Analysis of surface layers of hot – forging dies of skd61 steel fabricated by die – sinking electrical discharge machining using copper and titanium electrodes - Nguyen Huu Phan

Journal of Science and Technology 54 (5A) (2016) 19-26 ANALYSIS OF SURFACE LAYERS OF HOT – FORGING DIES OF SKD61 STEEL FABRICATED BY DIE – SINKING ELECTRICAL DISCHARGE MACHINING USING COPPER AND TITANIUM ELECTRODES Nguyen Huu Phan1, *, Banh Tien Long2, Ngo Cuong1 1Thai Nguyen Technical-economics College, Thinh Dan ward, Thai Nguyen city 2Hanoi University of Science and Technology, Hanoi, No. 1, Dai Co Viet, Hanoi *Email: phanktcn@gmail.com Received: 17 July 2016; Accepted for publication: 2 December 2016 ABSTRACT Electrical discharge machining (EDM) is a popular unconventional method for manufacturing tools, molds, and dies. Currently, the surface layer of the hot die after EDM needed to be polished to remove the layer of ≈ 50 μm. This is accepted according to practical experience and the lack of scientific basis because of limited research on the surface layer quality of the products after EDM and the study is focused on the evaluation of the product quality in the final processing. This study is aimed at the investigation of performance and structure of the surface layer in hot-forging die following die-sinking EDM. The machining conditions included the use of copper (Cu) and titanium (Ti) as electrode materials. A cross- sectional micrographic and hardness analysis was performed, as well as surface roughness measurements, in order to study the thermally affected zones of the hot-forging die surface layer of the SKD61 steel after EDM using oil as the dielectric fluid. The results showed that the performance of the die was reduced due to changes in the hardness and the chemical composition of the workpiece surface. The surface quality of hot-forging die after EDM with Ti or Cu electrode are similar. In this case, EDM using Ti electrode to improve the quality of hot die surface layer was unreasonable. Keywords: EDM, surface roughness, microscopic hardness, Titanium, Copper. 1. INTRODUCTION EDM is a reproductive shaping process in which the form of the electrode is mirrored onto the work piece. In EDM, removal of the unwanted material occurs by melting and vaporization. And there are no physical cutting forces between the electrode and the workpiece hence the tool need not be harder than the workpiece. This process use is particularly widespread in applications where very complex shapes in hard materials with a high geometrical and dimensional accuracy are required [1]. Therefore, EDM is widely used in the production of Nguyen Huu Phan, Banh Tien Long, Ngo Cuong 20 forging dies. However, the usefulness of the process is limited by its low machining efficiency and the poor finished surface quality [2]. Usually, the machined surface layer produced by EDM has different characteristics from those of the base metal and those of machined surfaces produced by conventional machining processes. The surface quality variation reduces the efficacy of the mold and the methods for improving the surface quality are required. The traditional machining processes (grinding, polishing, buffing, etc.) are typically used to remove the layer damaged after die-sinking EDM. In forging process, the lifespan of die is very important due to manufacturing cost and technical performance. The failure causes influencing working capacity of die are thermal fatigue, plastic deformation, wear, etc. In this causes, wear is the main factor affecting in hot forging process. Although wear cannot be eliminated, its effects can be minimized in some cases by electroplating, physical vapor deposition, or chemical vapor deposition. Since the different electrode materials have different effects, the choice of a suitable electrode material plays an important role in the EDM process. Prior research has shown that, during machining of the workpiece, the material is transferred from the electrode through plasma onto the workpiece surface [4]. The layer of surface modification using EDM process with electrode materials also carried out. The researcher implemented the experimental investigation to study the effects of composite electrodes on die life in EDM and Ti was used as an electrode material. They showed that the hardness of the layer containing TiC is much higher than that of base material which lead to improve wear resistance of die surface by a factor of three to seven times [5]. Surface modification during EDM is one of the many methods for improving a workpiece’s surface properties [6] and represents a future direction for EDM and surface modification research. In this study, the performance and properties of the surface material layer of a hot-forging die of SKD61 hardened steel after die sinking EDM are evaluated using the titanium and copper electrodes. In this analysis, the performance of the machined surface and the amount of material transferred from the electrode to the workpiece surface was observed. The EDM with Cu electrode is carried out to evaluate the effect of die-sinking EDM to surface quality of hot- forging die and this is the choice of direction for the next finishing machining. Ti electrode was tested to investigate the feasibility of the hot-forging die surface layer improvement by EDM using this metal. 2. EXPERIMENTAL PROCEDURE The experiments were conducted using a die-sinking EDM platform, model NC EDM 850 Suzhou Zhonghang Changfeng CNC Technology Co., Ltd., Jiangsu, China. The material used for workpiece was SKD61 (Japanese Industrial Standard) hot–die steel that is used extensively for hot-forged dies. The constituents of the steel, as determined by a chemical analysis, were: 0.40 % C, 0.47 % Mn, 0.98 % Si, 0.14 % Ni, 4.90 % Cr, 0.83 % V, 1.15 % Mo, 0.016 % Co, 0.00012 % S, 0.018 % P, and the balance was Fe. The workpiece dimension was 70×70×15 mm3. Before machining, the raw material had a microhardness of 490÷547 HV. The hot-forged die neck seal bearings of Honda motorbike had the shape shown in Figure 1. The electrode materials selected for this investigation were Ti and Cu. Copper has excellent electrical and thermal conductivities and it is a major commercial material. Titanium compounds have been applied extensively as materials for surface modification because of their hardness, abrasion resistance, high melting point, and low coefficient of friction [4]. The dielectric fluid used was oil (HD–1). Machining parameters are selected according to the requirements of the Pho Yen Mechanical Joint Stock Company of Viet Nam; these are provided in Table 1. Analysis of surface layers of hot-forging dies of SKD61 steel fabricated by die-sinking electrical 21 Table 1. Machining conditions. Parameter Value Intensity of discharge (A) 4 Pulse-on time (μs) 100 Pulse-off time (μs) 3 Dielectric Kerosene oil Polarity Positive Machining time 1h13’27” Voltage of discharge (V) 150 Electrode material Copper, Titanium Figure 1. Hot-forged die geometry used in this research. The following material parameters were studied during the course of this experiment: chemical composition, microstructure, surface hardness, surface roughness, and surface appearance. Three readings were taken for each work specimen to compute the final, average measurement. Surface roughness was measured using a SJ–400 from Mitutoyo, Japan. After EDM, the samples were cleaned and the cross-section of die-sink surface was machined. An optical microscope was used to study the change in the microstructure of the EDMed surface. The rest of the analysis was carried out on six samples using a scanning electron microscope (SEM, model JSM 6490, JEOL, Japan). The surfaces of the samples were cleaned prior to SEM analysis at three different magnifications: 100×, 500×, 1000×. To analyze the phase composition of the surfaces, selected workpieces were analyzed using X–ray diffractometry (XRD) over a 2θ range from 5° to 85° with a model Axiovert 40MAT from Carl Zeiss, Germany. Microhardness was measured on microhardness tester (model Indenta Met 1106) from Buehler, USA. The chemical compositions of the machined surfaces were analyzed using energy-dispersive X-ray spectroscopy (EDS, model JSM–6490LA, JEOL, Japan). 3. RESULTS AND DISCUSSION 3.1. Cross-section analysis following EDM The cross-sectional structure of the SKD61 steel surfaces fabricated using EDM with the titanium or copper electrodes exhibited three layers, as shown in Figure 2: the white layer, the Nguyen Huu Phan, Banh Tien Long, Ngo Cuong 22 heat-affected zone (HAZ) and the base metal. The white layer was the outermost light-colored layer with a relatively high thickness (Table 2): 12.03÷21.79 μm for the Cu electrode and 11.25 ÷ 22.77 μm for the Ti electrode, and it was distinct from the other layers. This layer forms when some of the molten material (from both the electrode and work piece) is not removed and is rapidly quenched by dielectric fluid. The white layer contains a high density of microscopic cracks that run across the total depth of the white layer, only seldom continuing into the layers beneath. The cracks are mostly perpendicular to the surface of the work piece. The larger microscopic crack size in the die fabricated using Ti electrode as compared to that produced using Cu electrode is evident from Figure 3. The microhardness values of the white layer in both types of specimens were quite similar (Figure 4): 453.7 HV for the Cu electrode and 464.1 HV for the Ti electrode. The values are lower than those obtained in the heat affected zone and base material. The forging dies and hot- mold dies always operate in high-temperature environments and under high shock pressure. Choosing the correct type and hardness of the die material and the surface-layer coating is very important for improving the working accuracy and functionality of the dies. Given the above results, the white layer presence reduces the working capacity of hot-die sinks. The HAZ zone was located beneath the white layer and is was difficult to observe clearly the properties. It wasn’t so thick as the white layer and the thickness is pointed in Table 2: 8.68 ÷ 13.84 μm Cu electrode and 8.54÷11.12 μm for the Ti electrode. In this layer, the material has been heated below the melting point of the material as in the recast layer. There were a few microscopic cracks with small depths in the heat-affected zone layer that were not parallel to the machined surface. The microhardness of the heat-affected zone was very high: 627.1 HV for the Cu electrode and 646.0 HV for the Ti electrode; these values were higher than that of the white layer and the base metal (570.5÷588.8 HV). The properties of the HAZ may alter the performance of the hot-forging die. Table 2. Depth of altered-metal-zone layers. Altered metal zone layers Depth (µm) Cu electrode Ti electrode White layer 12.03 ÷ 21.79 11.25 ÷ 22.77 Heat affected zone (HAZ) 8.68 ÷ 13.84 8.54 ÷ 11.12 Figure 2. The different layers formed on of the hot-forging die surface (a), Ti electrode (b), Cu electrode. Analysis of surface layers of hot-forging dies of SKD61 steel fabricated by die-sinking electrical 23 Figure 3. Cracks on of the hot-forging die surface (a), Ti electrode (b), Cu electrode. Figure 4. Microhardness measurements across transverse sections. 3.2. Chemical composition and X-ray diffraction patterns of machined surfaces The chemical composition of the machined surface layer was determined using EDS. The compositions and the XRD patterns of the surface layers are shown in Figures 5÷7. The compositions of the HAZ and base metal were unchanged in a result of EDM. The EDX analysis, employing the ZAF method for standardless quantitative analysis, of the chemical composition of the white layer indicated that it changed significantly. The EDX analysis of the white layer showed the presence of major constituent elements Fe, C, Mn, Si, V, Cr, and Mo in addition to Cu (from the Cu electrode) and Ti (from the Ti electrode), as shown in Figure 5. The carbon content in the white layer increased greatly from 0.40 % to 13.76 % for Cu electrode and to 11.43 % for Ti electrode. This is because, during the pulse, the thermal energy of the emitted sparks generated carbon cracking oil, thereby creating the carbon that entered the machined surface. The increase in carbon content improves the hardness and strength of the surface but reduces the toughness and ductility. The appearance of the electrode materials on the machined surface – Cu electrode: from 0.054 to 0.32 %, Ti electrode: from 0.053 to 1.98 % – is the result of the melted and evaporated electrode materials moving and sticking to the surface of the workpiece. Increased Cu and Ti contents can improve the corrosion resistance. The peaks corresponding to the different elements are shown in Figures 6 for both types of electrode materials. The Ti and Cu peaks confirm the presence of electrode material in the deposited layer. The intensity of Ti peak was greater than that of the Cu peak in workpiece. This indicates that more Ti accumulated at the surface and that the inner layer was richer. This situation is desirable since it produces a harder surface. The XRD analysis was carried to confirm the transfer of electrode material from the electrode and carbon from the dielectric fluid to the workpiece surface and, also, to identify the phases of the compounds formed during the EDM process; these results are shown in Figures 6a Nguyen Huu Phan, Banh Tien Long, Ngo Cuong 24 and b. Several compounds formed on the workpiece surface. The pattern indicated the presence of iron carbides (Fe7C3 and Fe3C) and molybdenum carbide (Mo3C7). The presence of Fe7C3 and Fe3C increase the hardness of the machined surface. The corrosion resistance and hardness of the machined surface can also be improved by Mo3C7. Figure 5. Chemical composition (C, Ti and Cu) of the machined hot die (a), %C (b), % electrode materials. Figure 6. EDX analysis of the hot-forging die surface layer (a), Ti electrode (b), Cu electrode. Figure 7. XRD patterns of the hot-forging die surface layer (a), Ti electrode (b), Cu electrode. 3.3. Topography of the machined surface Assuming that each spark leads to the formation of a spherical crater on the surface of the workpiece, the volume of metal removed per crater will be proportional to the cube of the crater depth. The surface accumulated many large craters created by the sparks generated during the pulse cycle, as shown in Figure 8a. The craters were on the radius of curvature (indicated by the arrows in Figure 8b) created when the melting and evaporating materials affected by the Analysis of surface layers of hot-forging dies of SKD61 steel fabricated by die-sinking electrical 25 dielectric fluid were simultaneously quenched and caused the outer surface tension. Many small hard particles appeared on the processed surface and adhered to it, causing an increase in the surface roughness. These spherical protrusions are particles of molten metal that were expelled molten workpiece and small amounts of electrode material form spheres during the discharge and later spatter and solidified on the workpiece surface. The particles formed as a result of the molten metal removal must be solidified at an extremely high rate. Otherwise, the molten metal surface tension would have rounded off the sharp edges. The cracks were formed due to the high thermal stresses prevailing at the specimen surface as it was cooled at fast rate after the discharge process. The average surface roughness of the hot-forging die surface after the die- sinking EDM process was 23.1÷26.3 µm. This result demonstrated that further polishing is required before use. Figure 8. Microstructures of the hot-forging die surface (a), Surface photographs (b), The form of craters and debris particles. 4. CONCLUSION The white layer had a low hardness and significant cracking that is detrimental for the forging die functionality. Thus, it is necessary to determine the effect of various parameters in order to reduce or eliminate the white layer formation on the machined surface. The thickness of the surface in polishing (≈ 50 μm) is not reasonable. This has removed HAZ layer, its hardness is highest, and it led to the durability of the mold is reduced and caused material losses. The results have shown that the use of Ti as an electrode material is not efficient. Besides, titanium is expensive, as compared to many other metals, is characterized by complexity of the extraction process, difficulty to melt that causes problems during fabrication. The machinability of titanium and its alloys is generally poor owing to several inherent properties of the materials. Therefore, the choice of electrode Ti is unreasonable in this case. The surface was covered with a hard layer produced in the EDM process consisting of carbides (TiC, WC, TaC, WC-Co, etc.) that enhanced the surface characteristics of the workpiece. Large amounts of Ti and C migrate from the electrode and dielectric fluid to the hot- forging die surface. However, TiC did not form on the machined surface layer. These results indicate a promising direction for the EDM research using electrodes TiC, WC and other carbides for successful surface modification. Acknowledgments. The authors would like to thank Prof. Hyungsun Kim - School of Materials Engineering - Inha University – Korea, Pho Yen Mechanical Joint Stock Company and Institute of Materials Science - VAST for the assistance during the research. Nguyen Huu Phan, Banh Tien Long, Ngo Cuong 26 REFERENCES 1. Abu Zeid O. A. - On the effect of electro-discharge machining parameters on the fatigue life of AISI D6 tool steel, J. Mater. Process. Technol. 68 (1997) 27–32. 2. Ho K. H., Newman S. T. - State of the art electrical discharge machining (EDM), International Journal of Machine Tools & Manufacture 43 (2003) 1287–1300. 3. Lee L. C., Lim L. C., Naryanan V., Venkatesh V. C. - Quantification of surface damage of tool steels after EDM, International Journal of Machinery Tools &Manufacture 28 (1987) 359–372. 4. Gangadhar A., Shunmugam M. S., Philip P. K. - Surface modification in electro discharge processing with a powder compact tool electrode, Wear 143 (1991) 45-55. 5. Moro T., Goto A., Mohri N., Saito N., Matsukawa K., Miyake H. - Surface modification process by electrical discharge machining with TiC semi-sintered electrode, Journal of Japanese Society of Precision Engineering 67 (2001) 114–119. 6. Yan B. H., Tsai H. C., Huang F. Y. - The effect in EDM of a dielectric of a urea solution in water on modifying the surface of titanium, International Journal of Machine Tools & Manufacture 45 (2005) 194-200. TÓM TẮT PHÂN TÍCH LỚP BỀ MẶT KHUÔN DẬP NÓNG BẰNG THÉP SKD61 SAU GIA CÔNG BẰNG XUNG ĐỊNH HÌNH VỚI ĐIỆN CỰC TITAN VÀ ĐỒNG Nguyễn Hữu Phấn1, *, Bành Tiến Long2, Ngô Cường1 1Trường Cao đẳng Kinh tế - Kỹ thuật Thái Nguyên, Phường Thịnh Đán, TP. Thái Nguyên 2Đại học Bách khoa Hà Nội, Số 1, Đại Cồ Việt, Hà Nội *Email: phanktcn@gmail.com Gia công tia lửa điện là phương pháp gia công phi truyền thống được sử dụng phổ biến trong gia công dụng cụ, khuôn mẫu. Hiện nay, lớp bề mặt khuôn dập nóng sau EDM thường phải đánh bóng với lượng dư ≈ 50 μm. Điều này được làm theo kinh nghiệm thực tế mà thiếu căn cứ khoa học do chưa có nghiên cứu nào được công bố về đánh giá chất lượng thực tế của lớp bề mặt của các khuôn này sau EDM. Các nghiên cứu chủ yếu tập trung vào đánh giá chất lượng sản phẩm này tại nguyên công gia công lần cuối. Hướng của nghiên cứu là khảo sát chất lượng và cấu trúc lớp bề mặt của khuôn dập nóng sau xung định hình. Điện cực đồng (Cu) và titan (Ti) được sử dụng trong nghiên cứu này. Cấu trúc mặt cắt ngang, độ cứng tế vi và nhấp nhô bề mặt được sử dụng đề nghiên cứu ảnh hưởng của năng lượng nhiệt đến lớp bề mặt của khuôn dập nóng làm bằng thép SKD61 sau EDM với dung dịch điện môi là dầu. Kết quả chỉ ra rằng, chất lượng bề mặt của khuôn dập nóng giảm do sự thay đổi của độ cứng tế vi và thành phần hóa học lớp bề mặt khuôn. Chất lượng bề mặt khuôn dập sau EDM với điện cực Ti tương tự với điện cực Cu. Việc sử dụng điện cực Ti để nâng cao chất lượng bề mặt bằng EDM của trường hợp này là không hợp lý. Từ khóa: EDM, nhám bề mặt, độ cứng tế vi, titan, đồng.