分层厚度和CuO-TiO_(2)含量对黏结剂喷射成形Al_(2)O_(3)陶瓷型芯性能的影响

Effect of Layer Thickness and CuO-TiO_(2)contents on the Properties of Al_(2)O_(3)Ceramic Core via Binder Jetting Formation

Al_(2)O_(3)陶瓷因其良好的耐高温性和化学稳定性,常被用作涡轮叶片铸造成形的型芯材料,但低温烧结的力学性能难以满足型芯零件的应用标准。为了解决该问题,本工作提出一种黏结剂喷射成形过程中以CuO-TiO_(2)作为液-固相烧结助剂实现Al_(2)O_(3)陶瓷型芯的低温力学性能的提升。详细探究了CuO-TiO_(2)含量(0~10%,质量分数)和分层厚度(60~100μm)对Al_(2)O_(3)陶瓷孔隙率、尺寸收缩率和力学性能的影响规律。结果表明,添加CuO-TiO_(2)既可以部分填充颗粒孔隙,又可以与Al_(2)O_(3)反应来提高坯体的综合性能。此外,低分层厚度(如60μm)有利于获得较优的性能。1500℃保温2 h烧结后各成分烧结体对应的相对密度较优值分别为30.64%±1.01%,55.33%±2.49%,53.68%±0.99%,54.31%±0.74%和55.50%±1.95%,对应的孔隙率由64.94%±1.2%逐渐下降到41.85%±1.82%。此时,纯Al_(2)O_(3)陶瓷X-Z轴尺寸收缩率均小于4%,而含CuO-TiO_(2)的坯体X-Z轴尺寸收缩率最大值分别小于17%,19%和17%。抗弯和压缩强度随着CuO-TiO_(2)含量增加先增加后减小,5.0%坯体获得最佳值分别为(29.4±1.57)MPa和(61.82±1.73)MPa,这与显微结构中气孔分布和Cu-Al-O系物相生成量有一定关系。

Introduction 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.Al_(2)O_(3)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 Al_(2)O_(3)ceramic cores.Therefore,a solid-liquid sintering additives,such as CuO and TiO_(2),was added to assist the sintering of Al_(2)O_(3)ceramic.In this work,influence of sintering additives contents ranged from 0 to 10%(mass fraction)and layer thickness ranged from 60-100μm on the porosity,microstructure,and mechanical properties of Al_(2)O_(3)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 Al_(2)O_(3)ceramics.Methods Irregular Al_(2)O_(3)powders were used as raw material,irregular CuO and ball-like TiO_(2)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 TiO_(2),0-10%(mass fraction)of CuO-TiO_(2)mixture was respectively weighted and added into Al_(2)O_(3)powders in the polystyrene jar with ethanol as medium,and then these powders with Al_(2)O_(3)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 18μm)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 60-100μm,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 1500℃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-7600F SEM and XRD-6100 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.Results and diScussion Al_(2)O_(3)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 Al_(2)O_(3)powders with 5.0%CuO-TiO_(2),60μm layer thickness and sintering at 1500℃for 2 h.The effects of CuO-TiO_(2)amounts and layer thicknesses on the properties of Al_(2)O_(3)ceramic cores were investigated.The relative density of green ceramics gradually increases with the contents of CuO-TiO_(2)increased from 0%to 10%under a low layer thickness.When sintered at 1500℃for 2 h,the porosity of ceramic with different content of CuO-TiO_(2)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 60μm,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 CuAlO_(2) and CuAl_(2)O_(4) by the chemical reaction between CuO/TiO_(2)and Al_(2)O_(3)can fill the pores.For pure Al_(2)O_(3)ceramic,the dimensional shrinkage of X-Z axis was less than 4%when layer thickness ranged between 60μm and 100μm,but that of Al_(2)O_(3)ceramics with 2.5%-10%CuO-TiO_(2)addition could reach about 17%.The surface roughness of Al_(2)O_(3)ceramics firstly decreased and then increased with the increase of CuO-TiO_(2)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 Al_(2)O_(3)ceramic firstly increased and then declined with the increase of CuO-TiO_(2)amounts,the maximum values were(29.4±1.57)MPa and(61.82±1.73)MPa,repetitively when CuO-TiO_(2)content equaled to 5.0%.Conclusions The BJAM Al_(2)O_(3)ceramic core with a suitable porosity and mechanical properties was obtained by regulating the amounts of CuO-TiO_(2)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-TiO_(2)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 CuAlO_(2) and CuAl_(2)O_(4) 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-TiO_(2)amounts,the optimal values of(6.00±0.63)μm and(7.00±0.78)μm were obtained at 5.0%Al_(2)O_(3)ceramic.The bending and compressive strength values similarly first increased and then declined with the CuO-TiO_(2)amounts,and the maximum values of(29.40±1.57)MPa and(61.82±1.73)MPa were obtained by the 5.0%Al_(2)O_(3)ceramic,which were mainly affected by the distribution of pores in the microstructure and the generation amounts of the second phases.