Constantly smaller components increase the demands of industrial manufacturing processes. Traditionally used manufacturing processes are increasingly reaching their limits in terms of complexity and size [1].

Alternative machining processes, especially for metal workpieces, are the so-called non-conventional machining processes. Non-conventional machining processes are alternative machining processes, especially for metal workpieces and among them, laser chemical machining (LCM) is very important [2]. Laser chemical machining combines the advantages of laser processing, such as precise and localized energy input, and the advantages of electrochemical processing with a gentle energy impact without significant heat impact [3], which is why the unprocessed microstructure of the materials is not affected [4].

In laser chemical machining, the workpiece is irradiated by a laser, which introduces local, precise and adequate energy in the form of heat into the workpiece. The workpiece is surrounded by electrolytes [5]. Depending on the laser intensity applied and the spot size of the laser, the induced temperature fields on the workpiece change the electrochemical potential that an anodic material dissolution is induced resulting in a material removal at the surface of the workpiece [6].

Within a suitable process window, a gentle ablation is generated without a detectable heat-affected zone and without remelting processes in the material, which is the main advantage of laser chemical machining compared to other laser based non-conventional ablation processes [4].

The process window of the laser chemical machining is strongly influenced by a variety of parameters [7], such as laser power, flow rate of the electrolyte, machining speed that is why irregularities of the ablation can occur when leaving the process window [8]. Since laser chemistry is mainly a temperature-dependent process, the quality of the ablation generated is mainly dependent on electrolyte boiling, along with the boiling bubble size and the dynamic behavior of the boiling bubbles [9].

In this work, the formation and the behavior of the bubbles are therefore examined more closely in order to identify the relationship between bubble formation and ablation quality. First, ablations with a variation of the process parameters, e.g. laser power and machining speed, are generated and recorded by a high-speed camera. The recorded videos are then analyzed with a focus on the bubble diameter, the adhering time of the bubbles on the workpiece and the bubble size after the bubble has detached from the workpiece.

A continuous-wave (cw) fiber laser with a wavelength of 1080 nm is used as the laser beam source. The Gaussian laser radiation is first collimated to a beam diameter of 8 mm, then further reduced to 2 mm by an inverted telescope and then focused with a lens system with a focal length of 53 mm. With this setup a focus diameter of 30 µm was realized.

Titanium 3.7024 was selected as treatment material, with a size of 20 mm × 20 mm × 1 mm. Ceramic (Al₂O₃) with a comparable absorption, surface quality and heat conductivity were selected as reference material, see Table 1. Reason for

this is that the surface quality is crucial for the number of nuclei. This means that more nuclei are formed in the recess on rougher surfaces. In addition, the absorption determines how much thermal power is introduced into the workpiece. The resulting temperature field correlates directly with the number of bubbles and the bubble diameter.

The laser powers were chosen in such a way that they cover all three regimes, no removal, non-disturbed removal and disturbed removal, of laser chemical material processing [7].

The workpieces were embedded in a closed chemical cell through which a 5 molar phosphoric acid was constantly flowing as electrolyte, see Figure 1. The flow rate of the acid was 3 m/s.

The electrolyte height of approx. 20 mm on the workpiece impairs the power density of the laser beam on the workpiece with a transmission of 0.68. During the experiments the movement of the laser processing head was ensured by x- and y-axes. For investigations linear ablation lines were generated with velocities of 50 µm/s and 100 µm/s, see Table 2.

A high-speed camera including a band pass filter was used to record the interaction zone between the electrolyte and the workpiece during the process, compare Figure 1. The shadow method was used for video recording. It describes a transmitted light method in which refractive index changes are made visible in transparent shadows, see Figure 2.

An illumination laser was used as the light source. The illumination unit consists of a pulsed diode laser with a wavelength of 810 nm, a widening optic for uniform illumination of the interaction zone.

The complete experimental set-up and the chosen experiment parameters are the same for all experiments, it is only varied in the laser power and the machining speed, see Table 2 and Figure 1.

The high-speed images were taken with 256 × 256 pixels and frame rates of 22,000 frames per second. One camera pixel corresponded to a square surface with a side length of 1.75 µm. The video sequences were evaluated using Matlab.

As shown in the shadow images, Figure 2, the workpiece represents the lower dark area and the electrolyte represents the bright area due to its transparency. On the electrolyte side, induced transmission optical densities can now be detected during the process. The density differences show the forming bubbles and streaks. All the frames were individually separated and analyzed. The recorded images were first converted into binary formats and edges were detected, see Figure 2. The geometrically closed surfaces were assumed to be circular and evaluated as bubbles.

In the evaluation, a distinction was made between two types of bubbles, the adhering bubbles and the exiting bubbles. An adhering bubble was detected when the bubble was in direct contact with the workpiece surface. The bubble diameter of the adhering bubble was determined at the moment of detachment, so that the bubble diameter of adhering bubbles is equal to the maximum bubble diameter. An exiting bubble was determined when the bubble lost contact with the workpiece and is at least 75 µm above the workpiece. The reason for this is that the bubbles collapse after the ascent, so the bubble size of the exiting bubbles was detected after the collapse. Before the collapse the bubbles have the same diameter as the adhering bubbles. Bubbles with a minimum diameter of 3 µm could be detected. In addition to the sizes, the number of both adhering and exiting bubbles was determined. With the help of the number of adhering bubbles and the recorded time period (5 s) the frequencies of the adhering bubbles were calculated (formation frequency).

Additionally, the time in which the workpiece surface is covered with bubbles was determined (bubble coverage time). For this purpose, the time was detected in which no bubble adheres to the surface of the workpiece in the area of the laser irradiation.

In the LCM process, bubbles of different sizes are produced which reduce the ablation quality and thus represent a barrier to laser chemical machining [8]. Analogous to the assumptions in [9], different size ranges of the bubbles were detected during the machining of Ceramics and Titanium, compare Figure 3 and Figure 9. This points to two different types of bubbles, process bubbles and boiling bubbles.

Process bubbles are formed due to the chemical reactions and are filled with hydrogen. These cannot be prevented during the laser chemical processing of metallic materials, such as Titanium, see Figure 3. The boiling bubbles are caused by the thermal influence of the laser on the workpiece surface, regardless of the workpiece material. The workpiece is heated in the interaction zone of the laser and the heat is transferred to the electrolyte. The boiling temperature of the

electrolyte is locally exceeded, so that bubble boiling occurs. When looking at the bubble sizes during the heating of Ceramics, see Figure 9, it is noticeable that bubbles of larger diameters are formed compared to the machining of Titanium, see Figure 3. Since no process bubbles are formed during the heating of Ceramics,

it can be concluded that bubbles with smaller diameters are process bubbles and bubbles with large diameters are boiling bubbles.

As can be seen in Figure 3 and Figure 4, high laser powers cause large diameters of adhering bubbles. This is due to the increased local temperature distribution on the workpiece. In [8] there is a proportional relationship between the temperature on the workpiece and the laser power. Due to heat transfer mechanisms, the electrolyte temperature also increases locally and a boiling process with increased bubble dynamics occurs.

If one considers the different processing regimes based on the definitions in [9], marked in Figure 13, no bubbles are formed if the ablation is not visible. In this case, the surface temperatures are not sufficient to locally break up the passive layer and initiate the chemical reactions necessary for ablation. If the temperature of the laser power is calculated in the “no removal” regime, see Table 3, the boiling temperature of the electrolyte is not reached.

By increasing the laser power, the undisturbed area is reached. If the surface temperatures of the workpiece are higher than the boiling temperature, see Table 3 and Table 4, local overheating of the electrolyte occurs, resulting in boiling bubbles.

If one looks at the recorded bubbles and the time in which the workpiece surface is covered by bubbles, see Figure 3 and Figure 13, one can see that both the bubble size and the time in which the workpiece surface is covered by bubbles increases, so that a boiling process can be assumed. From this it follows that boiling bubbles are also present in the undisturbed state and thus undisturbed laser chemical processing under ambient conditions is possible if boiling bubbles are present.

If the laser power is further increased, the disturbed state is reached. It is visible that due to a higher surface temperature, see Table 3 and Table 4, the number of boiling bubbles, the size of the boiling bubbles and the time in which the workpiece is covered by bubbles increases, see Figure 3 and Figure 13. These can be seen analogously at a higher processing speed of 100 µm/s.

Looking at the regimes of undisturbed and disturbed ablation, the formation of boiling bubbles can be seen in both regimes. This indicates that bubble formation and the boiling process itself is not an indication of the quality of ablation.

It could be shown that two different types of bubbles, process bubbles and boiling bubbles, occur during laser chemical processing, while the process bubbles do not affect the ablation quality.

Using video graphic analysis of bubble formation during chemical processing with the laser, it was shown that undisturbed ablation is possible even in the presence of boiling bubbles.

This work has been funded by the Project 403820352 “Steigerung der Prozesseffizienz der laserchemischen Bearbeitung durch Vermeidung der gasblasenbedingten Abtragsstörungen”. The authors gratefully acknowledge the financial support by the Deutsche Forschungsgemeinschaft.

The authors declare no conflicts of interest regarding the publication of this paper.

Simons, M., Radel, T., Kajsaravally, R.S. and Vollertsen, F. (2020) Influence of Laser-Induced Bubble Formation on Laser Chemical Machining. Journal of Surface Engineered Materials and Advanced Technology, 10, 21-33. https://doi.org/10.4236/jsemat.2020.102002