Comparison of microstructure and phase transformation of laser-welded joints in Al-10wt%Si-coated boron steel before and after hot stamping

The features of segregation and structure phase transformation of laser-welded joints in Al-10wt%Si-coated boron steel before and after hot stamping heat treatment were studied. Al segregation in the fusion zone is divided into two types. The first type is distributed unevenly in the fusion zone with a 1.28 wt%-0.30 wt% Al concentration. The microstructure of this zone comprises martensite (in Al-deficient zones) and bainite (in Al-rich zones). The second type is formed along the fusion boundary in a long comet shape with a high Al concentration (8.24-15.10 wt%). The microstructure of this zone comprises Fe3(Al,Si) with a single phase. The microstructures in the laser-welded fusion zone before and after hot stamping heat treatment were identical. The Al content in this zone decreases because Al diffuses toward the base metal during hot stamping heat treatment. On the other hand, after the hot stamping process, phase transformations can occur at the fusion boundary. Such transformations lead to microstructural changes from the Fe3(Al,Si) phase to the polygonal ferrite phase. It is clear that the Al contents in the segregation zone affect the ferrite formation in the fusion boundary and zone.