Mirostructure and properties of laser welded joints of dual phase and press-hardened steels

Abstract Tailor welding of 1000MPa-grade dual phase (DP) steels and 1500MPa-grade press-hardened steels (PHS) (22MnB5) with/without Al-Si coating was carried out using high power fiber laser. OM and SEM were used to observe the microstructure of welded joints. The microstructure of welded seam consisted of lath martensite and δ -ferrite because of the diffusion of Al-Si coating. Al element occurred segregaion in fusion line to form Fe-Al intermetallic compounds. The microstructural evolution in heat affected zone (HAZ) of two sides of dissimilar welded joints was similar: lath martensite (Coarse-grained HAZ) →lath martensite (Fine-grained HAZ) →martensite and ferrite (Intercritically HAZ) →tempered martensite and ferrite (Sub-critical HAZ). Softening occurred in both HAZs of dissimilar welded joints due to the forming of tempered martensite. Microhardness was tested by nano-indentation to evaluate the nano-mechanical properties of each phase in welded joints. Mechanical properties was measured by universal material testing machine. Both of the dissimilar welded joints, tensile fracture was at BM of DP980 side although Al-segregaion occurred at fusion line of DP980-PHS with Al-Si coating.