Studies quantify how cell structures increase the strength of 3D printed metals

The research group revealed that it contributed to the strength of micrometer-scale crystallographic lamellar structures and that it naturally, hierarchically, to nanometer-sized cell structures formed especially by metal structures, revealing that cell structures (cell-specific interfaces) are factors that cause extremely important strength.

Their paper, “Salient reinforcement effect of cells in the fusion treatment incol 718 of laser powder beds,” was published in a material research letter.

To clarify contributions individually, the research group, including the specially appointed Ishikawa Sichuan (master’s degree), was appointed Professor Ishimoto, and Professor Nakano Shita, Graduate School of Engineering, Osaka University, established a method for eliminating cell structures independently through layered structures.

As a result, the presence of lamellar structures increased strength by several percent, whereas cell structures increased strength by 40% (1.4-fold), revealing a very high reinforcement effect of cell structures.

The cellular structure reinforcement effects discovered in this study, combined with the reinforcement mechanisms and strength anisotropy that have been previously clarified in 3D printing materials, as well as shape-based functionality that 3D printing excels, are expected to break through the limitations of traditional mechanical functions and greatly expand the scope of artificially customized mechanical functions.

Metal 3D printing technology is attracting attention as a technology that allows you to create flexible shapes based on 3D data. Meanwhile, various alloys created by the LPBF method have higher strength than alloys created by traditional methods such as casting, and it has been reported that their material properties are increasing worldwide.

Against this background, there is a need to clarify and control the properties and properties of reinforcement factors to enhance the flexible design of the alloy.

However, because multiple unique structures coexist at various scales within objects created by metal 3D printing, it is difficult to separate the reinforcements caused by each unique structure, and quantitative identification of reinforcement factors has not been realized.

In 2021, the research group made the most of its artificial structure control technology developed for metal 3D printing, and successfully acquired the world’s first molded object, consisting of IN718’s micrometer-scale crystallographic lamellar structure and nanometer-sized cell structure.

They then came up with the idea that it is possible to separate contributions to reinforcement if the presence/absence of crystallographic layered structures and cellular structures are independently controlled.

The research group sought to eliminate both crystallographic layered and cellular structures. Crystallographic lamellar structures were eliminated by designing new scanning strategies. This structure has two plate-like regions where different crystal orientations overlap in one direction.

Faces in the build direction<011>Thick ones are called the main layer and face the build direction.<001>Thin is called sublayers. The two are spaced at approximately 100 μm apart. This lamellar structure is<001>Grains growing in the direction are born from the center bottom of the melt pool and form when sublayers are inserted between the main layers.

At the center bottom of the melting pool, heat flow occurs vertically downward,<001> <001>can extend in the build direction.

With the newly designed scanning strategy, researchers have this<001>By inhibiting the growth of the layer and shifting the pitch between layers at half intervals,<001>I managed to erase the orientation sub-layer. As a result, they obtained a single crystal with the same crystal orientation as the main layer (directed in the build direction).<011>).

Cellular structures are network-like solidification structures formed as a result of the ultra-strong solidification of the LPBF method (cooling rates reaching 107 k/s during coagulation), characterized by separation (heterogeneous composition) and accumulation of dislocations.

Therefore, cellular structures were eliminated by finding precise heat treatment conditions that eliminate heterogeneous composition through diffusion, promoting dislocation and annihilation, and inhibiting particle growth and recrystallization that caused changes in the texture of the crystals.

As a result, four types of samples were prepared by combining the presence or absence of layered structures and the presence or absence of cell structures.

A compression test was conducted and it was revealed that the lamellar structure contributed to an increase in strength (yield strength: the stress that the material begins to undergo permanent macroscopic deformation), and that the cell structure contributed to an increase of 40%.

This reveals that cellular structures are not merely solidified structures that represent quenching conditions, but are reinforcing factors that result in dramatic improvements in strength inherent in LPBF materials.

In the future, it is expected that cell structures will be actively utilized for the design of mechanical functions and for higher strengthening of alloys.

Furthermore, changing the direction of the load allows us to clarify the effect of the lamellar structure, making it an important reinforcement factor that can be introduced using the LPBF method.

Taking advantage of the characteristics of 3D printing, 3D printing is considered an alternative to a variety of products, including high shape flexibility and fewer parts.

On the other hand, the knowledge gained in this study on cellular structure enhancement means that replacing LPBF can not only lead to changes in manufacturing methods, but also dramatically improve the mechanical function of the product and its ultra-light weight.

Cell structures appear in many alloy systems during the LPBF process based on concentration distribution during solidification.

In other words, since this result can be applied to the various alloying materials that make up social infrastructure products, researchers hope that the ripple effects of this result will extend to a very wide range of industrial sectors.

Furthermore, this result is also of great academic importance, as the function and artificial control of 3D printed metal materials requires the construction of new theories such as elucidation of the formation mechanism of a particular structure, reinforcement mechanisms, and methods for controlling a particular structure.