Session: 16-01-04: Mechanical Performance III

Paper Number: 163814

Characterization and Experimentation of Additive Manufactured Layered Thin-Walled Elements for High Temperature Alloys. 

During the last decade, additive manufacturing was proved to be an adequate manufacturing method to produce bulk metallic parts. The unique resulting microstructure and the anisotropic mechanical behavior were examined thoroughly by the research community.  The current state of the art is investigating the performance of the cellular metallic parts produced by additive manufacturing. The current practice in cellular structure design optimization is to change the geometric parameters keeping the material parameters the same. Notable gaps between the finite element model (FEM) predictions and the experimental results were observed. This research aims to study the effect of the printed element dimensions on the resulting mechanical performance, and explain the resulting discrepancies between the finite element models and the experimental results.

 While transitioning the printed net shape from bulk metallic parts to cellular structures, the extrinsic net shape dimension is usually reduced to a sub-millimeter order. Along with this, to end up with a minimal surface roughness, a common practice in laser powder bed fusion additive manufacturing (LPBF) is to trace the outer boundary of the current layer using higher power and lower speed. This results in different power density input to the border compared to the interior of the part. As the produced part dimension reduces to few orders of the melt pool width (~100 µm). It is important to study the effect of this inhomogeneous energy input on the resulting microstructure and mechanical performance of the resulting parts as compared the previously studied elements of larger extrinsic dimensions.

The material studied in this research is Inconel 718 (IN718). Two different categories of elements are examined; sheets, in which only the element thickness is in the order of sub-millimeter, and struts, in which the element thickness and width are in the order of submillimeter. Miniature tensile specimens with the gauge length representing these two categories were printed using LPBF method and then stress relieved. For each category two different orientations are studied; parallel and perpendicular to the build direction.  The effect of the sample extrinsic dimensions on the resulting microstructure is examined by EBSD mapping. Firstly, the grain size is found to increase as the sample dimension decreases to 0.4 mm. Secondly, a gradient microstructure is observed with finer grains on the boundaries compared to the interior areas.  Thirdly, anisotropy in the grain structure is observed.  While the planes exposing multiple layers (planes with normal perpendicular to the layering direction) exposed columnar grains spanning multiple layers (both epitaxially and with slight tilt with length ~ 200 µm), the planes with a normal parallel to the built grains showed a periodic pattern in which the grains elongated to the sides of the melt pools (length ~ 80 µm), with finer grains in the melt pools overlapping areas.

 The effect of the previously mentioned change in the grain size as the sample extrinsic dimension changes, weather this change is along two major axes as in the sheets category or one major axis as in the struts category, on the tensile mechanical properties is studied. Furthermore, the effect of the sample orientation and testing temperature (up to 650 C^o) on the mechanical performance and failure mechanisms will be studied using the SEM in situ miniature tensile tester.

Presenting Author: George Z Voyiadjis Louisiana State University

Presenting Author Biography: George Z. Voyiadjis is the Boyd Professor at the Louisiana State University, in the Department of Civil and Environmental Engineering. This is the highest professorial rank awarded by the Louisiana State University System. He is also the holder of the Freeport-MacMoRan Endowed Chair in Engineering. He joined the faculty of Louisiana State University in 1980. He is currently the Chair of the Department of Civil and Environmental Engineering. He holds this position since February of 2001. He also served from 1992 to 1994 as the Acting Associate Dean of the Graduate School. He currently also serves since 2012 as the Director of the Louisiana State University Center for GeoInformatics (LSU C4G).
http://c4g.lsu.edu//
Elected to the European Academy of Sciences and Arts, 2021; Elected to the Academia Europaea Physics & Engineering Sciences, 2020; Elected to the European Academy of Sciences, 2019; Elected to the Korean Academy of Sci. and Eng., 2016; Elected to the Polish Academy of Sciences, 2013; He is the recipient of the 2008 Nathan M. Newmark Medal of the American Society of Civil Engineers ASCE-EMI&SEI and the 2012 Khan International Medal for outstanding life-long Contribution to the field of Plasticity. He was also the recipient of the of the ICDM2 Lifetime Achievement Medal for his significant contribution to Continuum Damage Mechanics, presented to him during the Second International Conference on Damage Mechanics (ICDM2), Troyes, France July 8-11, 2015. This is sponsored by the International Journal of Damage Mechanics and is held every three years. In 2022 he was the recipient of the American Society of Mechanical Engineers, ASME, Nadai Medal, of the Materials Division. He received the 2023 Blaise Pascal Medal for Engineering from the European Academy of Sciences. He recently received the American Society of Civil Engineers’ Engineering Mechanics Institute the 2024 Theodore von Kármán Medal. This Medal in particular is widely considered as one of the highest honors in all areas of engineering mechanics.

Authors:

George Z Voyiadjis Louisiana State University
Reem Abo Znemah Louisiana State University
Paul Wood The University of Derby