A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons

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1 Materials Transactions, Vol. 43, No. 6 (2002) pp to 1366 c 2002 The Japan Institute of Metals A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons De-Chang Tsai, Jenn-Ming Song, Truan-Sheng Lui and Li-Hui Chen Department of Materials Science and Engineering, National Cheng-Kung University, Tainan 701, R.O. China This study investigated the characteristics of the surface modification layer of SG cast irons using the electro-discharge machining (EDM). Experimental results indicated that the EDMed surface shows a continuous ridge appearance and the ridge density increases with higher graphite area fraction or nodule count. The modified layer with the high hardness of about Hv 1000 possesses a similar composition to the base material with near-eutectic composition. Microstructural analysis results showed that this rapidly solidified layer comprises mainly cellular γ Fe and intercellular metastable HCP ε phase. Micro-twins can be observed within the γ cells and their formation can be attributed to the residual tensile stress caused by thermal impact. (Received January 15, 2002; Accepted April 22, 2002) Keywords: electro-discharge machining, spheroidal graphite cast iron, surface modification, rapid solidification 1. Introduction Ferritic spheroidal graphite (SG) cast irons are widely used in engineering applications due to their superior properties such as good castability, heat resistance, damping capacity and excellent machinability. Therefore, their usage can be broadened to applications in gravity aluminum alloy casting 1, 2) molds and metal forming dies. Because of this, material removal of SG cast iron using the electro-discharge machining (EDM) method, which is a widely used technique for die making and precision machining, has been studied in our previous reports. 3, 4) These reports also showed that a resolidified surface layer with high hardness could be found close to the EDMed surface. Relevant reports have proposed using EDM as a surface modification technique on aluminum or ferrous alloys. 5 9) With regards to fairly soft metals, such as SG cast irons, it is desirable to machine precisely and surface modify during the same period utilizing the EDM method. Besides, the metallurgy of EDMed surface of ferrous materials has also been investigated by many researchers, 5 15) but the effects of the second phase, such as graphite, have not been examined. Considering the lack of the studies regarding this subject, the characteristics of the EDMed surfaces, as well as those of the modified surface layer, were explored in this study. In addition, the composition effects on the surface morphology were also examined. 2. Experimental Procedure SG cast irons with carbon contents from 2.1 mass% to 3.7 mass% and silicon contents from 2.0 mass% to 4.9 mass% were prepared in this study. The chemical composition and carbon equivalent (CE = %C + (%Si/3)) data are given in Table 1 with each specimen named in the table according to its carbon and silicon contents. The testing materials can be divided into two groups, including (1) C1 C5 with varying carbon content and a constant silicon content of about Corresponding author: samsong@ms24.url.com.tw Fig. 1 Microstructures of the SG irons: (a) 2.1C 2.7Si; (b) 3.0C 2.6Si; (c) 3.5C 2.8Si. 2.7 mass%; (2) S1 S4 with varying silicon content and a constant carbon content of about 3.5 mass%. For instance, the optical microstructures of the C1, C2 and C4 specimens are displayed in Fig. 1, showing that a higher graphite content and a finer graphite nodule as the carbon content is increased. The quantitative microstructural data of the testing materials are shown in Table 2. The equipment used in the EDM experiment is illustrated in Fig. 2 and Table 3 displays the settled experimental parameters. The discharge voltage variation between the cop-

A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons 1361 Table 1 Chemical composition of the SG irons (mass%). Designation C Si Mn P S Mg CE C1 2.1C 2.7Si 2.10 2.69 0.10 0.06 0.

2 A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons 1361 Table 1 Chemical composition of the SG irons (mass%). Designation C Si Mn P S Mg CE C1 2.1C 2.7Si C2 2.5C 2.6Si C3 3.0C 2.6Si S1 3.5C 2.0Si C4 (S2) 3.5C 2.8Si C5 3.7C 2.7Si S3 3.4C 4.4Si S4 3.5C 4.9Si Table 2 Quantitative data of microstructure (A g : Area fraction of graphite nodules; d g : Average diameter of graphite nodules; N g : Number of graphite nodules per mm 2 ). Designation A g (%) d g (µm) N g C C C C4(S2) C S S S Table 3 Experimental parameters in EDM. Electrode (+) Cu Specimen ( ) SG Cast irons Working area (mm 2 ) 100 Working time (min) 5 Charging current (A) 20 Pulse duration (µs) 100 Duty factor 0.5 Dielectric fluid Kerosene Fluid pressure (Kg/mm 2 ) 0.7 Duty factor is the ratio of pulse duration to a total pulse period. Fig. 3 Resolidified layer on the 3.5C 2.8Si specimen after EDM. 3. Results Fig. 2 Schematic diagram of the electro-discharge machiner. per electrode and the specimen was measured with an oscilloscope. The state of discharge craters on the EDMed surface was indicated by ridge density, which was detected by surface roughness measurer. The ridge is defined as the peak on the surface which is higher than the average roughness value (Ra). Surface morphology and the resolidified layer were observed by SEM. XRD and TEM were also used to analyze the microstructure of the resolidified layer. 3.1 Features of the EDMed surface Figure 3 shows a resolidified layer formed in the vicinity of the EDMed surface of 3.5C 2.8Si specimen. The hardness distribution tends to increase from the base material to the EDMed surface and then reaches an ultimate value above Hv1000 (Fig. 4). Figure 5 shows the thickness of the resolidified layer on the SG cast irons with various compositions, indicating the layer thickness increased with a higher C or Si content. The morphology of the EDMed surfaces is illustrated in Fig. 6, showing a wavy ridge pattern formed by overlapping craters. We also found that clustered graphite particles were located in the troughs in-between the ridges. Figure 7 indicates the ridge density also tends to increase with a higher C and Si content, and the influence of C content is more obvious.

1362 D.-C. Tsai, J.-M. Song, T.-S. Lui and L.-H. Chen Fig. 4 Distribution of microhardness near the EDMed surface of 3.5C 2.8Si specimen. Fig. 6 Apparence of the EDMed surfaces: (a) 2.1C 2.7Si; (b) 3.

3 1362 D.-C. Tsai, J.-M. Song, T.-S. Lui and L.-H. Chen Fig. 4 Distribution of microhardness near the EDMed surface of 3.5C 2.8Si specimen. Fig. 6 Apparence of the EDMed surfaces: (a) 2.1C 2.7Si; (b) 3.0C 2.6Si; (c) 3.5C 2.8Si (ridges are pointed out by arrows). Fig. 5 Thickness of the modified layer against carbon and silicon contents of the base materials. 3.2 Microstructural analysis of the resolidified layer Considering the microstructural features of the rapidly solidified layer thus produced, a near-eutectic specimen, 3.5C 2.8Si sample, was selected for further investigation. Figure 8 shows the magnified structure on the ridge region of the modified layer on 3.5C 2.8Si specimen displayed in Fig. 3, illustrating an extremely fine cellular structure with a thickness of about 50 µm. This photo also shows that this fine cellular structure grows along the through-thickness direction. The composition analysis results (Fig. 9) show that the resolidified layer comprised Fe, C and Si and the composition is similar to that of the base material. In additions, the element of the electrode (Cu) isn t detectable on the resolidified layer. Figure 10 displays the X-ray diffraction pattern on the EDMed surface of the 3.5C 2.8Si specimen, and shows that γ Fe and metastable HCP ε phase 16) were identified. The TEM analysis results shown in Figs. 11 and 12 demonstrate Fig. 7 Ridge density of the EDMed surface against carbon and silicon contents of the base materials. that the resolidified layer mainly consists of cellular γ Fe and intercellular ε phase. In addition, microtwins can be observed in the cellular γ Fe, as shown in Fig Discussion The EDM process is carried in a dielectric fluid, with a small gap between the workpiece and the electrode. The bulgy area could be considered causing a strong likelihood for discharge to occur. Since the temperature at the discharge point was estimated as exceeding 3000 C, rapid melting and evaporation occur on portions of the surface includ-

$A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons 1363 Magnified structure of the resolidified layer of 3.5C 2.8Si speci- Fig. 8 men. Fig. 10 X-ray diffraction pattern on EDMed surface.$ Only a small proportion of the melt produced by the discharge, about 15%, is carried away by the dielectric. 12) The remaining melt rapidly solidifies and forms a layer with an undulant surface.

Only a small proportion of the melt produced by the discharge, about 15%, is carried away by the dielectric. 12) The remaining melt rapidly solidifies and forms a layer with an undulant surface.

4 A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons 1363 Magnified structure of the resolidified layer of 3.5C 2.8Si speci- Fig. 8 men. Fig. 10 X-ray diffraction pattern on EDMed surface. Fig. 9 (a) EDS spectrum of the resolidified layer and (b) concentration distribution of Fe, C, Si on the cross section in the vicinity of modified surface. ing graphitic and ferritic regions. Only a small proportion of the melt produced by the discharge, about 15%, is carried away by the dielectric. 12) The remaining melt rapidly solidifies and forms a layer with an undulant surface. The flushing dielectric plays an important role in heat sinking during rapid solidification. Thus, the variations in hardness along the through-thickness direction of this rapidly solidified layer can be inferred to be the cooling rate gradient. With each discharge, a crater is formed on the workpiece Fig. 11 Cross-sectional structure perpendicular to the through-thickness direction of resolidified layer: (a) bright field image; (b) dark field image. due to the discharge impact generated during EDM. 18) With regards to SG cast irons, the EDM process is illustrated schematically in Fig. 14. At the first stage, the sparks and the melting occur successively (Fig. 14(a)), and then the residual melt solidifies. Considering the poor wetability between the melt and graphite, 17) the graphite particles are dewetting from the molten material, and thus the semi-molten graphite will be situated on the cavities of the EDMed surface (Fig. 14(b)). The ridges formed between the semi-molten graphite can be considered to be the next sparking position (Fig. 14(c)). Therefore, the graphite phase plays an important

$1364 D.-C. Tsai, J.-M. Song, T.-S. Lui and L.-H. Chen Fig. 14 EDM process in the case of SG cast irons. Fig. 12 Electron diffraction patterns taken from (a) cellular region, zone axis: [011] γ ; (b) intercellular region, zone axis: [01 11] ε.$ role in the surface roughness. As described in the results (Figs. 5 and 7), an increase in carbon content has a greater effect on the EDMed surface roughness and resolidified layer thickness.

role in the surface roughness. As described in the results (Figs. 5 and 7), an increase in carbon content has a greater effect on the EDMed surface roughness and resolidified layer thickness.

$That is, the ridge density is proportional to the graphite area fraction and inversely proportional to the graphite particle size.$

5 1364 D.-C. Tsai, J.-M. Song, T.-S. Lui and L.-H. Chen Fig. 14 EDM process in the case of SG cast irons. Fig. 12 Electron diffraction patterns taken from (a) cellular region, zone axis: [011] γ ; (b) intercellular region, zone axis: [01 11] ε. Fig. 13 Microstructure of resolidified layer under two beam [200] condition: (a) bright field image; (b) dark field image. Fig. 15 The relationship between surface roughness and graphite phase. role in the surface roughness. As described in the results (Figs. 5 and 7), an increase in carbon content has a greater effect on the EDMed surface roughness and resolidified layer thickness. As for surface roughness, this phenomenon can be explained by the data shown in Fig. 15. That is, the ridge density is proportional to the graphite area fraction and inversely proportional to the graphite particle size. Furthermore, it is well known that increasing carbon content will lead to a higher graphite area fraction and a smaller graphite particle size. The combined effects of graphite area and particle size caused by a higher carbon content lead to numerous discharging points and thus increase the ridge density. Silicon content also has a considerable influence on the graphite particle size and thus has a weaker effect on the surface roughness. The combined effect of carbon and silicon on the surface roughness can also be expressed as a function of carbon equivalent (Fig. 16). As for the thickness of the resolidified layer, increasing alloying content leads to a thicker layer, especially with regards to carbon content. This is closely related to the discharge density, which is the pulse duration divided by the unit machining time. A higher discharge density can improve the discharge efficiency and thus increase the amount of molten material. Figure 17 indicates that in the experimental materials of this

6 A Study on Electro-Discharge Machined Surfaces of Ferritic SG Cast Irons 1365 Table 4 Phases in rapidly solidified Fe C and Fe C Si alloys. Composition (wt%) Rapid cooling process Phases Reference Fe 4.2%C 1.5%Si Shock-tube method α, ε, γ,fe 3 C 21) Fe 3.8%C Gun method ε, amorphous phase 22) Fe 4.2%C 1.9%Si Gun method ε, amorphous phase 23) Fe 4.3%C Gun method ε, amorphous phase 24) Fe 4.2%C 3.6%Si Piston and anvil method ε, amorphous phase 25) Fe %C Splat cooled method α, ε, γ,fe 3 C 26) Fe %C RSR method α, γ,fe 3 C 27) Fe 3.0%C 4.0%Si Single roller method γ, ε 28) Fe %C %Si Single roller method Amorphous 29) Fe %C 0 7.2%Si RLA method α, α, ε, γ,fe 3 C, X 1,X 2 30) Fe 3%C 5%Si Single roller method α,fe 3 C, χ, β-mn type isostructure 31), 32) Fig. 16 Ridge density of EDMed surface against carbon equivalent value. Fig. 17 Ridge density of EDMed surface against discharge density. study, discharge density is proportional to the ridge density. This can be attributed to numerous discharge points caused by high graphite fraction area and nodule count, which increase the discharge density, and consequently the thickness of the resolidified layer is increased. Previous studies have reported the structure of EDMed surface layers in ferrous alloys. Austenite-martensite or austenite-ledeburite structures were found in the plain carbon steels and low alloy steels. 19, 20) Moreover, these reports also showed that martensite, ferrite, glassy phase and metastable carbide were observed in die steel. 6) In this study, microstructural analysis results concluded that the resolidified layer of EDMed SG cast iron comprised an extremely fine cellular γ Fe matrix and intercellular ε phase. All possible phases in the rapidly solidified Fe C Si alloys can be roughly summarized as γ phase, martensite, ferrite, amorphous phase, ε phase and other metastable phases (see Table 4) ) Since the high carbon content and high cooling rate can promote the stability of γ Fe, the cellular structure is formed as γ Fe, not martensite. 22) Furthermore, the formation of metastable ε phase is considered to be related to the cooling rate and composition. 24) Due to the high silicon content of the resolidified layer, ε phase is formed instead of cementite. 22) As described in Fig. 9, the composition of the resolidified layer is similar to that of the substrate, which is about 3.5 mass%c and 2.8 mass%si. Referring to the relationship between the composition and quenched-in phases of rapidly solidified Fe C Si alloys by the splat cooled method or the rotating-wateratomization process 30) (Fig. 18), the aforementioned structural difference of the recast layer between SG cast iron and other ferrous alloys can be recognized as resulting from the effects of C and silicon contents. Twins can be observed in the cellular γ Fe of the resolidified layer. Earlier electron-microscope observations have witnessed the occurrence of growth microtwins in evaporated FCC metal films. 33, 34) They also suggested that the formation of the microtwins was resulted from the accidental wrong stacking of atoms in the {111} faces. As for EDMed surface, it can be inferred that the twin structure is caused by the significant tension residual stress resulted from the heat generated 13, 35) by electro-discharging. 5. Conclusions (1) A modified surface layer with the hardness higher than Hv1000 of SG cast iron can be obtained on SG irons us-

7 1366 D.-C. Tsai, J.-M. Song, T.-S. Lui and L.-H. Chen Fig. 18 Composition and classification of rapidly quench-in phases in Fe C Si alloys (reproduced from Yamauchi. 30) ing the electro-discharge machining method. The ridge density and layer thickness increase with increasing carbon and silicon content. This is closely related to the graphite phase of the base material. (2) Microstructural investigation showed that the resolidified layer which possesses a similar composition to the base material exhibits a cellular structure growing along the heat transfer direction. And the structure was found to comprise mainly γ Fe in the cellular region and metastable ε phase in the intercellular region. Microtwins were observed in the cellular γ Fe. Acknowledgments The authors would like to thank the National Science Council of the Republic of China for financial supporting this research under Contract No. NSC E The authors also greatly acknowledge Dr. H. L. Huang for valuable comments in TEM analysis. REFERENCES 2) Metals Handbook, vol. 16, (ASM, Metals Park, Ohio, 1989) pp ) D. C. Tsai, T. S. Lui and L. H. Chen: Mater. Trans. 41 (2000) ) D. C. Tsai, T. S. Lui and L. H. Chen: AFS Trans. 107 (1999) ) H.-J. Kim and J. F. Wallace: Surf. Eng. 10 (1994) ) M. A. E.-R. Merdan and R. D. Arnell: Surf. Eng. 5 (1989) ) D. I. Pantelis, N. M. Vaxevanidis, A. E. Houndri, P. Dumas and M. Jeandin: Surf. Eng. 14 (1998) ) Y. Tsunekawa, M. Okumiya and N. Mohri: Mater. Sci. Eng. A174 (1994) ) Y. Fukuzawa and Y. Kojima: Materials and Manufacturing Process. 10 (1995) ) J. P. Kruth, L. Steven, B. Lauwers and K. U. Leuven: Annals of the CIRP 44 (1995) ) N. Mohri and Y. Tsunekawa: Annals of the CIRP 42 (1993) ) L. C. Lim, L. C. Lee, Y. S. Wong and H. H. Lu: Mater. Sci. Tech. 7 (1991) ) C. P. Tabrett: J. Mater. Sci. Let. 15 (1996) ) L. C. Lee, L. C. Lim, Y. S. Wong and H. H. Lu: J. Mater. Proc. Tech. 24 (1990) ) N. G. Meshcheriakov, G. Hubeny and E. Pink: Pract. Met. 25 (1988) ) R. C. Ruhl and M. Cohen: Acta Metall. 15 (1967) ) C. Wu and R. D. Pehlke: AFS Trans. 105 (1997) ) J. E. Greene and J. L. Guerrero-Alvarez: Metall. Trans. 5 (1974) ) H. K. Lloyd and R. H. Warren: J. Iron Steel Inst. 203 (1965) ) T. Minemura, A. Inoue, Y. Kojima and T. Masumoto: Proc. 5th Inter. Conference on Rapidly Quenched Metals, Sendai, Japan, Aug , 1981, ed. by T. Masumoto and K. Suzuki, (The Japan Inst. of Metals, 1982) pp ) R. C. Ruhl and M. Cohen: Trans. AIME 245 (1969) ) S. N. Singh and S. N. Ojha: Int. J. Rapid Solidification 7 (1992) ) I. R. Sare: Scr. Metall. 9 (1975) ) P. G. Boswell and G. A. Chadwick: J. Mater. Sci. 11 (1976) ) J. M. Dubois and G. Le Caer: Acta Metall. 25 (1977) ) I. Schmidt and E. Hornbogen: Z. Metallkde 69 (1978) ) O. A. Ruano, L. E. Eiselstein and O. D. Sherby: Metall. Trans. A13 (1982) ) H. Miyake, A. Okada, P. H. Shingu and R. Ozaki: Imono 57 (1985) ) A. Inoue, J. Saida and T. Masumoto: Mater. Trans., JIM 30 (1989) ) I. Yamauchi, Y. Nomura, I. Ohnaka and Y. Matsumoto: Tetsu-to-Hagané 73 (1987) ) K. H. Yang, M. H. Hon and K. C. Su: Scr. Metall. 24 (1990) ) K. H. Yang, M. H. Hon and K. C. Su: Scr. Metall. 24 (1990) ) L. Eiselstein, O. Ruand and O. Sherby: J. Mater. Sci. 18 (1983) ) J. W. Mattews and D. L. Allinson: Philos. Mag. 8 (1963) ) S. Sen and S. P. S. Gupta: J. Phys. D 11 (1978) ) J. C. Rebelo, A. Morao Dias, D. Kremer and J. L. Lebrun: J. Mater. Proc. Tech. 84 (1998) ) I. Ogata, Y. Mukoyama and M. Hihara: J. Jpn. Soc. Precision Eng. 57 (1991) ) Source Book on Ductile Iron, (ASM, Metals Park, Ohio, 1977) pp

Effect of Silicon Particles on the EDM Characteristics of Al Si Alloys

Materials Transactions, Vol. 43, No. 2 (2002) pp. 199 to 205 c 2002 The Japan Institute of Metals Effect of Silicon Particles on the EDM Characteristics of Al Si Alloys De-Chang Tsai, Truan-Sheng Lui and