Microstructure and grain growth direction of SRR99 single-crystal superalloy by selective laser melting

Abstract

Nickel-based Single Crystal (SX) superalloys have an excellent creep ductility and thermal fatigue resistance at elevated temperatures. But SX structure is difficult to be manufactured under the demand of increasing operating temperature and complex shapes. In this work, the feasibility of manufacturing Nickel-based SX-superalloys by selective laser melting (SLM) was investigated theoretically and experimentally. Firstly, the solidification conditions including temperature field, thermal gradient and solidification speed of the SLMed multi-track samples were calculated by an established finite element model to judge the theoretical feasibility based on the columnar to equiaxed transition (CET). Then, the single-track, triple-track and cuboid samples on a single-crystal SRR99 substrate were deposited to judge the experimental feasibility and explore their grain growth mechanism during SLM. The results showed that CET can be avoided during the whole process of SLM. The unidirectional SRR99 SX structure with a height of 2 mm can be successfully obtained by SLM. On horizontal section, the microstructures can be divided into [001] orientation zone, [010] orientation zone and overlapping zone with a typical secondary dendrite structure. On vertical section, the columnar dendritic structure along the dominant thermal gradient direction is present. The primary dendrite spacing of the SLMed samples (∼1–2 μm) is more than two orders of magnitude lower than that of the cast one (∼320 μm). So, SLM technology possesses the capacity to manufacture SX superalloy. When the deposited height reaches 6 mm, the misorientation angle is about 25° and a crack initiates. Finally, the grain growth mechanism of the SLMed SRR99 single-crystal superalloy was proposed based on the local thermal gradient, the existing well-aligned crystallographic orientation and the preferential grain growth direction.