Improving mechanical properties of the Al–Si coated press-hardened steel self-fusion joint by utilizing dual-beam laser welding

The Al–Si coated press-hardened steels (PHS) are increasingly used in the automotive industry, but their welding properties are significantly degraded due to frequent melting of the Al–Si coating into the molten pool. Al in the coating melts into the weld and undergoes local enrichment, inducing the formation of δ-ferrite in this region. The interface between such δ-ferrite and martensite acts as a crack initiation site, leading to premature weld fracture, which necessitates either removing the Al–Si coating before laser welding or adding filler wire during welding to dilute the Al content. In this study, a laser processing head with a beam splitting device was used to split a laser beam generated into two serially arranged beams for welding. Without removing the coating or adding additional filler wire, the mechanical properties of the laser-welded joints of the new thin-coated PHS have reached a level comparable to those of the base metal. The microstructure, fracture morphology, dynamic behavior, and morphological evolution of the molten pool under different welding conditions were investigated. The results showed that dual-beam laser welding with a spot spacing of 490 μm achieved an elongation of 9.21 %, representing an increase of 52.43 % compared with single-beam laser welding joints (5.67 %), and the tensile strength was almost identical to unwelded base metal. As the spot spacing increased, the joint elongation decreased gradually. In the fusion zone of single-beam joints, long and wide strips of δ-ferrite were observed, which contributed to brittle fracture and low elongation. As the spot spacing increased from 0 μm to 490 μm, 1000 μm, and 1500 μm, the area ratio of these δ-ferrite to martensite in the fusion line region is 6.84 %, 0 %, 0.56 %, and 2.55 %, respectively. The average grain size in the fusion line region of the single-beam joint was 8.13 μm, which was reduced to 7.79 μm in the dual-beam joint. The morphological characteristics of the molten pool and keyhole in top-view and longitudinal section were captured in real-time using a high-speed camera in combination with the “sandwich” welding method. The results demonstrated that the dual-beam laser enlarged the length and width dimensions of the melt pool by more than 15 % compared with the single-beam laser, and intensified melt flow within the pool, effectively suppressing macroscopic segregation of Al.