Influence of Al–Si coating on microstructure and mechanical properties of fiber laser welded and then press-hardened 22MnB5 steel

Abstract

The de-coated/coated 22MnB5 steel was laser welded and then press hardened in the present study, and the effect of Al–Si coating on microstructure and mechanical properties was comparatively investigated. The results showed that the fusion zone (FZ) of the de-coated joint was approximately composed of 90% (area fraction, the same as below) martensite and 10% (ferrite + bainite), while that of the coated joint contained about 40% martensite and 60% ferrite. The Al–Si coating dissolving in the FZ facilitated the formation of ferrite during the press hardening process. This mixed structure caused a low hardness of 300 HV in the FZ of the coated joint, and other regions composed of regular martensite showed hardness around 500 HV. The tensile strength of the de-coated joint was 1310 MPa, representing 89% joint efficiency relative to the base metal (BM). Ductile and brittle fracture characteristics were both observed. While only brittle fracture occurred for the coated joint showing poorer tensile properties. The digital image correlation (DIC) analysis revealed that the cracks initiated and finally cracked in the FZ for both joints. Both the break bending load and Erichsen value of the de-coated joint were also higher than those of the coated one. The martensite formed in the FZ of de-coated joint was uniform and contained high density of geometrically necessary dislocations (GNDs), which could lead to high strength and ductility due to the coordinated deformation among martensite grains. The large amount of ferrite in the FZ of coated joint led to larger grain misorientation and higher stress concentration.