Improving the mechanical properties of PHS laser welded joints by adding Ni foil to suppress δ-ferrite

Abstract Ni foils with various thickness were used as interlayer to achieve the laser tailored welding of 22MnB5 press-hardened steel (PHS) with Al-Si coating. The effect of Ni content on the microstructure, hardness and strength were studied. The results showed that Ni reduced the content of solidified primary δ phase in the fusion zone, and promoted the transformation from δ phase into γ phase. δ ferrite was absent with the Ni content of 4.7–19.5 wt.%. In addition, the fusion zone was in γ phase region during post-weld heat treatment at 950℃ because Ni can reduce the Ac3 temperature (the temperature at all ferrite was transformed into austenite). The microstructure of fusion zone after post-weld heat treatment was lath martensite with the Ni content of 4.7–11.2 wt.%. The average hardness was similar to base metal, and the tensile strength of welded joint was increased by ∼22% of as-received joint. Ni was effective to suppress the formation of δ ferrite in Al-rich fusion zone, which is potential to achieve the laser welding of PHS with Al-Si coating.