Highly Anisotropic Steel Processed by Selective Laser Melting
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For additive manufacturing of metals, selective laser melting can be employed. The microstructure evolution is directly influenced by processing parameters. Employing a high energy laser system, samples made from austenitic stainless steel were manufactured. The microstructure obtained is characterized by an extremely high degree of anisotropy featuring coarse elongated grains and a 〈001〉 texture alongside the build direction during processing. Eventually, the anisotropy of the microstructure drastically affects the monotonic properties of the current material.
KeywordsLaser System Austenitic Stainless Steel Additive Manufacturing 316L Stainless Steel Selective Laser Melting
Recently, techniques allowing for additive manufacturing of highly complex components have been gaining significant attention in both industry and academic research.[1, 2, 3] As no tools are required for processing, small to medium batches can be produced very efficiently. Polymers and metals can be processed depending on the technique employed; for processing of metals, wire-based techniques are available, but techniques employing a powder bed have the higher impact. Electron beam melting and selective laser melting (SLM®), both melting the powder locally accordingly to data provided by a model stemming from computer-aided design, are widely used nowadays.[1, 2, 3] From the academic point of view, the high degree of design freedom allowing for an extreme lightweight design and the aspect of microstructural design are very attractive.[4,5] The latter aspect is mainly influenced by process-related parameters such as scanning strategy and energy input. As has been shown by Thijs et al. for an aluminum alloy processed by selective laser melting in a very recent paper, the thermal gradient during cooling and the direction of heat flow are key parameters for microstructure evolution and design, respectively. Numerous metals and alloys have been processed by SLM®; aluminum and titanium alloys, nickel-based alloys, and stainless steels have been the subjects of recent work.[1, 2, 3, 4, 5, 6, 7, 8] Focusing on materials such as nickel-based alloys and austenitic steels, high-temperature applications are of interest. For such applications, a coarse-grained anisotropic microstructure is highly attractive. The current paper addresses this topic and introduces a highly anisotropic austenitic alloy 316L directly obtained from powder processed by SLM®. The conditions for obtaining such kind of microstructure are discussed in light of the processing parameters.
The material employed in the current study was face-centered cubic (fcc) 316L stainless steel. The initial powder with a mean particle size of 40 μm was supplied by SLM Solutions GmbH. For fabrication of cubical and tension specimens, a SLM®-280HL selective laser melting system in combination with MTT AutoFab software (Marcam Engineering GmbH) was used. The tensile specimens were built in the z-direction; thus, the loading axis was parallel to the built direction. Two Yttrium fiber lasers are employed in the current SLM® system, featuring maximum beam energies of 400 W and 1000 W, respectively. The layer thicknesses employed during processing differed according to the laser system used; 50-μm layers were selectively melted by the 400 W laser and up to 150 μm by the 1000 W laser. The samples used for mechanical testing were machined from cylindrical rods built with a diameter of 10 mm and a length of 65 mm, which then were machined to meet the required geometry featuring a gage section of 24 mm length at a diameter of 4 mm. The specimen geometry was based on the standard DIN 50125. A screw-driven testing rig was used for the tension tests, which were conducted in displacement control with a rate of 5 mm min−1. An optical extensometer was used to measure strains. For characterization of the microstructure of the different specimens, an X-ray diffraction system (XRD) and a scanning electron microscope equipped with an electron backscatter diffraction (EBSD) system were used. Macrotexture was characterized by XRD employing a Cu-Kα-source operated at 45 kV and 40 mA. Microtexture was characterized by EBSD at an acceleration voltage of 20 kV. For EBSD studies, the samples were electro-polished.
SLM® is a very promising technique for manufacturing of complex metal parts, but still suffers from process-induced imperfections, i.e., pores, undesired microstructures, and high residual stresses as well as relatively high processing times.[3,5,8] The latter aspect can be addressed by employing the 1000 W laser source. As this high energy laser source is capable of melting more than a common single layer of metal powder up to a total layer thickness of 200 μm, it allows for significant shortening of processing time and eventually processing cost. As will be shown in this paper, the high energy input by the 1000 W laser system additionally allows for a tailoring of the microstructure by inducing large columnar grains with a distinct orientation throughout the sample. The procedure for obtaining such microstructures using the current SLM® facility has been applied for patent.
In summary, the use of a 1000 W high energy laser system for selective laser melting of 316L stainless steel allows for the establishment of a coarse and strongly textured microstructure directly from the powder bed. Eventually, this unique microstructure strongly affects the mechanical behavior and consequently will be of high interest for future applications.
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