세계적인 환경 규제로 인해 마그네슘 합금과 같은 경량 소재에 대한 수요가 증가하고 있으며, 마그네슘 합금 소재의 다양한 산업계 적용을 위한 용접 및 접합 방식에 대한 연구도 지속적으로 수행되고 있다. 앞선 Part I 연구에서는 마그네슘 합금에 대한 파이버 레이저 Bead on Plate(BOP) 실험을 수행하여 맞대기 용접 조건의 확보를 위한 기초 연구를 수행하였으며, 본 연구에서는 Part I의 기초 BOP 실험에서 도출된 적합한 레이저 출력과 용접 속도를 바탕으로 두께 3mm의 AZ31B 마그네슘 합금에 대해 맞대기 용접을 시행하였고, 인장시험 및 경도시험을 수행한 후 기계 물성 데이터를 분석하였다. 분석 결과 레이저 출력 2.0 kW, 50 mm/s (Heat input)의 조건에서 항복강도 151.5 MPa, 인장강도 224.1 Mpa으로 우수한 인장, 항복강도를 얻을 수 있었다.

Titanium constitutes approximately 60% of the weight of steel and exhibits strength comparable to steel's but with a higher strength-to-weight ratio. Titanium alloys possess excellent corrosion resistance due to a thin oxide layer at room temperature; however, their reactivity increases above 600°C, leading to oxidation and nitridation. Welding titanium alloys presents challenges such as porosity issues. Laser welding minimizes the heat-affected zone (HAZ) by emitting high output in a localized area for a short duration. This process forms a narrow and deep HAZ, reducing the deterioration of mechanical properties and decreasing the contact area with oxygen. In this study, fiber laser welding was conducted on 8.0mm thick Ti-6Al-4V alloy using the Bead On Plate (BOP) technique. A total of 25 welding conditions were experimented with to observe bead shapes. The results demonstrated successful penetration within the 0.792mm to 8.000mm range. It was concluded that this experimental approach can predict diverse welding conditions for Ti-6Al-4V alloys of various thicknesses.

Liquified hydrogen is considered a new energy resource to replace conventional fossil fuels due to environmental regulations by the IMO. When building tank for the storage and transportation of liquified hydrogen, materials need to withstand temperatures of -253°C, which is even lower than that of LNG (-163°C). Austenitic stainless steel mainly used to build liquified hydrogen tank. When building the tanks, both the base material and welding zone need to have excellent strength in cryogenic condition, however, manual arc welding has several issues due to prolonged exposure of the base material to high temperatures. Laser welding, which has some benefits like short period of exposure time and decrease of thermal affected zone, is used many industries. In this study, laser bead on plate welding was conducted to determine the laser butt welding conditions for STS 304 and STS 316L steels. After the BOP test, cross-section observations were conducted to measure and compare four bead parameters. These tendency result of laser BOP test can be used as conditions laser butt welding of STS 304 and STS 316L steel.

Research into lightweighting to improve vehicle fuel efficiency and reduce exhaust emissions continues as environmental regulations become increasingly stringent. Magnesium alloys, chosen for their lightweight properties, are more than 35% lighter than aluminum alloys and also exhibit excellent mechanical characteristics. While magnesium alloys are commonly utilized in arc welding processes like GTAW and GMAW, they pose challenges such as high residual stresses and welding defects. Laser welding, on the other hand, offers the advantage of precise heat input, enabling deep and high-quality welds while minimizing welding distortion. In this study, fiber laser welding was employed to weld a 4.0mm thick AZ31B-H24 using the Bead on Plate technique. A total of 10 different welding conditions were tested with fiber laser welding, and the cross-sections of the weld beads were examined. Weld bead shapes were measured based on five parameters. The results allowed for an evaluation of the weldability of AZ31B-H24 using fiber laser welding.

Welding is a representative processing technology applied in many industrial sites due to its quality and convenience. In particular, fiber laser welding can be welded at a faster speed compared to arc welding, and there is an advantage in welding distortion, which is the most significant disadvantage of welding. In this study, the weldable thickness was predicted, and the optimal welding angle was estimated using simulations during the welding of the T-shape structure. The multi-layer heat source model proposed in the previous author's study was used, and the study was conducted using the proposed welding heat source under specific conditions of 4kw and 1.0m/min. As a result, it was predicted that high-quality welding would be possible when the thickness was 3mm or 4mm, and it was also confirmed that welding should be performed at an angle of 82.5° or more when welding a 3mm thick structure. As a follow-up study, we plan to build a welding heat source model under various conditions and conduct a study to derive welding conditions at various thicknesses.

Welding is one of representative manufacturing processes in the industrial field. Cryogenic storage containers are also manufactured through welding, and conversion to laser welding is issue in the field due to many advantages. Since welding causes thermal-elastic deformation, design considering distortion is required. Prediction of distortion through FEM is essential, but laser welding has difficulties in the field because there is no representative heat source model. The author presented the model that can cover various models using a multi-layer heat source model in previous studies. However the previous study has a limitation which is a welding heat source model must be derived after performing bead on plate welding. Thus this study was attempted to estimate the welding heat source parameters by comparing the shape of bead under various conditions. First, the difference between penetration shape and welding heat source parameters according to welding power was analyzed. The radius of the welding heat source increased according to the welding power, and the depth of the welding heat source also increased. The correlation between the penetration shape and the welding heat source parameter appears at a similar rate, however the follow-up research is necessary with more model data.

Due to environmental pollution, regulations on fossil fuels are required. There is a movement for the regulations by using LNG fueled propulsion ships. LNG is an eco-friendly fuel that does not emit NOx or SOx during combustion, but its boiling point is -163°C. Under that condition, the use of metal is restricted, and IMO defined applicable materials through IGC code. Among the metals, 9% nickel steel is one of excellent mechanical properties such as yield strength and tensile strength in cryogenic condition. Thus 9% nickel steel is widely used in cryogenic storage containers for ships. In addition, laser welding, which minimizes thermoelastic distortion by applying a concentrated heat source to a narrow area for a short period of time, is in the spotlight. So, this study is a basic research to predict and respond to thermal distortion during laser welding. Secondary version of the representative heat source model was derived through the author's previous research with STS304L, and the heat source model was derived by applying the heat source model to 9% nickel steel in this study. 9% nickel steel is a material that is in high demand and is widely used in the manufacture of cryogenic containers, so this study is expected to be able to respond immediately to the field.

9% nickel steel has remarkable mechanical properties in a cryogenic condition and is widely used in storage containers for LNG fueled ships. Demand for laser welding rather than conventional arc welding has grown to increase manufacturing efficiency. However there are various types of heat sources which are suggested by other researchers. With that, it is difficult to select a proper heat source shape for welding conditions. The author proposed a representative heat source model that can cover most of suggested heat source models through previous studies. Welding power was fixed at 4kW and the speed was changed to 1.0m/min, 1.5m/min, and 2.0m/min respectively. The shapes of the welding heat sources were derived, and the tendency of the main parameters was also deducted. It was observed that the width and depth of the weld bead decreased as the welding speed increased through welding experiment, parameters of welding heat source are changed linearly. Based on this study, it is expected that it will be possible to estimate the shape of the heat source under untested welding conditions.

Laser welding is used in various industries due to fast welding speed, and prediction with FEM is needed to design. The multi-layered heat source model that can cover various models has been recently proposed to analyze laser welding, but it takes a lot of time because more than 2,000 cases are compared to derive the results. In order to reduce the time, the simplified model should be suggested. In order to derive a simplified model, laser welding heat transfer analysis was performed using the welding direction length and the convection coefficient of the welding direction surface as parameters. As a result, the model whose difference is less than 0.1% compared to reference model is deducted. The analysis time was reduced by about 90% from 48 hours to 5 hours. Also difference of convection coefficient on the welding direction surface does not affect the temperature distribution much.

As laser welding has low thermal distortion and fast welding speed, the needs in the field is increasing in varous industries. Prediction of distortion and design with that prediction are very important for using welding process in the field. However many types of heat source models for laser welding are suggested, field engineers feels difffucult for using a proper model for a specific case. Thus I, author, suggested a representative model which can cover most of existing models with multi-layered heat source model. This method is very powerful, but there are much time consumption with analysis and comparision among more than 1,000 candidates. To solve these shortcomings, this study focused on to find the simplified model. In order to construct the simplified model, the length of the orthogonal direction to the welding direction was reduced from 300mm to 35mm, which makes time reduction of 75% with sustaining the quality of the original model. That can cover the disadvantage of multi-layered heat source model for laser welding, enhance the prediction of welding distortion after laser welding

Due to environmental pollution, regulations on existing petroleum-based fuels are increasing day by day. LNG is in the spotlight as an eco-friendly fuel that does not emit NOx or SOx, but its boiling point is -163°C, so it needs to be handled with care. Materials that can be used at the above temperature are defined by IMO through the IGC Code. Among them, 9% nickel steel has great advantages in yield strength and tensile strength under cryogenic conditions, but it is difficult to use in arc welding such as FCAW for various reasons. This study is a study to apply fiber laser welding to solve this problem. As a previous study, this study conducted a study to find a welding heat source. After performing bead on plate welding, the optimal heat source was derived by analyzing the shape of the bead and adjusting the parameters of the heat source model. In this case, by applying the multi-island genetic algorithm, which is a global optimization algorithm, not the intuition of the researcher, accurate results could be derived in a wide range.

Due to stricter environmental regulations of the International Maritime Organization(IMO), the number of ships fueled by Liquefied Natural Gas(LNG) is rapidly increasing. The International Code of the Construction and Equipment of Ships Carrying Liquefied Gases in Bulk(IGC Code) limits the material of tanks that can store cryogenic substances such as LNG. Among the materials listed in the IGC Code, ASTM A553M-17 has been recently adopted as a material for LNG fuel tank projects because of its excellent mechanical properties at cryogenic temperatures. In shipyards, this material is being used to build tanks through Flux Cored Arc Welding (FCAW). However, there is a problem that magnetization occurs during welding and there is a big difference in welding quality depending on the welding position. In order to overcome this problem, this study intends to conduct basic research to apply laser welding to ASTM A553M-17 material. In this study, a study on penetration (HAZ depth, Penetration) and welding defects during fiber laser welding according to three types of shielding gases(nitrogen, argon, and helium) was conducted. To this end, a Bead on plate(BOP) experiment was performed under four fiber laser conditions(Power, Speed) for each shielding gas and welding defects caused by the use of the shielding gas were compared through cross-sectional observation, and the penetration depth was analyzed.

The multi-layered heat source model is a model that can cover most of existing studies and can be defined with a simple formula. Based on the methodology performed in previous studies, the welding heat source was found through experiments and FEM under the welding power conditions of three cases and the parameters of the welding heat source were analyzed according to the welding power. In this study, parameters of fiber laser welding heat source according to welding power were searched through optimization algorithm and finite element analysis, and the correlation was analyzed. It was confirmed that the concentration of the welding heat source in the 1st layer was high regardless of the welding power, and it was confirmed that the concentration of the welding heat source in the 5th layer (last layer) increased as the welding power increased. This reflects the shape of the weld bead that appears during actual fiber laser welding, and it was confirmed that this study represents the actual phenomenon.

The penetration depth, bead height, width, and internal porosity were analyzed to select the perfect penetration conditions for the STS316L tube material with an outer diameter of 38.1mm and a thickness of 3.4 mm. The welding conditions to secure a penetration depth of 3.4mm or more were selected. In addition, a welding range in which underfill does not occur was selected. The range of the selected conditions is the condition of a welding speed of 0.75 to 1.25m/min with an output of 2.0kW. The selected welding conditions were applied to STS316L tube orbital welding, and as a result of cross-sectional inspection after welding, a welded part of less than 4% of complete penetration and porosity was secured. The strength of the weld was measured to be more than 800kgf, and the hardness of the weld was found to decrease compared to the base material. The decrease in the hardness of the weld is judged by the annealing effect of the heat treated base material.

In this study, a welding heat source model was presented and verified during fiber laser welding. The multi-layered heat source model is a model that can cover most of existing studies and can be defined with a simple formula. It consists of a total of 12 parameters, and an optimization algorithm was used to find them. As optimization algorithms, adaptive simulated annealing, multi island genetic algorithm, and Hooke-Jeeves technique were applied for comparative analysis. The parameters were found by comparing the temperature distribution when the STS304L was bead on plate welding and the temperature distribution derived through finite element analysis, and all three models were able to derive a model with similar trends. However, there was a deviation between parameters, which was attributed to the many variables. It is expected that a more clear welding heat source model can be derived in subsequent studies by giving a guide to the relational expression and range between variables and increasing the temperature measurement point, which is the target value.

Welding is the most widely used technology for manufacturing in the automobile, and shipbuilding industries. Fiber laser welding is rapidly introduced into the field to minimize welding distortion and fast welding speed. Although it is advantageous to use finite element analysis to predict welding distortion and find optimized welding conditions, there are various heat source model for fiber laser welding. In this study, a welding heat source was proposed using a multi-layered heat source model that encompasses most of the existing various welding heat source models: conical shape, curved model, exponential model, conical-cylindrical model, and conical-conical model. A case study was performed through finite element analysis using the radius of each layer and the ratio of heat energy of the layer as variables, and the variables were found by comparing them with the actual experimental results. For case study, by applying Adaptive simulated annealing, one of the global optimization algorithms, we were able to find the heat source model more efficiently.

The bead geometry according to the welding conditions was analyzed through the laser fillet welding experiment of 9% Ni steel, and the relationship between the shear strength and the five bead geometry measured by selecting the main bead geometry of the fillet weld was analyzed. Among the welding conditions, the welding conditions that directly affect the penetration depth are welding speed and laser power, and the working angle and beam position have a great influence on the formation of leg of vertical and horizontal members. The bead shape, which greatly affects the shear strength, is the horizontal member length, neck thickness, and weld length, and has a proportional relationship with the shear strength. As a result of confirming the relationship between shear strength and bead shape through the derivation of the trend line, it was confirmed that the length of the vertical member, whose R2 value was 0.92, was most closely related to the shear strength.

Natural gas is the most realistic fuel among eco-friendly fuels. Natural gas production is limited, and in Korea, it is supplied and utilized in the form of liquefied natural gas (LNG). In the case of LNG, the vaporization point is 163 degrees below zero, so ordinary metal cannot be used due to its brittleness. The International Maritime Organization (IMO) defines metals that can be used in the IGC Code, and is used for storage containers, transportation containers, etc. based on the metals. Welding is essential in the manufacture of large structures such as LNG storage tanks. In this study, weldability studies related to cryogenic materials were conducted. In Part I of this study, high-manganese steel and part II were studied for two types of stainless steel (STS304L, STS316L), and in Part III, aluminum (AL5083). During laser welding, the shape of the Bead on Plate (BOP) was analyzed, and a total of nine cases were analyzed using laser power and welding speed as variables. It was confirmed that the penetration and the width of the welding width were linearly proportional to the amount of heat input. Based on this study, it is possible to conduct a follow-up study to find the optimal welding conditions for butt welding and fillet welding.

As the part of efforts to respond to the environmental pollution, the demand for clean energy is increasing. Natural gas is one of the most realistic alternatives, and interest in the storage and transportation containers to utilize the natural gas is growing. The production area of ​​natural gas is limited, and in Korea, it is imported in the form of liquefied natural gas. LNG is a cryogenic state with a vaporization point of -163°C, so the ordinary metal cannot be used due to its brittleness. The international maritime organization (IMO) defines the metals that can handle LNG in the IGC Code, and the research is ongoing. This study was a preliminary study to check the weldability of related metals and confirmed the shape of bead on plate (BOP) during the laser welding for each material. In part I, a study was conducted on the high manganese steel, and in part II, a study was conducted on the materials of STS304L and STS316L among the stainless steel. Based on this study, it can be used as an optimal welding condition for the butt and fillet welding.

As demand for eco-friendly energy increases, demand for natural gas and Liquefied natural gas (LNG) storage technologies continues to increase. LNG is a cryogenic environment with a temperature of -163°C, so ordinary metals cannot be used due to brittleness. Accordingly, IGC Code designates the cryogenic materials such as Invar, STS304L, Al5083-0, and High Manganese Steel. For fabricating those materials, research on welding possibility is the most important. Thus this study focused on the possibility of laser welding of the cryogenic materials. The weldability of High Manganese Steel was researched in this paper, the shape and the dimensions of the beads after bead on plate (BOP) welding were observed. The experiment was conducted on a total of 25 cases with laser power and welding speed of 5 cases each, and the width, height, and penetration of the beads were confirmed. It was confirmed that the paramenter of bead increased linearly with the laser power, and the paramenters of bead increased linearly with decreasing welding speed. Based on this study, high manganese steel can be applied in various industries by applying it to butt welding.