Abstract
This article overviews the scientific results of the microstructural features observed in 316 L stainless steel manufactured by the laser powder bed fusion (LPBF) method obtained by the authors, and discusses the results with respect to the recently published literature. Microscopic features of the LPBF microstructure, i.e., epitaxial nucleation, cellular structure, microsegregation, porosity, competitive colony growth, and solidification texture, were experimentally studied by scanning and transmission electron microscopy, diffraction methods, and atom probe tomography. The influence of laser power and laser scanning speed on the microstructure was discussed in the perspective of governing the microstructure by controlling the process parameters. It was shown that the three-dimensional (3D) zig-zag solidification texture observed in the LPBF 316 L was related to the laser scanning strategy. The thermal stability of the microstructure was investigated under isothermal annealing conditions. It was shown that the cells formed at solidification started to disappear at about 800 °C, and that this process leads to a substantial decrease in hardness. Colony boundaries, nevertheless, were quite stable, and no significant grain growth was observed after heat treatment at 1050 °C. The observed experimental results are discussed with respect to the fundamental knowledge of the solidification processes, and compared with the existing literature data.
Highlights
Additive manufacturing (AM) techniques are recognized as manufacturing processes with the high ability to generate parts from three-dimensional (3D) CAD models that are impossible to produce through conventional methods
The microstructure of the laser powder bed fusion (LPBF) material is formed under the conditions of high temperature gradients and solidification rates, far from the ones of conventional materials
Specimens for Transmission electron microscopy (TEM) were electro-chemically prepared with Struers TenuPol-5 equipment using equipment using the procedure and the electrolyte recommended by Struers (Ballerup, Denmark)
Summary
Additive manufacturing (AM) techniques are recognized as manufacturing processes with the high ability to generate parts from three-dimensional (3D) CAD models that are impossible to produce through conventional methods. This is especially important for the manufacturing of end-use products, as the qualities and properties of the materials have to be the same as for conventional alloys. The microstructure of the LPBF material is formed under the conditions of high temperature gradients and solidification rates, far from the ones of conventional materials. This results in the formation of a nonequilibrium microstructure with a unique set of properties. A post-treatment is recommended to convert the LPBF colonial microstructure and to obtain the typical structures and properties for the corresponding conventional materials. The discussed phenomena can be expanded to other single-phase LPBF alloys solidifying without phase transformations in solid state
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