Machining Phenomena in EDM for Surface Modification with TiC Semi-sintered Electrode

Transcription

1 Machining Phenomena in EDM for Surface Modification with TiC Semi-sintered Electrode Toshio Moro, Akihiro Goto, Nagao Saito EDM system department, MITSUBISHI ELECTRIC CORP., Nagoya, Japan* Naotake Mohri Department of Precision Machinery Engineering, The University of Tokyo, Tokyo, Japan** Koji Akamatsu, Hisashi Yamada MC MACHINERY SYSTEMS, INC. Wood Dale, IL + and Toshio Sata Toyota Technological Institute, Nagoya, Japan ++ Abstract Electrical Discharge Coating (EDC) is one method of surface modification process for making a hard layer on the work piece, using the composite structure electrode made by metallic powder that forms carbide easily. In this research, semi-sintered product made of TiC powder is used as an electrode for surface modification. It is important to examine the electrode wear process because a large amount of powder from the worn electrode is expected to exist among the gap region and it becomes the source of generated layer on the work piece. As a result, the machine surface of the work piece is covered with TiC layer and it becomes functionally graded material. The wear length of semi-sintered electrode depends on electrode area. Especially, pulse duration does not affect on the amount of electrode wear in the same electrode area. The growth of the TiC layer on the work piece tends to stop at a certain thickness according to pulse duration but it dose not depend on the electrode area. From the hardness distribution of the machined surface, hardened region expands into the inside of original work piece. In this paper, some peculiarities in the surface modification phenomena by EDM with TiC semi-sintered electrode are discussed and the result of an application of the treatment to the tool edge of a punching die will be reported. Keywords: Electrical discharge machining, Electrode, Surface modification, TiC, Hard layer Introduction During these past 1 years, crack less surface of a work piece has been realized by the finishing electrical discharge machining (EDM) in which specially made electrode or powder mixed working fluid is used. Corrosion resistance and wear resistance of the work piece improves more. Besides, the trial to implant the substance in the gap onto the work piece was going to appear prosperous, and it become to produce new characteristics on the work piece[1][2][4][5][7][8][9]. For the surface modification by EDM, it is nec- essary to move and accumulate the in the gap region onto the top of the work piece. The substance in the gap region might be the decomposition product from tool electrode, the powder mixed from outside or the cracked carbon from working oil. As for the polarity of electrode, machining is done minus in case of all. For the use of the substance produced from an electrode, the electrode should be worn much through EDM. The electrode can be made by pressforming or sintering of fine powder being implanted to overcome the constraint of thermo-physical prosperities of material [3][6]. *EDM system department,mitsubishi ELECTRIC CORPORATION, NAGOYA WORKS, 1-14,Yadaminami5-chome, Higashi-ku, Nagoya ,Japan. moro@edm.mei.melco.co.jp **Department of Precision Machinery Engineering, The University of Tokyo, Graduate School of Engineering, Hongo, 7-3-1, Bunkyou-ku, Tokyo , Japan + MC MACHINERY SYSTEMS, INC. 15 Michael Drive Suite C Wood Dale, IL 6191, U.S.A ++ Toyota Technological Institute, Hisakata, Tempaku-ku Nagoyga , Japan

2 Electrode(-) Composite structure electrode : TiC Electrode(-) 25 2 Workpiece(+) Piled layer Workpiece(+) Fig 1: Principle of Surface Modification by EDM Table 1: Specifications of composite structure electrode Material TiC powder Purity 99. % Diameter of powder 2-5 u Forming and sintering conditions Pressure forming 98.1N/mm 2 Sintering process 9 1hr Size(mm) Transverse-rupture strength 14.1N/mm 2 Note) um :Mark um stands for micron meter. The surface layer of titanium carbide (TiC) gets possible to form on the face top of tungsten carbide (WC) if we use green compact of titanium hydride (TiH 2 ) as an electrode, and surface layer hardness reaches up to two thousand several hundred Hv. In this case the carbon in TiC which accumulated on the work surface, can be considered as the result of cracked carbon of working fluid which reacts with titanium. However, it is difficult to make large electrode because the compact was fragile even if it is made under high compacting pressure in use of Ti or TiH 2 powder. Besides, since these kinds of electrode wear too much, there is a tendency of generating rough surface on the work piece surface. Thermal conductivity of the electrode apparently falls, Table 2: Machining conditions for Surface Modification by EDM EDC conditions No.1 No.2 No.3 No.4 No.5 No.6 Discharge current: i e(a) Discharge duration: t e(us) Pulse interval time: t o(us) Duty factor: T(%) Electrode polarity (-) (-) (-) (-) (-) (-) Workpiece S45C S45C S45C S45C S45C S45C Note) us :Mark us stands for micron second. Wear length (u ) EDC conditions i e =8A, t e =8us, t o =128us :1mm 2 :4mm 2 :9mm Machining time ( 1 3 s) Fig 2: Relation between machining time and wear length and, besides, bonding strength between particles is smaller than base material. On this account the electrode material leaves it by discharge easily as shown in Fig 1. The products of semi-sintered green compact of TiC can be used for stable and large scale electrodes which leads to practical use in the market. In this paper, some peculiarities in the surface modification phenomena by EDM with TiC semisintered electrode are discussed and the result of an application of the treatment to the tool edge of a punching die will be reported. TiC Electrode Wear Phenomena In this research, semi-sintered product made of TiC powder is used as an electrode for surface modification. It is important to examine the electrode wear process because a large amount of powder from the worn electrode is expected to exist among the gap region and it becomes the source of generated layer on the work piece. The electrode forming and sintering conditions with its transverse rupture strength are shown in Table 1. Table 2 shows the electrical machining conditions for surface modification. Under these conditions, we confirm in advance that TiC layer of several um thickness can be provided on carbon steel under these machining conditions. And this coated layer has remarkable resistance to wear of die and cutting tool. The electrode wear length against machining time is shown in Fig 2 for each electrode area. These

3 Wear length (um) Wear length (um) Machining area=1mm 2 :te=2us,t=.59 :te=2us,t=.3 :te=8us,t=.59 :te=8us,t=.3 :te=16us,t=.59 :te=16us,t=.3 :te=2us,t=.3 :te=8us,t=.3 :te=2us,t=.59 :te=8us,t= Energy density (1 6 J/m 2 ) :te=16us,t=.3 :te=16us,t=.59 A=1mm 2 A=4mm 2 A=9mm Energy density ( 1 6 J/m 2 ) Fig 4: Wear length for different electrode area vs. energy density data are acquired in the machining with the electrodes of 1, 4, 9mm 2 under the condition of No.4 in Table 1. The machining time is set so that the number of discharges per electrode area is the same. The wear of electrode increases in proportion to the machining time for each electrode with its own area. In order to evaluate the effect of the machining time and the electrode area, we introduce energy density which represents supplied energy per unit area into the gap region. Energy density E(J/m 2 ) is defined by the following equation, E=e / n / A = U e / i e / T / t / A (1) where, e: energy consumed in gap region for a Average thickness (u ) EDC conditions ie=8a,te=2us,to=64us ie=8a,te=8us,to=128us A=1mm 2 A=4mm 2 A=9mm 2 A=1mm 2 A=4mm 2 A=9mm Machining time ( 1 3 s) Fig 3: Wear length for different pulse duration vs. energy density Fig 5: Relation between average thickness and 25 machining time ie=8a,te=2us,to=64us 2 A=1mm 2 A=4mm 2 T=.3 15 A=9mm2 T=.59 1 ie=8a,te=8us,to=128us single discharge, n: number of total discharges, A: electrode area, U e: discharge voltage, i e: discharge current, T: duty factor and t: machining time. In removal or additive machining, the data for the amount of machining might lay almost on a straight line if these are represented against the energy density. In other words, in this case, discharge disperses uniformly over the whole area of work piece surface, and the unit amount of removal or additive machining by a single discharge shows a constant. Fig 3 shows the electrode wear length against the energy density under the condition in Table 2 using sintered TiC electrode of 1mm 2. Electrode wear length increases in proportion to energy density and it means that the quantity of electrode wear increases according to the machining time as shown in Fig 2. Judging from this figure, the change of wear length seems less than 2% for the change of pulse duration te 2-16us under same duty factor. On the other hand, as for the generally used solid electrode or work piece, for example in the machining of steel with copper electrode, the removal rate of the steel (-) changes more than 2% for the change of pulse duration. Therefore, the effect of pulse duration is extremely little on the electrode wear in surface modification. It is considered that the semi-sintered electrode is removed by the first impact force with the vaporization of working fluid owing to each discharge before it is affected by heat duration because the electrode material is so fragile.

4 Average thickness (um) Machining area=1mm 2 :te=2us,t=.59 2 :te=2us,t= :te=8us,t= :te=8us,t=.3 :te=16us,t=.59 :te=16us,t= Energy density ( 1 6 J/m 2 ) Fig 6: Relation between average thickness and energy density Next, the relations between electrode wear length and energy density for different electrode area are shown in Fig 4. These data are acquired under the conditions of No.1 and No.4 in Table 1. We can observe in this figure that the wear length for each energy density gets larger according to the electrode area and short pulse duration gets larger. This suggests us that the impact force which acts on the electrode depends on the dispersion of discharges and vanish time of bubble due to each discharge. Thickness of Modified Layer As for the thickness of the modified layer, we adopt average height from the profile curve measured by a roughness meter. The resultant thickness given under No.1 and No.4 conditions in Table 2 with 1, 4, 9mm 2 electrode is displayed in Fig 5 in relation to machining time. The average thickness that is the amount of the accumulation per electrode area increases up to the limit thickness according to each machining conditions. The relation between average thickness and energy density for all conditions in Table 2 with 1mm 2 is shown in Fig 6. This Figure tells us that the thickness of piled layer does not depend on the electrode area and duty factor but depends on the pulse duration. In the surface modification by EDM the worn powder from the electrode is the main component of the piled layer on the work piece. The amount of electrode wear increases according to energy density as shown in Fig 3. It means that the amount of Average hardness (HV) EDC conditions i e =8A, t e =2us, t o =64us i e=8a, t e=8us, t o=128us HV=16(1-exp(-E/2 1 6 )) Energy density ( 1 6 J/m 2 ) Fig 7: Relation between average hardness and energy density powder supplied into the gap region per unit time is constant through the machining process. On the contrary, the thickness of the layer depends on pulse duration. Consequently, it is considered that the generation of piled layer is strongly caused by the heat input due to each discharge. Furthermore, the fact that the thickness of generated layer reaches to a limit in spite of stationary supply of worn powder in the gap indicates a special feature of modification by EDM. At the beginning of machining, the work piece is treated onto its virgin surface and the component of the surface gradually changes according to machining time. The top of the surface gets dense with TiC grains which are different from the original matrix of the work piece in thermal and mechanical properties. Therefore the thickness of piled layer reaches its limit in the machining process with constant conditions. Actually, piling does not occur onto the surface of complete sintered TiC under the same conditions in this research. Hardness of Modified Layer After machining at each time onto a plate of carbon steel (S45C) with a TiC semi-sintered electrode under the conditions of No.1, No.4, in Table 2, we measured the hardness of the work piece surface under 98mN of indentation force. The value of hardness averaged over 5 times are plotted in relation to energy density as shown in Fig 7. This graph shows that the data of the average hardness are on a curve gradually approaching the fixed value of about 2HV. And it does not depend on any machining

5 Microhardness (HV) Hard layer Machining area 2 2mm Machining time 32min ie=8a, te=2us, to=64us ie=8a, te=8us, to=128us Hardness of matrix Distance from surface (um) Fig 8: Microhardness distribution on the cross section of workpiece Matrix Boundary layer Fe-Kα Burr height (u ) Fig 1: Press die set for test shot EDC conditoins ie=8a te=2us,to=64us Thickness=5u 8 EDCoated die Punch shot number ( 1 4 ) Without coat Fig 11: Burr height of punched product vs. punch shot number Surface of matrix 1um Ti-Kα EPMA (electron probe micro analyzer). The result of the line analysis on Ti and Fe tells us that the machined layer is functionally graded even inside of the original work piece. Ti diffuses into the work piece through the machining, and this material distribution corresponds to the hardness distribution of the work piece. Fig 9: Result of EPMA on the cross section of hard layer conditions and electrode area. In order to evaluate the hardness of the inside of the work piece, we grind the work piece from the top surface and measure the hardness of the ground face at each depth. The measured hardness distributions are illustrated in the Fig 8 which are obtained by machining a 4mm 2 work piece under conditions No.1, No.4 in Table 2. This Figure shows that the hardness changes against the distance from the surface and it reaches inside of the original work piece. Fig 9 shows the result of an elemental analysis on the cross section of the work piece by an Effect of TiC Modified Layer on Press Die We tried surface modification by EDM onto the surface of cutting edge of a press die and a punch made from hardened tool steel (SKD11) with the hardness of HRC 6. With TiC semi-sintered electrodes, the surfaces of the punch and die are treated by EDM with planetary motion for 2min. The electrode of 2mm diameter is scanned around the surface of edges. The machining conditions are i e =8A, t e = 2us and t o =128us. Fig 1 shows a photo of a part of the die set. 4 holes of ellipses (5mmx3mm) can be seen in the center of the photo. Treated hole and non-treated hole are arrayed alternately. Silicon steel plates of

6 .5mm thickness are punched with this die and punch set. The height of burr of the punched products are measured for the evaluation of the die performance. Fig 11 shows the relation between the burr height and shot number as the result of the punching test. While the burr height of the products made by non-treated die increases according to the increase of shot number, the products made by surface modified by EDM keep the same burr height after hundreds of thousands. It shows that the die life increases obviously. Conclusions Some experiments of electrical discharge machining for surface modification were carried out onto carbon steel plate with TiC semi-sintered electrode. Summarizing the data given in the experiments, the following conclusions can be drawn. (1) The machine surface of the work piece is covered with TiC layer and it becomes functionally graded material. (2) The removal or additive machining rate can be represented with energy density. This shows the division of the effects of machining conditions on the machining performance. (3) The wear length of semi-sintered electrode depends on electrode area. Especially, pulse duration does not affect on the amount of removal in the same electrode area. It means that in the process of semi-sintered electrode wear, the electrode is removed mainly by the impact due to each discharge not by heating effect. (4) The growth of the TiC layer on the work piece tends to stop at a certain thickness according to pulse duration but it dose not depend on the electrode area. It is important to concern the effect of heating by each discharge taking consideration of thermal properties of piled layer. (5) From the hardness distribution of the machined surface, hardened region expands into the inside of original work piece. It corresponds to the distribution of Ti given by EPMA. (6) It is confirmed that the life of the die treated by EDM with TiC electrode increases through punching test of silicon steel. It is expected by applying this material coating method to die and cutting tool that we can industrialize this method as alternative technology of conventional PVD or CVD. 1 H.Narumiya, et.al. EDM by Powder Suspended Working Fluid. Proc.of ISEM-9, (1989) 5. 2 Y.Iino,et.al. Joining of Ceramic to Metal by use of an electric discharge machined surface. J. Materials References Science Letters 8(1989) N.Mohri, et.al. Metal Surface Modification by Electrical Discharge Machining with Composite Electrode. Annals of CIRP, 42,1,(1993) K,Masui, et.al. Surface Modification of Tool Steels by Alloying Method Using EDM Process. Proc. of ISEM-11, (1995) J.P.Kruth, et.al. Micro Structure Investigation and Metallographic Analysis of the White Layer of a Surface Machined by Electro Discharge Machining. Proc. of ISEM-11,(1995) H.Miyake, et.al. Improvement of Tool Life through Surface Modification by Electrical Discharge Machining. Proc. of ISEM-12, (1998) N.Mohri, et.al. Materials accretion by Electrical Discharge Machining with thin Electrodes. Proc. of the International Seminar on Improving Machine Tool Performance, (1998) H.Takezawa, et.al. Precision Machining by Feedback of Shape Data. Proc. of the 4th Japan- France Conference on Mechatronics, Kitakyushu, Japan, 2 (1998) N.Mohri, et al. A New Process of Additive and Removal Machining by EDM with a Thin Electrode. Annals of CIRP, 49,1,(2) 123.