IMPROVED METHODOLOGY FOR CALCULATING THE PROCESSES OF SURFACE ANODIC DISSOLUTION OF SPARK ERODED RECAST LAYER AT ELECTROCHEMICAL MACHINING WITH WIRE ELECTRODE

© V.I. Osipenko, A.P. Plakhotny, A.Y. Denisenko, 2014 manufactured and tested. The structural and technological solutions of joints that increase the carrying capacity from 90 to 130 % relative to the original method of connection are the results of experimental studies. Increase of interlayer structural strength is achieved through embedded connector. Technical recommendations on the adhesive sublayer on metal structural elements is offered; the conclusion about the impact on the viability of the joint is drawn as well.

Introduction.The EDM in general and carved finishing by wire electrode in particular have become an alternative to mechanic processing in tool production, manufacturing of complex-shape parts of dies, molds, etc.With further development of EDM technology a pertinently urgent task, as for any other type of metal processing, relates to improving properties of the original surface.The most effective way to provide the required surface finish qualities represents combined use of consecutively applied electrical discharge technology and electrochemical treatment with wire electrode.
Analysis of recent research and publications.According to many researchers in the field [1,2] the EDM process embodies a thermal one, when the material is removed by high energy electrical discharges.A concentrated heat flow directed onto a small workpiece surface area initiates both local melting and evaporation of the molten material.Depending on the time-dependent distribution of spark energy, only 10...20 % of the molten material is evaporated, and about 20...40 % is splashed out as a liquid from the wells under high pressure at breakdown path [3].The remaining molten material solidifies and creates a white modified layer surface.This layer, specific with a high microhardness contains residual stresses, having a network of cracks and microfractures and usually consists of wire electrode transferred material, oxides and carbides.Moreover, that surface roughness is high and depends on the technological conditions of pulsed current EDM process.Thus, there exists a need for improving the surface texture and removing the surface layer, ameliorating therefore the product performances.
The research goal is to develop a refined calculation methods for estimating the quantification process of anodic dissolution when speaking about EDM modified surface layer with uneven thickness electrochemical characteristics, through introducing the factor of alternating electrochemical equivalent of surface material layer.
Problem statement.
Let the anode layer I (Fig. 2) of thickness 1 h has a variable electrochemical equivalent v K , alternating with the depth within values from 1 the electrochemical equivalents for areas I and II are the same.Then with depth increasing the v K coefficient becomes a constant with value 2 K characteristic for the processed piece base material.
Let, while electrochemical dimensional treatment process, the anode surface dissolve, influenced by the electrostatic field of a round-section cathode having the radius r , distance from the anode H and spark gap δ (Fig. 2).
Provided the cathode potential value is U , the field intensity distribution on the anode surface, according to formula [4]  The electric field intensity component x E along the axis OX can be neglected because of its small value compared to y E .The depth of the dissolved anode material at each point with coordinate where ( ) j x -technological current density; t -dissolution delay; κ -electrolyte specific coductivity.
Equation (3) defines the curve 2 (Fig. 3) representing the groove profile at the given instance of dissolution time.Now we can find the coordinate 1 x of the curve 2 intersection with the straight line 1, as boundary between areas I and II, described using the equation 1 ( , ) .

Fig. 3. Shape of grooves on the anode surface at electrochemical dissolution: 1 -border between areas I and II having different electrochemical equivalent; 2 -groove profile at the given moment t of dissolution delay; 3, 4 -calculated and experimentally obtained groove profile at the final instance t 2 of dissolution delay
Supposing (3) equal to (4), we obtain Here we shall determine the dissolution time value at that the coordinate 1 x is found at character- istic positions.When 1 0 x = the moment, for which the surface EDM-modified layer will be dissolved at the 0 x = point is: So we can find the 2 x coordinate (Fig. 3) of the surface layer dissolution boundary point for the final time instance 2 t .Let 2 x a = α .We determine α, at that 1 2 ( ) Thus α represents an important parameter characterizing the width (along the anode surface) of the removed EDM-modified layer.
To find the groove maximum depth h , we divide into h two terms in areas I and II: where I m e a n 1 1

(0) ln
Next, we define the quantity of material removed during 2 t current action time that characterizes the productivity of electrochemical dimensional treatment process.Now we shall find the area of the dissolved material per unit of the anode thickness: Results.To correlate the analytic model with a real process of wire electrode electrochemical treatment, we effected series of experiments.The experimental study goal was to determine the geometrical parameters of grooves originated through processed pieces' surface layers electrochemical dissolution when wire-electrode being at fixed position.
The workpiece surface of 11,5 mm thickness (steel H12F1) has been preliminary modified as a result of three-fold electrical wire processing.Technological modes at first process approach: 100...130 A current amplitude, pulse current of 3,2 ms, frequency 22 kHz.The second approach, respectively: amplitude of 50...80A, duration 3,2 ms, frequency 22 kHz.The third one: amplitude 50 A, duration 2,5 ms, frequency 44 kHz.The final thickness of the EDM-modified layer ranged 30...50 mm.
Upon comparing the calculated and experimentally determined grooves' sizes we arrived to refine the numerical values of the steel surface layers electrochemical dissolution coefficients obtained under different technological conditions of processing.