Mechanism of Time Power Modulation Increasing Weld Depth in High Power Laser Welding

Researchers from the Hanover Laser Center and Leibniz University in Germany reported on the mechanism of increased welding depth during time power modulation in high-power laser beam welding. The related paper titled "Mechanisms of Increasing Welding Depth during Temporary Power Modulation in High Power Laser Beam Welding" was published in Advanced Engineering Materials.

Understanding the basic mechanism of increasing weld depth during time power modulation in laser welding allows for heterogeneous rotational welding without introducing accompanying turbulence, but enhances the mixing effect. A 16kW disc laser source was used to study stainless steel alloy 1.4301 and nickel based alloy 2.4856 rods with a diameter of 30mm. The modulation frequency is 0/50/100/200 Hz, and the laser beam power is low, medium, and high amplitude, respectively. The influence on processing and welding characteristics was studied through high-speed imaging with grayscale analysis, depth measurement of lock holes, metallographic slicing, and energy dispersive X-ray spectroscopy analysis. The results indicate that under the modulation frequency of 200Hz and high laser beam power amplitude, the depth of the lock hole remains at a high level, and the fundamental mechanism is related to the inertia of the lock hole. On this basis, a new welding mode with a constant lock hole depth is proposed. In addition, the welding depth can be increased by up to 20%, the saturation limit of modulation frequency has been determined, the mixing inside the weld seam has been strengthened, and a model for predicting welding depth based solely on surface width measurement results has been developed.

Laser beam welding has become a widely used welding process due to its high efficiency and low heat input. However, there are still several aspects that need improvement to enhance efficiency, and the most important one is to improve the stability of the process. The typical continuous welding process is accompanied by inherent keyhole oscillation. Especially when welding dissimilar metals, welding instability can lead to uneven distribution of alloy elements and decreased mechanical properties. Methods to improve the stability of laser welding processes include: 1) beam shaping; 2) Spatial modulation; 3) Time modulation.

Figure 1 Cross section of a 30mm diameter nickel based alloy rod with a 2.4856 conductive melted gold phase at the center of the specimen during heterogeneous laser welding.

Figure 2 Schematic diagram of experimental setup and processing area.

Figure 3 Cutting plane (red dashed line) and metallographic slice example: a) Cross section; b) Draw a red line along the longitudinal section to outline the shape of the weld pool and the location of the EDX scan (including the scanning direction).

Figure 4 Example program for high-speed image processing and weld width detection: a) High speed image with annotations and mask position indication; b) Weld width chart with average and calculated limit values.

Figure 5 shows the longitudinal and transverse sections of the butt joint 1.4301/2.4856, including the weld structure: a) without power modulation, Pcw=7kW； b) There is power modulation, Pav=6.7kW, π=0.73 (6.7 ± 3.9kW), f=200Hz.

Figure 6 The principle of protrusion evaporation and increased welding depth under time power modulation.

Figure 7 shows the chemical composition obtained through X-ray scanning.

Figure 8 shows the functional relationship between laser power and lock hole depth as a function of time with modulation frequency variation, 1.4301, Pav=3.5 kW, π=0.73 (3.5 ± 2.0 kW).

Figure 9 Formation mechanism of splashing and lock hole defects.

Power modulation of the stainless steel rod and nickel based alloy 1.4301/2.4856 butt joint can significantly increase the welding depth by nearly 20% and reduce the conductive melting at the center of the specimen. In addition, the enhancement of the mixture was clearly detected, which increased the uniformity of the welding chemical composition and helped reduce welding defects such as cracks. However, due to changes in the shape of the melt pool or improvements in process stability, the porosity will decrease without turbulence caused by time power modulation.

In addition, by measuring the width of the weld seam, the depth of the weld seam can be controlled based on the modulation frequency, which is very useful for manufacturing processes that utilize time power modulation.

Finally, researchers have discovered for the first time the exact mechanism by which welding depth increases with modulation frequency under constant average power conditions. These results should also apply to plate welding instead of round bar welding, and promote the development of time power modulation welding.

Future research could be based on the discovered mechanisms of action, including the impact of beam shaping on power modulation and convergence towards continuous wave welding processes, as welding depth may decrease with further frequency increase. In addition, the welding mode with constant lock hole depth should also be studied. Another method is to adjust the modulation of different rise and fall times to accommodate different durations of minimum and maximum lock hole depth levels. Due to the expected minimal additional increase in welding depth, this method is only suitable for special applications, such as materials that are highly sensitive to thermal loads.

Source: Yangtze River Delta Laser Alliance