Effect of Filler Wire on Laser Lap Welding of Al-Si Coated 22MnB5 Hot Stamping Steel

To suppress the accumulation of δ-ferrite and adjust the dilution of the Al-Si coating, 22MnB5 coated with Al-Si was laser lap welded with filler wire. The weld appearance, dilution of the coating, microstructure, and mechanical properties of the joints were studied. A mathematical model was established by energy conservation between the feeding speed and weld appearance and between the feeding speed and dilution of the base material. When the feeding speed was raised, the weld gradually changed from a tall cup shape to a bowl shape and the dilution of the coating decreased. This led to the reduction of the δ-ferrite. The fusion zone (FZ) was lath-like martensite (M). Four zones along the FZ to base material (BM) were formed in HAZ. The coarse grain heat-affected zone (CGHAZ) near FZ and the fine grain heat-affected zone (FGHAZ) consisted of martensite. The intercritical heat-affected zone (ICGHAZ) near BM consisted of martensite and a small amount of ferrite. The tempered zone (TZ) consisted of tempered martensite with the lowest hardness. Importantly, band-like (δ-ferrite) which had low strength and ductility occurred along the fusion boundary and became a weak region. The ultimate tensile strength was only 436 MPa without filler wire but reached 746 MPa with filler wire whose failure mode mixed dimpled and quasi-cleavage fracture.