Research On The Additive Manufacturing And Performance Of Aluminum-based Ceramic Casting Molds For Hollow Turbine Blades

Hollow turbine blades are one of the vital hot-end components of gas turbines（including aero-engines and heavy-duty gas turbines,etc.）,and their casting quality depends on the quality of ceramic molds.Currently,the core and the shell of ceramic molds are mostly prepared separately,which causes problems such as long production cycles,low yield and size control difficulty.Additive manufacturing（AM）has significant advantages in the preparation of ceramic molds,but it still faces the following scientific and technical problems:i)the preparation theory of raw materials suitable for ceramic mold AM is not yet clear;ii)AM and its post-processing processes suitable for the fabrication of ceramic molds are not yet mature;iii)The precision control and performance regulation mechanisms of AM ceramic molds are not yet clear.In response to these issues,the following researches have been carried out:（1）Stereolithography（SL）was proposed to prepare small-sized high-precision ceramic molds for aero-engine turbine blades.Related researches were carried out to address issues including large sintering shrinkage and difficulty in accuracy control of aluminum-based ceramic mold prepared by SL.Firstly,the effects of particle size distribution and sintering temperature on the properties of alumina mold materials prepared by SL were studied.It was found that adding an appropriate proportion of fine powder which filled the gaps among coarse powder could improve the comprehensive properties of the mold material.Alumina ceramic prepared using the 9:1 ratio of coarse and fine powder gradation and sintered at 1550°C possess relatively low sintering shrinkage,which were 6.36%（X/Y）and 11.39%（Z）along different directions,respectively.Its room temperature strength was 78.15±3.50 MPa,the apparent porosity was 30.12±0.08%,and the high temperature creep deformation was 4.44±0.45 mm,which met the requirements for casting purposes.Based on the optimized particle gradation,MgO was added as a sintering shrinkage compensator to further reduce the sintering shrinkage of the aluminum-based ceramic mold material.A mathematical model of the relationship between the MgO content and volumetric sintering shrinkage was established to provide a theoretical basis for the prediction of sintering shrinkage.The optimized MgO content was 6 wt%,and the linear shrinkage rates after sintering at 1550°C were 0.84±0.04%（X/Y）and 1.18±0.02%（Z）,the room temperature strength was 29.40±2.99 MPa,the apparent porosity was 36.82±0.08%,and the high temperature creep deformation was 4.60±0.18 mm,which all met the requirements for casting purposes.（2）Selective laser sintering（SLS）was proposed to prepare large-scale ceramic molds for heavy-duty gas turbine blades,and related researches were carried out to solve problems of sintering difficulty,poor material properties and difficulty in cleaning residual powder for the aluminum-based ceramic mold prepared by SLS.Firstly,spherical alumina powder and fumed Si O₂ were used to reduce the friction of composite powder and its bulk density and fluidity was improved.Silica sol and sub-micron alumina powder were used to prepare the infiltration slurry to improve the performance of the ceramic mold.The effect of the solid loading of the infiltration slurry on the weight gain efficiency,clogging behavior and mold properties was systematically studied.The linear shrinkage rates of the optimized ceramic mold along different directions were 1.51%（X/Y）and 2.03%（Z）,respectively.The apparent porosity was 30.817±0.004%,the room temperature flexural strength reached 29.84±6.06 MPa,the high-temperature（1550°C）flexural strength reached 18.10±1.25 MPa,and the creep deformation at 1550°C was0.01%,which all met the performance requirements for casting purposes.Based on the material with adequate properties,aiming at the difficulty in residual powder cleaning after SLS,a preparation route of the spliced ceramic mold was proposed on the premise of ensuring the integral formation of core and shell of the key parts of the turbine blade.The reliable connection of different parts of the ceramic mold was realized based on the optimized ceramic binder,and a spliced ceramic mold was designed and prepared based on the improved step joint.The results show that the flexural strength of the bonded ceramic mold reached 9.72±1.34 MPa,the casting mold has successfully passed the casting verification,and no leakage was found,which fully verified the feasibility of the spliced ceramic mold.This paper reveals the key influence mechanisms of different AM processes on the performance of ceramic molds and provides particular theoretical guidance and technical support for the popularization and application of AM ceramic molds.