Evolutions of microstructure and mechanical property of 308L stainless steel repaired by the local dry underwater wire arc additive manufacturing

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Local dry underwater wire arc additive manufacturing process (LDU-WAAM) was innovatively proposed to repair the damaged large underwater structures. A thorough investigation was performed to explore the microstructure and mechanical properties of 308L stainless steel multi-layer components prepared by LDU-WAAM technique. The results demonstrate that the microstructure of all components was composed of a significant amount of austenite and a minor amount of ferrite. Subjected to the repeated action of local arc heating and rapid water cooling, the multi-layer components were deposited into a stratified structure, each layer was mainly composed plenty of directional columnar crystals, and the adjacent layers were separated by a small number of equiaxed crystals. With the increase of deposited layer counts, the heat transfer paths from deposited metal to the substrate were gradually lengthened, which increased average grain size, LAGBs proportion, dislocation density, and austenite deformed twinning content of the multi-layer sample, also decreased its ferrite content. Hence, the microstructure of LDU-WAAM multi-layer component varied significantly from the bottom to the top, which led to the deteriorated mechanical properties along the building direction. In the 3-layer component, the ultimate tensile strengths of the first, second, and third layers were 554 MPa, 532 MPa, and 519 MPa, respectively. And the elongations of that were 25.3%, 23.2%, and 19.5%, respectively. The research results verify the feasibility of local dry underwater wire arc additive manufacturing process and provide a new process method for underwater on-site repair.