A study on the influence of grain size in electrochemical machining

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002~,~-"35"~ 81 01 005--13 S0200 2 Pergamon Pres~ Lid

A STUDY ON THE INFLUENCE OF GRAIN IN ELECTROCHEMICAL MACHINING

SIZE

and V. RADHAKRISHNAN~"

O, V. KRISHNA1AH CHETTY*

(Originall~ re~eized 10 October 1979: in final form 20 October 19801

Abstract - The metallurgical characteristics of the work material play an important role in Electrochemical Machining (ECM I. The effects of ~ orkpiece grain size and electrolyte velocity on the surface produced by ECM are discussed in this paper. Surface profile analysis together with scanning electron microscope studies brought out the significant role played by the above t~o parameters in surface production, particularl.~ on the short wavelength irregularities of the surface produced.

INTRODUCTION

THE PRINCIPLES of electrochemical machining are well known and understood. But the mechanism of surface production in ECM is a complex one. The basic principles of ECM deal with the removal of the material, in terms of the macrogeometry of the work surface. However in reality the microgeometry of the surface changes due to the heterogeneous transformations on the work surface leading to the formation of a surface topography characteristic of the process. The surface finish produced in ECM can vary widely depending on the selected parameters. Apart from the process parameters, one of the factors influencing the finish is the metallurgical structure of the work material. In brief, the overall mechanism of surface generation in ECM is not fully understood. Surface studies using a relocation machining fixture showed the influence of the electrolyte flow velocity on the macro- and microgeometry of the surface [1, 2, 3]. The influence of the workpiece structure on current density and metal removal rate has been reported [4, 5, 6]. Better metal removal rates have been observed for fine grained workpieces at higher flov, velocities [4]. The dependence of surface texture resulting from anodic dissolution of polycrystalline materials and single crystals on mass transfer conditions and crystal orientation has also been reported [7, 8, 9]. This paper deals with the role played by the workpiece grain size and the electrolyte flow velocity on the surface formation in ECM. Observations made by using the optical and scanning electron microscope have been quantified by the power spectrum analysis of the surface profiles and are presented in detail.

TABLE 1. Batch number

Annealing temperature (:C~

Soaking time (h)

Axerage ferrite grain size (/lm~

1 2 3 4

850 950 1200 1250

1 1 2 5

25.4 35.9 71.8 1440

Department of Mechanical Engineering, Indian Institute of Technology. Madras 600 036. India * Lecturer. "lProfessor. 57

58

O.V. KRISHNAIAHCHETTY and V. RADHAKRISHNAN

Ro

Flow Velocity

Rt

,urn

,urn

70

Zl

5.7

36

10

25

155

15

18

115

20

rn/s

Groan size : 71 8prn

200H,.~ 25Fro

l = 50Amps Y: 0 5rnrn t=10s J=102 A/cm 2

FIG. 1. Effect of electrolyte velocity on surface profiles.

EXPERIMENTAL

APPROACH

Experiments were conducted using a basic ECM set-up [1]. The electrolyte used was 25 % by weight brine solution. Steel of 0.42 % carbon was heat-treated to obtain different grain sizes and these were used as work material. The heat treatment details are given in Table 1. The grain size referred to is the ferrite grain size. These workpieces were of 25 mm diameter and for machining a 25 mm @ copper tool was used. The machined surface was checked for the roughness (R,, R t values) and the surface profile was traced and digitised for further analysis. RESULTS

AND DISCUSSION

Experiments conducted with flat and polished workpieces machined with an interelectrode gap (Y) of 0.5 mm showed some interesting trends with reference to the grain size and flow velocity. A set ofprofiles indicating the effect of electrolyte flow velocity (V) is shown in Fig. 1. These were obtained for a grain size of 71.8#m with a current density (J) of

Ro

Rt

Groin s~ze

50

33

Kz 0

3.5

2L.

~

25

15

t' L ~ L) ",-,.Jv~ ',,'vv,J

35.9

2s>,m2I-~L~^" , ±

"v~qV,,v,v./~

I=50Arnps

t= 10 s

V : 10 rn/s Y : O 5 m m J =10.2 A/crn2

FIG. 2. Effect of grain size of workpiece on surface profiles.

25 /.

Influence of Grain Size in ECM

59

10.2 A cm- 2 (current I = 50 A). The improvement in the finish with increasing flow velocity is clearly indicated by the profiles and their roughness values. Experiments with other grain sizes showed the same trend. The role of the workpiece grain size on the finish produced is brought out clearly in Fig. 2. These experiments were conducted with a flow velocity of 10 m sec- 1. The general trend of the results indicated that with a decrease in the grain size the finish improved. However, with 144.0/am grain size the observed values of surface roughness were lower than that obtained with 71.8/am grain size. One reason for this is due to the limited time of machining (10 sec) which is too short for machining out a full layer of grains of 144/am size. Figure 3 shows the combined effect of grain size and flow velocity on the roughness values, i.e. R, and R t. From this figure is could be observed that appreciable reduction in the roughness values starts near about 15 m see- 1 flow velocity, Below this velocity the grain size plays an important role in reducing the roughness. At velocities higher than 15msec-~ the role of grain size is insignificant as far as the surface roughness is concerned. Thus the effect of larger grains could be suppressed to a certain extent by increasing the electrolyte flow velocity. The trend of the R t values shown differs slightly from that of the Ra values and this is due to the inconsistency in the R t value which is obtained from two points in a profile. A relocation study on the volume machined showed that the metal removal rate increased with decreasing grain size [1.]. This is in line with the results reported by Kops and Quach [4].

I

I ~

-- - - - I ~ . ~ - - ~ " ~ . ~ . 1 ~ E :=.. ~4

I = 50Amps I Y= 0.5 rnrn n l

rt=l°" J = l ° 2 A ~ l

~

~

/ t GrQin size: o 2s,.,um b • 35.9 prn , ~ o! 71.0pm

"-

n,

'

5

i

I 10 Velocity,

15

20

15

20

rn/s

50I' '" I

t.O

30 E 20 - n-

10

10 Velocity, m/s

FIG. 3. Effect of velocity and grain size on R, and R, values of surface profiles.

(0)

'250Hm

{b}

P' 250/Jm

. {(:) , 250jura

(d)

(e)

,25o,,u

~250Hm -

FI0w direction

V= 5 m / s .

V = 10m/s

(f)

250pro,

{g} ,~250)urn4

V= 15m/s

V = 20 m/s

J = 10.2 A / c m 2

250~Jm (i) .~ : J -20./., A / c m 2

FIG. 4. Optical micrographs illustrating the effect of electrolyte velocity on surface topography. 60

Influence of Grain Size in ECM

61

Ftow direction

Ir

•

p e

i }

(O)

,20~um ,

2o/Jm ,

",

, 20~Jm ,

(d)

V=15m/s

(e)

I

V= 1 0 m / s

V= 5m/s

(c)

202am

(b}

V= 20m/s

, 20/Jm , J=10.2

20pro A/cm 2

/f}

'

'

FIe;. 5. Scanning electron micrographs and optical micrographs shov,ing the effect of velocity on surface topography.

62

O . V . KRISHNAIAHCHETTY and V. RADHAKRISHNAN

In order to observe the changes taking place on the three-dimensional surface, optical micrographs were taken after machining. Such optical micrographs of the workpieces of grain size 71.8 gm, machined at different flow velocities and current densities are shown in Fig. 4. The micrograph of the polished and etched workpiece (not machined by ECM) revealing the grain boundaries is shown in Fig. 4a. The surfaces on the left, i.e. b, c, d and e are obtained with different flow velocities, i.e. 5, 10, 15 and 20 m sec- ; respectively at a current density of 10.2 A cm- 2. To the right we have the micrographs obtained with a current density of 20.4 A cm- 2 for the same velocities. A vertical scanning of the two columns ofmicrographs shows a qualitative improvement in the finish. At 20 m sec- ; velocity with the lower current density the appearance of the surface is similar to that of the etched specimen shown in Fig. 4a, indicating thereby that etching conditions would have prevailed at this current density. With higher current density this was not observed. An explanation for such etching is given by Boden and Brook [10].

Flow direction

2rum

?,urn

t

1o) V= 5 m / s

(b) V:lOm/s

, 2/,Jm ,

t

(c)

(d)

V=15m/s

V= 2 0 r n / s J = 10.2 A / c m 2

FiG. 6. SEM pictures showing the effect of electrolyte velocity on short waves.

2~Jm

Influence of Grain Size in ECM

63

The topography of the surface observed could be explained by the theory proposed by Kops and Quach. Since the atoms along the grain boundaries possess higher free energy than the rest of the surroundings, in the active mode of dissolution the grain boundaries are attacked first. This leads to the formation of depressions and the surface becomes rough. The thickness of the diffusion layer is affected by the surface roughness and because of the depressions, the thickness of the diffusion layer tends to reduce further. In such a case, a rotating eddy is produced since the flow tends to separate from the surface. The metal ions on the upstream side of the valley will be picked up by the eddy and are transported away by the flow. Hence the rate of mass transfer in these areas depends on the velocity of the small eddies. These micro fluid disturbances, termed as 'rotating eddies' remove metal ions from the grain boundaries, thus accelerating the mass transfer and reducing the surface finish [4]. Any effort to increase the velocity reduces the diffusion layer thickness. For a flow velocity of 5 m sec- 1 the diffusion layer thickness and the viscous sub-layer thickness are 16.8 x 10 -4 mm and 15.7 x 10- ~ mm respectively. The corresponding values are 5.4 x 10 -4 mm and 5 x 10- 3 mm for a flow velocity of 20 m sec-1 [7, 11]. A direct result of increasing the velocity will be an increase in the level of turbulence. As a consequence to this, the effect of rotating eddies may be reduced resulting in a better finish. Such an improvement is observed in the finish as the velocity is increased (Figs. 4b-e). The trend was the same for other grain sizes. However, in all cases an improvement in the finish was noticeable with higher current densities for the same flow velocities. This can be qualitatively observed by scanning the figures in Fig. 4 horizontally. This is in tune with the known observation that a higher current density produces better finish. The topography of the surface has been governed by the micro eddy patterns which existed at the time of machining. For a detailed observation of the surface irregularities, SEM photographs of the specimens shown in Figs. 4(b-e) were taken and are presented in Figs. 5(a-d). Here again it is clear that the random surface produced by ECM is influenced considerably by the flow velocity. However, there is an interesting feature to be observed in Fig. 5(d). In this, some of the grains are seen spotted and some others have fine serrations on them. These serrations are uniform in certain places and non-uniform in other places. Micro-hardness tests revealed that the spotted grain and the grains with uniform serrations are ferrite grains (Figs. 5(e-f). This result indicates that the grain orientation has a significant effect on the short waves and supports the theory proposed by Kops and Quach [5, 6]. The sharp lameilar appearance of the short wavelength irregularities shown in Figs. 5ta, b) may be interpreted as the defect structure present in the grains. At such a microscopic level the orientation and individual structure of the grain determine the 'very short' wavelength irregularities. Specimens of different grain sizes, machined at lower velocities of 5 m sec- t and 10 m sec- ~ showed a lamellar appearance similar to Figs. 5(a, b). With increasing velocity of the electrolyte, these fine waves were modified. Quantitative evaluation of the amplitude of these waves was not possible due to the short wavelength of the irregularities (less than 1 pm) which were not reproduced by stylus instrument with a tip radius of 3/~m. However from Figs. 6(a-dt which show the enlargement of Figs. 5(a-d) it could be noticed in a qualitative fashion that these short wavelength irregularities are getting modified with increasing flow velocity of the electrolyte. Non-uniform short wavelength irregularities were observed on pearlitic grains. Ferrite grains dissolve more rapidly than pearlitic grains due to the presence of cementite which is passive in electrochemical reactions [12, 14]. Hence, as the ratio of pearlite to ferrite is increased, the non-uniform short waves increase making the surface rougher. For a better understanding of the role played by the flow velocity in obtaining a good finish, better experimental techniques are being developed [13]. Figure 7 shows the SEM micrographs of the specimens shown in Figs. 4(f-i). The investigations by Kops and Quach at high flow velocities (40-45 m sec- 1) brought out the influence ofworkpiece structure on current :lensity and metal removal rates [4, 5, 6]. The results presented in this paper are for a velocity range of 5-20 m sec- 1. Though the results obtained confirm the effect of grain size on the metal removal rate, the main point of interest has been on surface finish and its relation with flow velocity and grain size. Since the machining time was limited to 10 sec,jobs with the largest grain size were not fully machined

64

O.V. KRISHNAIAHCHETTY and V. RADHAKR1SHN~N Flow direction

20pro

20pro 4 (o)

'

"

(b)

V= 5 m / s

V =10m/s

I t

20pro ,

t•

k

,20prng (d)

(c) V=15m/s

V=20m/s J =20/, A/cm 2

FIG. 7. Scanning electron micrographs of components machined at 100 A current.

to remove one layer of grains. However with other sizes this time was sufficient. Experiments conducted on initially rough surface having random surface profiles (produced by shot blasting) again showed that the surface topography altered with the change in the electrolyte velocity. The SEM photographs of these workpieces machined at 100 A current (J = 20.4 A cm-2) are shown in Figs. 8(a-d). The surface characteristics showing the short waves are given in Figs. 8(e-h) at a higher magnification. The presence of pearlite in carbon steel gives a spongy appearance to the machined surface [12, 14]. Machining of Armco iron which is a single phase material gives a smooth surface. Even in this material the finish is found to improve with increasing electrolyte velocity. In Fig. 9 the SEM photographs of the surface of Armco iron together with the surface profiles taken are given. In order to quantify the observations, the power spectrum of the surface profiles were computed. In these computations, the very short waves which were only observed by the SEM could not be included due to the limitations in the resolution of the stylus instruments. The effect ofworkpiece grain size and the flow velocity on the power spectrum of the surface profiles is shown in Fig. 10. An increase in the grain size reduces the power of the short wave

V = 5 m/s

(e)

(a)

~pm

J = 20.4 A / c m 2

(g) V=15m/s

(c)

Fk;. 8. SEM photographs indicating the effect of electrolyte velocity on initial random surfaces.

(f) V=10m/s

ib)

; l o w direction

Ih) V-- 2Qm/s

[d)

5pro

m

.~

.~"

g

66

O. V. KRISHNAIAH CHETTY a n d V. RAI)HAKRISHNAN

\

.... '~

.

~, ~%~- .

qt ,20prn,

20 p r n

o) V = 5 m / s

b) V= 10m/s

k

\

c) V = 1 5 m / s

,20~Jm,

20pro

d) V=2Om/s

Flow velocity m/s

Rt pm

18.8

~

8.8

~,/,,--f ~

5.0

~

~

-~

5 10

~

15 20

5.0

200lJm I ' 25~Jm

Y=O.5mm [ = 100Amps t =10s.

J=204

A/cm2

FIG. 9. Scanning electron micrographs of Armco iron machined at differentelectrolytevelocities.

length irregularities (Fig. 10a) though the overall power is increased. This may be attributed to the reduced number of grain boundaries present on the surface ofa workpiece with larger grains. Figure 10(b) shows that the total power gets reduced as the flow velocity is increased. However it may also be observed that the reduction in the power content is more significant

Influence of Grain Size in ECM

67

G=71.Spm

Groin size {6 )=25/)pro

6= 1/J.)Jrn

V=5m/s

2

(a)

V=Sm/s

~lJ, , I000 500

L

l,lll,i 100 50

i

i

I ,,~L}, I 10 1000 5OO

V =10m/s

i

I,,,¢I, 100 50

I=50A

)

V= 15m/s

i I 10 1000 500 Wove length, )J rn

t=10s

Y =0 $rnm

V=20 m / s

I

100 50

10 1000 500

100

50

10

J=10 2 A / c m 2

(b)

FIG. 10, Effect of workpiece grain size and electrolyte velocity on power spectrum of surface profiles.

for the short waves. A similar trend was observed at a current density of 20.4 A cm -2. For understanding the effect of flow velocity on rotating eddies, workpiec,es with an initial ditch (produced by an indenter) were machined at different flow velocities. Figure 11 shows the SEM photograph obtained at flow velocities of 5 and 20 m sec- 1. It can be observed from Fig. 1 l(a), that the rotating eddies are able to transport the material from the depression. Because of the lower turbulence, the transported material moves slowly in a stratified form producing a streak near the edge of the depression. At a higher velocity, the turbulence being high, the effect of rotating eddies will be reduced and no streak appears near the edge (Fig. llb). The surface near the depression for 5 and 20 m sec-1 flow velocity is shown in Figs. 1 l(c, d) indicating the improvement in the finish with higher flow velocity. CONCLUSION

An attempt has been made in this paper to study the effect of workpiece grain size and electrolyte velocity on the surface production in ECM. It has been observed that the finish in terms of R= or R, values, is considerably influenced by the grain size of the work material. Workpieces with fine grained structure produce a better finish. However the effect of grain size can be offset to a certain extent by increasing the electrolyte flow velocity. The velocity used in the experiments reported in this paper varied from 5 to 20 m sec-1 which is well within the generally accepted range of 3-30 m sec-1 [15]. An increase in the flow velocity within this range showed a significant effect on the surface topography, some of which were even beyond the resolution of ordinary stylus instruments. Power spectrum analysis showed an overall improvement in the finish as the velocity was increased. In short, based on these qualitative and quantitative studies done on the surface the following conclusions could be drawn. 1. A decrease in workpiece grain size generally leads to an overall improvement in the

68

O. V. KRISHNAIAHCHETTY and V. RADHAKRISHNAN Flow direction L

a)

b)

c)

d) V= 5rn/s

V = 20m/s

J = 20.4 A/crn2

t = 20s

Fl6. I 1. SEM photographs illustrating the effectof rotating eddies.

surface finish. This effect is more significant at low flow velocities (V< 15 m sec-1). 2, When the current density is low and the flow velocity is high (V> 15 m sec-i) the surface topography resembles that of an etched component. 3. The size, orientation and defects in the grains influence the very short wavelength irregularities produced on the surface. 4. An increase in the grain size reduces the power of the short wavelength irregularities. 5. An increase in the flow velocity also reduces the power of the short wavelength irregularities. At the present stage of development, in any practical case, the structure of the workpiece material may not be one of the main factors in the selection of the optimum conditions for

Influence of Grain Size in ECM

69

achieving the required surface characteristics. However, its role on the surface production or metal removal is of sufficient importance. In future this may help in the precise prediction of the metal removal and surface characteristics. Acknowledgements - The SEM studies are made using Cambridge Stereoscan model $4-10 and the authors greatly appreciate the help rendered bx Mr. Lall. "l-extile Technology Department. l.l.T. Delhi, India.

[1] [2] [3] [4] [5] [6] [7] [8] [9] [10] [11] [12] [13] [143 [15]

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