Investigation on microstructure evolution of iron-based alloy via synchronous powder-feeding underwater laser additive

(a) Experimental equipment and schematic (b) Additive bulk material in SULC (c) Microstructure of SULC planar cladding (d) Schematic of SULC numerical model (e) Vapor distribution contour in SULC (f) Distribution of different size of grain in SULC additive bulk (g) Temperature contour of different coatings. [Display omitted] •Innovative preparation of iron-based bulk material underwater by synchronous powder-feeding underwater laser cladding, without using additional drainage structures.•The grain-scale structure comparison of additive structure between synchronous powder-feeding underwater laser cladding and conventional laser cladding was investigated.•The solidification structure and grain structure were analyzed and compared with conventional laser cladding by microstructure analysis.•A simulation study of the underwater laser heating process involving the effects of protective gas and water boiling was conducted. Underwater Laser Additive Manufacturing (ULAM) could further increase the size of synchronous powder-feeding underwater laser cladding (SULC) coatings to achieve underwater preparation devices and emergency repairs. In this paper, the homemade SULC system is used to prepare iron-based planar claddings and bulks, and the grain growth and structural characteristics of underwater additive bulk are obtained by micro-analysis. The grain's growth in the upper part of the bulk is similar to the planar cladding, and the area near the substrate has no obvious columnar grains due to the instability of the temperature gradient for multiple heat input. The EBSD results show that changes in grain size and dislocation density occurred in the additive interface. A periodic fine-grained band in the middle of the bulk appeared, the average hardness of the fine-grained band is 805Hv0.025, and the hardness of the larger equiaxed grain area is 770Hv0.025. Numerical simulation was utilized to obtain temperature field of the SULC laser heating process by introducing the protective airflow and boiling model. The maximum temperature in SULC is about 16% lower than in-air laser cladding, and the boiling model will not affect the temperature distribution. This paper provides theoretical support for the further realization of ULAM.