Study on electrochemical effects assisted magnetic abrasive finishing for finishing stainless steel SUS304

In order to obtain a high accuracy with high machining efficiency for finishing hard alloy metal material, we proposed a hybrid finishing method which is electrochemical (ECM) effects assisted magnetic abrasive finishing (MAF). In this study, the electrochemical magnetic abrasive finishing (EMAF process) was divided into EMAF stage and MAF stage. The metal surface can be easily finished with the passive films formed in electrochemical reactions. Simultaneously, the passive films can be removed by frictional action between magnetic brush and workpiece surface. Thus, the essence of EMAF process is to form and remove the passive films on the workpiece surface. This study focused on investigating the finishing mechanism and finishing characteristics of EMAF process. Through a series of experimental investigations, it can be confirmed that the finishing efficiency is remarkably improved by EMAF process. The optimal experimental result of EMAF process showed that the surface roughness was reduced to less than 30 nm from the original surface roughness 178 nm at 4 min in EMAF stage, and the surface roughness was finally reduced to 20 nm at 10 min in MAF stage. Additionally, we also found the finishing ability of magnetic abrasive decreased after 4 min EMAF stage.

MAF process has many advantages, such as good flexibility, self-sharpening of magnetic abrasive particles, as well as easy controllability. Especially, MAF process also has penetrating characteristics under the action of magnetic field. Thus, MAF process can be used not only for finishing regular-shaped workpiece [6][7][8], but also for finishing complex-shaped workpiece [9][10][11]. MAF process is also suitable for finishing various materials such as resin, glass, ceramics, metal [12][13][14][15].
Since the working life of abrasive particles is short, if the abrasive particles can't be promptly replaced, the machining efficiency of MAF process will decrease when MAF process is used to finishing hard alloy metal material or non-metallic materials with high hardness. In order to achieve optimal machining effect, the researchers not only optimized and improved the parameters of magnetic abrasive finishing, but also engaged in the research of combining magnetic abrasive finishing with a variety of processes [16][17][18][19][20]. In these composite processing methods related to magnetic abrasive finishing, electrolytic magnetic abrasive finishing (EMAF) has been proven to be an efficient hybrid finishing method by some researchers. Among them, B.H.
Yan proposed using EMAF process to finishing outer surface of tube, the results showed that EMAF with a high electrolytic current has better finishing characteristics than MAF [21]. Zou et al. developed special compound machining tools for EMAF process of plane workpiece and internal tube [22][23][24][25]. The electrolytic effects and mechanical finishing can synchronously perform through the compound machining tools in EMAF process [32]. The experimental results showed that not only the higher accuracy surface can be obtained, but also the machining efficiency can be significantly improved by EMAF process. They also respectively investigated the optimal combination of finishing parameters for plane workpiece and internal tube through a series of experiments.
In this study, a hybrid finishing method that electrochemical (ECM) effects assisted magnetic abrasive finishing is proposed. The electrochemical magnetic abrasive finishing (EMAF process) includes two different finishing stages which are the 1st finishing stage (EMAF stage) and the 2nd finishing stage (MAF stage). The high efficiency finishing of workpiece surface is realized in EMAF stage. Then, MAF stage as the 2nd finishing stage plays a role in achieving high precision finishing. According to the research contents of EMAF process, the system model of EMAF process is summarized in Figure 1. Before finishing, the initial values of workpiece surface such as roughness, hardness, undulation, profile accuracy, chemical properties are measured.
It can be seen that the combinations of mixed magnetic abrasive, rotational speed of machining tool, feeding speed of X-Y stage, working gap, concentration of electrolyte and working voltage etc. machining influencing factors need to be emphatically investigated in EMAF process. Finally, the surface roughness, undulation and profile accuracy are evaluated. The surface roughness before finishing and after finishing is measured by a contact type roughness meter (Surf-test SV-624-3D, Mitutoyo). The profile accuracy before finishing and after finishing is evaluated through a 3D non-contact optical profiling microscope (Zygo, New View 7300). The surface morphology and residual passive films are observed through a scanning electron microscope (HITACHI S4500). EDX analysis of surface composition is measured by a mini-scope (TM3030Plus).

Basic machining principle of EMAF process
The basic machining principle of EMAF process is shown in Figure 2 (Ⅰ). Figure 2 (Ⅲ(a)) shows original surface of workpiece with a lot of protruding portions before finishing. The EMAF stage is shown in Figure 2 (Ⅲ(b)). The protruding portions of workpiece are preferentially dissolved and the workpiece surface is finitely leveled [26][27][28][29]. Moreover, the surface hardness can be also reduced with the passive films form on the workpiece surface by electrochemical effect. Meanwhile, using the magnetic abrasive particles of magnetic brush to exert frictional force on the surface of workpiece, the passive films can be easily and rapidly removed.
The high-efficiency machining can be realized in EMAF stage since the hardness of passive films is smaller than the hardness of metal material. Generally, the growth rate of passive films from electrochemical effect is faster than the removal rate of passive films from mechanical action in the hybrid finishing stage. The residual passive films may affect the surface accuracy of workpiece. Hence, MAF process as the final finishing stage is performed after EMAF stage in order to obtain a high-precision surface quality, as shown in Figure 2 (Ⅲ(c)).

Figure 2
Schematic of EMAF process, (Ⅰ) the basic machining principle of EMAF process (Ⅱ) the forming mechanism of passive films in ECM process (Ⅲ) the whole process that the workpiece is finished by EMAF process By the way, the forming mechanism of passive films in ECM process as shown in Figure 2 (Ⅱ) [30]. The NaNO3 solution is adopted as the electrolyte in this study. Under electrochemical effect, electrolyte solution takes place ionization reactions which are shown as Eq. (1) and (2).
H2O → H + + OH - The cations move toward the cathode, anions move toward the anode. According to ionization tendency of cations and discharge ability of anions, the reaction shown as Eq. (3) occurs at the cathode, the reaction shown as Eq. (4) occurs at the anode. Hence, H2 generate at the cathode and O2 generate at the anode [31].

Experimental conditions in ECM process
We focused on investigating the influence of working voltage on the surface accuracy. Table 1 showed the detailed experimental conditions of ECM process. The original surface roughness Ra of workpiece was measured at approximate 0.16 ~ 0.2 μm. The working gap was adjusted to 1 mm, the rotational speed of compound machining tool was selected at 450 rpm, and the feeding speed of X-Y stage was adjusted to 5 mm/s. The working voltage was respectively set at 8 V, 10 V and 12 V. The electrolyte concentration was selected as 20 wt%. The finishing time of ECM process was selected at 4 min.

Experimental conditions of EMAF process
According to the experimental investigations of ECM process and the previous report [32], the finishing parameters of EMAF process were determined and shown in Table 2. The total finishing time of EMAF process was selected at 30 min. We focused on investigating the combinations of the EMAF stage time and

Measurement methods
After each finishing stage, the workpiece is air-dried and measured removal weight. Then, the surface contact-type surface roughness is used to measure the average surface roughness at these three locations (A, B, C) as shown in Fig.4. In addition, the macroscopic confocal images of finished surface were observed by SEM and 3D non-contact optical profiling microscope to evaluate the surface quality.

Figure 4
Observed locations of surface specimen Due to the generated heat is too high at 12 V working voltage case, the electrochemical machining can't be performed exceed 6 min. Through quantitatively comparing the change in surface roughness and material removal under various voltage conditions, it was noted that the surface roughness at 12 V working voltage case was remarkably significantly smaller than that under 10 V and 8 V working voltage cases; the material removal at 12 V working voltage case was also significantly more than that under 10 V and 8 V working voltage cases. Thus, it can be regarded that the smaller surface roughness and more material removal can be obtained at relatively higher working voltage condition. We also found that the passive films generated on the finished surface after ECM process. In ECM process, the surface roughness can be limitedly reduced at more than 60nm, but due to the formation of passive films, the mirror-finishing of workpiece surface can't be completed.

Change in surface morphology under different working voltage conditions
The surface morphology was obtained by a scanning electron microscope (SEM), which was shown in Figure 6. It can be seen that these were only initial texture lines on the workpiece surface before ECM process through Figure 6

Analysis of EDS and chemical mapping
Finally, we also respectively measured elemental ingredients of workpiece surface before and after ECM process by a mini-scope. The elemental ingredients of original surface were shown in Figure 7 (a). Figure 7 (b), (c) and (d) respectively showed that the elemental ingredients of finished surface after ECM process under different working voltage conditions. The measured results proved that the content of "Fe", "Cr" elements after ECM process was less than the elemental content of original surface. Moreover, the content of "Fe", "Cr" elements also decreased with the working voltage increased. The metal cations dissolved out from SUS304 workpiece surface under the electrochemical effect. Hence, the content of "Fe", "Cr" elements decreased after ECM process. According to Faraday law, the dissolved amount of metal cations can be quantitatively calculated by the formula (5).
Where, "W" is elution amount, "I" is electrolysis current, "M" is atomic weight, "n" is atomic valence. The dissolved amount of metal ions is proportional to electrolysis current and machining time. Thereby, "Fe", "Cr" elements of workpiece surface dissolved more at higher working voltage.

Figure 7 EDS of original surface and finished surface at different working voltage cases
Since the major composition of SUS304 material is constituted by Fe (70%), Cr (20%), Ni (10%) elements, Figure 8 described the chemical mappings of these three representative elements under various working voltage conditions. Fe, Cr, Ni elements were respectively represented by red, yellow and green. The color of these three elements was unitary and evenly distributed before ECM process. The black dots which were substances with unknown elements formed on the measured region and the distributions of these three elements were non-uniform after ECM process. Additionally, the quantity and size of black dots increased with the working voltage increased. The measuring results revealed that the content of each element decreased with the unknown elements increased since the original color area of each element decreased in ECM process.
It was also seen that the brightness of these three elements dimmed with the working voltage increased. It was considered that the metal elements began completely to dissolve from workpiece surface and the elution amount of metal elements was the most under 12 V working voltage case.

Experimental results of EMAF process
The experiments of EMAF stage were firstly performed before researching the EMAF process. Figure 10 showed As the passive films accumulated on the workpiece surface, the material removal rate before 4 min was remarkably more than that after 4 min under the condition of 12 V working voltage. results, it can be noted that the surface roughness drastically decreased in EMAF stage, the material removal in EMAF stage was obviously more than that in MAF stage. The material removal was approximately 7 ~ 10 mg in EMAF stage, and that was approximately 4 ~ 6 mg in MAF stage. The material removal rate in EMAF stage was nearly 7 times than that in MAF stage. Furthermore, the optimal experimental result of EMAF process showed that the surface roughness was reduced to less than 30 nm at 4 min EMAF stage. Then, it was reduced to 20 nm at 10 min MAF stage. However, the surface roughness almost didn't change after 10 min MAF stage.
In other words, the optimal surface accuracy can be obtained at 14 min EMAF process under the combination condition of 4 min EMAF stage combined with 26 min MAF stage. As the electrochemical effect was not enough in 2 min EMAF stage, and the electrochemical effect was excessive in 6 min, 8 min and 10 min EMAF stage. The 3D morphology of original workpiece surface and finished surface after various finishing stages was shown in Figure 12. It was seen that the protruding part of original surface was rapidly removed after EMAF stage, as shown in Figure 12 (b). However, some deep concave hairlines still remained on the finished surface after EMAF stage. Finally, it can be found that the residual concave hairlines were almost completely eliminated after MAF stage through the obtained 3D morphology of Figure 12 Figure   13 (b). The residual passive films were completely removed after MAF stage. Hence, it can be confirmed that MAF stage plays an essential role to achieve precision machining in EMAF process.

Different of electrochemical effect in ECM process and EMAF stage
In ECM process, the deposits and gas were generated in the electrolyte with the electrochemical reaction occurred. The deposits appeared dark green in the early stage of ECM process, the color of deposits deepened with the machining time increased. Finally, the generated deposits turned brown as shown in Figure 14 (a). Fe, Cr and Ni etc. metal cations dissolved out from workpiece surface by the electrochemical effect. With the generated O2, Fe x+ from divalent ion was oxidized to trivalent ion, Cr x+ from trivalent ion changed to hexavalent ion. The dissolved metal cations reacted with the free OHto generate the deposits, as shown in formula (6) ~ (11).
(a) Workpiece sample in ECM process (b) Workpiece sample in EMAF stage

Figure 14
The SUS304 workpiece was machined in ECM process and EMAF stage

Different of electrolysis current in ECM process and EMAF stage
In order to explore the reason for the weakening of the electrochemical reaction in EMAF stage, we used the current recorder to measure separately electrolysis current in EMAF stage and ECM process, which was shown in Figure 15. When the working voltage was 12 V, it was seen that the amplitude of electrolysis current in ECM process was obviously bigger than that in EMAF stage. The fluctuation of electrolysis current was severe before 45s ECM process, it drastically decreased and tended to stabilize after 45s ECM process. The peak value of electrolysis current reached to 14.6A in ECM process. However, the peak value of electrolysis current was 3.9A in EMAF stage. The fluctuation of electrolysis current was stable at 1.5 ~ 1.8A from beginning to end. The resistivity ρ of electrolyte became larger in EMAF stage since the workpiece surface was covered by the abrasive slurry. The conductivity σ can be calculated by the Eq. (12).
Through the Eq. (12), it was known that the conductivity σ and resistivity ρ were inversely proportional. Thus, the conductivity σ was smaller in EMAF stage than that in ECM process. Due to the difference of electrolysis current in EMAF stage and ECM process, the degree of electrochemical reaction was different in these two processes.

Figure 15
Electrolysis current in two different processing Figure 16 showed the photographs of the unfinished surface and the finished surface with three different finishing methods which were respectively traditional MAF process, ECM process and EMAF process. It can be seen that the mirror effect of finished surface by ECM process was not well. Although the surface roughness can rapidly reach to below 100 nm in ECM process, the brightness of finished surface was not well since the passive films formed on the finished surface. Additionally, it was also regarded that the mirror finishing can be achieved by both traditional MAF process and EMAF process.

Mechanical behavior of magnetic abrasive in EMAF stage
Though both traditional MAF process and EMAF process can achieve nano-scale mirror finished, the effective finishing area was approximately 90 × 30 mm 2 in MAF process and 80 × 24 mm 2 in EMAF process.
The effective finishing area in EMAF process was about 70% of the effective finishing area in MAF process. abrasive which attached to the iron powder was also lost with the iron powder was consumed. Thus, the effective finishing area in EMAF process was smaller than that in MAF process. Furthermore, according to the aforementioned SEM images in ECM process from Figure 6 and change in size of mixed magnetic abrasive in EMAF stage from Figure 17, it was easy to understand why the optimal surface accuracy can be obtained under the combination condition of 4 min EMAF stage and 26 min MAF stage. Since the electrochemical effect was not enough in 2 min EMAF stage and the size of mixed magnetic abrasive drastically decreased in 6 min, 8 min, 10 min EMAF stage (the finishing capacity of mixed magnetic abrasive was almost ignored after 6 min EMAF stage), the finishing efficiency was low in 2 min and 6 min, 8 min, 10 min EMAF stage. However, 4 min EMAF stage was considered as the optimal finishing time for EMAF process. The generated passive films from ECM process can be maximally removed by the mechanical machining in 4 min EMAF stage.

Conclusions
This paper proposed a hybrid finishing method which is electrochemical magnetic abrasive finishing (EMAF process) to finishing SUS304 workpiece. The essence of EMAF process is to form and remove the passive films from electrochemical effect. The main conclusions were summarized as follows: 1. The EMAF process includes EMAF stage and MAF stage in this study. The high efficiency finishing is realized in EMAF stage since the passive films form under electrochemical effect. MAF stage as the finally finishing stage plays a role in achieving high precision finishing. The experimental result showed that the surface roughness was reduced to less than 30 nm at 4 min EMAF stage, and it reached to 20 nm at 10 min MAF stage.
2. It was clarified that the passive films generated and the surface hardness was decreased under the electrochemical effect in ECM process. Moreover, the brown deposits generated with the electrochemical reaction occurred in ECM process. It was proved that the content of "Fe", "Cr" elements on the workpiece surface was measured to decrease after ECM process through the EDS results.
3. After EMAF stage, a small amount of passive films still existed on the finished surface. This is because that the electrochemical effect was stronger than the mechanical action in EMAF stage.
4. As the size of mixed magnetic abrasive drastically decreased in EMAF stage, the effective finishing area in EMAF process was smaller than it in MAF process. Therefore, the larger size in iron powder was considered to use in future research.