1. Introduction
Electrical discharge machining is a type of machining that consists of changing the geometric parameters, surface quality, and physical properties of the surface of a conductive workpiece under the influence of electric discharges between the workpiece and the tool electrode [
1,
2,
3,
4]. Despite the well-known effect of electrical discharges on the surface being machined, the nature of the processes between two electrodes remains unknown.
It should be noted that electrical discharge machining is different from electrochemical machining since the technologies differ in principles. Electrochemical machining is a method for machining electrically conductive materials by the anodic dissolution of the material in an electrolyte under an electric current. The method was proposed in 1911 by the famous Russian chemist E.I. Shpitalsky and modified by V.N. Gusev (W. Gussef), who first proposed conducting machining on the narrow interelectrode gaps (up to tenths of a millimeter) with forced electrolyte pumping [
5,
6]. The electrochemical discharge machining process is a hybrid machining process having advantages of electrochemical machining and electrical discharge machining and is suitable for nonconductive materials, where electric discharge takes place through the electrolyte and plays a critical role [
7]. Unlike these two technologies, electrical discharge machining allows changing the sizes and shapes of electrically conductive materials with high accuracy (±1–2 µm) in a chemically neutral medium of a liquid dielectric (deionized water, hydrocarbons) [
8,
9]. First developed and patented in 1986 [
10], electrical discharge machining techniques can cause electrical destruction of nonconductive materials. However, the developed techniques demonstrate very low efficiency. The maximum achieved depth of the machined holes in a non-oil medium for aluminum-based ceramics (Al
2O
3, AlN) does not exceed 700 µm [
11] (machining was conducted for a few hours with Ag nanoparticle suspension). The results for Si
3N
4 have never been reproduced [
12]. ZrO
2 is easily subjected to electrical discharge machining up to the depth of 5000–8000 µm in an oil medium [
13]. However, no one can explain why this technique never worked so efficiently with aluminum-based ceramics or dielectrics. The spectrum of techniques includes modification of workpiece conductivity using the following conductive agents:
Conductive nanoparticles introduced into nonconductive matrix [
14,
15], which have been known since the beginning of the 1980s;
Conductive nano- and microparticles introduced into the discharge gap [
11,
12,
16,
17,
18,
19], which were first proposed at the beginning of the 1980s (some of the authors even proposed modifying the interelectrode gap conditions by the introduction of nonconductive particles during machining of conductive materials [
20]);
Conductive adherent coating of nonconductive workpiece surfaces (auxiliary electrode) [
13,
14].
In all the proposed techniques, the problem of pulse reinitiation after the completed sublimation of additives from the discharge gap remains.
One way to solve this global problem of efficiency is to research the nature of the erosion process of conductive materials to understand better how the erosion process can be better designed using the chemical properties of chosen electrodes and working medium.
For many years, researchers have declared that the erosion process has thermal nature [
21,
22] or even mixed thermal and mechanical nature [
23] when the physical contact between electrodes is absent, in contrast to the mechanical nature of destruction during milling, turning, and punching operations (physical wear of the tool [
24,
25,
26,
27]).
Indeed, the nature of erosional destruction is of a mixed nature, but not thermomechanical; it is thermochemical. Since electrical discharge machining, electrochemical machining, and electrochemical discharge machining are contactless machining methods, there is always an interelectrode gap between the tool and workpiece electrodes, and the physical contact is absent. This allows machining materials despite their hardness or ductility with high precision [
5,
6,
7,
28].
The absence of mechanical nature is proved by observation of the even formed surfaces [
1,
2,
3,
4,
18,
21,
22] and various works devoted to the research of acoustic oscillations in the full range of spectra [
1,
29]. Results of research considering erosion products have shown the absence of mechanical contact between electrodes [
30,
31] and wire oscillations with an amplitude of about ±|9 +
i| nm (complex number) for steel workpieces and brass wire in water medium excited by working and idle pulses. Thus, the erosion process cannot have any thermomechanical nature since there is no mechanics.
Thermochemical nature is the novelty of this research, proved by X-ray spectroscopy and thermochemical equations of a thermally isolated system (Gibbs energy, entropy, and enthalpy of formation, equilibrium temperature).
The founders of the method, Prof. B.R. Lazarenko and Prof. N.I. Lazarenko [
28], declared the thermochemical nature of the phenomena. Since then, it has been raised only once for the combined method of mechanical milling assisted by electric discharge [
32] and repeated for electrical discharge machining of dielectric ceramics [
33]. A. Calka [
32] reported detected erosion products such as amorphous phases, nanocrystalline and quasicrystalline materials, supersaturated solid solutions, reduced minerals, high-surface-area catalysts, and reactive chemicals. The powder initiated solid–solid, solid–liquid, and solid–gas reactions, and a separate materials synthesis and the reaction of workpiece material in a gas atmosphere under discharge pulses were shown. Y. Guo [
33] reported on the evidence of the chemical reactions occurring during high-speed wire electrical discharge machining insulating zirconia using assisting electrode technique. Up to now, there have been no other works devoted to the analysis of thermochemical parameters between electrodes in the presence of heat and reactions with the formation of intermetallics.
For the first time, the Lazarenko spouses, who developed the electrical discharge machining method in 1942, mentioned the electrical and chemical nature of the method. Since then, it has been mentioned by only A. Calka and Y. Guo in their articles on chemical transformations occurring on machined surfaces. A. Calka plunged into this issue in sufficient detail from the material science point of view (eutectic, crystalline formation) but only touched on the topic of electrical discharge machining, whileY. Guo casually mentioned the thermochemical nature in the context of the machining of insulating ceramics. None of the authors who have investigated electrical erosion have gone so far as to explain the resulting modifications of the workpiece layers from a thermochemical point of view. There is no explanation of why the assisting alumina powder does not allow intensifying the machining of steel, why sparks and a cloud of black sediment are formed during electrical discharge machining of stainless steel, or why insulating zirconium dioxide can be machined in the hydrocarbon medium using the assistant electrode technique when aluminum oxide and nitride are very laborious to process.
This article presents a new, unique approach to understanding the deep nature of electrical erosion after the discharge channel has formed. It makes it possible to evaluate the erosion products; select them based on the properties of the electrical conductivity of the erosion products; and calculate theoretically the probability of the formation of certain erosion products based on the calculated Gibbs energy, entropy and enthalpy of formation, and equilibrium temperature.
Further understanding of the electrical discharge machining process is no longer possible without a thermochemical analysis of the interaction of the components of the electrodes and the dielectric medium. It will improve the efficiency of electrical discharge machining of conductive and nonconductive materials, give an exhaustive explanation of the observed phenomenon in the discharge channel, avoid combinations of the electrode and working medium materials that reduce the efficiency of electrical discharge machining, and predict the chemical content of the erosion products that assist or hamper machining.
This study is devoted to researching thermochemical interactions (phenomena) between tool electrode, workpiece, and dielectric medium components during wire electrical discharge machining; calculation of thermochemical parameters of erosion products’ formation; and substantiation of chemical reactions by thermochemical approach.
Research on the nature of electrical erosion wear was conducted for two types of structural materials, namely nickel-containing and non-nickel-containing materials, machined with a brass tool electrode in deionized water using spectroscopy and analytical research of the chemical interaction of electrodes and working medium components based on thermochemical parameters such as enthalpy, entropy, Gibbs energy, and equilibrium temperatures. Calculations are provided for the formation of conductive or nonconductive erosion products: Zn(OH)2, ZnO, NiO, Ni5Zn21.
The scientific novelty of the work is in the following:
The new data on thermochemical phenomena that occurred between electrodes and deionized water medium in the presence of plasma heat (10,000 °C);
Completed thermochemical analyses of chemical interactions at the surface and near-surface layers after electrical discharge machining.
The practical significance of the work is the development of the theoretical method of thermochemical prediction of erosion product chemical content to evaluate the electrical conditions in the discharge gap suitable for conductive and nonconductive materials.
The tasks of the study are the research of the surface topology of two structural materials, namely austenite anticorrosion X10CrNiTi18-10 (12kH18N10T) steel (nickel-contacting) and 2024 (D16) duralumin (non-nickel-containing), after electrical discharge machining with a brass wire tool electrode in a deionized water medium and the thermochemical analysis of chemical reactions between electrodes and working fluid (
Figure 1).
4. Discussion
The measured difference in the discharge gaps between two structural materials is related to their electrical properties. As can be seen from
Table 5, they have values of specific electrical resistance differing by a factor of ~14 (0.725
for steel and 0.052
for duralumin), which explains the difference in the discharge gap. Since duralumin is more conductive (electrical conductivity is the reciprocal of electrical resistance), the breakdown of the dielectric medium (deionized water) occurs at a greater distance than for stainless steel. According to generally accepted information about the typically recommended spark gaps for materials and electrical resistivity values, the lowest value of the conductive materials is listed for graphite, which also exhibits electrical anisotropy [
56], and the highest values for listed for silver [
57], metallic carbides (delocalized metal bond
d-element carbides such as Fe
xC
y, WC, TiC, V
xC
y, Cr
xC
y, and Ni
xC) [
58,
59], nitrides [
60,
61,
62], and superconducting intermetallics [
63]. The lowest values of the discharge gap correspond to the stable wire electrical discharge machining mode that allows more effective control of part geometry.
The elemental analyses showed that the quantity of zinc in the X10CrNiTi18-10 (12kH18N10T) steel sample is 1.84 times higher than that in the 2024 (D16) duralumin, which can be explained by chemical interaction between Zn of the wire and Ni in steel content. According to the theoretical prediction, it can be concluded that the Ni
5Zn
21 intermetallic compound (dark grey sediment) can be formed at the following ratio [
55]:
Let us calculate the entropy of the intermetallic compound in the crystalline form:
where
is the entropy of the crystalline phase of Ni
5Zn
21 intermetallic compound, J/(mol∙K);
Sc is the entropy of the crystalline phase of the intermetallic compound components, J/(mol∙K);
y is the molar fraction of the components, mol;
R = 8.31446261815324 J/(mol∙K); and the gas constant for a specific gas is
, where
M is the molar mass of Ni
5Zn
21 of 1.667 kg/mol. Then,
The entropy of the intermetallic compound formation is determined by the Boltzmann equation taking into account the Stirling theory and the corollary of Hess’s law [
64]:
when entropy is less than zero, reactions occur with a decrease in the degree of chaos (solidification).
The empirical value of electronegativities can be used to estimate the standard heats (enthalpy) of the formation of ionic and metallic compounds with the accuracy of 13.8–20.4% for intermetallics:
or
where
is the standard enthalpy of formation, kJ/mol;
z is a number of valence bonds; and
εA and
εB are empirical values of electronegativities that are 1.8 for Ni and 1.5 for Zn. The number of metallic bonds in the compound is
then
Gibbs energy will be
when
, the reaction at normal temperature (298 K) is possible in the forward direction from sublimated ions of metals, and the presence of high heat in the discharge channel (any other source of high heat such as laser beam or bombarding by fast atoms [
65,
66]) can only accelerate Ni
5Zn
21 (γ-phase) solidification. The oxide film formed on the transition metals can prevent the direct reaction under normal conditions. The practice shows that the reaction of Ni with Zn at temperatures above 1000 °C (melting points are 1453 °C for Ni, 419.6 °C for Zn, 1682 °C for NiO, and 1975 °C for ZnO; boiling point of Zn is 906.2 °C (
Table 5)) has an explosive character and is accompanied by a series of sparks in the discharge gap [
51,
67] that can be even acoustically monitored and registered [
1]. The thermochemical calculations are presented in
Table 8.
The enthalpy of the formation for reactions (2) and (3) is less than zero; thus, these reactions spontaneously take place with the release of heat, in contrast to the reaction with nickel (4).
Table 9 shows the calculated entropy, Gibbs energy, and equilibrium temperature for mentioned reactions.
In all reactions, the entropy is more than zero, corresponding to an increase in the degree of chaos (evaporation). That can be confirmed by the appearance of gas bubbles in the discharge gap observed during electrical discharge machining with a brass tool electrode and/or with a nickel-containing workpiece in a water medium. The calculated Gibbs energies for reactions (2) and (3) are almost similar when the entropy of formation is higher for ZnO formation. This means that reaction (3) is more probable than (2) in identical conditions. At the same time, Δ
G0298 of reaction (4) is above zero despite having higher entropy of formation than (3). This means that the reaction at normal conditions can only pass in the opposite direction and the formation of nickel oxide occurs with heat absorption only when heated to equilibrium temperatures (672.8 K). Thus, the reaction of Ni and Zn is more likely with the presence of nickel (
Figure 6).