钨粉对电火花表面强化模具钢性能的影响毕业论文外文翻译.doc

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1、附录1：英文资料Surface modification of die steel materials by EDM method using tungsten powder-mixed dielectric Sanjeev Kumara, , Uma Batrab,1 Keywords: Electrical discharge machining Powder-mixed EDM Surface modification Material transfer Tungsten powderabstract Surface modification by material transfer d

2、uring electrical discharge machining (EDM) has emerged as a key research area in the last decade. Material may be provided to the machined surface of the workpiece by the eroding tool electrode or by using powder-mixed dielectric. Breakdown of the hydrocarbon dielectric contributes carbon to the pla

3、sma channel which may also cause surface modification. The present work has investigated the response of three die steel materials to surface modification by EDM method with tungsten powder mixed in the dielectric medium. Taguchi experimental design technique was used to conduct the experiments on e

4、ach work material independently. Peak current, pulse on-time and pulse off-time were taken as variable factors and micro-hardness of the machined surface was taken as the response parameter. X-ray diffraction (XRD) and spectrometric analysis show substantial transfer of tungsten and carbon to the wo

5、rkpiece surface and an improvement of more than 100% in micro-hardness for all the three die steels. Presence of tungsten carbide (WC and W2 C) indicates that its formation is taking place in the plasma channel. Machining parameters for the best value of micro-hardness for each work material were fo

6、und to be the same.1. Introduction Electrical discharge machining (EDM) process has become the workhorse of the tool making industry for the precise machining of workpieces that conduct electricity. It plays a major role in the machining of dies, tools, etc. made of tungsten carbides or hard steels.

7、 The material can be machined in the hardened state and distortions resulting from heat treatment processes are eliminated 1. Although the mechanism of material erosion employed in EDM is still arguable 2, the widely accepted principle is the conversion of electrical energy into thermal energy throu

8、gh a series of discrete electrical discharges occurring between the electrode and workpiece immersed in a dielectric fluid 3. The insulating effect of the dielectric is important in avoiding electrolysis of the electrodes. Spark is initiated at the point of smallest inter-electrode gap by a high vol

9、tage, overcoming the dielectric breakdown strength of the small gap 4. Erosion of metal from both electrodes takes place there. After each discharge, the capacitor is recharged from the DC source through a resistor, and the spark that follows is transferred to the next narrowest gap. The cumulative

10、effect of a succession of sparks spread over the entire workpiece surface leads to its erosion, or machining to a shape which is approximately complementary to that of the tool 5. The electrical resistance of the dielectric influences the discharge energy and the time of spark initiation 6. If the r

11、esistance is low, an early discharge will occur. If it is large, the capacitor will attain a higher value of charge before the discharge spark occurs. The present work investigates the surface modification of three die steel materials by EDM with tungsten powder suspended in the dielectric medium. T

12、ungsten was selected because it is one of the important alloying elements in conventional die steel materials. L9 orthogonal array of Taguchi experimental design was used to conduct the experiments. Three levels of the three machining parameters, namely; peak current (Ip), pulse on-time (Pon) and pu

13、lse off-time (Poff) constituted the array. Negative polarity of the tool electrode and kerosene dielectric with side flushing was used for all the experiments. Increase in micro-hardness of the machined surface was taken as an indicator of surface alloying. 2. Background Most of the research works u

14、sing powder-mixed dielectric focus on improving the process parameters such as material removal rate (MRR), tool wear rate (TWR) and surface roughness. The study of the impact of such machining on surface modification began only about a decade ago 7. Furutani et al. 8 used titanium powder in kerosen

15、e dielectric and obtained titanium carbide layer of hardness 1600 HV on carbon steel with a negatively polarized copper electrode, 3 A peak current and 2 s pulse duration. A deposition method for solid lubricant layer of molybdenum disulphide by suspending its powder in the dielectric to produce par

16、ts for ultra high vacuum applications (such as space environment) has been proposed by Furutani and Shimizu 9. Using High Speed Framing Camera (HSFC) technique, Klocke et al. 10 found a larger plasma channel with aluminum powder-mixed dielectric in contrast to the standard dielectric. It was conclud

17、ed that in such cases, the discharge energy got distributed on a bigger workpiece surface. By adding urea to distilled water as the dielectric medium for machining titanium, Yan et al. 11 obtained TiN on the work surface which exhibited improved friction and wear characteristics. In order to address

18、 the problem of powder settling, Wu et al. 12 added a surfactant along with aluminum powder in the dielectric and observed a more apparent discharge distribution effect which resulted in a surface roughness Ra value of less than 0.2 m. Dielectric flow rate also had an important influence on the proc

19、ess capability 13. Powders suspended in the dielectric medium may get deposited on the machined surface either in free form or as carbides by combining with carbon from the breakdown of the hydrocarbon dielectric 14. Conductive powders enlarge the gap distance and improve the surface finish by reduc

20、ing spark energy and dispersing the discharges more randomly throughout the surface 15. Thickness of the recast layer is smaller and micro-cracks are reduced. Consequently, corrosion resistance of the machined surface is substantially improved. The type and concentration of the powder mixed in the d

21、ielectric has a direct bearing on the machining performance outputs 16. The available literature establishes that low peak current, shorter pulse on-time and negative polarity of the tool electrode favour the phenomenon of material transfer from powders mixed in the dielectric medium. However, the i

22、mpact of variation in pulse offtime as an independent parameter has not been investigated till date.3. Experimentation Experiments to investigate the migration of tungsten to the workpiece surface were conducted on three die steel materials by suspending tungsten powder in the dielectric medium. Tun

23、gsten forms hard, abrasion-resistant particles in tool steels and promotes hardness and strength at elevated temperatures. It can exist in different forms in die steels. It may dissolve in the ferrite or cementite phases of the ironcarbon system or it may be present as independent carbide in the for

24、m of WC, W2C, etc. or form intermetallic compounds with iron such as Fe3W2 or Fe2W 17. Fig. 1. SEM micrographs of (a) OHNS die steel, (b) D2 die steel and (c) H13 die steel. The workpieces of three die steel materials Oil-Hardening Non-Shrinkable (OHNS) type O2 die steel, High Carbon High Chromium (

25、HC-HCr) type D2 die steel and Hot work type H13 die steel were subjected to the standard hardening and tempering cycle 18. The samples of these steels were prepared using standard metallographic procedure. The microstructures were observed using a Scanning Electron Microscope (model JSM-840A, make J

26、EOL, Japan)having the range of magnification from 10 to 3,00,000. The microstructure of OHNS die steel (Fig. 1(a) consists of spheroidal cementite phase in the matrix of tempered martensite. In the micrograph of D2 high-carbon high-chromium die steel (Fig. 1(b), large undissolved carbides can be see

27、n in a tempered martensite matrix. The microstructure of H13 hot die steel (Fig. 1(c) consists of spheroidal cementite, which are smaller in size and higher in number as compared to that of OHNS die steel, embedded in tempered martensite. Table 1Original chemical composition of the work materialsEle

28、mentComposition (wt.%)OHNS die steelD2 die steelH13 die steelCarbon0.821.570.44Silicon0.180.191.04Manganese0.520.070.28Chromium0.4912.385.39TungstenVanadium0.190.961.13Molybdenum0.130.760.93Nickel0.050.090.19IronBalanceBalanceBalanceTable 2Machining parameters used for the experimentation. Sparking

29、voltage 1355% V Peak current 2, 4, 6 A Pulse on-time 5, 10, 20 s Pulse off-time 38, 57, 85 s Servo control Electro-mechanical Polarity Reverse (electrode negative) Dielectric Commercial grade kerosene Machining time 10 min for each cut Powder in dielectric Tungsten (particle size 3040 um) Powder con

30、centration 15 g/l Electrode Electrolytic copperChemical composition of the work materials is given in Table 1.It was determined using a Baird Optical Emission Spectrometer, model DV-6. The equipment uses argon gas and is capable of detecting elements in iron-base, copper-base and aluminum-base mater

31、ials to an accuracy of 0.0001%. Before machining, microhardness of each work material was measured at six different places and average values were noted. Machining of the workpieces was then carried out on Electrical Discharge Machine with conventional copper tool electrode. Time for each machining

32、cut was fixed at 10 min. The input machining parameters and their levels used for experimentation are given in Table 2. For using tungsten powdermixed dielectric, a small tank made of thin mild steel sheet was placed in the main machining tank to isolate it from the filtering system of the machine.

33、This tank was provided with a stirrer to keep the powder suspended uniformly in the dielectric throughout the machining cycle. A schematic diagram of machining set-up is shown in Fig. 2 XRD analysis was done on X-ray Diffractometer System, model ME210LA2 of Rigaku Corporation, Japan. The range of 2

34、from 5 to 100 was used at a scan speed of 5/min for each test.Fig. 2. Schematic diagram of machining set-up for powder-mixed dielectric.4. Results and discussionsAll the machined surfaces thus obtained were subjected to micro-hardness testing using a load of 9.807 N for a duration time of 20 s. This

35、 data was analyzed to find out the desirable combination of levels of the three input process parameters, their significance and relative contribution. Original micro-hardness and the best achieved micro-hardness after machining for each work material are given in Table 3. More than 100% increase in

36、 micro-hardness is observed for all the three work materials. Samples showing best value of micro-hardness for each combination of work material and method of machining were further subjected to: X-ray diffraction (XRD) analysis to find out the presence of additional elements and their phases; Scann

37、ing electron microscopy to analyze the structural features of the machined surfaces; Composition testing on Optical Emission Spectrophotometer to confirm material transfer and for quantitative analysis of the changes in the constituents of the machined surfaces. Spectrometric analysis of the surface

38、s after machining (Table 4) shows substantial pick-up of tungsten and carbon. From negligible amount of tungsten in the three work materials initially, it was possible to get 2.89% tungsten in OHNS die steel, 2.43% in D2 die steel and as much as 3.25% in H13 die steel. Within the limits of experimen

39、tal error, it can be stated that the amount of tungsten pick-up is almost identical and the original chemical composition of the die steel did not have any effect on this phenomenon. The presence of tungsten in the form of tungsten carbide and carbon pick-up indicates that the reaction is taking pla

40、ce in the plasma channel and it settles down on the machined surface during pulse off-time.4.1. Surface analysisTable 3Micro-hardness of the three work materials before and after machiningWork materialMicro-hardness (Vickers hardness number, HV)Before machiningAfter machining% increaseOHNS die steel

41、6071252106.3%D2 die steel6521410116.2%H13 die steel4651072130.5% All the three work materials reported more than 100% increase in micro-hardness (Table 3). Fig. 3 shows the SEM structure on the surface of OHNS die steel machined by EDM using tungsten powder suspended in kerosene dielectric. The EDM

42、parameters indicated in Fig. 3 are the best set that could be obtained after an optimization procedure. As can be seen from the topography, there are discrete craters along with some volcanic features, and many spherical droplets left on the EDmed surface, which indicates that the material removal m

43、echanism is melting and evaporation. The spherical droplets, seen in groups, range from 5 to 25 m in diameter. The solid behaviour is attributed to the discharge channel temperature being higher than that of molten particles ejected from the crater and nucleation generally starting internally 19. Th

44、e perfectly spherical particles were apparently re-solidified from the gaseous state, whereas irregularly recast layers were solidified. .from the liquid state. Previous studies of EDM surface morphology and debris have demonstrated that material was removed in both the liquid and gaseous state 20.

45、During the EDM process, the extremely high temperature produced by the electric discharge sparks rapidly vaporizes the dielectric fluid and creates pressure impulse around the tool electrode. The impact pressure and high thermal stresses generated by the discharge produces the craters and microcrack

46、s in the machining surface 21. Fig. 3. SEM of OHNS die steel after machining with tungsten powder-mixed dielectric(at Ip = 4 A, Pon = 10 s and Poff = 85 s).Fig. 4. XRD pattern of OHNS die steel after machining with tungsten powder-mixeddielectricTable 4Chemical composition of the work materials afte

47、r machining.ElementComposition (wt.%)OHNS die steelD2 die steelH13 die steelCarbon1.111.910.76Silicon0.190.171.07Manganese0.470.090.31Chromium0.4611.575.42Tungsten2.892.433.25Vanadium0.190.931.19Molybdenum0.120.780.91Nickel0.070.090.19IronBalanceBalanceBalance The upper material in extremely high temperature region will vaporize, while the lower material will melt. Two regions are clearly visible one is the original material and another