Suck-Joo Na

Implementation of real-time multiple reflection and fresnel absorption of laser beam in keyhole

A computational analysis of laser keyhole welding is achieved. The main driving force to make the molten pool as a narrow and deep keyhole is the recoil pressure induced by evaporation of the material. Also, the multiple reflection effect on the keyhole wall plays an important role in making the keyhole deeper and raising its total energy absorption rate. Multiple reflection and Fresnel absorption are implemented simultaneously with the proposed ray tracing technique in a discrete grid cell system during the simulation for every single time step. In particular, the Fresnel absorption model is chosen as an energy transfer mechanism from laser beam to workpiece. With all the governing equations including continuity, momentum and energy equation, the VOF method is adopted to trace the free surface of the molten pool. Simulation results are compared with the experimental ones to verify its validity. A pulsed Nd?:?YAG laser was used for keyhole welding experiments on mild steel plates of 7?mm thickness. It was observed that the generated keyhole maintains its solidified shape without any closing phenomenon both in the experiments and in the simulations.

Heat transfer and fluid flow in a partially or fully penetrated weld pool in gas tungsten arc welding

Abstract In this study, fluid flow driven by a combination of electromagnetic force, buoyancy force, arc drag force, and surface tension gradient is numerically analyzed for a partially or fully penetrated weld pool in stationary gas tungsten arc welding (GTAW). Irregular shape of the weld pool and the moving liquid–solid interface is handled by a boundary-fitted-coordinates technique. Surface tension on the top and the bottom free surfaces of the weld pool is found to play a critical role in determining the flow pattern in the weld pool. The calculated GTA weld bead geometry compares favorably with experimental results.

A study on the prediction of the laser weld shape with varying heat source equations and the thermal distortion of a small structure in micro-joining ☆

Abstract In the area of laser welding, numerous studies have been performed the past decades using either analytical or numerical approaches, or both combined. However, most of previous studies were process oriented and modeled differently in conduction and keyhole welding. In this research, the results of calculation using various heat source equations that have been proposed in previous studies were compared to the predictions of a new model. This model treats the problem of predicting, by numerical means, the thermo-mechanical behavior of laser spot welding for thin stainless steel plates. A finite element code, ABAQUS, is used for the heat-transfer analysis with a 3D model. Experimental studies of the laser spot welding have also been conducted to validate the numerical models presented. The results suggest that the predicted temperature profiles and weld dimensions vary according to the heat source equation of the laser beam. For this reason, it is essential to incorporate an accurate description of the heat source. Thermal and mechanical analyses of the laser micro-welding of a small structure are performed from these results.

Effect of various driving forces on heat and mass transfer in arc welding

Abstract In this study the heat transfer and fluid flow of the molten pool in stationary gas tungsten arc welding using argon shielding gas were investigated. Transporting phenomena from the welding arc to the base material surface, such as current density, heat flux, arc pressure, and shear stress acting on the weld pool surface, were taken from the simulation results of the corresponding welding arc. Various driving forces for the weld pool convection were considered: self-induced electromagnetic, surface tension, buoyancy, and impinging plasma arc forces. Furthermore, the effect of surface depression due to the arc pressure acting on the molten pool surface was considered. Because the fusion boundary has a curved and unknown shape during welding, a boundary-fitted coordinate system was adopted to precisely describe the boundary for the momentum equation. The numerical model was applied to AISI304 stainless steel and compared with the experimental results.

Application of Abel inversion in real-time calculations for circularly and elliptically symmetric radiation sources

New Abel inversion methods were developed for real-time calculations of emissions from circularly and elliptically symmetric radiation sources. Using the geometrical relationship between the plasma and the measuring device, an upper triangular area matrix was introduced to determine the local plasma emissivity from the measured irradiation value. Fast calculations of Abel inversion could be achieved using this area matrix since there was no need to fit the data for inversion. Inversion correctness was checked by the use of a given test function. This fast calculation algorithm could also be applied for the radiation source with the elliptical symmetry. This technique was applied to the measurement of arc plasma intensity in flat and V-grooved arc welding.

Development of irradiation strategies for free curve laser forming

Abstract Although forming sheet metal by laser-induced thermal stresses (laser forming) has been extensively studied, the research has mainly focused on a single angle forming process. The task of free curve laser forming of sheet metal is to determine a set of process parameters such as laser scanning paths, laser power and scanning speed that will make a given shape. Two methods were used for generating the laser scanning paths and the bending angles of each path. Each method was analyzed by computer simulation and the two methods were compared. Experiments verified the applicability of the proposed methods.

A study on vision sensors for seam tracking of height-varying weldment. Part 1: Mathematical model

Abstract Among the various types of sensors used in welding automation, arc and vision sensors are widely used for automatic seam tracking and quality control of arc welding processes. The development of a vision sensor which can provide 3D geometry information is needed especially for robot guidance in automatic welding of thin plates, because it is difficult to apply an arc sensor to the welding of thin plates. Vision sensors using optical triangulation have been widely used in various ways for automatic welding systems. The separation angle between the camera and the laser axis has been mainly considered in previous studies of the vision sensor with structured light. Their measuring efficiency is, however, considerably influenced by the different arrangements of the weldment, the CCD camera, and a diode laser of a vision sensor in three-dimensional space. Therefore to enhance the effectiveness of a vision sensor for height-varying workpieces, other geometrical parameters such as diagonal and tilt angles should also be investigated. In the present study, the data deficiency in the vision sensor falls into two classifications: the shadow effect and the missing field of view (FOV), and a mathematical model is proposed to estimate the occurrence of data deficiency.

A study on automatic seam tracking in pulsed laser edge welding by using a vision sensor without an auxiliary light source

Abstract Vision sensors adapted for automatic seam tracking in fusion welding have an auxiliary light source and a preview distance, which is the distance between the welding and the sensing position. The preview distance can generate tracking errors in automatic seam tracking, especially in micro welding of small parts. On the other hand, vision systems used for the weld pool monitoring are generally equipped with a highspeed camera and intense illumination. In this study, a vision sensor was applied to find the weld seam by monitoring the weld pool in pulsed Nd:YAG laser welding. The vision sensor observed the weld pool, and the weld seam was extracted from the image of the monitored weld pool. No auxiliary illumination source was used for monitoring the weld pool. The shape and brightness of the weld pool change temporally even in one pulse duration. To get a relatively clear image, the shutter of the CCD camera was synchronized with the start of each pulse and opened at a specific time delay from the laser pulse start. Experiments were carried out to find the appropriate moment of shutter triggering for relatively clear images. Seam tracking was successfully performed by using the information of the pool centerline in the edge joint, while the shape and brightness of the weld pool gave the first step toward the information on the status of weld quality.

A study on vision sensors for seam tracking of height-varying weldment. Part 2: Applications

Abstract Many efforts have been made to develop an automatic welding system in order to improve the quality and productivity in welding shipping containers and LNG (Liquefied Natural Gas) storage tanks which consist of many sheet metals having linear weldments with height variation. A vision sensor which can provide 3D geometry information about objects is investigated primarily, because it is difficult to apply the arc sensor to automatic welding of thin plates. However it is difficult to determine the proper configuration of sensory system including a CCD camera and diode laser to specific weldments because of the interference and relative motion among vision sensor, welding torch and weldment. Therefore, to develop a vision sensor with high performance for a specific workpiece, the sensory system should be analyzed for various configurations of sensors and weldment shapes by using a mathematical model. In this study, the measuring efficiency of vision sensor predicted by its model and the criterion for data deficiency was in significant agreement with experiments for typical corrugations of shipping containers and LNG tanks. It was found that the results obtained from the mathematical model were very useful in designing the vision sensor for automatic welding of linear weldments with height variation.

Detection of weld position and seam tracking based on Kalman filtering of weld pool images

This paper presents a novel method of detection of weld and seam tracking during gas tungsten arc welding (GTAW). The weld position equations are established based on an estimation of the centroid position of the weld pool images. A Kalman filter (KF) is applied to the weld pool images from a visual sensor to recursively compute the solution to the weld position equations. The estimation of the state provided by a Kalman filter can be used to estimate the weld feature location on the image plane at the next sampling time and its uncertainty. This allows adjusting the welding torch position in real time, which may sensibly reduce processing time and improve seam tracking accuracy. The effectiveness of the proposed algorithm in the presence of weld pool image noise has been tested by the computer simulations and actual welding experiments. Also, the results of experiments have demonstrated the robustness of weld position detection for seam tracking.