Influence of adjustable ring modes on molten pool behavior and microstructure of Al–Si coated boron steel in fiber laser welding

The influence of adjustable ring modes on the phase transition and microstructural evolution in conduction mode laser welding of Al–Si coated boron steel was investigated. Distinct geometric variations and molten pool behavior were observed through a ring-shaped beam effect. The ring beam introduced by ring-beam-mode (RBM) and dual-beam-mode (DBM) laser welding inhibited Al segregation into the fusion zone (FZ). The ring beam caused the Al molten particles to be driven towards the outer boundaries of the weld pool. A ring-beam-mode (RBM) and a dual-beam-mode (DBM) laser welding inhibited Al segregation into the FZ, resulting from a minimal temperature gradient and extension of the solidification time. Phase transition calculations demonstrated that a mount of residual ferrite within the FZ is significantly correlated with the Al concentration at specific temperature points, indicating a predominant role of a central-beam-mode (CBM) FZ in δ-ferrite formation. The RBM and DBM effectively suppressed Al segregation into the FZ, leading to a reduction in ferrite formation and the development of a refined microstructure characterized by a uniform and high density of geometrically necessary dislocations. Electron backscattered diffraction (EBSD) analysis revealed a higher proportion of low-angle grain boundaries in the RBM and DBM, attributed to the unique thermal distribution induced by the ring beam. These findings highlight the critical role of beam mode in tailoring the microstructure and properties of laser welds in Al–Si coated boron steel.