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Register a new userSimultaneous Spark Plasma Sintering of Multiple Complex Shapes
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Authors
Charles Mani`ere
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This work addresses the two great challenges of the spark plasma sintering (SPS) process: the sintering of complex shapes and the simultaneous production of multiple parts. A new controllable interface method is employed to concurrently consolidate two nickel gear shapes by SPS. A graphite deformable sub-mold is specifically designed for the mutual densification of the both complex parts in a unique 40 mm powder deformation space. An energy efficient SPS configuration is developed to allow the sintering of a large-scale powder assembly under electric current lower than 900 A. The stability of the developed process is studied by electro-thermal-mechanical (ETM) simulation. The ETM simulation reveals that homogeneous densification conditions can be attained by inserting an alumina powder at the sample/punches interfaces enabling the energy efficient heating and the thermal confinement of the nickel powder. Finally, the feasibility of the fabrication of the two near net shape gears with a very homogeneous microstructure is demonstrated.
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A new flash (ultra-rapid) spark plasma sintering method applicable to various materials systems, regardless of their electrical resistivity, is developed. A number of powders ranging from metals to electrically insulative ceramics have been successfully densified resulting in homogeneous microstructures within sintering times of 8-35 s. A finite element simulation reveals that the developed method, providing an extraordinary fast and homogeneous heating concentrated in the samples volume and punches, is applicable to all the different samples tested. The utilized uniquely controllable flash phenomenon is enabled by the combination of the electric current concentration around the sample and the confinement of the heat generated in this area by the lateral thermal contact resistance. The presented new method allows: extending flash sintering to nearly all materials, controlling sample shape by an added graphite die, and an energy efficient mass production of small and intermediate size objects. This approach represents also a potential venue for future investigations of flash sintering of complex shapes.
An energy efficient spark plasma sintering method enabling the densification of large size samples assisted by very low electric current levels is developed. In this method, the electric current is concentrated in the graphite foils around the sample. High energy dissipation is then achieved in this area enabling the heating and full densification of large (alumina) parts ({{O}} 40 mm) at relatively low currents (800 A). The electrothermal mechanical simulation reveals that the electric current needed to heat the large samples is 70 % lower in the energy efficient configuration compared to the traditional configuration. The presence of thermal and densification gradients is also revealed for the larger size samples. Potential solutions for this problem are discussed. The experiments confirm the possibility of full densification (96-99 %) of large alumina samples. This approach allows using small (and low cost) SPS devices (generally limited to 10-15 mm samples) for large size samples (40-50 mm). The developed technique enables also an optimized energy consumption by large scale SPS systems.
Graphite creep has high importance for applications using high pressures (100 MPa) and temperatures close to 2000 {textdegree}C. In particular, the new flash spark plasma sintering process (FSPS) is highly sensitive to graphite creep when applied to ultra-high temperature materials such as silicon carbide. In this flash process taking only a few seconds, the graphite tooling reaches temperatures higher than 2000 {textdegree}C resulting in its irreversible deformation. The graphite tooling creep prevents the flash spark plasma sintering process from progressing further. In this study, a finite element model is used to determine FSPS tooling temperatures. In this context, we explore the graphite creep onset for temperatures above 2000 {textdegree}C and for high pressures. Knowing the graphite high temperature limit, we modify the FSPS process so that the sintering occurs outside the graphite creep range of temperatures/pressures. 95 % dense silicon carbide compacts are obtained in about 30 s using the optimized FSPS.
The 3 heating modes are utilized to make ZrN powders have 3 different levels of the electric current density at the same temperature during spark plasma sintering (SPS). The constitutive equation of sintering for SPS is applied to the experimental porosity evolution of ZrN from three SPS modes, and this showed high electric current density increase the electric current assisted deformability value of ZrN pellets, resulting in a reduction of the flow stress. The electric current flow enhances the dislocation motion, which was experimentally proved and analyzed by modified Williamson-Hall equation applying to X-ray diffraction results, and the mechanical strength test of ZrN pellets.
The stability ofthe proportional--integral--derivative (PID)controlof temperature in the spark plasma sintering (SPS) process is investigated.ThePID regulationsof this process are tested fordifferent SPS toolingdimensions, physical parameters conditions,andareas of temperature control. It isshown thatthe PID regulation quality strongly depends on the heating time lag between the area of heat generation and the area of the temperature control. Tooling temperature rate maps arestudied to revealpotential areas forhighlyefficientPID control.The convergence of the model and experiment indicatesthat even with non-optimal initial PIDcoefficients, it is possible to reduce the temperature regulation inaccuracy to less than 4K by positioning the temperature control location in highlyresponsiveareas revealed by the finite-element calculationsof the temperature spatial distribution.
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