Paper received: 00.00.200x Paper accepted: 00.00.200x
Morphological - Functional Aspects of Electro-Discharge Machined Surface Textures
Georgios P. Petropoulos1 - Nikolaos M. Vaxevanidis2 - Miroslav Radovanovic3 - Carol Zoler4 1 University of Thessaly, Department of Mechanical and Industrial Engineering, Greece 2 School of Pedagogical & Technological Education, Department of Mechanical Engineering Educators, Greece 3 University of Niš, Faculty of Mechanical Engineering, Serbia 4 University of Petrosani, Faculty for Mechanical and Electrical Engineering, Romania
The present study concerns with the investigation of a set of "non-common " surface topography parameters of Electro-Discharge Machined surfaces that give differing aspects of texture related to morphological characterization and possible tribological applications. The parameters considered are the Abbott (bearing) curve parameter at 10 % of the raw unfiltered and the roughness profile height PtpicM and Rtp10%, respectively, the also bearing curve oriented Rk family of parameters (ISO 135652:1996), the skewness Rsk and the kurtosis Rku of the profile height distribution, the mean spacing Rsm, and the fractal dimension D. The correlation of the aforementioned parameters with pulse energy is examined to allow appropriate selection of machining conditions for producing functionally desirable Electro-Discharge Machined textures.
© 2009 Journal of Mechanical Engineering. All rights reserved.
Keywords: electrical discharge machining, steel, surface topography, surface roughness
0 INTRODUCTION
Technological advances and high strength requirements have led to an increasing use of materials with improved mechanical properties in industrial production. In the machining of these materials cutting processes exhibit several and serious shortcomings and are rivaled by the so-called "non-conventional" processes.
The most important advantage of Electrical Discharge Machining (EDM), one of the most widely applied non-conventional processes for high precision products, is that its effectiveness is independent of the mechanical properties of the machined materials on the convention that they are electrically conductive [1].
In EDM, for removing material from a machined part, a series of repetitive spark discharges between the electrode and the workpiece causes local melting and/or evaporation of material in the presence of a dielectric fluid and the resulted surface is characterized by overlapping craters and other topographic formations indicative of the intense thermal impact involved [2].
Besides the study of surface integrity changes (including surface topography) induced by EDM [2], of major interest is the correlation of the surface topography parameters with
machining conditions towards the advanced control and optimization of the EDM. 2-D and 3D profilometry is a promising tool for this purpose [3], together with the application of design of experiments methodology [4] and neural network models [5].
In previous works [2] and [6] to [7] emphasis was directed towards the multiparameter analysis of the Electro- Discharge Machined surface texture interrelationship to process parameters and statistical regression models were developed to correlate the machining conditions with the imparted surface finish characteristics.
The characterization and evaluation of engineering surface topography is a must for describing the functional behaviour and monitoring the quality of the products, as well as to control the manufacturing process performed. The measurement of surface topography has constituted a challenging metrological problem over the years. Since textures are complicated in form and to obtain a satisfying description at various levels, many parameters have been proposed.
The establishment of the "M" (central line) system has facilitated the communication between laboratory and industrial practice but too many parameters have been proposed; more than
*Corr. Author's Address: University of Thessaly, Volos, Greece, gpetrop@mie.uth.gr
95
one hundred in view of literature! [1]. The ISO-4287: 1997 standard comprises thirteen parameters for surface roughness and the relevant surface waviness parameters. This fact is attributed to the usually complicated form of surface textures and the need for obtaining a satisfying global description. As it is expectable, numerous research papers have been focused on better manipulation of these parameters in various manufacturing processes and the impact of process factors on surface characteristics (indicative refs [3] and [4]).
As dies, moulds and other components are nowadays often manufactured by EDM with high precision requirements, classical surface roughness parameters like Ra, Rt or Rz are not sufficient for providing a morphological characterization of surfaces. The consideration of more roughness parameters would be more useful to measure surface characteristics because of the aforementioned overlapping craters over the surfaces and the nature of the irregularities being completely different compared to conventional machining processes. Further, features of Electro-Discharge Machined surface profiles should be associated with functional characterization of the surface in case of experiencing contact used as machined or secondarily processed by a smoothening operation; to the authors' knowledge this topic is somehow overlooked in view of literature.
The present study concerns with the investigation of a set of "non-common" surface topography parameters of Electro- Discharge Machined surfaces that describe differing morphological characteristics and are sensitive to profile shapes.
Surface analysis followed deals with three aspects, namely: (a) correlation of bearing curve parameters with machining conditions and other surface texture parameters, (b) representation of the obtained bearing curves through polynomial fitting models and (c) description of bearing curves by the set of Rk parameters (ISO 135652:1996). The influence of the process variables on a number of amplitude (arithmetic and statistical) surface roughness parameters (Ra, Rsk, Rku), and the spacing parameter Rsm, which also provides differing aspects of surface properties, is also examined.
1 EXPERIMENTAL PART
1.1	Machining Conditions-Materials
Electro-Discharge Machining was performed on a HOSTEK SH-38GP (ZNC-P type) electro-discharge machine-tool with working voltage (Ve) of 30V and open circuit voltage of 100 V. Experiments were conducted in a typical oil dielectric (BP250) with electrolytic copper being used as the tool electrode (anode).
The pulse current, ie and the pulse-on time, tp considered to be the main operational parameters varied over a range from roughing to finishing, namely: /'e: 5, 10, 20, 30 A and tp: 100, 300, 500 |isec, thus resulting in 12 discrete pulse energies. The pulse energy was calculated by the formula: We = Ve Ie tp (Ve = 30 V).
Specimens of plain carbon steel Ck60 and an AISI D2 tool steel (Sverker 21) in the form of square plates of dimensions 70 x 70 x 10 mm were used as workpieces (cathode).
Ck60 is a popular structural steel with sufficient hardness and AISI D2 is a tool steel characterized by high wear resistance and high compressive strength.
Their chemical compositions are accordingly:
Ck 60: 0.60 % C, 0.35% Si, 0.80 Mn.
AISI D2: 1.50% C, 11.50 Cr, 0.80 V, 0.75 Mo.
1.2	Surface Texture Measurements
The "non-common" texture parameters used in this study are incorporated in the DIN EN ISO 13565-2: 1998 and ISO 4287:1997 standards, except the profile fractal dimension. Ra is considered for reference, as it is widely used.
The definition of the arithmetic and statistical parameters used according to ISO 4287:1997 is briefly given, as follows:
•	Ra (|im): average height of the profile
•	Rsm (| m): mean spacing of the profile peaks measured at the central line
•	Rsk : skewness (3nd order central moment) of the profile amplitude distribution
•	Rku : kurtosis (4th order central moment) of the profile amplitude distribution.
The surface texture parameters related to Abbott (bearing) curve parameters expressed by the ISO 4287:1997 represent the material to void relation at 10 % of the raw (unfiltered) and the
roughness profile height, and are Ptpio% and Rtp10<%, respectively. The raw parameter is used to provide an indicative description of integrated surface texture, considering at the same time both roughness and waviness; the latter is neglected in several cases, albeit it is functionally significant [22]. Of course, a distinction must be made between corresponding Abbott curves for the raw and the roughness profile.
Another way for evaluating characteristics of Abbott curves of roughness is through the "Rk" family of parameters defined in [8]. According to these standards a group of five parameters characterizes the following three components of surface roughness (Fig. 1): a) core by Rk, which stands for the depth of the roughness core profile b) peaks by Rpk representing the top portion of the surface to be quickly worn away and MR1 that is the upper limit of the core roughness and c) valleys by Rvk describing the lowest part of the surface which has the function of retaining the lubricant and MR2, the lowest limit of the core roughness.
P100
"40% —
MR1	MR2
Fig. 1. Definition of the family of "Rk" parameters
Regarding the determination of profile fractal dimension D, there is no yet internationally standardized procedure.
The multi-parameter surface texture analysis was performed using a Rank Taylor-Hobson Surtronic 3+ profilometer equipped with the Talyprof software. The cut-off length was selected at 0.8 mm whilst 40 measurements were conducted on every specimen at random directions, as it is known that Electro- Discharge Machining generates geometrically isotropic textures [9].
2 RESULTS AND DISCUSSION 2.1 Variation of Amplitude Parameters
Arithmetic average roughness (Ra) is by far the most commonly used parameter in surface finish measurement and for general quality, as well as process control. Despite its inherent limitations, is easy to be measured and offers a good overall description of height characteristics of a surface profile. The variation of average roughness, Ra with pulse energy is presented in figure 2. For Electro-Discharged Machined surfaces the variation of Ra with process operational parameters follows well-known patterns [10]; it increases when the pulse energy increases with a gradually lower rate. For medium and high pulse energies (We > 150 mJ).
?
.5 10
0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 2. Variation of average roughness Ra, with pulse energy, We
Electro-Discharged Machined surfaces of AISI D2 tool steel (Sverker 21®) are systematically rougher than the corresponding ones of Ck60. This observation can be correlated with differences in crater dimensions for the certain pulse energies, which in turn are physically linked with corresponding alterations of the heat source, i.e. the plasma channel and the resulting melting isothermal, as well as with the thermal properties (conductivity, diffusivity) of the material being machined [11].
The skewness parameter (Rsk) is typically used to measure the symmetry of the profile about the mean line providing an implication of the "fullness" or "emptiness" of the profile and is sensitive to the existence of deep valleys or high peaks [12]. Surfaces with a positive skewness, such as turned surfaces have fairly high spikes that protrude above a flatter average. Skewness correlates with the load carrying capability of a surface and negative values for the former
Sverker 2'
Ck fif
20
15

0
indicate pronounced bearing properties and favourable anti-wear behaviour; mostly for the running-in stage of function. The variation of skewness of EDMed surfaces with pulse energy is illustrated in figure 3; in general, it appears uncorrelated to the pulse energy. Judging from the measured skewness values, the EDMed profiles are revealed to be "empty" of material as indicated from the relatively high positive values, excluding some lower pulse energies; at higher energies skewness is almost stabilized at positive values; see also [7].
Pulse Energy We [mJ]
Fig. 3. Skewness of the heights distribution Rsk, versus pulse energy, We.
Kurtosis (Rsk) typically describes the sharpness of the probability density of the profile [7] and [12]. Practically, low kurtosis values indicate strong bumpy peaks that increase bearing capability and wear resistance, but this must be seen in association with skewness.
Measured values of this parameter are in the range of 2.5 to 3.5, see Fig. 4, indicating randomly distributed high peaks and low valleys but show poor correlation to pulse energy. Note also that for both Rsk and iku parameters higher values were measured for AISI D2 than the Ck60 specimens for the same medium and high pulse energies.
0 J-,-,-,-,-,
0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 4. Kurtosis of the heights distribution Rku, versus pulse energy, We
2.2 Spacing parameter
The spacing parameters measure the horizontal characteristics of the surface profiles. These parameters are of utmost importance in sheet metal pressing since they influence the lubrication conditions and the avoidance of surface defects such as scoring.
During the present study the mean spacing of the asperities (Rsm) was measured and the variation of this parameter with pulse energy is plotted in Fig. 5; it exhibits an increasing tendency. In combination with the number of the high spots of the profile, which decreases with pulse energy, it is concluded that EDMed surfaces processed at high pulse energies will permit a small number of contacts with a possible counterpart despite the increased roughness height.
* Sverker 21 —■— Ck 60
Pulse Energy We [mJ]
Fig. 5. Variation of the mean asperities spacing, Rsm with pulse energy We
2.3 Correlation of Bearing Curve Parameters with Machining Conditions
The Abbott (material ratio or bearing) curve is nowadays established as a mean for providing a working representation of the portions of the surface at different depths combining texture aspects related to contact area and contact mechanics, wear, lubricant retention and others [13]. Other views of the surface bearing capability can be provided by the skewness of the profile height distribution and the fractal dimension. These topographic parameters apart from connecting the EDM process to functional behaviour of the machined surfaces can also describe useful surface features and generally to give useful insights into the nature of roughness regarding the EDM process control.
Rtp% as a parameter refers to the bearing ratio at a specified height of the profile. The
bearing curve parameters corresponding to the raw and the roughness profile were selected at 10% depth in order to be functionally significant considering waviness as an also important component. The variation of Ptp10% and Rtp10o/„ parameters with pulse energy is presented in Figs. 6 and 7, respectively. Both parameteres increase when the pulse energy increases, over a relatively wide range. This behaviour implies that the bearing capability of the surface becomes pronounced at the same level, when intensifying the machining conditions. Furthermore, the similar trends shown by the unfiltered and the roughness profile Abbott curves indicate that besides roughness, waviness contributes to an extent in the rise of surface bearing capability exhibiting an increasing pattern. This fact was also confirmed in [7], attributed to intensified vibration between the electrode and the workpiece.
20
B- 30
Q.
¡5 25 a>
i §
? S 15 >
B 10
c 5
100	200	300	400
Pulse Energy We [mJ]
Fig. 6. Variation of Ptplmparameter of unfiltered profile with pulse energy, We
tg 5 c
<8 0

0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 7. Variation of Rtpl0%,parameter of roughness profile with pulse energy, We
Worth mentioning that for medium and high pulse energies, both Ptp10% and Rtp10% values measured for AISI D2 specimens are
systematically higher than the ones measured for Ck 60; a trend similar to one observed in Ra measurements.
A similar trend is followed if the Abbot parameters are considered with respect to Ra and Rsk. Note, that Rsk (skewness) can be considered as a rough measure of the surface bearing capacity and as it is shown generally correlates with the Rtp (Abbot curve) parameter.
2.4	Representation of Bearing Curves through Polynomial Fitting Models
Preliminary analysis and previous researches indicated that 3rd order polynomial fitting model gives a suceessful representation of bearing curves [13] and f15]. Given the mutual accordance, only the raw profile bearing curves fitting, for Ck 60 steel specimens, according to equation, y = ko + k1x + k2x2 + k3x3 is presented. Calculated fitting coefficients are shown in Fig. 8. An intense dropping trend of the cubic coefficient appears at high pulse energies. All these changes in the shape of the bearing curves are closely related to contact mechanics and wear behaviour [16].
2.5	Description of Bearing Curves by the Set of Rk Parameters
The Rk configuration is designed to divide the bearing ratio curve into three sections: the small peaks above the main plateaus, the plateaus themselves, and the deep valleys between plateaus; see Fig. 1.
These parameters describe the shape of the relevant bearing (material ratio or Abbott) curves and permit the distinction between plateau and valley portions of the surface along with the core characteristic of the curve, which corresponds to the stable portion of material in the surface after initial wear. A linear approximation of the bearing curve is provided: the depth of profile below 40% bearing area is taken to indicate the steady state wear status of the engine. They have been used in research works either [3] and [5]. The parameters were defined in paragraph 1.2. The functional behaviour in this way may be predicted and the control of the manufacturing process can be assisted.
Sverker 21
Ck 60
0
0
500
Sverker 21
Ck 60
25
S 20
15
> 10
60 55 50 45 40 35 30 0.1
■0.1
30 45 60 150 270	30 45 60 150 270
Pulse Energy We [mJ]	Pulse Energy We [mJ]
Fig. 8. Polynomial fitting coefficient against pulse energy, We (ko.' constant, kp linear, k2'
square, k^: cubic, material: Ck 60)
The variation of these parameters with pulse energy for all Electro-Discharge Machined specimens is plotted in Figs. 9 to 13. Judging from these plots it is obvious that the ISO 135652:1998 suits EDM, as the bearing curves are of a general "s"-shape and the correspondingly divided portions of the surface profile are distinctive, and the parameters vary in a monotonous way. The standard is successfully applied to "s"-shaped bearing curves in view of morphology because the profile statistical distribution does not markedly deviate from normality, but this does not imply necessarily a stratified texture [20] to [23]. And note however, that this standard was originally formulated for stratified textures and therefore the present results need further clarification, e.g. through friction and wear test on a pin-on-disc tribometer.
From the plots presented in Figs. 9 to 13 it is evident that for low pulse energies (We < 90 mJ) measured values for the two steels are quite similar; for medium and high pulse energies measured values for AISI D2 tool steel (Sverker 21®) are systematically higher than the corresponding ones for Ck 60. This trend was identified for all five parameters of the Rk family. Moreover, for high pulse energies (We > 270 mJ) Rvk, Mrj and Mr2 are almost constant and
characteristic for each material, see Fig. 11 to 13 respectively.
•Sverker 21	Ck 60
0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 9. Variation of Rk with pulse energy, We
•Sverker 21	Ck 60
10 8
I 6
1 4
0 100 200 300 400 500 Pulse Energy We [mJ]
Fig. 10. Variation of Rpk with pulse energy, We
40
0
2
0
-Sverker 21 —■—Ck 60
0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 11. Variation of Rvk with pulse energy, We
-Sverker 21 —■—Ck 60
30 25 5? 20 t 15
I 10
5 0

0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 12. Variation of MR1 parameter with pulse energy, We
♦ Sverker 21 —■—Ck 60
120 100 80 60 40 20 0
0	100	200	300	400	500
Pulse Energy We [mJ]
Fig. 13. Variation of MR2 parameter with pulse energy, We
2.6 Fractal Presentation
machining conditions [14] and [24], it was the only fractal paprameter considered in this study.
The fractal dimension D is an intrinsic property of the surface, which is scale independent and reflects, as aforementioned, the "complexity" of the profile structure. High values for fractal dimension D are relevant to higher complexity of the profile and as a consequence reinforce surface bearing capability.
A two dimensional self-affine fractal was assumed to represent the profiles of the EDMed parts and D was calculated via the power spectrum method. It appears almost insensitive to the pulse energy variation, except a rise at a low energy in the case of Ck60, see Fig. 14; such a phenomenon is consistent with results reported in [14]. The meaning of this is that the degree of complexity of the EDMed surfaces is almost independent of the machining conditions employed and over a wide range of their variation.
The fractal dimension values are significantly higher for Ck60. Certainly, more steel grades over a wider range of machining parameters have to be studied for clarifying the applicability of fractal geometry analysis in Electro-Discharge Machining.
□ 2 c
I 1,5
c
0}
! 1 b
m 0,5 it 0
200 300 Pulse Energy We [mJ]
Sverker 21
Ck 60
0
100
400
500
In addition to Abbot parameters, fractal-based methods for describing surface texture have attracted great interest because they can provide information that conventional surface roughness parameters cannot. Surface complexity can be represented under some hypotheses by fractal geometry. Fractal has been introduced to describe the micro roughness of surfaces generated by fracture, machined surfaces are as such, in order to provide evaluation by one or two parameters only.
The main fractal parameters are fractal dimension D and topothesy L. As fractal dimension is established to correlate with
Fig. 14. Variation of fractal dimension D with pulse energy, We
3 CONCLUSIONS
Close correlation exists between the tp bearing curve parameters of the raw and the roughness profile and the pulse energy regarded as the main machining variable. It was found that the Ptp and Rtp parameters increase monotonously with increase in pulse energy, whilst most of the Rk parameters exhibit a similar trend. Rsk and Rku parameters appear uncorrelated to pulse energy and moreover, the fractal dimension D appears insensitive to the pulse energy variation. Note that
R^ can be considered as a rough estimate of the surface bearing capability and as it is shown, generally is correlated with the tp (Abbot curve) parameters.
The bearing curves are represented by a 3rd degree polynomial model, which fits them satisfactorily. An intense falling trend of the cubic coefficient appears at high pulse energies. All these changes in the shape of the bearing curves are closely related to contact mechanics and wear behaviour of machined surfaces.
As far as the description of bearing curves by the set of Rk parameters defined in ISO 135652:1996 is concerned, it is indicated that these parameters can provide an adequate description of crucial portions of the resulted surface profile in relation to the machining conditions;
Regarding the correlation of the examined parameters with the machining conditions the Rsk, Rku and D appear uncorrelated over the whole range of machining conditions variation. A possible meaning of this is that these parameters are insensitive, more or less, to process factors and they correspond to essential qualitative characteristics of Electro-Discharge Machined surface texture and could make up a representative set of morphology or function oriented parameters. However, in order to verify this statement, more evidence is required in terms of wider variation of machining parameters and workpiece materials.
The aforementioned parameters give insight to different features of the Electro-Discharge Machined surfaces, both topographically and functionally and are of particular interest in controlling and optimising EDM operations. It would be possible to manufacture parts with certain surface roughness requirements using the results instead of trial and error.
Even in case that requirements exist for a second processing, by burnishing, grinding or other method, findings of such a kind will serve as inputs for a proper selection of process parameters.
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