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.

Butter layers of different thicknesses were successfully deposited on ferritic steel by using the tungsten arc welding (TIG) process. The effects of butter layer thickness on the microstructural characteristics, elemental distribution, and mechanical properties of underwater wet 16Mn/304L dissimilar welded joints were investigated. The results showed that the butter layer significantly changed the microstructure and elemental distribution of 16Mn/304L joints. As the thickness of butter increased, the heat-affected zone (HAZ) at the ferritic steel side changed from the original 16Mn steel to the ERNiCrMo-3 butter layer. The martensite content in HAZ also exhibited a downward trend. When the thickness of the butter layer exceeded 6 mm, the microstructure of HAZ at the ferritic steel side was composed of ferrite and pearlite, instead of quenched martensite. The microhardness of underwater dissimilar steel welded joints significantly reduced due to the absence of martensite. The addition of the butter layer increased the ultimate tensile strength from 515 MPa to 565 MPa. The results of this work could provide a robust basis for future applications of dissimilar steel structures.

In order to verify the feasibility of in situ repair of underwater local dry laser welding (ULDLW) on nuclear power reactor components, this work investigates the microstructure and mechanical properties of 304L austenitic stainless steel repaired by ULDLW using ER308L filler metal. Comprehensive comparison would be made between the ULDLW and conventional in-air laser welding to evaluate their applicability. The results demonstrate that the rapid cooling rate inherent to the underwater environment significantly influences solidification behavior and microstructural evolution. The weld metal (WM) solidifies in the ferritic–austenitic (FA) mode, with an increased proportion of lathy δ-ferrite at the expense of skeletal morphology compared to the in-air welds. Electron backscatter diffraction (EBSD) analysis reveals the substantial grain refinement in underwater welds, with average grain sizes of 39.4 μm versus 47.3 μm for in-air weld bead, accompanied by a higher fraction of low-angle grain boundaries (LAGBs). These microstructural modifications yield superior mechanical properties: underwater weld bead exhibits ultimate tensile strength (UTS) of 685.6 MPa, elongation of 57.5%, and impact toughness of 22.6 J, significantly exceeding the corresponding values for in-air welds (663.9 MPa, 51.8%, and 18.6 J, respectively). Fractographic analysis confirms ductile fracture mechanisms in both conditions. The enhanced performance is attributed to grain refinement strengthening via the Hall–Petch relationship and the increased LAGBs fraction, which impedes dislocation motion and crack propagation.

Ultrasonic frequency pulse assisted TIG welding (UFP-TIG) experiments were conducted to join Inconel 690 alloy (IN690) by adding Inconel 718 alloy (IN718) as the filler. The effect of the filler on the microstructure, mechanical properties, and ductility dip cracking (DDC) susceptibility of IN690 joints were investigated. The results show that a variety of precipitates, including MC-type carbide and Laves phases, are formed in the weld zone (WZ), which are uniformly dispersed in the interdendritic region and grain boundaries (GBs). The increase in the thickness of the IN718 filler facilitates the precipitation and growth of Laves phases and MC carbides. However, the formation of Laves phases in the WZ exhibits a lower bonding force with the matrix and deteriorates the tensile strength of IN690 joints. Due to the moderate content of Laves phases in the WZ, the IN690 joint with 1.0 mm filler reaches the maximum tensile strength (627 MPa), which is about 96.5% of that of the base metal (BM). The joint with 1.0 mm filler also achieves the highest elongation (35.4%). In addition, the strain-to-fracture tests indicate that the total length of cracks in the joint with the IN718 filler decreases by 66.49% under a 3.8% strain. As a result, the addition of the IN718 filler significantly improves the mechanical properties and DDC resistance of IN690 joints.

In this work, a laser lap-welded joint of galvanized steel/Mg and a laser lap-welded joint of galvanized steel/Mg assisted by ultrasonic vibration were compared. By adjusting the laser beam power and ultrasonic amplitude, the appropriate welding process parameters were obtained. The weld formation, microstructure and mechanical properties were studied and analyzed. The results indicated that the addition of ultrasonic vibration generated an excitation force with a certain frequency and amplitude on the weldment, making the molten metal in the molten pool produce ultrasonic forced vibration, and producing the effects of cavitation, acoustic streaming, mechanical stirring and heat, thus reducing welding residual stress and welding-deformation, porosity and incomplete-fusion defects. In addition, it can make the fusion zone transition evenly, improve the wettability, refine the weld grain, and reduce the average grain area from 583 μm² to 324 μm². Moreover, the distribution of Mg-Zn reinforcing phase at the interface was more uniform and denser, and the maximum tensile shear strength increased from 179.9 N/mm to 290 N/mm, indicating that the addition of ultrasonic vibration was conducive to improving the comprehensive mechanical properties of the joint.

This study addresses the challenges of porosity defects and grain coarsening inherent to local dry underwater laser welding of 304 austenitic stainless steel, a material widely used in nuclear power and marine engineering. To mitigate these issues, laser beam oscillation was integrated with a double-layer gas-curtain drainage system to secure a stable local dry environment. The individual effects of six key process parameters, including laser power, welding speed, wire feed rate, defocusing distance, oscillation frequency, and oscillation amplitude, on weld penetration, dilution ratio, and porosity were systematically investigated. Orthogonal design and range analysis quantified the relative significance of each parameter on weld morphology and defect formation. The results demonstrate that oscillation amplitude exerts the primary control over weld penetration, followed by defocusing distance, whereas oscillation frequency shows the least influence. Similarly, oscillation amplitude dominates porosity suppression, with defocusing distance acting as a secondary factor. Microstructural analysis indicated that high-frequency beam oscillation homogenizes the spatial energy distribution and induces forced convection within the molten pool. It is inferred that this dynamic stirring breaks up continuous columnar dendrites, decreases the average grain size from 32.37 to 21.52 μm (a 33.5% reduction), and effectively reduces porosity. Overall, this work provides empirical process data and microstructural control that may serve as a reference for future underwater laser repair applications.