Microstructure and mechanical properties of an ultra-high strength steel fabricated by laser hybrid additive manufacturing

35CrMnSiA is a type of ultra-high-strength steel that finds extensive application in key components characterized by geometrically regular structures and complex extensions, such as structural shells. Laser hybrid additive manufacturing (LHAM) is an advanced manufacturing technology that combines the benefits of traditional manufacturing and additive manufacturing. LHAM has the potential for rapid and cost-effective production of these components. However, the formation mechanism of the microstructure in different zones of 35CrMnSiA steel fabricated by LHAM and its impact on the overall tensile performance remain unclear, thereby tremendously restricting its application. To address this issue, this study employed wrought 35CrMnSiA steel as the base material and utilized directed energy deposition (DED) to fabricate defect-free LHAM parts. The microstructure of LHAM-fabricated 35CrMnSiA steel is location-related. The laser deposition zone comprised ferrite with internal and boundary-distributed carbides, whereas the substrate zone comprised martensite. The heat-affected zone exhibited a gradient microstructure between the two. The microstructure in the deposition zone was primarily governed by solidification behavior and thermal history during the deposition process, while the heat-affected zone was mainly influenced by recrystallization. The non-uniform characteristics of the microstructure significantly influenced its mechanical properties, resulting in a “cask effect,” namely, the tensile performance of LHAM-fabricated 35CrMnSiA steel depended on the weakest laser deposition zone. The ultimate tensile strength and elongation of LHAM-fabricated 35CrMnSiA steel are 959 MP and 15.2%, respectively. This study establishes a basis for controlling the microstructure and optimizing the performance of hybrid-manufactured ultra-high-strength steel.