Powder Materials Processing and Fundamental Understanding: Sintering
Sponsored by: TMS Materials Processing and Manufacturing Division, TMS: Powder Materials Committee
Program Organizers: Kathy Lu, Virginia Polytechnic Institute and State University; Eugene Olevsky, San Diego State University; Hang Yu, Virginia Polytechnic Institute and State University; Ruigang Wang, The University of Alabama; Isabella Van Rooyen, Pacific Northwest National Laboratory

Wednesday 8:30 AM
March 2, 2022
Room: 263C
Location: Anaheim Convention Center

Session Chair: Eugene Olevsky, San Diego State University; Hang Yu, Virginia Polytechnic Institute and State University

8:30 AM  Invited
Gravitation Effects on Sintering: Elisa Torresani1; Randall German1; Eugene Olvesky1; 1San Diego State University
    Powder components subjected to pressure-less sintering ideally homogeneously shrink and retain their original shapes. However, real-world materials processing is influenced by many factors (e.g. temperature non-uniformity, external friction forces, and gravity) which during sintering produce inhomogeneous densification and shape distortions in the final component. With the increasing interest in space exploration and planet colonization, the research and advancement in producing components and repairing in space conditions gain fundamental importance. During sintering, gravity imposes a non-uniform stress which, through its influence on pore buoyancy, grain compression, and substrate friction, affects the sintering densification, microstructure, properties, and dimensional uniformity. Ground-based and extraterrestrial sintering experiments on the liquid-phase sintering of tungsten heavy alloys are conducted, allowing to obtain densification and distortion data useful for extracting constitutive parameters needed to map the sintering response with and without gravity. It is shown that such models can enable predictions relevant to space-based repair and additive manufacturing.

9:00 AM  
Gravity-affected Sintering of 3D-printed Powder Components: Eugene Olevsky1; Elisa Torresani1; Randall German1; 1San Diego State University
    Present technologies of additive manufacturing (such as binder-jetting, stereolithography, robocasting, etc.) of complex-shape powder-components necessitate fine-tuning of sintering as applied to porous 3D-printing products. The densification of complex shapes requires control of the gravity-related phenomena to ensure a nearly full and distortion-free densification. The present study addresses these issues through the involvement of comprehensive finite element simulations, the determination of the additively manufactured powder specimens’ sintering behavior, and the experimental validation of the developed models describing sintering of 3D-printed objects. The presentation describes the application of a numerical approach based on continuum mechanics-based modeling of the gravity-induced distortions during sintering of 3D-printed powder components. The validation of the model is conducted through the comparison with the experimental results obtained for the sintering of the beam-shape components printed using ceramic stereolithography technology. A semi-analytical criterion, which can be used for sintered 3D-printed parts’ design recommendations, is derived.

9:20 AM  
Powder Metallurgy Fabrication of ZrHx Moderator and U-ZrHx Moderated Nuclear Fuel: Caitlin Taylor1; Erik Luther1; Adrian Wagner2; Thomas Nizolek1; Aditya Shivprasad1; Tarik Saleh1; 1Los Alamos National Laboratory; 2Idaho National Laboratory
    Metal hydrides have long been considered for neutron moderator applications in nuclear energy systems due to their high temperature stability compared to water, for example. Zirconium hydride (ZrHx) fabrication typically involves exposing a machined Zr component to hydrogen at elevated temperature. The volume expansion between metallic Zr and the hydride phase is approximately 16%, often resulting in cracking or large residual stress gradients. Some reactor concepts utilize uranium-zirconium hydride (U-ZrHx) moderated nuclear fuel. Fabrication of U-ZrHx traditionally follows a direct-hydriding route similar to that of ZrHx, but starting with U-Zr alloy. A powder metallurgy method for yttrium hydride (YHx) fabrication was recently developed at LANL by Shivprasad et al. In this work, we build on previous work to develop a method for fabricating ZrHx and U-ZrHx by powder metallurgy. The fabrication route and resulting microstructures will be discussed and compared to direct-hydrided material. LA-UR-21-26357.

9:40 AM  Invited
Sintering Based Production of Complex Shapes by the Coupling of Additive Manufacturing and Spark Plasma Sintering: Charles Maniere1; Geuntak Lee2; Elisa Torresani3; Sylvain Marinel1; Lise Durand4; Claude Estournès5; Eugene A. Olevsky3; 1Normandie University, ENSICAEN, UNICAEN, CNRS, CRISMAT, 14000, Caen, France; 2Powder Technology Laboratory, San Diego State University, San Diego, USA ; 3Powder Technology Laboratory, San Diego State University, San Diego, USA; 4CEMES, CNRS UMR 8011, Université de Toulouse, 29 rue Jeanne Marvig, 31055 Toulouse, France; 5CIRIMAT, CNRS-INP-UPS, Université Toulouse 3 – Paul Sabatier 118 route de Narbonne, F-31062 Toulouse cedex 9, France
    Fusion based additive manufacturing suffers from complex thermal history involving microstructures with columnar grains, cracks, week properties. On the opposite, Spark Plasma Sintering (SPS) allows the sintering under high pressures and low sintering temperature allowing obtaining small grains, metastable phases. However, SPS configuration does not allow producing complex shapes inhering the latter advanced properties. To solve this problem, we have developed an interface based approach coupling additive manufacturing and SPS to allow the fabrication of complex shapes by co-sintering of multiple powder beds. This topic has allowed the production of 7 patents since 2015 and involves advanced additive technologies like the SLA, FDM, binder jetting and advanced sintering models from predicting the powder co-sintering and the shape evolution. This presentation will cover the new interface approach context, the role of the numerical tool to predict the complex sintering behavior, and the presentation of the produced complex shapes and their microstructures.

10:10 AM Break

10:30 AM  
Stable Temperature Regulation in Spark Plasma Sintering Simulations: Runjian Jiang1; Elisa Torresani1; Eugene Olevsky1; 1San Diego State University
    The controllable spark plasma sintering (SPS) process requires a stable temperature regulation to predict the microstructure evolution and densification behavior accurately. A loop feedback control algorithm called proportional integral derivative (PID) is a practical simulation method and has achieved many remarkable outcomes in regular SPS process. However, the efficiency-driven industrial manufacturing inspires the modern sintering process to be time-saving and reproducible. The modified PID controller is to be developed for the accurate temperature regulation in some demanding sintering processes. Here the variable-coefficient PID controller is introduced as a precision temperature regulation tool to substitute the inefficient “trial and error” way. The foreseeable prospect of fractional order PID control (FOPID) is also discussed as a powerful tool for the ultra-fast temperature regulation in instantaneous sintering process. These attempts are expected to open a new window for the development of advanced sintering technologies.

10:50 AM  
Transparent Al2O3 Fabricated by Energy Efficient Spark Plasma Sintering: Cheolwoo Park1; Elisa Torresani1; Eugene A Olevesky1; Chris Haines2; 1San Diego State University/College of Engineering; 2US Army DEVCOM
    In this study, transparent alumina is produced using three types of experiments that are compared and analyzed to check sample properties and energy savings. The output power significantly decreases from 46% to 22% when a typical energy efficient configuration using a BN coated graphite foils is used, but most samples have cracks. The presented new method results in the prevention of cracks, full densification, and uniform transparency in alumina. Furthermore, consumption is 17% lower in the energy efficient configuration compared to the traditional configuration. Therefore, this method has potential for fabricating transparent ceramic of various materials with uniform transparency.

11:10 AM  
Sintering Anisotropy of Binder Jetting 316L: Alberto Cabo Rios1; Eduard Hryha2; Eugene Olevsky1; Peter Harlin3; 1SDSU; 2Chalmers University of Technology; 3Sandvik Additive Manufacturing
    The main objective of this study was to characterize the multiaxial sintering behavior of 316L stainless steel components manufactured using Binder Jetting (BJ). This was done by investigating how the orientation of the part related to the BJ building directions influence the sintering behavior. 316L stainless steel cubic samples (10x10x10 mm3) were manufactured using binder jetting printer (56.2% green density), then debinded and pre-sintered before dilatometry tests (57.4% pre-sintered density). Dimensional variation during sintering occurring along the three orthogonal axes of the cubes was studied using dilatometry. Results showed anisotropic shrinkage behavior during sintering with larger shrinkages along the building direction. Also, the microstructure evolution (secondary phases, grain size and porosity) was studied using light optical microscope-based image analysis and electron backscatter diffraction (EBSD). Enhanced shrinkage behavior was observed after ~1340°C that was correlated to the presence of δ-ferrite above this temperature, causing enhanced solid-state diffusion in the δ-ferrite.

11:30 AM  
Surface Modification of Micro Powders Using Plasma-based Reactors for Sintering of Copper-Chromium Alloys: Santiago Vargas1; Diana Galeano1; Carlos Castano1; 1Virginia Commonwealth University
    Surface modification of powders to form core/shell metal structures can drive new materials and structures for advanced sintering-based manufacturing of alloys not commercially available yet. In this work, copper powder was used as a core and modified by adding chromium shells using a custom-made magnetron sputtering DC and high power impulse magnetron sputtering system. Subsequently, the powders were sintered using various classical and advanced sintering approaches to understand the main sintering mechanisms and interfacial reactions. The conventional approach of mixed copper and chromium powders was used to contrast the resulting properties and microstructures. Both magnetron sputtering variants had a strong influence on the copper-chromium interface and chromium film structure. The activation energy during the sintering processes was influenced due to the atomic structure and diffusion phenomena occurring at the core-shell interface. Extensive surface characterization was performed to assess materials characteristics of powders after modifications and components after sintering.