# Process Control in Jet Electrochemical Machining of Stainless Steel through Inline Metrology of Current Density

^{1}

^{2}

^{*}

## Abstract

**:**

## 1. Introduction

_{3}solution as electrolyte, nozzle diameter d

_{n}= 100 µm, and nozzle speed of 500 µm/s, mean current density decreases from 2100 A/cm

^{2}for working gap a = 5 µm to 400 A/cm

^{2}for a = 100 µm for machining of EN 1.4541 [4]. In order to control the working gap, different strategies based on electrostatic probing before machining have already been studied. When the normal vector of the surface is calculated based on touching three points on the surface of workpiece to adjust working gap, the strategy is called “adjusting by normal vector”. In the “adjusting by grid” strategy, which is used for more complicated shape deviations, multiple points can be detected and the normal vectors of the corresponding areas can be calculated from the determined values. “Adjusting by reference points” is another strategy where an individual number of points along the removal geometry are detected. Besides, the working gap can also be controlled dynamically during the process where it is determined and adjusted when differing from a defined tolerance. This strategy is called “control dynamical” [5,14]. The precision of gap measurement increases when more points are detected. However, the measurement time becomes longer as well.

_{n}of 100 µm [4]. Several other process parameters, such as nozzle motion speed, nozzle diameter, electrolyte flow velocity, and electrolyte concentration, have significant influences on the machining result. The electrolyte flow is very important to ensure the complete removal of heat and gases produced by the reactions at either electrode and to let current flow to enable charge transport [15]. The nozzle diameter influences the distribution of current density and resulted removal geometry consequently. According to Schubert et al., it is shown that the depth of cut increases with nozzle diameter from 29 µm for d

_{n}= 60 µm to 77 µm for d

_{n}= 200 µm [16]. The type and concentration of salt in the electrolyte are chosen depending on the material and the need to provide sufficient conductivity to assure the dissolution of the workpiece material with adequate removal rate [17]. Table 1 shows the values of applied process parameters for different materials and the achieved results. Electrolyte type is usually selected based on workpiece material. According to the table, NaCl solutions are mostly used in Jet-ECM as a nonpassive electrolyte and NaNO

_{3}with the mass concentration of 20% to 30% as a passive electrolyte. Besides, nozzles diameters range from 100 µm to 510 µm; working gaps are also in the same range. Common potential values for Jet-ECM is up to 60 V, and nozzle speeds amounting to 1000 µm/s have been studied. These parameters result in up to 250 µm of depth and the Sa roughness of less than 1.5 µm. In this study, the used parameters were chosen with regards to Table 1.

## 2. Materials and Methods

^{4}C/mol) and z the electrochemical valence of an ion of the ablated material. The calculation of the electric charge Q results from the integration of the time-dependent electric current I(t) over the processing time t. Mathematically, this relationship is described according to Equation (2) [24] where ${t}_{1}$ and ${t}_{2}$ are the times correspond to the start and the end of machining process.

_{sp}represents a material constant, which is calculated according to Equation (4) [21].

_{eff}= V/Q and the specific dissolution volume V

_{sp}, according to Equation (5) [3].

_{E}[23].

## 3. Experimental Setup

#### Design of Experiments

## 4. Results

#### 4.1. Single Grooves

_{m}[11]. As an example, the current density developed during the machining of single grooves with the machining voltage of 60 V as function of the nozzle displacement for the analyzed working gaps is shown as point diagram in Figure 6A. It can be seen in Figure 6A that no major changes in the current density were seen during machining single grooves over a plane workpiece surface.

_{m}[A/cm

^{2}]

^{2}, while a further increase in mean current density results in an increasing roughness. An adequate control of the mean current density offers the possibility for finish-machining in order to achieve a predefined surface roughness Sa.

#### 4.2. Intersecting Grooves

_{min}of the five intersecting positions are displayed as a function of the depth of the premachined groove for all the analyzed voltages. The point diagram shows that the minimum mean current density decreases linearly with increasing depth of the premachined groove. Hence, the minimum mean current density can be considered as an indicator for the value of surface deviations depending on the removal depth of the premachined grooves. As can be seen in Figure 11, the linearity of changes of the minimum current density with depth of premachined grooves is independent of machining voltage and therefore, with low or high machining voltages, the amount of surface deviations can be characterized.

_{r}was calculated from the difference between the maximum depth in the intersection and the depth of the premachined groove. Figure 12 shows the calculated relative depths as function of the minimum current density.

_{r}[µm] = 1.08 µm + 0.053 × J

_{min}[A/cm

^{2}]

^{2}and increases again at further increase in minimum current density, although the slope of changes is less significant than the slope determined for single grooves as can be seen in Figure 13.

#### 4.3. Parallel Grooves

^{2}. This can be explained by the changes of current density distribution where by the increase of lateral gap, the actual working gap decreases and more material is ablated from the side of the groove rather than the bottom. According to Figure 15 this corresponds to the value at a lateral gap of approximately 120 µm, up to which the premachined groove affects the average mean current density when machining the subsequent groove. Hence, at a further increase in average mean current density only slight changes were detected.

## 5. Conclusions

- depth changes linearly with current density and
- surface roughness decreases with the increase in current density and then increases again.

- minimum current density over intersections changes proportionally to the depth of premachined grooves for each machining voltage level, which can be used as a monitoring tool for the first groove depth;
- the relative depth of intersections showed linear changes with the minimum current density over the intersection. Therefore, minimum depth over intersection can be applied for the prediction of the relative depth; and
- the depth and the mean current density of subsequent parallel grooves changes linearly with the lateral gap. This enhance the process monitoring with useful data of the actual lateral gap as well as the depth by monitoring the mean current density.

## Author Contributions

## Funding

## Acknowledgments

## Conflicts of Interest

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**Figure 5.**Schematics of machining directions for (

**A**) intersecting and (

**B**) parallel grooves, the electrolyte is not shown.

**Figure 6.**Mean current density as a function of the nozzle displacement (

**A**) and average mean current density as a function of working gap for U = 60 V (

**B**).

**Figure 8.**Aerial roughness Sa as function of the mean current density in Jet-EC milling of single grooves.

**Figure 10.**Mean current density as function of the nozzle displacement during Jet-EC milling of subsequent grooves with a voltage of 60 V crossing premachined grooves with different depths.

**Figure 11.**Minimum mean current densities of intersections as a function of the depth of premachined grooves for differing voltages.

**Figure 13.**Aerial roughness (Sa) as function of the minimal current density in Jet-EC milling of intersecting grooves measured in the center of the intersections.

**Figure 15.**Average mean current density of the subsequent grooves as function of the lateral gap from the premachined parallel groove.

**Figure 16.**Depth of second parallel groove as function of (

**A**) mean current density and (

**B**) lateral gap.

**Table 1.**Applied process parameters and achieved results in previous studies for different materials.

Parameter | Value | |||||||||
---|---|---|---|---|---|---|---|---|---|---|

Workpiece Material | Co [18] | WC [18] | WC- 6% Co [18] | Nimonic 80A [19] | Ti-6Al-4V [20] | EN 1.2379 [21] | EN 1.4301 [21] | EN 1.4541 [21] | EN 1.5920 [21] | Brass, Cu39Zn2Pb [22] |

Nozzle inner diameter (µm) | 100 | 100 | 100 | 100 | 250 | 100 | 510 | |||

Electrolyte | 20% NaCl | 20% NaCl | 20% NaCl | 20% NaCl | 2–4 M NaNO_{3} | 30% NaNO_{3} | 2.3 M NaNO_{3} | |||

Working gap (µm) | 100 | 100 | 100 | 100 | 500 | 100 | 500 | |||

Voltage (V) | 50 | 50 | 10–55 | 1–56 | - | 56 | - | |||

Nozzle speed (µm/s) | 200 | 200 | 200 | 150 | 0 Machining time: 10s | 200–1000 | 500 | |||

Depth of removal (µm) | 40 | < 1 | 4–5 | 300 | 50–250 | 75–240 | 60–230 | 60–220 | 100–250 | 150 µm/C |

Surface roughness (µm) | - | - | Ra < 0.65 | - | - | 0.35 < Ra < 0.45 | 0.1 < Ra < 0.15 | 0.15 < Ra < 0.33 | 0.3 < Ra < 0.45 | 0.3 < Sa < 1.5 |

Parameter | Value | |
---|---|---|

Workpiece material | EN 1.4301 | |

Nozzle inner diameter | 100 µm | |

Electrolyte | 30% NaNO_{3} | |

Electrolyte supply rate | 10 mL/min | |

Working gap | single grooves | 100, 200, 300, 400, and 500 µm |

intersecting grooves | 100 µm | |

parallel grooves | 100 µm | |

Voltage | single grooves | 30, 40, 50,60, 70, 80, and 90 V |

intersecting grooves | 30, 40, 50, 60, and 70 V | |

parallel grooves | 60 V | |

Nozzle speed | 200 µm/s |

Process and Geometry Parameter | Value | ||||
---|---|---|---|---|---|

Machining voltage (V) | 70 | 60 | 50 | 40 | 30 |

Depth of groove (µm) | 59 | 52 | 44 | 36 | 28 |

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**MDPI and ACS Style**

Yahyavi Zanjani, M.; Hackert-Oschätzchen, M.; Martin, A.; Meichsner, G.; Edelmann, J.; Schubert, A.
Process Control in Jet Electrochemical Machining of Stainless Steel through Inline Metrology of Current Density. *Micromachines* **2019**, *10*, 261.
https://doi.org/10.3390/mi10040261

**AMA Style**

Yahyavi Zanjani M, Hackert-Oschätzchen M, Martin A, Meichsner G, Edelmann J, Schubert A.
Process Control in Jet Electrochemical Machining of Stainless Steel through Inline Metrology of Current Density. *Micromachines*. 2019; 10(4):261.
https://doi.org/10.3390/mi10040261

**Chicago/Turabian Style**

Yahyavi Zanjani, Matin, Matthias Hackert-Oschätzchen, André Martin, Gunnar Meichsner, Jan Edelmann, and Andreas Schubert.
2019. "Process Control in Jet Electrochemical Machining of Stainless Steel through Inline Metrology of Current Density" *Micromachines* 10, no. 4: 261.
https://doi.org/10.3390/mi10040261