2025 Volume 66 Issue 1 Pages 113-116
Highly efficient and high quality pure copper welding with the blue diode laser, which has high optical absorption into pure copper, is expected to be realized. It is necessary to clarify the interaction between copper and laser beam when a blue diode laser is focused and irradiated on pure copper, and we focused on the blue diode laser-induced plume generated during copper welding. In this study, as a basic research to elucidate the interaction between the laser beam and the blue diode laser induced plume generated during welding of pure copper, spectroscopic analysis of the plume was performed to identify the plume constituent elements and their spatial distribution was experimentally clarified. The emission lines in the spectra were found to be Cu and CuO, indicating that the composition of the plume varies spatially.
This Paper was Originally Published in Japanese in J. Japan Inst. Copper 63 (2024) 171–175. Figure 2 was slightly modified.
Pure copper is widely used in motor coils and batteries due to its high electrical and thermal conductivity, and the progress of pure copper joining processing technology is essential to improve product performance. Blue diode lasers have been attracting attention in recent years. Conventionally used near-infrared lasers have a low light absorption rate of 5–10% in pure copper, making stable and efficient welding difficult.
However, blue diode lasers have a high light absorption rate of about 60% in pure copper, making welding with high heat input efficiency possible. Therefore, blue diode lasers are attracting attention as an essential welding light source for realizing a carbon-neutral society [1]. Development of high-power blue diode laser sources has been undertaken around the world in recent years, but there are few research reports on the basic physics of welding using blue diode lasers to achieve highly efficient pure copper welding. Although several studies have been conducted around the world, including by our research team, on the efficiency with a laser converts light energy into thermal energy on the surface of pure copper, and on the flow of pure copper after melting [2–4], no research has been reported that focuses on the plume (a general term for “plasma” generated when metal vapor at the processing point is further heated by the laser beam at the processing point, “fumes” that are fine dust generated when metal vapor cools, and “spatter”, which is flying molten metal) that is generated above the processing point during pure copper welding.
In this study, the plume generated when welding pure copper sheets by focusing a blue diode laser is defined as the “blue diode laser induced plume”. In a study on stainless steel welding using a high-power near-infrared fiber laser, it was reported that the laser is diffused, absorbed, and refracted by the plume, reducing the energy density (amount of energy per unit area at the processing point) of the laser that reaches the processing point [5]. A similar phenomenon may occur in pure copper welding using a blue diode laser, and there is a concern that this will be a factor that prevents highly efficient and stable pure copper welding. Therefore, if the interaction mechanism between light and particles in the blue diode laser induced plume generated during pure copper welding can be clarified, it is believed that this will lead to the realization of more efficient and stable pure copper welding. As a first step in elucidating the interaction mechanism, this study focused on the plasma and fumes in the blue diode laser induced plume and aimed to experimentally identify their composition through spectroscopic measurements. We designed and manufactured a detection optical system that can measure spectroscopic measurements by resolving them in time and space, and measured the spectroscopic spectrum of the plume. The obtained emission lines may be due to the target copper, copper oxide, or atmospheric gas, so we changed the atmospheric gas and performed spectrometry to identify the emission lines. In addition, since there is not enough database of emission lines for copper oxide, we identified them by actually performing spectrometry on the plume generated by irradiating copper oxide powder with a blue diode laser.
Figure 1 shows the experimental setup. The light source used was a blue diode laser (manufactured by Shimadzu Corporation) with a maximum output of 1500 W, which was spatially combined with three blue diode lasers with an output of 500 W. The laser irradiation conditions are shown in Table 1. Oxygen-free copper was used as the target, and the laser was irradiated while Ar gas was sprayed from a shielding gas nozzle in the atmosphere. The spontaneous emission of the plume during the processing was captured with a high-speed camera. The observation conditions for the spectrometry are shown in Table 2. A detection system was constructed so that the measurement field of the spectrometer (HR2000+, manufactured by Ocean Photonics) was set to Φ100 µm, and the spectrometry was performed from a position 0.2 mm directly above the processing point to a height of 12 mm above. A microscope (VHX-7000, Keyence) was used to observe the sample surface and cross section after the laser irradiation.
Schematic diagram of experimental setup.
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Figure 2 shows (a) the surface and (b) cross-sectional photographs of a pure copper plate irradiated with a focused blue diode laser, and (c) a high-speed camera image. The weld width was 1.02 mm and the depth was 0.86 mm. The surface of the molten pool was uneven, and the spatter adhesion was observed. The cross-sectional shape of the molten area indicates that a keyhole mode welding was performed. An observation with a high-speed camera showed that strong light emission was observed directly above the processing point, and the intensity decreased as it moved upward. A spatter scattering was also observed. The height position of the spectroscopic measurement was recorded on the image obtained by superimposing the high-speed camera observation images and averaging them over time. Since the goal of this study is to identify the emission lines, the spectroscopic spectra described in the following sections are spectra obtained by time-averaging 880 ms during the laser irradiation.
Melting area shape and plume generation behavior. (a) Surface and (b) cross-sectional image of pure copper plate after blue diode laser welding and (c) high speed camera images of blue diode laser induced plume.
In order to identify the constituent elements of the blue diode laser induced plume, a blue diode laser was irradiated onto a pure copper plate using Ar, N2, Air, and O2 as shielding gases, and the optical spectrum was measured at a position 4 mm above the processing point in the wavelength range from 510 nm to 650 nm. The spectra for each gas used are shown in Fig. 3. Emission line peaks were measured at wavelengths of 510.53, 514.63, 523.72, 528.72, 539.17, 547.33, 555.48, 569.96, 578.09, 606.48, and 616.37 nm, and there was no change in the wavelength of the emission line spectra for all gases. Therefore, it was found that no emission lines were generated due to the ionization of the atmospheric gas.
Spectrum of laser induced plume of pure copper at the detected height of 4 mm when Ar, N2, Air and O2 gas was used as shielding gas.
Next, the results of isolating the emission line spectra caused by Cu and CuO are described. When welding in the atmosphere using a shielding gas nozzle, it is difficult to completely block the oxygen in the air at and above the processing point, so the blue diode laser was irradiated in a simple chamber replaced with Ar gas, and the emission line spectrum was measured. Figure 4 shows the spectrum measured when a shielding gas nozzle was used and when measured in a chamber. In an Ar gas atmosphere where oxygen was blocked, emission lines were observed at wavelengths of 510.53, 515.54, 521.91, 529.63, 569.96, and 578.09 nm, and compared with the results of previous studies [6, 7], these spectra were found to be neutral lines of Cu. On the other hand, when a shielding gas nozzle was used, emission lines were observed at wavelengths of 523.72, 539.17, 547.33, 555.48, 606.48, and 616.37 nm in addition to the above. Based on the results of previous research [8], the wavelengths of 606.48 and 616.37 nm are spectra derived from copper oxide, but we decided to identify the other spectra by actually irradiating CuO with the laser. Figure 5 shows the results of measurements taken by irradiating CuO powder with a blue diode laser in a chamber filled with Ar gas. As a result, peaks were observed at 14 wavelengths, namely 510.53, 515.54, 523.72, 529.63, 539.62, 545.06, 548.24, 553.22, 555.48, 569.96, 578.09, 589.37, 606.48, and 616.37 nm. Among these measurements, the wavelengths 510.53, 515.54, 529.63, 569.96, and 578.09 nm were determined to be neutral lines of Cu by comparison with the results of irradiation on pure copper, and the wavelengths 523.72, 539.62, 545.06, 548.24, 553.22, and 555.48 nm was identified as an emission line derived from Cu but not from Cu generated under the influence of oxygen, i.e., an emission line derived from CuO.
Spectrum of laser induced plume of pure copper at the detected height of 4 mm when argon gas was flowed in the atmosphere using a shield gas nozzle (black line), and when the atmosphere in the chamber was replaced with argon gas (gray line).
Spectrum of laser induced plume of CuO (black line) and Cu (gray line) at the detected height of 4 mm when the atmosphere in the chamber was replaced with argon gas.
The above experiment enabled us to identify all of the clear spectral emission lines in the plume generated by irradiating pure copper with a focused blue diode laser, and found that they were all neutral lines of Cu and CuO. Furthermore, under the laser irradiation conditions used in this experiment, which were an output of 1400 W and a spot diameter of 300 µm, it was found that all of the emission line spectra were neutral lines of Cu and CuO, indicating that no ionization of Cu or the ambient gas occurred, and that no plasma was generated by the laser on the metal vapor.
3.3 Spatial distribution of blue diode laser induced plumeFigure 6 shows the results of investigating the spatial composition distribution in the vertical direction of the blue diode laser induced plume generated during the pure copper welding. (a) is the spectrum measured at a position 0.2 mm above the processing point, and (b) is the result measured at a position 4 mm above the processing point. Based on the wavelengths of the emission lines identified in the previous experiment, the dotted line indicates the neutral line of Cu and the dashed line indicates the emission line of CuO. Ar gas was used as the shielding gas, and the welding was performed using a shielding gas nozzle. Although the neutral line of Cu was observed at a height of 0.2 mm directly above the processing point, no peak was observed for CuO. In addition to the neutral line of Cu, the emission line of CuO was observed at a position 4 mm above the processing point. Next, Fig. 7 shows the relationship between the height of the measurement point measured by spectroscopy and the intensity ratio of the emission line of Cu with a wavelength of 515.54 nm and the emission line of CuO with a wavelength of 606.48 nm. The intensity ratio of Cu and CuO in the plume was not constant, and it was found that Cu was the most abundant at 98.5% directly above the processing point, and the proportion of CuO tended to increase upward. At a height of 12 mm, CuO accounted for 97.6%. These results suggest that the CuO in the plume was not generated at the laser irradiation point, but rather that the generated plume combined with oxygen in the atmosphere and was oxidized as it moved upward.
Spectrum of laser induced plume of pure copper at the detected height of (a) 0.2 mm and (b) 4 mm. Gray dotted line was the spectrum of neutral Cu atom. Gray dashed line was the spectrum of CuO.
Ratio of spectral intensity of Cu (515.54 nm) and CuO (606.48 nm) in the blue diode laser-induced plume.
This study experimentally identified the elements contained in the blue diode laser induced plume during the pure copper welding and clarified its spatial distribution. In the future, we believe that further discussion of energy loss due to the interaction between the plume and the laser will be possible by investigating the correlation between the temporal intensity change of the blue diode laser induced plume and the welding results. In addition, we will also measure the plume generated when a near-infrared laser is irradiated on pure copper, and by comparing the results with the results of blue diode laser irradiation, we hope to clarify the differences depending on the laser wavelength that excites the plume. Finally, the establishment of technology for stable welding of pure copper using a laser is a technology that is strongly desired by industry. If we can understand the interactions between the plume and the laser light and achieve feedback control of the laser through in-process monitoring, we hope that this will lead to the further promotion of the use of blue diode lasers.
As a fundamental study to clarify the interaction between the blue diode laser induced plume generated during pure copper welding and the laser light, we performed a spectroscopic analysis of the plume and experimentally clarified the identification and spatial distribution of the plume constituent elements.
This study was supported by JSPS Research Activity Start-up Support JP23K19180. We would like to express our sincere gratitude to Nichia Corporation for their cooperation in carrying out this study.
