In this study, a fiber laser at a wavelength of 1070 nm with different beam shapes (spot, dough-nut, and spot-wobble) was used to weld thin 316 L stainless steel foils. The welding speed was varied from 400 to 1000 mm/s in the absence of shielding gas. The weld geometry, microstructure, lap shear strength, and crystallographic grain structure of the micro-joints were analyzed and correlated with the beam shape and welding speed. The results indicate that the laser beam shape significantly affected the weld width and penetration depth, and the best welding speed was 500 mm/s. This study proved for the first time that a spot-wobble laser beam could achieve better mechanical properties and microstructural characteristics than a doughnut beam during the high-power laser welding of thin-foil stainless steel plates.

In this study, the laser cutting characteristics were analyzed according to the shape of the back side of the specimen, and the laser cutting characteristics were compared according to the thickness of the edge (10 mm, 20 mm, and 30 mm). A Yb-YAG laser was used in this study, and the cutting target was STS304 with a thickness of 50 mm, and the cutting process was analyzed using a high-speed camera. In the experiment, it was found through image analysis that the cutting performance was excellent at 30 mm thickness of the edge. In order to analyze this reason, a thermal conduction analysis (numerical simulation) was performed, and it was confirmed that the thicker thickness of the edge caused a preheating effect during laser cutting due to a large amount of heat accumulation. This effect can be used as a reference for the initial processing state while cutting thick metals as it is a characteristic that has not been revealed before.

This article presents an extended Cartesian space robot control framework that features a virtual force tracking impedance control to enhance the end‐effector trajectory tracking performance. Initially, the concept of a virtual surface is introduced, which is assumed to be at some constant distance from the desired end‐effector trajectory. This virtual surface generates a virtual contact force when interacting with the torque‐controlled robot end‐effector. The interaction is then manipulated using an impedance control model to track a constant desired force. If the robot end‐effector deviates from the desired trajectory, the constant force‐tracking impedance control generates a compliance trajectory that regulates the end‐effector movements, constraining it to the desired trajectory. For robust force tracking, impedance parameters are optimally tuned using a closed‐loop dynamic model incorporating both robot and impedance dynamics. Additionally, super twisting sliding mode control (STSMC) is integrated to overcome uncertainties and the impact of robot dynamics on force‐tracking performance. Experimental validation confirms the theoretical claims of the proposed approach. It demonstrates that force‐tracking impedance control improves the end‐effector trajectory tracking by quickly reacting to the dynamic trajectories compared to position control only and effectively maintains it on the desired trajectories.

Laser shock peening creates compressive residual stress on the surface of the material, reducing stress corrosion cracking and increasing fatigue life. FE simulation of laser shock peening is an effective way to determine the mechanical effects on the material. In conventional FE simulations of laser shock peening, explicit analysis is used while pressure loads are applied and switched into implicit analysis to dissipate kinetic energy. In this study, static damping was adopted to dissipate kinetic energy without conversion into implicit analysis. Simulation of a single laser shock and multiple shocks was performed, and deformation and minimum principal stress were compared to evaluate the static damping effect. The history of the internal and kinetic energy were analyzed to compare the stabilization time depending on the damping value. Laser shock peening experiments were also performed on stainless steel 304 material. The residual stress of the specimen was measured by the hole drilling method and it was compared to the FE simulation result. The residual stress from the experiment and the simulation results showed similar distributions in the depth direction. Anisotropic residual stress distribution due to the laser path was observed in both results.

While dismantling nuclear power plants, the reactor vessel internal is cut underwater using mechanical and thermal cutting. In laser thermal cutting, assist gas must be used to remove melted metal; consequently, a large number of radioactive aerosols can be generated. To reduce the generation of aerosols, the assist gas pressure should be lowered. However, below the pressure limit, the molten metal is not well-removed from the cut surface and fails to cut. In this study, an assist gas visualization experiment was performed to find a condition for the gas to flow well inside the cut surface, even at low pressures. The top kerf width, nozzle type, distance between nozzle and specimen, and assist gas pressure were selected as process parameters, and in the case of large top kerf width condition, assist gas was able to penetrate deeply. In the actual laser-cutting experiment, the laser beam focus position was set to −20 mm and −30 mm. In the case of −30 mm, the top kerf width was widened due to the characteristics of the laser beam profile, and cutting was successful even though the assist gas pressure was lowered by 20%.

Despite the dismantling structures that are submerged to significant depths of water during the decommissioning of nuclear power plants, there is limited research on deep-water laser cutting processes. A self-designed pressurized chamber was used in this study and successfully conducted the world’s first laser cutting experiment in a simulated 10 m water depth environment. laser cutting was performed in a 10 m underwater environment, and the cutting efficiency was compared to that observed in a 1 m underwater environment. Therefore, A 100 mm thickness of 304 stainless steel was successfully cut underwater, and the highest cutting speed of 100 mm/min was achieved. The result indicates that, as the cutting speed increased during underwater laser cutting, both the heat input and the mass flow rate of the assist gas decreased, resulting in a narrower rear kerf width and an ineffective evacuation of the molten metal.