A team of Japanese-based researchers is researching the use of selective laser melting (SLM) technology to print single-crystal structures made of pure nickel (Ni) in 3D.
The demand for 3D printed superalloys like Inconel has been growing over the years. These high-temperature metals have excellent mechanical properties, corrosion resistance, and wear resistance, which typically operate in the 500 ° C + temperature range. Therefore, they are common in the aerospace sector, where they are used to manufacture jet engine components such as turbine blades.
Single-crystal turbine blades are capable of operating at higher temperatures than their crystalline counterparts, but the additional fabrication of Ni-based single-crystal superalloys has been difficult so far. Although they can be processed by electron beam melting (EBM), the use of laser technologies such as SLM usually requires the use of a single crystal seed (building plate).
Now, a team from Japan’s National Institute of Materials Science, Kyushu University, and Osaka University has used a flat-bottomed laser profile to print a single-crystal Ni 3D Ni-seed.
Difficulties in controlling grain formation
When printing with SLM, it is a difficult process to fine-tune the grain boundaries, remove tension, and closely control the homogeneity of the fabric texture. This can be attributed to the high thermal gradient inherent in the technology, through which the top layer heats up quickly, the bottom layers are cooler and there is limited heat conductivity in between. Repeated thermal cycles result in high tensile and dislocation densities, resulting in dynamic internal crystallization and the formation of new grains.
For more accurate microstructure control, especially for single crystals, users need to establish a scanning strategy or use a single crystal seed from the beginning.
According to the researchers, Gaussian-based beam profiles are commonly used in SLM to control textures and microstructures, but there is currently no research focused on upper beam flat profiles.
Homogeneous single crystal nickel for 3D printing
In this study, the Japan-based team used an SLM Solutions SLM 280 HL system to produce hollow Ni structures in the Ar environment. First, polycrystalline Ni plates were used to study the behavior of single-track flat laser tracks, which allowed the team to calculate the shape of subsequent melting pools.
The depths of the melting pool were measured and the geometries of the melting pool observed were used to print cylindrical specimens made by Niz on 304 polycrystalline stainless steel plates. Each specimen was crushed and polished, and the microstructures were analyzed using microstructural electron microscopy (SEM) images and electron scattering diffraction (EBSD).
By optimizing the planetary melting pool, the team was successful in relying on a single-crystal homogeneous single-crystal structure made of pure Ni without relying on a single-glass plate. The researchers point out that the work provides new guidelines that can be used to control microstructures and their associated properties in the SLM process, especially to ensure single crystal structures. In future research, the findings will be implemented with a wider range of metal alloys.
Further details of the study can be found in the article “Manufacture of single crystal of pure nickel by selective laser melting with a high flat laser beam”.
The world of additional metal fabrication is rich and diverse in research. A team from Tsinghua University and the National University of Singapore has investigated the effects of fluid flow on the mechanical properties of 3D printed metal parts. Although factors such as temperature gradient and solidification rate have been well studied, the effects of fluid flow in the melting pool of a 3D printed part have not yet been investigated.
Elsewhere, a team of international researchers recently delved into the basic physics behind metal 3D printing in order to better understand the flaws of printed parts. In critical industries where parts performance is critical, pores can affect the limits of the types of parts that can be printed. Therefore, there is a constant need to develop better techniques for detecting and mitigating errors in 3D printing.
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The highlighted image shows the IPF maps and textures of the 3D printed specimens built in flat 3D. Image via Kyushu University.