Kozo Yasuda

High-power COIL and Nd:YAG laser welding

We have constructed a laser welding system, which enabled high-power laser welding by combining three laser beams of 1 µm wavelength. Its wavelength enables optical silica fibers transmission and the flexible system. The heart of this system consists of a 4 kW and a 6 kW Nd:YAG lasers and a 10 kW class Chemical Oxygen-Iodine Laser (COIL) beams of 6 kW Nd:YAG laser and COIL are combined in a coaxial beam and its maximum average power is 19 kW. The third laser beam, 4 kW Nd:YAG laser beam, is added obliquely from the same side of workpiece or oppositely from the reverse one. The effects of various welding parameters were investigated, such as the laser power, pulse modulation, and so on. As a result of the welding test with the 6 kW Nd:YAG laser, it was clarified that the pulse wave (PW) has good efficiency of deeper penetration at low welding speed. When the combined beam with CW COIL and PW Nd:YAG laser was used, 20 mm penetration on the stainless steel could be achieved at a welding speed of 1 m/min. By adding the third laser beam, the both side welding on 30mm thickness plate could be achieved.

Thick plate welding with Nd:YAG laser and COIL

In the field of heavy industries, many products are made of thick metal parts. Nd:YAG laser has been recently developed up to 10 kW. Nd:YAG laser has the characteristics of the optical fiber transmittance and the good absorption by the metal surface, so that it is expected to apply Nd:YAG laser to thick plate welding. This study presents the thick plate welding with Nd:YAG laser and COIL (Chemical Oxygen-Iodine Laser). We have developed a coaxial beam combining system with beams of Nd:YAG laser and COIL. The maximum average power of the combined beam was 19 kW. Welding tests of 304 stainless steel plates were carried out. The effects of various welding parameters were investigated, such as the laser power and pulse modulation. As a result, it was clarified that the pulse wave has good efficiency of deeper penetration as compared to continuous wave at low welding speed. When the combined beam was used, 20 mm penetration depth on the stainless steel could be obtained in high aspect ratio at welding speed of 1m/min. When the combined beams and another Nd:YAG laser beam whose power was 4 kW were used, both side welding on 30 mm thickness plate could be achieved.

Development of numerical simulation technique for laser welding

We have been developing numerical simulation technique for laser welding. This study present some results obtained recently on the improvements of simulation models and numerical procedures, experimental data for code validation and the results of comparison of experiments with numerical analysis. We added the surface evaporation model under boiling temperature and introduced the level-set method as accurate and low numerical diffusion tracking procedure. We extend our gas phase model as enable to treat multi species of gas and mutual diffusion process. Two kinds of experiment were carried out in this study for code validation. One is the experiment to observe the surface tension driven convective flow and the other is to confirm the threshold of laser power density to form the keyhole. In these experiments, 6 kW YAG laser is used in CW mode and test pieces of aluminum alloy A1050P are irradiated in an inert atmospheric box. We calculated the heat conduction type of welding using improved code based on the actual welding conditions including the surface tension and surface evaporating models. And we simulated a series of transient behavior of molten pool irradiated by high laser power density.