Alumino-thermic additions to the flux-cored wire can be utilized to enhance welding heat input, improve welding productivity, and minimize the dependency on power sources in the air environment. In this article, the influence of Al/Fe₂O₃ thermite on electrical dependency, microstructural characteristics, and mechanical performance during the underwater wet welding of Q235 steel was investigated. The results revealed that adding exothermic flux enhanced the underwater wet welding appearance. The basicity of the slag steadily decreased from 1.77 to 0.73 because of the formation of Al₂O₃. Thermite increased the quantity and diameter of inclusions in the underwater wet weld metal. The excessive addition of Al/Fe₂O₃ thermite resulted in agglomerated inclusions in the weld metal. Significant microstructural changes from grain boundary ferrite to acicular ferrite were mainly caused by the de-oxidation products of the thermite process. When thermite was added to the flux-cored wire, the ultimate tensile strength first increased and then decreased, while the microhardness of the underwater wet welds showed an obviously increasing trend. The addition of exothermic flux additions to the flux-cored wire can help generate chemical heat and increase the energy required to melt the wire. This study established a link between thermite addition and joint performance, paving the way for the development and application of thermite-assisted self-shielded flux-cored wire.

The ultrasonic-frequency pulse underwater wet welding (UFP-UWW) process was achieved through a constant-voltage-mode power source connecting an ultrasonic-frequency pulse power source in parallel. The arc voltage and welding current waveforms, sound signal variations, microstructural characteristics and mechanical properties at different parameters were investigated. The results showed that the ultrasonic-frequency pulse voltage and current of the UFP-UWW process displayed a periodic high-frequency oscillation on the based values of the conventional UWW process. The arc stability of the UFP-UWW process improved owing to the fact that the proportions of the unstable arc burning region could be reduced to 1.56% after the introduction of the ultrasonic-frequency pulse current. No significant changes in weld width and penetration were observed while the weld dilution rate increased to 54.2% for the combination of 40 V–30 kHz, compared with the results of the conventional UWW process. The flux-cored arc (FCA) welding arc in the air had the same frequency response to the ultrasonic excitation signal, which verified the existence of the ultrasonic-frequency vibration induced by the periodic high-frequency electromagnetic forces. The application of the ultrasonic-frequency pulse produced finer columnar grains in the welds with an average length of 315 μm, although the amount of pro-eutectoid ferrite and acicular ferrite varied little. The mechanical properties of the welded joints were also noticeably enhanced with the application of different ultrasonic excitation frequencies. The optimum tensile strength and impact toughness of the welded joint were improved by 6.7% and 21.7% when the applied ultrasonic excitation voltage was 40 V for a pulsed frequency of 30 kHz. These results facilitate the application of ultrasonic arc welding technology in the marine field.