While the adoption and development of metal 3D printing continues to grow, so too does research into different types of metals that can be 3D printed, including steel. The aerospace and automotive industries in particular would have many applications for 3D printing components with high-performance steel. Now, for the first time, a research team at the University of Kassel in Germany has used additive manufacturing to process a steel alloy with extremely high damage tolerance, which will help in promoting safety and reliability of 3D printed metal parts.

These increased qualities will open up many fields of application, according to materials scientist Prof. Dr.-Ing. Thomas Niendorf, who has been a professor of metallic materials at the university since 2015. This project by his Emmy Noether research group was funded by the German Research Foundation (DFG).

“Applications in the aerospace and automotive industries, current drivers behind the technological development of 3D printing, will benefit considerably,” said Professor Niendorf in a translated quote. “3D metal printing will open up new areas on this basis.”

This isn’t the first time steel alloys have been used for 3D printing, but Professor Niendorf’s group combined the electron beam melting (EBM) process with a new starting material to create higher quality steel products. Together with fellow researchers from the TU Bergakademie Freiberg, his research group developed both the alloy and the process, and published their results in a paper, titled “Design of novel materials for additive manufacturing – Isotropic microstructure and high defect tolerance,” in the journal Scientific Reports. In addition to Professor Niendorf, co-authors include J. Günther, F. Brenn, M. Droste, M. Wendler, O. Volkova, and H. Biermann.

The abstract reads, “Electron Beam Melting (EBM) is a powder-bed additive manufacturing technology enabling the production of complex metallic parts with generally good mechanical properties. However, the performance of powder-bed based additively manufactured materials is governed by multiple factors that are difficult to control. Alloys that solidify in cubic crystal structures are usually affected by strong anisotropy due to the formation of columnar grains of preferred orientation. Moreover, processing induced defects and porosity detrimentally influence static and cyclic mechanical properties. The current study presents results on processing of a metastable austenitic CrMnNi steel by EBM. Due to multiple phase transformations induced by intrinsic heat-treatment in the layer-wise EBM process the material develops a fine-grained microstructure almost without a preferred crystallographic grain orientation. The deformation-induced phase transformation yields high damage tolerance and, thus, excellent mechanical properties less sensitive to process-induced inhomogeneities. Various scan strategies were applied to evaluate the width of an appropriate process window in terms of microstructure evolution, porosity and change of chemical composition.”

Prof. Dr. Thomas Niendorf in front of a 3D metal printer. [Image: Andreas Fischer]

The study addresses the challenges surrounding microstructural control and presents the team’s first results on EBM processing with their new material, which they developed using a TRIP (TRansformation Induced Plasticity) steel alloy. This type of alloy, thanks to its special deformation mechanisms, holds up very well, and the heat from the EBM process helps to avoid any unpredictable material properties, resulting in a significantly better inner material structure that protects against possible damage.

Arcam A2X

According to the paper, “In the present study it is demonstrated that the EBM processed CrMnNi steel exhibits excellent tensile properties even when large process-induced defects are present. Furthermore, it is revealed that the alloy undergoes phase transformation upon process inherent cooling and heating, respectively. This finally results in a fine-grained microstructure without pronounced texture.”

The specimens were 3D printed on an Arcam A2X, and the EBM process, coupled with the team’s new material, works well for small, complex components. While we know that titanium alloys are popular in the worldwide 3D metal printing market, the steel components that the researchers 3D printed don’t require any reworking, which makes them less expensive to manufacture.

Professor Niendorf, whose main area of research centers around the search for new processes and materials well-suited for 3D printing, believes that there is, according to the university, “tremendous potential for the German economy in 3D metal printing.”

“German manufacturers are leaders in the production of metal powders and the construction of 3D laser melting equipment,” said Professor Niendorf.

SEM micrographs of fracture surfaces: (a) as-built specimen after tensile testing; (b,c and d) showing magnified details of defects marked with white dashed rectangles in (a); (e) heat-treated specimen after tensile testing; (f) and (g) showing magnified views of defects marked with white dashed rectangles in (e).

Discuss this and other 3D printing topics at 3DPrintBoard.com or share your thoughts below. 

[Source/Images: University of Kassel]

 

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