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Abstract

Taguchi’s L₉ orthogonal array has been effectively used to study the effect of process parameters such as voltage, feed rate and electrolyte concentration on material removal rate in context of two different types of electrolyte, namely, aqueous NaCl solution and CuSO₄ mixed aqueous NaCl solution. The results indicated that Cu²⁺ ions formed due to electrochemical reactions prevent the oxidation of Fe²⁺ to Fe³⁺ and catalyze the anodic dissolution of iron during machining. The experimental results were analyzed using analysis of variance method to investigate the significance and percentage contribution of individual process parameters on performance characteristics.

Keywords

Electrochemical machining aqueous NaCl parameters material removal rate

Introduction

Electrochemical machining (ECM) has got an industrial importance due to its capability of controlled atomic level metal removal.¹ It is an anodic dissolution process based on electrolysis, where the application of a more traditional process is not convenient. ECM has been successfully employed in aerospace, automobile industries and now gaining much importance in the electronics and other high-tech industries for the fabrication of micro-components.^2,3 Mask-less and through-mask electrochemical micromachining techniques have been successively used in thin films and foils of materials that are difficult to machine by other methods.^4,5 ECM is a low-voltage (5–25 V) machining process which offers high metal removal rate and also capable of machining hard conductive materials into complicated profiles without any thermal damages, thus suitable for mass production work with low labor requirements.^6,7 The dissolution rate is highly reliant on the selection of electrolytes and its current-carrying capacity. On increasing the concentration of electrolyte solution, dissolution rate also increases, but excess concentration allows the crystal formation, which may damage the accessories of ECM and reduce the volume of electrolyte in flow pipes. The conductivity of electrolyte depends not only on the concentration but also on the ionic interaction. Thus, the current-carrying process done by the base electrolyte is small, but H⁺ and OH⁻ ions produced in electrolysis of water play an important role.^8,9 The achievement of higher dissolution rate in ECM is a strong research base which is possible with a change in the composition of electrolyte solution to promote catalytic effect during dissolution.¹⁰

During ECM of iron at low current density, it has been observed that Fe⁺ cation formed very easily, but it is highly unstable and immediately oxidizes into Fe²⁺ state. Increase in current density leads to simultaneous production of Fe²⁺ and Fe³⁺, and at higher current density, the apparent valence of iron increases above three.¹¹ Therefore, stabilizing Fe²⁺ in the aqueous solution is a challenge during dissolution.

Experimental details

Experimental setup and principle of ECM

The experimental setup of ECM is shown in Figure 1

Figure 1.

Experimental setup.

ECM is an anodic dissolution process that works on the principle of Faraday’s law. During machining of iron in the presence of aqueous NaCl electrolyte solution, the following chemical reactions are observed.¹²

Reactions at cathode

N a^{+} + e^{-} \Leftrightarrow Na

Na + H_{2} O \Leftrightarrow NaOH + H^{+}

2 H^{+} + 2 e^{-} \Leftrightarrow H_{2}

It shows that only hydrogen gas will evolve at cathode.

When pure iron is being machined electrochemically, the following reactions would occur^13,14

Fe \Leftrightarrow F e^{2 +} + 2 e^{-}

F e^{2 +} + 2 C l^{-} \Leftrightarrow FeC l_{2}

F e^{2 +} + 2 (OH)^{-} \Leftrightarrow Fe (OH)_{2}

FeC l_{2} + 2 (OH)^{-} \Leftrightarrow Fe (OH)_{2} + 2 C l^{-}

2 C l^{-} \to C l_{2} + 2 e^{-}

2 FeC l_{2} + C l_{2} \Leftrightarrow 2 FeC l_{3}

H^{+} + C l^{-} \Leftrightarrow HCl

2 Fe (OH)_{2} + H_{2} O + O_{2} \Leftrightarrow 2 Fe (OH)_{3} ↓

Fe (OH)_{3} + 3 HCl \Leftrightarrow FeC l_{3} + 3 H_{2} O

FeC l_{3} + 3 NaOH \Leftrightarrow Fe (OH)_{3} + 3 NaCl

It shows that during ECM of iron in NaCl electrolyte, iron is removed as Fe(OH)₂ and precipitated as sludge, while sodium chloride is recovered back. Due to further reactions, formation of Fe(OH)₃ takes place and hence, it is confirm that iron exist in Fe²⁺ and Fe³⁺ states during dissolution.

Determination of Fe²⁺ and Fe³⁺ ions in electrolyte solution

The electrolyte solution containing Fe²⁺ and Fe³⁺ ions was collected. Fe²⁺ ions were determined directly by titrating a known volume of iron electrolyte solution with K₂Cr₂O₇ in acidic medium (HCl)

C r_{2} O_{7}^{2 -} + 6 F e^{2 +} + {14 H}^{+} = 2 C r^{3 +} + 6 F e^{3 +} + 7 H_{2} O

Internal indicator N-phenyl anthranilic acid was used to mark the end point. Fe³⁺ ions were determined after all of them are reduced into Fe²⁺ ions with SnCl₂ in the presence of hot HCl

S n^{2 +} + 2 F e^{3 +} = S n^{+ 4} + 2 F e^{2 +}

The solution was then cooled and the excess SnCl₂ was removed by adding HgCl₂ solution

2 H g^{+ 2} + S n^{+ 2} + C l^{-} = S n^{+ 4} + H g_{2} C l_{2} (white ppt)

Titration of known volume of standard solution was done using standard solution of K₂Cr₂O₇ in acidic medium. From the volume of K₂Cr₂O₇ used, the total amount of Fe²⁺ and Fe³⁺ ions was determined. The amount of Fe³⁺ ions was determined by subtracting the amount of Fe²⁺ which is determined earlier.

Material removal rate (MRR) during ECM is greatly influenced by dissolution valence. As the dissolution valence decreases, MRR increases. In this article, an approach is made to enhance the electrochemical dissolution of iron through control of valency (transition); therefore, in this direction, the use of CuSO₄ mixed aqueous NaCl electrolyte solution is suggested which dissociates further to form Cu²⁺ on passing electric current through the circuit.

The dissolution limit of iron by Cu²⁺ ions can be justified by considering the standard electron potential E° for Cu²⁺, Fe/Fe²⁺ and Fe/Fe³⁺ described as follows¹⁵

C u^{2 +} + 2 e^{-} \to Cu E \circ = + 0.34 V

F e^{2 +} + 2 e^{-} \to Fe E \circ = - 0.44 V

F e^{3 +} + e^{-} \to F e^{2 +} E \circ = + 0.77 V

As E° for Cu²⁺ → Cu is more positive than Fe²⁺ → Fe, Cu²⁺ will oxidize Fe to Fe²⁺. However, as E° for Cu²⁺ → Cu is less positive than Fe³⁺ → Fe²⁺, Cu²⁺ will not oxidize Fe²⁺ to Fe³⁺.

Electrolyte solution

The results of electrical conductance of NaCl at different concentrations in water are shown in Figure 2. The concentration of 20 g (0.5 M) of NaCl/L of water was taken as the optimum concentration to test the concentration requirement of CuSO₄. Conductance was measured for different concentrations of CuSO₄ in selected concentration of NaCl in water. As shown in Figure 3, at 0.8 g of CuSO₄ per 100 mL of aqueous NaCl, PH value is adequate to promote the dissolution of iron. Mixing of CuSO₄ with the aqueous NaCl gives the saturated solution of Na₂SO₄ and CuCl₂. As the potential difference is applied between the workpiece and the tool, CuCl₂ further dissociates to form Cu²⁺ ions.

Figure 2.

Conductance versus NaCl concentration.

Figure 3.

PH versus CuSO₄ concentration (g per 100 mL of aqueous NaCl).

Machining conditions

The selected machining parameters on the basis of performance characteristics are shown in Table 1.

Table 1.

Machining conditions for analysis.

Machining conditions	Specifications
Constant parameters
Inter electrode gap	0.010 mm
Power supply	2–50 V DC
Electrolyte flow rate	8 L/min
Electrolytes and their concentration	Aqueous NaCl (20 g/L of water) and CuSO₄ mixed aqueous NaCl solution (0.8 g per 100 mL of solution)
Tool material	Brass
Workpiece	Iron
Machining time	2 min
Variable parameters
Applied voltage	10–18 V
Feed rate	0.06–0.3 mm/min
Electrolyte concentration	10–30 g/L

Selection of machining process parameters

Table 2 shows the machining parameters and selected levels for experimental procedure.

Table 2.

Process parameter and their levels.

Parameters	Code	Levels
		1	2	3
Voltage (V)	A	10	14	18
Feed rate (mm/min)	B	0.06	0.18	0.3
Electrolyte concentration (g/L)	C	10	20	30

Measurement of MRR

The initial weight of the workpiece was taken for calculation of MRR. Keeping the flow rate constant at 8 L/min, the rest of the parameters are set according to Table 1 for each run. Workpiece was kept horizontal, and a cylindrical electrode was used for machining. Gap between the tool and the workpiece was maintained carefully to avoid choking. The electrode was fed continuously toward the workpiece during machining and time was recorded. After machining, cavity was formed on the workpiece. The final weight of the workpiece was taken, and MRR was calculated as per the following formula

MRR = \frac{initialweight - finalweight}{density \times totaltime}

(1)

Experimental work

The design resulted in a total of 18 experiments, which are performed at a supply voltage of 10–18 V, 10–30 g/L of electrolyte concentration and 0.06–0.3 mm/min feed rate as the values for the control variables. The responses measured are the MRR. Scheme of the experiments is shown in Table 3.

Table 3.

Taguchi’s L₉ orthogonal array for MRR.

Exp. no.	A	B	C	MRR (mm³/min)
				In presence of aqueous NaCl solution	In presence of CuSO₄ mixed aqueous NaCl solution
1	10	0.06	10	1320	1506
2	10	0.18	20	2346	2670
3	10	0.3	30	3762	4326
4	14	0.06	20	2220	2568
5	14	0.18	30	3570	4140
6	14	0.3	10	3684	4308
7	18	0.06	30	3216	3780
8	18	0.18	10	3600	4266
9	18	0.3	20	3918	4644

MRR: material removal rate.

Results and discussion

Analysis of variance when machining in the presence of NaCl electrolyte solution

The analysis of variance (ANOVA) has been performed to determine the significance and percentage contribution of individual process parameter on MRR during machining of iron in aqueous NaCl solution. F-test presented in Table 4 assesses that the most significant factors contributing toward the MRR are the feed rate and the voltage with percentage contribution of 55 % and 28.3 %, respectively. Graphical representation of percentage contribution of each parameter is shown in Figure 4.

Table 4.

ANOVA for MRR (NaCl solution as electrolyte).

Source	DF	Sum of squares	Mean of squares	F-value	p-value	Contribution (%)
Voltage (V)	1	1,821,606	1,821,606	20.42	0.006	28.3
Feed rate (mm/min)	1	3,538,944	3,538,944	39.68	0.001	55
Electrolyte concentration (g/L)	1	629,856	629,856	7.06	0.045	9.8
Error	5	445,946	89,189			6.9
Total	8	6,436,352
S = 298.646 R−Sq = 93.07% R−Sq(adj) = 88.91%

DF: degrees of freedom.

Figure 4.

Contribution of the parameters (aqueous NaCl as electrolyte).

Figure 5 shows the main effect plot of the MRR depicting the effect of various machining parameters on MRR while machining in the presence of aqueous NaCl solution. As seen from the plot obtained, the MRR increased with increase in both voltage and feed rate. This is due to the fact that with increase in voltage, the current increases in the inter electrode gap, thus increasing the MRR. Feed rate is another important parameter. Increase in feed rate results in decrease in the conducting path between the workpiece and the tool resulting in high current density, thus enhancing the rapid anodic dissolution. Although the higher concentration of NaCl is favorable for better MRR, the larger number of ions associated during the machining process increases the machining current, but excess concentration allows the crystal formation which reduces the volume of electrolyte in flow pipes and also affects the dissolution rate. ECM of iron in the presence of NaCl electrolyte promotes the oxidation of Fe²⁺ to Fe³⁺. The maximum MRR obtained during machining of iron in aqueous NaCl solution recorded was 3918 mm³/min.

Figure 5.

Main effect plot for signal-to-noise ratios (aqueous NaCl as electrolyte).

Regression equation

\begin{matrix} MRR = & - 658 + 137.8 voltage + 6400 feed rate \\ + 32.4 electrolyte concentration \end{matrix}

(2)

Equation (2) shows that feed rate is the dominant factor affecting the MRR.

The prediction accuracy of the developed models is shown in Figure 6 which shows that the predicted values for MRR are very close with the experimental values and points are scattered very close to the best fit line.

Figure 6.

Normal probability plot for MRR (aqueous NaCl as electrolyte).

ANOVA when machining in the presence of CuSO₄ mixed aqueous NaCl electrolyte solution

Percentage of contribution of each parameter on MRR during ECM of iron in CuSO₄ mixed aqueous NaCl electrolyte solution is shown in Table 5 and represented graphically in Figure 7.

Table 5.

ANOVA for MRR (CuSO₄ mixed aqueous NaCl solution as electrolyte).

Source	DF	Sum of squares	Mean of squares	F-value	p-value	Contribution (%)
Voltage (V)	1	2,923,224	2,923,224	24.86	0.004	31.8
Feed rate (mm/min)	1	4,903,296	4,903,296	41.69	0.001	53.3
Electrolyte concentration (g/L)	1	781,926	781,926	6.65	0.050	8.5
Error	5	588,050	117,610			6.4
Total	8	9,196,496
S = 342.943 R−Sq = 93.61% R−Sq(adj) = 89.77%

DF: degrees of freedom.

Figure 7.

Contribution of parameters (CuSO₄ mixed aqueous NaCl as electrolyte).

Figure 8 shows the main effect plot of MRR depicting the effect of machining parameters on MRR while machining in the presence of CuSO₄ mixed aqueous NaCl solution. Addition of 0.8 g of CuSO₄ per 100 mL of aqueous NaCl proves to be very effective in improving the MRR of the workpiece. As seen from the graph obtained, the MRR improved significantly with increase in both the feed rate and the voltage. Maximum increase in MRR was observed as 18.7 % for the same set of machining conditions.

Figure 8.

Main effect plot for signal-to-noise ratios (CuSO₄ mixed aqueous NaCl as electrolyte).

Regression equation

\begin{matrix} MRR = & - 942 + 174.5 voltage + 7533 feed rate \\ + 36.1 electrolyte concentration \end{matrix}

(3)

Equation (3) shows that voltage is the dominant factor affecting the MRR.

It can be seen in Figure 9 that all the points on the normal plot lie close to the straight line (mean line). This implies that the data are fairly normal with small deviation from the normality which shows the effectiveness of the developed model.

Figure 9.

Normal probability plot for MRR (CuSO₄ mixed aqueous NaCl as electrolyte).

Conclusion

The electrochemical characteristics of iron in aqueous NaCl solution and CuSO₄ mixed aqueous NaCl electrolyte have been analyzed experimentally to investigate the influence of process parameters on MRR. The process parameters, such as voltage, feed rate and electrolyte concentration, were successfully controlled. The different combinations of these parameters were used for the experimentation in order to determine their influence on MRR. The experiment was performed by varying all parameters in combination as per L₉ orthogonal array. The experimental observations support the conclusion that the presence of Cu²⁺ ions in the electrolyte solution prevents the further oxidation of Fe²⁺ to Fe³⁺ and enhances the low valence dissolution of iron. Design of experiments and ANOVA helped in identifying the significant parameters affecting MRR. The best combination of the parameters are voltage = 18 V, feed rate = 0.3 mm/min and electrolyte concentration = 20 g/L when using CuSO₄ mixed aqueous NaCl solution as electrolyte. The maximum MRR obtained was 18.7 % higher when compared with aqueous NaCl electrolyte for the same set of working conditions.

Footnotes

Acknowledgements

The author(s) express their sincere thanks to the Department of Applied Chemistry BIT Extension Centre Deoghar for their cooperation to conduct the experiments in order to observe the catalytic behavior of Cu²⁺ ions.

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship and/or publication of this article.

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Experimental investigations of catalytic effect of Cu 2+ during anodic dissolution of iron in NaCl electrolyte

Abstract

Keywords

Introduction

Experimental details

Experimental setup and principle of ECM

Reactions at cathode

Determination of Fe2+ and Fe3+ ions in electrolyte solution

Electrolyte solution

Machining conditions

Selection of machining process parameters

Measurement of MRR

Experimental work

Results and discussion

Analysis of variance when machining in the presence of NaCl electrolyte solution

ANOVA when machining in the presence of CuSO4 mixed aqueous NaCl electrolyte solution

Conclusion

Footnotes

Acknowledgements

Declaration of conflicting interests

Funding

References

Determination of Fe²⁺ and Fe³⁺ ions in electrolyte solution

ANOVA when machining in the presence of CuSO₄ mixed aqueous NaCl electrolyte solution