It is reported that researchers from BIAS Bremer Institution f ü r angewandte Strahltechnik GmbH in Germany have reported a comparative study of laser deep penetration welding processes for pure nickel using blue and infrared light wavelengths. The related research was published in Welding in the World under the title "Process comparison of laser deep penetration welding in pure nickel using blue and infrared wavelengths".
Compared with infrared laser radiation, the Fresnel absorption rate in the visible blue spectral range is significantly increased, making it suitable for thermal conduction mode welding of materials such as copper and nickel. Recently, a blue laser source with a wavelength of 445 nm has emerged, whose power and beam parameters are sufficient to exceed the intensity threshold of laser deep penetration welding. Compared with heat conduction mode welding, in laser beam deep penetration welding, the total absorption is significantly increased due to multiple reflections inside the lock hole. However, since the absorbed energy per reflection inside the lock hole is wavelength dependent, it can be assumed that the selection of laser wavelength will cause changes in the local energy distribution inside the lock hole, thereby altering its dynamics. To investigate this issue, researchers conducted laser beam deep penetration welding experiments on 2.4068 pure nickel using infrared laser sources and blue laser sources with comparable beam characteristics. The experiment was monitored and compared through multi-sensor devices and metallographic analysis. The use of a blue laser beam can reduce sputtering volume, increase porosity, and significantly alter acoustic emission, thus proving the hypothesis for pure nickel.
Figure 1: The measured caustics and one-dimensional and two-dimensional intensity curves on the focal plane of the laser beam used.
Figure 2: Sample size and design (the sample needs to be replaced after each welding to allow the sample temperature to drop to room temperature before the next welding)
Figure 3: Left: Experimental schematic diagram; Right: Image of experimental setup
Figure 4: Left: High speed video raw frames used for splash detection; Center: identified areas of interest; Right: Detected splashes
Figure 5: Left: High speed video raw frame used for measuring lock hole area; Left second: Detected lock hole area; Right two: measured lock hole area; Right: Definition of Lock Hole Area Radius Deviation
Research has shown that the comparison of carbon dioxide laser sources and solid-state laser sources with different wavelengths has a significant impact on keyhole dynamics, but this cannot be entirely attributed to changes in the Fresnel absorption coefficient caused by plasma absorption. In order to further clarify the relevant effects, this study aims to separate the effects of plasma absorption and Fresnel absorption coefficient changes on keyhole dynamics by using lasers of different wavelengths. The hypothesis studied by researchers is that in nickel laser beam deep penetration welding, the laser wavelength changes from 1030 nm to 445 nm, and the Fresnel absorption coefficient increases accordingly. This will cause changes in the local energy distribution inside the lock hole, thereby altering the dynamics of the lock hole, including the wave motion of the lock hole opening, the formation of splashes, acoustic emission, and the resulting porosity. To verify this hypothesis, experimental monitoring and comparison were conducted on nickel plates using lasers of the two wavelengths mentioned above. In this study, nickel was found to be more suitable than copper because the Fresnel absorption coefficient significantly increased from infrared to blue wavelengths. However, compared to copper laser beam welding, which can only observe unstable processes, researchers have developed a constant deep penetration welding process. This makes the welding process more comparable.
Figure 6: Average weld depth (upper figure) and average weld width (middle figure) as a function of laser power and wavelength; Characteristic metallographic cross-section (as shown in the figure below)
Figure 7: Etching the longitudinal section of the gold phase, with a significant increase in welding depth
Figure 8: Spectral Reference
This study conducted laser beam deep penetration welding experiments on 2.4068 pure nickel using an infrared laser beam source with a wavelength of 1030nm and a blue laser beam source with a wavelength of 445nm. The beam characteristics of these two laser beams were comparable. In each case, two different laser powers were used, with the same welding depth compared to samples welded using their respective other wavelengths, to investigate the hypothesis that changing the laser wavelength would alter the local energy distribution and dynamics inside the lock hole, including fluctuations in the lock hole opening, formation of splashes, acoustic emission, and resulting porosity. The experiment was monitored and compared through metallographic analysis and multi-sensor setup (including splash tracking, lock hole area tracking, and airborne acoustic emission measurement), and the results confirmed this hypothesis.
1. Changing the laser wavelength from 1030 nm to 445 nm will alter the dynamic of the laser beam deep penetration welding lock hole for pure nickel.
2. When welding pure nickel, the effect of Fresnel absorption coefficient on welding penetration decreases with the increase of aspect ratio when the laser beam wavelength changes from infrared wavelength to blue wavelength.
3. Compared with the wavelength of the blue laser beam, using an infrared laser beam with a lower Fresnel absorption coefficient can reduce the porosity of nickel welds.
4. For laser beam deep penetration welding of nickel, compared with welding processes using infrared laser beam wavelengths, using blue wavelengths with higher Fresnel absorption coefficients can reduce spatter and improve process stability.
5. Through airborne acoustic analysis, significant differences can be detected when welding nickel using blue wavelength and infrared wavelength.
Source: Yangtze River Delta Laser Alliance
