Study on δ-ferrite evolution and properties of laser fusion zone during post-weld heat treatment on Al-Si coated press-hardened steel

Laser welding of Al-Si coated press-hardened steel (PHS) was still a challenging due to the formation of δ-ferrite in the fusion zone. However, it was ambiguous on the δ-ferrite evolution during post-weld heat treatment (PWHT). In this study, Al-Si coated PHS was welded by laser fiber and then heat-treated at various temperatures (650 °C, 950 °C and 1050 °C), and the microstructure evolution was investigated by in situ-observation. Mechanical properties and formability of welded joints was analyzed. The results showed that δ-ferrite did not go through phase transformation during PWHT regardless of any temperature, but with a grain boundary flatted and movement. The presence of coarse δ-ferrite after PWHT was the responsible for the decreasing of tensile strength and formability of welded joints. The initiation crack was easy to generate in the phase interface between δ-ferrite and martensite to form a local brittle fracture zone finally. This study results provides a new theoretical criterion for the properties improvement on the laser welding of PHS without removing Al-Si coating.