In the welding of spheroidal ductile cast iron and gray cast iron, brittle phases such as ledeburite and martensite form in the vicinity of fusion boundary. These phases not only promote welding cracks and reduce joint ductility but also increase the susceptibility to porosities in the weld metal. To improve joint ductility and reduce blow holes in cast iron welding joints, six commercially available Ni-based filler metals and an additional nine originally developed Ni-based filler metals were evaluated. Using these filler metals, butt joints and bead-on-plate specimens of spheroidal graphite cast iron (FCD400-18) and flake graphite cast iron (FC250, FC300) were produced. Tensile testing of the joints revealed that a Ni–Cr alloy filler metal containing 13.77 mass% Cr exhibited the most favorable mechanical properties. The tensile strengths of the FCD400-18, FC250, and FC300 joints all exceeded the minimum specified values of the respective base metals, and the maximum elongation reached up to 14.3 % for the FCD400-18 joints. Furthermore, filler metals developed by adding approximately 4 mass% Si and 2 mass% Al to this Ni–Cr alloy were confirmed by radiographic testing (RT) to eliminate blow holes completely in FC250 bead-on-plate specimens. Microstructural observations at the fusion boundaries showed a clear trend in which larger average ledeburite layer thickness correlates with lower elongation, with a high coefficient of determination (R² = 0.7813). To predict the average ledeburite layer thickness, regression analysis was conducted using the Ni equivalent, Cr equivalent, and the melting onset temperature of the filler metals as independent variables. The analysis revealed that the melting onset temperature had the strongest influence, demonstrating that lowering the melting onset temperature of the filler metal is effective for improving joint ductility.

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Gas entrapment is a critical issue in laser metal deposition (LMD), leading to porosity and degradation of the mechanical properties of the clad beads. In this study, the mechanism of gas entrapment was investigated based on the dynamic interaction between powder particles and the molten pool, and a suppression guideline was established using the Weber number. The Weber number under each cladding condition was calculated by measuring the size and velocity of solid powder particles using high-speed imaging, and estimating the molten pool properties (density and surface tension) from the measured surface temperature. When the carrier gas flow rate exceeded a certain value, the porosity of the clad bead increased sharply, indicating the onset of gas entrapment. By correlating the porosity with the Weber number, the critical Weber number was found to be approximately 15–25 for Ni-based, Co-based, and Fe-based alloys. Regardless of material, gas entrapment was suppressed by maintaining the Weber number below approximately 15. Reducing the powder particle size alone did not decrease the Weber number, because it increased both the particle velocity and its standard deviation. The most effective approach for suppressing gas entrapment was to slow down the powder particles velocity. This can be achieved by reducing the carrier gas flow rate, increasing the cross-sectional area of the carrier gas path, or using a low-density carrier gas. The findings demonstrate that gas entrapment in LMD can be systematically and consistently characterized by the Weber number, thereby providing a unified framework for optimizing processing conditions to obtain dense clad beads.

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In TIG welding, if oxygen is present in the molten metal in the range of 70 to 300 ppm, the surface tension of the hot side of the molten metal becomes greater than that of the cold side, and the molten metal flows from the edge of the molten pool to the center of the molten pool, resulting in deep penetration. In this study, this phenomenon was used to perform full butt welding of thick carbon steel plates without using arc air gouging. First, a weld metal with a high oxygen content was successfully obtained by MAG welding using Ar+40%O₂ as a shielding gas. Next, this weld metal was welded to a carbon steel plate using a bead-on-plate method, and performed TIG welding on this bead, resulting in a weld bead with a penetration of D: 4.3 mm and D/W: 0.61. In addition, with the conventional X-groove, the bevel surface of the base metal was melted by the arc, raising the liquid level of the molten pool, and convection occurred from the liquid level, resulting in shallow penetration at the root. Therefore, by providing a flat surface at the root of the groove, the bevel surface of the base metal was not melted by the arc, resulting in complete welding with deep penetration. Furthermore, weld joint performance tests using these welding conditions and groove shapes were conducted, and it was confirmed that there were no problems with the weld joint.

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