Key Findings
- Focusing on the micrometer-scale crystallographic lamellar structure and nanometer-scale cellular structure of higher-strength alloy objects created by metal 3D printing, the researchers succeeded in quantitatively extracting the respective effects on the strength of the objects. It was revealed that the cellular structure is the causal factor that results in large srengthening effect.
- While the crystallographic lamellar structure only increased strength by a few percent, the cellular structure increased strength by 40% (1.4 times) .
- The reason why alloy objects produced by laser powder bed fusion (LPBF) indicate extremely high strength had remained unclear due to the presence of multiple unique structures. However, it became possible to isolate the cell structure through heat treatment, and the lamellar structure through the design of a unique scanning strategy .
- It is expected that this will dramatically improve the mechanical functions of products and be applied to alloy materials in a wide range of industrial fields.
Outlines
A 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 conducted quantitative individual analysis on the contribution to 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.
In order to clarify the contributions individually, the research group 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 by 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.
Fig. 1
The structure of the LPBF-formed object obtained from IN718 alloy, in which a micrometer-scale crystallographic lamellar structure and a nanometer-scale cellular structure coexist. The green and red parts in (a) have different crystal orientations. The arrangement of the cellular structure is shown in (b), and it is found to be the structure causing the high strength of the LPBF material.
Research Background
Metal 3D printing technology has been attracting attention as a technology that can create products with flexible shape 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 (Fig. 1). 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.
Research Contents
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 <011> facing the build direction (green in Fig. 1) is called the main layer, and the thinner one with <001> facing the build direction (red in Fig. 1) 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 <001> 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 downward, which makes it easy for <001> to grow in the build direction, which is the antiparallel direction (i.e., sub-layer is easy to form). In the newly designed scanning strategy, we succeeded in inhibiting this <001> growth and erasing the <001>-oriented sub-layer by shifting the pitch between layers by half an interval (Fig. 2). As a result, we obtained a single crystal with the same crystal orientation as the main layer (<011> 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 (Fig. 3).
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 (Fig. 4, top table). 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% (Fig. 4).
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 strengthning of alloys.
Furthermore, the effects of the lamellar structure can be made apparent by changing the direction of the load, making it a very important strengthening factor that can be introduced using the LPBF method.
Fig. 2
The newly designed scanning strategy successfully eliminated the red sub-layer and eliminated the lamellar structure, resulting in a single crystal with a <011> orientation in the build direction, which has the same crystal orientation as the main layer of the lamellar structure.
Fig. 3
Segregation and dislocations were successfully eliminated by heat treatment. Meanwhile, the crystal orientation (lamella structure) remained unchanged. Combined with the results in
Fig. 2, this shows that the lamellar structure and cellular structure were successfully eliminated independently. The arrowheads in (c, f) are markers added to analyze the same location before and after heat treatment.
Fig. 4
Mechanical test results for four types of samples with and without lamellar structure and cellular structure. The presence of cellular structure showed a 40% increase in strength.
Social Impact of this research result
Taking advantage of the characteristics of 3D printing, such as high flexibility of shape and less 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, we 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.
Notes
The article, Remarkable Strengthening Effects of Cells in Laser Powder Bed Fusion-Processed Inconel 718 was published in "Materials Research Letters" at DOI: https://doi.org/10.1080/21663831.2025.2522801
This research was conducted as part of the "Custom Mechano-Functional Control by Formation of Specific Interfaces via Metal 3D Printing―Learning from Hierarchical Anisotropic Architecture in Bone"(Representative: Takayoshi Nakano) (Project number: JPMJCR2194) project of the Japan Science and Technology Agency (JST), Strategic Basic Research Program CREST Team-Based Research: "Elucidation of Nanoscale Dynamic Behavior and Mechanical Property Mechanisms for the Creation of Innovative Mechanical Functional Materials" (Research Director: Kohzo Ito).
