Investigation of the underwater laser directed energy deposition technique for the on-site repair of HSLA-100 steel with excellent performance

Abstract A novel underwater directed energy deposition (UDED) technique was developed for the on-site repair of marine equipment in an underwater environment. A special drainage nozzle was integrated with a laser cladding head to form the underwater manufacturing tool which ensured the successful repair of the damage zone on an HSLA-100 steel plate. The influence mechanisms of inherently complex heat-treatment cycles and kinetics involved in the UDED process on the microstructural formation/evolution process were systematically investigated using both experimental and numerical approaches. The mechanical properties, including microhardness, nanohardness, tensile strength and Charpy impact toughness of the UDED repaired samples were investigated and the results were compared with those repaired by in-air directed energy deposition (DED). The experimental results show that the rapid cooling rate accelerated by the water quenching effect promoted the formation of columnar austenite and the subsequent fine martensite. Compared with the DED samples, the activation energy values for the nucleation and growth of the precipitates in UDED samples were much lower due to the existence of high-density crystal defects and large internal residual stresses. The numerical results indicate that compared with DED, the cooling rate significantly increased and the peak temperature decreased during UDED. The average microhardness of the samples prepared by UDED (315–334 HV) was higher than that prepared by DED (307 HV). The tensile properties and impact toughness of the as-repaired samples by UDED were equal or even better than those obtained by DED owing to the formation of fine martensite and high dislocation densities in the UDED samples. This work clarifies the difference in metallurgical processes in air and underwater environments, which can provide guidance for future structural manipulation of materials additively manufactured in underwater environments.