| Based on the principle of layer-wise forming, selective laser sintering (SLS) makes the powder directly into three-dimensional solid parts using computer aided design and manufacturing. SLS is one of the most potential technology in the field of Additive Manufacturing (AM). Nowadays, much attention is paid to fabricating ceramic parts via SLS due to the enlarged application domain and high value of ceramic parts. But so far, it exists problems of low density, poor mechanical properties and manufacturing precision while forming ceramic parts by SLS. And it is also not systemic enough in the research of powder material preparation, forming process and mechanism, and furnace sintering. So it is unable to meet the requirements of additive manufacturing complex shape ceramic parts with high performance.In view of the above background, this study aiming at the fabricating ceramic parts with high density, high performance and high precision, systematically studied the method of preparing ceramic composite powder, introduced the Cold Isostatic Pressing (CIP) technology into the post-process of SLS, with subsequent debinding and Furnace Sintering (FS) technology. SLS/CIP/FS hybrid technology was used for fabricating ceramic parts. Granulated Al2O3powder, nano-ZrO2powder and Gaobai clay were selected as research objects, it covered granulated and nano powder of modern ceramics as well as the clay materials of traditional ceramics. So it has the universal application of SLS/CIP/FS technology in other kinds of ceramics. Three typical ceramic, Al2O3, ZrO2, Gaobai clay ceramic parts or products were manufactured by SLS/CIP/FS. Densification mechanism, shrinkage, microstructure and mechanical properties of the three ceramics were studied by DSC thermal analysis, TG thermogravimetric analysis, SEM, X-ray diffraction (XRD), bending strength and Vickers-hardness; based on modified Cam-Clay, Drucker-Prager/Cap and SOVS models, numerical simulation size change of SLS ceramic parts in CIP and FS process was carried on the Abaqus finite element analysis platform taking Al2O3as an example. The results were compared with the test in order to further improve the accuracy of SLS/CIP/FS process and optimize forming process of ceramic parts. The research contents and conclusions are below:(1) Preparation, SLS/CIP/FS forming, densification and properties characteristics of Al2O3-Polyvinyl Alcohol (PVA)-Epoxy Resin E06(ER6) composite powder were studied. According to the material properties and market characteristics of Al2O3ceramics, the method of coating and mechanical mixing were combined to prepare Al2O3-PVA-ER6composite powder, the optimized SLS forming process parameters (preheating temperature, laser power, scanning speed, scanning space and layer thickness) were determined as53℃、21W、1600mm/s、100μm、150μm; based on optimal SLS process, the effect law of the different CIP holding pressure on the Al2O3blank size, density was analysised, After comprehensive considering of the forming efficiency and forming properties the reasonable process of CIP holding pressure determined as200MPa; In the basis of the TG curve of PVA, ER6, the reasonable debinding was determined; according to the Al2O3ceramic material’s characteristics of high temperature sintering, experiments of high temperature sintering process were carried out to determine the reasonable holding temperature and holding time of FS process, respectively as1650℃and120min; optimized SLS/CIP/FS forming process parameters was adopted to manufacture Al2O3ceramic parts, the relative density and bending strength reached92%and175MPa.(2) Preparation method, SLS/CIP/FS forming and characteristics of nano-ZrO2ceramic composite powder were studied. According to the two kinds of binder of crystalline polymer stearic acid and nylon12, take solvent evaporation and solvent precipitation method to prepare two kinds of nano ceramic composite materials. The optimized SLS forming process parameters laser energy density were determined as0.264J/mm2and0.415J/mm; according to the thermal debinding mechanism as well as the TG curve of PA12and stearic acid, respectively determine the reasonable thermal debinding process; under the same conditions of sintering at high temperature, the forming parts of nano-ZrO2coated nylon12had higher relative density and Vickers hardness, respectively reached97%and1180HV1, radial and axial shrinkage rate were58.1%and67.8%.(3) The SLS/CIP/FS method and the traditional process of "colored pottery" was combined to manufacture the traditional ceramic products with Gaobai clay powder. Studied evolution law and mechanism of the laser energy on the clay’s shrinkage rate and density during SLS/CIP/FS forming to determine the best laser energy density as0.3575J/mm2; using SEM, XRD to study effects of the FS sintering temperature on the shrinkage rate, density and Vickers-hardness, with increasing sintering temperature, density of clay sample gradually increased and the shrinkage and Vickers-hardness increased firstly and then decreased, when T=1350℃, Vickers-hardness reached855.8HV1, when T=1450℃, the relative density reached97%. Finally, using the optimized SLS/CIP/FS process parameters, we got the "Yellow duck" ceramic products.(4) Based on devised clay Cam-Clay, Drucker-Prager/Cap and viscoplastic SOVS model, take Al2O3as an example, the hydrostatic compressive stress-volumetric plastic strain curve and the thermal expansion curves of the sintering shrinkage rate with time, temperature changes were obtained through the experiment, the size change of SLS ceramic parts during CIP and FS process were simulated on the Abaqus finite element analysis platform, and the results were compared with the test, the simulation error were controlled within8%and2%, which make foundation for improving the precision and optimization of ceramic parts hybrid forming process of SLS/CIP/FS.In summary, three kinds of typical ceramic parts were manufactured by SLS/CIP/FS, which can provide references for other ceramic forming. It promoted the SLS technology extension to high density, high performance and high precision ceramic parts with complex shape and have good market prospects. It has certain significance to advancement of SLS technology in our country and further application in ceramic industry. |
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