Frequency-dependent microstructural evolution and strengthening in transverse oscillating laser welding of martensitic-ferritic dissimilar stainless steels

Oscillating laser beam welding (OLBW) offers advantages in reducing the temperature gradient and promoting equiaxed grain formation in the fusion zone (FZ), thereby enabling the welding of dissimilar stainless steels. This study systematically investigated the effects of transverse oscillation frequency on the macro-morphology, microstructure, and mechanical properties of dissimilar stainless steels welds between martensitic and ferritic steels. The results show that OLBW not only significantly widens the weld but also effectively ameliorates surface welding defects such as incomplete filling compared with continuous laser welding (CLW). In addition, laser oscillation facilitates the transformation of the solidification microstructure in FZ from coarse columnar grains to fine columnar grains and equiaxed grains; nevertheless, yet a higher oscillation frequency does not always contribute to grain refinement. Furthermore, by optimizing thermal distribution, OLBW effectively tailors the microhardness of the weld zones, with hardness peaking at 50 Hz before declining at higher oscillation frequencies due to decelerated cooling rates and incomplete martensitic transformation. Further, compared with CLW, laser oscillation contributes to the improvement of the tensile strength and elongation of the welds, but has no effect on the yield strength. The fracture surface is characterized by a large number of equiaxed dimples and micro-voids, indicating typical ductile fracture. This study provides a new method and theoretical basis for laser welding of dissimilar stainless steels.