Characterization and Simulation of the Microstructure of Additively Manufactured Hastelloy X Parts with Columnar Grain Structure

Abstract

One of the well-known AM techniques is laser powder-bed fusion (LPBF), which enables engineers to benefit from the concept of “design freedom” while designing complex geometries compared to other conventional methods. This versatile powder-bed process has been used to manufacture parts with adjustable parameters such as laser power, scanning velocity, and layer thickness to be optimized to print defect-free parts. To achieve preferable part quality with no porosity, it is necessary to have consolidated layers made of single lines. Thus, an in-depth understanding of the effect of key process parameters on fast solidification is required to produce condensed layers leading to high-quality parts with low porosity and high strength. Poorly selected parameters result in creating materials with an undesirable mechanical property. In as-manufactured parts, mechanical properties are mostly governed by the formed microstructure. For example, varying grain size, phase formation, crystallographic texture, etc. can all result in changes to the mechanical responses of the material. All AM processes are extremely expensive in comparison with the subtractive method. However, having less waste material during LPBF, as well as the opportunity to reduce the weight through topology optimization of the target geometry, make an opportunity to compete with traditional methods for expensive materials such as Ni-base superalloys, titanium alloys, and high strength steels. Among these materials, Ni-base alloys offer homologues working temperature resulting in high-temperature strength and low creep rate. Besides, alloying elements such as Cr provide good corrosion resistance even in high temperatures. One of the most important applications of Ni-base superalloy can be observed in the hot section of aircraft engines, where more than fifty percent of this specific section is made of Ni-base superalloys. In this thesis, the microstructure formation with respect to the mechanical response of the fabricated material during rapid solidification of the LPBF process is studied. Having an estimated microstructure prior to the fabrication step is helpful in terms of saving time, money, and material compared to the tedious trial and error or statistical approach to find the required process parameters. Microstructure estimation can be achievable by utilizing a proper phase-field model which is capable of mimicking the epitaxial grain growth during the LPBF process. A comprehensive review has been done under the topic of microstructural analysis of Ni-base superalloys from the available literature. The effect of laser power and scanning velocity on the melt pool formation has been studied independently. In addition, the effectiveness of the combined parameter of Linear Energy Density (LED) on the melt pool geometry and microstructure of Hastelloy X single tracks has been investigated. Also, a possible nucleation mechanism is discovered during the single-track study, and its significant effect on the microstructure of the melt pool is investigated. With the knowledge gained during the single track study, a phase-field model is utilized to simulate the as-built grain structure of the Hastelloy X part. The model focuses on epitaxial grain growth and nucleation, which is observed from the previous chapter. Results reveal that the model is powerful in terms of predicting grains dimension, grains morphology, and columnar structure formation. Based on the mentioned study, a set of process parameters (including laser power, scanning velocity, layer thickness, and hatch distances) was identified to fabricate almost defect-free parts. In the next step, different solidification patterns with the same mentioned process parameters have been utilized to control the as-built microstructure of LPBF-made parts. The mechanical response was observed to be significantly different from different microstructures in texture and morphology. The gained knowledge can be used to manufacture a functionally graded part produced by the LPBF process in the future. Lastly, a heat-treatment study is performed on LPBFed samples. It is intended to rebuild most of the as-built microstructure to restore the original soft properties of Hastelloy X. Results show that static recrystallization is possible right after the manufacturing process (LPBF). The effect of the static recrystallization on the grain structure and mechanical response is investigated. Results reveal that the recrystallization process can alleviate the anisotropy in mechanical properties.