A team of Japanese-based researchers is studying the use of selective laser melting (SLM) technology for 3D printing on single crystal structures made of pure nickel (Ni).
Over the years, there has been a growing demand for Ni-based 3D printed superalloys, such as Inconel. These high-temperature metals are often characterized by their excellent mechanical properties, corrosion resistance and creep resistance, usually operating in the temperature range of 500 ° C +. As such, they are preferred in the aerospace sector, where they are used to manufacture jet engine components such as turbine blades.
Monocrystalline turbine blades are able to operate at much higher temperatures than their crystalline counterparts, but the additive production of Ni-based single crystal superalloys has proved difficult so far. Although they can be processed by electron beam melting (EBM), the use of laser-based technologies such as SLM usually requires the use of a single-crystal germ (building plate).
The team from Japan’s National Institute of Materials Science, Kyushu University and Osaka University now uses a flat-tipped laser profile to 3D print single-crystal Ni without the need for seeds at all.
Difficulties in controlling grain formation
When printing with SLM, carefully controlling grain boundaries, stress suppression, and part texture texture homogeneity is a difficult process. This may be due to the steep thermal gradient inherent in the technology, in which the top layer heats up quickly, the bottom layers are cooler and there is limited thermal conductivity between them. Repetitive thermal cycles lead to a high density of deformations and dislocations, which leads to internal dynamic crystallization and the formation of new grains.
For more precise microstructural control, especially for single crystal production, users need to either refine their scanning strategy or use single crystal seed from the outset.
According to researchers, Gaussian-based beam profiles are commonly used to control textures and microstructures in SLM, but there are currently no studies focused on flat-topped beam profiles.
3D printing of homogeneous single crystal nickel
In the present study, the Japan-based team uses the SLM Solutions SLM 280 HL system to produce pure Ni structures in Ar medium. First, polycrystalline Ni plates were used to study the behavior of single flat-tipped laser tracks, which allowed the team to evaluate the shape of subsequent melt pools.
The depths of the melt basin were measured and the observed geometries of the melt basin were used to print cylindrical specimens made of Ni on 304 polycrystalline stainless steel plates. Each specimen was ground and polished and the microstructures were examined using a scanning electron. microscopy (SEM) and electron back diffraction (EBSD).
By optimizing the planar melt, the team was able to print a homogeneous single-crystal structure made of pure Ni without relying on a single crystal plate. The researchers argue that the work provides new insights into how different parameters can be used to control the microstructure and related properties in the SLM process, especially to provide single crystal structures. In future studies, the findings will be applied with a wider range of metal alloys.
Further details of the study can be found in the article entitled “Production of pure crystals of pure nickel by selective laser melting with a flat-beam laser beam”.
The world of metal additives is rich and diverse in research. A team from Tsinghua University and the National University of Singapore recently studied the effect of fluid flow on the mechanical properties of metallic 3D printed parts. While factors such as temperature gradient and curing rate have been well studied, the effects of fluid flow in the melt of the 3D printed part have not yet been studied.
Elsewhere, a team of international researchers has recently delved deeper into the fundamental physics behind metallic 3D printing, all in the hopes of better understanding defects in printed parts. In critical industries, where the performance of the parts is paramount, the pores may be a constraint on the types of parts that can be printed. Therefore, there is a constant need to develop better techniques for detecting and mitigating defects in 3D printing.
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The presented image shows IPF maps and textures of 3D printed flat-topped copies. Image through Kyushu University.