Inconel 625, a nickel-based superalloy widely used in key components in aerospace, energy, and other fields, is of great significance for the repair of local damage occurring under harsh service conditions. Laser directed energy deposition (L-DED) provides an efficient approach for the precise repair of high-performance components. However, the inherent rapid non-equilibrium solidification process of this technology easily leads to the formation of epitaxially grown columnar dendrite structures in the repaired region, along with the segregation of elements such as Nb and Mo between dendrites, resulting in the precipitation of brittle Laves phases and carbides and the formation of high residual stress. These microstructural defects often cause the deterioration of mechanical properties (especially plasticity) of the repaired region, and form a significant property gradient with the substrate, thereby restricting the overall service reliability and service life of the repaired component. Therefore, how to regulate and optimize the microstructure and properties of the repaired region through effective post-heat-treatment processes becomes a key scientific issue for improving the quality of laser additive repair.

To address this issue, this study pre-fabricated grooves on IN625 alloy substrates to simulate actual damage, and used laser directed energy deposition technology for in situ repair (Fig.2). By designing and implementing a post-repair solution treatment and aging treatment regime, and comprehensively using various characterization methods such as optical microscopy (OM), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), Vickers hardness testing, and room-temperature tensile testing, this study systematically compared and analyzed the microstructure evolution patterns and mechanical property responses of the repaired region and the substrate in both the as-deposited and heat-treated states.

The results showed that the as-deposited repaired region exhibited a typical columnar dendrite morphology epitaxially grown along the heat flow direction, and Laves phases enriched in Nb and Mo, as well as a small amount of MC-type carbides, were clearly observed between dendrites. After the optimized solution and aging treatment, the microstructure of the repaired region underwent a fundamental transformation. The original columnar dendrite structure was broken and transformed into a uniform and fine equiaxed grain structure. Most of the harmful Laves and δ phases were fully dissolved, while a large amount of nanoscale γ″ strengthening phase was precipitated within the matrix (Fig.4～Fig.7). This microstructure optimization behavior significantly improved the mechanical properties of the material. Tensile test data indicated that the ultimate tensile strength of the heat-treated repaired region was improved, while the elongation markedly increased from 16.40% in the as-deposited state to 39.17% (Fig.9～Fig.10). Microhardness test results showed that the hardness distribution in the heat-treated repaired region became more uniform, and the hardness gradient between it and the substrate was significantly reduced (Fig.8). The mechanical property transition at the interface became smoother, which helped alleviate stress concentration and improve interfacial bonding stability.

In summary, this study confirms that implementing post-repair solution and aging heat treatment is an effective post-treatment strategy for Inconel 625 alloy repaired by laser directed energy deposition. This process can not only effectively eliminate the unfavorable microstructures caused by rapid solidification, such as dissolving brittle Laves phases and transforming columnar crystals into equiaxed crystals, but also actively introduce γ″ strengthening phases to achieve precipitation strengthening. Ultimately, while improving the strength of the repaired region, this treatment enables a significant improvement in its plasticity and toughness, and markedly reduces the property gradient between the repaired region and the substrate. This study provides a clear direction for process optimization and a theoretical basis for laser additive repair of IN625 alloy components, which is of great value for promoting the reliable application of this technology in the remanufacturing field of critical components in high-end equipment.