Get 20M+ Full-Text Papers For Less Than $1.50/day. Start a 14-Day Trial for You or Your Team.

Learn More →

Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing

Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing Adv. Manuf. (2019) 7:353–363 https://doi.org/10.1007/s40436-019-00273-2 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 1,2 2,3 2 2 2 • • • • Wei-Jun Zhu Guo-Qiang Tian Yang Lu Kai Miao Di-Chen Li Received: 26 February 2019 / Revised: 3 April 2019 / Accepted: 21 July 2019 / Published online: 26 September 2019 The Author(s) 2019 Abstract The precision casting method based on alumina- Keywords Alumina-based ceramic core  Leaching based ceramic cores is one of the main techniques used to Alkali solution  Additive manufacturing (AM) manufacture hollow turbine blades. Additive manufactur- Stereolithography ing (AM) technology provides an alternate solution to fabricating ceramic cores quickly and precisely. As the complexity of the structure increases and the strength of the 1 Introduction material improves, the leaching process of the cores becomes more complicated. This study proposes a com- Currently, directional column- and single-crystal hollow pound pore-forming method to increase the porosity of turbine blades for aero engines use ceramic cores with high ceramic cores by adding a preformed-pore agent and performance (e.g., silica, alumina). Because of their high materials that convert to easy-to-corrode phases. The pre- chemical stability and good mechanical properties at high formed-pore agents (e.g., carbon fibers) can be burned off temperatures, alumina-based ceramic cores are commonly during sintering to form pores before the leaching, and the used in specific fields [1–3]. Traditionally, fabrication of easy-to-corrode phases (e.g., CaCO , SiO , b-Al O ) can the ceramic cores involves time-consuming procedures and 3 2 2 3 be leached firstly to form pores during the leaching process. low processing flexibility. The preparation of integral The pores formed in the aforementioned two stages ceramic molds based on additive manufacturing (AM) increase the contact area of the cores and leaching solution, technology provides a new method for fabricating ceramic thus improving the leaching rate. In the current study, the cores with complex cooling channels efficiently [4]. additive amount of the preformed-pore agent was opti- Removing alumina-based ceramic cores, as represented by mized, and the effect of the easy-to-corrode phases on the chemical leaching, is the key process after casting. How- comprehensive properties of the cores was then compared. ever, high chemical stability makes the removal of cores Based on this, the corresponding model was established. particularly difficult. However, as the complexity of the structure increases and the strength of the material improves, the leaching process of the AM-based cores becomes more difficult [5]. One method to improve the chemical reaction is by selecting superior leaching solu- & Wei-Jun Zhu wjzhu@xjtu.edu.cn tions. Some types of effective leaching solutions indeed exist (i.e., hydrofluoric acid), which can concert Al O into 2 3 School of Mechanical Engineering and Automation, Beihang 3? soluble Al to remove alumina-based ceramic cores effi- University, Beijing 100191, People’s Republic of China ciently [6]. The basis reaction equation is Li AlF 3 6- State Key Laboratory for Manufacturing Systems - ? 2Al O = 3LiAlO ? 2AlF . However, the F ion has 2 3 2 3 Engineering, Xi’an Jiaotong University, Xi’an 710049, severely harmful effects on the human body and environ- People’s Republic of China ment. Therefore, this method has been abandoned for many School of Aeronautical Engineering, Zhengzhou University years [7]. Alkali-boiling leaching is now a commonly used of Aeronautics, Zhengzhou 450046, People’s Republic of method. The main research direction is now divided into China 123 354 W.-J. Zhu et al. two categories. One is to increase the leaching pressure, 2 Materials and methods thereby increasing the temperature of the alkali solution and thus the reaction reactivity. High pressure can result in 2.1 Raw materials increased depth of the alkali solution into the cores, thereby increasing the contact area of the alkali solution and the Alumina-based ceramic core samples were shaped using cores [8–10]. The other research direction focuses on gelcasting technology [4, 13, 14]. A ceramic slurry with a improving the pore structure by adding pore-forming u = 60% solid content was prepared according to the agents to the cores to prefabricate pores, or by adding gradations listed in Table 1, and the fused alumina was materials that will convert to easy-to-corrode phases, which used in a technically pure form. The as-shaped ceramic can be dissolved in advance in the process of leaching to core’s green body was further treated through freeze drying form pores, thus increasing the contact area of the alkali [15–17], high-temperature sintering, and a dipping pro- solution and the cores. cessing [18]. The freeze-drying process was conducted in a For the method of improving leaching pressure, signif- DTY-1SL vacuum freeze dryer. Then, the core samples icantly improving the equipment is necessary. The exis- sized (60 mm 9 10 mm 9 4 mm) were subjected to a re- tence of high pressure typically requires equipment of a baking treatment at 1 500 C for 0.5 h to model the high standard, which often poses safety hazards. On the pouring condition during a real investment casting. contrary, the method of introducing the pore structure is Through this treatment, the core samples could be used for more efficient. This method can help to improve the the etching experiment. porosity of the cores before and during the process of The material used to preform pores was a carbon fiber leaching, thus increase the contact area between the cores that was easy to burn with a diameter of approximately and leaching liquid. Still, the ceramic cores must meet the 7–10 lm and an average length of approximately 500 lm. synthesis requirements of high-temperature strength, The materials that can convert to easy-to-corrode phases deflection, sintering shrinkage rate, and so on. Thus, this included CaCO , b-Al O , and SiO powders. The mean 3 2 3 2 research represents a comprehensive optimization under a particle sizes of the CaCO , b-Al O , and SiO powders 3 2 3 2 multi-objective constraint. In theory, the higher the were 5 lm, 5 lm, and 40 lm, respectively. The carbon porosity, the higher the leaching rate. However, the sin- fiber was mixed with the ceramic slurry at w = 1%, 2% and tering shrinkage rate is bound to rise, and the high-tem- 3%, respectively, of the total mass, and the corresponding perature performance should deteriorate. Therefore, mass of the alumina powder was deducted from the 40 lm conducting systematic research on the pore-forming series. The CaCO powder was mixed with the ceramic methods without considerably affecting other properties is slurry at w = 2%, 4%, 6% and 8%, respectively, of the total necessary. mass, and the corresponding mass of the alumina powder Therefore, in this study, different pore-forming methods was deducted from the 5 lm series. The b-Al O powder 2 3 are tested systematically. In addition, a compound method was mixed with the ceramic slurry at w = 1%, 2%, 3% and of pore-forming is introduced, which involves adding an 4%, respectively, of the total mass, and the corresponding easy combustible material to achieve preformed pores mass of the alumina powder was deducted from the 5 lm while also adding materials to the cores that convert to easy series. The SiO powder was mixed with the ceramic slurry corrosion phases that can be removed prior to the process at w = 1%, 2%, 3% and 4%, respectively, of the total mass, of leaching; this is likely to increase the porosity consid- and the corresponding mass of the alumina powder was erably. This method will likely achieve maximum porosity deducted from the 40 lm series. during leaching without affecting other properties, thus achieving a higher leaching rate. In this study, the leaching rate of alumina-based ceramic cores is improved without affecting other properties. This is the basis for the study of the cores and promotes the Table 1 Particle gradation of the alumina-based ceramic slurry for gelcasting technology of precision casting of hollow turbine blades [11, 12]. Size/lm u (Solid content)/% 100 20 40 40 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 355 2.2 Sampling preparation 2.3 Leaching process Previous studies conducted by our group developed an During the removal process, the ceramic core samples were AM-based method to fabricate ceramic cores [3, 4, 14], and immersed in the w = 70% KOH solution [5]. The etching the procedure for this is shown in Fig. 1. Directly driven by experiment was conducted in an atmospheric Monel kettle CAD digital data, the mold of the turbine is formed by at a constant temperature of 220 C. stereolithography (SL), which is one of the AM processes, A series of experiments were performed to investigate and the ceramic slurry is formed by the gelcasting method. the relationships between the leaching rate and pore- After freeze drying, the ceramic core is formed by high- forming methods. In addition, the open porosity, sintering temperature sintering. The hollow blade can be cast using shrinkage rate, high-temperature strength, and high-tem- the strengthened ceramic core (see Fig. 1). perature deflection of the cores with pore-forming additive First, the resin molds for the test samples were prepared materials were measured and analyzed synthetically. using an AM apparatus (SPS600B, Xi’an Jiaotong University, Xi’an, China) with a photosensitive resin (SPR 2.4 Testing 8981, Zhengbang Technology Co., Ltd., Zhuhai, China). A ceramic slurry with a low viscosity (less than 1 Pas) and During the experiments, the open porosity of the core high solid loading (u = 60%) was prepared by ball-milling samples was measured by the Archimedes method, and the for 40 min. After degassing for 5 min, the ceramic slurry sintering shrinkage rate was measured by Vernier callipers. was poured into the resin mold and then polymerized The high-temperature strength was tested at 1 500 Cinan in situ to form wet green bodies under the action of the HSST-6003QP high-temperature stress-strain testing initiator and catalyst. After freeze drying for 24 h, the dried machine (Sinosteel Luoyang Institute of Refractories green bodies were placed in a furnace, heated to 1 100 C, Research Co., Ltd., China). The high-temperature deflec- and maintained at that temperature for 3 h to pyrolyze the tion was tested in a TDV-1600PC high-temperature ther- resin prototype and organic monomer polymers. Finally, mal deformation testing machine (Sinosteel Luoyang Institute of Refractories Research Co., Ltd., China) using to promote mullitization, the samples were sintered at 1 400 C for 3 h. ceramic samples of a nominal size of 2 mm 9 6mm 9 120 mm. The samples were mounted on a silicon-nitride ceramic fixture whose support spacing was 100 mm within the chamber of the testing machine that had been heated at Fig. 1 Procedure of AM-based method to fabricate ceramic cores (within dotted-lined box) 123 356 W.-J. Zhu et al. 1 500 C. It was maintained for 30 min, and then decreased to room temperature. Deflection tests were then performed by measuring the drop distance of the interme- diate point of the samples. The microstructure was ana- lyzed using scanning electron microscopy in an SU-8010 from Hitachi Ltd. 3 Results and discussion 3.1 Effects of preformed pores on the performances of the cores The effect of the carbon fibers on the microstructures of the ceramic molds is illustrated in Fig. 2. The presence of the Fig. 3 Open porosities of samples with different carbon fiber carbon fibers in the green bodies, as shown in Fig. 2a, led additives to a significant increase in the number of pores in the ceramic molds. It is worth mentioning that more open pores could be expected due to the high-aspect-ratio mor- phology of the carbon fibers, which would considerably enhance the leaching performance. However, the increase in porosity could possibly impair other performances of the ceramic core (e.g., the surface quality and mechanical properties). Therefore, the effects of the carbon fibers should be carefully investigated, and some trade-off must be made to balance all performances. The results of the open porosities of the core samples with the carbon fiber additive are illustrated in Fig. 3. The open porosity of the core samples without the carbon fiber additive was u = 31.3%, and it increased when the carbon fiber content was increased, indicating u = 34.6% for w = 3%. The experimental results of the leaching rate influenced by the carbon fiber additive are illustrated in Fig. 4 Effects of carbon fiber additives on the leaching rate Fig. 4. The leaching rate of the core samples without the carbon fiber additive was 13.46%/h, and it increased along with the carbon fiber, indicating v = 18.48%/h for w = 3%. Fig. 2 Microstructures of the ceramic molds affected by the carbon fibers a carbon fibers as prepared in the green body, b pores introduced by the carbon fibers after burning out 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 357 Fig. 5 CT images of pores filling in the ceramic molds with different carbon fiber additives a w = 3%, b w = 2%, c w = 1% (circled zones indicate defects) Table 2 Performances of the samples before and after the carbon fiber was added w (Carbon High-temperature High-temperature Sintering fiber)/% strength/MPa deflection/mm shrinkage rate/% 0 23.8 0.41 0.13 1 23.2 0.43 0.15 With the carbon fiber additive, the viscosity of the ceramic slurry increased, which decreased the liquidity of the ceramic slurry when it was gelcasted, resulting in an insufficiently fine structure in the mold. Figure 5 shows the filling results of the tip part in the mold with different carbon fiber additives. As illustrated in Figs. 5a and b, the Fig. 6 Effect of the CaCO additive on the leaching rate viscosity of the ceramic slurry is too high when the additive is w = 3% and 2%, respectively, and the tip parts could not from 13.46%/h to 15.05%/h without affecting the mold- be filled, leaving obvious defects in the cores. However, filling capacity of slurry and other properties. when the additive was w = 1%, the viscosity was moderate and the mold could be fully filled, as illustrated in Fig. 5c. 3.2 Effect of the compound method of pore-forming The addition of the carbon fiber can improve the leaching on the performances of the cores performance to some extent, but the increase in porosity can damage the mechanical properties. In addition, the theo- To increase the leaching rate further, based on w =1% retical solid phase content of the cores is reduced after the carbon fiber additive, materials that could convert to easy- loss of the carbon fiber, and the sintering shrinkage rate to-corrode phases were continuously added, which were increases, thus affecting the precision of the mold and leached prior to the process of leaching, with pores sub- blades. The other properties of the samples with w =1% sequently left in situ. The porosity increased further when carbon fiber were tested as presented in Table 2. It can be the materials were being leached, thus the leaching rate seen that with the carbon fiber additive, the high-tempera- was further improved. ture strength was slightly lower, and the high-temperature deflection and sintering shrinkage rate increased slightly. 3.2.1 CaCO additive However, these changes can be ignored. Therefore, the current study determined that the optimal additive amount Figure 6 illustrates the influence rule of CaCO on the of the carbon fiber was w = 1%. The leaching rate increased leaching rate. Figure 6 indicates that with the increase of 123 358 W.-J. Zhu et al. Fig. 7 Microstructures on the sample surfaces after leaching a with w(CaCO ) = 8%, b without CaCO additive 3 3 Fig. 9 Microstructure on the sample surface after sintering with w(CaCO ) = 8% additive Fig. 8 Effect of the CaCO additive on the sintering shrinkage rate CaCO , the leaching rate improved. The leaching rate of the core samples without the CaCO additive was 15.05%/ h, and it increased when the amount of CaCO additive was increased, with 21.42%/h for w = 8%. Figure 7 illustrates the microstructure morphologies of ceramic samples after leaching without CaCO and with w =8% CaCO . As shown in Fig. 7, the surface break degree of the samples with w = 8% CaCO additive was greater than that of the samples without CaCO , indicating that the cores with CaCO could be leached more easily. However, after CaCO was added, the sintering shrinkage rate of the samples significantly increased. Fig- ure 8 illustrates the effect of CaCO on the sintering shrinkage rate, which increased from 0.15% to 5.59% when w(CaCO ) = 8% was added. The sintering shrinkage rate increased significantly Fig. 10 Effect of the b-Al O additive on the leaching rate 2 3 when CaCO was added. This was because CaO, which was resulted from the decomposition of CaCO , reacts with process involves liquid phase sintering, which causes sin- Al O to generate calcium aluminate in the particle contact tering densification, thus increasing the sintering shrinkage 2 3 area. Calcium aluminate can be easily leached, but this rate. As illustrated in Fig. 9, the microstructure 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 359 Fig. 11 Microstructures on the sample surfaces after leaching a without and b with w(b-Al O )= 1% 2 3 Fig. 12 Effects of the b-Al O additive on the high-temperature 2 3 strength and deflection Fig. 14 Effect of the SiO additive on the leaching rate turbine blade’s precision casting requirements is difficult. Therefore, CaCO cannot be used to improve the leaching performance of ceramic cores. 3.2.2 b-Al O additive 2 3 The crystal structure of b-Al O is relatively loose, and the 2 3 additive’s density is 3.31 g/cm , which is less than the 3.97 g/cm of a-Al O . Therefore, the porosity will increase 2 3 as b-Al O converts to a-Al O after sintering at 1 500 C. 2 3 2 3 Therefore, the cores with b-Al O can be easily leached. 2 3 Figure 10 illustrates the influence rule of b-Al O on the 2 3 leaching rate, and shows that with the increase of b-Al O , 2 3 the leaching rate was improved. The leaching rate of the Fig. 13 Effect of the b-Al O additive on the sintering shrinkage rate 2 3 core samples without the b-Al O additive was 15.05%/h, 2 3 and it increased when the amount of b-Al O additive was 2 3 morphology of the unleached sample with w(CaCO )= 8% 3 increased, indicating 24.01%/h for w = 4%. additive means that the density is very high and the Figure 11 illustrates the microstructure morphologies of porosity is very small, indicating that CaCO promotes 3 ceramic samples after leaching without b-Al O and with 2 3 densification sintering. This means that meeting the hollow w(b-Al O ) = 1%. As shown in Fig. 11, the surface of the 2 3 123 360 W.-J. Zhu et al. Fig. 15 Microstructures of the sample surface after leaching a without and b with w(SiO )= 3% Figures 12 and 13 illustrate the influence rules of b- Al O on the high-temperature strength, high-temperature 2 3 deflection, and sintering shrinkage rate. With the addition of b-Al O , the high-temperature strength decreased, and 2 3 the high-temperature deflection and sintering shrinkage rate increased. When the mass fraction of b-Al O additive 2 3 increased to 2%, the high-temperature strength decreased considerably, and the deflection increased rapidly. How- ever, when the mass fraction of b-Al O additive was 1%, 2 3 the leaching rate reached 17.46%/h, and the high-temper- ature strength, deflection, and sintering shrinkage rate were 20.89 MPa, 0.47 mm and 0.17%, respectively, which met the hollow turbine blade’s precision casting requirements. Fig. 16 Effects of the SiO additive on the high-temperature strength and deflection 3.2.3 SiO additive Figure 14 illustrates the influence rule of SiO on the leaching rate. Figure 14 indicates that the leaching rate of the core samples without the SiO additive was 15.05%/h, and it increased when the amount of the SiO additive was increased, indicating 22.04%/h for w = 4%. Figure 15 illustrates the microstructure morphologies of ceramic samples after leaching without SiO and with w(SiO ) = 3%. As shown in Fig. 15, the surface of the samples with w(SiO ) = 3% additive was greater than that of the samples without SiO , indicating that the cores with SiO could be more easily leached because the porosity was improved when SiO was leached in the early stage of the leaching process. Figures 16 and 17 illustrate the influence rules of SiO on the high-temperature strength, high-temperature Fig. 17 Effect of the SiO additive on the sintering shrinkage rate deflection, and sintering shrinkage rate. With the addition of SiO , the high-temperature strength decreased, and the samples with w(b-Al O ) = 1% additive was greater than 2 3 high-temperature deflection and sintering shrinkage rate that of the samples without b-Al O , indicating that the 2 3 increased. When the mass fraction of SiO additive cores with b-Al O could be more easily leached because 2 3 increased to 4%, the high-temperature strength decreased the porosity was improved by having b-Al O leached in 2 3 significantly, and the deflection increased rapidly. The the early stage of the leaching process. 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 361 Fig. 18 Schematic of the increase of the leaching rate by the compound pore-forming method 3.3 Mechanism of pore-forming methods to improve leaching performance In theory, the higher the porosity, the larger is the contact area between the leaching liquid and the cores and the higher is the leaching rate. Based on the w(C fiber) = 1%, adding materials that can convert to easy-to-corrode phases can improve the leaching performance of the cores. This is because the compound pore-forming method can achieve preformed pores prior to the leaching. In addition, the easy corrosion phases are leached in advance, and thus more pores are generated, thus increasing the leaching rate. The principles behind the process are illustrated in Fig. 18. In a typical condition, combining the two mech- anisms, such as performed pores and easy-to-corrode Fig. 19 Effects of b-Al O and SiO additives on the leaching rate 2 3 2 phases, improve the leaching performance. For the former, carbon fibers are used as the additives and open pores are produced by burning out carbon fibers during sintering. For reason for this is that the presence of a few low-melting- the latter, more pores are introduced by eroding the easy- point phases in SiO can help to reduce the high-tem- to-corrode phases, such as CaCO , b-Al O , and SiO . 3 2 3 2 perature strength, accelerate the creep, and promote the The chemical reaction during the process is sintering, leading to an increased sintering shrinkage rate. Simultaneously, SiO transformed into cristobalite during 2 2KOH þ Al O ¼ 2KAlO þ H O: ð1Þ 2 3 2 2 the sintering process [19], which severely shrank when The removal efficiency is described quantitatively as the cooled to 180–270 C. The shrinkage could easily pro- leaching rate by measuring the dry weights before and after duce microcracks, thus damaging the high-temperature the removal process, and the leaching rate (v) is calculated strength and improving the sintering shrinkage rate. by the following equation However, when the mass fraction of SiO additive was m  m 1 2 3%, the leaching rate reached 20.92%/h, and the high- m ¼  100%; ð2Þ m  t temperature strength, deflection, and sintering shrinkage rate were 20.60 MPa, 0.48 mm and 0.20%, respectively, where m and m are the weights of the core samples before 1 2 which met the hollow turbine blade’s precision casting and after the leaching process, respectively, and t is the requirements. etching time. 123 362 W.-J. Zhu et al. Table 3 Performances of alumina samples with different additives Samples Performances Sintering shrinkage High-temperature High-temperature Leaching -1 rate/% strength/MPa deflection/mm rate/(%h ) a-Al O 0.13 23.80 0.41 13.46 2 3 a-Al O ? C(w = 1%) 0.15 23.20 0.43 15.05 2 3 a-Al O ? C(w = 1%) ? CaCO Too high – – – 2 3 3 a-Al O ? C(w = 1%) ? b-Al O (w = 1%) 0.17 20.89 0.47 17.46 2 3 2 3 a-Al O ? C(w = 1%) ? SiO (w = 3%) 0.20 20.60 0.48 20.92 2 3 2 3.4 Comparison of leaching performances leaching performance could be expected by optimizing the with different pore-forming methods formula of the two additives of the same mechanism. Further studies are recommended to investigate the effects A comparison of the effects of b-Al O and SiO additives of combining different additives on the leaching perfor- 2 3 2 on the leaching rate is illustrated in Fig. 19. A comparison mances of ceramic cores. of comprehensive performances of the different samples is Acknowledgements This work was supported by the National Nat- presented in Table 3. ural Science Foundation of China (Grant No. 51505457), the National Science and Technology Major Project (Grant No. 2017-VII-0008- 0101), the Key Research and Development Program of Shaanxi 4 Conclusions Province (Grant No. 2018ZDXM-GY-059), the Open Fund of State Key Laboratory of Manufacturing Systems Engineering (Grant No. SKLMS2016013), the Fundamental Research Funds for the Central The effects of different pore-forming methods on the Universities, and the Youth Innovation Team of Shaanxi Universities. leaching rates of alumina-based ceramic cores was studied, and the effects of different pore-forming methods on the Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea other properties of the cores was discussed. The addition of tivecommons.org/licenses/by/4.0/), which permits unrestricted use, carbon fiber increased the porosity of cores, thus improving distribution, and reproduction in any medium, provided you give the leaching performance. When the amount of added appropriate credit to the original author(s) and the source, provide a carbon fiber was w = 1%, the porosity and leaching rate link to the Creative Commons license, and indicate if changes were made. increased to 32.4% and 15.05%/h, respectively, whereas other performances (e.g., filling capacity and mechanical properties) were maintained. CaCO could improve the References leaching performance, but it resulted in an extremely high sintering shrinkage rate. b-Al O and SiO increased the 2 3 2 1. Najjar YSH, Alghamdi AS, Al-Beirutty MH (2004) Comparative porosity, thus improving the leaching performance. When performance of combined gas turbine systems under three dif- the mass fraction of added b-Al O and SiO were 1% and ferent blade cooling schemes. Appl Therm Eng 24(13): 2 3 2 1919–1934 3%, respectively, the leaching rate increased to 17.46%/h 2. Shi X, Agnew B, Che D (2010) Performance enhancement of and 20.92%/h, respectively, without hampering other conventional combined cycle power plant by inlet air cooling, properties. As previously mentioned, carbon fiber, CaCO , 3 inter-cooling and LNG cold energy utilization. Appl Therm Eng 30(14–15):2003–2010 b-Al O , and SiO are all additives that improve the 2 3 2 3. Lu ZL, Cao JW, Jing H et al (2013) Review of main manufac- leaching performance of a-Al O -based ceramic cores. 2 3 turing processes of complex hollow turbine blades. Virtual Phys However, the mechanisms of improvement are different. Prototyp 8(2):87–95 Carbon fibers are used to preform pores (open pores in the 4. Wu H, Li D, Tang Y et al (2010) Rapid fabrication of alumina- case), whereas CaCO , b-Al O , and SiO are based on based ceramic cores for gas turbine blades by stereolithography 3 2 3 2 and gelcasting. J Mater Process Technol 209(18–19):5886–5891 introducing easy-to-corrode phases into the a-Al O 2 3 5. Zhang H, Lu ZL, Zhe JI et al (2017) Basis for the alkaline matrix. In this study, the effect of every additive was dis- removal process design of the alumina-based ceramic core. cussed, and combinations of additives were also investi- J Ceram Soc Jpn 125:616–622 gated by mixing carbon fibers with CaCO , b-Al O , and 6. Arendt RH, Borom MP, Huseby IC et al (1978) Molten salt leach 3 2 3 for removal of inorganic cores from directionally solidified SiO additive. However, combining two additives of the eutectic alloy structures. General Electric Company, Boston same mechanism (e.g., CaCO with b-Al O , b-Al O with 3 2 3 2 3 7. Kruglov EP, Kochetova GK (2007) Improvement of a techno- SiO ) was not conducted in this study. With an increase in logical process for ceramic core removal out of internal cavities the number of design variables, further improvement of the 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 363 of aircraft GTE turbine blade castings. Rus Aeronaut (Iz VUZ) Guo-Qiang Tian received his 50(2):227–229 Ph.D. degree in Mechanical 8. Parille DRSW, Earle AA (1998) Method of core leach. United Engineering from Xi’an Jiao- States: United Technologies Corporation, Hartford tong University, P. R. China. He 9. Sangeeta D, Niskayuna N (1998) Method of dissolving or is currently a lecturer at leaching ceramic cores in airfoils. United States: General Electric Zhengzhou University of Aero- Company, Schenectady nautics, P. R. China. His 10. Schlienger MEN, Baldwin Michael D, Eugenio A (2004) Method research interest is additive and apparatus for removing ceramic material from cast compo- manufacturing technology of nents. United States: Rolls-Royce Corporation, Indianapolis ceramic mold. 11. Wu HH, Li DC, Tang YP et al (2011) Improving high tempera- ture properties of alumina based ceramic cores containing yttria by vacuum impregnating. Mater Sci Technol-lond 27(4):823–828 12. Lu Z, Tian G, Wan W et al (2016) Effect of in situ synthesised mullite whiskers on the high-temperature strength of Al O -based 2 3 ceramic moulds for casting hollow turbine blades. Ceram Int 42:18851–18858 Yang Lu received his M.S. 13. Tian G, Lu Z, Miao K et al (2015) Formation mechanism of degree in Mechanical Engi- cracks during the freeze drying of gelcast ceramic parts. J Am neering from Xi’an Jiaotong Ceram Soc 98(10):3338–3345 University, P. R. China. He is 14. Miao K, Lu Z, Cao J et al (2016) Effect of polydimethylsiloxane currently an Engineer at Tianjin on the mid-temperature strength of gelcast Al O ceramic parts. 2 3 infinity Industrial Technology Mater Design 89:810–814 Co., Ltd, P. R. China. His 15. Kiennemann J, Chartier T, Pagnoux C et al (2005) Drying research interest is additive mechanisms and stress development in aqueous alumina tape manufacturing technology (3D casting. J Eur Ceram Soc 25(9):1551–1564 printing). 16. Fukasawa T, Ando M, Ohji T et al (2010) Synthesis of porous ceramics with complex pore structure by freeze-dry processing. J Am Ceram Soc 84(1):230–232 17. Zhang D, Zhang Y, Xie R et al (2012) Freeze gelcasting of aqueous alumina suspensions for porous ceramics. Ceram Int 38(7):6063–6066 18. Qin Y, Pan W (2009) Effect of silica sol on the properties of Kai Miao received his Ph.D. alumina-based ceramic core composites. Mater Sci Eng, A degree in Mechanical Engi- 508(1–2):71–75 neering from Xi’an Jiaotong 19. Kim YH, Yeo JG, Choi SC (2016) The effect of fused silica University, P. R. China. He is crystallization on flexural strength and shrinkage of ceramic cores currently a Post-Doctor at Xi’an for investment casting. J Korean Ceram Soc 53:246–252 Jiaotong University, P. R. China. His research interest is additive manufacturing tech- Wei-Jun Zhu received his nology (3D printing). Ph.D. degree in Mechanical Engineering from Xi’an Jiao- tong University, P. R. China. He is currently an Associate Pro- fessor at Beihang University, P. R. China. His research interests include additive manufacturing technology (3D printing) and its Di-Chen Li received his Ph.D. applications in aerospace. degree in Mechanical Engi- neering from Xi’an Jiaotong University, P. R. China. He is currently a Professor and Director of State Key Lab for Manufacturing System Engi- neering at Xi’an Jiaotong University, P. R. China. His research interests include addi- tive manufacturing technology (3D printing), Bio-fabrication and Shaping of composite materials. http://www.deepdyve.com/assets/images/DeepDyve-Logo-lg.png Advances in Manufacturing Springer Journals

Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing

Download PDF
11 pages

Loading next page...
 
/lp/springer-journals/leaching-improvement-of-ceramic-cores-for-hollow-turbine-blades-based-NVZpC790J7

References (23)

Publisher
Springer Journals
Copyright
Copyright © 2019 by The Author(s)
Subject
Engineering; Manufacturing, Machines, Tools, Processes; Control, Robotics, Mechatronics; Nanotechnology and Microengineering
ISSN
2095-3127
eISSN
2195-3597
DOI
10.1007/s40436-019-00273-2
Publisher site
See Article on Publisher Site

Abstract

Adv. Manuf. (2019) 7:353–363 https://doi.org/10.1007/s40436-019-00273-2 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 1,2 2,3 2 2 2 • • • • Wei-Jun Zhu Guo-Qiang Tian Yang Lu Kai Miao Di-Chen Li Received: 26 February 2019 / Revised: 3 April 2019 / Accepted: 21 July 2019 / Published online: 26 September 2019 The Author(s) 2019 Abstract The precision casting method based on alumina- Keywords Alumina-based ceramic core  Leaching based ceramic cores is one of the main techniques used to Alkali solution  Additive manufacturing (AM) manufacture hollow turbine blades. Additive manufactur- Stereolithography ing (AM) technology provides an alternate solution to fabricating ceramic cores quickly and precisely. As the complexity of the structure increases and the strength of the 1 Introduction material improves, the leaching process of the cores becomes more complicated. This study proposes a com- Currently, directional column- and single-crystal hollow pound pore-forming method to increase the porosity of turbine blades for aero engines use ceramic cores with high ceramic cores by adding a preformed-pore agent and performance (e.g., silica, alumina). Because of their high materials that convert to easy-to-corrode phases. The pre- chemical stability and good mechanical properties at high formed-pore agents (e.g., carbon fibers) can be burned off temperatures, alumina-based ceramic cores are commonly during sintering to form pores before the leaching, and the used in specific fields [1–3]. Traditionally, fabrication of easy-to-corrode phases (e.g., CaCO , SiO , b-Al O ) can the ceramic cores involves time-consuming procedures and 3 2 2 3 be leached firstly to form pores during the leaching process. low processing flexibility. The preparation of integral The pores formed in the aforementioned two stages ceramic molds based on additive manufacturing (AM) increase the contact area of the cores and leaching solution, technology provides a new method for fabricating ceramic thus improving the leaching rate. In the current study, the cores with complex cooling channels efficiently [4]. additive amount of the preformed-pore agent was opti- Removing alumina-based ceramic cores, as represented by mized, and the effect of the easy-to-corrode phases on the chemical leaching, is the key process after casting. How- comprehensive properties of the cores was then compared. ever, high chemical stability makes the removal of cores Based on this, the corresponding model was established. particularly difficult. However, as the complexity of the structure increases and the strength of the material improves, the leaching process of the AM-based cores becomes more difficult [5]. One method to improve the chemical reaction is by selecting superior leaching solu- & Wei-Jun Zhu wjzhu@xjtu.edu.cn tions. Some types of effective leaching solutions indeed exist (i.e., hydrofluoric acid), which can concert Al O into 2 3 School of Mechanical Engineering and Automation, Beihang 3? soluble Al to remove alumina-based ceramic cores effi- University, Beijing 100191, People’s Republic of China ciently [6]. The basis reaction equation is Li AlF 3 6- State Key Laboratory for Manufacturing Systems - ? 2Al O = 3LiAlO ? 2AlF . However, the F ion has 2 3 2 3 Engineering, Xi’an Jiaotong University, Xi’an 710049, severely harmful effects on the human body and environ- People’s Republic of China ment. Therefore, this method has been abandoned for many School of Aeronautical Engineering, Zhengzhou University years [7]. Alkali-boiling leaching is now a commonly used of Aeronautics, Zhengzhou 450046, People’s Republic of method. The main research direction is now divided into China 123 354 W.-J. Zhu et al. two categories. One is to increase the leaching pressure, 2 Materials and methods thereby increasing the temperature of the alkali solution and thus the reaction reactivity. High pressure can result in 2.1 Raw materials increased depth of the alkali solution into the cores, thereby increasing the contact area of the alkali solution and the Alumina-based ceramic core samples were shaped using cores [8–10]. The other research direction focuses on gelcasting technology [4, 13, 14]. A ceramic slurry with a improving the pore structure by adding pore-forming u = 60% solid content was prepared according to the agents to the cores to prefabricate pores, or by adding gradations listed in Table 1, and the fused alumina was materials that will convert to easy-to-corrode phases, which used in a technically pure form. The as-shaped ceramic can be dissolved in advance in the process of leaching to core’s green body was further treated through freeze drying form pores, thus increasing the contact area of the alkali [15–17], high-temperature sintering, and a dipping pro- solution and the cores. cessing [18]. The freeze-drying process was conducted in a For the method of improving leaching pressure, signif- DTY-1SL vacuum freeze dryer. Then, the core samples icantly improving the equipment is necessary. The exis- sized (60 mm 9 10 mm 9 4 mm) were subjected to a re- tence of high pressure typically requires equipment of a baking treatment at 1 500 C for 0.5 h to model the high standard, which often poses safety hazards. On the pouring condition during a real investment casting. contrary, the method of introducing the pore structure is Through this treatment, the core samples could be used for more efficient. This method can help to improve the the etching experiment. porosity of the cores before and during the process of The material used to preform pores was a carbon fiber leaching, thus increase the contact area between the cores that was easy to burn with a diameter of approximately and leaching liquid. Still, the ceramic cores must meet the 7–10 lm and an average length of approximately 500 lm. synthesis requirements of high-temperature strength, The materials that can convert to easy-to-corrode phases deflection, sintering shrinkage rate, and so on. Thus, this included CaCO , b-Al O , and SiO powders. The mean 3 2 3 2 research represents a comprehensive optimization under a particle sizes of the CaCO , b-Al O , and SiO powders 3 2 3 2 multi-objective constraint. In theory, the higher the were 5 lm, 5 lm, and 40 lm, respectively. The carbon porosity, the higher the leaching rate. However, the sin- fiber was mixed with the ceramic slurry at w = 1%, 2% and tering shrinkage rate is bound to rise, and the high-tem- 3%, respectively, of the total mass, and the corresponding perature performance should deteriorate. Therefore, mass of the alumina powder was deducted from the 40 lm conducting systematic research on the pore-forming series. The CaCO powder was mixed with the ceramic methods without considerably affecting other properties is slurry at w = 2%, 4%, 6% and 8%, respectively, of the total necessary. mass, and the corresponding mass of the alumina powder Therefore, in this study, different pore-forming methods was deducted from the 5 lm series. The b-Al O powder 2 3 are tested systematically. In addition, a compound method was mixed with the ceramic slurry at w = 1%, 2%, 3% and of pore-forming is introduced, which involves adding an 4%, respectively, of the total mass, and the corresponding easy combustible material to achieve preformed pores mass of the alumina powder was deducted from the 5 lm while also adding materials to the cores that convert to easy series. The SiO powder was mixed with the ceramic slurry corrosion phases that can be removed prior to the process at w = 1%, 2%, 3% and 4%, respectively, of the total mass, of leaching; this is likely to increase the porosity consid- and the corresponding mass of the alumina powder was erably. This method will likely achieve maximum porosity deducted from the 40 lm series. during leaching without affecting other properties, thus achieving a higher leaching rate. In this study, the leaching rate of alumina-based ceramic cores is improved without affecting other properties. This is the basis for the study of the cores and promotes the Table 1 Particle gradation of the alumina-based ceramic slurry for gelcasting technology of precision casting of hollow turbine blades [11, 12]. Size/lm u (Solid content)/% 100 20 40 40 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 355 2.2 Sampling preparation 2.3 Leaching process Previous studies conducted by our group developed an During the removal process, the ceramic core samples were AM-based method to fabricate ceramic cores [3, 4, 14], and immersed in the w = 70% KOH solution [5]. The etching the procedure for this is shown in Fig. 1. Directly driven by experiment was conducted in an atmospheric Monel kettle CAD digital data, the mold of the turbine is formed by at a constant temperature of 220 C. stereolithography (SL), which is one of the AM processes, A series of experiments were performed to investigate and the ceramic slurry is formed by the gelcasting method. the relationships between the leaching rate and pore- After freeze drying, the ceramic core is formed by high- forming methods. In addition, the open porosity, sintering temperature sintering. The hollow blade can be cast using shrinkage rate, high-temperature strength, and high-tem- the strengthened ceramic core (see Fig. 1). perature deflection of the cores with pore-forming additive First, the resin molds for the test samples were prepared materials were measured and analyzed synthetically. using an AM apparatus (SPS600B, Xi’an Jiaotong University, Xi’an, China) with a photosensitive resin (SPR 2.4 Testing 8981, Zhengbang Technology Co., Ltd., Zhuhai, China). A ceramic slurry with a low viscosity (less than 1 Pas) and During the experiments, the open porosity of the core high solid loading (u = 60%) was prepared by ball-milling samples was measured by the Archimedes method, and the for 40 min. After degassing for 5 min, the ceramic slurry sintering shrinkage rate was measured by Vernier callipers. was poured into the resin mold and then polymerized The high-temperature strength was tested at 1 500 Cinan in situ to form wet green bodies under the action of the HSST-6003QP high-temperature stress-strain testing initiator and catalyst. After freeze drying for 24 h, the dried machine (Sinosteel Luoyang Institute of Refractories green bodies were placed in a furnace, heated to 1 100 C, Research Co., Ltd., China). The high-temperature deflec- and maintained at that temperature for 3 h to pyrolyze the tion was tested in a TDV-1600PC high-temperature ther- resin prototype and organic monomer polymers. Finally, mal deformation testing machine (Sinosteel Luoyang Institute of Refractories Research Co., Ltd., China) using to promote mullitization, the samples were sintered at 1 400 C for 3 h. ceramic samples of a nominal size of 2 mm 9 6mm 9 120 mm. The samples were mounted on a silicon-nitride ceramic fixture whose support spacing was 100 mm within the chamber of the testing machine that had been heated at Fig. 1 Procedure of AM-based method to fabricate ceramic cores (within dotted-lined box) 123 356 W.-J. Zhu et al. 1 500 C. It was maintained for 30 min, and then decreased to room temperature. Deflection tests were then performed by measuring the drop distance of the interme- diate point of the samples. The microstructure was ana- lyzed using scanning electron microscopy in an SU-8010 from Hitachi Ltd. 3 Results and discussion 3.1 Effects of preformed pores on the performances of the cores The effect of the carbon fibers on the microstructures of the ceramic molds is illustrated in Fig. 2. The presence of the Fig. 3 Open porosities of samples with different carbon fiber carbon fibers in the green bodies, as shown in Fig. 2a, led additives to a significant increase in the number of pores in the ceramic molds. It is worth mentioning that more open pores could be expected due to the high-aspect-ratio mor- phology of the carbon fibers, which would considerably enhance the leaching performance. However, the increase in porosity could possibly impair other performances of the ceramic core (e.g., the surface quality and mechanical properties). Therefore, the effects of the carbon fibers should be carefully investigated, and some trade-off must be made to balance all performances. The results of the open porosities of the core samples with the carbon fiber additive are illustrated in Fig. 3. The open porosity of the core samples without the carbon fiber additive was u = 31.3%, and it increased when the carbon fiber content was increased, indicating u = 34.6% for w = 3%. The experimental results of the leaching rate influenced by the carbon fiber additive are illustrated in Fig. 4 Effects of carbon fiber additives on the leaching rate Fig. 4. The leaching rate of the core samples without the carbon fiber additive was 13.46%/h, and it increased along with the carbon fiber, indicating v = 18.48%/h for w = 3%. Fig. 2 Microstructures of the ceramic molds affected by the carbon fibers a carbon fibers as prepared in the green body, b pores introduced by the carbon fibers after burning out 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 357 Fig. 5 CT images of pores filling in the ceramic molds with different carbon fiber additives a w = 3%, b w = 2%, c w = 1% (circled zones indicate defects) Table 2 Performances of the samples before and after the carbon fiber was added w (Carbon High-temperature High-temperature Sintering fiber)/% strength/MPa deflection/mm shrinkage rate/% 0 23.8 0.41 0.13 1 23.2 0.43 0.15 With the carbon fiber additive, the viscosity of the ceramic slurry increased, which decreased the liquidity of the ceramic slurry when it was gelcasted, resulting in an insufficiently fine structure in the mold. Figure 5 shows the filling results of the tip part in the mold with different carbon fiber additives. As illustrated in Figs. 5a and b, the Fig. 6 Effect of the CaCO additive on the leaching rate viscosity of the ceramic slurry is too high when the additive is w = 3% and 2%, respectively, and the tip parts could not from 13.46%/h to 15.05%/h without affecting the mold- be filled, leaving obvious defects in the cores. However, filling capacity of slurry and other properties. when the additive was w = 1%, the viscosity was moderate and the mold could be fully filled, as illustrated in Fig. 5c. 3.2 Effect of the compound method of pore-forming The addition of the carbon fiber can improve the leaching on the performances of the cores performance to some extent, but the increase in porosity can damage the mechanical properties. In addition, the theo- To increase the leaching rate further, based on w =1% retical solid phase content of the cores is reduced after the carbon fiber additive, materials that could convert to easy- loss of the carbon fiber, and the sintering shrinkage rate to-corrode phases were continuously added, which were increases, thus affecting the precision of the mold and leached prior to the process of leaching, with pores sub- blades. The other properties of the samples with w =1% sequently left in situ. The porosity increased further when carbon fiber were tested as presented in Table 2. It can be the materials were being leached, thus the leaching rate seen that with the carbon fiber additive, the high-tempera- was further improved. ture strength was slightly lower, and the high-temperature deflection and sintering shrinkage rate increased slightly. 3.2.1 CaCO additive However, these changes can be ignored. Therefore, the current study determined that the optimal additive amount Figure 6 illustrates the influence rule of CaCO on the of the carbon fiber was w = 1%. The leaching rate increased leaching rate. Figure 6 indicates that with the increase of 123 358 W.-J. Zhu et al. Fig. 7 Microstructures on the sample surfaces after leaching a with w(CaCO ) = 8%, b without CaCO additive 3 3 Fig. 9 Microstructure on the sample surface after sintering with w(CaCO ) = 8% additive Fig. 8 Effect of the CaCO additive on the sintering shrinkage rate CaCO , the leaching rate improved. The leaching rate of the core samples without the CaCO additive was 15.05%/ h, and it increased when the amount of CaCO additive was increased, with 21.42%/h for w = 8%. Figure 7 illustrates the microstructure morphologies of ceramic samples after leaching without CaCO and with w =8% CaCO . As shown in Fig. 7, the surface break degree of the samples with w = 8% CaCO additive was greater than that of the samples without CaCO , indicating that the cores with CaCO could be leached more easily. However, after CaCO was added, the sintering shrinkage rate of the samples significantly increased. Fig- ure 8 illustrates the effect of CaCO on the sintering shrinkage rate, which increased from 0.15% to 5.59% when w(CaCO ) = 8% was added. The sintering shrinkage rate increased significantly Fig. 10 Effect of the b-Al O additive on the leaching rate 2 3 when CaCO was added. This was because CaO, which was resulted from the decomposition of CaCO , reacts with process involves liquid phase sintering, which causes sin- Al O to generate calcium aluminate in the particle contact tering densification, thus increasing the sintering shrinkage 2 3 area. Calcium aluminate can be easily leached, but this rate. As illustrated in Fig. 9, the microstructure 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 359 Fig. 11 Microstructures on the sample surfaces after leaching a without and b with w(b-Al O )= 1% 2 3 Fig. 12 Effects of the b-Al O additive on the high-temperature 2 3 strength and deflection Fig. 14 Effect of the SiO additive on the leaching rate turbine blade’s precision casting requirements is difficult. Therefore, CaCO cannot be used to improve the leaching performance of ceramic cores. 3.2.2 b-Al O additive 2 3 The crystal structure of b-Al O is relatively loose, and the 2 3 additive’s density is 3.31 g/cm , which is less than the 3.97 g/cm of a-Al O . Therefore, the porosity will increase 2 3 as b-Al O converts to a-Al O after sintering at 1 500 C. 2 3 2 3 Therefore, the cores with b-Al O can be easily leached. 2 3 Figure 10 illustrates the influence rule of b-Al O on the 2 3 leaching rate, and shows that with the increase of b-Al O , 2 3 the leaching rate was improved. The leaching rate of the Fig. 13 Effect of the b-Al O additive on the sintering shrinkage rate 2 3 core samples without the b-Al O additive was 15.05%/h, 2 3 and it increased when the amount of b-Al O additive was 2 3 morphology of the unleached sample with w(CaCO )= 8% 3 increased, indicating 24.01%/h for w = 4%. additive means that the density is very high and the Figure 11 illustrates the microstructure morphologies of porosity is very small, indicating that CaCO promotes 3 ceramic samples after leaching without b-Al O and with 2 3 densification sintering. This means that meeting the hollow w(b-Al O ) = 1%. As shown in Fig. 11, the surface of the 2 3 123 360 W.-J. Zhu et al. Fig. 15 Microstructures of the sample surface after leaching a without and b with w(SiO )= 3% Figures 12 and 13 illustrate the influence rules of b- Al O on the high-temperature strength, high-temperature 2 3 deflection, and sintering shrinkage rate. With the addition of b-Al O , the high-temperature strength decreased, and 2 3 the high-temperature deflection and sintering shrinkage rate increased. When the mass fraction of b-Al O additive 2 3 increased to 2%, the high-temperature strength decreased considerably, and the deflection increased rapidly. How- ever, when the mass fraction of b-Al O additive was 1%, 2 3 the leaching rate reached 17.46%/h, and the high-temper- ature strength, deflection, and sintering shrinkage rate were 20.89 MPa, 0.47 mm and 0.17%, respectively, which met the hollow turbine blade’s precision casting requirements. Fig. 16 Effects of the SiO additive on the high-temperature strength and deflection 3.2.3 SiO additive Figure 14 illustrates the influence rule of SiO on the leaching rate. Figure 14 indicates that the leaching rate of the core samples without the SiO additive was 15.05%/h, and it increased when the amount of the SiO additive was increased, indicating 22.04%/h for w = 4%. Figure 15 illustrates the microstructure morphologies of ceramic samples after leaching without SiO and with w(SiO ) = 3%. As shown in Fig. 15, the surface of the samples with w(SiO ) = 3% additive was greater than that of the samples without SiO , indicating that the cores with SiO could be more easily leached because the porosity was improved when SiO was leached in the early stage of the leaching process. Figures 16 and 17 illustrate the influence rules of SiO on the high-temperature strength, high-temperature Fig. 17 Effect of the SiO additive on the sintering shrinkage rate deflection, and sintering shrinkage rate. With the addition of SiO , the high-temperature strength decreased, and the samples with w(b-Al O ) = 1% additive was greater than 2 3 high-temperature deflection and sintering shrinkage rate that of the samples without b-Al O , indicating that the 2 3 increased. When the mass fraction of SiO additive cores with b-Al O could be more easily leached because 2 3 increased to 4%, the high-temperature strength decreased the porosity was improved by having b-Al O leached in 2 3 significantly, and the deflection increased rapidly. The the early stage of the leaching process. 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 361 Fig. 18 Schematic of the increase of the leaching rate by the compound pore-forming method 3.3 Mechanism of pore-forming methods to improve leaching performance In theory, the higher the porosity, the larger is the contact area between the leaching liquid and the cores and the higher is the leaching rate. Based on the w(C fiber) = 1%, adding materials that can convert to easy-to-corrode phases can improve the leaching performance of the cores. This is because the compound pore-forming method can achieve preformed pores prior to the leaching. In addition, the easy corrosion phases are leached in advance, and thus more pores are generated, thus increasing the leaching rate. The principles behind the process are illustrated in Fig. 18. In a typical condition, combining the two mech- anisms, such as performed pores and easy-to-corrode Fig. 19 Effects of b-Al O and SiO additives on the leaching rate 2 3 2 phases, improve the leaching performance. For the former, carbon fibers are used as the additives and open pores are produced by burning out carbon fibers during sintering. For reason for this is that the presence of a few low-melting- the latter, more pores are introduced by eroding the easy- point phases in SiO can help to reduce the high-tem- to-corrode phases, such as CaCO , b-Al O , and SiO . 3 2 3 2 perature strength, accelerate the creep, and promote the The chemical reaction during the process is sintering, leading to an increased sintering shrinkage rate. Simultaneously, SiO transformed into cristobalite during 2 2KOH þ Al O ¼ 2KAlO þ H O: ð1Þ 2 3 2 2 the sintering process [19], which severely shrank when The removal efficiency is described quantitatively as the cooled to 180–270 C. The shrinkage could easily pro- leaching rate by measuring the dry weights before and after duce microcracks, thus damaging the high-temperature the removal process, and the leaching rate (v) is calculated strength and improving the sintering shrinkage rate. by the following equation However, when the mass fraction of SiO additive was m  m 1 2 3%, the leaching rate reached 20.92%/h, and the high- m ¼  100%; ð2Þ m  t temperature strength, deflection, and sintering shrinkage rate were 20.60 MPa, 0.48 mm and 0.20%, respectively, where m and m are the weights of the core samples before 1 2 which met the hollow turbine blade’s precision casting and after the leaching process, respectively, and t is the requirements. etching time. 123 362 W.-J. Zhu et al. Table 3 Performances of alumina samples with different additives Samples Performances Sintering shrinkage High-temperature High-temperature Leaching -1 rate/% strength/MPa deflection/mm rate/(%h ) a-Al O 0.13 23.80 0.41 13.46 2 3 a-Al O ? C(w = 1%) 0.15 23.20 0.43 15.05 2 3 a-Al O ? C(w = 1%) ? CaCO Too high – – – 2 3 3 a-Al O ? C(w = 1%) ? b-Al O (w = 1%) 0.17 20.89 0.47 17.46 2 3 2 3 a-Al O ? C(w = 1%) ? SiO (w = 3%) 0.20 20.60 0.48 20.92 2 3 2 3.4 Comparison of leaching performances leaching performance could be expected by optimizing the with different pore-forming methods formula of the two additives of the same mechanism. Further studies are recommended to investigate the effects A comparison of the effects of b-Al O and SiO additives of combining different additives on the leaching perfor- 2 3 2 on the leaching rate is illustrated in Fig. 19. A comparison mances of ceramic cores. of comprehensive performances of the different samples is Acknowledgements This work was supported by the National Nat- presented in Table 3. ural Science Foundation of China (Grant No. 51505457), the National Science and Technology Major Project (Grant No. 2017-VII-0008- 0101), the Key Research and Development Program of Shaanxi 4 Conclusions Province (Grant No. 2018ZDXM-GY-059), the Open Fund of State Key Laboratory of Manufacturing Systems Engineering (Grant No. SKLMS2016013), the Fundamental Research Funds for the Central The effects of different pore-forming methods on the Universities, and the Youth Innovation Team of Shaanxi Universities. leaching rates of alumina-based ceramic cores was studied, and the effects of different pore-forming methods on the Open Access This article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://crea other properties of the cores was discussed. The addition of tivecommons.org/licenses/by/4.0/), which permits unrestricted use, carbon fiber increased the porosity of cores, thus improving distribution, and reproduction in any medium, provided you give the leaching performance. When the amount of added appropriate credit to the original author(s) and the source, provide a carbon fiber was w = 1%, the porosity and leaching rate link to the Creative Commons license, and indicate if changes were made. increased to 32.4% and 15.05%/h, respectively, whereas other performances (e.g., filling capacity and mechanical properties) were maintained. CaCO could improve the References leaching performance, but it resulted in an extremely high sintering shrinkage rate. b-Al O and SiO increased the 2 3 2 1. Najjar YSH, Alghamdi AS, Al-Beirutty MH (2004) Comparative porosity, thus improving the leaching performance. When performance of combined gas turbine systems under three dif- the mass fraction of added b-Al O and SiO were 1% and ferent blade cooling schemes. Appl Therm Eng 24(13): 2 3 2 1919–1934 3%, respectively, the leaching rate increased to 17.46%/h 2. Shi X, Agnew B, Che D (2010) Performance enhancement of and 20.92%/h, respectively, without hampering other conventional combined cycle power plant by inlet air cooling, properties. As previously mentioned, carbon fiber, CaCO , 3 inter-cooling and LNG cold energy utilization. Appl Therm Eng 30(14–15):2003–2010 b-Al O , and SiO are all additives that improve the 2 3 2 3. Lu ZL, Cao JW, Jing H et al (2013) Review of main manufac- leaching performance of a-Al O -based ceramic cores. 2 3 turing processes of complex hollow turbine blades. Virtual Phys However, the mechanisms of improvement are different. Prototyp 8(2):87–95 Carbon fibers are used to preform pores (open pores in the 4. Wu H, Li D, Tang Y et al (2010) Rapid fabrication of alumina- case), whereas CaCO , b-Al O , and SiO are based on based ceramic cores for gas turbine blades by stereolithography 3 2 3 2 and gelcasting. J Mater Process Technol 209(18–19):5886–5891 introducing easy-to-corrode phases into the a-Al O 2 3 5. Zhang H, Lu ZL, Zhe JI et al (2017) Basis for the alkaline matrix. In this study, the effect of every additive was dis- removal process design of the alumina-based ceramic core. cussed, and combinations of additives were also investi- J Ceram Soc Jpn 125:616–622 gated by mixing carbon fibers with CaCO , b-Al O , and 6. Arendt RH, Borom MP, Huseby IC et al (1978) Molten salt leach 3 2 3 for removal of inorganic cores from directionally solidified SiO additive. However, combining two additives of the eutectic alloy structures. General Electric Company, Boston same mechanism (e.g., CaCO with b-Al O , b-Al O with 3 2 3 2 3 7. Kruglov EP, Kochetova GK (2007) Improvement of a techno- SiO ) was not conducted in this study. With an increase in logical process for ceramic core removal out of internal cavities the number of design variables, further improvement of the 123 Leaching improvement of ceramic cores for hollow turbine blades based on additive manufacturing 363 of aircraft GTE turbine blade castings. Rus Aeronaut (Iz VUZ) Guo-Qiang Tian received his 50(2):227–229 Ph.D. degree in Mechanical 8. Parille DRSW, Earle AA (1998) Method of core leach. United Engineering from Xi’an Jiao- States: United Technologies Corporation, Hartford tong University, P. R. China. He 9. Sangeeta D, Niskayuna N (1998) Method of dissolving or is currently a lecturer at leaching ceramic cores in airfoils. United States: General Electric Zhengzhou University of Aero- Company, Schenectady nautics, P. R. China. His 10. Schlienger MEN, Baldwin Michael D, Eugenio A (2004) Method research interest is additive and apparatus for removing ceramic material from cast compo- manufacturing technology of nents. United States: Rolls-Royce Corporation, Indianapolis ceramic mold. 11. Wu HH, Li DC, Tang YP et al (2011) Improving high tempera- ture properties of alumina based ceramic cores containing yttria by vacuum impregnating. Mater Sci Technol-lond 27(4):823–828 12. Lu Z, Tian G, Wan W et al (2016) Effect of in situ synthesised mullite whiskers on the high-temperature strength of Al O -based 2 3 ceramic moulds for casting hollow turbine blades. Ceram Int 42:18851–18858 Yang Lu received his M.S. 13. Tian G, Lu Z, Miao K et al (2015) Formation mechanism of degree in Mechanical Engi- cracks during the freeze drying of gelcast ceramic parts. J Am neering from Xi’an Jiaotong Ceram Soc 98(10):3338–3345 University, P. R. China. He is 14. Miao K, Lu Z, Cao J et al (2016) Effect of polydimethylsiloxane currently an Engineer at Tianjin on the mid-temperature strength of gelcast Al O ceramic parts. 2 3 infinity Industrial Technology Mater Design 89:810–814 Co., Ltd, P. R. China. His 15. Kiennemann J, Chartier T, Pagnoux C et al (2005) Drying research interest is additive mechanisms and stress development in aqueous alumina tape manufacturing technology (3D casting. J Eur Ceram Soc 25(9):1551–1564 printing). 16. Fukasawa T, Ando M, Ohji T et al (2010) Synthesis of porous ceramics with complex pore structure by freeze-dry processing. J Am Ceram Soc 84(1):230–232 17. Zhang D, Zhang Y, Xie R et al (2012) Freeze gelcasting of aqueous alumina suspensions for porous ceramics. Ceram Int 38(7):6063–6066 18. Qin Y, Pan W (2009) Effect of silica sol on the properties of Kai Miao received his Ph.D. alumina-based ceramic core composites. Mater Sci Eng, A degree in Mechanical Engi- 508(1–2):71–75 neering from Xi’an Jiaotong 19. Kim YH, Yeo JG, Choi SC (2016) The effect of fused silica University, P. R. China. He is crystallization on flexural strength and shrinkage of ceramic cores currently a Post-Doctor at Xi’an for investment casting. J Korean Ceram Soc 53:246–252 Jiaotong University, P. R. China. His research interest is additive manufacturing tech- Wei-Jun Zhu received his nology (3D printing). Ph.D. degree in Mechanical Engineering from Xi’an Jiao- tong University, P. R. China. He is currently an Associate Pro- fessor at Beihang University, P. R. China. His research interests include additive manufacturing technology (3D printing) and its Di-Chen Li received his Ph.D. applications in aerospace. degree in Mechanical Engi- neering from Xi’an Jiaotong University, P. R. China. He is currently a Professor and Director of State Key Lab for Manufacturing System Engi- neering at Xi’an Jiaotong University, P. R. China. His research interests include addi- tive manufacturing technology (3D printing), Bio-fabrication and Shaping of composite materials.

Journal

Advances in Manufacturing – Springer Journals

Published: Sep 26, 2019

There are no references for this article.