Advanced laser directed energy deposition of Q690D high-strength steel using flux-cored wire: Microstructure evolution and mechanical performance

Abstract

High humidity and corrosive marine environments present substantial challenges to the durability and repair of high-strength steels. In this study, a structurally sound and defect-free repair of offshore-grade Q690D high-strength low-alloy steel was successfully achieved using a combination of flux-cored wire and laser wire-directed energy deposition technology. The repaired region was predominantly composed of lath martensite in both the deposition zone and the heat-affected zone. During the deposition process, the flux-cored wire facilitated the in-situ formation of nanoscale Ti₂C and TiC precipitates with core-shell structures through metallurgical reactions with the surrounding matrix. The repaired specimens exhibited a tensile strength of approximately 807 MPa, comparable to that of the base metal, along with a significantly enhanced flexural strength of ∼1626 MPa, primarily due to the synergistic interaction between nano-precipitates, lath martensite structures, and dislocation movement. However, Charpy impact energy at −20 ℃ decreased to approximately 52.4 % of the base material, owing to the embrittling effects of lath martensite and oxide inclusions, which facilitate void nucleation and crack propagation. These results validate the application potential of flux-cored wire assisted laser wire-directed energy deposition technology for repairing high-strength steels in marine-relevant environments, and reveal a coupled strengthening and embrittlement mechanism. This provides both mechanistic insight and practical guidance for the restoration of other structural alloys with similar service demands.