Additive manufacturing and performance research of aluminum-based ceramic cores

As a ceramic core for forming the cooling structure of the intrinsic cavern of turbine blades, the quality of tubular turbine blades is directly affected by their accuracy and performance. In this paper, based on the DLP photocuring technology, the 50% by volume alumina porcelain suspensions with low viscosity were successfully prepared. Secondly, the effects of printing parameters on the curing characteristics were explored to obtain the best printing parameters. The alumina porcelain properties were analyzed for the heating treatment process effect. As the temperature increases, its mechanical properties increase. In addition, the porosity of the patulous orifice reached 48%. The flexural strength of the alumina porcelain component was 26 MPa, which met the requirements of a high-temperature compound metal blade for the porcelain core.


Introduction
The aero-engine is the core component of the spacecraft, and the turbine blade that determines the actual performance of the engine is one of the most critical parts of the aero-engine [1][2][3].Manufacturing a complex cooling structure inside the blade to reduce the wall temperature of the turbine blade and improve its performance has become a critical link in advanced engine manufacturing.The shape of the internal cavity of the blade determines the cooling method of the blade, and the complexity of the inner cavity depends on the complex structure that the ceramic core can manufacture.Therefore, the ceramic cores become one of the bottlenecks in making blades.The conventional fabricating process of porcelain core mainly includes heated pressing injection molding, transfer molding core, grouting molding, and gel injection molding.The ceramic core produced by traditional methods has the merit of eximious fabrication exactness, uniform size, and mass production.However, they also have a long development cycle and the process is complex and challenging to control [4].It is difficult to manufacture hollow turbine blades with complex cavities.It needs to design and manufacture expensive molds.Therefore, it is urgent to explore a new ceramic core manufacturing method to make up for the defects of the traditional preparation process.The emergence of ceramic additive manufacturing technology (3D printing) provides an excellent solution to these problems in time.
Stereolithography additive manufacturing technology has the characteristics of high manufacturing precision, considerable design freedom, fast forming speed of complex structures, and fast iteration of products [5][6][7].It can compensate for the shortcomings of ceramic core manufacturing complex cavity structures, complex processes, long cycles, and high costs.Light curing technology reduces the process of core pressing, mold preparation, body trimming, and molding filler preparation, thereby reducing the time cost of mold manufacturing and core processing, which provides excellent convenience for rapid verification and iteration of turbine blades.Therefore, the application of photocuring technology to manufacture ceramic cores has unique technical advantages and potential, and it has also become one of the research hotspots of ceramic core manufacturing.
Many universities and research institutions have used photocuring ceramic additive manufacturing technology to prepare ceramic cores.Halloran's team at the University of Michigan [8] successfully prepared an integrated ceramic core-shell by the SLA process using fused silica as a printing material.The in-layer plane size of the printed blank is about 0.7% smaller than the model size, while the size perpendicular to the printing layer is 0.3% larger than the model size.After sintering, the sintering shrinkage in these two directions is about 10.7 ± 0.2%.Bae et al. [9] studied the cristobalite conversion rate and strength of the integrated core at elevated temperatures.The results show that due to the shrinkage of β-type to α-type during the transformation process, many micro-cracks are formed.Regarding aluminum-based ceramics, Li et al. [10] explored the affection of temperature on the properties of alumina ceramics manufactured by photocuring 3D printing technology.When the sintering temperature was 1280°C, the flexural strength of the alumina porcelain component was 24 MPa.The porosity of the patulous orifice was 37.6%, while the shrinkage rates in the long, wide, and high directions were 2.1%, 2.3%, and 3.8%, respectively.The research on the aluminum-based core is relatively few and not systematic.Therefore, detailed research is necessary.
Based on the DLP photocuring technology, this paper focuses on the key issues affecting the manufacturing accuracy in the additive fabricating process of aluminum-based ceramic cores and explores the slurry properties, sintering process control, and molding accuracy control of alumina porcelain cores.

2.1
Slurry preparation Alumina porcelain suspension contains alumina powder (D 50 = 500 nm), light-sensitive colophony, and affix.The alumina suspension can be up to 50% by volume.Figure 1 (a) exhibits the alumina porcelain particle size distribution.Alumina powder has a lognormal granum allocation.Figure 1 (b) shows the alumina porcelain scanning electron microscope (SEM) micrograph.Based on the previous work [11], photosensitive resins were prepared.Acidic copolymers are used as dispersants.The addition of dispersant is 2 wt.% of the total weight of the ceramic powder.Irgacure 819 photoinitiator and photoinhibitor purchased from Aladdin were used.The amount of photoinitiator and photoinhibitor was 0.5 wt.% and 0.02 wt.% of light-sensitive colophony, respectively.

2.2
3D printing preparation process Al 2 O 3 particles, photosensitive resin, photoinitiator, and the dispersing agent were blended in a planetary mill for 5 hours.Alumina porcelain printing suspension was then defoamed in a vacuum for 10 minutes.Alumina ceramic samples were fabricated by the DLP fabrication machine.The machine has a 405 nm ray origin and a 50 μm pixel DLP chip.

2.3
Degreasing Based on thermogravimetric analysis, the photosensitive resin thermolysis is localized at 450°C.At 600°C, the photosensitive resin thermolysis is essentially finished.So we set the temperature holding point at 450°C and the temperature holding point at 600°C.First of all, the alumina porcelain raised the temperature to 450°C, and the warming velocity was 0.5 °C/min.The alumina porcelain was kept warm for 120 minutes.Second, the alumina porcelain was enhanced to 600 °C at a warming velocity of 1 °C/min.The alumina porcelain was kept warm for 120 minutes.Finally, the alumina porcelain was then enhanced to 1000°C for 120 minutes at a warming velocity of 2 °C/min.The sintering alumina porcelain was set to decrease to 600°C with a cooling velocity of 2 °C/min.The samples were then cooled naturally to room temperature.

2.4
Characterization The laser particle size analyzer was employed to analyze the alumina porcelain particle size and distribution (Winner 2000ZDE).The TGA equipment was used to investigate the weight loss behavior of the suspension when heated to 1000°C (NETZCH STA 449F5).By using the Archimedes method, the porosity and density of alumina ceramics were tested.SEM (Hitachi S-8020) was employed to examine the printed alumina porcelain fracture.The Instron 5965 machine was utilized for the bending test.An average of ten samples is taken for each flexural test result.

3.1
Performance analysis of slurry Before printing alumina-based ceramic cores by the DLP process, preparing photocuring slurry with good curing performance, suitable viscosity, and high solid content is necessary.Well-dispersed photocuring slurry is the key to obtaining high-density and uniform microstructure of ceramic products.It is also an essential factor in improving the macroscopic properties of materials.The first step was to conFigure the 50% by-volume alumina porcelain suspension.The viscosity of the slurry was tested as shown in Figure 2. When no dispersant is added, The alumina porcelain suspension exhibits high viscosity, with a static viscosity reaching 27,500 cp.Viscosity decreases as the shear rate increases.When 2wt.% BYK111 was added, the rheology of the slurry was improved.To further control the thixotropic properties of the slurry, thixotropic agents were added to the slurry.Before the printing process, it is essential to assess the curing depth of the alumina suspension, with the corresponding outcomes presented in Figure 3 and Table 1.After a curing time of 0.01 s, the alumina slurry can achieve a curing depth of 100 μm.As the illumination time increases, the curing depth is further improved.When the illumination time is 0.52 s, the single-layer printed sample warps.
The single-layer printed sample cracks when the illumination time increases to 0.56 s.When the illumination time increases to more than 0.6 s, the single-layer printed sample appears overexposed.Finally, the printing parameters were selected as 0.4 s and the printing layer thickness was 50 μm, which can ensure that each layer can be fully bonded.Thermogravimetric analysis should be carried out before degreasing the printed body.The appropriate degreasing process should be selected according to the thermal degradation curve so that the organic matter can be slowly degraded and discharged from the body to reduce the phenomenon of degreasing cracking.Thermogravimetric analysis of the suspension is shown in Figure 4.The decomposition of most photosensitive resins typically occurs at 450°C, with the process essentially completed by 600°C.To allow the resin to decompose adequately during the heating process, the heating rate should be set relatively slowly before 450°C.After 600°C, the resin in the printing body is completely volatilized.

3.2
Performance analysis of sintered samples The experiment involved examining the shrinkage, density, and porosity of alumina porcelain under various sintering temperatures, as detailed in Table 2.The alumina porcelain shrinkage rate is also enhanced by raising the sintering temperature.The abnormality at 1400°C is attributed to the sintering expansion phenomenon observed in the alumina porcelain.Furthermore, the shrinkage of alumina porcelain is consistent across all three dimensions, which can ensure that the accuracy of the spline is within the predictable range after sintering.Concurrently, with the elevation of sintering temperature, there is a corresponding increase in the alumina porcelain density.The trend is in line with the rate of contraction.(Each data point in  5 shows the microstructure of the alumina body after degreasing and sintering at different temperatures.There is no apparent sintering polymerization phenomenon of alumina ceramic particles after degreasing.There are some voids between alumina ceramic particles.The connection between large particles depends on the adhesion of some small alumina ceramic particles, resulting in a loose distribution of ceramic particles.At 1300°C, the alumina ceramic particles began to neck further, and the temperature also reached the diffusion activation energy of the atoms and began to diffuse and rearrange.The alumina ceramic particles sintered at 1300°C and 1400°C changed from spherical to non-spherical.The alumina porcelain particles sintered at 1500°C are further tightly bonded and the ceramic material is dense.The alumina porcelain density is enhanced by increasing the alumina ceramic sintering temperature.The alumina porcelain properties are improved by the close arrangement of the ceramic grains. .The bending strength of alumina after different heat treatment temperatures is shown in Figure 6.The ceramic bending strength increases as the temperature increases.The flexural strength is 24 MPa at 1300°C.The scanning electron microscope also shows that many pores are initially sintered, but not dense sintered.At the same time, it is observed from the picture that the particle shape of alumina begins to change from spherical to non-spherical.When the temperature rises to 1400°C, many spherical particles are transformed into non-spherical particles, which expands the sample volume, and the bending strength increases slightly, only 26 MPa.The alumina porcelain particles are tightly bonded at 1500°C.The ceramic grains are closely arranged.The flexural strength upgraded to 105 MPa.Combined with the performance of the core, the appropriate bending strength is between 20 and 30 MPa.Considering its dissolution performance, its porosity needs to be greater than 35%.Therefore, the 1300°C and 1400°C samples can meet the requirements of the core.The alumina porcelain core was heated to 1400°C and the shrinkage rate was degraded.This temperature is the best heat treatment temperature for aluminum-based cores.

3.3
Printing accuracy analysis There are many thin-walled structures in the core model.Before printing the ceramic core of a complex structure, the limiting conditions of the photocuring process on thin-walled structures should be first explored.As shown in Figure 7 8, there are seven components.The two cylinders of the alumina porcelain components are suspended.It can be seen from the physical map that although it can be printed, the interlayer bonding could be better, and there is deformation, indicating that the printing method has specific requirements for the shape and size of the suspended part.It can be seen from these components that the accuracy of 3D printing is very high, and the hollow circular hole has a high resolution.Figure 9 shows the morphology of the pore structure in detail.It can be seen that the difference between the printed apertures and the apertures designed for the model is very small.After printing, the pores of 300 μm shrink to 263 μm, and the pores of 600 μm shrink to 516 μm.By designing different pore size structures for printing, it is found that the pore size after printing is less different from the pore size designed by the model.Precise control of the delicate pores in the core can be achieved by the model compensation method.

Conclusions
In this paper, based on the DLP photocuring technology, alumina porcelain suspension of 50% by volume with low viscosity was successfully prepared.The printed body has a lower shrinkage rate after heat treatment at 1400°C.The porosity of the patulous orifice reached 48%.The flexural strength of the alumina porcelain component was 26 MPa, which met the requirements of the high-temperature compound metal blade for the porcelain core.At the same time, by designing and printing the typical and fine structure in the core model, the limiting conditions in the printing process are obtained, which lays a theoretical foundation for preparing high-precision complex structure ceramic cores.

Figure 2 .
Figure 2. The viscosity of alumina suspension.Before the printing process, it is essential to assess the curing depth of the alumina suspension, with the corresponding outcomes presented in Figure3and Table1.After a curing time of 0.01 s, the alumina slurry can achieve a curing depth of 100 μm.As the illumination time increases, the curing depth is further improved.When the illumination time is 0.52 s, the single-layer printed sample warps.

Figure 3 .
Figure 3. Single-layer cured sample photos of alumina slurry under different illumination time.Thermogravimetric analysis should be carried out before degreasing the printed body.The appropriate degreasing process should be selected according to the thermal degradation curve so that the organic matter can be slowly degraded and discharged from the body to reduce the phenomenon of degreasing cracking.Thermogravimetric analysis of the suspension is shown in Figure4.The decomposition of most photosensitive resins typically occurs at 450°C, with the process essentially completed by 600°C.To allow the resin to decompose adequately during the heating process, the heating rate should be set relatively slowly before 450°C.After 600°C, the resin in the printing body is completely volatilized.

Figure 5 .
Figure 5. Microstructure of alumina green body after degreasing (a) and sintered alumina porcelain at different temperatures (b) 1300°C, (c) 1400°C and (d) 1500°C.The bending strength of alumina after different heat treatment temperatures is shown in Figure6.The ceramic bending strength increases as the temperature increases.The flexural strength is 24 MPa at 1300°C.The scanning electron microscope also shows that many pores are initially sintered, but not dense sintered.At the same time, it is observed from the picture that the particle shape of alumina begins to change from spherical to non-spherical.When the temperature rises to 1400°C, many spherical particles are transformed into non-spherical particles, which expands the sample volume, and the bending strength increases slightly, only 26 MPa.The alumina porcelain particles are tightly bonded at 1500°C.The ceramic grains are closely arranged.The flexural strength upgraded to 105 MPa.Combined with the performance of the core, the appropriate bending strength is between 20 and 30 MPa.Considering its dissolution performance, its porosity needs to be greater than 35%.Therefore, the 1300°C and 1400°C samples can meet the requirements of the core.The alumina porcelain core was heated to 1400°C and the shrinkage rate was degraded.This temperature is the best heat treatment temperature for aluminum-based cores.

Figure 6 .
Figure 6.The flexure strength of alumina green body after sintering at different temperatures.
(a), three different thin-walled structures with wall thicknesses of 200 μm, 375 μm, and 500 μm are designed.When the wall thickness is 200 μm, the printed sample cannot be well-formed, as shown in the left diagram of Figure 7 (b), and the complete ring cannot be formed.It can achieve good forming when the wall thickness is 375 μm and 500 μm.The sample of model C was further observed by an optical microscope, as shown in Figure 7 (c).It was found that the thin-walled structure can be well formed and has high forming accuracy.

Figure 7 .
Figure 7. Three kinds of thin-walled models and printing effect diagram (a) Three thin-walled types of models, (b) Sample schematic diagram after printing, (c) Optical electron microscopy of Model C printed sample.Before printing complex model cores, simple models should be printed to observe the effects of different structural designs, printing effects, and sintering results.As shown in Figure8, there are seven components.The two cylinders of the alumina porcelain components are suspended.It can be seen from the physical map that although it can be printed, the interlayer bonding could be better, and there is deformation, indicating that the printing method has specific requirements for the shape and size of the suspended part.It can be seen from these components that the accuracy of 3D printing is very high, and the hollow circular hole has a high resolution.

Figure 8 .
Figure 8. Model of typical core structure and printing effect diagram (a) model, (b) photo of the corresponding sample.Figure9shows the morphology of the pore structure in detail.It can be seen that the difference between the printed apertures and the apertures designed for the model is very small.After printing, the pores of 300 μm shrink to 263 μm, and the pores of 600 μm shrink to 516 μm.By designing different pore size structures for printing, it is found that the pore size after printing is less different from the pore size designed by the model.Precise control of the delicate pores in the core can be achieved by the model compensation method.

Figure 9 .
Figure 9. Models with different pore sizes and printing effect diagram (a) model, (b) scanning electron microscope diagram corresponding to different pore sizes.

Table 1 .
The curing depth of alumina slurry under different illumination time.

Table 2
is derived from 10 samples and averaged.)

Table 2 .
The shrinkage, density, and porosity of alumina porcelain.