**Abstract**

This chapter presents research on pulsed electrochemical micromachining of stainless steel. Suitable equipment to study the process is described as well as a fitting procedure to machine and measure the variables involved. The pulse on-time must be maintained in the order of ns to achieve a good current confinement since the tool is active. Some experiments were carried out to assess the most important variables of the process: current confinement, surface roughness, material removal rate and efficiency. The current confinement has been observed to worsen when the pulse on-time increases, as well as the surface roughness. The material removal rate and efficiency increase with the voltage amplitude and the pulse on-time. The voltage amplitude must be higher than 12 V so that the phenomenon of passivation does not affect the process. There is a compromise in the choice of the variables, so a suitable combination of parameters is determined in order to achieve a good material removal rate with an acceptable result.

**Keywords:** pulsed electrochemical micromachining, current confinement, material removal rate, efficiency

### **1. Introduction**

Microfabrication consists of obtaining products or parts with features at microor submicroscale, therefore requiring very narrowly controlled material removal. Microfabrication has been widely used for the manufacture of holes in injectors, fluidic microchemical reactors requiring microscale pumps, micromoulds and many more applications, as described by Brousseau et al. [1]. Microfabrication plays an increasingly important role in the miniaturisation of components from biomedical applications to manufacturing sensors. Surfaces to be obtained are slots, complex surfaces, microholes, etc. Combinations of those features must frequently be achieved in the industry of microelectronics. These parts are very often manufactured by conventional processes with all the limitations and problems involved, such as tool wear, inaccuracy due to low rigidity of the tool, heat generated by the process, etc. With the development of MEMS and multiple benefits of the microsystems, microproducts are widely accepted in various fields of applications like aerospace, automotive, biomedicine, etc. [2]. In this context, non-conventional processes, and especially electrochemical micromachining, acquire greater significance due to their specific characteristics to avoid the problems of conventional processes.

Since the first years of developments in electrochemistry, electrochemical methods have played an important role in precision technologies to machine structures and parts. In the 1950s, electrochemical machining arose as the most widely used

technique to manufacture complex geometries, such as turbine blades, generally in dense materials. The ease of application of this technology along with the inherent advantages of the process, such as good surface roughness, promoted its application to more advanced processes in the field of micromechanics, microelectronics and micro-systems [3]. Electrochemical deposition techniques were used as standard technology to deposit copper to obtain connections in high performance circuits while lithographic techniques, LIGA, are used to manufacture micromoulds [4, 5].

Electrochemical micromachining is a highly specialised process used in the aerospace industry. Today, it is starting to be used in other industries, where difficult-to-manufacture parts, complex surfaces and components of a microscopic scale need to be obtained. Electrochemical micromachining is today widely used for manufacturing semiconductor elements and thin metallic films [6]. In addition, electrochemical micromachining can be easily hybridised with other processes to broaden the process capabilities and material processing window [7].

Analogous to conventional electrochemical machining (ECM), pulsed electrochemical micromachining (PECMM) is a controlled process of anodic dissolution to remove the material with current densities in the order of 105 A/m2 between the tool (cathode) and the workpiece (anode) through the electrolyte [8]. PECMM uses a pulsed voltage signal and must be analysed per pulse according to the structure of the Helmholtz/Gouy-Chapman/Stern double layer [9], which can be modelled as a resistance in parallel with a capacitor. This model has provided good results in experiments and indicates that the current is used at first to charge the capacitor (capacitive current). When the charge is high enough, that is, when its voltage is high enough, some current will flow to be used in the anodic dissolution process (faradaic current) since the polarisation or overpotential will have a significant value. Therefore, two stages can be distinguished in each pulse. The first part of the pulse on-time is a transient period in which the current is used in the polarisation of the double layer, which has to be high to achieve fast polarisation. The second stage is a steady period in which the current is used mainly for the anodic dissolution. In this context, what seems most fitting is that the transient process (non-faradaic) should be very short and the steady process (faradaic) very long. In addition, the intermittent supply of voltage provides idle time to flush the hydrogen bubbles and sludge from the machining zone and also increases control over the dissolution process [10]. However, a long steady period decreases the accuracy of the process as the current confinement under the tool tip worsens when this period is lengthened. Therefore, a compromise in the time of the steady-state period is required. By solving the differential equation of the equivalent circuit, the expression of the current as a function of time is obtained. The resulting time constant is the product of the electrolyte resistivity, the capacity of the double layer and the distance between the interelectrode gap (IEG) [8].

$$
\pi = \rho \cdot \text{IEG} \cdot \text{c}\_{\text{DL}} \tag{1}
$$

**241**

as the electrolyte.

*Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

current will flow.

**2. Experimental set-up**

shows a sketch of this equipment.

electrochemical micromachining. One of them is to use a tool in which all the surface is isolated except for the tip. This method ensures that all the current flows from the tip and that the cavity obtained is equal to that tip, since this current is responsible for the anodic dissolution of the material. Another method is the use of ultrashort voltage pulses, usually shorter than 100 ns. This method achieves high accuracy by confining the faradaic current density under the tool due to the incomplete charge of the double layer in areas far from the part through which a very low

An important phenomenon which affects the process is the formation of a passive oxide layer that hinders the anodic dissolution [11]. The characterisation of this phenomenon is very important, since some processes like electropolishing are performed more adequately in passivation conditions [12]. When this takes place, the voltage applied has to be above a threshold value to cause effective machining [13]. It can also be avoided by adding acid to the electrolyte, such as HCl or H2SO4, which dissolves the passive layer. This layer can be considered an additional electrical resistance in the equivalent circuit, which prevents the current from being confined under the tool tip [14]. According to this explanation, the current which flows from the sides of the tool finds a similar resistance to that which flows from

Significant advances have been made in the research of this process on many materials such as aluminium, titanium, steel and copper [15, 16]. Stainless steel is a very important material to be used in any type of microcomponent, but dissolution is difficult since its chemical properties are not very suitable for this process. Some of the existing studies were performed specifically on stainless steel [17–20]. Nevertheless, the pulse on-time used in those cases is too high to obtain a good confinement of the current. Furthermore, there are few studies in which the size of the tool is as small as a few microns. Though some work has been done in order to control the process by varying the main parameters [21], there is a huge amount of work to be done to characterise this process correctly as regards the values of the parameters in order to obtain a good result in terms of current confinement, surface roughness, material removal rate (MRR) and experimental set-up. In this work, a broad study has been made of the results of PECMM in stainless steel with pulse

The experiments performed for the study were made by means of equipment that allows accuracy and ease of handling of tools and parts to be achieved. **Figure 1**

The equipment for the experiments rests on an anti-vibrations Table TMC, which provides a floating bench that prevents the tool and the part from oscillating. The position of the recipient is controlled by a three-dimensional (3D) nanometric positioning system based on a piezoelectric technology and with a resolution of 1 nm. There is a system of recirculation for the electrolyte, which flows constantly through the cell to a tank from which it is pumped to the cell after passing through a filter. Thus, the particles that appear in the cell are constantly being removed from the electrolyte. Experiments were performed in a solution of NaNO3 at 2% in weight

The material of the workpiece is AISI 304 stainless steel and the tool is made of Tungsten, 99.7% high purity. The tools are pins with a very small tip, measuring about 5 μm in diameter. The tool tip is sharpened by means of anodic dissolution in which the tungsten pin is used as the anode and the sheet of stainless steel as the

the tool tip and therefore the current is spread over a broad surface.

on-time values in the order of ns as a function of the main variables.

A high value of the constant time will cause the current lines to spread over a broad area from the tool tip, thus reducing the accuracy of the process. Therefore, a low pulse-on time must be chosen to achieve accuracy.

By causing the tool to move towards the workpiece, the material is removed under its tip, since the current density is higher at a lower distance between the tool and workpiece, and thus, the geometry of the tool is copied as a cavity in the workpiece. As compared with other processes, PECMM is a high-precision technique to obtain holes of a small diameter or to obtain crack-free microcomponents without any residual stress. There are two methods of achieving accuracy with

#### *Pulsed Electrochemical Micromachining in Stainless Steel DOI: http://dx.doi.org/10.5772/intechopen.93750*

*Nanofibers - Synthesis, Properties and Applications*

technique to manufacture complex geometries, such as turbine blades, generally in dense materials. The ease of application of this technology along with the inherent advantages of the process, such as good surface roughness, promoted its application to more advanced processes in the field of micromechanics, microelectronics and micro-systems [3]. Electrochemical deposition techniques were used as standard technology to deposit copper to obtain connections in high performance circuits while lithographic techniques, LIGA, are used to manufacture micromoulds [4, 5]. Electrochemical micromachining is a highly specialised process used in the aerospace industry. Today, it is starting to be used in other industries, where difficult-to-manufacture parts, complex surfaces and components of a microscopic scale need to be obtained. Electrochemical micromachining is today widely used for manufacturing semiconductor elements and thin metallic films [6]. In addition, electrochemical micromachining can be easily hybridised with other processes to

broaden the process capabilities and material processing window [7].

to remove the material with current densities in the order of 105

Analogous to conventional electrochemical machining (ECM), pulsed electrochemical micromachining (PECMM) is a controlled process of anodic dissolution

tool (cathode) and the workpiece (anode) through the electrolyte [8]. PECMM uses a pulsed voltage signal and must be analysed per pulse according to the structure of the Helmholtz/Gouy-Chapman/Stern double layer [9], which can be modelled as a resistance in parallel with a capacitor. This model has provided good results in experiments and indicates that the current is used at first to charge the capacitor (capacitive current). When the charge is high enough, that is, when its voltage is high enough, some current will flow to be used in the anodic dissolution process (faradaic current) since the polarisation or overpotential will have a significant value. Therefore, two stages can be distinguished in each pulse. The first part of the pulse on-time is a transient period in which the current is used in the polarisation of the double layer, which has to be high to achieve fast polarisation. The second stage is a steady period in which the current is used mainly for the anodic dissolution. In this context, what seems most fitting is that the transient process (non-faradaic) should be very short and the steady process (faradaic) very long. In addition, the intermittent supply of voltage provides idle time to flush the hydrogen bubbles and sludge from the machining zone and also increases control over the dissolution process [10]. However, a long steady period decreases the accuracy of the process as the current confinement under the tool tip worsens when this period is lengthened. Therefore, a compromise in the time of the steady-state period is required. By solving the differential equation of the equivalent circuit, the expression of the current as a function of time is obtained. The resulting time constant is the product of the electrolyte resistivity, the capacity of the double layer and the distance between the

=⋅ ⋅ IEG *DL*

A high value of the constant time will cause the current lines to spread over a broad area from the tool tip, thus reducing the accuracy of the process. Therefore, a

By causing the tool to move towards the workpiece, the material is removed under its tip, since the current density is higher at a lower distance between the tool and workpiece, and thus, the geometry of the tool is copied as a cavity in the workpiece. As compared with other processes, PECMM is a high-precision technique to obtain holes of a small diameter or to obtain crack-free microcomponents without any residual stress. There are two methods of achieving accuracy with

*c* (1)

τ ρ

low pulse-on time must be chosen to achieve accuracy.

A/m2

between the

**240**

interelectrode gap (IEG) [8].

electrochemical micromachining. One of them is to use a tool in which all the surface is isolated except for the tip. This method ensures that all the current flows from the tip and that the cavity obtained is equal to that tip, since this current is responsible for the anodic dissolution of the material. Another method is the use of ultrashort voltage pulses, usually shorter than 100 ns. This method achieves high accuracy by confining the faradaic current density under the tool due to the incomplete charge of the double layer in areas far from the part through which a very low current will flow.

An important phenomenon which affects the process is the formation of a passive oxide layer that hinders the anodic dissolution [11]. The characterisation of this phenomenon is very important, since some processes like electropolishing are performed more adequately in passivation conditions [12]. When this takes place, the voltage applied has to be above a threshold value to cause effective machining [13]. It can also be avoided by adding acid to the electrolyte, such as HCl or H2SO4, which dissolves the passive layer. This layer can be considered an additional electrical resistance in the equivalent circuit, which prevents the current from being confined under the tool tip [14]. According to this explanation, the current which flows from the sides of the tool finds a similar resistance to that which flows from the tool tip and therefore the current is spread over a broad surface.

Significant advances have been made in the research of this process on many materials such as aluminium, titanium, steel and copper [15, 16]. Stainless steel is a very important material to be used in any type of microcomponent, but dissolution is difficult since its chemical properties are not very suitable for this process. Some of the existing studies were performed specifically on stainless steel [17–20]. Nevertheless, the pulse on-time used in those cases is too high to obtain a good confinement of the current. Furthermore, there are few studies in which the size of the tool is as small as a few microns. Though some work has been done in order to control the process by varying the main parameters [21], there is a huge amount of work to be done to characterise this process correctly as regards the values of the parameters in order to obtain a good result in terms of current confinement, surface roughness, material removal rate (MRR) and experimental set-up. In this work, a broad study has been made of the results of PECMM in stainless steel with pulse on-time values in the order of ns as a function of the main variables.
