Study quantifies how cellular structures enhance strength in 3D-printed metals

A research group conducted quantitative individual analysis on the contribution to the strength of the micrometer-scale crystallographic lamellar structures and nanometer-sized cellular structures that are formed spontaneously, hierarchically, and specifically by metal 3D printing technology, and revealed that the cellular structure (cell-specific interfaces) is a factor that brings about extremely significant strengthening.

Their paper, “Remarkable Strengthening Effects of Cells in Laser Powder Bed Fusion-Processed Inconel 718,” was published in Materials Research Letters.

In order to clarify the contributions individually, the research group, including Taichi Kikukawa (Master’s Course), Specially Appointed Professor Takuya Ishimoto, and Professor Takayoshi Nakano of the Graduate School of Engineering at the University of Osaka, established a method to independently eliminate the cellular structure by heat treatment and the lamellar structure by designing a unique scanning strategy.

As a result, while the presence of the lamellar structure increased the strength by a few percent, the cellular structure increased the strength by 40% (1.4 times), revealing the extremely high strengthening effect of the cellular structure.

The strengthening effect of the cellular structure discovered in this study, combined with the strengthening mechanism and strength anisotropy that have been clarified so far in 3D printing materials, as well as the shape-based functionality that 3D printing excels at, is expected to break through the limitations of conventional mechanical functions and greatly expand the scope of artificially customized mechanical function control.

Metal 3D printing technology has been attracting attention as a technology that can create products with flexible shapes based on 3D data. On the other hand, it has been reported that various alloys created by the LPBF method have higher strength than alloys created by conventional methods such as casting, and there is growing interest worldwide in their material properties.

Against this background, there is a demand to clarify and control the properties and characteristics of strengthening factors to increase the strength of alloys and flexible designing of strength.

However, since multiple unique structures coexist at various scales inside objects created by metal 3D printing, it is difficult to isolate the strengthening caused by each unique structure, and quantitative identification of strengthening factors has not been realized.

In 2021, the research group made full use of the artificial structure control technology in metal 3D printing that they have developed, and succeeded in obtaining the world’s first molded object consisting of a micrometer-scale crystallographic lamellar structure and a nanometer-sized cellular structure in IN718.

They then came up with the idea that if the presence/absence of the crystallographic lamellar structure and the cellular structure were independently controlled, it would be possible to isolate their contributions to strengthening.

The research group attempted to eliminate both the crystallographic lamellar structure and the cellular structure. The crystallographic lamellar structure was eliminated by designing a new scanning strategy. This structure has two plate-like areas with different crystal orientations overlapping in one direction.

The thicker one with facing the build direction is called the main layer, and the thinner one with facing the build direction is called the sub-layer. The two are arranged with an interval of about 100 μm. This lamellar structure is formed when crystal grains that grow in the direction are born from the center bottom of the melt pool, and the sub-layer is inserted between the main layers.

At the center bottom of the melt pool, a heat flow occurs vertically downwards, which makes it easy for to grow in the build direction, which is the antiparallel direction (i.e., a sub-layer is easy to form).

In the newly designed scanning strategy, the researchers succeeded in inhibiting this growth and erasing the -oriented sub-layer by shifting the pitch between layers by half an interval. As a result, they obtained a single crystal with the same crystal orientation as the main layer ( oriented in the build direction).

The cellular structure is a network-like solidification structure formed as a result of ultra-rapid solidification in the LPBF method (the cooling rate during solidification reaches 107 K/s), and is characterized by segregation (uneven composition) and accumulation of dislocations.

Therefore, the cellular structure was eliminated by finding precise heat treatment conditions that eliminate the uneven composition through diffusion, promote the rearrangement and annihilation of dislocations, and suppress grain growth and recrystallization that cause changes in the crystal texture.

As a result, four types of samples were prepared by combining the presence or absence of a lamellar structure and the presence or absence of a cellular structure.

Compression tests were performed and it was revealed that the lamellar structure contributed to an increase in strength (yield strength: the stress at which the material begins to undergo permanent macroscopic deformation) of several percent, while the cellular structure contributed to an increase of as much as 40%.

This makes it clear that the cellular structure is not simply a solidification structure that represents the quenched state, but a strengthening factor that brings about a dramatic improvement in strength which is unique to LPBF materials.

In the future, it is expected that the cellular structure will be actively utilized in the design of mechanical functions and higher strengthening of alloys.

Furthermore, the effects of the lamellar structure can be made apparent by changing the direction of the load, making it an important strengthening factor that can be introduced using the LPBF method.

Taking advantage of the characteristics of 3D printing, such as high flexibility of shape and fewer parts, 3D printing is being considered as a replacement for various products.

On the other hand, the knowledge gained in this study on strengthening by cellular structures means that replacement with LPBF has the potential to bring about not only a change in manufacturing method, but also a dramatic improvement in the mechanical function of the product and its ultra-lightweighting.

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

In other words, since this result can be applied to various alloy materials that make up social infrastructure products, the researchers expect the ripple effects of this result to extend to an extremely wide range of industrial fields.

Furthermore, since the elucidation of the functions and artificial control of 3D printed metal materials requires the construction of new theories such as the elucidation of the formation mechanism of specific structures, the strengthening mechanism, and the control method of specific structures, this result is also of great academic significance.