Underwater laser directed energy deposition of NV E690 steel

Powder-based laser direct metal deposition (DMD), one of the directed energy deposition (DED) techniques, was applied in air and underwater to repair pre-machined NV E690 steel plates. Systematic investigations on the effects of the underwater environment and the ambient pressures (0.01 MPa–0.35 MPa) on the microstructure evolution, phase transformation, and mechanical properties were conducted. The water quenching effect in the underwater direct metal deposition (UDMD) refined the grain size and increased the dislocation density and lath martensite content in the deposited material. The theoretical models of the underwater pressurized nitriding process and the precipitation kinetics of (Ti, V) N particles were established. Moreover, the microstructure evolution and the mechanical properties of other UDMD repaired samples did not show obvious relation with the underwater ambient pressures. This investigation not only provides an efficient candidate for the underwater restoration technique but also bridges marine engineering and emerging DMD technology.