Study on Electrolytic Magnetic Abrasive Finishing for Finishing Stainless Steel SUS304 Plane with a Special Compound Machining Tool

Journal of Manufacturing and Materials Processing

Article Study on Electrolytic Magnetic Abrasive Finishing for Finishing Stainless Steel SUS304 Plane with a Special Compound Machining Tool

Xu Sun and Yanhua Zou *

Graduate School of Engineering, Utsunomiya University, 7-1-2 Yoto, Utsunomiya, Tochigi 321-8585, Japan; [email protected] * Correspondence: [email protected]; Tel.: +81-028-689-6057

Received: 25 April 2018; Accepted: 19 June 2018; Published: 26 June 2018

Abstract: In order to improve the finishing efficiency of traditional plane magnetic abrasive finishing (MAF), we have previously proposed an effective plane MAF process combined with an electrolytic process and developed a special compound machining tool for the electrolytic magnetic abrasive finishing (EMAF). In this research, the EMAF process is divided into two finishing steps. The first finishing step is the EMAF step, and a single MAF step constitutes the second finishing step. The machinability of SUS304 material can be improved through the formation of passive films from the electrolytic process in the EMAF step. Meanwhile, the passive films can be rapidly and easily removed by friction between the magnetic particles and the workpiece-generated mechanical machining force. Thus, the surface finishing efficiency can be greatly improved. Compared with a dedicated MAF machining tool or a dedicated electrolytic machining tool, this special compound machining tool can synchronously achieve MAF and electrolytic processes to make the processing more convenient. This study focuses on exploring mechanical finishing characteristics of the special compound machining tool through MAF experiments. Additionally, EMAF experiments are conducted under the optimal mechanical finishing conditions. The experimental results of the EMAF process show that the surface roughness Ra can be reduced to less than 30 nm at the 4-min EMAF step, and it can be further reduced to 20 nm at the 10-min MAF step.

Keywords: plane magnetic abrasive finishing; electrolytic magnetic abrasive finishing; compound machining tool; finishing efficiency; surface roughness

1. Introduction With the development of semiconductor-related and biotechnology-related industries, the demands for surface quality and machining efficiency are becoming higher and higher. Therefore, the finishing operation is more and more important in the whole production process. Among the many precision machining methods, magnetic abrasive finishing (MAF) is undoubtedly proven to be an effective precision processing method [1–4]. Though the MAF process, as a kind of ultra-precision machining method, can achieve nanoscale surface finishing, the polishing efficiency decreases when a hard metal surface is polished by the MAF process, as compared to other processing methods [5–7]. Thereby, diamond is usually used to polish hard metal surfaces in the MAF process in order to ensure the polishing efficiency. As a result, the polishing costs become high. In recent years, in order to improve surface accuracy and polishing efficiency, researchers have not only optimized and improved the parameters of magnetic abrasive finishing, but also dedicated efforts to explore magnetic abrasive finishing combined with a variety of processes. In these MAF compound processing methods, electrolytic/electrochemical magnetic abrasive finishing (EMAF) has been proposed and conducted

J. Manuf. Mater. Process. 2018, 2, 41; doi:10.3390/jmmp2030041 www.mdpi.com/journal/jmmp J. Manuf. Mater. Process. 2018, 2, 41 2 of 13 by a few researchers. Among them, Zou et al. developed magnetic abrasive finishing combined with electrolytic process methods to respectively polish the plane workpiece and internal surface of a tube. A large number of experimental results have indicated that the polishing efficiency of a metal material can be significantly improved through magnetic abrasive finishing combined with the electrolytic process [8,9]. Pandey et al. also proved that chemo-ultrasonic assisted magnetic abrasive finishing is effective through a series of experimental investigations, and a regression model was developed to predict the percentage change in surface roughness in terms of significant processing factors [10]. Yadava et al. investigated the effect of electrolytic current, magnetic flux density, and the rotational speed of workpiece on material removal and surface roughness through electrochemical dissolution and magnetic abrasive machining [11]. Based on the abovementioned reports, it can be considered that the finishing efficiency of a metal material can be improved by the EMAF process. Currently, the EMAF method is usually divided into synchronous finishing and steps finishing. The synchronous EMAF process is realized by a special compound machining tool [12]; the steps EMAF process involves passive films being formed on the workpiece surface by an oxidizing agent before the experiment, after which they are removed by a dedicated magnetic machining tool [13]. However, the development of EMAF process compound machining tools is very scarce. Hence, a special compound machining tool was developed before this study. The MAF and electrolytic processes are two important finishing parts of the EMAF process. The electrolytic process takes a primary role in removing material and reducing the surface hardness of a workpiece for the EMAF process [14–17]. Generally, the roughness of the metal surface can be minimized in a shorter finishing time by the electrolytic process. However, the mirror polishing is difficult to be achieved by only the electrolytic process due to the generation of passive films on the surface. The synchronous MAF process is used to remove the generated passive films from the electrolytic process in order to achieve precision machining. The MAF and electrolytic processes are synchronously performed in the first finishing step, then the MAF process as a final process continues to be performed in the second finishing step in order to obtain a perfect surface profile accuracy with a high machining efficiency. In this study, we focused on exploring the mechanical finishing characteristics of the special compound machining tool through MAF experiments. Additionally, EMAF experiments were conducted under the optimal mechanical finishing conditions. The experimental results of EMAF process demonstrated that the efficiency of precision machining can be significantly improved.

2. Processing Principles In this research, the EMAF process includes two finishing steps, which are respectively the first finishing step (EMAF step) and the second finishing step (MAF step). The machining principle of the EMAF process is shown in Figure1. Figure1a shows the original surface with a lot of initial hairlines before finishing. Figure1b shows the finished surface after the EMAF step. When turning on DC constant voltage power, the protruding portions are preferentially leveled and passive films are formed on the workpiece surface by the electrolytic process. Simultaneously, the magnetic abrasive particles of a magnetic brush are used to exert friction on the workpiece surface so the passive films can be effectively removed. Also, the hardness of the passive films is smaller than the hardness of SUS304 stainless material [18]. Thus, the efficiency of precision machining can be improved through the first finishing step—the EMAF process. However, a few passive films still exist on the finished surface after the EMAF step. The residual passive films affect the surface accuracy. Hence, the MAF step represents a final process after the EMAF step that is used to completely remove all passive films in order to improve the surface accuracy of the workpiece. Figure2c shows the finished surface after the MAF step. J. Manuf. Mater. Process. 2018, 2, 41 3 of 13 J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 3 of 13 J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 3 of 13

Figure 1.1. Schematic of the electrolytic/electrochemical magnetic magnetic abrasive finishing ﬁnishing (EMAF) process. Figure 1. Schematic of the electrolytic/electrochemical magnetic abrasive finishing (EMAF) process.

Figure 2. Three-dimensional views of the electrolytic magnetic compound machining tool. Figure 2. Three-dimensional views of the electrolyticelectrolytic magnetic compound machining tool. 3. Experimental Setup 3. Experimental Setup 3. Experimental Setup The three-dimensional views of the electrolytic magnetic compound machining tool are shown The three-dimensional views of the electrolytic magnetic compound machining tool are shown in FigureThethree-dimensional 2. In order to be able views to ofsynchronously the electrolytic achieve magnetic the compoundMAF and electrolytic machining processes, tool are shown four in Figure 2. In order to be able to synchronously achieve the MAF and electrolytic processes, four inmagnetic Figure2 poles. In order (Φ6 mm) to be and able a tocross-shape synchronously cathode achieve (30 × 3 the × 3 MAF mm) and are electrolyticembedded in processes, the bottom four of magnetic poles (Φ6 mm) and a cross-shape cathode (30 × 3 × 3 mm) are embedded in the bottom of magneticthe electrolytic poles (magneticΦ6 mm) and compound a cross-shape machining cathode tool (30. The× 3 ×electrode3 mm) are connects embedded to a incopper the bottom ring with of the a the electrolytic magnetic compound machining tool. The electrode connects to a copper ring with a electrolyticwire. The current magnetic flows compound from the machining negative tool. pole The of electrodethe DC power connects source to a copperto the ringelectrode with aof wire. the wire. The current flows from the negative pole of the DC power source to the electrode of the Thecompound current machining flows from tool the negativethrough polethe carbon of the DCbrush power connected source to thethe electrodecopper ring. of the Additionally, compound compound machining tool through the carbon brush connected to the copper ring. Additionally, machiningthe magnetic tool brush through forms the at carbonthe magnetic brush poles. connected to the copper ring. Additionally, the magnetic the magnetic brush forms at the magnetic poles. brushFigure forms at3 theshows magnetic the experimental poles. setup of electrolytic magnetic abrasive finishing. The Figure 3 shows the experimental setup of electrolytic magnetic abrasive finishing. The workpieceFigure is3 showsa SUS304 the plane experimental that is 100 setup mm of in electrolytic length, 100 magnetic mm in width, abrasive and finishing. 1 mm in The thickness. workpiece The workpiece is a SUS304 plane that is 100 mm in length, 100 mm in width, and 1 mm in thickness. The iselectrolytic a SUS304 planemagnetic that compound is 100 mm inmachining length, 100 tool mm is in fixed width, on and the 1chuck mm in of thickness. a milling The machine. electrolytic The electrolytic magnetic compound machining tool is fixed on the chuck of a milling machine. The magneticrotational compound direction machiningand velocity tool of is the fixed compound on the chuck machining of a milling tool machine. are controlled The rotational by the direction milling rotational direction and velocity of the compound machining tool are controlled by the milling andmachine. velocity The of SUS304 the compound plane workpiece machining as tool an are anode controlled is fixed by in the the milling container machine. and Theconnected SUS304 to plane the machine. The SUS304 plane workpiece as an anode is fixed in the container and connected to the workpiecepositive pole as of an the anode DC ispower fixed source. in the container The NaNO and3 electrolyte connected solution to the positive is supplied pole from of the aDC nozzle power by positive pole of the DC power source. The NaNO3 electrolyte solution is supplied from a nozzle by source.a pump, The and NaNO the flow3 electrolyte rate of the solution electrolyte is supplied is controlled from through a nozzle a by flow a pump, meter. andThe theworkpiece flow rate and of a pump, and the flow rate of the electrolyte is controlled through a flow meter. The workpiece and thecontainer electrolyte are laid is controlled on the X-Y through stage. a flowThe meter.feedin Theg trajectory workpiece and and velocity container of arethe laidX-Y onstage the X-Yare containercontrolled areby alaid numerical on the control X-Y stage. (NC) programThe feedin system.g trajectory and velocity of the X-Y stage are controlled by a numerical control (NC) program system.

J. Manuf. Mater. Process. 2018, 2, 41 4 of 13 stage. The feeding trajectory and velocity of the X-Y stage are controlled by a numerical control (NC) programJ. Manuf. Mater. system. Process. 2018, 2, x FOR PEER REVIEW 4 of 13

Figure 3. External photo of the experimental setup. Figure 3. External photo of the experimental setup.

4. Experimental Results and Discussions 4. Experimental Results and Discussion

4.1. Experimental Procedure and Conditions of the MAF Process The MAFMAF process process as as an an important important finishing finishing part ofpart the of EMAF the EMAF process, process, as the mechanical as the mechanical finishing characteristicsfinishing characteristics of the electrolytic of the electrolytic magnetic compoundmagnetic compound machining machining tool first need tool to first be exploredneed to bybe theexplored MAF processby the experiments.MAF process Therefore, experiments. we focused Therefore, on investigating we focused theon mixedinvestigating magnetic the abrasive, mixed rotationalmagnetic speedabrasive, of therotational machining speed tool andof the the mach workingining gap. tool The and detailed the working experimental gap. The conditions detailed of experimental conditions of the MAF process are shown in Table 1. The original surface roughness the MAF process are shown in Table1. The original surface roughness Ra of the workpiece is measured toRa beof approximatelythe workpiece 0.16~0.2is measuredµm. Theto be mixed approximately magnetic abrasive0.16~0.2 respectivelyμm. The mixed consists magnetic of quantitative abrasive electrolyticrespectively iron consists powder of quantitative with 330 µm, electrolytic 149 µm, 75 ironµm inpowder mean diameterwith 330 andμm, quantitative 149 μm, 75 #4000,μm in #6000,mean #8000,diameter #10000 and WAquantitative particles. In#4000, order #6000, to ensure #8000, that # the10000 machining WA particles. power isIn adequate, order to theensure working that gapthe ismachining respectively power adjusted is adequate, to 1 mm andthe working 2 mm. To gap obtain is respectively a higher surface adjusted accuracy to 1 and mm higher and machining2 mm. To efficiency,obtain a higher the rotational surface speed accuracy of the and machining higher toolmachining is respectively efficiency, selected the rotational at 230 rpm speed and 450 of rpm.the Themachining finishing tool time is respectively of the MAF processselected isat limited 230 rpm at 60and min. 450 Afterrpm. eachThe finishing finishing step,time of the the finished MAF surfaceprocess isis measured limited at and 60 observed min. After by aeach contact finishi roughnessng step, meter the (Mitutoyo,finished surface in 1-20-1 is Sakado, measured Takatsu, and Kawasaki,observed by Japan) a contact and three-dimensionalroughness meter (Mitutoyo, (3D) non-contact in 1-20-1 optical Sakado, profiling Takatsu, microscopy Kawasaki, (Wyko-Vision Japan) and NT1100,three-dimensional in 1-1-30 Shibadaimon, (3D) non-contact Minato, optical Tokyo, profiling Japan) microscopy at these three (Wyko-Vision locations, as shownNT1100, in Figurein 1-1-304. Shibadaimon, Minato, Tokyo, Japan) at these three locations, as shown in Figure 4. Table 1. Experimental conditions of the magnetic abrasive finishing (MAF) process. Table 1. Experimental conditions of the magnetic abrasive finishing (MAF) process. × × WorkpieceWorkpiece SUS304SUS304 Plane Plane (100 (100 × 100100 × 1 mm)mm) Original Roughness Ra 0.16~0.2 µm Original Roughness Ra 0.16~0.2 μm Electrolytic iron powder: 75 µm, 149 µm, 330 µm Electrolytic iron powder: 75 μm, 149 μm, 330 μm Mixed Magnetic Abrasive WA particles: #4000, #6000, #8000, #10000 Mixed Magnetic Abrasive WA particles:Oily #4000, grinding #6000, fluid #8000, #10000 Working Gap Oily grinding1 mm, 2 mm fluid FeedingWorking Speed Gap of Stage 1 mm,5 mm/s 2 mm FeedingRotational Speed Speed of Stage of Tool 2305 rpm, mm/s 450 rpm RotationalFinishing Speed Timeof Tool 230 rpm,60 450 min rpm Finishing Time 60 min

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FigureFigure 4. 4. ObservedObserved locations locations of of surface surface specimen. specimen. Figure 4. Observed locations of surface specimen. 4.2.4.2. Experimental Experimental Results Results of of the the MAF MAF Process 4.2. Experimental Results of the MAF Process Figure 5a–c respectively shows the change in surface roughness Ra under the conditions of the Figure5a–c respectively shows the change in surface roughness Ra under the conditions of the iron powderFigure 5a–cwith respectively mean diameters shows of the 330 change μm, 149in surface μm, 75 roughness μm mixed Ra underwith different the conditions sizes ofof WAthe iron powder with mean diameters of 330 µm, 149 µm, 75 µm mixed with different sizes of WA particles. particles.iron powder Through with respectivelymean diameters comparing of 330 μthem, experimental149 μm, 75 μ resultsm mixed in withFigure different 5a–c, it sizes can beof WAseen Through respectively comparing the experimental results in Figure5a–c, it can be seen that the optimal thatparticles. the optimal Through surface respectively roughness comparing can be obtainedthe experimental under the results conditions in Figure of 330-5a–c,μm it ironcan bepowder seen surface roughness can be obtained under the conditions of 330-µm iron powder mixed with #4000 WA mixedthat the with optimal #4000 surfaceWA particles, roughness 149- canμm beiron obtained powder under mixed the with conditions #8000 WA of 330-particles,μm iron and powder 75-μm particles, 149-µm iron powder mixed with #8000 WA particles, and 75-µm iron powder mixed with ironmixed powder with mixed#4000 WAwith particles, #10000 WA 149- particles.μm iron powderThen, the mixed best withexperimental #8000 WA results particles, under and different 75-μm #10000 WA particles. Then, the best experimental results under different diameters of iron powder diametersiron powder of iron mixed powder with #10000conditions WA wereparticles. compar Thened,, theas shownbest experimental in Figure 5d. results The undercombination different of conditions were compared, as shown in Figure5d. The combination of 149- µm iron powder mixed 149-diametersμm iron of powderiron powder mixed conditions with #8000 were WA compar particed,les as can shown be consideredin Figure 5d. as Thethe combinationoptimal mixed of with #8000 WA particles can be considered as the optimal mixed magnetic abrasive by comparing the magnetic149-μm abrasiveiron powder by comparing mixed with the #8000best experimental WA particles results can beshown considered in Figure as 5d.the optimal mixed bestmagnetic experimental abrasive results by comparing shown in the Figure best 5experimentald. results shown in Figure 5d.

(a(a) ) (b)

Figure 5. Cont.

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((cc)) ((dd))

Figure 5. Change in surface roughness Ra under the conditions of iron powder with different FigureFigure 5. 5.Change Change in surfacein surface roughness roughnessR under Ra under the conditions the conditions of iron of powder iron withpowder different with diametersdifferent diameters mixed with different sizes ofa WA abrasive; comparison of the best experimental results mixeddiameters with mixed different with sizes different of WA sizes abrasive; of WA comparison abrasive; comparison of the best of experimental the best experimental results under results the under the conditions of iron powder with different diameters: (a) iron powder with a mean diameter conditionsunder the ofconditions iron powder of iron with powd differenter with diameters: different diameters: (a) iron powder (a) iron with powder a mean with diameter a mean ofdiameter 330 µm of 330 μm mixed with different sizes of abrasive grains; (b) iron powder with a mean diameter of 149 mixedof 330 with μm mixed different with sizes different of abrasive sizes of grains; abrasive (b) irongrains; powder (b) iron with powder a mean with diameter a mean of diameter 149 µm of mixed 149 μm mixed with different sizes of abrasive grains; (c) iron powder with a mean diameter of 75 μm withμm differentmixed with sizes different of abrasive sizes grains;of abrasive (c) iron grains; powder (c) iron with powder a mean with diameter a mean of diameter 75 µm mixed of 75 withμm mixed with different sizes of abrasive grains; (d) comparison of best experimental results under differentmixed with sizes different of abrasive sizes grains; of abrasive (d) comparison grains; of(d best) comparison experimental of best results experimental under different results diameters under differentdifferent diameters diameters of of iron iron powder. powder. of iron powder.

FigureFigure 6 6 shows shows the the effectseffects ofof thethe machiningmachining tool’s different rotationalrotational speedsspeeds onon thethe changechange in in Figure6 shows the effects of the machining tool’s different rotational speeds on the change in surfacesurface roughness roughness RRa a andand materialmaterial removalremoval MM inin thethe 60-min MAF process.process. ItIt cancan bebe seenseen thatthat thethe surface roughness R and material removal M in the 60-min MAF process. It can be seen that the surfacesurface roughness roughness R Raa aat at 450 450 rpmrpm rotationalrotational speedspeed waswas obviously smallersmaller thanthan thethe surfacesurface roughness roughness R R surfaceRRa aat at 230 230 roughness rpm rpm rotational rotationala at 450 speedspeed rpm in rotationalin thethe 60-min60-min speed MAFMAF was process. obviously In smalleraddition, than thethe the materialmaterial surface removalremoval roughness M M at ata at450450 230 rpm rpm rpm rotational rotational rotational speedspeed was inwas the alsoalso 60-min muchmuch MAF moremore process. thanthan the In addition,material theremovalremoval material MM at removalat 230230 rpmrpmM atrotationalrotational 450 rpm rotationalspeed.speed. speed was also much more than the material removal M at 230 rpm rotational speed.

Figure 6. Change in surface roughness Ra and material removal M at the machining tool’s different Figure 6.6. ChangeChange in surfacesurface roughnessroughness RRa and material removal M at thethe machiningmachining tool’s different rotational speeds in the 60-min MAF process.a rotational speedsspeeds inin thethe 60-min60-min MAFMAF process.process.

TheThe macroscopic macroscopic confocal confocal imagesimages ofof thethe finished surface at the 60-min60-min MAFMAF processprocess under under the the conditionsThe macroscopic of various confocalrotational images speeds ofof thethe finishedmachining surface tool are at theshown 60-min in Figure MAF 7. process It can be under clearly the conditions of various rotational speeds of the machining tool are shown in Figure 7. It can be clearly conditionsseen that the of variouscolor distribution rotational of speeds the finished of the machining surface was tool mainly are shown green, inyellow, Figure and7. It red can at be 450 clearly rpm seen that the color distribution of the finished surface was mainly green, yellow, and red at 450 rpm seenrotational that the speed, color while distribution it was mainly of the finishedgreen, yellow, surface red, was and mainly blue at green, 230 rpm yellow, rotational and red speed. at 450 Thus, rpm rotational speed, while it was mainly green, yellow, red, and blue at 230 rpm rotational speed. Thus, rotationalit was regarded speed, whilethat the it was finished mainly surface green, accuracy yellow, red, at 450 and rpm blue rotational at 230 rpm speed rotational was slightly speed. Thus, better it it was regarded that the finished surface accuracy at 450 rpm rotational speed was slightly better wasthan regarded the finished that surface the finished accuracy surface at 230 accuracy rpm rotational at 450 rpm speed. rotational Moreov speeder, it wascan be slightly clearly better seen that than than the finished surface accuracy at 230 rpm rotational speed. Moreover, it can be clearly seen that the initial hairlines of the workpiece surface have almost completely been removed at both 450 rpm the initial hairlines of the workpiece surface have almost completely been removed at both 450 rpm

J. Manuf. Mater. Process. 2018, 2, 41 7 of 13 the ﬁnished surface accuracy at 230 rpm rotational speed. Moreover, it can be clearly seen that the J. Manuf. Mater. Process. 2018, 2, x FOR PEER REVIEW 7 of 13 initialJ. Manuf. hairlines Mater. Process. of the 2018 workpiece, 2, x FOR PEER surface REVIEW have almost completely been removed at both 450 rpm 7 andof 13 230and rpm230 rotationalrpm rotational speed speed conditions. conditions. However, However, a few scratcheda few scratched hairlines hairlines remained remained on the ﬁnished on the surfacefinishedand 230 in surfacerpm the 230rotational in rpm the rotational 230 speed rpm conditions. speed rotational case. Thus,However,speed it case. can a be Thus,few recognized scratched it can thatbe hairlines recognized a higher remained surface that a accuracy onhigher the cansurfacefinished be obtained accuracy surface at canin relatively the be obtained230 higherrpm atrotational rotationalrelatively speed speedhigher case. conditions.rotational Thus, speed it can conditions. be recognized that a higher surface accuracy can be obtained at relatively higher rotational speed conditions.

Figure 7. Macroscopic confocal images of the finished surface after the 60-min MAF process under Figure 7. Macroscopic confocal images of the ﬁnished surface after the 60-min MAF process under the theFigure conditions 7. Macroscopic of various confocal rotation imagesal speeds of theof the finished machining surface tool. after the 60-min MAF process under conditions of various rotational speeds of the machining tool. the conditions of various rotational speeds of the machining tool.

Figure 8 shows the change in surface roughness Ra and material removal M under various Figure8 8 shows shows thethe change change in in surface surface roughness roughness RRaa and materialmaterial removal MM underunder variousvarious working gaps. It was recognized that the surface roughness Ra under a 1-mm working gap was workingworking gaps.gaps. It was recognized that the surface roughness Ra under a 1-mm working gap waswas remarkably smaller than the surface roughness Ra under a 2-mm working gap in the 60-min MAF remarkablyprocess.remarkably Furthermore, smaller than the thethe material surfacesurface removal roughnessroughness M under Raa under the a1-mm 2-mm working working gap gap was in thealso 60-min60-min much MAFmoreMAF process.thanprocess. the Furthermore,materialFurthermore, removal the the material Mmaterial under removal theremoval 2-mmM M underworking under the thegap. 1-mm 1-mm working working gap gap was was also also much much more more than thethan material the material removal removalM under M under the 2-mm the 2-mm working working gap. gap.

Figure 8. Change in surface roughness Ra and material removal M under different working gaps in Figure 8. Change in surface roughness Ra and material removal M under different working gaps in Figurethe 60-min 8. Change MAF process. in surface roughness Ra and material removal M under different working gaps in the 60-minthe 60-min MAF MAF process. process. The macroscopic confocal images of the finished surface after the 60-min MAF process under The macroscopic confocal images of the finished surface after the 60-min MAF process under the conditionsThe macroscopic of different confocal working images gaps of the are finished shownsurface in Figure after 9. theThrough 60-min comparing MAF process the under finished the the conditions of different working gaps are shown in Figure 9. Through comparing the finished conditionssurface profile of different after the working 60-min MAF gaps areprocess shown under in Figure the conditions9. Through of comparing different working the finished gaps, surface it was surface profile after the 60-min MAF process under the conditions of different working gaps, it was profilerecognized after that the 60-minthe finished MAF processsurface underaccuracy the at conditions a 1-mm working of different gap working was obviously gaps, it wasbetter recognized than the recognized that the finished surface accuracy at a 1-mm working gap was obviously better than the thatfinished the finishedsurface accuracy surface accuracyat a 2-mm at working a 1-mm gap. working Furthermore, gap was it obviously can be clearly better seen than that the the finished initial finished surface accuracy at a 2-mm working gap. Furthermore, it can be clearly seen that the initial surfacehairlines accuracy of the finished at a 2-mm surface working were gap. almost Furthermore, completely it can removed be clearly in seenthe 1-mm that the working initial hairlines gap case. of hairlines of the finished surface were almost completely removed in the 1-mm working gap case. theHowever, finished many surface initial were hairlines almost completelystill existed removed on the finished in the 1-mm surface working in the gap2-mm case. working However, gap manycase. Thus,However, it can many be recognizedinitial hairlines that stilla higher existed surface on the accuracyfinished surfacecan be inobtained the 2-mm at workingrelatively gap smaller case. workingThus, it gapcan conditions.be recognized that a higher surface accuracy can be obtained at relatively smaller working gap conditions.

J. Manuf. Mater. Process. 2018, 2, 41 8 of 13 initial hairlines still existed on the ﬁnished surface in the 2-mm working gap case. Thus, it can be recognizedJ. Manuf. Mater. that Process. a higher 2018, 2 surface, x FOR PEER accuracy REVIEW can be obtained at relatively smaller working gap conditions. 8 of 13

Figure 9. Macroscopic confocal images of the finished surface under the conditions of various Figure 9. Macroscopic confocal images of the ﬁnished surface under the conditions of various working working gaps after the 60-min MAF process. gaps after the 60-min MAF process.

4.3. Experimental Procedure and Conditions of the EMAF Process 4.3. Experimental Procedure and Conditions of the EMAF Process According to the investigated mechanical machining characteristics of the compound According to the investigated mechanical machining characteristics of the compound machining machining tool, it can be regarded that the effect of the rotational speed and working gap on the tool, it can be regarded that the effect of the rotational speed and working gap on the mechanical mechanical machining characteristics are very obvious in the MAF process. Based on the machining characteristics are very obvious in the MAF process. Based on the experimental results of experimental results of the MAF process, the detailed experimental conditions of the EMAF process the MAF process, the detailed experimental conditions of the EMAF process were decided and are were decided and are shown in Table 2. The original surface roughness Ra of the workpiece is shown in Table2. The original surface roughness Ra of the workpiece is measured to be approximately measured to be approximately 0.16~0.2 μm. The mixed magnetic abrasive consists of quantitative 0.16~0.2 µm. The mixed magnetic abrasive consists of quantitative electrolytic iron powder with electrolytic iron powder with a mean diameter of 149 μm and quantitative #8000 WA particles. The a mean diameter of 149 µm and quantitative #8000 WA particles. The working gap is adjusted to working gap is adjusted to 1 mm, the rotational speed of compound machining tool is selected at 1 mm, the rotational speed of compound machining tool is selected at 450 rpm, and the feeding speed 450 rpm, and the feeding speed of the X-Y stage is adjusted to 5 mm/s. The working voltage is of the X-Y stage is adjusted to 5 mm/s. The working voltage is selected to be 12 V. The electrolyte selected to be 12 V. The electrolyte concentration is selected as 20 wt % NaNO3 electrolyte solution. concentration is selected as 20 wt % NaNO electrolyte solution. The total finishing time of the EMAF The total finishing time of the EMAF process3 is limited to 30 min. This total finishing time is a process is limited to 30 min. This total finishing time is a combination of the 4-min EMAF step and combination of the 4-min EMAF step and 26-min MAF step. The finished surface is respectively 26-min MAF step. The finished surface is respectively measured and observed after the 4-min EMAF measured and observed after the 4-min EMAF step, each 10-min MAF step, and the remaining step, each 10-min MAF step, and the remaining 6-min MAF step. 6-min MAF step. Table 2. Experimental conditions of the EMAF process. Table 2. Experimental conditions of the EMAF process. WorkpieceWorkpiece SUS304SUS304 plane plane (100 (100 ×× 100100 ×× 11 mm) mm) Original Roughness Ra 0.16~0.2 µm Original Roughness Ra 0.16~0.2 μm µ ElectrolyticElectrolytic iron iron powder: powder: 149 149 μmm Mixed Magnetic Abrasive Mixed Magnetic Abrasive WAWA particles: particles: #8000 #8000 Oily grinding fluid Oily grinding fluid Working Gap 1 mm Working Gap 1 mm Feeding Speed of Stage 5 mm/s Feeding Speed of Stage 5 mm/s Rotational Speed of Tool 450 rpm Rotational Speed of Tool 450 rpm Working Voltage 12 V Working Voltage 12 V Electrolyte Concentration NaNO3 electrolyte solution, 20 wt % Electrolyte Concentration NaNO3 electrolyte solution, 20 wt % Finishing Time (30 min) 4-min EMAF step + 26-min MAF step Finishing Time (30 min) 4-min EMAF step + 26-min MAF step 4.4. Experimental Results and Discussions of the EMAF Process 4.4. Experimental Results and Discussions of the EMAF Process Figure 10 shows the change in surface roughness Ra and material removal M under the condition Figure 10 shows the change in surface roughness Ra and material removal M under the of the combination of the 4-min EMAF step and 26-min MAF step. It can be noted that the surface condition of the combination of the 4-min EMAF step and 26-min MAF step. It can be noted that the roughness Ra drastically decreased in the EMAF step, while the material removal M in the EMAF surface roughness Ra drastically decreased in the EMAF step, while the material removal M in the step was obviously more than the material removal M in the MAF step. The material removal M was EMAF step was obviously more than the material removal M in the MAF step. The material removal M was approximately 7.5 g in the 4-min EMAF step; the material removal M was less than 5 g in the 26-min MAF step. In other words, the material removal rate in the 4-min EMAF process was nearly 9.75 times that in the 26-min MAF step. Furthermore, the surface roughness Ra could be reduced to less than 30 nm at the 4-min EMAF step. Then, it could be reduced to 20 nm at the

J. Manuf. Mater. Process. 2018, 2, 41 9 of 13 approximately 7.5 g in the 4-min EMAF step; the material removal M was less than 5 g in the 26-min MAF step. In other words, the material removal rate in the 4-min EMAF process was nearly 9.75 times R thatJ. Manuf. in theMater. 26-min Process. MAF 2018, 2 step., x FOR Furthermore, PEER REVIEW the surface roughness a could be reduced to less 9 than of 13 30J. Manuf. nm at Mater. the 4-minProcess.EMAF 2018, 2, step.x FOR Then,PEER REVIEW it could be reduced to 20 nm at the 10-min MAF step, after which 9 of 13 there10-min was MAF little step, change after to which the surface there was roughness little changeRa. Therefore, to the surface it was roughness found that R thea. Therefore, optimal surface it was accuracy10-minfound thatMAF can the step, be optimal obtained after whichsurface via the there accuracy 14-min was EMAF littlecan bechange process obtained tounder the via surface the condition14-min roughness EMAF of theR a.process Therefore, combination under it was ofthe a 4-minfoundcondition EMAFthat of the the step optimal combination and a surface 26-min of MAFaccuracya 4-min step. EMAF can be step obtained and a 26-minvia the MAF14-min step. EMAF process under the condition of the combination of a 4-min EMAF step and a 26-min MAF step.

Figure 10. Change in surface roughness Ra and material removal M under the condition of the Figure 10. Change in surface roughness Ra and material removal M under the condition of the Figure 10. Change in surface roughness Ra and material removal M under the condition of the combinationcombination ofof thethe 4-min4-min EMAFEMAF stepstep andand 26-min26-min MAFMAF step.step. combination of the 4-min EMAF step and 26-min MAF step. The non-contact 3D measurements of the original surface and finished surface after different The non-contact 3D measurements of the original surface and finished surface after different finishingThe non-contactsteps under 3Dthe measurements condition of the of combinthe originational surfaceof the 4-minand finished EMAF stepsurface and after 26-min different MAF finishing steps under the condition of the combination of the 4-min EMAF step and 26-min MAF step finishingstep are shownsteps under in Figure the condition 11. Through of the the combin macrosationcopic of theconfocal 4-min image EMAF (a), step many and 26-mindense initialMAF are shown in Figure 11. Through the macroscopic confocal image (a), many dense initial hairlines can stephairlines are showncan be inclearly Figure seen 11. onThrough the workpiece the macros surfacecopic beforeconfocal finishing. image (a),It can many be seendense that initial the be clearly seen on the workpiece surface before finishing. It can be seen that the protruding part of the hairlinesprotruding can part be ofclearly the original seen on surface the workpiece can be rapidly surface removed before afterfinishing. the EMAF It can step,be seen according that the to original surface can be rapidly removed after the EMAF step, according to the macroscopic confocal protrudingthe macroscopic part ofconfocal the original image surface (b). Yet, can some be rapidly deep concaveremoved hairlines after the wereEMAF still step, retained according on the to image (b). Yet, some deep concave hairlines were still retained on the finished surface after the EMAF thefinished macroscopic surface confocalafter the imageEMAF (b). step. Yet, Then, some itdeep was concavefound thathairlines the depth were stillof residual retained concave on the step. Then, it was found that the depth of residual concave hairlines obviously became shallow after finishedhairlines surfaceobviously after became the EMAF shallow step. after Then, the itMAF was step, found as thatshown the in depth the macroscopicof residual confocalconcave the MAF step, as shown in the macroscopic confocal image (c). Yet, the number of scratched hairlines hairlinesimage (c). obviously Yet, the number became of shallow scratched after hairlines the MAF on the step, finished as shown surface in slightlythe macroscopic increased confocalafter the on the finished surface slightly increased after the MAF step. This is because the finished surface was imageMAF step. (c). Yet, This the is becausenumber the of scratchedfinished surface hairlines wa ons excessively the finished polished surface by slightly the magnetic increased brush. after the excessivelyMAF step. This polished is because by the the magnetic finished brush. surface was excessively polished by the magnetic brush.

Figure 11. Non-contact three-dimensional (3D) measurement of the unfinished surface and finished Figuresurface 11. after Non-contact different finishing three-dimensional steps under (3D) the measurem measurement conditionent of of the the combination unfinishedunfinished surfaceof the 4-min and finishedfinished EMAF surfacestep and afterafter 26-min differentdifferent MAF finishing step.finishing steps steps under under the the condition condition of theof the combination combination of the of 4-minthe 4-min EMAF EMAF step andstep 26-minand 26-min MAF MAF step. step. SEM photographs of the original surface before the EMAF process are shown in Figure 12a. The SEMinitial photographs hairline of theof theunfinished original surfacesurface canbefore be theclearly EMAF seen process through are the shown SEM in photographs. Figure 12a. TheFigure initial 12b hairlineshows an of SEMthe unfinished photograph surface of the canfinis behed clearly surface seen after through the 4-min the EMAFSEM photographs. step. It was Figurefound that12b showsa small an amount SEM photographof micropores of stillthe finisexistedhed onsurface the finished after the surface. 4-min Moreover,EMAF step. this It wasalso foundindicated that that a small the action amount of theof microporesMAF process still canno existted completely on the finished remove surface. the passive Moreover, films generated this also indicatedby the electrolytic that the actionprocess. of Anthe SEMMAF photograph process canno of tthe completely finished surfaceremove after the passivethe 26-min films MAF generated step is byshown the electrolytic in Figure 12c.process. Compared An SEM with photograph the original of the surface finished of the surface workpiece, after the it 26-mincan be MAFclearly step seen is shownthat the in initial Figure hairline 12c. Compared of the surface with was the almostoriginal completely surface of removed.the workpiece, Furthermore, it can be it clearly also can seen be that the initial hairline of the surface was almost completely removed. Furthermore, it also can be

J. Manuf. Mater. Process. 2018, 2, 41 10 of 13

SEM photographs of the original surface before the EMAF process are shown in Figure 12a. The initial hairline of the unfinished surface can be clearly seen through the SEM photographs. Figure 12b shows an SEM photograph of the finished surface after the 4-min EMAF step. It was found that a small amount of micropores still existed on the finished surface. Moreover, this also indicated that the action of the MAF process cannot completely remove the passive films generated by the electrolyticJ. Manuf. Mater. process. Process. 2018 An, SEM2, x FOR photograph PEER REVIEW ofthe finished surface after the 26-min MAF step is shown 10 of in13 Figure 12c. Compared with the original surface of the workpiece, it can be clearly seen that the initial hairlineconfirmed of thethat surface the MAF was step almost plays completely an essential removed. role in Furthermore,achieving precision it also canmachining be confirmed in the that EMAF the MAFprocess. step plays an essential role in achieving precision machining in the EMAF process.

Figure 12. SEM images of the original surface and finished surface after different finishing steps. Figure 12. SEM images of the original surface and ﬁnished surface after different ﬁnishing steps.

4.5. Discussions 4.5. Discussions The magnetic brush is formed on a magnetic pole through mixed magnetic abrasive orderly The magnetic brush is formed on a magnetic pole through mixed magnetic abrasive orderly arrangements in the direction of the magnetic field lines. A single magnetic particle along the arrangements in the direction of the magnetic field lines. A single magnetic particle along the magnetic magnetic equipotential line direction generates a force Fx and along the magnetic force line equipotential line direction generates a force Fx and along the magnetic force line direction generates a direction generates a force Fy, which are calculated by Equations (1) and (2) as follows: force Fy, which are calculated by Equations (1) and (2) as follows: Fx = КD3χμ0H(∂H⁄ ∂x) (1) 3 Fx = KD χµ0H(∂H/∂x) (1) Fy = КD3χμ0H(∂H⁄ ∂y) (2) 3 Fy = KD χµ0H(∂H/∂y) (2) where К is the correction coefficient of the volume, D is the diameter of the magnetic particle, χ is wherethe susceptibilityK is the correction of the magnetic coefficient particle, of the μ volume,0 is the permeabilityD is the diameter of the of vacuum, the magnetic H is the particle, magneticχ is thefield susceptibility intensity, and of ∂H the⁄ ∂x magnetic and ∂H particle,⁄ ∂y are µthe0 is gradients the permeability of the magn of theetic vacuum, field intensityH is the in magneticthe x and fieldy directions, intensity, respectively. and ∂H/∂x andThe ∂magneticH/∂y are field thegradients intensity ofH theas an magnetic important field parameter intensity infor the magneticx and y directions,force Fx and respectively. Fy, and can be The calculated magnetic by field the intensity followingH equation:as an important parameter for magnetic force Fx and Fy, and can be calculated by the following equation: H = B/μ (3) where B is the magnetic flux density and μ isH the= B/ magneticµ permeability of the medium. It is not(3) difficult to see that the magnetic intensity B directly affect the magnetic force. Therefore, different whereshapes Bofis machining the magnetic tools flux have density different and mechanicalµ is the magnetic finishing permeability characteristics. of the medium. It is not difficultAccording to see thatto Equation the magnetic (3), it intensity can be seenB directly that magnetic affect the field magnetic strength force. H is Therefore,proportional different to the shapesmagnetic of flux machining density tools B. Figure have different13 shows mechanical the analysis finishing of the magn characteristics.etic flux density of the electrolytic magneticAccording compound to Equation machining (3), it tool can beand seen traditional that magnetic magnetic field machining strength H tool,is proportional conducted tousing the magneticMagnet Version flux density 7 software.B. Figure The 13 maximum shows the element analysis size of theof the magnetic mesh was flux set density to 0.001 of the m. electrolyticFigure 13a magneticshows that compound the magnetic machining flux density tool and of traditional the electrolytic magnetic magnetic machining compou tool, conductednd machining using tool Magnet was Versionrelatively 7 software.strong at Thethe magnetic maximum poles; element the sizemagnetic of the flux mesh density was set at tothe 0.001 magnetic m. Figure pole 13 edgea shows was thatapproximately the magnetic 1 T flux and density the magnetic of the electrolytic flux density magnetic at the magnetic compound pole machining center was tool approximately was relatively strong0.5 T. Additionally, at the magnetic the poles; magnetic the magnetic flux density flux gradually density at weakened the magnetic farther pole away edge wasfrom approximately the magnetic 1poles. T and Figure the magnetic 13b shows flux densitythat the at magnetic the magnetic flux pole density center (approximately was approximately 1 T) 0.5of T.the Additionally, traditional themagnetic magnetic machining flux density tool graduallywas relatively weakened strong farther on awaythe edge from of the the magnetic magnetic poles. pole, Figure whit 13 itb shows(approximately that the magnetic 0.7 T) was flux densityrelatively (approximately weak at the 1 center T) of the of traditional the magnetic magnetic pole. machining Figure 13c,d tool respectively shows the analysis of the magnetic flux density with mesh grid for the electrolytic magnetic compound machining tool and traditional magnetic machining tool. It was noted that the overall magnetic field strength of the electrolytic magnetic compound tool was obviously smaller than that of the traditional magnetic machining tool by comparing analytical results. In other words, it can be regarded that the mechanical finishing characteristics of the electrolytic magnetic compound tool were worse than those of the traditional magnetic machining tool.

J. Manuf. Mater. Process. 2018, 2, 41 11 of 13 was relatively strong on the edge of the magnetic pole, whit it (approximately 0.7 T) was relatively weak at the center of the magnetic pole. Figure 13c,d respectively shows the analysis of the magnetic flux density with mesh grid for the electrolytic magnetic compound machining tool and traditional magnetic machining tool. It was noted that the overall magnetic field strength of the electrolytic magnetic compound tool was obviously smaller than that of the traditional magnetic machining tool by comparing analytical results. In other words, it can be regarded that the mechanical finishing characteristics of the electrolytic magnetic compound tool were worse than those of the traditional J.magnetic Manuf. Mater. machining Process. 2018 tool., 2, x FOR PEER REVIEW 11 of 13

(a) (b)

Figure 13. Using Magnet Version 7 software to analyze thethe magnetic fluxflux density of the electrolytic magnetic compoundcompound machining machining tool tool and and traditional traditional magnetic magnetic machining machining tool: (tool:a) magnetic (a) magnetic flux density flux densityof electrolytic of electrolytic magnetic magnetic compound compound machining machining tool; (b) magnetictool; (b) magnetic flux density flux of density traditional of traditional magnetic magneticmachining machining tool; (c) magnetic tool; flux(c) magnetic density with flux mesh density grid forwith electrolytic mesh grid magnetic for electrolytic compound machiningmagnetic compoundtool; (d) magnetic machining flux densitytool; (d with) magnetic mesh grid flux for density traditional with magnetic mesh grid machining for traditional tool. magnetic machining tool. Though the overall magnetic field strength of the electrolytic magnetic compound tool was Though the overall magnetic field strength of the electrolytic magnetic compound tool was obviously smaller than that of the traditional magnetic machining tool, both the surface quality obviously smaller than that of the traditional magnetic machining tool, both the surface quality and and material removal rate were very significant when using the electrolytic magnetic compound material removal rate were very significant when using the electrolytic magnetic compound machining tool in the EAMF step. This is because the generated electrolytic action by the electrode machining tool in the EAMF step. This is because the generated electrolytic action by the electrode of the compound machining tool effectively made up for the lack of magnetic force generated at the of the compound machining tool effectively made up for the lack of magnetic force generated at the magnetic poles. Yet, the surface roughness reached a minimum when the finishing time was 14 min for magnetic poles. Yet, the surface roughness reached a minimum when the finishing time was 14 min the EMAF process. This is probably due to the effects of the electrolytic process and magnetic abrasive for the EMAF process. This is probably due to the effects of the electrolytic process and magnetic that reached the finishing balance. Hence, the surface roughness no longer declined. Since a large abrasive that reached the finishing balance. Hence, the surface roughness no longer declined. Since amount of metal ions dissociated from the surface of the workpiece by the electrolytic reactions in a large amount of metal ions dissociated from the surface of the workpiece by the electrolytic the EMAF step, the material removal rate in the 4-min EMAF step was higher than that in the 26-min reactions in the EMAF step, the material removal rate in the 4-min EMAF step was higher than that MAF step. in the 26-min MAF step.

5. Conclusions This paper mainly reported the investigation of the mechanical finishing characteristics of the electrolytic magnetic compound machining tool and the experimental results of the EMAF process under the condition of the combination of a 4-min EMAF step and a 26-min MAF step. The main conclusions can be summarized as follows: 1. The mechanical finishing characteristics of the electrolytic magnetic compound machining tool were determined through MAF process experiments and the analysis of the magnetic field strength. The experimental results of the MAF process show that a smaller surface roughness Ra and more material removal M can be obtained at a relatively higher rotational speed (450 rpm) and a relatively smaller working gap (1 mm) for the compound machining tool.

J. Manuf. Mater. Process. 2018, 2, 41 12 of 13

5. Conclusions This paper mainly reported the investigation of the mechanical finishing characteristics of the electrolytic magnetic compound machining tool and the experimental results of the EMAF process under the condition of the combination of a 4-min EMAF step and a 26-min MAF step. The main conclusions can be summarized as follows: 1. The mechanical finishing characteristics of the electrolytic magnetic compound machining tool were determined through MAF process experiments and the analysis of the magnetic field strength. The experimental results of the MAF process show that a smaller surface roughness Ra and more material removal M can be obtained at a relatively higher rotational speed (450 rpm) and a relatively smaller working gap (1 mm) for the compound machining tool. Furthermore, the combination of 149-µm iron powder and #8000 WA particles can be regarded as the optimal mixed magnetic abrasive for the compound machining tool. 2. Through the experimental results of the EMAF process under the condition of the combination of a 4-min EMAF step and a 26-min MAF step, it can be confirmed that the surface roughness Ra drastically decreased in the EMAF step; the material removal M in the EMAF step was nearly 9.75 times that in the MAF step. Moreover, it was also recognized that the surface roughness Ra can be reduced to less than 30 nm by the 4-min EMAF step. Then, the surface roughness Ra can be reduced to 20 nm by the 10-min MAF step. In other words, the surface roughness Ra can reach 20 nm from the original surface roughness Ra of 178 nm after the 14-min EMAF process. 3. Since the overall magnetic field strength of the electrolytic magnetic compound tool was obviously smaller than that of the traditional magnetic machining tool, the mechanical finishing characteristics of the developed electrolytic magnetic compound machining tool were worse than those of the traditional magnetic machining tool. However, the electrolytic action generated by the electrode of the compound machining tool was effectively able to make up for the lack of magnetic force generated at the magnetic poles.

Author Contributions: Y.Z. conceived of the presented idea. X.S. designed the experimental set-up, conducted the experiments and analyzed the magnetic flux density of the electrolytic magnetic compound machining tool and traditional magnetic machining tool. After that, X.S. also wrote the manuscript and responded the reviewer comments. The two authors provided critical feedback and helped shape the research and manuscript. Funding: This research received no external funding. Acknowledgments: We acknowledge valuable contributions and support from the Utsunomiya University Creative Department for Innovation (CDI). We also acknowledge partial contribution from graduates Takumi Kojo for this study. Conflicts of Interest: The authors declare no conflict of interest.

References

1. Shinmura, T.; Takazawa, K.; Hatano, E.; Matsunaga, M. Study on magnetic abrasive finishing. Ann. CIRP 1990, 39, 325–328. [CrossRef] 2. Ihar, I.; Nakano, E.; McLamore, E.; Schueller, J.K.; Toyoda, K.; Umetsu, K.; Yamaguchi, H. Cleanability of milk deposits on inner stainless steel tubing surfaces prepared by magnetic abrasive finishing. Int. J. Eng. Agric. Environ. Food 2017, 10, 63–68. [CrossRef] 3. Hashimoto, F.; Yamaguchi, H.; Krajnik, P.; Wegener, K.; Chaudhari, R.; Hoffmeister, H.W.; Kuster, F. Abrasive fine-finishing technology. CIRP Ann. Manuf. Technol. 2016, 65, 597–620. [CrossRef] 4. Zou, Y.H.; Jiao, A.Y.; Aizawa, T. Study on plane magnetic abrasive finishing process-experimental and theoretical analysis on polishing trajectory. Adv. Mater. Res. 2010, 126, 1023–1028. [CrossRef] 5. Martinez, S.; Lamikiz, A.; Ukar, E.; Calleja, A.; Arrizubieta, J.A.; Lopez de Lacalle, L.N. Analysis of the regimes in the scanner-based laser hardening process. Opt. Lasers Eng. 2017, 90, 72–80. [CrossRef] 6. Tabernero, I.; Lamikiz, A.; Ukar, E.; Lopez de Lacalle, L.N.; Angulo, C.; Urbikain, G. Numerical simulation and experimental validation of powder flux distribution in coaxial laser cladding. J. Mater. Process. Technol. 2010, 210, 2125–2134. [CrossRef] J. Manuf. Mater. Process. 2018, 2, 41 13 of 13

7. Lopez de Lacalle, L.N.; Rodriguez, A.; Lamikiz, A.; Calleja, A.; Alberdi, R. Five-Axis Machining and Burnishing of Complex Parts for the Improvement of Surface Roughness. Mater. Manuf. Process. 2011, 26, 997–1003. [CrossRef] 8. Sun, X.; Zou, Y.H. Development of magnetic abrasive finishing combined with electrolytic process for finishing SUS304 stainless steel plane. Int. J. Adv. Manuf. Technol. 2017, 92, 3373–3384. [CrossRef] 9. Ridha, M.M.; Zou, Y.H.; Sugiyama, H. Development of a new internal finishing of tube by magnetic abrasive finishing process combined with electrochemical machining. Int. J. Mech. Eng. Appl. 2015, 3, 22–29. 10. Sihag, N.; Kala, P.; Pandey, P.M. Chemo Assisted Magnetic Abrasive Finishing: Experimental Investigations. Procedia CIRP 2015, 26, 539–543. [CrossRef] 11. Judal, K.B.; Yadava, V. A study of electrochemical magnetic abrasive machining process. Int. J. Manuf. Technol. Manag. 2013, 27, 142–153. [CrossRef] 12. Liu, G.Y.; Guo, Z.N.; Jiang, S.Z.; Qu, N.S.; Li, Y.B. A study of processing Al 6061 with electrochemical magnetic abrasive finishing. Procedia CIRP 2014, 14, 234–238. [CrossRef] 13. Sihag, N.; Kala, P.; Pandey, P.M. Experimental investigations of chemo-ultrasonic assisted magnetic abrasive finishing process. Int. J. Precis. Technol. 2015, 5, 246–260. [CrossRef] 14. Pa, P.S. Super finishing with ultrasonic and magnetic assistance in electrochemical micro-machining. Electrochem. Acta 2009, 54, 6022–6027. [CrossRef] 15. Fang, J.; Jin, Z.; Xu, W.; Shi, Y. Magnetic electrochemical finishing machining. J. Mater. Process. Technol. 2002, 129, 283–287. [CrossRef] 16. Kim, J.D.; Xu, Y.M.; Kang, Y.H. Study on the characteristics of magneto-electrolytic-abrasive polishing by using the newly developed nonwoven-abrasive pads. Int. J. Mach. Tools Manuf. 1998, 38, 1031–1043. [CrossRef] 17. Strehblow, H.H. Passivity of metals. Adv. Electrochem. Sci. Eng. 2003, 8, 271–374. 18. Yasuo, K. Ultra-Precision Machining by Electrolytic Complex Method; Aipishi: Tokyo, Japan, 1994; pp. 111–116. (In Japanese)

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