Electrical Discharge Machining Surface Research Articles

レーザビームを用いた冷間ダイス鋼放電加工面の改質 残留応力解放機構の解明

The method of laser surface modification with melting has been proposed for surface modification of electrical discharge machined surfaces. This method can eliminate defects, such as cracks and micro-craters, by melting and solidifying heat-affected zones using a laser beam. However, the tensile stress, which ranges from 550MPa to 850MPa, is retained in the laser-irradiated surface, and residual stress can be removed by aging treatment. In this study, the residual stress relief mechanism was experimentally examined, and the following results were obtained. The tensile residual stress on the laser-treated material is removed due to the reason that the retained austenite structure is transformed into a martensite one. This transformation is achieved by precipitating the carbides at the grain boundaries. The relationship between the aging temperature and aging time was experimentally determined when the residual stress on the laser-treated material was removed by aging treatment. It was found that the residual stress on the laser-treated material can be removed after a short time of 5 minutes by performing aging treatment at 600°C.

EDM surface effects on the fatigue strength of a 15 vol% SiC p/Al metal matrix composite material

The effects of electrical discharge machining (EDM) on the surface quality and subsequent performance of a 15 vol% SiC particulate reinforced A356 Al under monotonic and fatigue loading conditions are investigated. Surface and subsurface conditions after EDM surface sparking of the material were compared with the same material after careful surface polishing. Tensile tests were performed on the as-received material and the EDM surface sparked material to ascertain effects of EDM processing on basic monotonic properties. High cycle constant stress amplitude fatigue tests were conducted on the polished material and on the sparked material, and fractographic analysis was performed to study the mechanisms of fatigue fracture. EDM sparking was found to dramatically increase surface roughness and cause slight subsurface softening in the microstructure below the outer recast layer. Fatigue strength is notably reduced by EDM processing, with greater degradation resulting from higher material removal rates (MRRs).

放電加工によるチタン合金の着色仕上げ 第２報 発色機構の解析

In the previous report, we proposed a new coloring method for titanium alloy by Electrical Discharge Machining (EDM) in dielectric water and discussed the coloring mechanism for titanium alloy. The colored surface could be caused by an interference phenomenon in the anodic oxide film formed with electrolytic affection during EDM. In this study, further investigation of the coloring mechanism on the EDM surface was carried out by calculating the spectral reflectance of interference light based on the “multiple beam interference” theory. As a result, it was shown that the color of the EDM surface strongly resembled the interference color of spectral reflectance. It was therefore confirmed that the surface coloring by EDM was caused by an interference phenomenon in the anodic oxide film. Moreover, the oxide film thickness depended on the average working voltage.

Development of a lifting wavelet representation for surface characterization

This paper reviews the existing numerical analysis methods and their problems in surface metrology. Based on the requirements of functional analysis of surfaces, this paper proposes a lifting wavelet representation for extraction of different components of a surface. The theory of the lifting wavelet is introduced and a fast algorithm is developed. Different frequency components of the surface can be separated, extracted and then reconstructed according the intended requirements of functional analysis. The surface textures can be highlighted and multi-scalar topographical features can be identified and clearly recovered. In order to verify the behaviour of the new model, a computer simulation based on sinusoidal and triangular waveforms is used. Case studies are conducted using a series of typical surfaces of engineering and bioengineering, such as planes, cylinders and curves, measured by contact (stylus) and non-contact (phase-shifting interferometry) instruments, to demonstrate the feasibility and applicability of using the lifting wavelet model in the analysis of these surfaces.

Effect of Graphite Nodules on the EDM Machinability of Ferritic SG Cast Irons

This investigation examines how graphite nodule counts and average nodule diameter affects the material removal rate (MRR) of Electrical Discharge Machining (EDM). Experimental results indicate that the MRR increased with a decreasing average graphite diameter and increasing graphite nodule count. This phenomenon was attributed to a rigged EDMed surface with a higher ridged density produced. However, the ridged density increased continuously with the number of graphite nodules, thus increasing the material removal rate. Furthermore, there is an essential relationship between MRR and pulse on time for all specimens with a pulse on time for the maximum MRR of 100 μs. The agreement correlation in the quantitative data of ridged density suggests that while the EDM surface feature is similar depends on both pulse on time and graphite nodule counts.

Calculation of the fractal dimensions of machined surface profiles

The optimal method calculating the fractal dimension of a machined surface profile is discussed in this paper. Through the analysis of the results calculated by use of the Weiestrass-Mandelbrot fractal function with known fractal dimensions, the variation method is found to be the best one for calculating the fractal dimensions of the surface profiles among the power spectrum method, the cover method and the rectangular cell counting method. By this method, the fractal dimensions of the surface profiles machined by turning, electrical discharge machining and grinding are evaluated. Furthermore, the relation between the fractal dimensions and the machining conditions are also investigated. In the case of turning, the fractal dimensions of surface profiles decrease with the change of the feed rate whereas the nose radius. The fractal dimensions of the electrical discharge machining surface profiles hardly vary with an increase of the peak discharge currents and the pulse durations. The fractal dimensions of the surface profiles machined by grinding do not change with the grain sizes of grinding wheels.

粉末混入放電加工における表面生成機構の研究

The surface generated by electrical discharge machining (EDM) appears to be generally mat. However, the machined surface in EDM with silicon powder mixed fluid becomes glossier and has a smaller surface roughness than that in conventional EDM with kerosine type fluid, which leads to the omission of finish hand lapping of metal mold. The quality of machined surface significantly depends on the kind of workpiece material, but the surface generation mechanism has not yet been made clear sufficiently.In this study, the effect of silicon powder mixing on the surface generation mechanism is experimentally investigated, analyzing the shape of crater generated by a single pulse discharge, the surface roughness, the machined surface and so on. Main conclusions obtained are as follows:(1) The gap distance in EDM with silicon powder mixed fluid is larger than that with kerosine type fluid, because of the lower resistivity of the former and the influence of silicon powder arrangement in the gap.(2) EDM with silicon powder mixed fluid leads to smaller undulation of a crater, because of the smaller impact force acting on the workpiece due to evaporation and expansion of machining fluid.(3) The more the precipitated carbides in the workpiece are, the larger the surface roughness is, since the carbides come off because of crack propagation due to the frequent heat impact during EDM.(4) EDM with silicon powder mixed fluid results in the stable machining without short circuit between the electrode and the workpiece.

EDM Surface Characterization of a Ceramic Composite TiB2/SiC

Electrical Discharge Machined (EDM) hole surface characteristics of 20 percent Titanium diboride (TiB2) particulate and Silicon carbide (SiC) matrix composite material were investigated. The EDM hole surfaces produced by using brass, copper and graphite were examined by scanning electron microscopy (SEM) and surface profilometry to determine the surface characteristics. As-machined surfaces showed the microcracks in the recast layer, and the individual TiB2 grains were exposed on the surface. The depth of the recast structure on the surface has varied from 8 μm to less than a micrometer and was approximately proportional to the amount of impinging energy or power input. The EDM process appears to be a promising method of producing an excellent surface in electrically conductive TiB2/SiC composite under slow cutting conditions.

Surface Topography Investigations by Fractal Analysis of Spark-Eroded, Electrically Conductive Ceramics

Several ceramics, two metals, and graphite were electric discharge (ED) machined using two different machining conditions. The ED machined surfaces are studied using scanning electron microscopy (SEM) and profilometry. There is a brief discussion of the basic principles of the electric discharge machining (EDH) process and of fractal analysis. Relations between EDM conditions, workpiece material, microscopic analysis, and fractal and conventional analysis of profiles, are found. Conventional relations between EDM parameters and surface roughness are not confirmed with ceramic work-pieces. Fractal analysis has some potential to illuminate the relation between the workpiece, EDM process, and the resulting surface.

Electrical discharge machining of carbon fibre composite materials

An investigation has been made into the feasibility of using Electrical Discharge Machining (EDM) as a means of machining carbon fibre composite materials. Machining was preformed at various currents, pulse durations and with different tool materials and polarities. There exists an optimum material removal rate with peak current and the pulse-on time. However, electrical discharge machining of carbon fibre composite materials should be done at low current density as high current density would cause the epoxy resin to smear over the surface leading to reduced material removal rate and rapid deterioration of the EDM surface. It is found that copper electrodes perform better than graphite electrodes in terms of tool wear, and tools with positive polarity give higher material removal rate and lower tool wear ratio.

放電加工における応力分布に関する研究 ＩＶ 焼戻し処理効果と応力分布形態

In electric discharge machining (EDM), as the residual stress that generates on surface of the workpiece has direct effects upon the quality of products, more detailed investigations are required.In previous papers, the relationships between the residual stress distribution and the electrical conditions (energy supplying duration, peak discharge current), the materials of workpiece (mild steel, cold work die steel) and machining forms (impulse and repetitional discharge machining) etc, were reported by the same authers.The residual stress generated on EDM surface is mainly caused by thermal stress and transformation stress resulting from thermal action of electric discharge. Accordingly, the magnitude and contribution form of stress are closely related to the mechanical characteristics and metallic structure of the workpiece. Therefore, it is very important to make clear the relation between the residual stress and heat treatment processes of the material (workpiece).This paper examined the distribution form of residual stress with a main point to the tempering effects after hardening in the case of the cold work die steel, and discussed the cause for generation of residual stress being observed the cross section of the workpiece (machined by EDM) employing scanning microscope.

Some metallurgical aspects of die failure

A previous examination of spark-eroded surfaces in industrial operation has enabled some relevant factors to be studied in the laboratory. The endurance limit of hot-worked die steels at working temperature has been found to correlate with die life when fracture is the controlling failure mechanism. Also, metallurgical examination shows that the endurance limit is related to the microstructural features. The use of electrical-discharge machining (EDM) has a significant effect on the endurance limit. A mechanism of oxidation-aided wear has been considered for dies which wear out-of-tolerance. The possible wear of the EDM surface by plucking has been examined.

The microstructure of electric discharge machined WC-Co

The electrical discharge machined surfaces of several sintered WCCo alloys were examined at room temperature by x-ray diffraction and optical and electron microscopy. In addition to the hexagonal WC, both W 2C and the face-centered-cubic modification of WC were found. The relative amounts of W 2C and WC(FCC) vary inversely with capacitance. Upon annealing, both phases disappear and are replaced by the η-phase (W 3Co 3C). Certain features of the microstructure are tentatively associated with the extraneous phases, but further interpretation of the x-ray and metallographic results requires a more specific study of the. respective roles of cobalt and carbon in the surface reactions.