CAREER: Understanding multiscale sintering kinetics and microstructural evolution in binder-based metal additive manufacturing

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NON-TECHNICAL SUMMARY: The process of using heat to turn powder into a porous solid without fully melting said powder is a manufacturing approach known as “solid-state sintering” (SSS). This technology forms the backbone of many important industrial processes for metal and ceramic materials such as powder metallurgy (P/M), metal injection molding (MIM), and field-assisted sintering (FAST). Overall, SSS platforms are advantageous because they do not require high temperatures to fully melt metal or ceramic powders which mean less energy is required to manufacture parts by sintering than by other standard processing methods. However, long-standing challenges still exist for sintering which include issues such as porosity, shrinkage and other changes in final shape that aren’t easily predicted due to events that occur at the scale of micro- and nanometers which aren’t well understood. This research project is revealing fundamental sintering mechanisms across multiple length scales by directly imaging powder surfaces and the internal structure of powders undergoing sintering in real-time using high powered x-ray experiments and high magnification electron microscopes. These state-of-the-art approaches are providing new knowledge on the way defects form and how internal structures change. Binder jet 3D printing (BJ3DP) is used as an example process since BJ3DP is of significant interest to the automotive industry but research findings contribute to new design strategies for a variety of SSS applications. This project also facilitates the training of undergraduate and graduate students in advanced manufacturing to address the current and growing skills gap in the U.S. This is being accomplished by exposing students to state-of-the-art characterization tools, engaging them in manufacturing research and involving them in professional development opportunities with industrial partners in the automotive industry. Additionally, this project is broadening participation through the recruitment and mentoring of graduate students hailing from underrepresented populations and hand-on community outreach events for students in grades K-12 in collaboration with the University of Michigan Museum of Natural History.TECHNICAL SUMMARY: Solid-state sintering (SSS) facilitates the efficient production of metal and ceramic materials, however, long-standing challenges remain due to a lack of fundamental insight of the dominant mechanisms facilitating internal microstructural development. Since the overall driving force for sintering (reduction of interfacial surface energy) can be facilitated by at least six different and potentially competing mechanisms, a robust understanding of SSS has been largely stymied by the absence of in-situ data. This research is employing novel in-situ x-ray computed tomography (XCT) and high energy diffraction microscopy (HEDM) to directly image 3D particle and internal microstructural evolution with micron-scale resolution during SSS in novel binder jet 3D printing (BJ3DP). Binder jet is used as an exemplar due to its strong potential for implementation in the automotive industry. Results are being combined with electron microscopy to understand the primary diffusion mechanisms at work during SSS, and how they are influenced by process, feedstock, and/or material factors. Findings are being used to test fundamental hypotheses on densification and grain-growth in BJ3DP and serve as calibration data for physics-based models. Overall, this new knowledge is enabling enhanced prediction of microstructure evolution applicable to a variety of SSS processes with strong industrial relevance such as powder metallurgy, metal injection molding, and field-assisted sintering. Integrated educational modules are being employed to reduce the currently growing manufacturing skills gap in the U.S. through research experiences for undergraduate and graduate students and professional development opportunities for students with the automotive industry. Educational and outreach activities include actively recruiting and training underrepresented minorities and women and providing accessible, hands-on activities for students in K-12 in collaboration with the U-M Natural History Museum to inspire interest in advanced manufacturing and STEM.This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.