materials
Article Electrochemical and X-ray Examinations of Erosion Products during Dressing of Superhard Grinding Wheels Using Alternating Current and Ecological Electrolytes of Low Concentration of Chemical Compounds
Marcin Goł ˛abczak 1,* , Andrzej Goł ˛abczak 2 and Barbara Tomczyk 3
1 Department of Production Engineering, Institute of Machine Tools and Production Engineering, Lodz University of Technology, Stefanowskiego Str. 1/15, 90-924 Łód´z,Poland 2 Department of Management Engineering, Institute of Social and Technical Sciences, State Vocational University in Włocławek, 3 Maja Str. 17, 87-800 Włocławek, Poland; [email protected] 3 Department of Mechanics and Building Structures, Institute of Civil Engineering, Warsaw University of Life Sciences, Nowoursynowska Str. 166, 02-787 Warszawa, Poland; [email protected] * Correspondence: [email protected]; Tel.: +48-600-814-706
Abstract: This article introduces significant cognitive and usable values in the field of abrasive tech- nology especially in the development of new methods of the electrochemical dressing of superhard grinding wheels with metal bonds. Cognitive values mainly concern the elaboration of the theoretical backgrounds of the electrochemical digestion of compounds of grinding wheel metal bond and Citation: Goł ˛abczak,M.; Goł ˛abczak, gumming up products of the cutting surface of grinding wheel (CSGW). Cognitive values also deal A.; Tomczyk, B. Electrochemical and with determining the mathematical relationships describing the influence of technological conditions X-ray Examinations of Erosion of dressing on shaping of cutting abilities of superhard grinding wheels. On the other hand, the Products during Dressing of useful values refer to the industry implementation of the elaborated method and equipment for Superhard Grinding Wheels Using the electrochemical dressing of suparhard grinding wheels using alternating current (ECDGW-AC). Alternating Current and Ecological Electrolytes of Low Concentration of The cost of the device for the realization of this process is low and can be applied in the production Chemical Compounds. Materials 2021, conditions. The novel achievements presented in the article are: the elaboration of a new method and 14, 1375. https://doi.org/10.3390/ equipment for electrochemical dressing of superhard grinding wheels (ECDGW-AC), the selection of ma14061375 electrolytes of low concentration of chemical compounds, tests concerning the digestion of grinding wheel metal bond compounds and gumming up products of CSGW using X-ray analysis, as well Academic Editors: Tomasz Ta´nski as the determination of chemical reactions taking place during elaborated new dressing process, and Przemysław Snopi´nski the elaboration of mathematical relationships describing influence of technological conditions of this process on dressing speed and shaping of cutting abilities of superhard grinding wheels, and Received: 9 February 2021 the performance of technological tests of dressing of superhard grinding wheels using ECDGW-AC Accepted: 10 March 2021 method. The elaborated method can be used in ambient temperature and does not cause thermal Published: 12 March 2021 damages of abrasive grains of cutting surface of grinding wheel and is useful not only for dressing super hard grinding wheels but also for correcting their geometrical deviations. Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in Keywords: superhard grinding wheels; electrochemical dressing; operational properties; ecological published maps and institutional affil- iations. electrolytes; electrochemical reactions; erosion products
1. Introduction Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. Metal bonded grinding wheels made of superhard abrasive materials (e.g., natural This article is an open access article crystal diamond-CD and synthetic diamond-SD or cubic boron nitride-CBN) fulfill sig- distributed under the terms and nificantly modern requirements concerning grinding processes challenges, especially the conditions of the Creative Commons precise machining of hard metal alloys or composites [1–3]. Attribution (CC BY) license (https:// The rational and economical exploitation of these kind of grinding wheels is condi- creativecommons.org/licenses/by/ tioned by overcoming the difficulties in shaping their macro- and microgeometry in the 4.0/).
Materials 2021, 14, 1375. https://doi.org/10.3390/ma14061375 https://www.mdpi.com/journal/materials Materials 2021, 14, 1375 2 of 22
sharpening process, both for the manufacturers and users of grinding wheels. These diffi- culties mainly result from specific physico-mechanical properties of the grinding wheels including the especially high hardness of abrasive material and metal bond strength [4,5]. For this reason, the use of conventional methods for dressing of superhard grinding wheels, e.g., dressing with diamond single-grain dressers, diamond rotating dressers, kneading with a hard roller, etc., is ineffective and uneconomical [1,6,7]. The wear of grinding wheels in the process of grinding with superhard grinding wheels occurs as a result of the phenomenon of gumming up of the cutting surface of the grinding wheel (CSGW) by grinding products and abrasive wear of abrasive grains [2,8–10]. In order to counteract the occurrence of the grinding wheel gumming up, the grinding wheel is impregnated with molybdenum compounds (MoS2)[11,12]. However, the wear of the abrasive grains of the grinding wheel requires their removal from the CSGW in the process of dressing with erosive methods. The analyses of the source publications and the authors’ investigation results indicate the significant role of erosive methods of superhard grinding wheels dressing in solving the difficulties in forming the cutting abilities of su- perhard grinding wheels, e.g.,: electrochemical dressing [13–15], electrolytic in-process dressing—ELID [16,17], electrochemical dressing with foil electrodes [18], electrodischarge dressing [19–21], electrodischarge and electrochemical dressing [13,22–24], and laser dress- ing [25]. Among the erosive methods for dressing superhard grinding wheels there are very attractive and effective electrochemical methods based on the application of alternating current (ECDGW-AC) [22–24,26–28]. However, the disadvantage of the electrochemical methods is the necessity of using high concentration electrolyte solutions of chemical compounds (15 ÷ 40%). The presence of electrolytes solutions creates a significant nuisance value for the environment and surroundings, including the corrosive effect of electrolytes on grinder assembly, harmfulness for grinder operators, ecological limitations while utilizing used electrolytes solutions, and challenges arising from grinding hazardous waste [22,24,28,29]. The level of nuisance of electrolytes solutions depends on the chemical composition and concentration of chemical compounds. In order to limit these nuisances, we have analyzed the theoretical literature and carried out the following range of experimental tests: the selection of non-toxic chemical compounds in electrolytes solutions, the reduction of concentration of these compounds in electrolyte, and the application of protective additives reducing corrosivity of grinder assembly.
2. Materials and Methods 2.1. Main Aim of the Investigations The aims of the investigations carried out were the elaboration of a new method and equipment for the electrochemical dressing of superhard grinding wheels (ECDGW- AC), the selection of chemical compound of low concentrated electrolytes solutions, the determination of electrochemical reaction in process of anodic digestion of components of metal bond of grinding wheel and grinding materials, the X-ray identification (TUR-M62, Carl Zeiss AG, Jena, Germany) of products arising in electrochemical erosion, and an assessment of the effectiveness of grinding wheels dressings. In the following chapter, detailed information concerning experimental conditions and examples of the investigation results are presented.
2.2. Characteristics of the Abrasive Tools and the Machine Tool For the investigations, the precision universal tool grinding machine Tacchella 4AM (Pabianice, Poland) and the diamond superhard grinding wheels with a metal bond of technical designation S3020 175 × 6 × 3 × 50 D125/100 M100 (“VIS” S.A., Warszawa, Poland) were used. Their main characteristics are depicted in Table1. Materials 2021, 14, x FOR PEER REVIEW 3 of 23
2.2. Characteristics of the Abrasive Tools and the Machine Tool For the investigations, the precision universal tool grinding machine Tacchella 4AM (Pabianice, Poland) and the diamond superhard grinding wheels with a metal bond of Materials 14 2021, , 1375 technical designation S3020 175 × 6 × 3 × 50 D125/100 M100 (“VIS” S.A., Warszawa3, ofPo- 22 land) were used. Their main characteristics are depicted in Table 1.
Table 1.1. Characteristics ofof thethe grindinggrinding wheelswheels usedused inin thethe investigations.investigations. Technical Designation S3020 175 × 6 × 3 × 50 D125/100 M100 Technical Designation S3020 175 × 6 × 3 × 50 D125/100 M100 ManufacturerManufacturer “VIS” “VIS”S.A. (Warszawa, S.A. (Warszawa, Poland) Poland) Type ofType Grinding of Grinding WheelWheel Flat diamond Flat diamond cup grin cupding grinding wheel wheel External diameterExternal D = 125 diameter mm, inner D = diameter 125 mm, innerd = 50 diameter mm, abra- Dimensions Dimensions sive layer width dw == 506 mm, mm, thickness abrasive layerof abrasive width layer w = 6 x mm, = 3 mm Abrasive Grain Type Monocrystallinethickness of synthetic abrasive diamond layer x = 3 mm Abrasive GrainAbrasive Fracture Grain Number Type Monocrystalline125 synthetic diamond AbrasiveBond Grain Fracture NumberMetal 125 Grains ConcentrationBond 100 Metal Grains Concentration 100 Grinding Machine Tacchella 4AM fum Ponar-Pabianice (Poland) Grinding Machine Tacchella 4AM fum Ponar-Pabianice (Poland)
2.3. Electrochemical Method of Grinding Wheels Dressing Using Alternating Current 2.3. Electrochemical Method of Grinding Wheels Dressing Using Alternating Current The electrochemical method for superhard grinding wheels dressing based on appli- The electrochemical method for superhard grinding wheels dressing based on appli- cations of an alternating current has been proposed in [3,4,6]. The presence of specially cations of an alternating current has been proposed in [3,4,6]. The presence of specially selected electrolytes with low concentration of chemical compounds (about 5%) is re- selected electrolytes with low concentration of chemical compounds (about 5%) is required quired in this method. in this method. The dressing process of the face superhard grinding wheel consists of the anodic di- The dressing process of the face superhard grinding wheel consists of the anodic gestion of the metal bond—5 and gumming up products of cutting surface of superhard digestion of the metal bond—5 and gumming up products of cutting surface of superhard grinding wheel (CSGW)—6, with the use of alternating current applied from the current grinding wheel (CSGW)—6, with the use of alternating current applied from the current power supply—1 to two carbon electrodes—2. The digestion process of the face superhard power supply—1 to two carbon electrodes—2. The digestion process of the face superhard grinding wheel (metal bond and gumming up products CSGW) proceeds in the inter- grinding wheel (metal bond and gumming up products CSGW) proceeds in the inter- electrode gap—S, to which the low concentrated electrolyte solution—ES is supplied un- electrode gap—S, to which the low concentrated electrolyte solution—ES is supplied under pressureder pressure of specially of specially selected selected chemical chemical compound compound (Figure (Figure1). 1).
VGW
3 5
4 6
S le ES
1
2
Figure 1. TheThe scheme scheme of of the the electrochemical electrochemical dressing dressing of of grinding grinding wheel wheel using using alternating alternating current current (ECDGW-AC):(ECDGW-AC): 1—alternating1—alternating currentcurrent power power supply, supply, 2—carbon 2—carbon electrodes, electrodes, 3—grinding 3—grinding wheel wheel frame, frame,4—abrasive 4—abrasive grains, 5—metalgrains, 5— bond,metal 6—gumming bond, 6—gumming up products up products of cutting of surface cutting of surface superhard of super- grinding wheelhard grinding (CSGW), wheel 7—spindle; (CSGW),V GW7——speedspindle; ofVGW grinding—speed wheel,of grindingS—inter-electrode wheel, S—inter gap,-electrodele—distance gap, lbetweene—distance electrodes, betweenES electrodes,—electrolyte ES— solutionelectrolyte of specially solution selectedof specially chemical selected compounds. chemical com- pounds. The process of anodic digestion of the grinding wheel compounds has a cyclical nature because of the alternating current change conditions of the grinding wheel polarization, which alternately is the anode or cathode, adequate to the variation frequency of the alternating current (Figure2). Materials 2021, 14, x FOR PEER REVIEW 4 of 23
The process of anodic digestion of the grinding wheel compounds has a cyclical na- Materials 2021, 14, 1375ture because of the alternating current change conditions of the grinding wheel polariza- 4 of 22 tion, which alternately is the anode or cathode, adequate to the variation frequency of the alternating current (Figure 2).
Figure 2. (a) EquivalentFigure 2. (a electric) Equivalent circuit electric diagram circuit of the diagram dressing of process the dressing using process ECDGW-AC using method; ECDGW (-bAC,c) Course of current method; (b,c) Course of current variation on electrodes; Re—resistance of electrolyte, RGW—re- variation on electrodes; Re—resistance of electrolyte, RGW—resistance of grinding wheel. sistance of grinding wheel. The digestion of the grinding wheel compounds takes place during positive polar- The digestionization of of the the grinding grinding wheel wheel compounds (when it is takes the anode). place during During positive the negative polari- periods of the zation of the grindinggrinding wheelwheel (when (when it it is is the the cathode), anode). theDuring so-called the negative cathode polarizationperiods of the of the grinding grinding wheelwheel (when takes it is place. the cathode), This polarization the so-called favors cathode tearing polarization off the passive of the layergrinding from the CSGW wheel takes place.(metal This oxides polarization and other chemicalfavors tearing compounds) off the passive arising duringlayer from anodic the process. CSGW The removal (metal oxidesof and the other passive chemical layer fromcompounds) the CSGW arising is a favorable during anodic process process. because The it increasesremoval polarization of the passiveresistance layer from in the the CSGW interelectrode is a favorable gap proce whichss slows because down it increases the digestion polarization speed of grinding resistance in wheelthe interelectrode compounds gap and which decreases slows the down dressing the digestion efficiency. speed The mainof grinding advantages of the wheel compoundsECDGW-AC and decreases method the are dressing the commutator-free efficiency. The power main supply advantages to the of grinding the EC- wheel, lower DGW-AC methodcost of are power the commutator supply unit,- cathodefree power activation supply of to the the grinding grinding wheel wheel, subjected lower to dressing, cost of powerapplication supply unit, of cathode diluted activation solutions ofof electrolytes,the grinding andwheel displaying subjected insignificant to dressing, noxiousness Materials 2021, 14, x FORapplication PEER REVIEW offor diluted the surroundings solutions of andelectrolytes, the natural and environment. displaying insignificant noxiousness 5 of 24
for the surroundingsThe and work the stand natural constructed environment. for the investigations consists of an appropriately adapted The worktool stand grinder constructed equipped withfor the the investigations electrochemical consists attachment of an and appropriately systems enabling control adapted tool andgrinder measurements equipped with (Figure the 3electrochemical). attachment and systems enabling control and measurements (Figure 3).
6 7 8 9 12 Tool Stepping 5 electrode motor feed unit
1 11 13
4
14 10
2 5 Workpiece
Figure 3. The scheme of a stand for grinding wheels dressing using the ECDGW-AC method: 1—grinding wheel, 2— workpiece, 3—Figureelectrolyte 3. TheThe supplying scheme scheme nozzle,of of a stand 4— inductivefor grinding sensor, wheels 5— tooldressing -electrode, using 6 —the electrode ECDGW ECDGW-AC holder,-AC method: 7—tool -1 1—grindingelectrode—grinding wheel, wheel, 2 2—— feed unit, 8—guideworkpiece,-screw, 3 3—electrolyte —9—electrolytestepping supplying motor, 10— nozzle, AC power 4 4—inductive—inductive unit, 11— sensor, sensor,LC generator, 5 5—tool-electrode,—tool-electrode, 12—computer, 6 6—electrode—electrode 13—spindle, holder, holder, 14 7 7—tool-electrode——tool-electrode feed unit, 8 8—guide-screw,—guide-screw, 9 9—stepping—stepping motor, motor, 10 10—AC—AC power unit, unit, 11 11—LC—LC generator, generator, 12 12—computer,—computer, 13 13—spindle,—spindle, 14 14—— headstock, le—distance between electrodes, ES—electrolyte solution of specially selected chemical compounds, S—inter- headstock, le—distance between electrodes, ES—electrolyte solution of specially selected chemical compounds, S—inter- electrode gap, ΔH—the thickness of the grinding wheel layer removed in the ECDGW-AC process. electrode gap, ∆H—the thickness of the grinding wheel layer removed in the ECDGW-AC process. The stand provides sharpening of grinding wheels using either stationary electrode (and variable spacing of electrodes) or an electrode with a step feed (providing stabiliza- tion of the spacing of electrodes). The linear grinding wheel decrement (ΔH) was deter- mined using the contactless inductive sensor (4) equipped with LC generator (11). The stabilization of the inter-electrode gap (S) was provided by the microcomputer regulator (12) by means of the stepping motor (9) and the guide-screw (8) feed—in the set of elec- trodes (7) to the grinding wheel (1). Basic parameters and results of the process were reg- istered with the microcomputer which governed the process according to the special com- puter software.
2.4. Theoretical Analysis of the ECDGW-AC Process For theoretical considerations, a physical model of the ECDGW-AC process, cf. Fig- ures 1 and 2, has been developed. The following simplifying assumptions have been made for the ECDGW-AC process model [3,4,6]: The grinding wheel digested components (metal bond and products glazing the CSGW) are treated as a homogeneous material. The physical properties of the electrolyte solution in the inter-electrode gap are con- stant. The distribution of the electrical potential in the inter-electrode gap has a linear char- acter. There are no hydrodynamic interfering phenomena of continuity of electrolyte flow in the inter-electrode gap. The total polarity of the electrodes does not depend on the current density. Taking into account the assumptions given above, it may be stated that Ohm’s law is satisfied in the ECDGW-AC process, while the current density (i) on the anode is defined by the following equation [22]:
Materials 2021, 14, 1375 5 of 22
The stand provides sharpening of grinding wheels using either stationary electrode (and variable spacing of electrodes) or an electrode with a step feed (providing stabilization of the spacing of electrodes). The linear grinding wheel decrement (∆H) was determined using the contactless inductive sensor (4) equipped with LC generator (11). The stabilization of the inter-electrode gap (S) was provided by the microcomputer regulator (12) by means of the stepping motor (9) and the guide-screw (8) feed—in the set of electrodes (7) to the grinding wheel (1). Basic parameters and results of the process were registered with the microcomputer which governed the process according to the special computer software.
2.4. Theoretical Analysis of the ECDGW-AC Process For theoretical considerations, a physical model of the ECDGW-AC process, cf. Figures1 and2, has been developed. The following simplifying assumptions have been made for the ECDGW-AC process model [3,4,6]: • The grinding wheel digested components (metal bond and products glazing the CSGW) are treated as a homogeneous material. • The physical properties of the electrolyte solution in the inter-electrode gap are constant. • The distribution of the electrical potential in the inter-electrode gap has a linear character. • There are no hydrodynamic interfering phenomena of continuity of electrolyte flow in the inter-electrode gap. • The total polarity of the electrodes does not depend on the current density. Taking into account the assumptions given above, it may be stated that Ohm’s law is satisfied in the ECDGW-AC process, while the current density (i) on the anode is defined by the following equation [22]: U − E i = χ e (1) S
where χ—the specific conductance of the electrolyte, Ue—the inter-electrode voltage, E— the total overpotential, S—the thickness of the inter-electrode gap. For the ECDGW-AC process, the model using a stationary electrode (with a vari- able thickness of the inter-electrode gap) and the rate of digestion of the grinding wheel components is described by the following differential equation:
dS U sinωt − E = χk e (2) dt v S
where kv—the coefficient of electrochemical workability of the grinding wheel components, Ue—the effective value voltage of the inter-electrode voltage. To determine the thickness of the inter-electrode gap in the period tϑ < 0, T/2 corre- sponding to “the positive semi-wave” of the working voltage (Figure4), Equation (2) must be integrated in the interval ti < t < 1/2T − ti, with an initial condition S(t = 0) = S0. The integral equation takes the following form:
S 1/2T t Z Z Z i SdS = kvχ(Umsinωt − E)dt − 2 kvχ(Umsinωt − E)dt (3)
S0 0 0
Determining constants D and CE: 1 2 1 E E E D = Umkvχ, CE = sin arcsin − arcsin (4) π 2 Um 2πUm Um
The solution to Equation (3) leads to the following equation: 2 2 1 E S1 − S0 = 2DT − − 2CE (5) π 4Um Materials 2021, 14, 1375 6 of 22 Materials 2021, 14, x FOR PEER REVIEW 6 of 23
FigureFigure 4.4. AA curvecurve ofof thethe inter-electrodeinter-electrode voltage voltageU Ummand and inter inter inter-electrode inter-electrode gap gap during during ECDGW-AC ECDGW- AC process, SGW—surface of grinding wheel, ti—the time of start of digestion process. process, SGW—surface of grinding wheel, ti—the time of start of digestion process.
TheThe Equationintegral e (5)quation describes takes a changethe following in the inter-electrodeform: gap thickness. It can be seen
that the elementary푆 layers1/ of2푇 the grinding wheel will be digested푡푖 for successive semi-waves of the working voltage. Thus, according to relationship (5), a change in the inter-electrode ∫ 푆푑푆 = ∫ 푘 휒(푈 푠푖푛 휔푡 − 퐸)푑푡 − 2 ∫ 푘 휒(푈 푠푖푛 휔푡 − 퐸)푑푡 (3) gap thickness is described by a푣 series푚 of Equation (6): 푣 푚 푆0 0 0 1 E Determining constants 2D and2 CE: Sn − S0 = 2DTn − − CE (6) π 4Um 1 2 1 퐸 퐸 퐸 퐷 = 푈푚푘푣휒, 퐶퐸 = 푠푖푛 ( 푎푟푐푠푖푛 ) − 푎푟푐푠푖푛 (4) where Tn—the time of dressing needed휋 for the2 removal푈푚 of the2휋 grinding푈푚 wheel푈푚 layer on the part of its circumference corresponding to the width of the tool electrode b. By introducing The solution to Equation (3) leads to the following equation: an effective time of dressing toe, determined by Equation (7) and following transformations, Equation (8) defining the inter-electrode2 2 gap thickness1 퐸 was obtained: 푆1 − 푆0 = 2퐷푇 ( − − 2퐶퐸) (5) 휋 4푈푚 b t = t (7) The Equation (5) describes a changeoe ino theπD inter-electrode gap thickness. It can be seen that the elementary layers of the grinding wheel will be digested for successive semi- wherewavest oof—the the working actual time voltage. of dressing, Thus, Daccording—the grinding to relationship wheel diameter. (5), a change in the inter- electrode gap thickness is describeds by a series of Equation (6): 1 E 2 1 퐸 Sn = 2 S0 −2 2D − − 2Ce toe (8) 푆푛 − 푆0 = 2퐷π푇푛 ( 4U−m − 퐶퐸) (6) 휋 4푈푚
whereSince 푇푛— inthe the time ECDGW-AC of dressing process needed the for inequality the removalE ofU them is grinding satisfied, wheel Equation layer (8) on can the bepart reduced of its circumference to the following corresponding simpler form: to the width of the tool electrode b. By introducing an effective time of dressing 푡표푒 , determinedq by Equation (7) and following transfor- 2 mations, Equation (8) defining the Sintern =-electrodeS0 − 2Dt gapoe thickness was obtained: (9) 푏 The thickness of the abrasive layer푡 removed= 푡 ∆ H in the ECDGW-AC process is de-(7) 표푒 표 휋퐷 fined by: q where 푡표—the actual time of dressing, D—the grinding2 wheel diameter. ∆H = Sn − S0 = S0 − 2Dtoe − S0 (10)
The effective time of dressing determining2 1 the퐸 technological removal of the digested 푆푛 = √푆0 − 2퐷 ( − − 2퐶푒) 푡표푒 (8) layer of the grinding wheel ∆H is defined by:휋 4푈푚
Since in the ECDGW-AC process the∆H inequality(∆H + 2S 퐸) ≪ 푈 is satisfied, Equation (8) can t = 0 푚 (11) be reduced to the following simpleroe form: 2D
Materials 2021, 14, x FOR PEER REVIEW 7 of 23
2 푆푛 = √푆0 − 2퐷푡표푒 (9)
The thickness of the abrasive layer removed ΔH in the ECDGW-AC process is de- fined by:
2 훥퐻 = 푆푛 − 푆0 = √푆0 − 2퐷푡표푒 − 푆0 (10)
The effective time of dressing determining the technological removal of the digested layer of the grinding wheel ΔH is defined by: Materials 2021, 14, 1375 7 of 22 훥퐻(훥퐻 + 2푆0) 푡 = (11) 표푒 2퐷 For the model of the ECDGW-AC process making use of the continuous feed of the For the model of the ECDGW-AC process making use of the continuous feed of the electrode, changes in the inter-electrode gap are compensated by an automatic system of electrode, changes in the inter-electrode gap are compensated by an automatic system of the electrode feed (Figure 5). the electrode feed (Figure5).
fO f PC GENERATOR LC AMPLIFIER O PROGRAMMER
PC 8031
L OPTOISOLATOR HOST COMPUTER STEP
POWER DEGREE
STEPPER
MOTOR
Figure 5. AA schematic schematic diagram of system for control of tool tool-electrode-electrode feed. Therefore, the inter-electrode gap thickness in the ECDGW-AC process is constant, Therefore, the inter-electrode gap thickness in the ECDGW-AC process is constant, Vdig = constant. The thickness of the grinding wheel layer removed in the ECDGW-AC Vdig = constant. The thickness of the grinding wheel layer removed in the ECDGW-AC process can be determined from the following equation: process can be determined from the following equation:
U푈e푒 ∆H∆퐻==k푘χ푣휒 t푡표푒 ((12)12) v 푆0 oe S0 where Ue—the effective value voltage of the inter-electrode gap. where Ue—the effective value voltage of the inter-electrode gap.
2.5. Selection Selection of of Electrolytes Electrolytes Solutions The sselectionelection of chemical compounds and concentration of electrolytes solutions used in the the electrochemical electrochemical dressi dressingng of grinding wheel are conditioned by many mutually exclusive criteria. These These are are,, among among others: others: good good electrochemical electrochemical properties properties of of electrolyte, electrolyte, lowlow nuisance nuisance for for surroundings surroundings and environment (e.g. (e.g.,:,: in aspect of reprocessing the used electrolytes and and dressing dressing products, products, limitations limitations of corrosivity of corrosivity of the of grinding the grinding machine machine units, units,a fire hazard,a fire hazard, safety, safety, and the and costs the of costs electrolytes of electrolytes and their and shelf their life) shelf [22 life),26, 28[22,26,28]]. . FulfillingFulfilling all the above mentioned cri criteriateria is a very difficultdifficult and complex research task.task. Therefore, Therefore, we we have have focused focused on on selecting electrolytes fulfilling fulfilling the most important requirements,requirements, namely: the the reduction reduction of of nuisance nuisance for for surroundings surroundings and and the the environment, limitation of corrosivity of the grinding machine units, and assurance of the effectiveness of the grinding wheels dressing. Efforts to limit the electrolytes nuisance for surroundings and the environment consist of elimination from their chemical composition toxic and hazardous compounds (e.g.,: nitrides and chromates). Limitations of the corrosivity of the grinding machine units have been achieved by reduction of chemical compounds concentration in solution (to about 5%) and application of the protective additions. Investigations carried out allow for the elaboration of five multicomponent solutions of electrolytes [3,4,14] whose physicochemical properties are presented in Table2. Materials 2021, 14, 1375 8 of 22
Table 2. Physicochemical properties of electrolytes solutions.
Chemical Composition and Symbol of Electrolyte Solution (ES) No. Electrolyte Properties E1 E2 E3 E4 E5
NaCl, KNO2, NaCl, KNO2, NaCl, KNO2, NaCl, KNO3, NaCl, KNO3, 1. chemical composition KNO2, KNO3, KNO2, KNO3, KNO2, KNO3, Na2O3, Na2O3 CaCO3 Na2CO3 Na2CO3 2. concentration of solution [%] 5 5 5 5 5 3. protective addition [%] - - 0.5 1 - 4. density of electrolyte [g/cm3] 1.04 1.039 1.039 1.042 1.043 5. pH of electrolyte 9.7 10.81 10.80 10.81 10.90
3. Results and Discussion 3.1. Studies on Electrolytes Applied for the ECDGW-AC Process The investigations concern the determination of electrochemical equilibrium condi- tions and rates of metal digestion (being components of both metal bond and products loading on the CSGW) in selected solutions of electrolytes, the estimation of minimal den- sity of these metals dissolution current, and an assessment of the corrosive effect of these electrolytes on constructional units of the grinder. The research was based on the digestion of samples of metals making up the metal bond and products loading on the CSGW in 5 dif- ferent electrolyte solutions E1–E5 (containing up to 5% of the following compounds: NaCl, KNO2, KNO3, Na2CO3, CaCO3, and a protective agent) and chrono-volt-amperometric measurements. The dissolution of Cu, Sn and Ag, i.e., the components of metal bond of a (S3020 175 × 6 × 3 × 50 D125/100 M100) grinding wheel, as well as Co, Ti and W, which are components of products loading on the CSGW, formed during sintered carbides (type S20S) grinding, was within the scope of the studies. The usefulness of the selected electrolytes was estimated on the basis of the rates of individual metal digestion. The value of the current corresponding to the equilibrium state between the reactions of anodic metal digestion and cathodic reduction of the electrolyte solution components was the factor characterizing the metal dissolution rate in the electrolytes. The values of equilibrium potentials and digestion current were determined based on cyclic volt-amperograms and Tafel’s relationship plots. The comparative assessment of the effect of individual electrolytes on grinding wheel components dissolution rate is depicted in Figure6. Their analysis indicates that the chem- ical composition of electrolyte solutions and the applied protective agent (present in the electrolytes E3 and E4) substantially influence the rate of the grinding wheel components digestion. It is particularly evident in the case of Cu and Sn, which are components of the grinding wheel metal bond, and Co, which binds sintered carbides loading on the CSGW. The ratio of the highest to the lowest rate of these metals digestion in selected electrolytes amounted to almost 3, and the rate was the highest for the E5 electrolyte, followed by the rates of dissolution in E1, E2, E3, and E4. The studies proved that appropriate electrolytes provide digestion of all the metals present either in grinding wheel metal bond (Ag, Cu, Sn) or in products loading on the CSGW (Co, W), that enables selection of the best electrolyte solution, optimal for the ECDGW-AC process actual conditions. The only exception is Ti, which undergoes passivation and forms a layer of oxides on the CSGW. The dissolution of all components of metal bond, grinding wheel and Co, which binds the sintered carbides (WC and TiC) results in the simultaneous removal of Ti oxides from the CSGW within the ECDGW-AC process. This fact was confirmed by X-ray studies on the EDGW–AC process products resulting from electrochemical reactions. The executed chrono-volt-amperometric studies and the analysis of their results revealed that the minimum current density enabling digestion of all the metals being components of the grinding wheel was equal to about 0.3 Acm−2 (Figure7). MaterialsMaterials2021 2021, 14, 14, 1375, x FOR PEER REVIEW 910 of of 22 24
FigureFigure 6. 6.The The effecteffect ofof electrolyteelectrolyte compositioncomposition (Table(Table2 )2 on) on the the digestion digestion rate rate of of the the grinding grinding wheel wheel components:components: ( a(a)) Grinding Grinding wheel wheel metals metals bond; bond; (b ()b Metals) Metals in in products products of of loading loading in thein the cutting cutting surface sur- offace grinding of grinding wheel wheel (CSGW). (CSGW).
(a) (b)
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(a) (b)
(c) (d)
(e) (f)
Figure 7. The chrono-volt-amperometricchrono-volt-amperometric plots plots of of the the grinding grinding wheel wheel components: components: (a) ( Copper;a) Copper; (b) ( Tin;b) Tin; (c) Silver; (c) Silver; (d) Titanium; (d) Tita- nium;(e) Cobalt; (e) Cobalt; (f) Tungsten. (f) Tungsten.
3.1.1.3.1.1. Electrochemical Electrochemical Reactions Reactions and and Erosion Erosion Products of the ECDGW ECDGW-AC-AC Process TheThe a analysisnalysis of of electrochemical reactions pathways concerned concerned the the digestion of all metals,metals, either being components ofof aa grindinggrinding wheelwheel (S3020(S3020 175 175× ×6 6× × 33× × 5050 D125/100D125/100 M100M100)) metal metal bond bond or or present present in in waste materials resulting resulting from from sintered carbides (S20S) (S20S) grinding,grinding, found found in in the the CSGW. CSGW. Experiments Experiments on on these these metals metals digestion digestion were were run run in the in thela- boratorylaboratory scale scale using using an an electrolyzer electrolyzer potentiostat/galvanostat potentiostat/galvanostat (Eco (Eco Chemie Chemie-Metrohm-Metrohm Au- Au- tolab B.V., Utrecht, The Netherlands) for measurements and the chrono-volt-amperometric
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tolab B.V., Utrecht, The Netherlands) for measurements and the chrono-volt-amperomet- ric method [3]. The chrono-volt-amperometric plots of these metals digestion in the se- lected electrolytemethod solution [3 ].(E3) The are chrono-volt-amperometric collected in Figure 7. plots of these metals digestion in the selected The curveselectrolyte above present solution variations (E3) are in collectedthe electrode in Figure potentials7. E(t) during digestion, corresponding to actualThe period curves aboveof grinding present wheel variations sharpening in the by electrode means potentialsof the ECDGWE(t) during- digestion, AC method (aboutcorresponding 15 min). to actual period of grinding wheel sharpening by means of the ECDGW-AC The identificationmethod of (about the erosion 15 min). products was done using X-ray phase analysis [3]. The identification of the erosion products was done using X-ray phase analysis [3]. Examples of the diffraction patterns of the grinding wheel components dissolution are Examples of the diffraction patterns of the grinding wheel components dissolution are illustrated in Figure 8. illustrated in Figure8. Cu (copper) - sample Cu 01 - electrodeposit
Sn (tin) Ag (silver)
Co (cobalt) Ti (titanium)
Figure 8. X-ray diffraction patterns of the grinding wheel erosion products: (a) Copper—the sample; (b) Copper— Figure 8. X-ray diffraction patterns of the grinding wheel erosion products: (a) Copper—the sam- electrodeposit;ple; ( (bc) C Tin;opper (d)— Silver;electrodeposit (e) Cobalt;; ( (cf)) T Titanium;in; (d) Silver Electrolyte; (e) Cobalt solution; (f) Titanium; E3. Electrolyte solution E3. 3.1.2. Copper Digestion 3.1.2. Copper Digestion The analysis of variations in the electrode potential (the sample) (Figure7a) indicates The analysisthat of thevariations lowest in current the electrode density (0.003potential Acm (the−2 )sample) corresponds (Figure to digestion7a) indicates of copper yielding −2 that the lowest currentits cations density Cu+ (0.003and Cu Acm2+, which) corresponds react with to thedigestion solution of copper components. yielding The observation its cations Cu+ and Cu2+, which react with the solution components. The observation of the of the electrode surrounding revealed generation of a blue precipitate of CuCl2. When electrode surroundingthe process reveale ofd digestion generation was of accomplished, a blue precipitate the whiteof CuCl coating2. When of the CuCl pro- on the electrode cess of digestionsurface was accomplished, was present. Thethe white majority coating of products of CuCl slid on offthe theelectrode electrode surface surface and settled was present. Theon majority the electrolyzer of products bottom. slid After off the complete electrode sedimentation, surface and thesesettled products on the changed their color to brown. X-ray identification of these products (Figure8a,b) revealed that on the
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electrode surface mainly CuCl and 3Cu(OH)2CuCl2 were formed, and the precipitate from the electrolyte solution (after sedimentation and drying) was mainly composed of Cu2O, CuO, and Cu7Cl4(OH)10H2O. Taking into consideration the results of the process of anodic dissolution of copper analysis, the following electrochemical reactions were proposed [28]:
Cu → Cu+ + e E = +0.279 (13)
Cu → Cu2+ + 2e E = +0.093 (14) Cu2+ + Cl− + e → CuCl E = +0.300 (15) 2+ − Cu + 2Cl → CuCl2 (16) Cu + Cl− → CuCl E = −0.105 (17) − 2CuCl + H2O 21C4 Cu2O + 2Cl + 2H (18) − + 2Cu2O + 4H2O + 2Cl 3Cu(OH)2CuCl2 + 2H + 4e (19) E = + 0.207 − 0.0295 pH − 0.0295 (Cl−) (20) The reactions presented above as well as X-ray examinations proved that after rela- tively short period of time (about 5 h) these reactions products (in the form of precipitate) incubated in the electrolyte solution are converted mainly into copper oxides and the com- plex compound Cu7Cl4(OH)10H2O. This reaction pathway is favorable for the electrolyte solution stability and possibility of its regeneration. A small amount of Cu7Cl4(OH)10H2O, detected in the solution, indicated minimally reduced concentrations of Cl− and OH− ions in the electrolyte solution.
3.1.3. Tin Digestion The analysis of variation in the potential E(t) (Figure7b) proves that, already at the lowest digestion current density (i = 0.0038 Acm−2), Sn2+ and Sn4+ cations are released. The values of potentials corresponding to this current density are approximately equal to respective standard potentials (for Sn2+ E = −0.23 V and for Sn4+ E = 0.382 V). During these reactions, a white, sponge-like coating on the surface of electrode is formed. An increase in the dissolution current density (up to i = 0.048 Acm−2) results in an increase of the electrode potential and, after polarization time of about 600 s, the electrode is almost dissolved on the boundary of its submerging in the electrolyte solution. White, porous precipitate composed of tin oxides and hydroxides is observed on the electrode surface. The presence of these compounds was confirmed by X-ray analysis (Figure8c). Taking into consideration these results, the following reactions were proposed [28]:
2+ Sn Sn + 2e E = −0.382 (21)
2+ 4+ Sn Sn +2e E = −0.092 (22) 4+ Sn Sn + 4e E = −0.232 (23) + Sn + 2H2O → Sn(OH)2 + 2H + 2e E = −0.742 (24) + Sn + 2H2O → SnO2 + 4H + 4e E = −0.752 (25) − − Sn + 3OH HSnO2 + H2O + 2e E = −1.152 (26) − − 2- HSnO2 + 3OH + H2O Sn(OH)6 + 2e E = −1.172 (27)
Sn(OH)2 → SnO + H2O (28) The results of X-ray examination and the pathway of electrochemical reactions indicate that the process of anodic silver dissolution may lead to a decrease in Cl− ions concentration in the electrolyte solution, because AgCl is the main product of the reactions. Materials 2021, 14, 1375 13 of 22
3.1.4. Silver Digestion The variation in potential E(t) (Figure7c) shows that for low current densities ( i = 0.001 ÷ 0.0025 A cm−2), the electrode potentials are constant, and their values correspond to the electrode reaction of silver in the presence of Cl− and OH− ions. In this case, a grey, well-sticking to the electrode surface, AgCl coating was found (chlorine-silver electrode). An enhancement of the current density up to 0.16 A cm−2, resulted in an increase of the potential up to 0.55 V, which corresponded to the anodic silver digestion. The absence of silver ions in the electrolyte solution was also revealed. Higher current densities caused a white coating to form, which diffused to the solution. Most probably, it was composed of AgCl or Ag2CO3. Their presence was confirmed by X-ray analysis of the products of dissolution formed on the electrode surface and the precipitate present in the solution (Figure8d). The main component of the precipitate formed in the solution appeared to be AgCl together with small amount of AgO. For the analyzed process of anodic silver digestion, the following electrochemical reactions were proposed [28]:
+ Ag Ag + e E = +0.557 (29) − Ag + Cl AgCl E = −0.200 (30) − 2Ag + 2OH Ag2O+ H2O + 2e E = +0.102 (31) − Ag2O + 2OH 2AgO + H2O + 2e E = +0.358 (32) − 2AgO + 2OH Ag2O3 + H2O + 2e E = +0.498 (33) 2- 2Ag + CO3 Ag2CO3 + 2e E = +0.218 (34)
Ag2CO3 → Ag2O + CO2 (35) The results of X-ray examination and the pathway of electrochemical reactions indicate that the process of anodic silver digestion may lead to a decrease in Cl− ions concentration in the electrolyte solution since AgCl is the main product of the reactions.
3.1.5. Cobalt Digestion The variation in the cobalt electrode potential E(t) (Figure7e) indicates that for a small current density its potential of digestion is negative. The sample’s surface was coated with a pink precipitate, which diffused into the electrolyte solution, thus suggesting the possibility of cobalt hydroxide synthesis. An increase in current density up to i = 0.293 Acm−2 results in a rapid cobalt conversion to Co+3 ions. For this current density, products of digestion were rapidly removed from the sample surface, and after polarization time equal to 500 s, the sudden increase in the potential, accompanied by the complete sample digestion at the borderline of its submerging in the electrolyte solution, were observed. The course of digestion indicates that the products of electrochemical reactions are probably as follows: cobalt chloride, cobalt hydroxide, which is poorly soluble, and cobalt carbonate. An X- ray analysis of dissolution products (Figure8e) confirmed the presence of the both first compounds, it means CoCl2 and Co(OH)2. The results of cobalt digestion process permit to anticipate the following reactions [28]:
2+ Co Co + 2e E = −0.522 (36)
3+ Co Co + 3e E = +0.088 (37) 2+ 3+ Co Co + e E = +1.598 (38) 2- Co + CO3 CoCO3 + 2e E = −0.932 (39) − Co + 2OH Co(OH)2 + 2e E = −0.972 (40) − Co(OH)2 + OH Co(OH)3 + e E = −0.0724 (41) Materials 2021, 14, 1375 14 of 22
+ Co + 2H2O → Co(OH)2 + 2H + 2e E = −0.562 (42) 2+ − Co + 2Cl → CoCl2 (43) 2+ 2− Co + C03 → CoCO3 (44) An analysis of the reactions presented above points to a possibility of a decrease in OH− and Cl− ions concentration in the electrolyte solution.
3.1.6. Titanium Carbide Digestion The variation in potential E(t) (Figure7d) suggests that the oxide layer is formed on the electrode surface for the low current density. This conclusion is confirmed by a white coating on the surface of the sample. The presence of titanium oxide was also detected using X-ray analysis (Figure8f). Digestion of this compound is based on its decomposition [28]:
− + TiC + 6OH → TiO2 + CO2 + 2H + 2H2O + 8e (45)
The reactions leading to titanium oxides synthesis during the process of dissolution are probably as follows [28]:
2+ Ti Ti + 2e E = −1.862 (46)
2+ 3+ Ti Ti + e E = −0.612 (47) + Ti + 2H2O TiO2 + 4H + 4e E = −1.102 (48) + Ti + H2O TiO + 2H + 2e E = −1.962 (49) 2+ + Ti + H2O TiO + 2H + 4e E = −1.122 (50) 3+ 2+ + Ti + H2O TiO + 2H + e E = −0.142 (51) Titanium oxides point to the passivation of this metal. Because its ions are generated at negative potential values, it is likely that they are present in the electrolyte solution.
3.1.7. Tungsten Carbide Digestion The variation in potential E(t) (Figure7f) indicates that for low current densities, the electrode potential is negative. Its value equal to −0.25 V corresponds to the reaction of tungsten ionization. An increase in the current density results in the passivation of the sample’s surface, analogously to titanium. The dissolving of tungsten carbide leads to its decomposition to tungsten acid and carbon dioxide, according to the following equation [28]: − 2− + WC + 10OH → WO4 + CO2 + 2H + 4H2O + 10e (52) During the process of tungsten dissolution, a dark coating (oxides layer) was observed on the surface of the electrode, together with a relatively high anodic potential (about 2.5 V), appearing already at low current density. This is the evidence of the tungsten passivation during the process. The following tungsten reactions in the applied electrolyte solution are possible [28]: + W + 2H2O WO2 + 4H + 4e E = −0.362 (53) + W + 3H2O WO3 + 6H + 6e E = −0.332 (54) The presented reactions prove that tungsten oxides and carbon dioxide are the prod- ucts of tungsten carbide dissolution. The investigations revealed that all the metals, both being components of the grinding wheel metal bond (Ag, Cu, Sn) and cobalt, which is the binding metal for sintered carbides (TiC–Co and WC–Co), loading on the CSGW, undergo the anodic dissolution. Dissolution of cobalt results in decomposition of the carbides TiC and WC, mainly to CO2 and metal oxides, which are passive against the solution. However, their passivity has no deleterious Materials 2021, 14, x FOR PEER REVIEW 16 of 24
W + 3H2O ⇄ WO3 + 6H+ + 6e E = −0.332 (54) The presented reactions prove that tungsten oxides and carbon dioxide are the prod- Materials 2021, 14, 1375 15 of 22 ucts of tungsten carbide dissolution. The investigations revealed that all the metals, both being components of the grind- ing wheel metal bond (Ag, Cu, Sn) and cobalt, which is the binding metal for sintered carbides (TiC–Co and WC–Co), loading on the CSGW, undergo the anodic dissolution. effect on the grinding wheel sharpening, since dissolution of the metals-components of the Dissolution of cobalt results in decomposition of the carbides TiC and WC, mainly to CO2 metaland metal bond oxides, (Ag, Cu,which Sn) are and passive cobalt, against causes the theirsolution. removal However, from their CSGW. passivity The mainhas no products ofdeleterious electrochemical effect on reactions the grinding corresponding wheel sharpening, to the since dissolution dissolution process of the are metals chlorides,-com- oxides andponents carbonates of the metal of the bond dissolved (Ag, Cu, Sn metals.) and cobalt, The compounds causes their removal listed above from CSGW mainly. The form easy- to-filtermain products precipitates of electrochemical in the electrolyte reactions solution, corresponding which to are the beneficial dissolution for process regenerating are the solutionchlorides, and oxides utilizing and carbonates the waste of the products dissolved of themetals. grinding The compounds wheels dressing listed above process. It wasmainly revealed form easy that-to the-filter anodic precipitates dissolution in the electrolyte of metals maysolution, be accompaniedwhich are beneficial by a decreasefor in Clregenerat− and OHing the− ions solution concentration and utilizing in the the waste electrolyte products solution. of the grinding This negativewheels dressing effect may be eliminatedprocess. It was with revealed the use that of properthe anodic chemicals dissolution and of by metals controlling may be the accompanied ion concentration by a and pHdecrease of the in electrolyte Cl− and OH solution.− ions concentration in the electrolyte solution. This negative effect may be eliminated with the use of proper chemicals and by controlling the ion concentra- 3.2.tion Theand NoxiousnesspH of the electrolyte of the ECDGW-AC solution. Process for the Surroundings
3.2. TheThe Noxiousness process of of dressing the ECDGW of superhard-AC Process grindingfor the Surroundings wheels using the ECDGW-AC method requires the application of electrolytes that are noxious for the surroundings and the The process of dressing of superhard grinding wheels using the ECDGW-AC method naturalrequires environment.the application Theirof electrolytes negative that effect are noxious consists, for among the surroundings other things, and ofthe the nat- corrosive actionural environment. on the grinding Their machinenegative effect units consists, and the among noxiousness other things, for the of operators the corrosive of the ac- grinding machinestion on theand grinding for the machine surroundings. units and the There noxiousness are also ecologicalfor the operators restrictions of the grinding and difficulties relatedmachines to and the for utilization the surroundings. of dressing There waste are also and ecological regeneration restrictions of electrolytes. and difficulties The degree ofrelated the problems to the utilization mentioned of dressing above waste depends and regenera mainlytion on of the electrolytes. chemical The composition degree of of the chemicalthe problems compounds mentioned and above their depends concentration mainly on in the the chemical electrolyte. composition The investigations of the chem- carried outical confirmedcompounds theand possibilitytheir concentration to limit in this the noxiousnesselectrolyte. The in investigations the process ofcarried grinding out wheel dressingconfirmed especially the possibility by eliminating to limit this toxicnoxious compoundsness in the process from the of electrolytegrinding wheel composition dress- (e.g., nitritesing especially and chromates) by eliminating and toxic reducing compounds the chemical from thecompound electrolyte composition concentration (e.g. to, ni- about 5%. Thetrites corrosive and chromates) action and of electrolytes reducing the on chemical the grinding compound wheel concentration units waslimited to about by 5%. the use of The corrosive action of electrolytes on the grinding wheel units was limited by the use of electrolytes of a high pH of 9.4–12.5, the selection of their chemical composition. and the electrolytes of a high pH of 9.4–12.5, the selection of their chemical composition. and the use of protective agents. Examples of the results of the laboratory investigations into the use of protective agents. Examples of the results of the laboratory investigations into the raterate of corrosion (det (determinedermined by bythe thesingle single-sweep-sweep voltammetry voltammetry method) method) of steel ofsheet steel 815 sheet 815 inin differentdifferent electrolytes electrolytes are are shown shown in F inigure Figure 9. 9.
Figure 9. The effect of the electrolyte composition on the corrosion rate of constructional steel; Electrolytes as in Table2.
The slowest rate of corrosion was obtained for electrolytes E4 and E3, which contained a protective agent. The effect of the remaining electrolytes, different composition of chemical compounds can be ordered as follows: E2, E1, and E5. It should be noted that the use of protective agents can also attenuate the effectiveness of electrolytes in the dressing process. Therefore, these two processes should be analyzed simultaneously. Also, the investigations concerning the description of electrochemical reactions and identification of the products formed in the dressing process are continued. Their knowledge is of great Materials 2021, 14, 1375 16 of 22
significance in view of the requirements of the environmental protection and the possibility of electrolyte regeneration.
3.3. Experimental Investigations of the ECDGW-AC Process The comprehensive results of the experimental studies on the ECDGW-AC process and the description of the test stand contain the authors’ earlier publications [4,6]. The paper sample presented the results of the experimental investigations aiming at the verification of the mathematical relationships established and the assessment of the effectiveness of the ECDGW-AC process using a stationary tool-electrode and, comparatively, with its feed stabilizing the inter-electrode gap thickness in the ECDGW-AC process. The automatic feed of the tool electrode is affected by a control system whose diagram is shown in Figure5. An inductive contactless sensor L interacting with a frequency generator LC was used for the measurement of the inter-electrode gap thickness. The tool electrode feed is affected by a step motor controlled by a computer. Materials 2021, 14, x FOR PEER REVIEW 18 of 24 The characteristic results of the investigations of the ECDGW-AC process are presented in Figure 10.
Figure 10. A comparison of the theoretical calculations and the experimental investigations of the Figure 10. A comparison of the theoretical calculations and the experimental investigations of the grinding layer ΔH removed in the ECDGW-AC dressing process: (a) Dressing with a stationary grindingelectrode; ( layerb) Dressing∆H removed with a step in feed the electrode. ECDGW-AC dressing process: (a) Dressing with a stationary electrode; (b) Dressing with a step feed electrode. They refer to the comparison of the theoretical calculations of the dissolution rate of the cutting abrasive layer (ΔH) with the results of the experimental investigations. This comparison of the investigation results confirms the significant effect of the tool electrode feed on the effectiveness of the ECDGW-AC process. The tests of dressing of a grinding wheel (S3020 175 × 6 × 3 × 50 D125/100 M100) with the feed of the tool electrode have shown an increase in the effectiveness of the dressing process by about 45% compared with the dressing process with a stationary electrode. The application of the feed of the tool electrode in the ECDGW-AC process has a favorable effect on the reduction of losses of the dissolution current and stability of the dressing process [28]. The investigations car- ried out have also confirmed the good conformity of the theoretical calculations with the experimental investigations. The differences in the rate of digestion did not exceed 50% in the case of dressing with a stationary electrode and about 20% when the feed of the elec- trode was used, which testifies the correct development of the ECDGW-AC process. The
Materials 2021, 14, 1375 17 of 22
They refer to the comparison of the theoretical calculations of the dissolution rate of the cutting abrasive layer (∆H) with the results of the experimental investigations. This comparison of the investigation results confirms the significant effect of the tool electrode feed on the effectiveness of the ECDGW-AC process. The tests of dressing of a grinding wheel (S3020 175 × 6 × 3 × 50 D125/100 M100) with the feed of the tool electrode have shown an increase in the effectiveness of the dressing process by about 45% compared with the dressing process with a stationary electrode. The application of the feed of the tool electrode in the ECDGW-AC process has a favorable effect on the reduction of losses of the dissolution current and stability of the dressing process [28]. The investigations Materials 2021, 14, x FOR PEER REVIEWcarried out have also confirmed the good conformity of the theoretical calculations18 with of 23 the experimental investigations. The differences in the rate of digestion did not exceed 50% in the case of dressing with a stationary electrode and about 20% when the feed of the electrode was used, which testifies the correct development of the ECDGW-AC process. Theexamples examples of experimental of experimental data data confirming confirming the the possibility possibility of of reduction inin axialaxial run—outrun—out ofof a a face face of of a a grinding grinding wheel wheel in in the the ECDGW-AC ECDGW-AC process process are are shown shown in in Figure Figure 11 11..
FigureFigure 11. 11.The The comparison comparison ofof experimentalexperimental andand theoreticaltheoretical valuesvalues ofof thethe reductionreduction ofof thethe run—outrun—out deviation (εO) of a grinding wheel in the ECDGW-AC process. deviation (εO) of a grinding wheel in the ECDGW-AC process.
TheThe comparison comparison of of experimental experimental and and theoretical theoretical values values of of the the coefficient coefficient of of reduction reduction inin deviation deviation of of the the run—out run—out of aof grinding a grinding wheel wheel in the in ECDGW-AC the ECDGW dressing-AC dressing process. process. They referThey to refer dressing to dressing of a diamond of a diamond grinding grinding wheel ofwheel the known of the known initial value initial of value face run-outof face runεO.- Theout dressingεO. The dressing process was process carried was out carried with the out electrode with the as electrode the stationary as the tool.stationary In the coursetool. In ofthe the course dressing, of the the dressing, face run-out the faceε(t) andrun- theout thickness ε(t) and the of the thickness removed of layerthe removed of the grinding layer of wheelthe grinding (∆H) were wheel determined. (ΔH) were determined. TheThe analysis analysis of of the the results results confirms confirms that that reduction reduction of the of deviationthe deviation (εO) in (ε theO) in ECDGW- the EC- ACDGW process-AC process is possible is possible and points and to points conformity to conformity of the theoretical of the theoretical calculations calculations with experi- with mentalexperimental data. data. TheThe ECDGW-AC ECDGW-AC process process effectiveness effectiveness in in the the forming forming of theof the macro-and macro-and microgeometry microgeom- cuttingetry cutt surfaceing surface grinding grinding wheel (CSGW)wheel (CSGW) was assessed was assessed by stereometric by stereometric parameters, parameters, such as: S H thesuch static as: cuttingthe static edges cutting ( N ),edges the kinematic (SN), the cuttingkinematic edges cutting ( N), edges and the (H averageN), and the maximum average thickness of a chip (T ). They were determined using the computer analysis of circumferen- maximum thicknessN of a chip (TN). They were determined using the computer analysis of tialcircu representationmferential representation of the CSGW profileof the andCSGW simulation profile ofand the simulation grinding process. of the grinding Peaks of pro- the µ CSGWcess. Peaks profile, of ofthe which CSGW the profile, minimum of which height the was minimum equal to 1 heightm, and was which equal were to present1 μm, and at µ awhich depth were up to present 30 m fromat a depth the nominal up to 30 surface μm from CSGW the nominal surface, surface were identified CSGW surface, as the static were cutting edges (SN)[28]. Examples of the results of the investigations illustrating the effect identified as the static cutting edges (SN) [28]. Examples of the results of the investigations of the ECDGW-AC process on the formation of the CSGW macro- and micro-geometry are illustrating the effect of the ECDGW-AC process on the formation of the CSGW macro- presented in Figure 12. and micro-geometry are presented in Figure 12.
Materials 2021, 14, x FOR PEER REVIEW 19 of 24
examples of experimental data confirming the possibility of reduction in axial run—out of a face of a grinding wheel in the ECDGW-AC process are shown in Figure 11.
Figure 11. The comparison of experimental and theoretical values of the reduction of the run—out deviation (εO) of a grinding wheel in the ECDGW-AC process.
The comparison of experimental and theoretical values of the coefficient of reduction in deviation of the run—out of a grinding wheel in the ECDGW-AC dressing process. They refer to dressing of a diamond grinding wheel of the known initial value of face run- out εO. The dressing process was carried out with the electrode as the stationary tool. In the course of the dressing, the face run-out ε(t) and the thickness of the removed layer of the grinding wheel (ΔH) were determined. The analysis of the results confirms that reduction of the deviation (εO) in the EC- DGW-AC process is possible and points to conformity of the theoretical calculations with experimental data. The ECDGW-AC process effectiveness in the forming of the macro-and microgeom- etry cutting surface grinding wheel (CSGW) was assessed by stereometric parameters, such as: the static cutting edges (SN), the kinematic cutting edges (HN), and the average maximum thickness of a chip (TN). They were determined using the computer analysis of circumferential representation of the CSGW profile and simulation of the grinding pro- cess. Peaks of the CSGW profile, of which the minimum height was equal to 1 μm, and which were present at a depth up to 30 μm from the nominal surface CSGW surface, were identified as the static cutting edges (SN) [28]. Examples of the results of the investigations Materials 2021, 14, 1375 illustrating the effect of the ECDGW-AC process on the formation of the CSGW macro18 of 22- and micro-geometry are presented in Figure 12.
Figure 12. The stereometric parameters CSGW in ECDGW-AC process: (a) Static cutting edges; (b) Kinematic cutting
edges; (c) Maximum chip thickness; Grinding conditions: VGW = 25 m/s, Vf = 5 m/min, a = 0.02 mm; Dressing conditions: 2 VGW = 25 m/s, i = 6 A/cm , Ue = 18.7 V. This is the comparison of the stereometric parameters of the CSGW for different conditions of the grinding wheel (S3020 175 × 6 × 3 × 50 D125/100 M100), it means for the grinding wheel glazed in the test of grinding of sintered carbides (S20S) and further subjected to dressing by the ECDGW-AC method. The static edges (SN) were assessed basing on the changes in their total number (ΣSN) and the average depth of deposition (Z) in the CSGW (Figure 12a). The most important changes in these parameters were observed in the initial period of dressing, in which the removal of the wheel layer (ΣHN) is approximately 20 µm (Figure 12b). On the completion of the ECDGW-AC process, very favorable changes in these parameters were obtained: a decrease in ΣSN by approximately 46% and an increase in the depth of deposition of the static edges by approximately 140%. The kinematic cutting edges HN (Figure 12b) were identified from a set of static edges basing on the coefficient of “simulation of the grinding process”—k, which is the condition of penetration of the CSGW into the workpiece [28]: r Vf a k = 2 (55) VGW DGW
where VGW—speed of grinding wheel, Vf—speed of longitudinal feed, a—depth of grind- ing, DGW—equivalent grinding wheel diameter. Under the experimental grinding conditions [28]: k = 0.712 × 10−4. An analysis of the investigation results shows that the tendency of changes in ΣSN (Figure 12a) and Z in the CSGW is similar to that of the static edges, since by using the ECDGW-AC process a decrease in the number of kinematic edges by approximately 51% and an increase in the depth of their deposition in the CSGW by approximately 85% was obtained. This favorable direction of changes in the ΣSN and z parameters confirms the effectiveness of the ECDGW-AC process in restoring the cutting ability of the grinding wheel subjected to dressing. The average maximum thickness of chips (TN) is determined by the kinematic edges (HN) and distance between them (LN), according to the equation:
TN = kLN − (HN − HN−1) (56)
where LN—distance between kinematic edges. An analysis of the changes in this parameter (Figure 12) confirms the effectiveness of the ECDGW-AC process in forming the CSGW, particularly in restoring the cutting ability of the grinding wheel. Following the dressing by ECDGW-AC method, a change in this parameter is observed consisting of the histogram shift of distribution of TN into a zone of greater dimensions of the chip. This attests to an increase in the proportion of the cutting edges actively participating in chip formation and a limitation of the number of apparently active cutting edges. The loading of grinding products on the CSGW is one of the forms of superhard grinding wheels wear. An X-ray method of the modified external standard, developed by the author, was used in order to assess the degree of loading the CSGW with Materials 2021, 14, 1375 19 of 22
grinding products. Some examples of investigation results, confirming the effectiveness of the ECDGW-AC process for the removal of products loading on the CSGW of the (S3020 Materials 2021, 14, x FOR PEER REVIEW 20 of 23 175 × 6 × 3 × 50 D125/100 M100) grinding wheel, glazed in test of sintered carbides (S20S) grinding, are shown in Figure 13.
FigureFigure 13. 13.Effect Effect ofof dressing time on on the the degree degree of of gumming gumming up up products products of ofcutting cutting surf surfaceace of su- of perhard grinding wheel (CSGW). superhard grinding wheel (CSGW).
TheirTheir analysis analysis indicates indicates that that the the most most intensive intensive removal removal of of products products loading loading on on the the CSGWCSGW occurs occurs in in the the initial initial period period of dressing, of dressing, approximately approximately 2 min. 2 Themin complete. The complete removal re- ismoval obtained is obtained after 5 to after 6 min 5 to of 6 dressing.min of dressing. A layer A of layer the grinding of the grinding wheel ofwheel an approximate of an approx- thicknessimate thickness 10 µm is 10 removed µm is removed over this over dressing this dressing period. Theperiod. results The confirm results theconfirm effectiveness the effec- oftiveness the ECDGW-AC of the ECDGW process-AC in process the removal in the of removal grinding of productsgrinding gummingproducts gumming up the CSGW. up the CSGW. 4. Conclusions 4. ConclusionsThe presented investigation results confirm the possibility of the application of super- hard grindingThe presented wheels investigation (ECDGW-AC) results solutions confirm of low the concentration possibility electrolytesof the application of chemical of su- compoundsperhard grinding during the wheels electrochemical (ECDGW-AC dressing) solutions process. of low The concentration determined chemical electrolytes com- of positionchemical of compounds low concentration during solutionsthe electrochemical of electrolytes dressing (containing process. about The d 5%etermined of chemical chem- compounds)ical composi assurestion of thelow requiredconcentration efficiency solutions of the of dressing electrolytes process (containing of grinding about wheels 5% of usingchemical ECDGW-AC compounds) method. assures Solutions the required of electrolytes efficiency are of characterized the dressing byprocess lower of nuisance grinding forwheels surroundings using ECDGW and the-AC environment method. Solutions and low corrosivityof electrolytes of the are grinding characterized machine by units. lower Electrochemicalnuisance for surroundings studies allow and for the elaborationenvironment of and the courselow corrosivity of electrochemical of the grinding reactions ma- duringchine units. ECDGW-AC Electrochemical process. studies However, allow X-ray for the investigations elaboration canof the be course used for of electrochem- the precise identificationical reactions of during the erosion ECDGW products.-AC process. It is However, demonstrated X-ray that investigations an anodic dissolutioncan be used isfor observedthe precise for allidentification metals composing of the erosion the metal products. bond of It grinding is demonstrated wheel and that grinded an anodic materials. disso- lutionThe is main observed products for all of metals coursed composing electrochemical the metal reactions bond of are grinding chlorides, wheel oxides, and hydrox- grinded idesmaterials. and carbonates of digestion metals. The above mentioned chemical compounds are presentThe in solutionsmain products of electrolytes of coursed as electrochemical sediments which reactions can be easilyare chlorides, filtered out.oxides, Their hy- chemicaldroxides analysis and carbonates does not revealof digestion the presence metals. of The chemical above compoundsmentioned chemical which can compounds be harm- fulare to present the environment in solutions or of for electrolytes the operators as sediments of the grinding which machines can be easily Investigation filtered out. results Their confirmchemical possibility analysis of does reduction not reveal of corrosive the presence influence of chemic of solutionsal compounds of electrolytes which on can the be corrosivity of the grinding machine units by application of alkali solutions of pH ≥ 9.4 and harmful to the environment or for the operators of the grinding machines Investigation protective additives. results confirm possibility of reduction of corrosive influence of solutions of electrolytes The theoretical consideration and the experimental investigations presented here have on the corrosivity of the grinding machine units by application of alkali solutions of pH ≥ shown a significant effect of the feed of the tool–electrode on the effectiveness of grinding 9.4 and protective additives. wheel dressing in the ECDGW-AC process. The application of the proper feed of the tool The theoretical consideration and the experimental investigations presented here have shown a significant effect of the feed of the tool–electrode on the effectiveness of grinding wheel dressing in the ECDGW-AC process. The application of the proper feed of the tool electrode leads to reduction of the dissolution current losses. Moreover, the efficiency of the dressing process increases.
Materials 2021, 14, 1375 20 of 22
electrode leads to reduction of the dissolution current losses. Moreover, the efficiency of the dressing process increases. The ECDGW-AC method is particularly predisposed for forming of the CSGW macro- and micro-geometry, removing the grinding products loading on the CSGW, and for correct- ing the CSGW profile. The investigations have confirmed that this kind of dressing method can be recommended for dressing of superhard grinding wheel. The implementation of the ECDGW-AC method requires equipping the grinding machine with a cheap and simple electrochemical unit. Currently, there are realized industrial tests concerning the implementation elaborated dressing method of superhard grinding wheels (ECDGW-AC) in factory producing abrasive tools. The elaborated method of electrochemical dressing of superhard grinding wheels (ECDGW-AC) has received polish patents no PL 169942 and PL 190236 [30,31].
Author Contributions: A.G. performed the conception of the work, investigation, methodology, validation and founding acquisition; M.G. performed formal analysis, resources, writing-original draft preparation, writing-review, editing, visualization and supervision; B.T. performed software, data curation and interpretation of data for the work. All authors have read and agreed to the published version of the manuscript. Funding: This research received no external funding. Institutional Review Board Statement: Not applicable. Informed Consent Statement: Not applicable. Data Availability Statement: Data is contained within the article. Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
CBN Cubic boron nitride CD Crystal diamond CSGW Cutting surface of gringing wheel ECDGW-AC Electrochemical dressing of suparhard grinding wheels using alternating current SD Synthetic diamond ES Electrolyte solution of specially selected chemical compounds a Depth of grinding, mm D Grinding wheel external diameter, mm DGW Equivalent grinding wheel diameter, mm E Total overpotential, V HN Kinematic cutting edges, µm k Coefficient of “simulation of the grinding process” Coefficient of electrochemical workability of the grinding wheel components, k v mm3A−1mm−1 le Distance between electrodes, mm LN Distance between kinematic edges, mm Re Resistance of electrolyte, Ω RGW Resistance of grinding wheel, Ω S Thickness of inter-electrode gap, mm −1 ∑SN Static cutting edges, mm SGW Surface of grinding wheel ti Time of start of digestion process, min to Actual time of dressing, min toe Effective time of dressing, min TN Average maximum thickness of a chip, µm Tn Time of dressing needed for the removal of the grinding wheel layer, min Ue Effective value voltage of the inter-electrode gap, V −1 Vdig Speed of digestion, mms Vf Speed of longitudinal feed, ms−1 Materials 2021, 14, 1375 21 of 22
−1 VGW Speed of grinding wheel, ms Z Average depth of deposition of the static edges, µm χ Specific conductance of the electrolyte, Sm−1 ε Face run-out, µm ∆H Thickness of the grinding wheel layer removed in the ECDGW-AC process, µm.
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