A prediction model of the welding process of Ti-6Al-4V titanium alloy was established by using the finite element method, which was used to evaluate the phase composition, residual stress and deformation of the welded joints of Ti-6Al-4V sheets with different processes (including tungsten inert gas welding, TIG, and laser beam welding, LBW). The Ti-6Al-4V structures of TIG welding and LBW are widely used in marine engineering. In order to quantitatively study the effects of different welding processes (including TIG welding and LBW) on the microstructure evolution, macro residual stress and deformation of Ti6Al4V titanium alloy sheets during welding, a unified prediction model considering solid-state phase transformation was established based on the ABAQUS subroutine. In this paper, LBW and TIG welding experiments of 1.6 mm thick Ti-6Al-4V titanium alloy sheets were designed. The microstructure distribution of the welded joints observed in the experiment was consistent with the phase composition predicted by the model, and the hardness measurement experiment could also verify the phase composition and proportion. From the residual stress measured by experiment and the residual stress and deformation calculated by finite element simulation of LBW and TIG weldments, it is concluded that the effect of phase transformation on residual stress is mainly in the weld area, which has an effect on the distribution of tensile and compressive stress in the weld area. The overall deformation of the welded joint is mainly related to the welding process, and the phase transformation only affects the local volume change of the weld seam. Importantly, the phase composition and residual stress, which are scalar fields, calculated by the established model can be introduced into the numerical analysis of structural fracture failure as input influence factors.

For the multi-layer and multi-pass welding process of the heavy plate, the hydrogen diffusion behavior was numerically simulated to study the effect of solid-state phase transition (SSPT) on the hydrogen diffusion in the thickness direction, and the influence of the residual stress-induced diffusion after SSPT. The calculation results were compared with the experimental results. The comparison shows that the distribution of hydrogen concentration in the direction of thickness was in good agreement. The position with the most severe cold cracking sensitivity was located at a 20–30 mm depth from the top surface in this article. After welding, the hydrogen concentration in this position was kept at a high level for a long time under the effect of the size-constraint effect of the heavy plate and the existence of welding residual stress gradient. In addition, the SSPT reduced the residual stress level of weld metal (WM) significantly, increased that of the heat affected zone (HAZ), and the hydrogen was redistributed under the influence of stress. In the process of phase transformation, the parameters of hydrogen diffusion property of the material changed dramatically in a short time, the hydrogen diffusion coefficient increased in order of magnitude, and the solubility decreased in order of magnitude. This directly led to the upward diffusion of hydrogen in WM, and produced a self-gathering effect. For a welded joint of heavy plate, the self-gathering effect between passes was effective in the short-range and ineffective in the long-range.

This study investigates the effects of welding parameters and the addition of a buffer layer on the morphology, microstructure, mechanical properties, and corrosion resistance of the overlay layer during overlay welding. This paper uses Q235 steel as the base material, ER309L as the buffer layer, and ER347 as the overlay layer to conduct process experiments on overlay welding component, aiming to obtain optimal process parameters. The effects of welding line energy and weld bead overlap rate on the morphology, dimensions, and dilution rate of the overlay layer were analyzed. Furthermore, the influence of the presence or absence of the buffer layer on the microstructure, mechanical properties, and corrosion resistance of the overlay layer was investigated. The microstructure and morphology of the overlay layer were characterized by optical microscopy (OM), scanning electron microscopy (SEM) and energy-dispersive spectroscopy (EDS). Mechanical and electrochemical tests were also performed to evaluate the mechanical and corrosion resistance properties of ER347 stainless steel weld overlays. The results showed that the optimal process parameters were successfully obtained, which ensured sound weld bead formation while minimizing dilution. The addition of the buffer layer (ER309L) improved the bonding quality of the overlay welding component interface, reduced element dilution in the overlay layer, significantly improved hardness distribution, and reduced sudden changes in hardness in the fusion zone, thereby optimizing the mechanical properties of the ER347 stainless steel overlay layer. After adding the buffer layer, the corrosion current density decreased from 6.23 × 10⁻⁵ A·cm⁻² to 2.21 × 10⁻⁵ A·cm⁻², and the corrosion potential increased from −1.049 V to −0.973 V, effectively reducing the corrosion risk of the overlay component. This study innovatively introduced a buffer layer in the process of overlay welding austenitic stainless steel on low-carbon steel and investigated the impact of the overlay welding process on the overlay layer, thereby contributing to a comprehensive understanding of the overlay welding process from multiple perspectives.

The hydrogen diffusion in welding was investigated by using thermal-mechanical-hydrogen diffusion sequential coupled procedures based on finite element method. A self-gathering effect induced by the solid-state phase transformation was discovered. Because of the self-gathering effect, the hydrogen concentration in weld metal was accumulated to a peak value which can be larger than the initial hydrogen concentration in molten pool, and subsequently the hydrogen concentration in heat affect zone was redistributed. In multi-pass welding, the gathered effect not only happened inside a weld pass, but also in the inter-pass, which further increased the sensitivity of the hydrogen-assisted cold cracking. Controlling should be adopted to restrain the hydrogen accumulation. Welding stress evolution during the solid-state phase transformation process had limited effect on the hydrogen diffusion.