Compared with the preparation process of blade turbine using precision investment casting (PIC), additive manufacturing technology, especially binder jetting additive manufacturing (BJAM), possess many advantages such as the decreased cost, simplified process, easily preparation of complex porous materials, and diversified selectivity of binder types, therefore, it has been widely used to fabricate the core parts. Al₂O₃ ceramic owns better oxidation and corrosion resistance, highly serviced temperature and high hardness, is widely used as the core materials. However, the solid sintering method is hard to meet the mechanical requirement of Al₂O₃ ceramic cores. Therefore, a solid-liquid sintering additives, such as CuO and TiO₂, was added to assist the sintering of Al₂O₃ ceramic. In this work, influence of sintering additives contents ranged from 0 to 10% (mass fraction) and layer thickness ranged from 60-100 μm on the porosity, microstructure, and mechanical properties of Al₂O₃ ceramic cores were investigated. the results could construct a theoretical foundation for the rapid and low-cost printing of high-performance ceramic cores, and further expand the application scope of Al₂O₃ ceramics.

Irregular Al₂O₃ powders were used as raw material, irregular CuO and ball-like TiO₂ powders were used as liquid-solid phase sintering additives. Based on the mass fraction of 4∶1 corresponding to the eutectic temperature of CuO and TiO₂, 0-10% (mass fraction) of CuO-TiO₂ mixture was respectively weighted and added into Al₂O₃ powders in the polystyrene jar with ethanol as medium, and then these powders with Al₂O₃ balls (weight ratio of ball to powder was 5∶1) were ball milled at 180 r/min for 2 h. After milling, the mixed powder was dried at 90 ℃ for 12 h and then sieved through 800-mesh screen (about 18 μm) to gain the eventual printed powders. The complex parts were printed by a binder jetting 3D printer, the corresponding parameters were the layer thickness of 60-100 μm, inkjet amount of 60%-80% and inkjet number of 2 Ps. After printing, the powders were cured at 120 ℃ for 4 h, and then the samples were firstly sintered to 600 ℃ for 1 h with a heating rate of 5 ℃/min, then further sintered to 1 500 ℃ for 2 h with a heating rate of 5 ℃/min. Finally, the samples were cooled to ambient temperature with naturally cooling.

The thermogravimetric curve of binder was measured by simultaneous thermal analyzer. The relative density and porosity of samples were measured via Archimedes' methods. The microstructure and element distribution of powders, sintered parts and fracture surface were observed via a JSM-7 600F SEM and XRD-6 100 X-ray diffractometer. The three-dimensional surface roughness of sample was measured by laser scanning confocal microscope. The bending and compressive strengths were determined using a universal testing machine.

Al₂O₃ ceramic cores with an optimal porosity of 43.76% ± 0.72%, bending strength of (29.4 ± 1.57) MPa, and compressive strength of (61.82±1.73) MPa were obtained when Al₂O₃ powders with 5.0% CuO-TiO₂, 60 μm layer thickness and sintering at 1 500 ℃ for 2 h. The effects of CuO-TiO₂ amounts and layer thicknesses on the properties of Al₂O₃ ceramic cores were investigated. The relative density of green ceramics gradually increases with the contents of CuO-TiO₂ increased from 0% to 10% under a low layer thickness.

When sintered at 1 500 ℃ for 2 h, the porosity of ceramic with different content of CuO-TiO₂ mixture equaled to 64.94% ±1.2%, 42.36% ±1.44%, 43.76% ±0.72%, 42.21% ±0.35%, and 41.85% ±1.82%, respectively, when the layer thickness was 60 μm, which can be attributed to that the sintering process could decline the surface energy of particles and rise the particles contact, and the generation of liquid phases such as CuAlO₂ and CuAl₂O₄ by the chemical reaction between CuO/TiO₂ and Al₂O₃ can fill the pores. For pure Al₂O₃ ceramic, the dimensional shrinkage of X-Z axis was less than 4% when layer thickness ranged between 60 μm and 100 μm, but that of Al₂O₃ ceramics with 2.5%-10% CuO-TiO₂ addition could reach about 17%. The surface roughness of Al₂O₃ ceramics firstly decreased and then increased with the increase of CuO-TiO₂ amounts, and roughness of X-Y surface was better than that of X-Z surface due to the press and smoothing from powder roller and the filling effect of liquid phases. The bending and compressive strength of the sintered Al₂O₃ ceramic firstly increased and then declined with the increase of CuO-TiO₂ amounts, the maximum values were (29.4±1.57) MPa and (61.82±1.73) MPa, repetitively when CuO-TiO₂ content equaled to 5.0%.

The BJAM Al₂O₃ ceramic core with a suitable porosity and mechanical properties was obtained by regulating the amounts of CuO-TiO₂ and layer thicknesses. Results showed that ceramics with lower layer thickness could obtain relatively better performance. The relative density of sintered samples increased by 11.72%, 65.66%, 58.82%, 51.24% and 54.81% with the increase of CuO-TiO₂ amounts compared with the green bodies. And the sintered porosity gradually decreased from 64.94 ± 1.20% to 41.85±1.82%, which was attributed to the generation of CuAlO₂ and CuAl₂O₄ phases. Additionally, the roughness values of X-Y surface were better than that of X-Z surface of all samples, but the roughness firstly decreased and then increased with the increase of CuO-TiO₂ amounts, the optimal values of (6.00 ± 0.63) μm and (7.00±0.78) μm were obtained at 5.0% Al₂O₃ ceramic. The bending and compressive strength values similarly first increased and then declined with the CuO-TiO₂ amounts, and the maximum values of (29.40±1.57) MPa and (61.82±1.73) MPa were obtained by the 5.0% Al₂O₃ ceramic, which were mainly affected by the distribution of pores in the microstructure and the generation amounts of the second phases.