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Quality of surface
Specificity of the ECM process defines the quality features of the processed surface. Formation of the surface microrelief during the ECM unlike processing by cutting is mostly defined by chemical structure of the processed material, chemical composition, concentration, temperature and velocity of electrolyte fluid. Power and thermal factors practically do not participate in the facial layer formation. The facial layer is formed as a result of electrochemical dissolution of a material and chemical influence of environment. The roughness of the processed surface which is the most important geometric characteristic of fatigue strength, depending on conditions and parametres of ECM mode changes in a wide range from Rz 40 micron to Ra 0,01 micron. For the majority of constructional materials while using traditional ECM methods there is no technological difficulty to get roughness Ra 0,4 … 0,8 micron. In case of use of the technological method of microsecond impulse bipolar ECM by vibrating EDM electrode you can achieve roughness of Ra 0,01 … 0,05 micron and in some cases even less. Thus, the modern level of the ECM development exceeds or at least is comparable with the majority of the well-known methods of finishing processing on achievable roughness parametres.
View of optically smooth surface of 20 mm in diameter (on the left), received by an impulse bipolar ECM method and its micro-profile chart (on the right), received by scanning probe microscope (Ra 0,0025 micron, Rz 0,05 micron)
The surface roughness is not a unique and main parametre of its working capacity. In particular at cyclic loadings the greatest interest represent such indicators, as the form of fine irregularities, degree of roughness evenness. The ECM of steels creates a microrelief with a smoother contour of roughness, than grinding. In case of absence of original roughness and macrodefects of flying-off type the values of roughness parametres after the ECM practically do not depend (within accuracy of measurements carried out) on the accepted direction of measurement, that essentially distinguishes the ECM from methods of processing by cutting for which the certain grooves orientation from a tool edge is characteristic.
The surface is not a unique and main parametre of its working capacity. In particular at cyclic loadings the greatest interest represent such indicators, as the form of fine irregularities, degree of roughness evenness. The ECM of steels creates a microrelief with a smoother contour of roughness, than grinding. In case of absence of original roughness and macrodefects of flying-off type the values of roughness parametres after the ECM practically do not depend (within accuracy of measurements carried out) on the accepted direction of measurement, that essentially distinguishes the ECM from methods of processing by cutting for which the certain grooves orientation from a tool edge is characteristic.Electrochemical heterogeneity of the basis of a particle and its borders, caused by distinctions in its chemical composition, structure and tension of the material, is the reason of intergranular corrosion. Reduction of corrosion on borders of grains at low temperature of electrolyte fluid and high current density is explained by braking of diffusion processes in anode layer because of increase in viscosity of electrolyte fluid and, as consequence, reduction of potential difference and dissolution velocity of various parts of the alloy surface. We will mark that there is some critical current density above which corrosion is not observed, for example, for nickel alloys it is within 20 … 60 A/cm² (depending on alloy grade).
During the ECM process, hydrogen evolves at the cathode, both as a result of reduction reaction, and water molecules breakdown. Hydrogen absorption of titanium and some other alloys is called by ability of Ti, V, Nb, Zr, Th, Ta, Pd, La, Ce to adsorb exothermically atomic hydrogen. Hydrogen absorption depends on current density and form, hydrogen indicator ðÍ and solution temperature. With increase in current density and pulse ratio, increase of pH to 10...12 hydrogen, absorption of titanium alloys decreases. Hydrogen absorption of technically pure titan it is not revealed by experiment at high density of a peak current (j> 100A/cm²) and microsecond (50-500 µs) impulses.
Hydrogen dissolution in steel and nickel alloys has an endothermic nature, because of this hydrogen evolves from steel at normal temperature and pressure. Thus, hydrogen absorption of steels and nickel alloys during the ECM process is insignificant.
The surface cold working, practically inevitable at machining operation of parts is a result of plastic deformations and thermal emission in the working area. One of features of the ECM process is the minimum heat-force impact on the processed surface that creates preconditions to absence of the surface cold working that proves to be true by results of X-ray analysis of crystal structure and micro-hardness measurement.
Estimating influence of the ECM on fatigue strength at harmonic loads, it is necessary to mark decrease of dissipation of tests results peculiar to the method. As at calculations they are usually guided by the minimum values of durability, this feature should be considered as one of advantages of the ECM method. Evenness of the surface roughness and absence of residual pressure in the facial layer can serve as an explanation to this. For the surfaces processed by mechanical methods such important factors for fatigue strength as separate defects of a micro-profile in the form of scratches, furrows and superficial residual pressure, are subject to considerable fluctuations of casual character.
It is known that special modes of impulse bipolar ECM allow to create facial layers on processed surfaces of parts from the chemical elements included as alloying into the structure of the processed material. Thus, it is possible to raise, for example, the chrome concentration in the facial layer at the ECM of chromiferous steels and alloys.
The increase in quantity of chrome at the processed surface after bipolar microsecond electrochemical processing of chromiferous steels is confirmed with results of definition of a chemical composition of the facial layer made using various methods.
Results of measurements show that the facial layer enriched by chrome has more smoothed microrelief and a smaller roughness.
In case of use of chromiferous surfaces in adjoint pairs of friction, shape-generating equipment (punches, matrixes), etc. the friction coefficient decreases and fatigue strength, wear resistance and corrosive resistance increases. For example, durability of the punches made by machines of the "ET" series from the chisel tool steel, used at manufacturing of fixing arrangements, (according to the data given by "Autonormal" plant, Belebej city) raises in 10 … 30 times in comparison with the similar punches manufactured by traditional technology.
