The research objective of this Grant Opportunity for Academic Liaison with Industry (GOALI) project is to apply and integrate theories in mechanical engineering and materials science to the Freeze-form Extrusion Fabrication process for fabrication of graded composite structures. Missouri University of Science and Technology in partnership with Boeing will research the science and technology of using this novel additive manufacturing process to fabricate graded composite structures having controllable gradients between zirconium carbide (an ultra-high temperature ceramic) and tungsten (a refractory metal). Multiple colloidal pastes will be made from combinations of different raw materials (zirconium oxide, carbon, tungsten, carbon and tungsten carbide), and they will be extruded by multiple extruders in varying ratios, simultaneously and continuously, to produce components containing a graded zirconium carbide/tungsten architecture as designed after reaction sintering. The research will focus on understanding rheology and surface chemistry for development of high solids loading and well dispersed colloidal pastes; modeling, analysis and control of the freeform fabrication process; and evaluation of the dimensional accuracy, mechanical properties and microstructure of the fabricated parts after post-processing.
This project is expected to generate scientific and technical knowledge on the Freeze-form Extrusion Fabrication process for fabricating graded composite structures. This knowledge will be useful to other extrusion-based freeform fabrication processes. The developed technology will enable tool-less fabrication of 3D components with functionally graded materials, thereby increasing material flexibility and reducing manufacturing costs and times for low-volume components that have complex geometry and demand high performance.
The research objective of this project was to apply and integrate theories in mechanical engineering and materials sciences for the development of a novel additive manufacturing process called the Freeze-form Extrusion Fabrication (FEF), to fabricate three-dimensional parts having controllable gradients between an ultra-high temperature ceramic and a refractory metal. The research focused on scientific understanding of paste rheology in relation to surface chemistry, modeling and analysis of paste extrusion, feedback control of past extrusion process, and machine design for fabricating 3D parts with functionally graded materials using the FEF process. The research was carried out by the Missouri University of Science and Technology in collaboration with The Boeing Company. Conducting this project involved 3 professors, 6 graduate students, 5 undergraduate students, 3 post-docs, and 1 technician from Missouri S&T, and 3 research engineers from Boeing. This project has provided valuable experiences to all of the team personnel in terms of applying and integrating theories in different disciplines to the development of an advanced additive manufacturing process. This project contributes to research and education resources through journal publications, conference presentations, and patent filing. We have already published 2 papers in professional journals and 4 papers in conference proceedings. Additionally, we currently have 1 journal paper under review, 3 journal papers in preparation, and 1 patent to be filed in December 2012. The main research activities and findings are summarized below: Three types of water soluble binders, i.e., PVA, aquazol, and methylcell, were examined for their binder effectiveness in aqueous pastes. Methylcell was found to be the most efficient binder among the three tested, for transforming the rheological behavior of a ceramic paste into that of a pseudoplastic with a high yield stress. A triple-extruder FEF system was designed and developed, with a prototype physical system manufactured for experimental verification of the developed analytical models, control algorithms, and numerical simulations. The FEF system development included a triple-extruder mechanism design, extrusion modeling and control, extrusion planning for desired composition gradients, and software coding for control and system integration. The paste extrusion process was modeled by characterizing the paste viscosity using a modified Herschel-Bulkley model. An analytical model representing the relationship between plunger velocity and extrusion force in the paste extrusion process was developed using the modified Herschel–Bulkley model and the Navier–Stokes equation. Based on the analytical model, a constitutive law was established for the process of extruding aqueous-based ceramic pastes. A series of experiments and simulations were conducted to validate the analytical model, and good agreement between the simulation and experimental results was obtained. An intelligent control methodology was developed utilizing a hybrid extrusion force-plunger velocity controller. It includes a plunger velocity controller used to ensure steady extrusion flow rate and an extrusion force controller used to regulate the start and stop of extrusion. The two controllers are coupled so as to provide the capabilities of extrusion on demand, material composition control, and air bubble release compensation. The functionality of the developed hybrid controller was demonstrated in fabricating monolithic and functionally graded parts from multiple aqueous pastes using the FEF process. The effectiveness of the developed multi-extruder FEF system was demonstrated first by fabricating limestone (CaCO3) parts with graded colors and then by fabricating parts with graded compositions between alumina (Al2O3) and zirconia (ZrO2). After the fabricated part was post-processed, energy dispersive spectroscopy (EDS) measurements were taken on the sintered part to examine the compositional changes across the graded specimen in comparison with a control set of specimens with known Al2O3-ZrO2 compositions. The FEF process was investigated to build FGM parts grading between ZrC (a ultra-high temperature ceramic) and W (a refractory metal). Aqueous-based colloidal suspensions of ZrC and W were developed for use in the FEF process to fabricate test bars graded from 100%ZrC to 50%W-50%ZrC (percent in volume). After FEF processing, the W and ZrC materials were co-sintered at 2300°C and then characterized to determine the resulting density, flexure strength, and microstructure of the sintered specimen. The FEF fabricated test bars achieved relative density in the range of 47-70% and flexural strength in the range of 25-70 MPa.
