Junsu Ahn - Academia.edu (original) (raw)
Papers by Junsu Ahn
Welding in the World, 2016
In the laser keyhole welding process, one ray can have several reflections and absorptions during... more In the laser keyhole welding process, one ray can have several reflections and absorptions during the process. This is called multiple-reflection and is considered to be one of the important physical phenomena in the laser keyhole welding process. To calculate the multiple-reflection in laser keyhole welding simulations, a ‘Ray tracing’ methodology is used. Through ray tracing, several ray properties, such as the current location of a ray, the location of reflection, and the reflected direction, can be calculated. This study investigated the numerical simulations of two ray tracing methods, the Direct Search Method (DSM) and Progressive Search Method (PSM). There are differences between the two ray tracing methods. PSM uses a real equation for the surface and ray direction vector while DSM uses a discriminant. A reflected ray is moved step by step in PSM, but only once by the discriminant in DSM. The reflected ray starts from the real contact point in PSM, but from the center of the contacted sub-cell in DSM. PSM can depict the multi-path of the ray by employing a set of sub-rays concept. On the other hand, DSM cannot depict the multi-path of the ray, because it uses a single ray definition. Therefore, PSM is a more physical numerical method and can depict various phenomena, such as transmission and scattering. PSM has a higher accuracy in laser keyhole welding simulations. In PSM, two new models were applied, the transmission and scattering models. All simulations were performed with the computational fluid dynamics (CFD) method including the volume of fluid (VOF) technique. Simulations using the DSM and PSM were performed to compare these two methods.
Journal of the Korean Welding and Joining Society, 2009
A roll projection welding machine is introduced to fabricate metallic sandwich plate consisting o... more A roll projection welding machine is introduced to fabricate metallic sandwich plate consisting of a structured inner sheet with projection-like shape and a pair of skin sheets. To fabricate the metallic sandwich plate of consistent and good quality, two process monitoring methods are introduced; dynamic resistance monitoring and skin sheet temperature monitoring. Dynamic resistance monitoring has no time delay but gives only averaged value over plate width. Skin sheet temperature monitoring has certain amount of time delay but is good for predicting weld quality of specified position. By the two complementary monitoring methods, the characteristics of the new welding process is successfully understood.
Welding in the World, 2009
The metal sandwich plate has many good properties such as high specifi c stiffness, high specifi ... more The metal sandwich plate has many good properties such as high specifi c stiffness, high specifi c strength, good impact absorption, effective thermal insulation and soundproofi ng. Several bonding methods such as brazing, adhesive bonding, projection welding and adhesive-assisted projection welding are tested for fabrication of the metal sandwich plate. Finally, a new bonding method, 3-layer roll projection welding, is introduced to fabricate the metal sandwich plate of large area for the plate width of 270 mm. The new method uses a pair of roll electrodes like in seam welding; however, the workpiece width is much wider than that of seam welding by more than ten times. Projection welding of large area is made at two internal interfaces of the 3-layer weldment, consisting of a structured inner sheet of projection-like shape and a pair of skin sheets, when it passes through the gap between the roll electrodes. The weld quality is inspected by peeling test for various welding conditions, in order to fi nd the optimal welding parameters. The welding sequence is to be synchronized for the given welding speed and pattern type of structured inner sheet to provide the consistent welding condition.
Welding in the World, 2009
It is well known that the laser beam can be absorbed and/or scattered during its path from laser ... more It is well known that the laser beam can be absorbed and/or scattered during its path from laser head to workpiece surface in laser-arc hybrid welding. It is believed that absorption is generated mainly by inverse Bremsstrahlung in arc plasma of laser-arc hybrid welding, while scattering is governed by Rayleigh scattering in the metal vapour region, formed by laser high-power irradiation on metallic surface. A particle-based numerical simulation technique for these kinds of laser-matter interaction is introduced to predict the redistribution of the incident laser beam energy. The methodology for laser scattering is adopted from the photon mapping technique which is widely used in the rendering process of computer graphics in recent years. The shape of the keyhole in high-power welding is also considered to observe the effect of multiple laser beam reflections in the keyhole.
Journal of Materials Processing Technology, 2010
Ship hull plate forming by flame torch plays an important role in shipbuilding process. In shipya... more Ship hull plate forming by flame torch plays an important role in shipbuilding process. In shipyards, the line heating method is one of the major processes carried out by workers to form the plates into desired shapes. Since this process depends on the skilled workers' experiences and knowhow, there is a lot of variation in the products. There also could be some difficulties in communicating with each other. Hence, it is necessary to develop an automation system to increase not only the plate forming productivity, but also product quality. For automated plate forming, it is required to know where and how much to heat on the plate. Thus, the inverse problem should be solved to obtain process parameters such as torch travel speed and the heating positions. In this study, the deformed curve shape by flame heat was geometrically considered so that the appropriate geometrical parameters can be obtained. Then, the relationship between the bending angle and the radius of the curvature of the deformed curve shape were established. In addition, the process parameters were obtained by using the relationship between geometrical parameters and process parameters. Lastly, experiments of the two-dimensional plate forming considering deformed curve shape with the curvature were performed. The results of the formed shape were similar to the target shape.
Applied Surface Science, 2013
A numerical study of nanosecond laser ablation process for semitransparent material was performed... more A numerical study of nanosecond laser ablation process for semitransparent material was performed. A heat source model using ray tracing is suggested for three-dimensional simulations of laser material processing based on the volume-of-fluid (VOF) method. The model is capable to describe both the ray transmission into the material and the reflections from the material surface. In the computational implementation of the model, a stochastic approach was introduced to avoid the recursive branching of an incident ray into transmitted and reflected rays. Since the ray tracing highly depends on the shape of the target material, proper surface reconstruction method is also considered. For the spatially continuous representation of the free surface of the material, the piecewise linear surface of the VOF method was converted into the level set surface reconstructed by the interpolation of the signed distance function. The applicability of the model was validated by example simulations and experiments on polyimide workpiece with nanosecond laser.
Welding in the World, 2016
In the laser keyhole welding process, one ray can have several reflections and absorptions during... more In the laser keyhole welding process, one ray can have several reflections and absorptions during the process. This is called multiple-reflection and is considered to be one of the important physical phenomena in the laser keyhole welding process. To calculate the multiple-reflection in laser keyhole welding simulations, a ‘Ray tracing’ methodology is used. Through ray tracing, several ray properties, such as the current location of a ray, the location of reflection, and the reflected direction, can be calculated. This study investigated the numerical simulations of two ray tracing methods, the Direct Search Method (DSM) and Progressive Search Method (PSM). There are differences between the two ray tracing methods. PSM uses a real equation for the surface and ray direction vector while DSM uses a discriminant. A reflected ray is moved step by step in PSM, but only once by the discriminant in DSM. The reflected ray starts from the real contact point in PSM, but from the center of the contacted sub-cell in DSM. PSM can depict the multi-path of the ray by employing a set of sub-rays concept. On the other hand, DSM cannot depict the multi-path of the ray, because it uses a single ray definition. Therefore, PSM is a more physical numerical method and can depict various phenomena, such as transmission and scattering. PSM has a higher accuracy in laser keyhole welding simulations. In PSM, two new models were applied, the transmission and scattering models. All simulations were performed with the computational fluid dynamics (CFD) method including the volume of fluid (VOF) technique. Simulations using the DSM and PSM were performed to compare these two methods.
Journal of the Korean Welding and Joining Society, 2009
A roll projection welding machine is introduced to fabricate metallic sandwich plate consisting o... more A roll projection welding machine is introduced to fabricate metallic sandwich plate consisting of a structured inner sheet with projection-like shape and a pair of skin sheets. To fabricate the metallic sandwich plate of consistent and good quality, two process monitoring methods are introduced; dynamic resistance monitoring and skin sheet temperature monitoring. Dynamic resistance monitoring has no time delay but gives only averaged value over plate width. Skin sheet temperature monitoring has certain amount of time delay but is good for predicting weld quality of specified position. By the two complementary monitoring methods, the characteristics of the new welding process is successfully understood.
Welding in the World, 2009
The metal sandwich plate has many good properties such as high specifi c stiffness, high specifi ... more The metal sandwich plate has many good properties such as high specifi c stiffness, high specifi c strength, good impact absorption, effective thermal insulation and soundproofi ng. Several bonding methods such as brazing, adhesive bonding, projection welding and adhesive-assisted projection welding are tested for fabrication of the metal sandwich plate. Finally, a new bonding method, 3-layer roll projection welding, is introduced to fabricate the metal sandwich plate of large area for the plate width of 270 mm. The new method uses a pair of roll electrodes like in seam welding; however, the workpiece width is much wider than that of seam welding by more than ten times. Projection welding of large area is made at two internal interfaces of the 3-layer weldment, consisting of a structured inner sheet of projection-like shape and a pair of skin sheets, when it passes through the gap between the roll electrodes. The weld quality is inspected by peeling test for various welding conditions, in order to fi nd the optimal welding parameters. The welding sequence is to be synchronized for the given welding speed and pattern type of structured inner sheet to provide the consistent welding condition.
Welding in the World, 2009
It is well known that the laser beam can be absorbed and/or scattered during its path from laser ... more It is well known that the laser beam can be absorbed and/or scattered during its path from laser head to workpiece surface in laser-arc hybrid welding. It is believed that absorption is generated mainly by inverse Bremsstrahlung in arc plasma of laser-arc hybrid welding, while scattering is governed by Rayleigh scattering in the metal vapour region, formed by laser high-power irradiation on metallic surface. A particle-based numerical simulation technique for these kinds of laser-matter interaction is introduced to predict the redistribution of the incident laser beam energy. The methodology for laser scattering is adopted from the photon mapping technique which is widely used in the rendering process of computer graphics in recent years. The shape of the keyhole in high-power welding is also considered to observe the effect of multiple laser beam reflections in the keyhole.
Journal of Materials Processing Technology, 2010
Ship hull plate forming by flame torch plays an important role in shipbuilding process. In shipya... more Ship hull plate forming by flame torch plays an important role in shipbuilding process. In shipyards, the line heating method is one of the major processes carried out by workers to form the plates into desired shapes. Since this process depends on the skilled workers' experiences and knowhow, there is a lot of variation in the products. There also could be some difficulties in communicating with each other. Hence, it is necessary to develop an automation system to increase not only the plate forming productivity, but also product quality. For automated plate forming, it is required to know where and how much to heat on the plate. Thus, the inverse problem should be solved to obtain process parameters such as torch travel speed and the heating positions. In this study, the deformed curve shape by flame heat was geometrically considered so that the appropriate geometrical parameters can be obtained. Then, the relationship between the bending angle and the radius of the curvature of the deformed curve shape were established. In addition, the process parameters were obtained by using the relationship between geometrical parameters and process parameters. Lastly, experiments of the two-dimensional plate forming considering deformed curve shape with the curvature were performed. The results of the formed shape were similar to the target shape.
Applied Surface Science, 2013
A numerical study of nanosecond laser ablation process for semitransparent material was performed... more A numerical study of nanosecond laser ablation process for semitransparent material was performed. A heat source model using ray tracing is suggested for three-dimensional simulations of laser material processing based on the volume-of-fluid (VOF) method. The model is capable to describe both the ray transmission into the material and the reflections from the material surface. In the computational implementation of the model, a stochastic approach was introduced to avoid the recursive branching of an incident ray into transmitted and reflected rays. Since the ray tracing highly depends on the shape of the target material, proper surface reconstruction method is also considered. For the spatially continuous representation of the free surface of the material, the piecewise linear surface of the VOF method was converted into the level set surface reconstructed by the interpolation of the signed distance function. The applicability of the model was validated by example simulations and experiments on polyimide workpiece with nanosecond laser.