In laser welding of steel sheets, hydrogen dissolves into the weld metal. Since the elongation at break decreases when hydrogen dissolves into steel, the weld may fracture if significant deformation is applied immediately after welding. For this reason, in press forming of laser welded TWB, it is necessary to ensure sufficient time between welding and forming. On the other hand, there is no simple method for non-destructively measuring the amount of hydrogen remaining in the laser weld bead. However, if the analytical solution of the diffusion equation describing hydrogen evolute from the weld bead and the diffusion coefficient are known, it is possible to estimate the required waiting time between welding and press forming. While there are numerous methods for measuring the diffusion coefficient, no effective method for measuring the diffusion coefficient at the laser welds is currently known. Therefore, the authors attempted to determine the diffusion coefficient at the laser welds. Hydrogen dissolved during laser welding localizes within the weld metal. It then diffuses into the heat-affected zone. However, due to the thinness of the steel sheet, the hydrogen is released into the atmosphere before it can diffuse widely into the base metal. Therefore, if the diffusion coefficient is determined based on the diffusion behavior of hydrogen dissolved during welding, it can be considered as the diffusion coefficient of the welds. The authors derived an equation representing the amount of hydrogen diffusing from the laser-welded metal and determined the diffusion coefficient to reproduce the evolution behavior from the welds. The determined values are diffusion coefficients at 50 ℃, with 5.76×10^-5 mm²/s for 0.18C steel welding and 1.28×10^-4 mm²/s and 1.54×10^-4 mm²/s for two types of 0.14C steel welding, respectively.

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Among various additive manufacturing technologies, wire arc additive manufacturing (WAAM) is one of the most suitable methods for producing large-scale aluminum components, owing to its high deposition rate. However, achieving high-quality components by WAAM remains challenging, due to the heterogeneous microstructures and defects formed during WAAM process. To improve the performance of components, understanding and controlling the formation of the microstructure and defects is essential. In this study, therefore, the main objective was to clarify the effect of heat input on grain morphology and the relationship between microstructure and mechanical properties of Al–Mg component manufactured by cold metal transfer-based WAAM process. Wall specimens were fabricated using Al–Mg wire (ER5356) with three different heat inputs, and their microstructures and mechanical properties were examined. The dominant grain morphology transitioned from feathery grains to columnar grains and finally to equiaxed grains. The number of porosities decreased with decreasing heat input. The component fabricated at the medium heat input exhibited superior tensile properties, i.e., ultimate tensile strength and elongation, by simultaneously suppressing the formation of detrimental feathery grains and porosities. These findings demonstrated that appropriate control of heat input can change the grain morphology and suppress the formation of porosities, thereby improving the tensile properties of Al–Mg components manufactured by WAAM.

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