This study compared the mechanical properties of the NAB (Ni-Al-bronze) material fabricated using wire arc additive manufacturing (WAAM) technology with those of the cast NAB. Using a CMT (cold metal transfer) welding process, this study analyzed the bead shape for six welding conditions, determined an appropriate bead shape, and fabricated a square bulk NAB material using the bead shape. For a mechanical properties comparison, the study obtained two test specimens per welding direction from the fabricated bulk NAB material, and compared those with the cast NAB materials. In the tensile test, the deposited NAB material showed significantly better results than the cast NAB; furthermore, the deposited NAB material showed better performance than the cast NAB material in the Vickers hardness test, impact test and wear test. In addition, the deposited NAB showed anisotropy depending on the welding direction, and showed high tensile strength, hardness and shock absorption in the longitudinal direction of the welding line.

Estimating the thermo-elasto-plastic deformation by arc welding through finite element analysis has been used in various industrial fields. The Goldak heat source model is one of the most important and widely used models in finite element analysis, and its parameters are estimated based on the results of previous studies and tests. Part I of this study focused on the adequate heat source model, and the study for the welding deformation with the moving heat source will be done on the latter research. This study used the parameters of Goldak’s heat source model, weld efficiency, and the location of the heat source as design variables, and defined the Heat Affected Zone (HAZ) boundary line of Bead on Plate (BOP) welding as the target. BOP welding was performed using SS400 plates, the HAZ boundary line was determined based on examining the shape of the cross-section, and the optimization condition was that temperature inside the boundary line exceeded 727 °C while the temperature outside the line did not exceed 727 °C during the welding process. During this process, a multi-island genetic algorithm (non-linear global optimization method) was used to obtain the optimal results out of 1000 candidate groups, in which the HAZ boundary was similar to the experimental results. Applying a global optimization algorithm to determine the parameters of the most important heat source model to analyze welding deformation is significant, and this may be applied in various industrial fields that use welding including shipbuilding, aviation, and machinery industries.

The wire arc additive manufacturing (WAAM) process used to manufacture aluminum parts has a number of variables. This study focuses on the effects of the heat input and the current and voltage ratio on the deposition efficiency. The effects of the heat input and current and voltage ratio (V/A) on the bead geometry were analyzed, depending on the cross-sectional geometry of the deposition layers, for nine different deposition conditions. The deposition efficiency was also analyzed by analyzing the cross-sectional geometry of the thin-wall parts made of aluminum. The heat input range was about 2.7 kJ/cm to 4.5 kJ/cm; the higher the heat input, the higher the deposition efficiency. The maximum deposition efficiency achieved in this study was 76%. The current and voltage ratio was used to quantify the portion of voltage (V) in the total heat input (Q), and the effect on the bead geometry was analyzed. As the portion of voltage in the quasi heat input decreased by about 10%, it was found that the deposition efficiency was decreased by 1% to 3%.

As the environmental regulations on ship emissions by the International Maritime Organization (IMO) become stricter, the demand for a ship powered by liquefied natural gas (LNG) is rapidly increasing worldwide. Compared to other materials, high-manganese steel has the advantages of superior impact toughness at cryogenic temperatures, a low thermal expansion coefficient, and a low-cost base material and welding rod. However, there is a limitation that the mechanical properties of a filler material are worse than those of a base material that has excellent mechanical properties. To solve these shortcomings, a basic study was performed to apply fiber laser welding with little welding deformation and no filler material to high-manganese steel. The relationship between laser welding parameters and penetration shapes was confirmed through cross-section observation and analysis by performing a bead on plate (BOP) test by changing laser power and welding speed, which are the main parameters of laser welding. In addition, the welding performance was evaluated through mechanical property tests (yield strength, tensile strength, hardness, cryogenic impact strength) of a welding part after performing the high-manganese steel laser butt welding experiment. As a result, it was confirmed that the yield strength of a high-manganese steel laser welding part was 97.5% of that of a base metal, and its tensile strength was 93.5% of that of a base metal.

The International Maritime Organization (IMO) is tightening regulations to reduce greenhouse gas emissions from ship operations. As a result, the number of vessels using liquefied natural gas (LNG) as fuel has increased rapidly. At this time, ASTM A553-1 (9% nickel steel) is being used as a tank material for storing LNG as fuel because of its higher strength than other cryogenic materials. Currently, shipyards are manufacturing LNG fuel tanks using the flux cored arc welding (FCAW) method using 9% nickel steel material. However, fabrication through FCAW has two drawbacks. The first is that the welding filler is 20 times higher cost than the base metal, and the second is that the total production cost increases because the thickness of the tank increases due to the strength drop near the heat affected zone (HAZ) after welding. The laser welding of A553-1, which does not require additional welding fillers and has no yield and tensile strength reduction in the HAZ, can overcome the drawbacks of FCAW and ensure price competitiveness. Through the study of Part I (penetration shape by bead on plate), the penetration characteristics of laser welding were studied and the optimized welding conditions of 15 mm thickness of A553-1 were obtained. With optimized conditions, butt laser welding tests of A553-1 material were conducted in this study, and mechanical properties, which are tensile/yield strength, hardness, bending strength, and impact property on the cryogenic temperature of the weld zone after laser beam welding, are confirmed by comparing those after FCAW. In the case of tensile/yield strength, hardness, and bending strength at weldment, the values of laser beam welding (LBW) are higher than those of FCAW, and the value of the impact test after FCAW is higher than that of LBW, but both values are satisfied in ASTM. Through these conclusions, it is confirmed that there are no mechanical property problems in replacing the existing FCAW with LBW.

With the need for eco-friendly energy increasing rapidly due to global environmental issues, there is a rapidly increasing demand for liquefied natural gas (LNG). LNG is liquefied at minus 163 degrees Celsius, and its volume decreases to 1/600, giving it a relatively higher storage and transport efficiency than gaseous natural gas (NG). The material for the tanks that store cryogenic LNG must be a material with high impact toughness at cryogenic temperatures. Invar, which contains 36% nickel and has a very low coefficient of thermal expansion, is used for the membranes and corner structures of LNG cargo holds. The cross-shaped Invar structure used in an LNG cargo hold is manufactured through manual tungsten inert gas (TIG) fillet welding, which causes welding distortion and weldability problems. This study is a feasibility study that aims to reduce welding distortion, increase weldability with welding speed, and reduce the steps in an existing process by half by replacing the existing manufacturing method with automatic fiber laser fillet welding. Laser welding using fiber laser parameters are controlled for 1.5 and 3.0 mm thick Invar materials and weldability is secured through cross-section observation. Then, the optimal welding conditions with top and back beads secured are derived through a trial and error method.