-12.58252) + (15.41196-

12.58252) + (14.14563-12.58252) + (15.30181-

12.58252)

= 23.54098

And the calculated value of SR = =

0.066519%, which is very close to the experimental

value with only 5.7% error. SEM micrograph (Fig. 4)

of the machined surface, at the setting shows a smooth

surface without any micro-cracks.

Fig. 4. SEM micrograph after machining with cryogenic

treated aluminium electrode at 1000x

(Peak current = 5 A, Pulse On-time = 150 µs, Duty Factor =

80%, Gap Voltage = 40 V)

After the cryogenic treatment of tool electrode, there

is a substantial decrease in tool wear which is due to

the strengthening of crystal structure and reduction in

the lattice defects. More importantly, tool wear has

decreased even at higher values of peak current and it has a profound positive effect on the surface finish of

the machined surface also. A comparison of the

results obtained before and after cryogenic treatment

has been presented in Table 5.

Table 5. Comparison of tool wear and optimal combination

of factors

Electrode

Material

Type of

Electrode

Tool

Wear (%)

Optimum

combination of factors

Aluminium

Non Cryogenic

Treated

0.0922 A2B2C2D2

Cryogenic

Treated 0.066519 A1 B2 C2 D2

4. Conclusions Experiments were conducted on H11 hot die steel by

EDM using non-cryogenic treated aluminium

electrode and cryogenic treated aluminium electrode.

The following conclusions can be drawn from this

experimental work:

(1) Cryogenic treatment significantly affects the

performance of EDM aluminium electrode.

(2) After comparing the results for surface

roughness before and after deep cryogenic

treatment of the aluminium electrode, 27.85%)

improvement is reported.

(3) It is found that the optimum combination of the

process parameters also changes as a result of

cryogenic treatment.

(4) Tool wear of machined surface is critical in

EDM. Since cryogenic treatment has a

significant positive effect on this parameter, it

can be recommended that EDM tool electrodes

should be cryogenically treated.

(5) More experiments need to be conducted on

other varieties of die steel materials like oil

hardening non-shrinkable (OHNS) die steels and high carbon high chromium die steels to

conclusively establish the positive effect of

deep cryogenic treatment on the performance

of EDM tool electrodes.

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