Comparison investigation on microstructure and properties of dissimilar welds by laser alignment welding and laser offset welding between Al–Si coated 1.5 GPa and novel 2 GPa press hardened steels

Press hardened steel (PHS) plays a key role in the manufacture of anti-collision structural components. The formation of δ-ferrite is a suffering issue for the laser welding of Al–Si coated PHS. Oscillating laser was used to weld Al–Si coated 1.5 GPa PHS and novel 2 GPa PHS, and the effect of laser offset on the microstructure and properties of the dissimilar welded joints was studied. The results showed that a perfect weld profile was achieved by laser offset welding (LOW), without any welding defects. The δ-ferrite formed in as-received welds of laser alignment welding (LAW) with high Al content (up to 2.9 wt.%), but it disappeared with the laser offset to 2 GPa PHS, and the maximum Al content in the segregation zone reduced to 1.2 wt.%. After post-welding heat treatment, the δ-ferrite was coarsened and the α-ferrite formed in the secondary Al-rich area for the high A_c3 temperature, but the α-ferrite was few and fine in LOW welds. The hardness in the LAW welds was lower than that in the LOW welds, due to the presence of δ-ferrite, as well as less carbon content and Ti and V alloying elements. The tensile strength (1561 MPa) and elongation (5.4%) with LOW were higher than those with LAW (1490 MPa, 3.1%), and the fracture occurred in the Al–Si coated PHS. It is indicated that adjusting the laser offset is effective to prevent the formation of δ-ferrite and is potential to avoid the formation of α-ferrite. It also provides a wide heat treatment temperature window for the dissimilar welds of 1.5 GPa PHS and novel 2 GPa PHS.