Collaborative Research: Fabrication of Columnar-Grained Microstructures and Single Crystals via Directional Recrystallization of Additively-Manufactured Nickel-based Alloys

Dartmouth College, Hanover NH

Investigators

Abstract

Ni-based superalloys were developed in the 1940s and have been continuously improved ever since, culminating in the current single-crystal jet turbine blades made by directional solidification. However, the cost of such turbine blades exceeds $15,000 and replacement costs when a jet engine is overhauled can be hundreds of thousands to millions of dollars. Thus, a cost-effective method of producing Ni-based superalloy turbine blades is imperative. This award supports research into methods to produce columnar-grained structures or single crystals of nickel alloys by directional recrystallization (DR), a solid-state process, of additively manufactured (AM) material, in which the complex structures are built layer-by-layer by powder fusion. The goal is to investigate if production of a single crystal Ni-based superalloy turbine blade is feasible via the integrated route of AM and DR, and to understand the physics underlying such a processing route. The technology, although focused on Ni alloys, is nonspecific and could be used for other materials, and easily scaled up. The project will train undergraduates and Ph.D. students. In addition, undergraduates from Smith College and from the University of Massachusetts will undertake an annual workshop on AM technology. To gain a fundamental understanding of DR processing of AM materials, the project will use several printing strategies to make different AM microstructures using laser powder bed fusion and laser directed energy deposition of Ni-Al alloys, and the microstructures before and after DR processing will be characterized. The project will build a new DR system specifically for this purpose. Several scientific questions will be addressed: (1) Are carbides or similar insoluble particles necessary to prevent equiaxed grain growth and thus enable columnar-grained structures to grow? (2) Are the columnar structures produced by primary recrystallization or secondary recrystallization? (3) Can this technology grow columnar grains or single crystals at high hot-zone velocities? Previous work has indicated that the upper hot zone velocity to propagate columnar grains is substantially higher than that required to nucleate them. (4) Can this technology use a spiral growth selector built into an AM sample to provide grain selection during DR to produce single crystals? (5) Can single crystals or columnar-grained structures with complex geometries, such as hollow components used for air-cooled Ni-based superalloy turbine blades, be produced by the integrated approach of AM and DR? If successful, this project can generate new understanding in the manufacturing of high-performance alloys, which will advance aerospace industries and alloy manufacturing. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.