Al-Si-coated press-hardened steel is a type of ultra-high-strength steel having both lightweight and safety properties. Owing to the presence of the Al-Si coating, Al segregation and ferrite formation occur in the weld during conventional laser welding, which significantly deteriorates the mechanical properties of the welded joints. The inhibition of Al segregation and ferrite formation is key to improving the mechanical properties of welded joints. In this study, based on the stirred melt pool characteristics of an oscillating laser beam and dilution effect of the welding wire, oscillating laser welding with a filler wire was used to improve the overall weld formation and reduce the ferrite formation in the weld. Compared with conventional laser welding, after hot stamping, the average hardness, tensile strength, and elongation of the welded joints assembled with oscillating laser filler wire welding were increased to 471 HV, 1561 MPa and 3.1%, respectively, which were increased by 20.2%, 8.4%, and 61.5% compared with those of conventional laser-welded joints.
The effects of the oscillation frequency on the weld formation, microstructure, and mechanical properties of joints were investigated using oscillating laser welding with Al-Si-coated 22MnB5 steel filler wire. Oscillation laser welding process with filler wire was compared with conventional laser welding to investigate the suppression of ferrite in the weld. Optical microscopy and scanning electron microscopy were used to analyze the weld formation, microstructure, chemical composition, and fracture morphology of the joints. The weld hardness was measured using a hardness tester, and the mechanical properties of the joint after hot stamping were measured using a universal stretching machine and extensometer. The equilibrium phase diagram of the laser welding process was calculated using the Pandat software.
The absence of filler material during conventional laser welding resulted in a concave weld surface. Oscillating laser welding with filler wire improved the weld formation; subsequently, the weld seam was fuller (Fig. 4). Compared with conventional laser welding, oscillating laser welding process with filler wire significantly reduced the area ratio of coating in the weld and diluted the Al content in the weld. The Al mass fraction in the weld decreased from 2.16% to 1.38% (Table 4), and the primary δ-ferrite volume fraction in the weld pool decreased from 82% to 71% (Fig. 12). The formation of δ-ferrite could be significantly inhibited by the oscillating laser welding process with filler wire. After hot stamping, the content of α-ferrites in the weld decreased significantly with an increase in the oscillation frequency from 0 Hz to 200 Hz (Fig. 6). Compared with conventional laser-welded joints, the hardness, tensile strength, and elongation of the welded joint assembled with oscillating laser filler wire welding increased by 20.2%, 8.4%, and 61.5%, respectively.
In this study, 1.5 mm thick Al-Si coated 22MnB5 steel was welded using the oscillating laser with filler wire, and the resulting characteristics were compared to those obtained with conventional laser welding. The cross-sectional shape of the conventional laser welded seam was “Y” shaped, and the upper and lower surfaces were concave. The volume fraction of α-ferrite in the weld after hot stamping reached 36.73%, and the hardness, tensile strength, and elongation of the welded joint were 392 HV, 1440 MPa, and 1.92%, respectively. The shape of the weld was improved by using oscillating laser welding with filler wire. The content of Al in weld was diluted and content of δ-ferrite was reduced. When the oscillation frequency increased from 140 Hz to 200 Hz, the volume fraction of α-ferrite in the weld decreased from 30.14% to 11.51%. As the oscillation frequency continued to increase to 320 Hz, the fluctuation of α-ferrite content in the weld became not significant. After optimizing the parameters, the hardness, tensile strength, and elongation of the joint increased to 471 HV, 1561 MPa, and 3.1%, respectively. Because α-ferrite formed in the weld under both welding processes after hot stamping, the joints all fractured at the weld. The brittle zone of the fracture that occurred in the conventional laser-welded joint was the cleavage plane with the river pattern, and the ductile zone was small and shallow. The area ratio of the brittle zone was greater than 80%, which was typical of brittle fractures. Compared with that in the conventional laser welding, the area ratio of the ductility zone in the fracture of the welded joint in oscillating laser filler wire welding was more than 90%; accordingly, the ductility of the welded joint was improved.
