Single-crystal structure formation in laser directed energy deposited Inconel 718 through process parameter optimization and substrate orientation tuning

Abstract

A transverse grain boundary perpendicular to the applied loading direction is normally considered a main reason for the deterioration of the mechanical properties of nickel-based superalloys at high temperatures. Therefore, reducing or even eliminating these types of grain boundaries can effectively delay failure and improve high-temperature mechanical properties. In this study, the feasibility of process parameter optimization and substrate rotation was explored for fabricating single-crystal Inconel 718 specimens during laser directed energy deposition (LDED). The study determined that optimizing the process parameters was dependent on the ability to enlarge the [001] region at the bottom of the melt pool as much as possible. Simultaneously, ensuring that the remelting depth exceeded the stray grain region height at the top of the melt pool was necessary. Therefore, that the stray grains can be fully erased during the subsequent deposition process. Accordingly, an Inconel 718 single-walled specimen (height: 20 mm) with a full single-crystal structure was successfully fabricated using LDED for the first time. However, this approach remains insufficient for fabricating a block with a single-crystal structure, as SGs appear readily in the overlapping regions. Substrate rotation was further considered, where ensuring that one side of the melt pool was in the [001]-grain region was critical. Although the other side of the melt pool featured SGs, they were eliminated through the following overlapping process, as the SG region in the current melt pool corresponded to the [001]-grain region in the next melt pool. Through these two approaches, a small-format single-crystal block with dimensions of 27 × 8.3 × 1.3 mm (length × width × height) was successfully fabricated. Because the Inconel 718 superalloy is not specifically designed for single-crystal structure generation, a large-format block with a single-crystal structure still cannot be fabricated using these approaches. Nevertheless, the findings remain insightful because they demonstrate a wider range of variable microstructures achievable with additive manufacturing processes than with traditional forming processes such as casting or forging and may provide more opportunities for improving the mechanical properties. In addition, the preparation of a small-format single-crystal structure has significant applications in repairing damaged components such as aeroengine blades.