Experimental investigation on migration of material during EDM of die steel (T215 Cr12)

Journal of

Materials Processing Technology

ELSEVIER

Journal of Materials Processing Technology 56 (1996) 439-451

EXPERIMENTAL INVESTIGATION ON MIGRATION OF MATERIAL DURING EDM OF DIE STEEL (T215 Cr12)

J.S. Soni) and G. Chakravertib " Defence Research & Development Lab., Hyderabad, India b Mech. Eng. Dept., JNTU College of Eng., Hyderabad, India

ABSTRACT This paper presents the Scanning Electron Microscopic (SEM) investigation on changes in chemical composition of resolidified layers of both the tools and the workpieces as well as debris. An investigation has also been made on variation of micro-hardness, depth ofresolidifiedlayer and heat affected zone (HAZ) with pulse current and electrode rotation. This change in chemical composition occurs due to migration of material from either of the electrodes during electro-discharge machining of high carbon high chromium die steel (hardened) with rotating copper-tungsten tool electrode. The machiningcharacteristicsof the material depend on the various machiningparameters like discharge current, pulse duration, voltage, dielectric and electrode materials. The rotation of electrode during macbining also contributes towards these. The experiments were conducted by varying discharge current and electrode rotation to study the effect of these parameters on alloying of tool and workpiece surfaces. An effort has also been made to compare these results with that of stationary electrode. The experiments were statistically designed and a mathematical model of the process has been developed. It is foundthat appreciable amount of elements has migrated from the tool electrode to workpiece and vice versa and got alloyed in the resolidified layer. The chemical composition of debris also changed due to the pick up of elements from both the electrodes.

I. INTRODUCTION The electro-discharge machining technique (EDM) has been developed significantly over last two decades and is being widely used for production of tools, complex shapes, thin slots and micro holes and particularly for machining materials which are difficult to machine by conventional methods. Though significant advances have taken place in EDM technology, a number of problems associated with its use remain unresolved. For example, tool electrode shape degeneration and occurence of side sparks affect work cavity ref.[1]. Substantial changes can occur in the workpiece surface which may affect the performance of the finished parts ref.[2]. The high current discharge leaves a large crater having large diameter and in random location. At the sametime, a white layer caused b y rapid heating and cooling, which also results in high residual tensile stresses ref.[3-6], heat affected zone ref. [7,8], changes in the composition of eroded surfaces and surface crackingref.[3,9]. The white layer and cracks as they are not deep enough may be removed by using fine cutting conditions and subsequent polishing. 0924-0136/96/$15.00 © 1996 Elsevier Science S.A. All rights reserved

SSD10924-0136 (95) 01858-C

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J.S. Soni, G. Chakraverti /Journal ~'Materials Processing Technology 56 (1996) 439-451

Analytical models for the computation of chemical composition, hardness and depth of resolidified layer after migration of material are not available. Even experimental investigations are limited to a elementary research. 2. PREVIOUS WORK Only a few authors have studied the migration of material during EDM in the past. A number of authors have relJorted that the surfaces of the eroded electrodes are of the material which considerably differs from the initial one by its chemical composition and the properties. The surface consists of the dielectric pyrolysis products and of the alloy between the matrix and the opposite electrode. The material of the workpiece can diffuse into the tool surface and influence its wear resistance which can have even a negative effect ref.[1-5]. Several others ref.[9-11] have noticed the presence of a considerable quantity of opposite electrode material in the surface treated and debris produced by EDM. Roethel [9] has investigated the mechanism of mass transfer of electrode material and determined the changes in the zone of thermal influence. Pandey and Jilani [12] presented a thermal model on plasma ch~nn el growth and thermally damaged surface layer. They observed that the transverse section of workpieces has three distinct zones: a) White layer, b) heat affected zone (HAZ), and c) unaffected parent metal. The changes in chemical composition often remain confined to within the resolidified layer which was supported by others ref.[ll-16]. 3. EXPERIMENTAL DETAILS Fine Sodick Mark 5 NC EDM with servo-control and rotating heads with variable speed for electrode was used to conduct the experiments. The work and electrode materials properties are given in Table 1. Other boundary conditions were : Work material and chemical composition

Work shape and size Tool material Tool shape and size Dielectric Pulse duration Voltage Current Electrode rotations Type of machining

High carbon high chromium die steel IS-T215 Cr12 (C 2.15, Mn 0.37, Si 0.22, Cr 12, Mo 0.8, V 0.8) Square shape and size 25x25x10mm Copper-tungsten (W 80, Cu 20) Cylindrical shape of diameter 3mm Kerosene (commercial grade) 20 micro seconds (ps) 90 V 3,9 and 15 Amperes (A) 0, 500, 750 and 1000 rev/min Blind and through holes macbining. For through holes machining the workpieces were pre-drilled with 2ram diorneter holes

4. MEASUREMENTS The spark machined test pieces were thoroughly washed with acetone before observing them under Scanning Electron Microscope. SEM micro-graphs were taken for debris and surface textures. Qualitative Energy Dispersive Spectroscopic chemical analyser was used to measure the amount of migrated alloying elements for electrode, workpiece and debris. Electron Probe Micro-Analyser was used to confirm the above observations.

J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

The workpieces were cut through half of the hole and mounted on bakelite for the microexamination of the transverse section. The mounting will prevent the rounding of edge during polishing of the workpieces. The transverse sections were observed under optical microscope. The micro-hardne~, depth ofresolidifiedlayer and heat affected zone were measured on Vicker micro-hardness tester with an inbuilt optical micrometer. Table I Work & electrode materials properties

Properties Density, Kg/m 3 Hardness, HRc Thermal conductivity, W/M°K Electrical resistivity, p~cm Melting temperature, °C Boiling temperature, °C

Electrode Material 14700 170 BHN 139 45 3380 5555

Work material 7700 52-55 209 70 1536 2860

5. DESIGN OF EXPERIMENTS The experimental work was p|snned on the basis of statistical design of experiment~ Hence, a complete 32factorial experimental model was formulated with full randomis~tion with the help of a random number table ref. [17]. To simplify the data, the levels were set at equal intervals and the resulting orthogonal contrasts facilitate easy computation of the regression coefficients ref.[18]. The design scheme of experiments plan along with the factors and their coded levels are given in Table 2. The regre~fm coefficients and the coefficient ofcorre!_~fion(r) were computed according to the standard statistical procedure. A complete analysis ofvariance (ANOVA) was performed to test the significance of the obtained coefficients at 5% level of significance. The procedure is illustrated in Appendix. Table 2 Design scheme of the experiment Response Resolidified layer, (ram) Micro-hardness, (VPN)

Factors x 1 Current, (A) x2 Electrode rotation (rev/min)

Coded levels -1 0 1 3 500

9 750

Remarks

15 Pulse duration constant 1 0 0 0 (20ps)

Cracking behaviour Surface texture Debris formation Migration of material The adequacy of the proposed model was tested by comparing the variation due to regression (909945.3 from Appendix) with the error (778.969). The calculated ratio of these variances (F) is highly significant. This proves the adequacy of the model.

6. RESULTS AND DISCUSSION 6.1 Micro exRmination of transverse section of workpiece Fig. l(a&b) shows the micrographs of the transverse section of die steel workpieces mahined with copper-tungsten electrode with positive electrode polarity.

441

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J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

A white layer representing resolidified layer and the heat affected zone can be seen adjacent to the steel matrix. During machiningofblind hole the thickness of white layer increases with stationary electrode when compared with rotating electrode. This is attributed to the improved flushing with rotating electrode due to which the material cools at faster rate and this results in decrease in the heat affected zone. The white layer exhibits modified structure caused mainly by the reaction products due to cracking of the dielectric and alloying with the tool material. In the present c~se, high carbon (C,2.15%) high chromium die steel is used as one of the workpieces and hence carburisation of iron occurs when it is machined in kerosene. The white layer formed shows the presence ofmartensite and retained austenite ref.[7]. The spark machined surface is not always uniform and homogeneous, but irrespective of the energy input, a severely damaged surface can result.

a) 3A, 201Js, 0 rev/min

,

30 l~m

b) 3A, 20}ls, 750 rev/min

SEM micrographs of a transverse section of EDM holes

c) 3A, 20ps, 0 rev/min r

150 Fm

Fig. 1 S E M EDM

d) 3A, 20ps, 500 rev/min 30 l~m .

!

SEM micrographs of EDM surface micrographs of a transverse section of E D M surface

r:

holes and

,

J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

Fig. 1 (a&b) also shows that the white layer thickness is non uniform and discontinuous. Fissures and voids are also formed. Fig. l(c) depicts formation of surface which dearly indicates a different pattern of solidification. Collapse of bubble and spread splats are observed (Fig. ld). The cracks are also formed on the machined surface which are confined to resolidified layer. 6.2 Modification ofsurfaee due to migration of material

6.2.1 Micro examination of workpiece surface The composition of the workpiece surface was checked by Qualitative Energy Dispersive Spectroscopy (EDS) on Scanning Electron Microscope and transverse section by Electron Probe Micro-Analyser (EPMA). }00 s e c s

Counts

1000 Fe

900 BOO 7O0

Cr

600 500 400 300 200' ~00 0 0

2

3

~

i

5

i

i

,

7

8

9

keV

~0

a) Original surface 100 ~ e c s

Counts

1o9o 900

Fo 800 700 600 Cr" 500 400 300 200 lO0 keV

0 0

!

~

3

4

5

6

b) 3A, 20 /~, 750 rev/min

Fig. 2 Energy Dispersive Spectroscopic (EDS) pattern showingthe relative intensities of various elements present in the surface Workpiece : Die steel, Through hole machining

443

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J.S. Soni, G. Chakraverti / Journal ~'Materials Processing Technology 56 (1996) 439-451

The elements present in the surface are clearly indicated by the peaks corresponding to their energy levels. Tungsten and copper have migrated from tool electrode and alloyed with iron and chromium ofworkpieces. It is also evident from Fig. 2 that an appreciable amount of tungsten has migrated when compared with copper. It is noticed from Table 3 that 176-483 counts (peak values) of tungsten have migrated from tool electrode to die steel workpiece during through hole machining while it is more in blind hole machining which has gone upto 802 counts. Chromium counts are lesser in the later case. The migration of copper from tool electrode is limited to 23 to 60 counts. There is no appreoiable change during blind hole machining. The migration of tungsten is very high when compared with copper.

BQ~e Die Sleet

C 50 p m -~

Fig. 3 EPMA film from cross section of workpiece showing presence of carbon EPMA of transverse section shown in Fig. 3 clearly indicates that the migrated elements are spread in resolidified layer and restricted to that. Traces of carbon in the deposited layer are also clearly seen. 6.2.2 Micro examination of tool electrode surface Fig. 4 shows a EDS spectra for original surface of tool electrode and for migration of elements from workpiece to tool electrode surface. Fig. 4(b) shows the presence of iron (1254 counts) and chromuium (413 counts) on tool electrode surface during machining of die steel. Hence, it clearly indicates that the migration of iron is more than tungsten (Table 4). 6.3 Micro examination of debris The material is ejected from the workpiece surface in the form of powder particles after solidification of gas bubbles (a mixture of gases of metal and dielectric) and liquid metals. The SEM micrographs (Fig.5) show that most of the particles are spherical and they exhibit a wide range of sizes. These particles are either solid or hollow spheres and some deviation is also observed in some particles. Fig. 5(a&b) showsthe debris particles ofdiesteel collected from dielectric where as Fig. 5(c&d) shows the particles remain attached on workpiece surface. The die steel debris particles collected from dielectric by keeping a magnet near die steel workpiece are attracted by the magnet and attached on surface. This powder was mounted on aluminium mounts with the help of a fLxing agent. EDS pattern (Fig. 6) shows peaks of iron, chromium and tungsten which clearly indicate the migration of tungsten from electrode. The counts of elements present in the debris particles are given in Table 5. The counts of tungsten varies from 70 to 130 where as chromium varies from 320

J.S. Soni. G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

445

to 489 counts. This is due to the high melting temperatures of tungsten and chromium. EPMA of debris (Fig. 7) also confirms the presence of iron, chromium and tungsten. The presence of carbon also noticed during E P M ~ Table 5 shows some traces of copper in the debris. Table 3 Migration of electrode material to workpiece Current A

Pulse duration ~s

Counts of elements present in matrix Fe Cr W Cu

Electrode rotation rev/min Through hole

9 15 3 9 3 9 3 9

500 500 750 750 1000 1000 0 0

20 20 20 20 20 20 20 20

1378 1355 1236 1067 1186 1207 1408 1432

389 395 404 334 408 357 431 408

483 252 427 273 261 291 176 193

56 25 60 44 28 48 24 23

873 1216 477 1009

254 577 363 492 142 327 273 802

37 42 28 44

Blind hole 3 3 3 3

20 20 20 20

500 750 1000 0

6.4 Micro hardness and depth of resolidified layer 6.4.1 Through hole machining The top layer of EDM surfaces are subjected to very high temperatures and fast cooling rates, resulting in a hard resolidified layer. The variation of micro hardness, depth ofresolidified layer and HAZ with current and electrode rotation is shown in Figs. 8-11. It is observed from Fig. 8 that the surface hardness increases significantly with electro-discharge machining. This increase is more at low and high currents but electrode rotation does not affect appreciably. It is observed (Fig. 9) that the depth ofresolidified layer & HAZ increase with current and rotation has no appreciable effect. Table 4 Migration of workpiece material to electrode Current A

Pulse duration ps

Electrode rotation rev/min

Counts of elements present in matrix W Cu Fe Cr

Through hole 3 15

20 20

0 750

1570 3826

150 510 408 1254

163 413

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J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439--451

Table 5 Chemical composition of die steel debris Current A

Pulse duration ]as

Electrode rotation rev/min

3 9 9 9 15

20 20 20 20 20

750 500 750 1000 1000

Counts of e l e m e n t s present in matrix Cr W Cu

Fe 2050 1488 1656 1621 1728

489 326 320 310 363

90 117 70 Some traces 130 observed 106

JOO secs

Counts

4800

4200

3600

3000

2400

IB00

1200 Cu 600

Y

•

"

,,

-

S

,

6

~

,

e

.

9

t

keV

lo

a) Original surface of Cu W tool electrode 1000 900 800 700 600

Fe

500 400 300

,oat;

200

0

o

t

2

keV

"~ , 5 6 ~ e =J io b) 9A, 20 ps, 0 rev/min SuFface of tool electrode after machining die steel work piece

Fig. 4 Energy Dispersive Spectroscopic (EDS) pattern showing the relative intensities of various elements present in the surface of tool electrode during through hole machining

J.S. Soni, G, Chakraverti / Journal of Materials Processing Technology 56 (1996) 439~151

447

6.4.2 Blind hole machining Figs. 10&ll depict the variation of surface hardness, resolidified layer and HAZ with electrode rotation for blind and through holes machining. Fig. 10 shows that the hardness is more with through hole machining due to improved flushing of debris and presence of fresh dielectric. A deviation is observed at 750 rev/min which is due to the combined effect of rotation and hole conditions. Fig. 11 shows that the depth is decreased during through hole machining which can be attributed to the higher thermal conductivity of die steel. The experimental model suggested and the responses obtained, correlate well as given in Table 6 and Appendix. It is clear that the current and rotation do have some effect on the depth of resolidified layer but their effect is not appreciable on micro hardness.

a) 3A, 20jas, 750 rev/min Elliptical particle

15 l~m

b) 9A, 20~s, 500 rev/min Group of spherical particles

Debris particles collected from dielectric

8 ~m t

15 pm r

i

c) 3A, 20jas, 1000 rev/min Hollow spherical particle

d) 15A, 20ps, 1000 rev/min Group of spherical particles

Debris particles still attached on workpiece surface Fig. 5 SEM micrographs of debris particles

I

448

J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439--451

Table 6 Regression coefficients for surface micro hardness and depth of resolidified layer & HAZ for high carbon high chromium die steel Measured Response

Regressioncoefficients 130 131

Micro hardness (VPN)

Depth of resolidified layer & HAZ

731.111 ±595.060 0.061 ±0.00000

Fratlo 132

29.833 ±178.518 0.008 ±0.00000

~11

27.500 4.833 ±178.518 i535.554 0.017 0.002 ±0.00000 ±0.00000

~22

64.833 -1.250 ±535.554 ±267.177 0.003 0.003 ±0.00000 ~0.00000

(ram)

~00 s e e s

Counts ~000

Fe ~800 ~600 ~00 1200

~000 BOO 600

CP

400 200 0 0

t

2

3

4

5

A

keY

6

9

e

10

a) 3A. 20 #s, 750 rev/min 100 S e E S

Counts

2000 1800 1600 Fe t400 1200 1000 BOO 600 ,400 Cr 200

W t

1

~

3

keY

,~

5

6

7

~12

e

b) 9A, 20/~s, 1000 rev/min

Fig. 6 Energy Dispersive Spectroscopic (EDS) pattern showing the relative intensities of various elements present in the debris Through hole machining

1168.141 3485.793

J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

Fa

Er

W

15 A 20 I~s 750 rev/min

BASE

DIE STEEL )

Fig. 7 EPMA film from die steel powder (debris) showing presence of iron, chromium, tungsten and carbon.

7. CONCLUSIONS In the light of this work, it is possible to draw the following conclusions : (1) An appreciable Amount of material migrates from tool electrodes to workpieces. (2) An appreciable Amount of material migrates from the workpieces to tool electrodes. (3) The migration of tungsten from tool electrode to workpiece is more in blind hole machining than through hole machining. (4) The debris of die steel is alloyed with the tool electrode material. (5) The surface hardness increases significantly.

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J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

450

900

THROUGH HOLE MACHINING

"IHROUGH~

PULSE DURATION :

PULS£ DURAI1ON :

20 p s

ELECTRO0[ ROTATION : 0

rev/mln

0

MACHINING 20 p m

ELECTROOEROTATION : 0 rav/mtn

~;

O

500 rev/n~n

850

0

750 rev/min

.~

1000 rev/min

C]

E

E

8OO

0,15 ad

7~o 0.10

700

0.05

650

I

I

I

I

I

3

6

9

12

15

go

I

I

I

I

3

6

9

12

CURRENT, A

CURRENT, A

Fig. 8 Variation of micro hardness with current and electrode rotation

PULSE CURRENT

:3A

PULSE DURATION

:

Fig. 9 Variation of depth ofresolidified layer & HAZ with current and electrode rotation

20/~m BUNO,POLE

0

850

E E

80O

ad

0.15

--

Pt~.~ CURRENT

:3^

PULSE DURAnON

: 20/~s

mROUCH HOLE

0.10

700

0.05

I,

6,50

250

I 500

I 750

R[CTRO~ ROTATION.rev/min

Fig. I0 Variation of micro hardness with electrode rotation

I 1000

A

--

0 0

I

I

i,

I

25O

5OO

75O

IOO0

EI.[C~00C ROTATION, rev/m~n

Fig. 11 Variation of depth of resolidifiedlayer & H A Z with electrode rotation

REFERENCES

[1]

[2] [3]

J.S. Soni and G. ChAkraverti, Production of Surface Finish and Accuracies with Rotating Electrode in Electro-Discharge Machining, Proc. Conf. 14th A I M T D R , IIT Bombay, 1990, p. 297. J.S. Soni and G. ChRkraverti, Physico-MechAnical Effect on Electro-Discharge Machined Surface ofHigh Carbon High Chromium Die Steel,J of Inst.ofEngrs. (India)-PR,Vol. 71, July, 1990, p. 19. J.S. Soni and G. ChMrraverti, Micro Examination of Craters and Debris Formed on E D M Surfaces, J of Metals, Materials and Processes, Vol. 3, No 3, 1991, p. 177.

J.S. Soni, G. Chakraverti / Journal of Materials Processing Technology 56 (1996) 439-451

[4] [5] [6] [7] [8] [9]

[10] [11] [12]

[13] [14] [15] [16] [17]

[18]

J.S. Soni and G. Chakraverti, Cracking Behaviour of High Carbon High Chromium Tool Steel During EDM-A Study, Proc. Conf. 14th AIMTDR, IIT, Bombay, 1990, p. 263. K. Kagaya, Y. Oishi and K. Yada, Micro-Electro-Discharge Machining Using Water as a Working Fluid-1 : Micro-hole Drilling, Prec. Eng., Vol.8, No 3, 1986, p. 157. T. Sato, T. Mizutani, K. Yonemachi andK. Kawate, The Development of an Electro-Discharge Machine for Micro-hole Boring, Prec. Eng., Vol.8,, No 3, 1986. p. 163. M.L. Jeswani, Physico-Mechanical Characteristics of Spark Machined Surfaces, Proc. Conf. 9th AIMTDR, IIT, Kanpur, 1980, p. 32. V.S.R. Murthi and P.I~ Philip, An Analysis of Debris in Ultrasonic Assisted EDM, Wear, Vol. 117, No 2, 1987, p. 241. F. Roethel, L. Kosec and V. Garbajs, Contribution to the Micro-Analysis of the Spark Eroded Surfaces, Annals of the CIRP, Vol. 25, No 1, 1976, p. 135. J.D. Ayers and Moore Kathy, Formation of Metal Carbide Powder by Spark Machining of Reactive Metals, Metallurgical Transactions, Vol. 15A, 1984, p. 1117. Koshy George, P.I~ Philip and Geddam A, Hardening of Surface Layers Using Electric Discharge Techniques, Proc. Conf. l l t h AIMTDR, IIT, Madras, 1981, p. 315. P.C. Pandey and S.T. Jilani, Plasma Channel Growth and the ResolidifiedLayer in EDM, Prec. Eng., Vol. 8, No 2, 1986, p. 104. Rajnish Prakash and U.I~ Gupta, A Study of the Isopulse Spark Eroded Surfaces of some Tool Steels, Proc. Conf. 10th AIMTDR, 1982, p. 351. A. Erden, Effect of Materials on the Mechanism of Electric Discharge Machining (EDM), J of Engineering Materials and Technology, Vol. 105, 1983, p. 133. J.R, CrookaU and B.C. Khor, Residual Stresses and Surface Effects in Electro-Discharge Machining, Proc. Conf. 13th IMTDR, 1972, p. 331. A. Gangadhar, M.S. Shunmugam and P.K. Philip, Surface Modification in Electro Discharge Processing with a Powder Compact Tool Electrode Wear, Vol. 143, No 1, 1991, p. 45. Cochran, G. William and COx, M. Gertrude, Experimental Design, Asea Publishing House, Bombay, 1977. M.N. Das and N.C. Giri, Design and Analysis of Experiments, Willey Eastern Ltd, Hyderabad, 1986.

Appendix The estimates of the coefficients and the test of their significance are presented in an ANOVA table. Hypothesis tested are : Ha : ~q=0, FLlx~.0.os~= 4.84 ( Micro hardness for die steel ) Coefficient estimated 13o

Estimate 13ij

d.o.f

S.S.

M.S.

731.111

1

5441334.457

5441334.457

Fcal 6985.304 **

131

29,833

1

5340A67

5340.167

6.855 *

[32

27.500.

1

4537.500

4537.500

5.825 *

~11

4.833

1

46.721

46.721

0.05998

1322

64.833

1

8406.705

8406.705

10.792 *

~12

-1.25 0

1

6.250

6.250

6

5459671.800

909945.300

Due to regression About regression

11

8568.657

778.969

Total

17

5468240.457

321661.200

* Significant

( P < 5~ )

** Highly significant ( P < 1~ )

Coefficient of correlation,'r = 1 (Micro hardness) ----1--

0.008 1168.141"*

SSabout regr SS total 8568.657 5468240.457

= 0.9984

451