Effect of external applied magnetic field on microstructures and mechanical properties of laser welding joint of medium-Mn nanostructured steel

Medium-Mn nanostructured steel developed recently demonstrates attractive mechanical property and high potential for structural application. In this study, a new welding method with external magnetic field assisted laser was applied to investigate the weldability of this steel. The effects of external applied magnetic field on the microstructure and tensile behavior of the joint were studied. Our results show that the external magnetic field significantly enhances the ultimate strength by 43.9% and dramatically turns the brittle fracture into ductile with a more than tripled elongation rate during uniaxial tension tests. The prior-austenite grain size in weld with external magnetic field is more than twice finer than those in the laser weld, together with the elimination of the Mn segregation at prior austenite grain boundary (PAGB), significantly improve the stability of the prior austenite and lead to an increased volume fraction of retained austenite in weld. Our analyses manifest that the thermoelectric magnetic convection (TEMC) generating from the coaction between the external magnetic field and the internal thermoelectric current is the main reason for the alteration of the microstructures and mechanical properties. Promotion of the heat and mass transfer driven by the interdendritic TEMC reduces the segregation at PAGB and decreases the grain size. The improvement of the strength originates from the refined grain-size and promoted mechanical-induced martensite transformation, while the reduction of the segregation and increased volume fraction of retained austenite contribute to the enhanced ductility.