1 Introduction

Currently, directional column- and single-crystal hollow turbine blades for aero engines use ceramic cores with high performance (e.g., silica, alumina). Because of their high chemical stability and good mechanical properties at high temperatures, alumina-based ceramic cores are commonly used in specific fields [1,2,3]. Traditionally, fabrication of the ceramic cores involves time-consuming procedures and low processing flexibility. The preparation of integral ceramic molds based on additive manufacturing (AM) technology provides a new method for fabricating ceramic cores with complex cooling channels efficiently [4]. Removing alumina-based ceramic cores, as represented by chemical leaching, is the key process after casting. However, high chemical stability makes the removal of cores 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 solutions. Some types of effective leaching solutions indeed exist (i.e., hydrofluoric acid), which can concert Al2O3 into soluble Al3+ to remove alumina-based ceramic cores efficiently [6]. The basis reaction equation is Li3AlF6 + 2Al2O3 = 3LiAlO2 + 2AlF3. However, the F− ion has severely harmful effects on the human body and environment. Therefore, this method has been abandoned for many years [7]. Alkali-boiling leaching is now a commonly used method. The main research direction is now divided into two categories. One is to increase the leaching pressure, thereby increasing the temperature of the alkali solution and thus the reaction reactivity. High pressure can result in increased depth of the alkali solution into the cores, thereby increasing the contact area of the alkali solution and the cores [8,9,10]. The other research direction focuses on improving the pore structure by adding pore-forming agents to the cores to prefabricate pores, or by adding materials that will convert to easy-to-corrode phases, which can be dissolved in advance in the process of leaching to form pores, thus increasing the contact area of the alkali solution and the cores.

For the method of improving leaching pressure, significantly improving the equipment is necessary. The existence of high pressure typically requires equipment of a high standard, which often poses safety hazards. On the contrary, the method of introducing the pore structure is more efficient. This method can help to improve the porosity of the cores before and during the process of leaching, thus increase the contact area between the cores and leaching liquid. Still, the ceramic cores must meet the synthesis requirements of high-temperature strength, deflection, sintering shrinkage rate, and so on. Thus, this research represents a comprehensive optimization under a multi-objective constraint. In theory, the higher the porosity, the higher the leaching rate. However, the sintering shrinkage rate is bound to rise, and the high-temperature performance should deteriorate. Therefore, conducting systematic research on the pore-forming methods without considerably affecting other properties is necessary.

Therefore, in this study, different pore-forming methods are tested systematically. In addition, a compound method of pore-forming is introduced, which involves adding an easy combustible material to achieve preformed pores while also adding materials to the cores that convert to easy corrosion phases that can be removed prior to the process of leaching; this is likely to increase the porosity considerably. This method will likely achieve maximum porosity 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 technology of precision casting of hollow turbine blades [11, 12].

2 Materials and methods

2.1 Raw materials

Alumina-based ceramic core samples were shaped using gelcasting technology [4, 13, 14]. A ceramic slurry with a φ = 60% solid content was prepared according to the gradations listed in Table 1, and the fused alumina was used in a technically pure form. The as-shaped ceramic core’s green body was further treated through freeze drying [15,16,17], high-temperature sintering, and a dipping processing [18]. The freeze-drying process was conducted in a DTY-1SL vacuum freeze dryer. Then, the core samples sized (60 mm × 10 mm × 4 mm) were subjected to a re-baking treatment at 1 500 °C for 0.5 h to model the pouring condition during a real investment casting. Through this treatment, the core samples could be used for the etching experiment.

Table 1 Particle gradation of the alumina-based ceramic slurry for gelcasting
Full size table

The material used to preform pores was a carbon fiber that was easy to burn with a diameter of approximately 7–10 μm and an average length of approximately 500 μm. The materials that can convert to easy-to-corrode phases included CaCO3, β-Al2O3, and SiO2 powders. The mean particle sizes of the CaCO3, β-Al2O3, and SiO2 powders were 5 μm, 5 μm, and 40 μm, respectively. The carbon fiber was mixed with the ceramic slurry at w = 1%, 2% and 3%, respectively, of the total mass, and the corresponding mass of the alumina powder was deducted from the 40 μm series. The CaCO3 powder was mixed with the ceramic slurry at w = 2%, 4%, 6% and 8%, respectively, of the total mass, and the corresponding mass of the alumina powder was deducted from the 5 μm series. The β-Al2O3 powder was mixed with the ceramic slurry at w = 1%, 2%, 3% and 4%, respectively, of the total mass, and the corresponding mass of the alumina powder was deducted from the 5 μm series. The SiO2 powder was mixed with the ceramic slurry at w = 1%, 2%, 3% and 4%, respectively, of the total mass, and the corresponding mass of the alumina powder was deducted from the 40 μm series.

2.2 Sampling preparation

Previous studies conducted by our group developed an AM-based method to fabricate ceramic cores [3, 4, 14], and the procedure for this is shown in Fig. 1. Directly driven by CAD digital data, the mold of the turbine is formed by stereolithography (SL), which is one of the AM processes, and the ceramic slurry is formed by the gelcasting method. After freeze drying, the ceramic core is formed by high-temperature sintering. The hollow blade can be cast using the strengthened ceramic core (see Fig. 1).

Fig. 1
figure 1

Procedure of AM-based method to fabricate ceramic cores (within dotted-lined box)

Full size image

First, the resin molds for the test samples were prepared using an AM apparatus (SPS600B, Xi’an Jiaotong University, Xi’an, China) with a photosensitive resin (SPR 8981, Zhengbang Technology Co., Ltd., Zhuhai, China). A ceramic slurry with a low viscosity (less than 1 Pa·s) and high solid loading (φ = 60%) was prepared by ball-milling for 40 min. After degassing for 5 min, the ceramic slurry was poured into the resin mold and then polymerized in situ to form wet green bodies under the action of the initiator and catalyst. After freeze drying for 24 h, the dried green bodies were placed in a furnace, heated to 1 100 °C, and maintained at that temperature for 3 h to pyrolyze the resin prototype and organic monomer polymers. Finally, to promote mullitization, the samples were sintered at 1 400 °C for 3 h.

2.3 Leaching process

During the removal process, the ceramic core samples were immersed in the w = 70% KOH solution [5]. The etching experiment was conducted in an atmospheric Monel kettle at a constant temperature of 220 °C.

A series of experiments were performed to investigate the relationships between the leaching rate and pore-forming methods. In addition, the open porosity, sintering shrinkage rate, high-temperature strength, and high-temperature deflection of the cores with pore-forming additive materials were measured and analyzed synthetically.

2.4 Testing

During the experiments, the open porosity of the core samples was measured by the Archimedes method, and the sintering shrinkage rate was measured by Vernier callipers. The high-temperature strength was tested at 1 500 °C in an HSST-6003QP high-temperature stress-strain testing machine (Sinosteel Luoyang Institute of Refractories Research Co., Ltd., China). The high-temperature deflection was tested in a TDV-1600PC high-temperature thermal deformation testing machine (Sinosteel Luoyang Institute of Refractories Research Co., Ltd., China) using ceramic samples of a nominal size of 2 mm × 6 mm × 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 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 intermediate point of the samples. The microstructure was analyzed 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 carbon fibers in the green bodies, as shown in Fig. 2a, led 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 morphology 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.

Fig. 2
figure 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

Full size image

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 φ = 31.3%, and it increased when the carbon fiber content was increased, indicating φ = 34.6% for w = 3%. The experimental results of the leaching rate influenced by the carbon fiber additive are illustrated in 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. 3
figure 3

Open porosities of samples with different carbon fiber additives

Full size image
Fig. 4
figure 4

Effects of carbon fiber additives on the leaching rate

Full size image

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 viscosity of the ceramic slurry is too high when the additive is w = 3% and 2%, respectively, and the tip parts could not be filled, leaving obvious defects in the cores. However, when the additive was w = 1%, the viscosity was moderate and the mold could be fully filled, as illustrated in Fig. 5c.

Fig. 5
figure 5

CT images of pores filling in the ceramic molds with different carbon fiber additives aw = 3%, bw = 2%, cw = 1% (circled zones indicate defects)

Full size image

The addition of the carbon fiber can improve the leaching performance to some extent, but the increase in porosity can damage the mechanical properties. In addition, the theoretical solid phase content of the cores is reduced after the loss of the carbon fiber, and the sintering shrinkage rate increases, thus affecting the precision of the mold and blades. The other properties of the samples with w = 1% carbon fiber were tested as presented in Table 2. It can be seen that with the carbon fiber additive, the high-temperature strength was slightly lower, and the high-temperature deflection and sintering shrinkage rate increased slightly. However, these changes can be ignored. Therefore, the current study determined that the optimal additive amount of the carbon fiber was w = 1%. The leaching rate increased from 13.46%/h to 15.05%/h without affecting the mold-filling capacity of slurry and other properties.

Table 2 Performances of the samples before and after the carbon fiber was added
Full size table

3.2 Effect of the compound method of pore-forming on the performances of the cores

To increase the leaching rate further, based on w = 1% carbon fiber additive, materials that could convert to easy-to-corrode phases were continuously added, which were leached prior to the process of leaching, with pores subsequently left in situ. The porosity increased further when the materials were being leached, thus the leaching rate was further improved.

3.2.1 CaCO3 additive

Figure 6 illustrates the influence rule of CaCO3 on the leaching rate. Figure 6 indicates that with the increase of CaCO3, the leaching rate improved. The leaching rate of the core samples without the CaCO3 additive was 15.05%/h, and it increased when the amount of CaCO3 additive was increased, with 21.42%/h for w = 8%.

Fig. 6
figure 6

Effect of the CaCO3 additive on the leaching rate

Full size image

Figure 7 illustrates the microstructure morphologies of ceramic samples after leaching without CaCO3 and with w = 8% CaCO3. As shown in Fig. 7, the surface break degree of the samples with w = 8% CaCO3 additive was greater than that of the samples without CaCO3, indicating that the cores with CaCO3 could be leached more easily.

Fig. 7
figure 7

Microstructures on the sample surfaces after leaching a with w(CaCO3) = 8%, b without CaCO3 additive

Full size image

However, after CaCO3 was added, the sintering shrinkage rate of the samples significantly increased. Figure 8 illustrates the effect of CaCO3 on the sintering shrinkage rate, which increased from 0.15% to 5.59% when w(CaCO3) = 8% was added.

Fig. 8
figure 8

Effect of the CaCO3 additive on the sintering shrinkage rate

Full size image

The sintering shrinkage rate increased significantly when CaCO3 was added. This was because CaO, which was resulted from the decomposition of CaCO3, reacts with Al2O3 to generate calcium aluminate in the particle contact area. Calcium aluminate can be easily leached, but this process involves liquid phase sintering, which causes sintering densification, thus increasing the sintering shrinkage rate. As illustrated in Fig. 9, the microstructure morphology of the unleached sample with w(CaCO3) = 8% additive means that the density is very high and the porosity is very small, indicating that CaCO3 promotes densification sintering. This means that meeting the hollow turbine blade’s precision casting requirements is difficult. Therefore, CaCO3 cannot be used to improve the leaching performance of ceramic cores.

Fig. 9
figure 9

Microstructure on the sample surface after sintering with w(CaCO3) = 8% additive

Full size image

3.2.2 β-Al2O3 additive

The crystal structure of β-Al2O3 is relatively loose, and the additive’s density is 3.31 g/cm3, which is less than the 3.97 g/cm3 of α-Al2O3. Therefore, the porosity will increase as β-Al2O3 converts to α-Al2O3 after sintering at 1 500 °C. Therefore, the cores with β-Al2O3 can be easily leached.

Figure 10 illustrates the influence rule of β-Al2O3 on the leaching rate, and shows that with the increase of β-Al2O3, the leaching rate was improved. The leaching rate of the core samples without the β-Al2O3 additive was 15.05%/h, and it increased when the amount of β-Al2O3 additive was increased, indicating 24.01%/h for w = 4%.

Fig. 10
figure 10

Effect of the β-Al2O3 additive on the leaching rate

Full size image

Figure 11 illustrates the microstructure morphologies of ceramic samples after leaching without β-Al2O3 and with w(β-Al2O3) = 1%. As shown in Fig. 11, the surface of the samples with w(β-Al2O3) = 1% additive was greater than that of the samples without β-Al2O3, indicating that the cores with β-Al2O3 could be more easily leached because the porosity was improved by having β-Al2O3 leached in the early stage of the leaching process.

Fig. 11
figure 11

Microstructures on the sample surfaces after leaching a without and b with w(β-Al2O3) = 1%

Full size image

Figures 12 and 13 illustrate the influence rules of β-Al2O3 on the high-temperature strength, high-temperature deflection, and sintering shrinkage rate. With the addition of β-Al2O3, the high-temperature strength decreased, and the high-temperature deflection and sintering shrinkage rate increased. When the mass fraction of β-Al2O3 additive increased to 2%, the high-temperature strength decreased considerably, and the deflection increased rapidly. However, when the mass fraction of β-Al2O3 additive was 1%, the leaching rate reached 17.46%/h, and the high-temperature 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. 12
figure 12

Effects of the β-Al2O3 additive on the high-temperature strength and deflection

Full size image
Fig. 13
figure 13

Effect of the β-Al2O3 additive on the sintering shrinkage rate

Full size image

3.2.3 SiO2 additive

Figure 14 illustrates the influence rule of SiO2 on the leaching rate. Figure 14 indicates that the leaching rate of the core samples without the SiO2 additive was 15.05%/h, and it increased when the amount of the SiO2 additive was increased, indicating 22.04%/h for w = 4%.

Fig. 14
figure 14

Effect of the SiO2 additive on the leaching rate

Full size image

Figure 15 illustrates the microstructure morphologies of ceramic samples after leaching without SiO2 and with w(SiO2) = 3%. As shown in Fig. 15, the surface of the samples with w(SiO2) = 3% additive was greater than that of the samples without SiO2, indicating that the cores with SiO2 could be more easily leached because the porosity was improved when SiO2 was leached in the early stage of the leaching process.

Fig. 15
figure 15

Microstructures of the sample surface after leaching a without and b with w(SiO2) = 3%

Full size image

Figures 16 and 17 illustrate the influence rules of SiO2 on the high-temperature strength, high-temperature deflection, and sintering shrinkage rate. With the addition of SiO2, the high-temperature strength decreased, and the high-temperature deflection and sintering shrinkage rate increased. When the mass fraction of SiO2 additive increased to 4%, the high-temperature strength decreased significantly, and the deflection increased rapidly. The reason for this is that the presence of a few low-melting-point phases in SiO2 can help to reduce the high-temperature strength, accelerate the creep, and promote the sintering, leading to an increased sintering shrinkage rate. Simultaneously, SiO2 transformed into cristobalite during the sintering process [19], which severely shrank when cooled to 180–270 °C. The shrinkage could easily produce microcracks, thus damaging the high-temperature strength and improving the sintering shrinkage rate. However, when the mass fraction of SiO2 additive was 3%, the leaching rate reached 20.92%/h, and the high-temperature strength, deflection, and sintering shrinkage rate were 20.60 MPa, 0.48 mm and 0.20%, respectively, which met the hollow turbine blade’s precision casting requirements.

Fig. 16
figure 16

Effects of the SiO2 additive on the high-temperature strength and deflection

Full size image
Fig. 17
figure 17

Effect of the SiO2 additive on the sintering shrinkage rate

Full size image

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 mechanisms, such as performed pores and easy-to-corrode 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 the latter, more pores are introduced by eroding the easy-to-corrode phases, such as CaCO3, β-Al2O3, and SiO2.

Fig. 18
figure 18

Schematic of the increase of the leaching rate by the compound pore-forming method

Full size image

The chemical reaction during the process is

$$2\text{KOH} + \text{Al}_{2} \text{O}_{3} = 2\text{KAlO}_{2} + \text{H}_{2} \text{O}.$$
(1)

The removal efficiency is described quantitatively as the leaching rate by measuring the dry weights before and after the removal process, and the leaching rate (v) is calculated by the following equation

$$\nu = \frac{{m_{1} - m_{2} }}{{m_{1} \times t}} \times 100\% ,$$
(2)

where m1 and m2 are the weights of the core samples before and after the leaching process, respectively, and t is the etching time.

3.4 Comparison of leaching performances with different pore-forming methods

A comparison of the effects of β-Al2O3 and SiO2 additives on the leaching rate is illustrated in Fig. 19. A comparison of comprehensive performances of the different samples is presented in Table 3.

Fig. 19
figure 19

Effects of β-Al2O3 and SiO2 additives on the leaching rate

Full size image
Table 3 Performances of alumina samples with different additives
Full size table

4 Conclusions

The effects of different pore-forming methods on the leaching rates of alumina-based ceramic cores was studied, and the effects of different pore-forming methods on the other properties of the cores was discussed. The addition of carbon fiber increased the porosity of cores, thus improving the leaching performance. When the amount of added carbon fiber was w = 1%, the porosity and leaching rate increased to 32.4% and 15.05%/h, respectively, whereas other performances (e.g., filling capacity and mechanical properties) were maintained. CaCO3 could improve the leaching performance, but it resulted in an extremely high sintering shrinkage rate. β-Al2O3 and SiO2 increased the porosity, thus improving the leaching performance. When the mass fraction of added β-Al2O3 and SiO2 were 1% and 3%, respectively, the leaching rate increased to 17.46%/h and 20.92%/h, respectively, without hampering other properties. As previously mentioned, carbon fiber, CaCO3, β-Al2O3, and SiO2 are all additives that improve the leaching performance of α-Al2O3-based ceramic cores. However, the mechanisms of improvement are different. Carbon fibers are used to preform pores (open pores in the case), whereas CaCO3, β-Al2O3, and SiO2 are based on introducing easy-to-corrode phases into the α-Al2O3 matrix. In this study, the effect of every additive was discussed, and combinations of additives were also investigated by mixing carbon fibers with CaCO3, β-Al2O3, and SiO2 additive. However, combining two additives of the same mechanism (e.g., CaCO3 with β-Al2O3, β-Al2O3 with SiO2) was not conducted in this study. With an increase in the number of design variables, further improvement of the leaching performance could be expected by optimizing the formula of the two additives of the same mechanism. Further studies are recommended to investigate the effects of combining different additives on the leaching performances of ceramic cores.