This study investigates the laser welding of 1.4 mm thick hot-stamped steel with Al?Si coating using nickel-based alloy filler metal. The microstructural differences in the weld joint under varying laser powers and feeding rates are systematically compared and analyzed. Additionally, the influence of alloying elements on the performance of the joints is thoroughly explored. The results demonstrate that Ni and Co, as strong austenite-stabilizing elements in the nickel-based filler metal, effectively mitigate the adverse effects of aluminum (Al), preventing peritectic transformation and the formation of ferrite in the weld. When the mass fraction of Ni in the weld exceeds 3.8%, the segregation of the Al element is entirely suppressed, and the weld exhibits a fully martensitic structure before and after hot stamping. Furthermore, when the mass fraction of Ni in the weld is 6.47%, second-phase particles, such as M₂₃C₆ and TiN precipitates, are observed in the martensitic matrix, further enhancing the strength of the weld. The optimal welding joint exhibits an average hardness of 535 HV and a tensile strength of 1643.5 MPa, comparable to the base metal. The joint is broken at the base metal.

The current research primarily focuses on using metal foil filling technology with a single alloying element to improve the laser welded joint strength of hot-stamped steel with Al?Si coating. However, the application of Ni foil filling technology is limited in its application scope in industrial production due to the complexity of the process and the uncontrollable content of introduced Ni. In view of this, this study proposes the use of nickel-based alloy welding wires for laser welding. By adjusting laser power and wire feeding speed, the alloy element content in the weld metal can be controlled effectively.

From Figs. 2(d)?(f), it is evident that increasing the laser power further increases the amount of filler metal melted, resulting in a significant reduction of δ-ferrite within the weld seam of sample 2#, with only a small amount of small-sized particles present near the surface and fusion line. In sample 3#, an increased feeding speed leads to a higher Ni content in the weld seam. Figure 2 (g)?(i) shows that the weld seam of sample 3# exhibits a nearly fully martensitic structure, representing a significant improvement compared with that of samples 1# and 2#. Figure 3 displays the microstructure of the weld seam after laser welding and hot forming using nickel-based filler metal. From Figs. 3(a)?(c), it is clear that after hot forming, the weld seam of sample 1# transitions from the original δ ferrite to a mixed structure of martensite and α ferrite, with ferrite mass fraction around 18%. Figure 8 presents the tensile properties and fracture locations of the weld joints (1#, 2#, and 3#) using nickel filler metal. The tensile curves reveal that the tensile strengths of all three groups are over 1630 MPa, comparable to the strength level of the base material. The curves for all specimens show significant plastic deformation, indicating a considerable improvement in tensile properties.

The use of nickel-based alloy welding wires significantly improves the microstructure of weld seams. When the mass fraction of Ni in the weld exceeds 3.8%, aluminum segregation is effectively suppressed, resulting in a fully martensitic structure both before and after hot forming. With the mass fraction of Ni increases to 6.47%, second-phase particles such as M₂₃C₆ and TiN precipitate in the weld, with solid solution strengthening effects significantly enhancing the mechanical properties of the joint. Additionally, austenite-stabilizing elements such as Ni and Co suppress the adverse effects of aluminum on the austenite phase, ensuring the complete peritectic transformation and suppressing δ ferrite formation. The optimal welding joint (sample 3#) welded with nickel-based wire exhibits excellent mechanical properties. The average hardness of the weld area reached 535 HV, and the tensile strength was up to 1643.5 MPa, comparable to the base metal. Furthermore, the fracture occurring on the base metal side indicates improved ductility of the welded joint. The uniform distribution of alloying elements such as Ni, Co, and Cr within the weld plays a critical role in enhancing the joint’s performance. Ni enhances weld strength by lowering the austenite transformation temperature and expanding the austenite phase field. The synergistic effects of Co and Cr further promote structural uniformity and optimal second-phase distribution. These elements significantly improve the mechanical properties of the weld joint through mechanisms such as solid solution strengthening and grain refinement.