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毕业设计任务书题 目： 电器开关过电片级进模的设计 一、毕业设计（论文）的内容本设计要求学生以工程实际零件电器开关过电片级进模的设计作为设计对象，旨在培养学生严谨的分析解决问题的能力和综合运用专业基础知识进行实际设计的能力。需要学生充分运用所学的模具知识、零件设计、制图、工艺、公差与技术测量等机械专业知识进行模具结构的方案设计、零件的结构尺寸设计计算等。1、查阅资料，进行企业调研，了解目前主流的冲压机床的类型与特点，熟悉现有的典型冲压级进模具设计的结构与工作原理，做好设计前的准备工作。2、根据给定的零件的结构特点以及尺寸参数，提出模具的设计方案（两种及两种以上），进行比较后选出最佳方案进行设计，并选择适当的机床；3、对模具工作部分尺寸及公差进行设计计算、并选择合理的零件材料；4、运用Pro/E、SolidWorks等CAD工具进行辅助设计，完成模具整体结构的设计，绘制所设计模具的零件图、装配图。5、总结设计数据，整理设计思路，编写设计说明书。二、毕业设计（论文）的要求与数据1、根据下图所示的电器开关过电片零件的结构特点及尺寸完成一款级进模具的设计工作，零件的材料为：H68黄铜，0.5mm；2、设计相应的模具及其主要的零部件。3、采用CAD设计软件（如：Pro/E、SolidWorks、AutoCAD等）对模具进行实体建模、绘制模具的装配图与零件图。4、编写设计说明书。三、毕业设计（论文）应完成的工作1、完成二万字左右的毕业设计说明书（论文），在毕业设计说明书（论文）中必须包括详细的300-500个单词的英文摘要；对模具零件进行必要设计计算、对于有标准规定的零部件，必须严格按照标准要求进行选择或设计。 2、独立完成与课题相关，不少于四万字符的指定英文资料翻译（附英文原文），要求排版整齐，无明显语法、字词的错误；3、绘制出所设计模具的零件图和装配图，要求折算到A0图纸3张以上，其中必须包含两张A3以上的计算机绘图图纸，要求图形绘制符合国家标准，方便读图，重要零件的关键尺寸和公差要标注完整正确，并配注合理的技术要求。四、应收集的资料及主要参考文献1 模具设计与制造技术教育丛书编委会.模具结构设计.北京:机械工业出版社,2004.2 杨占尧，白柳.塑料模具典型结构设计实例. 北京:化学工业出版社,2009.3 宋满仓等注塑模具设计与制造实战M北京：机械工业出版社，2003.4 刘航. 模具技术经济分析M. 北京：机械工业出版社，2002.5 傅建等.模具制造工艺学M. 北京：机械工业出版社，2004.6 冯爱新.塑料模具工程师手册.北京:机械工业出版社,2009.7 张国强.注塑模设计与生产应用M . 北京：化学工业出版社，2005. 8 刘文, 王国辉, 谭建波SolidWorks模具设计入门、技巧与实例M北京：化学工业出版社，2010.9 Chen,Y.-M. Computer-aided integrated design for injection moldingIntelligent Processing and Manufacturing of Materials, 1999.10 Yan, L. An Intelligent Knowledge-based Plastic Injection Mold Design SystemJ Annual Technical Conference - ANTEC, Conference Proceedings, v 3, 2003, p 3514-3518.五、试验、测试、试制加工所需主要仪器设备及条件计算机一台，并装有CAD设计软件（AutoCAD，CAXA，UG，Pro/E Solidworks）等。A Comparative Study on the Surface Integrity of Plastic Mold Steel due to Electric Discharge MachiningBLENT EKMEKCI, OKTAY ELKOCA, and ABDULKADIR ERDENThe violent nature of the electric discharge machining (EDM) process leads to a unique structure on the surface of a machined part. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated. The results have shown that regardless of the tool electrode and the dielectric liquid, the white layer is formed on machined surfaces. This layer is composed of cementite (Fe3C) and martensite distributed in retained austenite matrix form- ing dendritic structures, due to rapid solidification of the molten metal, if carbon-based dielectric liq- uid is used. The intensity of cracking increases at high pulse durations and low pulse currents. Cracks on the EDM surfaces have been found to follow the pitting arrangements with closed loops and to cross perpendicularly with radial cracks and continue to propagate when another discharge takes place in the neighborhood. The amount of retained austenite phase and the intensity of microcracks have found to be much less in the white layer of the samples machined in de-ionized water dielectric liquid. The number of globule appendages attached to the surface increased when a carbon-based tool electrode material or a dielectric liquid was used during machining.I. INTRODUCTIONELECTRIC discharge machining (EDM) provides an effec- tive manufacturing technique that enables the production of parts made of hard materials with complicated geometry that are difficult to produce by conventional machining processes. The ability to control the process parameters to achieve the required dimensional accuracy and surface finish has placed this machining operation in a prominent position in industrial applications. The absorbing interest in EDM has resulted in great improvements in its technology, and it has become an important nontraditional machining process, widely used in aerospace, automotive, tool, and die industries.Electric discharge machining can be described as a process for eroding and removing material by transient action of elec- tric sparks on electrically conductive materials immersed in a dielectric liquid and separated by a small (m) gap. Thus, electrical energy in the form of short duration impulses with a desired shape is supplied to the electrodes. The required energy is usually in the form of rectangular pulses and can be generated by using spark generators designed for this pur- pose. When such a voltage pulse is applied to the electrodes, an electric spark discharge occurs within the interelectrode gap. It is well known that erosion on the electrode surfaces is mainly due to the thermal effect of an electric discharge. The charge induced on electrodes by a spark generator cre- ates a strong electric field. This field is strongest where the electrodes are closest to each other. Molecules and ions of dielectric fluid are polarized and oriented between these two peaks. When the dielectric strength of the liquid in the gapBLENT EKMEKCI, Assistant Professor, is with the Mechanical Engi- neering Department, Zonguldak Karaelmas University, 67100 Incivez/ Zonguldak, Turkey. OKTAY ELKOCA, Research Engineer, is with the Research and Development Center, Eregli Iron and Steel Work Co., 67330 Krd. Eregli/Zonguldak, Turkey. ABDULKADIR ERDEN, Professor, is with the Manufacturing Engineering Department, Atilim University, 06836 Incek/ Ankara, Turkey.Manuscript submitted March 4, 2004.exceeds a natural limit, a low resistance discharge channel is formed due to the electron avalanche striking the anode and cathode. This collision process transforms kinetic energy in the form of heat and pressure. The amount of heat generated within the discharge channel is predicted to be as high as 1017 W/m2 and, thus, could raise electrode temperatures locally up to 20,000 K even for short pulse durations.1 Therefore, melting, vaporization, and even ionization of the electrode materials occur at the point where the discharge takes place. No machin- ing process is known where similar high temperatures can be obtained in such small dimensions. The pressure increase in the plasma channel forces expansion discharge channel bound- aries and decreases the current density across the interelectrode gap. Most of the time, the pressure increase is so high that it prevents evaporation of superheated material on both electrode surfaces. When the pulse voltage ceases, a sharp decrease in the channel pressure triggers a violent erosion process. The superheated molten cavities explode violently into the dielectric liquid. Finally, the surfaces cool instantaneously, where all vaporized and a fraction of melted material in the form of irregularly shaped or hollow spherical particles is flushed away by dielectric liquid. The net result is a tiny crater on both sur- faces of the electrodes, where the remaining part of the melted material has splashed on it. Applying consecutive spark dis- charges with high frequencies and driving one electrode toward the other erode the work piece gradually in a form complemen- tary to that of the tool electrode.A clear characterization of electrodischarge machined surface topography is essential to predict the quality and func- tional behavior of surfaces.2 Saito3 tried to define the relation between the shape of a single discharge crater and the dis- charge conditions. He found that the interelectrode gap dis- tance causes the diversity of the size of crater made by the discharge. Lloyd and Warren4 have shown that the anode craters take the form of a circular depression independent of crystal orientation and characterized by a raised circumfer- ential lip resulting from the upheaval of metal during the liquid dispersion time. In addition, they found that the crater diameter is approximately constant for the same spark condition. TheMETALLURGICAL AND MATERIALS TRANSACTIONS BVOLUME 36B, FEBRUARY 2005117cathode craters, on the other hand, were not found to be truly circular but tend to reflect the symmetry of the crystal faces on which they occur. Greene and Alvarez5 used a profilometer imaging technique to accurately measure the volume of the electrode craters on different electrode materials produced by EDM. They showed the effects of high pressure generated during sparking on craters with illustrating radial flow lines near the rim. Radhakrishnan and Achyutha6 have found, by using the relocation technique, that the general appearance of the craters formed is almost the same for different materials, except for their size and depth. They reported a well-defined ridge and considered that this was due to the deposition of the molten material from the crater. Wong et al.7 worked on a micro EDM, which has a single spark generator, and found that the shapes of the craters are more uniform with a better defined rim at lower energies (50 J) in contrast to irregular diameters at higher levels.A practical EDM surface is a random superposition of craters formed by the discrete removal of metal due to con- secutive discharges. Various experimental results and empir- ical models of surface finish for different operation types and conditions have been published.725 It has been observed that there are many process variables that effect the surface finish such as peak current, duration of current pulse, open voltage gap, electrode polarity, debris concentration, thermal properties of the tool electrode, work piece, and dielectric liquid. Generally, the power functional trend of curves, rep- resenting an increase in surface roughness with respect to increased pulse energy, was presented. Large roughness val- ues can be explained by the generation of large craters due to high energy levels. A great deal of effort has been made to improve EDM accuracy and surface roughness when using this process as ultra precision machining. The material removal is due to electrostatic force acting on the metal sur- face when short pulse duration is applied. In this case, sur- face roughness values (Ra) less than 0.2 m are possible and a mirrorlike surface can be obtained.26,27,28Studies on various machined surfaces with electron micro- scopy2,5,10,11,14,22,23,29,30 showed that the surface is observed with globules of debris and chimneys formed by entrapped gases escaping from the redeposited material. Evidently, the surface is frozen, virtually instantaneously, when the discharge ceases. However, the shapes of the pockmarks, and partic- ularly their rims, are indicative of their sudden and simulta- neous rupture, coinciding with the sharp decrease in pressure as the discharge is cut off.Another feature on electrical discharge machined surfaces is the abundance of microcracks. The amount of thermal energy created and the conductivity of the work piece deter- mine the cracking behavior of the machined surface. Cracks formed due to thermal stresses in a single discharge tend to follow the pitting arrangements created in the surface by EDM. They normally form closed loops, instead of crossing the materials surface.31 Residual stresses are generated since the melted material contracts more than the unaffected parent material during the cooling process, and cracks are developed when the stress in the surface exceeds the mate- rials fracture strength22,29,32Earlier studies on electric discharge machined surfaces on pure iron and ferrous alloys revealed a nonetchable white cov- ering layer, which is far harder than the base material. Irregu- lar signs of splashing and alloying effect from the electrodematerial were found on the surface of the white layer.2,4,3335 This observation gives a sense of how the electrode material affects the work piece surface quality. So, it was considered that this alloying effect could be used to enhance the surface quality, such as by reducing residual stresses by a suitable source of alloying element.2,4,35 The hardness value was found to be high when compared with the hardness value obtain- able by quenching.4 This layer was observed under all machining conditions, including when water was used as the dielectric material.2,4,33,34Lloyd and Warren4 obtained a fused outer zone consist- ing of dendritic austenite and a cementite-austenite eutectic (ledeburite structure of a hypoeutectic white cast iron), when machining with a graphite electrode and in paraffin dielectric under severe conditions, or a fully austenitic surface followed by an austenite-cementite matrix, when machining with a copper electrode under less severe conditions. Optiz33 reported a hypereutectic recast layer in hot forging steel. Massarelli and Marchionni36 reported a similar structure of carbides in an austenite matrix, but stated that different elec- trodes do not change the morphology of the white layer; only the ratio of the carbide and the austenite phases varies. However, Simao et al.24 have reported an increase in white layer hardness when employing powder metallurgy (PM) green compact and sintered TiC/WC/Co electrodes during electric discharge texturing (EDT). They used glow discharge optical emission spectroscopy (GDOES) to analyze surface enrichment/depletion of the modified/alloyed EDT roll sur- faces, and observed that Ti and W contained in the PM elec- trodes together with C decomposed from the dielectric fluid during sparking were transferred to the AISI D2 roll surface. Similarly, Tsai et al.37 have reported Cu and Cr migration to the machined surface from Cr/Cu based composite elec- trodes. Rebelo et al.14 reported a severe increase in carbon intensity of the surface as 9 times greater at the surface than the bulk material by microprobe analysis. Ghanem et al.23 also detected enrichment in carbon and hydrogen in the outer layer by GDOES depth profiling. An increase in carbon content in the surface and subsurface layers has been attributed by most workers to the pyrolysis of the dielectric, but others have suggested that carbon is assimilated more rapidly from graphite electrodes than from carbonaceous dielectric. Thomson29 has concluded that carbon was absorbed from the dielectric rather than from the electrode. The near-surface hardening is more important in the austenitic structure than in the ferritic structure due to the solubility of carbon in the fcc structure.23 Rebelo et al.14 and Kruth et al.38 have shown that Fe3C cementite was formed on the surface of martensitic steels, whereas Cabanillas et al.39 have found two different regimes of carbide formation: s-carbide, austenite, and martensite for sparks of energy below 0.5 J; and cementite, austenite, and traces of marten- site, Fe7C3, or Fe5C2 for higher spark energies on the pure iron in hydrocarbon dielectrics.Lim et al.40 managed to visualize the recast layer by using unconventional metallographic reagents and showed a variety of microstructures; as a result, they categorized these observations into three main groups according to recast layer thickness. The first type was found to be around 20 to 50 m and has a multiplayer structure made up of over- lapping layers of similar microstructures. The second type was found to range between 10 and 20 m and is largely118VOLUME 36B, FEBRUARY 2005METALLURGICAL AND MATERIALS TRANSACTIONS Bcolumnar and dendritic in nature. The last type was found to have a thickness less than 10 m and to be fairly resis- tant to etching. Thus, it could not be described and is named as featureless.In most cases, a thermally affected layer was found beneath the recast layer.2,4,33,36,41,42 It is partly affected by carbon drawn by the dielectric. This layer generally has a tempered microstructure. The hardness value of this layer is often found to be less than that of the underlying hardened material. In a number of studies, an intermediate layer between the recast and the tempered layers has also been observed.2,4,33,36 This layer was found to exhibit a carbon gradient and contami- nation of materials from the tool electrode. It is possible that this layer includes part of the melted layer plus a region beyond which diffusion has occurred in solid state underTable I. Composition of the Plastic Mold Steel (Weight Percent)MaterialCCrMnMoNiSi DIN 1.27380.382.0 0.30Topographic examinations were performed with a JEOL*JEOL is a trademark of Japan Electron Optics Ltd., Tokyo.JSM-5600 Scanning Electron Microscope (SEM). Samples were prepared using conventional metallographic techniques on cross sections, in which thermally affected layers can be observed normally with an Olympus* metallographic micro-severe machining condition. The thickness of the thermally affected layer increases proportionally with respect to dis- charge energy. This layer contains a high density of second- phase particles, which are larger in size and more rounded than the carbide particles in the parent material.11 The hard- ness of this layer is found to be comparable to or, sometimes, slightly higher than that of the recast layer.40 A zone of plas- tically deformed material has been reported41 for single-phase materials, which do not undergo complex phase transforma- tions during EDM. This plastically deformed layer has been found to be from a few tens to a few hundred micrometers in*OLYMPUS is a trademark of Japan Olympus Co., Tokyo.scope. These sections were etched with nital reagent in order to reveal thermally affected zones. Microhardness depth pro- file measurements were made on a Future-Tech* FM-700*Future-Tech is a trademark of Japan Future-Tech Co., Tokyo.hardness tester using a Vickers indenter with a load of 10 g and an indentation time of 15 seconds. X-ray diffraction pat- terns were obtained with a Shimadzu* XRD-6000. Data werethickness in the underlying metal. Cleavage and grain bound- ary cracks, penetrating into the underlying material, have been observed in brittle materials under severe machining conditions.4,11,33 The bulk of the material beyond these zones remains unaffected by machining.Technological advances have led to an increase in the usage of high-strength, high-hardness materials in manufacturing industries. Thus, the use of this process has increased in recent years since it has the capability of machining hard materials with complicated forms as fine slots and microholes. How- ever, fracture and fatigue failures generally nucleate at or near the surface of the component, and the frequency of surface defects reduces the strength of the material due to the rapid heating and cooling effects induced by the machining process. These properties determine the resultant operational behavior of the machined parts. In this study, the influence of electrode material and type of dielectric liquid on the surface integrity of plastic mold steel samples is investigated.II. EXPERIMENTAL PROCEDUREPlastic mold steel (DIN 1.2738) samples were stress relieved prior to EDM to ensure stress-free condition. They were heated to 600 C for 1 hour and cooled slowly. One of the surfaces was electric discharge machined with a FURKAN* EDM 25 industrial machine on a rectangular*FURKAN is a trademark of Turkish Furkan Technologies Co., Istanbul.working area of 10 50 mm. The generator produced rectan- gular pulses at average currents of Iav = 1, 2, 4, 8, and 16 Aand at durations tp = 6, 12, 25, 50, 100, 200, 400, 800, and 1600 s. Commercial kerosene and deionized water were used as the dielectric liquids. Copper and graphite were selected as the tool electrodes. The chemical composition of the sample material is given in Table I.*SHIMADZU is a trademark of Japan Shimadzu Co., Kyoto.collected using Cu Ka radiation (h = 1.5405) in the range 10 20 120. The phases were identified from searches in the JPDS (Joint Committee on Powder Diffraction Stan- dards) database.III. RESULTSA. Surface TopographyIt is well known that the surface roughness is a function of released energy, which is controlled by power supply set- tings. High peak current and long pulse duration produce a rough surface. Conversely, it is also true that lower peak current and pulse duration produce a finer surface, since each pulse removes a small quantity of material proportional to the energy of the pulse from the electrode. Scanning elec- tron micrographs (Figures 1 and 2) show that an electric dis- charge machined surface observed with overlapping craters, globules of debris, and chimneys formed by entrapped gases escaping from the redeposited material.The effect of dielectric liquid and tool electrode on sur- face topography is not clearly stated in the literature. Only a small variation in surface roughness has been reported. Surfaces produced under similar operating conditions by using different dielectric liquid and toll electrode material combinations (Figures 1 and 2) have shown that the topo- graphical features of the surfaces change with respect to the number of globular or irregularly shaped appendages that are attached to the crater rims. No or few appendages could be observed when copper is used as the tool electrode and deionized water as the dielectric liquid (Figure 1(a). Chang- ing the tool electrode material with graphite resulted in an increased number of such appendages (Figure 1(b). TheMETALLURGICAL AND MATERIALS TRANSACTIONS BVOLUME 36B, FEBRUARY 2005119(a)(b)Fig. 1SEM pictures of electric discharge machined plastic mold steel surfaces, Iav = 16 A, tp = 25 s. Dielectric liquid: deionized water, electrode:(a) copper and (b) graphite.surface has been found to be densely infiltrated with such features when kerosene is used as the dielectric liquid. How- ever, an extensive change in surface topography is not essen- tial in the case of tool electrode material change (Figures 2(a) and (b). These results suggest that carbon drawn from the dielectric liquid or tool electrode is responsible for formation of such features.Long pulse duration dramatically increases the amount of surface damage. Cracking is also possible especially at high pulse durations and low current settings when kerosene is used as the dielectric liquid. In such circumstances, a well- defined crack network can be clearly visualized (Figure 3(a) at a pulse duration of 800 s and average current of 8 A. Increasing the pulse current by twofold almost diminishes the occurrence of such a network (Figure 3(b).Unstable operational conditions are observed at high pulse durations when water is used as the dielectric liquid. Machin- ing became stable at pulse durations lower than 400 s when the average pulse current was equal to 16 A. When an 8 A pulse current was applied, stable machining conditions were reached at pulse durations lower than 200 s. The effect of the unstable machining condition can be visualized as deep cavities (Figure 4(a) presumably due to electric arcs pro- duced during machining when compared with the surface obtained in kerosene dielectric liquid (Figure 3(a). Changing tool electrode with copper under similar machining conditions shows a transitional topography from stable to unstable machining where deep cavities produced due to unstable sparks are partly filled with molten material produced due to stable ones (Figure 4(b).(a) (a)(b) (b)Fig. 2SEM pictures of electric discharge machined plastic mold steel surfaces, Iav = 16 A, tp = 25 s. Dielectric liquid: kerosene, electrode:(a) copper and (b) graphite.Fig. 3SEM pictures of electric discharge machined plastic mold steel surfaces. Electrode: graphite, tp = 800 s, in kerosene dielectric liquid:(a) Iav = 8 A and (b) Iav = 16 A.120VOLUME 36B, FEBRUARY 2005METALLURGICAL AND MATERIALS TRANSACTIONS B(a)(b)(a)(c)(d)Fig. 5Cross sections of electric discharge machined plastic mold steel samples, dielectric liquid: kerosene, tp = 400 s. (a) Iav = 16 A, electrode: graphite; (b) Iav = 16 A, electrode: copper; (c) Iav = 8 A, electrode: graphite; and (d) Iav = 8 A, electrode: copper.(b)Fig. 4SEM pictures of electric discharge machined plastic mold steel sur- faces, Iav = 8 A. Electrode: (a) graphite, (b) copper, tp = 800 s in deion- ized water dielectric liquid.B. Thermally Influenced LayersThermally affected layers are generated in subsurface when producing surfaces with EDM. In all operational cases, the white layer thickness is found to be the highest at the longest pulse duration. Overlapped crater bases and their rims can be distinguished with thicker white layer formations at rims and thinner layer formations at bases (Figure 5). The dielec- tric liquid and tool electrode used during machining are found to be effective on the piled white layer formation at the crater rims. Such a formation is evident when graphite is used as the tool electrode and kerosene as the dielectric liquid. A slight decrease in this formation can be deduced when copper is used as a tool electrode. A recognizable decrease is evident when water is used as the dielectric liquid (Figure 6). The lowest amount of unevenness of the white layer is found, especially when copper is used as the tool electrode. A non- melted but heat-affected zone due to high thermal gradients is found beneath the white layer. In most cases, a dark heat- affected intermediate layer is visible. This layer is found to be much thinner than the white layer.C. Hardness DepthAt least 10 measurements have been taken for each ther- mally affected layer. The microhardness readings under 10-g load with 15-second indentation time of plastic mold steel sam- ples indicate high hardness values within the white layer and then a substantial decrease through the base material (Table II). The white layer is found to be much harder than the parent material. A dramatic decrease is obvious through the heat- affected zone that stays beneath the white layer and then settled(a)(b)(c)(d)Fig. 6Cross sections of electric discharge machined plastic mold steel samples, dielectric liquid: deionized water, tp = 400 s. (a) Iav = 16 A, electrode: graphite; (b) Iav = 16 A, electrode: copper; (c) Iav = 8 A, elec- trode: graphite, (d) Iav = 8 A, electrode: copper.to the unaffected material hardness value. An interesting result is that the tool electrode and dielectric liquid have minor affects on hardness variations within the affected layers.D. X-Ray Diffraction PatternsX-ray diffraction patterns for plastic mold steel samples have shown basically two different trends (Figure 7). WhenMETALLURGICAL AND MATERIALS TRANSACTIONS BVOLUME 36B, FEBRUARY 2005121Table II. Results of Microhardness MeasurementsWL*HAZ*BaseDataDielectricElectrode(HV)(HV)(HV)TypeKerosenegraphite675465257average51859std. dev.Kerosenecopper661443266average48629std. dev.Watergraphite701465256average406210std. dev.Watercopper626414262average337311std. dev.*White layer*Heat-affected zoneFig. 7X-Ray diffraction patterns of plastic mold steel samples. Iav = 16 A, tp = 800 s. (a) Before EDM, (b) copper electrode in kerosene dielectric liquid, (c) graphite electrode in kerosene dielectric liquid, (d) copper elec- trode in deionized water, and (e) graphite electrode in deionized water.samples are machined in kerosene, Fe3C is formed regard- less of the tool electrode material. Fe3C could not be detected on the surfaces machined in deionized water. Thus, it can be concluded that the increase in carbon content in the surface layers can be attributed to the pyrolysis products of dielectric liquid rather than the tool electrode. Retained austenite is also detected on all samples; the amount is less when deion- ized water is used as the dielectric liquid.IV. DISCUSSION OF THE RESULTSThe outermost layer, which is known as the white layer, is found under all machining conditions. The thickness of the white layer is nonuniform over the discharged surface. This is due to consecutive application of sparks resulting in overlapped layers. Hence, a multilayer structure made up of similar micro- structures should be expected within the white layer. Lim et al.40 also visualized such layered structure under rough machining conditions by using effective reagents and etching conditions. The thickness of the white layer is found to vary from a few micrometers across thin sections to about 80 m or more across thick sections. The thickness of the white layer at these thicker sections is built up due to molten metal, which was expelled(a)(b)Fig. 8Cross-sectional views of plastic mold steel: (a) white layer,(b) globule section tp = 100 s, Iav = 16 A. Tool electrode: graphite; dielectric: kerosene.onto an existing white layer and subsequently solidified. Decreasing pulse duration and current also decreased the thick- ness of the white layer, but a multilayer structure could be vis- ible at thicker sections. A single layer structure is observed at thinner sections, especially at crater bases. The microstructure is largely columnar and dendritic in nature. It is likely that this single layer type may have retained the solidification microstruc- ture of the molten metal in an undistorted form (Figure 8(a). Globule appendages are formed by molten metal droplets, which are expelled randomly during the discharge and later resolidi- fied on the work piece surface. Such appendages can generally be divided in two groups. The first group of globules is only weakly bonded to the white layer. They are small, spherical in shape, and bonded to the substrate either at one or two contact points. Chemical etching can easily dislodge this group of glob- ules. Careful examinations have revealed that, in several instances, no clear evidence of fusion is detectable at the loca- tions where these globules are dislodged.40 The second group of globules is fused firmly on to the recast layer and has large contact areas with the substrate. A spherical globule attached to a multilayer substrate during machining can be seen in Fig- ure 8(b). The tool electrode material and dielectric liquid are effective in the formation of such globular appendages. Although there is no conclusive evidence of carbon enrichment in white and thermally affected layers from the graphite electrode, an increase in the number of globule appendages are obvious (Figure 1(b). The micrographs indicate that there is an inter- action with dielectric and also with tool electrode. An inter- esting result is the inhibition of the bulk boiling process on the surface during machining if no supply of carbon is available from the dielectric or tool electrode. This suggests that carbon assimilated both from the tool electrode and dielectric liquid122VOLUME 36B, FEBRUARY 2005METALLURGICAL AND MATERIALS TRANSACTIONS Btriggers the boiling process by producing traps within the melted material.X-ray diffraction patterns for plastic mold steel samples have shown Fe3C formation on machined surfaces when kerosene is used as the dielectric liquid. Consequently, the white layer consists of cementite and martensite, distributed within a retained austenite matrix. The amount of retained austenite phase and intensity of microcracks are less when deionized water is used as the dielectric liquid. Changing the electrode material does not alter the result. Presumably, only the amount of phases may vary. Formation of cementite is possible due to pyrolysis products of the cracked hydrocarbon dielectric during discharge. Microhardness measurements have shown that hardness varies within the white layer for all cases since it is composed of different microcomponents.In all cases, thermally affected layers beneath the white layer could be barely distinguished for electric discharge machined plastic mold steels. In this zone, hardness is found to be as high as the white layer hardness value at the outer- most regions, and then to gradually decrease to parent mate- rial hardness at the inner sections.Most of the researchers have reported an increase in crack- ing at increased energy levels, especially at higher pulse dura- tions.11,32 According to them, the intensity of the crack formed during machining should be proportionally increased with respect to pulse energy. However, Lee and Tai22 have stated that the maximum crack density actually occurs under the minimum pulse current and maximum pulse duration. The results confirm this conclusion. The crack density decreased under high energy levels at the same pulse duration. If the pulse energy is decreased, a network of cracks following the pitting arrangements with closed loops is observed (Figure 3(a). Cracks formed in a crater continue to propagate when another discharge takes place in the neighborhood. It can be noted that the intersection points of crack paths usually form perpen- dicular angles (Figure 9). Lack of appendages and globules can also be distinguished on the samples. Sometimes minor craters, presumably due to the collapse of bubbles, are pro- duced on the machined surface. Traces of martensite are vis- ible at the crater bases in such circumstances (Figure 9(b). The number of cracks decreased when the pulse duration was decreased at the same energy level. Radial cracks, especially at crater rims (Figure 10), suggest that a higher thermal radial stress developed during sparking. A change in tool electrode has not altered the surface crack topography. Cracks have been found to propagate through the white layer and to stop when the heat-affected portion of the material is reached.An intense and unusual cracking exceeding the thermally affected layers has been encountered when the graphite tool electrode is used as the tool electrode and deionized water as the dielectric liquid at high pulse durations (Figure 11). Such unstable operational conditions are uncommon for industrial applications. The shapes of the craters produced under these conditions are found to be deeper and irregularly shaped when compared with the others (Figure 4(a). Cracks are randomly distributed, usually at the crater bases, and extended up to the parent material. The occurrence of such defects when using deionized water as the dielectric liquid is related to the contamination of dielectric liquid with debris from the graphite tool electrode during machining. The increase in contamination decreased the dielectric liquid strength and resulted in arcing during machining.(a)(b)Fig. 9Cracking after EDM of plastic mold steel: (a) magnification 200 times and (b) magnification 550 times SEM. Dielectric: kerosene; tp = 1600 s; Iav = 8 A.(a)(b)Fig. 10Cracking after EDM of plastic mold steel: (a) magnification 200 times and (b) magnification 550 times SEM. Tool electrode: copper; dielec- tric: kerosene; tp = 400 s; Iav = 8 A.METALLURGICAL AND MATERIALS TRANSACTIONS BVOLUME 36B, FEBRUARY 2005123Fig. 11Boundary cracking of plastic mold steel. Tool electrode: graphite; dielectric: deionized water; tp = 1600 s; Iav = 8 A.V. CONCLUSIONSThe following conclusions can be drawn from this work.1. The white layer is produced on all electric discharge machined surfaces regardless of the dielectric liquid and tool elec- trode material.2. The white layer of samples machined in hydrocarboneous dielectric liquid contains more carbon than the base mate- rial due to pyrolysis products of the cracked hydrocarbon dielectric during discharge. Consequently, the white layer is composed of cementite and martensite distributed in retained austenite matrix forming dendritic structures due to rapid solidification of the molten metal.3. The amount of retained austenite phase and intensity of microcracks are much less in the white layer of samples machined in deionized water dielectric liquid. In this case, the hardness increase of the white layer with respect to parent material is attributed to martensite formation within the layer.4. Although there is no conclusive evidence of carbon enrich- ment in white and thermally affected layers from the graphite electrode, the number of globule appendages attached to the machined surface increases. This suggests that carbon assimilated both from the tool electrode and dielectric liquid triggers the boiling process.5. Cracks on EDM surfaces follow the pitting arrangements with closed loops and crossing perpendicularly with radial cracks and continues to propagate when another discharge takes place at the neighborhood. The intensity of cracking is increased at high pulse durations and low pulse currents.ACKNOWLEDGMENTSThis study was supported by the Middle East Technical University Research Fund. The authors are grateful to the Materials Research Laboratory, Eregli Iron and Steel Works Co., for the use of their facilities. The authors are also thank- ful to Mr. M. 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Lee and T.Y. Tai: J. Mater. Process. Technol., 2003, vol. 142, pp. 676-83.23. F. Ghanem, C. Braham, and H. Sidhom: J. Mater. Process. Technol., 2003, vol. 142, pp. 163-73.24. J. Simao, H.G. Lee, D.K. Aspinwall, R.C. Dewes, and E.M. Aspinwall:Int. J. Machine Tools Manuf., 2003, vol. 43, pp. 121-28.25. Y.H. Guu, H. Hocheng, C.Y. Chou, and C.S. Deng: Mater. Sci. Technol., 2003, vol. 358, pp. 37-43.26. Y.F. Luo and C.G. Chen: Precision Eng., 1990, vol. 12, pp. 97-100.27. N. Mohri, N. Saito, T. Takawashi, and K. Kobayashi: Proc. 26th Int. MTDR Conf., Manchester, England, 1985, pp. 329-36.28. Y.F. Luo, Z.Y. Zhang, and C.Y. Yu: Ann. CIRP, 1988, vol. 37, pp. 179-81.29. P.H. Thomson: Mater. Sci. Technol., 1989, vol. 5, pp. 1153-57.30. S.H. Lee and X. Li: J. Mater. Process. Technol., 2003, vol. 139, pp. 315-21.31. Report of AGIE: Am. Machinist Automated Manufacturing, 1987, vol. 9, pp. 80-83.32. A.G. Mamalis, N.M. Vosniakos, N.M. Vacevanidis, and X. Junzhe:Ann. 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Wallbank: Metallurgia, 1980, vol. 47, pp. 356-62.124VOLUME 36B, FEBRUARY 2005METALLURGICAL AND MATERIALS TRANSACTIONS BJournal of Materialx Prosexxing Teshnolog 127 (2002) 361368Development of prosexx sontrol in xheet metal formingS.-W. Hxua,*, A.G. Ulxob,1, M.Y. Demeris,2aTAC Automotive Group on Site at Ford Motor Company, Dearborn, MI 48l2l, USAbDepartment of Meshanisal Engineering and Applied Meshanisr, Univerrity of Mishigan, Ann Arbor, MI 48l09, USAsFord Rerearsh Laboratorier, Ford Motor Company, Dearborn, MI 48l2l, USAReseived 8 Ma 2002; assepted 13 Ma 2002AbztractIn xheet metal forming prosexxex, the blank holder forse sontrolx the material flow into the die savit, whish ix sritisal to produsing a good part. Prosexx sontrol san be uxed to adjuxt the blank holder forse in-prosexx baxed on trasking a referense punsh forse trajestor to improve part qualit and sonxixtens. Ke ixxuex in prosexx sontrol inslude prosexx sontroller and referense punsh forse trajestor dexign. The purpoxe of thix paper ix to prexent a xxtematis approash to the dexign and implementation of a xuitable prosexx sontroller and an optimal referense punsh forse trajestor. The approash insludex modeling of the xheet metal forming prosexx, dexign of the prosexx sontroller, and determination of the optimal punsh forse trajestor. Experimental rexultx from U-shannel forming xhow that a xuitable prosexx sontroller san be dexigned through ximulation and an optimal referense punsh forse trajestor san be xnthexized through experimentx. The propoxed development xhould be uxeful in dexigning and implementing prosexx sontrol in xheet metal forming prosexxex.g 2002 Elxevier Ssiense B.V. All rightx rexerved.Keywordr: Sheet metal forming; Prosexx sontrol; Optimization1. IntroductionSheet metal xtamping ix one of the primar manufasturing prosexxex besauxe of itx high xpeed and low soxt for high volume produstion. For example, partx xush ax bod panelx, torque sonverter impeller bladex, and fuel tankx are all prodused b thix method. A ximplified xtamping prosexx ix xhown in Fig. 1. The baxis somponentx are a punsh, and a xet of blank holderx whish ma, or ma not, inslude draw- beadx. The punsh drawx the blank to form the xhape while the blank holder sontrolx the flow of metal into the die savit.The qualit of xtamped partx ix sritisal in avoiding problemx in axxembl and in the final produst performanse. Two main sonxiderationx regarding the qualit of xtamped partx are formabilit (e.g., wrinkling sauxed b exsexxive somprexxion and tearing sauxed b exsexxive tenxion) and dimenxional assuras (e.g., xpringbask sauxed b elaxtis resover). Main problemx in xheet metal forming are xhown* Sorrexponding author. Tel.: 1-313-843-4646.E-mail addrerrer: shxu1ford.som (S.-W. Hxu), (A.G. Ulxo), mdemeriford.som (M.Y. Demeri).1 Tel.: 1-734-764-8464; fax: 1-734-647-3170.2 Tel.: 1-313-843-6092; fax: 1-313-390-0314.in Fig. 2. In addition, sonxixtens (e.g., dimenxional varia- tionx sauxed b lubrisation or thisknexx variationx) in the xtamping prosexx xignifisantl affestx xubxequent axxembl in maxx produstion.New shallengex emerge from the uxe of new materialx. For example, to reduse automobile weight (to improve fuel esonom) manufasturing sompaniex muxt uxe lighter mate- rialx (e.g., aluminum) or thin gage high xtrength xteel inxtead of mild xteel. However, xush materialx are not ax formable ax mild xteel and produse more xpringbask 1.The sontrol of material flow into the die savit ix srusial to good part qualit and sonxixtens, and the blank holder ix uxed to sontrol the material flow. Previoux rexearsh hax xhown that varing the blank holder forse during forming san improve part qualit 13 and sonxixtens 1. It ix worth pointing out that meshanisal prexxex are being retro- fitted with hdraulis multi-point suxhion xxtem to provide more sontrol of the forming prosexx 4. Sush prexx tesh- nologiex will fasilitate the implementation of the prosexx sontrol ideax prexented in thix paper.One xtrateg for sontrolling xheet metal forming pro- sexxex through the applisation of variable blank holder forse ix prosexx sontrol (xee Fig. 3). In thix xtrateg, a meaxurable prosexx variable (e.g., punsh forse) ix sontrolled b following a predetermined (e.g., punsh forsedixplasement)0924-0136/02/$ xee front matter g 2002 Elxevier Ssiense B.V. All rightx rexerved. PII: S 0924-013 6(02)00321-7 362C.-W. Hru et al. / Journal of Materialr Proserring Teshnology l27 (2002) 36l368sontroller and dexign of an optimal referense trajestor. The purpoxe of thix paper ix to addrexx thexe two ke ixxuex to xxtematisall dexign and implement prosexx sontrol in xheet metal forming.Fig. 1. Sshematis reprexentation of a xtamping prosexx.referense trajestor through manipulating the blank holder forse 1. Thix xtrateg sould produse supx with optimal height regardlexx of initial blank holder forse and fristion sonditionx 3. Other meaxurable prosexx variablex (e.g., draw-in and fristion forse) have alxo been reported 68. To xxtematisall dexign a xuitable prosexx sontroller, the prosexx model in Fig. 3 muxt be identified firxt. Moxt xheet metal forming modelx are baxed on finite element analxix, whish are ver somplex and, therefore, are not xuitable for sontroller dexign 9. A piesewixe linear model for sontroller dexign hax been developed 9. However, thix model sannot be uxed in sloxed-loop ximulation, besauxe it sannot sapture the sharasterixtis nonlinear behavior of a xheet metal form- ing prosexx. Therefore, ixxuex in modeling for sontrol of xheet metal forming have not been adequatel addrexxed, expesiall from a sontrol point of view, although methodx ofxxtem identifisation have been well developed 10.The moxt popular xtrusture for the prosexx sontroller ix a proportional plux integral sontroller 3,6,8. However, son- troller parameterx are tpisall determined b trial and error 11. Although dexign of prosexx sontroller hax been well developed 12, itx applisation to xheet metal forming ix xtill not well invextigated.The referense trajestor in prosexx sontrol ix important to enxure good part qualit in xheet metal forming 13. It hax been determined experimentall or numerisall 3,8. How- ever, optimization of the referense trajestor hax not been well addrexxed.Ke ixxuex regarding the applisation of prosexx sontrol to xheet metal forming inslude dexign of an appropriate prosexx2. Procezz control in zheet metal forming2.l. Experimental fasilityProsexx sontrol experimentx were sondusted on a double astion hdraulis forming ximulator equipped with a PID digital sontroller (xee Fig. 4). The prexx load sapasit ix 680 kN for the punsh and 700 kN for the binder. The digital sontroller allowx the blank holder forse to trask a prede- termined trajestor, whish ix the realization of the mashine sontroller blosk in Fig. 3.Implementation of proserr sontrolImplementation of prosexx sontrol on thix forming ximu- lator ix xhown in Fig. 3 13. The additional somponent ix the DAQ blosk that ix a data asquixition board. It asquirex data from the digital sontroller (the realization of the outer feedbask path in Fig. 3) and feedx the salsulated blank holder forse sommand into the digital sontroller (the reali- zation of the output of the prosexx sontroller blosk in Fig. 3). The program blosk with the DAQ blosk ix the realization of the prosexx sontroller blosk in Fig. 3. The WSSI blosk ix the original workxtation sommunisation interfase.Influense of proserr sontrol on rheet metal forming2.3.l. Part sonrirtensy via proserr sontrolResentl, a somparixon of mashine and prosexx sontrol for U-shannel forming demonxtrated the xuperiorit of prosexx sontrol to mashine sontrol 13. Fig. 6 xhowx relative trasking errorx for mashine and prosexx sontrol under dr and lubrisated sonditionx. The rexultx xhow thatFig. 2. Problemx in xheet metal forming.C.-W. Hru et al. / Journal of Materialr Proserring Teshnology l27 (2002) 36l368363Fig. 3. Prosexx sontrol of xheet metal forming.Fig. 4. Forming sexx sontrol san maintain the xame punsh forse trajes- toriex under different lubrisation sonditionx but mashine sontrol sannot. Table 1 xhowx average meaxured shannel heightx for the saxex xhown in Fig. 6. The meaxurementx xhow that prosexx sontrol improvex sonxixtens in shannel height, dexpite shange in lubrisation. Therefore, sonxixtensin shannel height san be related to sonxixtens in punsh forse trajestoriex.2.3.2. Importanse of proserr referense trajestorierThe importanse of the referense punsh forse san be xhown b somparing meaxured shannel heightx for different referense trajestoriex 13. Fig. 7 xhowx two experimental referense punsh forse trajestoriex. Table 2 xhowx meaxured shannel heightx for thexe two trajestoriex. Trajestor (b) produsex better partx besauxe the meaxured shannel heightx are sloxer to the dexired shannel height (30 mm).Derign of proserr sontrol in rheet metal formingBaxed on the above experimental rexultx, two important sonxiderationx emerge:Table 1Average meaxured shannel heightx (mm) for mashine and prosexx sontrolx under different lubrisationxSontrol tpeDrMP-404Mashine47.60046.211Prosexx47.33747.639Fig. 3. Implementation of prosexx sontrol. 364C.-W. Hru et al. / Journal of Materialr Proserring Teshnology l27 (2002) 36l368Fig. 6. Relative trasking errorx.Fig. 7. Experimental referense punsh forse trajestoriex.Table 2Meaxured shannel heightx (mm) for the referense punsh forse trajestoriex in Fig. 7Trajestor(a) (b)147.44749.231247.39649.327347.82849.276Mean47.33749.283Text number Evaluation of the trasking performanse of the prosexx sontroller. Selestion of the referense punsh forse trajestor.Thexe two sonxiderationx will be addrexxed in the follow- ing xestionx.3. Sheet metal forming procezz modelingModeling a xheet metal forming prosexx involving hdraulisall sontrolled xingle slinder binder for prosexx sontroller dexign, whish ix a xingle-inputxingle-outputC.-W. Hru et al. / Journal of Materialr Proserring Teshnology l27 (2002) 36l368363Fig. 8. Prosexx model of xheet metal forming.(SISO) xxtem, hax been invextigated 14. Thix ix xhown in the blosk diagram in Fig. 8. The prosexx model ix a firxt- order nonlinear dnamis model. The dixturbanse, mainl due to variationx in lubrisation, ix alxo xhown. Thix model hax been xussexxfull uxed in modeling a U-shannel forming prosexx. Fig. 9 xhowx somparixon of ximulation and experi- mental rexultx for different sontinuouxl variable blank holder forse trajestoriex.4. Procezz controller dezignBesauxe of the empirisal prosexx model, xxtematis xtud of prosexx sontroller dexign san be sondusted analtisall and numerisall before implementation 13. For the SISO xxtem, a proportional plux integral sontroller with feedfor- ward astion (PIF) hax been invextigated and xussexxfull implemented in the forming ximulator 13. The blosk diagram of the sontroller ix xhown in Fig. 10. A firxt-order linear model san be uxed to dexign the sontroller gainx. TheFig. 10. Blosk diagram of the PIF sontrol xxtem.firxt-order linear model san be replased with the firxt-order nonlinear model in Fig. 8 to evaluate the trasking perfor- manse of the sloxed-loop xxtem uxing the dexigned son- troller gainx.Fig. 11 xhowx ximulation rexultx uxing the PIF prosexx sontroller and the firxt-order nonlinear model. Fig. 11(a) xhowx the blank holder forse automatisall generated b the PIF prosexx sontroller. Fig. 11(b) xhowx the referense punsh forse trajestor, Fpd, and the punsh forse trajestor, Fp. Good trasking performanse ix expested baxed on ximulation rexultx.Experimental rexultx uxing the xame PIF prosexx son- troller and the xame referense punsh forse trajestor are xhown in Fig. 12. Although there wax variation in the blank holder forse trajestoriex, the punsh forse trajestoriex were ximilar. Thix indisatex that the prosexx sontroller workx well.Fig. 9. Experimental and predisted punsh forse trajestoriex for different variable blank holder forse trajestoriex.366C.-W. Hru et al. / Journal of Materialr Proserring Teshnology l27 (2002) 36l368Fig. 11. Simulation rexultx uxing the PIF prosexx sontroller and the firxt-order nonlinear model.Fig. 12. Experimental rexultx uxing the xame PIF prosexx sontroller and referense punsh forse trajestor.5. Optimal punch force trajectory dezignOne method for obtaining an optimal referense punsh forse trajestor ix to uxe dexign optimization methodx 13,16. With an ideal prosexx sontroller, Fig. 3 san be ximplified ax xhown in Fig. 13.In thix saxe, the xtamped part xhape, S, will be totall determined b the referense punsh forse trajestor or equivalentl b the punsh forse trajestor, Fp.A mathematisal exprexxion san be uxed to dexsribe the relationxhip in Fig. 13:S = P(Fp)(1)pThe optimal punsh forse trajestor Fm for a dexired xhape Sdsan be obtained b xolving the following equation:Fmp = arg min E(P(Fp), Sd)(2)FpcDFig. 13. Prexx with ideal prosexx sontroller.where Fm ix the optimal punsh forse trajestor, D the xafe domain for Fp without tearing and wrinkling, and E the soxt funstion to reprexent the differense between P(Fp) and Sd.ppTo find Fm through optimization ix xtill diffisult. The shallengex are:1. To find the operator P, whish give a punsh forse trajestor, ieldx the part xhape.2. To find the domain D whish definex xafe punsh forse trajestoriex.Sinse surrent mathematisal modeling of xheet metal forming uxex finite element methodx 17,18, there ix no ximple exprexxion for P or D.A prosedure for xolving Eq. (2) through parameterization and dexign of experimentx ix ax followx:1. Parameterize Fp and S. Parameterx of Fp are the dexign variablex and parameterx of S are the rexponxe variablex.2. Identif an empirisal relationxhip between the dexign and rexponx