Optimization of Process Parameters in Electropolishing of SS 316L Utilizing Taguchi Robust Design

Abstract

In electropolishing, the material removal rate is frequently neglected, as this process is primarily focused on surface finish, and yet, it is crucial for manufacturing metallic sheets. Solutions are required to enhance the material removal rate while maintaining surface quality. This work introduces an electropolishing technique that involves suspending ethanol in an electrolyte solution and employing a magnetic field during machining processes. The Taguchi approach is utilized to determine the ideal process parameters for enhancing the material removal rate of SS 316L electropolishing through a L9 orthogonal array. Pareto analysis of variance (ANOVA) is utilized to examine the four parameters of the machining process: applied voltage, ethanol concentration, machining gap variation, and the magnetic field of the electrolyte. The results demonstrate that the applied voltage, the incorporation of ethanol in electropolishing, and a reduced machining gap significantly increase the material removal rate; however, the introduction of a magnetic field did not notably increase the material removal rate.

1. Introduction

Medical-grade stainless steel 316L (SS 316L) is widely employed in numerous biomedical applications owing to its exceptional mechanical strength and cost-effectiveness [1,2]. Traditional machining of this material suffers from inadequate surface integrity due to elevated cutting pressures and temperatures at tool–work contact, resulting in significant tool wear [3,4]. This frequently leads to elevated machining expenses and subpar surface quality [5]. Non-conventional machining methods are regarded as superior for machining SS 316L [6].

A range of unconventional methods has been established, including electrochemical machining, electrical discharge machining and chemical machining [7,8]. Among them, electropolishing is particularly suitable for the fabrication of metallic materials, especially stainless steel 316L [9]. Electropolishing, as a novel machining technique, provides exceptional opportunities for the treatment and shaping of robust industrial materials such as SS 316L [10]. The lack of physical contact between the tool and the workpiece material results in negligible or absent mechanical and thermal stresses [11]. In the electropolishing process, any hard conductive material can be machined using a softer tool material [12]. Furthermore, it offers numerous advantages over alternative machining methods, including smooth and crack-free surfaces, burr-free finishes, little tool wear, an elevated material removal rate, and the capability to fabricate complex geometric shapes [13]. The material removal method is predicated on Faraday’s law of electrolysis, wherein anodic dissolution of the workpiece occurs within the electrolyte [14]. In electropolishing, a voltage of 5–30 V is delivered across the minimal cutting gap between the tool (cathode) and the workpiece (anode) [15].

Electropolishing is often performed in concentrated acid electrolytes, including phosphoric acid, sulphuric acid, perchloric acid, and their combinations, depending on the materials of the workpiece [16]. Lu [17] conducted electropolishing and magnetoelectropolishing surface treatments on SS 316L in a solution of 20% HClO₄ and 80% CH₃COOH (vol%). A magnetic field with a flux density of 0.3 T was applied to the surface of the electropolishing workpiece. An elevated chromium content and a more protective passive coating can be attained with MEP treatment, which accelerates the selective dissolution of magnetic components like iron and nickel during electropolishing under a magnetic field. Using synchrotron X-ray imaging and multiphysics modelling, Mi et al. [18] studied how electromagnetic pulses (which affect the solidification dynamics of intermetallic phases) improve phase distribution, fluid flow, and microstructures. This refers to magnetoelectropolishing, whereby an applied magnetic field improves ion transport and surface homogeneity during electropolishing. Magnetic fields in electropolishing increase surface smoothing by influencing electrolyte flow and ion removal, whereas EMPs maximize metal solidification by managing phase transitions, hence producing more homogeneous and defect-free surfaces. Both systems improve material qualities and performance by using electromagnetic forces.

Hryniewicz examined the corrosion characteristics of SS 316L following magnetoelectropolishing at 0.35 T in a sulphuric/orthophosphoric acids electrolyte mixture, and found out that magnetoelectropolishing elevated the chromium content of the surface and enhanced corrosion resistance [19]. Given the environmental and safety concerns associated with prevalent acid-based electrolytes for electropolishing, there is a significant demand for the development of new eco-friendly and safe polishing electrolytes that yield high surface quality [20].

An eco-friendly electrolyte is provided and filled in the machining gap to facilitate current flow and to transport heat and dissolved metal [21]. The rate of anodic dissolution is influenced by electrical characteristics, electrolyte attributes, the distance of the machining gap and material qualities [22]. Researchers have conducted extensive studies to determine the various degrees of process parameters for enhanced electropolishing performance. Kim employed the Taguchi method to ascertain the optimal parameters for the electropolishing process of titanium [23]. They found that the voltage exerted the most substantial impact on surface roughness, directly affecting the thickness and microstructure of the oxide layer; however, excessive voltage resulted in surface damage. In addition, according to the Taguchi robust design, the ideal parameters for titanium electropolishing are a voltage of 16 V, a process period of 25 min, a temperature of 35 °C, an ethylene glycol ratio of 2, and a concentration of 30% additional distilled water.

Likewise, environmentally sustainable electrolytes are being explored for titanium; similar to stainless steel studies, elevated temperatures were utilized without altering the flux density, and the previous research was not optimized. Consequently, due to these deficiencies, the current study determined the optimal conditions for the electropolishing of SS 316L using NaCl-based solutions enhanced with ethanol, applying the Taguchi robust design approach. This method involved conducting an analysis of variance (ANOVA) and enhancing the signal-to-noise ratio through Taguchi’s “the-larger-the-better” criterion. The Pareto ANOVA technique was employed to assess the influence of four machining factors (applied voltage, ethanol concentration in the electrolyte, machining gap, and magnetic field) and their levels on achieving a high material removal rate. In addition, surface analysis was performed utilizing a surface roughness tester, and the outcomes of each treatment were compared.

2. Materials and Methods

2.1. Materials

SS 316L served as the working material in all experiments conducted in this investigation. Figure 1 illustrates the surface picture of the unpolished SS 316L, exhibiting an average surface roughness of 0.142 $R_a$. SS 304 sheets were utilized as the cathode (tool electrode).

Table 1. Chemical composition of SS 304.

Component Elements	C	Si	Mn	P	S	Ni	Cr	Mo
(%)	0.03	1	2	0.045	0.03	12–15	16–18	2–3

and

Table 2. Chemical composition of SS 316L.

Component Elements	C	Cr	Mn	Si	P	S	Ni	N	Fe
(%)	0.07	17.5–19.5	2	1	0.045	0.015	8–10.5	0.1	Balance

present the chemical compositions of SS 316L and SS 304, which were utilized in this investigation. The sheets measuring 200 $\mathsf{\mu }\mathrm{m}$ in thickness were cut into rectangular samples measuring 20 mm × 10 mm and 200 µm. To maintain uniform initial surface roughness throughout all trials, these specimens were utilized in their received condition from the manufacturer. The substrates were subjected to ultrasonic cleaning with ethanol for 10 min to eliminate contaminants. Ethylene glycol (EG, 99%), ethanol (99%) and sodium chloride (NaCl, 99%) were acquired from Merck & Co., Inc., Rahway, NJ, USA, and utilized as received.

2.2. Electrolytes

The electrochemical polishing process of SS 316L was examined utilizing electrolyte solutions composed of ethylene glycol, NaCl, and ethanol at varying concentrations. Ethylene glycol was combined with NaCl to create a 1 M electrolyte solution. Ethanol was incorporated into the electrolyte solution at concentrations of 0, 10, and 20 (vol.%). Each electrolyte was generated by quantifying the specific volume of the component in a beaker and agitating at 300 rpm for 30 min.

2.3. Experimental Setup

The electropolishing procedure utilized SS 316L sheet as the working electrode and SS 304 as the tool electrode, immersed in 50 mL of electrolyte solution at varying potentials of 6 V, 8 V, and 10 V for a duration of 30 min, as seen in Figure 2. All electropolishing tests were performed at room temperature. The gap between the workpiece and the tool was adjusted to 5 mm, 10 mm, and 20 mm. A magnetic field with a flux density of 0.4 T and 0.5 T was produced using a neodymium magnet. The electrolyte solution was not agitated, as the turbulence interfered with the process. Subsequent to electropolishing, the samples were sequentially rinsed with pure water and ethanol and ultimately dried at an ambient temperature.

2.4. Material Removal Rate

The material removal rate following electropolishing was derived from the material removals and the recorded electropolishing durations in $\mathrm{m}\mathrm{g}/\mathrm{m}\mathrm{i}\mathrm{n}$ (1).

$$Material removal rate={\displaystyle \frac{W_b-W_a}{Machining time}}.$$

(1)

$W_b$ represents the weight of the workpiece prior to electropolishing, while $W_a$ denotes the weight of the workpiece subsequent to the electropolishing procedure. Weight variations $W_b-W_a$ were documented post-electropolishing utilizing a precision balance (Fujitsu FS AR 210, Tokyo, Japan).

2.5. Surface Characterization

2.5.1. Surface Roughness

In addition to gloss and reflectivity, which are subjectively assessed visually, surface roughness was identified as a critical criterion for measuring the impact of each machining parameter following the electropolishing of SS 316L samples. A Digital SRT-6210S Surface Roughness Tester (SRT-6210S, GuangZhou Landtek Instruments Co., LTD, Guangzhou, China) was utilized to analyze the surface morphology post-electropolishing of each sample. The arithmetic mean height ($R_a$) in μm was assessed on each electropolished surface at three distinct locations for enhanced dependability with a sampling length of 0.8 mm.

2.5.2. Surface Morphology

A digital microscope (Aven Tools Mighty Scope, Ann Arbor, MI 48108, USA) with a magnification of 500× was employed for a more accurate assessment of the morphology and identified irregularities of the electropolished surfaces.

2.5.3. Water Contact Angle Measurement

At room temperature, the water contact angle (WCA) of the polished surfaces was assessed by employing a single 10 μL droplet of water to measure the wettability. The digital microscope obtained an image of the water droplet’s contact angle and calculated it. To guarantee accuracy, we measured the contact angles of each membrane three times and subsequently computed the average.

2.6. Taguchi Robust Design and Electropolishing Parameters

The Taguchi robust design is an optimization methodology used within the design of experiments, utilizing statistical principles and techniques. This method was selected for this study since it facilitates effective quality enhancement by identifying ideal factors and assessment levels through the signal-to-noise ratio. The improvement in material removal rate is attained with less experimentation, thereby decreasing both experimental duration and expense. The electropolishing of the SS 316L specimens was optimized through Taguchi’s robust design. The electropolishing parameters of applied voltage, ethanol concentration in the electrolyte, applied magnetic field, and machining gap were identified as factors, each with designated values as variables. The Taguchi robust design was utilized in both the experimental design and statistical analysis.

2.6.1. Factors and Levels

The experimental factors (applied voltage, ethanol concentration in the electrolyte, applied magnetic field, and machining gap) were set at three levels, as indicated in

Table 3. Designed factors and levels for electropolishing parameter.

Control Factor	Levels
	1	2	3
Applied Voltage (V)	6	8	10
Ethanol concentration (vol. %)	0	10	20
Magnetic field (T)	0	0.4	0.5
Machining Gap (cm)	0.5	1.0	1.5

. The applied voltage was set at levels 1, 2, and 3, corresponding to 6, 8, and 10 Volts, respectively.

2.6.2. Orthogonal Array Selection and Factor Assignments

An L₉ orthogonal array (four parameters, in three levels) was employed in this study. The applied voltage (V), ethanol content (vol.%), magnetic field (T), and machining gap (mm) data were all repeated three times under the same conditions in order to observe the data in the experiment with reliability. The instrumental parameters displayed in

Table 4. Designed Taguchi orthogonal array for electropolishing parameters.

Experiment No.	Variable Process
	Applied Voltage (V)	Ethanol Concentration (%)	Magnetic Field (T)	Machining Gap (cm)
1	6	0	0	0.5
2	6	10	0.4	1.0
3	6	20	0.5	1.5
4	8	0	0.4	1.5
5	8	10	0.5	0.5
6	8	20	0	1.0
7	10	0	0.5	1.0
8	10	10	0	1.5
9	10	20	0.4	0.5

were determined using an L₉ orthogonal array. A trial condition with the level of factors is represented by each row in this table.

2.6.3. Signal-to-Noise Ratio

For output characteristics and the signal/noise ratio (S/N), the Taguchi approach uses the terms “signal” and “noise” to denote the desired signal value and the unwanted noise value, respectively. The S/N ratio equation is contingent upon the criteria employed for optimizing quality features. For the material removal rate, the larger-the-better type characteristic is used because the maximum material removal rate is desirable. This research utilized the results of applied voltage (V), ethanol concentration (vol.%), magnetic field (T), and machining gap (mm) to compute the relevant signal-to-noise (S/N) ratio, as expressed in the following Equation (2):

$$S/N_S=-10{log}_{10}\left[{\displaystyle \frac{1}{n}}\left({\sum }_{i=1}^n{\displaystyle \frac{1}{y_i^2}}\right)\right],$$

(2)

where $y_i$ represents the observations of the quality characteristic under various noise conditions, and $n$ is the number of experiments conducted.

3. Results and Discussion

Table 5. Material removal rate and S/N ratio.

Experiment No.	Material Removal Rate (µm/s)			Mean (mg/min)	Standard Deviation	S/N Ratio (dB)
	1	2	3
1	0.69	0.55	0.77	0.67	0.11	−3.71
2	0.62	0.88	0.97	0.82	0.18	−2.18
3	0.83	0.83	0.81	0.82	0.01	−1.69
4	0.82	0.76	0.92	0.83	0.08	−1.66
5	1.72	1.72	2.15	1.86	0.25	5.28
6	1.29	1.11	1.37	1.26	0.13	1.9
7	1.42	1.78	1.65	1.62	0.18	4.05
8	1.09	1.17	1.31	1.19	0.11	1.42
9	3.99	4.37	4.80	4.39	0.41	12.77

encapsulates the findings of the material removal rate from three iterations and the S/N ratio for each L₉ orthogonal array subsequent to the completion of the nine experimental matrixes. The mean S/N ratio for each factor, as presented in

Table 6. Average S/N ratio by factor levels (dB).

	Factor
	Applied Voltage (V)	Ethanol Concentration (vol.%)	Magnetic Field (T)	Machining Gap (cm)
Level 1	−7.58	−1.32	−0.39	14.34
Level 2	5.52	4.52	8.93	3.77
Level 3	18.24	12.98	7.64	−1.93
Max-Min	25.82	14.30	9.32	16.27
Average	5.39

, can be derived from the numerical data in Table 5. Figure 2 illustrates the average signal-to-noise ratio for each level, together with the individual effects of each factor, referred to as major effects.

3.1. Optimal Level Combination for Each Factor

This study aims to enhance the material removal rate by identifying the ideal amount for each element. The best level for each factor can be identified by the level that exhibits the maximum signal-to-noise ratio value. Figure 3 and

Table 7. Optimal condition for micro powder mixed dielectric in µ-EDM.

Factor	Level
Applied Voltage	10 V
Ethanol Concentration	20 vol.%
Magnetic Field	0.4 T
Machining Gap	0.5 cm

indicate that the optimal combination of each factor includes an applied voltage of 10 V, an ethanol concentration of 20 vol.%, a magnetic field of 0.4 T, and a machining gap of 0.5 cm, all of which significantly enhance the material removal rate.

Table 8. Pareto ANOVA analysis for material removal rate in electropolishing of SS 316L.

Factor		Applied Voltage	Ethanol Concentration	Magnetic Field	Machining Gap	Total
Sum of factor level	1	−7.58	−1.32	−0.39	14.34	64.72
	2	5.52	4.52	8.93	3.77
	3	18.24	12.98	7.64	−1.93
Square of difference (S)		1000.08	310.17	153.01	408.93	1872.18
Degrees of Freedom (φ)		2	2	2	2
S/φ		500.04	155.08	76.50	204.46	936.09
Contribution ratio (%)		53.42	21.84	16.57	8.17
Cumulative (%)		53.42	75.26	91.83	100
Optimum combination of significant factor levels		Applied voltage, 10 V Machining gap, 0.5 cm Ethanol concentration, 20 vol.% Magnetic field, 0.41 T
		The optimal level of each critical element is the level that maximizes the aggregate of signal to noise ratios.
Remarks on optimum combinations		The significant variables are selected from the left-hand side of the Pareto diagram above, which collectively contribute approximately 90%.

presents the Pareto ANOVA for the material removal rate during the electropolishing of SS 316L. The critical elements selected from the left side of the aforementioned Pareto diagram, presented in Table 8 and Figure 4, collectively represent approximately 90% of the total. The flux density’s contribution to this experiment exceeds 90%, as indicated in Table 8, demonstrating that the magnetic field is not pivotal in attaining a high material removal rate. Hence, it is necessary to set the machining conditions as: applied voltage of 10 V, a machining gap of 5 mm, and an ethanol concentration of 20 vol.%.

Three verification experiments were conducted to assess the repeatability of the optimal combination of machining parameters outlined in

Table 9. Material removal rate of SS electropolishing after verification experiments.

Verification Experiment	Material Removal Rate (mg/min)
	1	2	3
- Applied voltage, 10 V - Machining gap, 0.5 cm - Ethanol concentration, 20 vol.% - Magnetic field, 0.41 T	5.73	6.11	6.38
Mean (mg/min)	6.07

. The highest average material removal rate, 6.07 mg/min, is derived from the validation test results presented in Table 5. The combination of the levels for each factor can be confirmed as accurate and designated as the best parameters for SS 316L electropolishing.

3.2. Effect of Applied Voltage on the Electropolishing Process

The critical factors in Pareto ANOVA analysis are selected from the left-hand side, collectively accounting for approximately 90% of the total contribution. According to the Pareto analysis presented in Table 8, the applied voltage has been identified as a critical parameter in the electropolishing process, demonstrating a significance of 53.42% in achieving substantial material removal.

In the process of electropolishing, the current density–voltage curve is typically employed to analyze the dissolution mechanism of the anodic material [24]. Figure 5 illustrates the current density–voltage curve of the electropolishing process for several materials, incorporating the etching, passivating, polishing, and gas evolution zones. In the etching zones, the workpiece is directly dissolved. The mechanically polished surface leads to the development of pitting on the metal surface in this area. In the passivating zone, the current density gradually declines with higher voltage due to the formation of a passive oxide layer on the anodic surface. In the gas evolution zone, the passive oxide layer deteriorates as the voltage increases, and anodic dissolution occurs with the generation of oxygen. The presence of oxygen bubbles trapped on the workpiece surface facilitates the occurrence of pitting in this area, which is consequently referred to as the pitting zone. This process induces rapid anodic dissolution, therefore, as the applied voltage rises, the machining current in the inter-electrode gap escalates, resulting in an improved material removal rate [25]. The applied voltage accounts for 53.42% of the significance level, and the statistical analysis confirmed the density–voltage correlation to achieve a high clearance rate.

3.3. Effect of Machining Gap on the Electropolishing Process

Table 8 indicates that the machining gap is a substantial impact (21.84%) in enhancing the material removal rate. A narrower machining gap width (5 mm) between the anode and cathode results in an increased material removal rate, as illustrated in Figure 6. The gap width between the electrode and the workpiece directly affects the current conditions and the discharge residues of the electrolyte. A narrow interelectrode gap generates a strong electric field, leading to elevated current density and thus, significant material removal. However, a reduced interelectrode distance has some drawbacks. Firstly, it is likely to induce a short circuit [26], and secondly, the flushing of the electrolyte becomes progressively more difficult due to the prolonged duration of electropolishing, which negatively affects the precision of the process.

3.4. Effect of Ethanol Concentration on the Electropolishing Process

This study utilizes ionic liquids, a mixture of ethylene glycol and salt, along with ethanol as an addition, with concentration being one of the variables examined for its impact on enhancing material removal.

Table 8 presents the Pareto ANOVA analysis, indicating that the concentration of ethanol in the electrolyte solution (ethylene glycol-NaCl) significantly influences the material removal rate by 16.57%. This component is regarded as a crucial parameter in material removal, as it cumulatively surpasses 90% following the application of voltage and the machining gap. As the concentration of ethanol rises, the material removal rate of 316L samples escalates, suggesting that ethanol exerts a facilitative influence on dissolution. This is due to the fact that ethanol lowers the solubility of ferrous ions in comparison to water; it increases the dissolving rate and is therefore essential for the electropolishing of SS 316L [27,28]. Higher acidity of ethanolic solutions promotes the SS 316L deterioration during electropolishing. Moreover, ethanol affects important electrolyte characteristics including viscosity, conductivity, and mass transfer, therefore optimizing the metal removal mechanism. Ethanol reduces surface tension, thereby facilitating gas bubble dissociation and increasing ion mobility at the metal–electrolyte interface, so promoting a more homogeneous dissolving process [29].

Moreover, ethanol changes the anodic dissolution mechanism, therefore preventing overly strong localized etching and producing a surface finish with the best possible material removal rate [30]. It also diminishes the creation and dimensions of oxygen bubbles produced during anodic reactions, enhancing electrolyte–metal contact and polishing uniformity [31]. The physicochemical effects collectively reveal the moderate yet significant impact of ethanol concentration identified in the Taguchi-ANOVA analysis, confirming its mechanistic function in enhancing electropolishing efficacy.

3.5. Effect of Magnetic Field on the Electropolishing Process

Numerous attempts have been made to explore the magnetoelectropolishing technology by examining the impact of incorporating a magnetic field into the electropolishing process. Electropolishing and magnetoelectropolishing were conducted on commercially available titanium during the experiment. The experimental results demonstrated that electropolishing alters the surface chemistry and morphology by creating a porous structure, whereas magnetoelectropolishing yields a microgranular structure. By affecting mass transfer, gas bubble dynamics, and electrolyte flow, a magnetic field applied in electropolishing improves the material removal rate. Lorentz forces produced by the interaction of the magnetic field with the electric current in the electrolyte cause magnetohydrodynamic convection, hence lowering the diffusion layer thickness and accelerating the movement of dissolved metal ions away from the surface. This increases the efficiency of anodic dissolution, hence raising the MRR. Furthermore, the magnetic field enhances gas bubble detachment, therefore avoiding their obstruction of active polishing sites and guaranteeing a consistent material removal procedure. Furthermore, the field changes electrolyte conductivity and ion mobility, so optimizing the electrochemical interactions engaged in metal dissolution. The best magnetic field intensity found in this work was 0.4 T. On the other hand, increasing the magnetic field strength over 0.4 T produces a Lorentz force so strong that it disturbs the electrolyte’s convective flow and causes turbulence [29,32,33]. This outcome is consistent with other studies in which, mostly due to better mass transfer and bubble removal efficiency, introducing a magnetic field boosts the material removal rate compared to traditional electropolishing [30].

In this work, while the magnetic field contributes to an increased material removal rate, it is not as significant a parameter as the applied voltage, machining gap, and ethanol concentration in the electrolyte, as shown in Table 8. The Pareto ANOVA figure demonstrates that the contribution ratio of the magnetic field to the increase in material removal rate is merely 8.17%, suggesting that this parameter is not relevant for achieving a greater material removal rate.

3.6. Effect of Electropolishing Process on Water Contact Angle

One of the most important characteristics of a metal surface in a biomedical device application is its hydrophilicity. The surface hydrophilicity of the manufactured electropolished surface can be examined using the water contact angle of the surface. In general, a hydrophilic surface denotes a smaller contact angle—one that is less than 90⁰. Figure 6 displays the electropolished membrane’s contact angles. The electropolished membranes with the best machining parameters are the most hydrophobic, as seen in Figure 7. The most hydrophilic surface was achieved with an applied voltage of 8 V, a 0% ethanol content, a 0.41 T magnetic field, and a 15 mm machining gap.

3.7. Effect of Electropolishing Process on Surface Finish

Table 10. The result of surface roughness for electropolished SS 316L.

Experiment No.	1	2	3	4	5	6	7	8	9	Optimal Condition
Surface roughness (µm)	0.56	0.57	0.77	0.57	1.13	0.77	1.06	1.64	1.09	0.71

presents the surface roughness outcomes derived from the instrumental parameters shown in Table 4, together with the identified optimal parameter (Table 10). The optimal electropolishing parameters established by material removal rate were an applied voltage of 10 V, a machining gap of 5 mm, an ethanol concentration of 20 vol.%, and a magnetic field strength of 0.4 T. Under these conditions, the average surface roughness was 0.7 $R_a$, which is substantially greater than the minimum value of 0.56 $R_a$ derived from the experimental parameters. In electropolishing, an increased material removal rate typically results in greater surface roughness, indicating that while rapid material removal can expedite the process, it frequently yields a less smooth surface finish; thus, the quicker the material is removed, the more irregular the surface may become [34].

Figure 8 shows the surface morphology of the electropolished SS 316L. As presented in Figure 8, the choice of machining parameters in electropolishing significantly influences the final surface topography and the material removal distribution within the electropolished region. Consequently, the choice of optimal electropolishing parameters is essential, as shown in this study, resulting in a notable decrease in surface roughness when compared to other surfaces processed at a low material removal rate.

According to Table 10, experiment no. 1 exhibited the lowest surface roughness at 0.56 $R_a$, coinciding with the lowest material removal rate observed. Table 10 indicates that the minimum surface roughness of the electropolished SS 316L (0.56 $R_a$) was observed in trial no. 1, which also showed the lowest material removal rate. Additionally, the morphology exhibited some inconsistencies, attributed to the lack of applied voltage and the absence of ethanol in the electrolyte.

Improving the material removal rate in electropolishing directly addresses key manufacturing constraints such as production efficiency, cost reduction, and surface quality. A higher MRR reduces processing time, allowing manufacturers to increase throughput while maintaining high-quality surface coatings. This is critical in industries like medical devices, where precision and fluidity are essential. Additionally, an enhanced MRR lowers energy consumption and electrolyte usage, reducing overall production costs while ensuring conformance with industry standards for corrosion resistance and biocompatibility.

4. Conclusions

Electropolishing involves the novel application of voltage, ethanol concentration in the electrolyte, varying inter-electrode gaps, and the implementation of a magnetic field to enhance the material removal rate. The Taguchi approach was introduced to identify the ideal process parameters for enhancing the material removal rate. The principal findings of this investigation are encapsulated as follows:

• The application of increased voltage significantly enhanced the material removal rate during the electropolishing of SS 316L.
• The presence of high-concentration ethanol in the electrolyte accelerates dissolution initiation and leads to a more pronounced dissolution effect, hence increasing the material removal rate.
• The machining gap plays a crucial role in enhancing the material removal rate. The machining gap generates a robust electric field, resulting in increased current density and thus, a substantial enhancement in the material removal rate.
• The application of a magnetic field (0.4 T) influences the enhancement of the material removal rate. However, increasing it beyond 0.4 T will induce the turbulent flow of the electrolyte, resulting in a diminished material removal rate.
• The Taguchi analysis results indicate that to attain a high material removal rate, the following machining parameters must be established: an applied voltage of 10 V, ethanol addition at a concentration of 20 vol.%, a machining gap of 10 mm, and a magnetic field strength of 0.4 T. Nevertheless, the Pareto ANOVA analysis indicated that the application of a magnetic field was not a significant factor in determining the achievement of a high material removal rate.
• The limitation of this study is that the conclusion established in the Design of Experiment (DOE) cannot be applied to an alternative workpiece material or tool in the electropolishing process. As a result, it is essential to examine the mechanism of material removal in order to improve the reliability of the results. Another disadvantage of this work is the utilization of the L9 orthogonal array, which fails to capture potentially significant interaction effects among parameters. Integrating factorial design, or multi-response optimization can mitigate the constraints of the L9 orthogonal array.