Microstructure evolution and mechanical performance optimization of 30CrMnSiNi2A steel processed by laser powder bed fusion combined with heat treatment

In this study, 30CrMnSiNi2A high-strength steel components were successfully fabricated by laser powder bed fusion (LPBF) technology. The microstructure evolution was effectively tailored by the control of laser energy density, which can be adjusted by the laser processing parameters, including laser power, scanning speed, hatch spacing and scanning layer thickness. The effects of laser energy densities on the molten pool morphology, densification, phase constitution and mechanical properties of as-built samples were systematically analyzed. It can be found that when the laser energy density reached 96.2 J/mm3, a maximum relative density of 99.4% can be achieved, while the corresponding microhardness, yield strength (YS) and ultimate tensile strength (UTS) reached 478.3 HV, 1560 MPa and 1159 MPa, respectively. Notably, compared with the mechanical properties of traditional castings reported in earlier studies, these performances were enhanced by 75%, 20.9%, and 46.8%, respectively. Particularly, the scanning layer thickness exhibited the most significant impact on the mechanical properties. This was primarily ascribed to the transition of melt pool from the conduction-dominated regime to the stable keyhole conduction mode at higher laser energy density, where can effectively enhance wetting and remelting to suppress the defect formation and improve the material integrity. During the LBPF process, the retained austenite was gradually transformed into martensite, while the reduced layer thickness can enhance the melt pool fluidity to promote the formation of flat and uniform scanning tracks, and the densification and mechanical performance can be improved. The melt pool boundaries and structural inhomogeneities can be further eliminated by the subsequent heat treatment, leading to the grain growth of austenite phase. After heat treatment, the average microhardness (411.8 HV) and UTS (1341 MPa) were decreased, whereas the elongation was enhanced from 10.1% to 16.1%. Meanwhile, the fracture mode was also shifted from mixed brittle-ductile fracture to ductile fracture. It is anticipated that the fabrication of 30CrMnSiNi2A component with high density and superior mechanical properties can provide a theoretical and practical foundation for the high-performance manufacturing of complex high-strength steel parts in the fields of aerospace and automotive industry.