8.2 Experimental part and analysis of it
8.2.5 Fifth step
The last step of the study is measuring of reflection coefficient using power meter. All of set-up parameters completely the same like in table 12. New set-up parts were drawn in AutoCAD software (figure 39). Black-painted plate was replaced by the power meter (figure 40). Sensitive zone of the power meter was positioned according to the shape location from the table 12. Table 13 shows the reflection coefficient in the percentage meaning for different angles and power. figure 41 illustrated dot plots based on Table 13. Linear approximation was used to show trend line.
Table 13. Results of the fifth step.
Wire angle, ° Laser power, kW Reflected power, kW Reflection coefficient, %
1.3 0.290 22.31
Table 13 continues. Results of the fifth step.
Wire angle, ° Laser power, kW Reflected power, kW Reflection coefficient, %
1.8 0.300 15.94
1.9 0.355 16.67
60
2.0 0.380 18.68
Figure 39. Drawing of new metallic parts for the set-up. 1 – Front-side protective cover plate for the power meter; 2 – Back-side fix cover plate for the power meter; 3, 4, 5 – Parts of support structure for the power meter.
Figure 40. Set-up to measure intensity of the reflected light. 1 - laser processing head of ytterbium laser system “YLS-10000-S4”; 2 - Feed nozzle with carbon-manganese steel wire
“OK Autrod 12.64”; 3 – Compact Power Monitor PRIMES CFM-F (800-1100 nm); 4 - Light catching surface nearby sensitive area.
Figure 41. Dot plots illustration of the reflection coefficient with approximation lines for different wire angles and laser power.
9 RESULTS AND DISCUSSION
It can be seen from the table 9 that small beam diameter in combination with high WFR can destroy the plate for the reason that small beam size in comparing with bigger one gives higher meaning of power density. Equation 1 describes the dependence between power density and the size of an irradiation area , where is a laser power, is an absorption 𝑞 𝑆 𝑃 𝐴 coefficient. Equation 2 shows the dependence between the irradiation area and the meaning 𝑆 of beam diameter :𝑑 dependent on beam diameter. It is a reason why the focal position was changed and the beam diameter was increased with the aim of do not damage plate:
(3)
Power meter, which was used in the set-up (figure 40), has sensitive area with diameter of 100 mm, so it was very important to explore if there are any significant changes in shape distribution for the same power but with different interaction time. That is why it was decided to use 2 seconds interaction time against 5 seconds due to the reason that no significant changes were observed (figure 30).
For the reason of limited time to access the laboratory, the decision about stable FWR was made to cut experimental part for this study. It was necessary to find, which of the available WFR meanings are better in case of limitation in sensitive are of the power meter. From the figure 31 it was concluded that WFR of 4 m/min leads to the smallest thermal shape of reflected beam for any of the chosen angles.
The main role of the table 12 is to show the location of the reflected beam falling and on which distance from the working table it is. It can be seen that the shape location almost unchanged with a slight increasing in laser power. However, the position of it influences significant changes with the changes in wire angle. The base law of geometrical optics can easily explain this fact.
From the results of power meter measurements, the reflection coefficient was found as the ratio of the laser power to the reflected power. It can be seen from the table 13 and figure 41 that in case of using “OK Autrod 12.64” wire 30° angle gives the biggest reflection in comparison with 45° and 60°. It can be mentioned that when power increases, reflection coefficient becomes smaller. This fact is described by the mechanism of the wire surface formation when it is melted. figure 42 shows surface of the wire using different power with the angle of 45°. It can be seen, that with 1.4 kW the wire does not fully melt in the area of laser beam and has some kind of “protrusion” in the end (green line). For this reason, light almost does not scatter inside a liquid zone of the metal and it is reflected with a bigger value in compare with 2.0 kW. Significant part of the light that can be reflected goes through the liquid metal droplet (orange zone) and experiencing scattering; part of the scattered photons goes with almost the same direction like the laser beam, which is reflected from the wire.
However, most of them scattered in other directions, especially in the direction to the base metal. If compare reflection coefficient for 1.4 kW and 2.0 kW power in case 45° angle (table 13), it loses approximately 24% of overall reflection. With power increasing, reflection has to become lower. Based on the geometrical optics, it is suggested that red zones on figure 42 shows approximately directions of the reflected light, that is come from the laser head, which is above the wire. This assumption explains why the reflected zone in case of 1.4 kW looks more spherical than the zone of 2.0 kW (figure 43).
Graphs on figure 41 shows dependences between power and reflection coefficient for angles of 45° and 60°. The linear approximations of them looks the same like in Salminen`s research [1], but they have inverse meanings in comparison with graphs that he got (figure 44). It can be explained by the figure 45. There are shown two scenarios, when the laser beam area fully fall into the wire surface and when it is not fall on it. Picture A described Salminen`s approach, picture B described approach of this study. Salminen used constant power of 5 kW and started from low WFR then moving to bigger meanings. It leads to the fact, that with
increasing in WFR, wire has no more time to melt with the same speed for the reason that wire volume that has to be melted is increased. The opposite approach is used in this study.
That is why graphs look like inverse to each other.
Figure 42. Photos of different conditions of the wire for 1.4 kW and 2.0 kW with constant WFR using angle of 45°. Green line - “protrusion” in the end of the wire; red zone – possible directions of the reflected beam; orange zone – light scattering.
Figure 43. Thermal photo of the beam shapes for 1.4 kW and 2.0 kW with constant WFR using angle of 45°.
Figure 44. Comparison of two graphs. A – Salminen’s results [1]; B – Figure 41. Green line – approximation for 60°; Red line – approximation for 45°.
Figure 45. Illustration of different experimental approaches. A1, A2 - Salminen’s approach;
B1, B2 – the approach of this study.
10 SUMMARY
The approach to measuring reflection coefficient was developed in the study. It was found that interaction time has no influence on the character of the reflection. According to the thermal photos from figure 31 it can be concluded, that reflection is growing when WFR is increasing. With a constant WFR and the angle, the reflection coefficient is decreased with increasing in power. This correlation has linear character. Wire angle has significant influence on the reflection.
For the reason that all of the results are proofed by comparison with Salminen’s results [1]
and by analysis of high-speed and thermal images (figures 42, 43), it can be concluded that the wire surface has one of the major influence on the reflection coefficient in case of using constant laser-wire combination. WFR and laser power have influence on changes in the geometry of wire surface, but have no influence on the reflection by themselves. It is suggested that for different wire and laser combination the results will not be the same due to the different absorption coefficient.
Based on the results, it can be offered to future studies to go deeper into the sphere of droplet and wire monitoring throughout the melting process following the example of study done by Shao, Wang, and Zhang [43]. The research was successful, all targets were achieved, and the results were analyzed. Analytical approach of results was applied and conclusions were done.
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