1 Introduction

With the rapid development of aerospace, automotive, and communications industries, light alloy castings, such as aluminum, and magnesium alloy die castings have been widely used in these fields [1,2,3]. However, in order to further pursue the structural rationality and lightweight of the key parts, the light alloy die castings tend to be integrated and complicated, resulting in more complex internal cavity channels inside the castings, which put forwards higher requirements for the core materials used to form complex hollow structure castings [4,5,6]. Normally, the sand cores cannot meet the conditions of industrial applications due to their lower strength and poor collapsibility, while the ceramic cores have poor collapsibility in spite of sufficient strength [7,8,9,10,11].

The water-soluble salt cores have attracted considerable attention due to their excellent properties including high strength, low gas evolution, and good water-soluble collapsibility, which are desired for the preparation of complex hollow composite structure castings [12,13,14,15]. Liu and Gong et al. [16, 17] prepared KNO3-based water-soluble salt cores with bending strength of 42–46 MPa for zinc alloy die castings by gravity casting method. Yaokawa et al. [13] fabricated salt cores composed of alkali carbonate and alkali chloride with bending strength of 20–30 MPa for aluminum alloy die castings by gravity casting method. Jelínek et al. [18] reported NaCl and KCl salt cores with bending strength of 5.3–10 MPa through squeezing technology using alkali silicates as a binder. Sakoda et al. [19] prepared NaCl and KCl salt cores with bending strength of 20–37 MPa for aluminum alloy die castings by pressing sintering method. Among all these researches, the bending strength of the salt cores fabricated by gravity casting method or pressing sintering method had exceeded 20 MPa, which can be used for the manufacture of aluminum alloy die castings. However, these traditional methods have some disadvantages, such as necessary molds, high cost, and high energy consumption. Especially, the geometric design and structural complexity of salt core products are limited by the current forming processes, which seriously impedes its industrial application. Therefore, it is of great significance to explore a new forming process to prepare complex water-soluble salt cores with high strength and low cost.

Material extrusion (ME), as a kind of additive manufacturing technology, can freely form parts without molds [20, 21], which provides a new direction for the preparation of complex water-soluble salt cores with a low cost. In the ME technology, the solid powder materials are made into a uniformly dispersed slurry, and then the slurry is continuously extruded to form components under certain pressures according to the designed geometry [22, 23]. In recent years, this technology has been used to prepare high porosity structural parts and biomedical scaffolds with NaCl, CaCl2, and CuSO4 as the matrixes [24,25,26]. Due to the excellent water solubility of inorganic salt materials, all these studies have focused on the spatial network structure but ignoring improving the mechanical properties of the salt-based templates. The salt core materials and sintering process are critical to the mechanical properties of the water-soluble salt core. However, there are few reports on the preparation of high-strength water-soluble salt cores through the ME method.

In this paper, a novel method was developed to fabricate high-strength salt cores by material extrusion process using Na2SO4 powder and NaCl powder as the matrix material, and the bauxite powder was selected as the reinforcing material. The effects of bauxite powder content and liquid phase sintering parameters on the bending strength, linear shrinkage, water solubility, and porosity of the salt cores were investigated. The optimized process parameters were obtained. Finally, the complex water-soluble salt core components with high strength were prepared under optimized experimental conditions. This method provides a new way to obtain complex water-soluble salt cores with high strength and low cost, which has important theoretical and practical significance for the water-soluble salt cores in industrial development.

2 Materials and methods

2.1 Raw materials

In this study, water-soluble sodium sulfate powder (Na2SO4, 95 wt.% purity, D50 = 12.5 μm) and sodium chloride powder (NaCl, 99 wt.% purity, D50 = 75 μm) were selected as the matrix materials. Bauxite powder (D50 = 45 μm, Zhengzhou Xiangyu Casting Material Co., Ltd., China) was selected as the reinforcing material. Anhydrous ethanol (EtOH) (analytical purity, Sinopharm Chemical Reagent Co., Ltd., China) was adopted as a carrier liquid for preparing the salt-based slurry. Polyvinyl pyrrolidone (PVP) (Shandong Yousuo Chemical Technology Co., Ltd., China) was used as a binder, and methyl silicone oil was used as a release agent. The compositions of the salt-based slurry are listed in Table 1, and the mole percentage of Na2SO4 and NaCl is 30:70 mol%.

Table 1 Compositions of the salt-based slurry
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2.2 Preparation process

Figure 1 depicts the schematic diagram of preparing water-soluble salt cores via ME technology. The preparation process of the salt cores mainly includes salt-based slurry preparation, extrusion forming, and sintering. The solid powder (Na2SO4, NaCl, and bauxite) was first added to an anhydrous ethanol solution with 15 wt.% PVP and stirred manually, and then ball milling was carried out in a planetary mixer for 8 h at 180 rpm to obtain a stable salt-based slurry. To follow, the salt-based slurry was transferred to an extrusion device made by our research group, which is modified from a dispenser machine and driven by air pressure. The salt-based slurry was extruded according to the designed route by pneumatic actuation to form the required samples at room temperature, and the printing parameters of the ME technology are shown in Table 2. Finally, the salt core green body was dried in drying oven at 50 ℃ for 8 h, and the dried green body was sintered at the set temperature to obtain salt core samples.

Fig. 1
figure 1

Schematic diagram of the ME technology to prepare water-soluble salt cores

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Table 2 Printing parameters for salt-based slurry
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2.3 Measurement and characterization

A stress-controlled rheometer (DHR-2, TA, USA) was used to test the rheological behaviors of the salt-based slurry with a parallel plate of 25 mm in diameter and a testing gap of 1000 μm. An ElectroPuls all-electric dynamic and fatigue test system (ElectroPuls E1000, Instron, USA) was used to measure the bending strength of the salt cores using a three-point-bending method with a span of 30 mm and a pressure head loading speed of 0.5 mm/min, and five samples (50 × 7 × 7 mm in length, width, and height, respectively) were measured in each group to reduce the error. The water solubility rate (K) of the salt cores was calculated by Eq. (1):

$${K}=\frac{m}{s\times t}$$
(1)

where m and s represent the mass and total surface area of the salt core samples, respectively, and t is the dissolution time of the salt core samples in still water at 40 ℃. The linear shrinkage (L) of the salt core samples was measured according to Eq. (2):

$${L}=\frac{{l}_{1}-{l}_{2}}{{l}_{1}}\times 100\%$$
(2)

where l1 and l2 represent the length of dried samples and sintered samples, respectively. The moisture rate (φ) of the salt core samples was calculated according to Eq. (3):

$${\varphi }=\frac{{m}_{1}-{m}_{0}}{{m}_{0}}\times 100\%$$
(3)

where m0 represents the original mass of the salt core samples and m1 represents the mass of the salt core samples exposed to the air (relative humidity, 60–70%) for 2 days. The bulk density (Db), open porosity (Po), and closed porosity (Pc) of the salt core samples were tested using the Archimedes method with the equations, respectively, displayed in Eqs. (4) and (5) [27]:

$${{P}}_{{o}}=\frac{{M}_{1}-{M}_{0}}{{M}_{1}-{M}_{2}}\times 100\%$$
(4)
$${{P}}_{{c}}=\frac{{M}_{0}-{M}_{2}-{M}_{o}\times {\rho }_{0}/{\rho }_{r}}{{M}_{1}-{M}_{2}}\times 100\%$$
(5)

where M0 represents the mass of dried salt core samples, M1 is the mass of kerosene-saturated salt core samples in air, and M2 is the mass of samples in kerosene. ρ0 is the density of kerosene (0.8 g/cm3), and ρr is the real density of the salt core samples. Five tests were performed for each material state to ensure good reproducibility.

Thermogravimetric and differential thermal analysis (TG–DTA, PerkinElmer Instruments) were performed on the green body after drying to determine the sintering process, and the heating temperature was increased from room temperature to 640 °C at a rate of 10 °C/min. The compositions of the salt core samples were analyzed using X-ray diffraction (XRD-7000S, Japan) with a Cu Kα radiation. The micromorphology of the salt core samples pretreated by carbon sputtering was observed using a scanning electron microscope (SEM, Quanta 200) equipped with an energy dispersive spectrometer (EDS).

3 Results and discussion

3.1 Material extrusion of salt cores

Rheological properties of the salt-based slurry are critical to fabricate the salt cores using a ME method, and the rheological behaviors of salt-based slurry with different bauxite powder contents are shown in Fig. 2. It can be observed that the salt-based slurry with different bauxite powder contents exhibits shear thinning property, which means that the salt-based slurry can be extruded smoothly from the nozzle, and the extruded slurry will be rapidly deposited and formed without shear force. Meanwhile, it can also be seen that the viscosity and shear stress of the slurry decrease as the content of bauxite powder increases. The reason is that the density of bauxite powder (3.45 g/cm3) is greater than that of NaCl (2.16 g/cm3) and Na2SO4 (2.68 g/cm3). Increasing of bauxite powder content, the volume fraction of the solid powder (Na2SO4, NaCl, and bauxite) decreases, leading to the decrease of the solid content of the salt-based slurry.

Fig. 2
figure 2

Rheological behaviors of salt-based slurry with different bauxite powder contents: a viscosity versus shear rate and b stress versus shear rate

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The salt core green body prepared by ME technology was then sintered in order to obtain excellent mechanical property. The sintering process of the salt core green body was determined by the TG–DTA results, as shown in Fig. 3. It can be seen from Fig. 3a that the PVP in salt core green body is decomposed at about 350 ℃, and with the further increase of temperature, the salt core green body begins to melt at about 620 ℃, which is in good agreement with the solidus temperature of NaCl-Na2CO3 binary phase diagram [28]. The liquid phase sintering temperature was set to 630 ℃ based on our previous experiments. Therefore, the sintering process of the salt core green body was carried out according to Fig. 3b. First, the salt core green body was calcined at 350 ℃ for 1 h with a heating rate of 1 ℃/min to remove the PVP organic matter and then elevated the temperature to 600 ℃ at a heating rate of 3 ℃/min, and the temperature was kept for 60 min to ensure the same temperature inside and outside the salt cores. Finally, it was heated to 630 ℃ and kept for 15 min, 30 min, and 45 min, respectively, to complete the liquid phase sintering and obtain the salt core samples.

Fig. 3
figure 3

Sintering process of salt core green body: a TG–DTA curves and b sintering temperature versus sintering time

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3.2 Performance characteristics

Figure 4 shows the effects of bauxite powder content at various sintering time on bending strength and linear shrinkage of the salt core samples. It is evident that the bauxite powder content and sintering time have a remarkable influence on the bending strength and linear shrinkage of the salt core. As illustrated in Fig. 4a, when the sintering time is 15 min, the bending strength of the salt core is less than 9 MPa, and the bending strength decreases with the increase of bauxite powder content. When the sintering time is 30 min and 45 min, respectively, the bending strength of the salt core increases first and then decreases as the content of bauxite powder increases. For example, when the sintering time is 30 min, the bending strength of the salt core increases dramatically from 14.56 to 24.43 MPa with increasing the bauxite powder content from 0 to 20 wt.% and then decreases to 18.68 MPa as the bauxite powder content further increase to 30 wt.%. It should be noted that the salt core samples without bauxite powder (0 wt.%) are destroyed by excessive melting when the sintering time is 45 min.

Fig. 4
figure 4

Effects of bauxite powder content at various sintering times on the salt core samples: a bending strength and b linear shrinkage

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As illustrated in Fig. 4b, when the sintering time is constant, the linear shrinkage of the salt core decreases with the increase of bauxite powder content. When the content of bauxite powder is constant, the linear shrinkage increases with the increase of sintering time. For example, when the sintering time is 30 min, with the content of bauxite powder increasing from 0 to 30 wt.%, the linear shrinkage decreases from 10.5 to 4.2%. It is well-known that the bending strength and linear shrinkage are the main properties of the salt core. First, the salt core is a brittle material and needs high strength to resist the impact of liquid metal during high-pressure die casting. Second, low shrinkage is necessary to ensure the precision of castings.

Figure 5 shows the effects of bauxite powder content at various sintering times on open porosity and close porosity of the salt core samples. As can be seen, the open porosity decreases with the increase of sintering time, while the close porosity shows the opposite trend. When the content of bauxite powder is 20 wt.%, the open porosity decreases dramatically from 21.97 to 3.08% with increasing the sintering time from 15 to 45 min, while the closed porosity increases from 4.5 to 9.58%. When the sintering time is 15 min, the salt core possesses a higher open porosity and a lower closed porosity, which is caused by insufficient sintering time. When the sintering time is 30 min and 45 min, respectively, the open porosity increases slowly and then sharply with the increase of bauxite powder content. Such phenomenon might result from two factors. On the one hand, the bauxite powder is a refractory material that will improve the thermal stability of the salt core. On the other hand, the excessive bauxite powder is prone to agglomeration and would reduce the fluidity of the liquid phase [29].

Fig. 5
figure 5

Effects of bauxite powder content at various sintering times on the salt core samples: a open porosity and b close porosity

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Figure 6 depicts the effects of bauxite powder content at various sintering times on water solubility rate and moisture rate of the salt core samples. As shown in Fig. 6, the water solubility rate and the moisture rate display an overall trend of decreasing with the increase of sintering time and bauxite powder content, respectively. When the sintering time is 30 min, as the bauxite powder content increases from 0 to 30 wt.%, the water solubility rate of the salt core decreases from 336.7 to 152.6 (g/min·m2), and the moisture rate decreases from 0.49 to 0.27%. When the content of bauxite powder is 20 wt.%, with the increases of sintering time from 15 to 45 min, the water solubility rate and the moisture rate of salt core decrease from 322.1 (g/min·m2) to 205.4 (g/min·m2) and from 0.42% to 0.28%, respectively. For the water-soluble salt core, the higher water solubility rate is beneficial to the removal from the castings, and the lower moisture rate is conducive to the storage of the salt core. Generally, the higher the densification degree of the salt core, the lower its water solubility rate and moisture rate. Furthermore, the bauxite powder as an insoluble material can also reduce the water solubility rate and moisture rate of the salt core.

Fig. 6
figure 6

Effects of bauxite powder content at various sintering times on the salt core samples: a water solubility rate and b moisture rate

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In conclusion, when the sintering time and the bauxite powder content are 30 min and 20 wt.%, respectively, the obtained salt core samples possess excellent comprehensive properties, which the bending strength, linear shrinkage rate, water solubility rate, and moisture rate are 24.43 MPa, 6.3%, 207.6 (g/min·m2), and 0.29%, respectively.

3.3 Microstructure analysis

Figure 7 shows the XRD patterns of the salt core samples with different bauxite powder contents sintered for 30 min. It can be observed that the salt cores are mainly composed of NaCl phase, Na2SO4 phase, and Al2O3 phase, and there is no other new phase detected, which indicates that the bauxite powder has not reacted with the salt melt and stably exists in the salt core matrix.

Fig. 7
figure 7

XRD pattern of the salt core with different bauxite powder contents sintered for 30 min

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Figure 8 shows the EDS map analyses of the salt core samples with 20 wt.% bauxite powder at 30-min sintering. It can be seen that the elements of Na, Cl, Al, S, and O are clearly distributed in the salt core, and the NaCl (Cl element) and bauxite powders (Al element) are relatively dispersed in the salt core. Furthermore, it can also be observed that the bauxite powder is distributed around the NaCl phase.

Fig. 8
figure 8

EDS map analyses of salt core samples with 20 wt.% bauxite powder sintered for 30 min

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Figure 9 shows the microstructures of the salt core samples with 20 wt.% bauxite powder at different sintering times. When the sintering time is 15 min, as shown in Fig. 9a and d, a large number of open pores are visible, and only a small amount of solid powder was sintered into agglomerates. When the sintering time is 30 min, as shown in Fig. 9b and e, the pores of the salt core remarkably reduce, and the densification degree increases. In addition, some closed pores are observed in the salt core. As the sintering time increases to 45 min, the densification degree of salt core is further improved, as shown in Fig. 9c and f. This is because increasing the sintering time can increase the liquid phase volume, thereby enhancing the efficiency of particle rearrangement and mass transfer. However, the shrinkage will be generated during the solidification of the liquid phase, resulting in some closed pores in the salt core, most of which are spherical or nearly spherical.

Fig. 9
figure 9

Microstructures of salt core samples with 20 wt.% bauxite powder at different sintering times: a, d 15 min; b, e 30 min; and c, f 45 min

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Generally, ceramic powder as a reinforcing material can improve the solidification structure of the salt cores [16, 29]. The microstructures of the salt core samples with different bauxite powder contents at 30-min sintering are shown in Fig. 10, and Table 3 lists the EDS results of the salt cores selected in Fig. 10. According to the results of EDS analysis, it suggests that the components of the salt core are NaCl, Na2SO4, and bauxite powder, which matches the results of XRD and EDS map analyses. It can be seen from Fig. 10a–d that the NaCl phase is significantly refined with the increase of bauxite powder content, and the NaCl phase changes from coarse lath crystals to fine bulk crystals gradually. The grain refinement can promote the strength of the salt core, which can be explained by the Hall–Petch Eq. (6) [30], where σ represents the yield strength of the salt cores, D represents the mean grain size, σ0 is the intrinsic yield strength, and K is the material constant.

Fig. 10
figure 10

Microstructures of salt core samples with different bauxite powder contents sintered for 30 min: a 0 wt.%; b 10 wt.%; c 20 wt.%; and d 30 wt.%

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Table 3 EDS results of the selected positions of the salt core samples corresponding to Fig. 10
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$$\sigma ={\sigma }_{0}+{KD}^{-1/2}$$
(6)

The appropriate ceramic powder can improve the performance of the salt core, but excessive addition may have the opposite effect [29, 31]. Many defects were observed when the bauxite powder content is 30 wt.%, as shown in Fig. 11, resulting in a decrease in the bending strength of the salt core.

Fig. 11
figure 11

Microstructures of salt core samples with 30 wt.% bauxite powder at different sintering times: a 30 min and b 45 min

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Figure 12 depicts the images of the salt core samples with 20 wt.% bauxite powder under different sintering times. The side profile of the salt core samples is displayed in Fig. 12a–d, and the cross-section profile of the salt core samples is displayed in Fig. 12e–h. It is clear that the deformation degree of salt core increases with prolonging the sintering time. When the sintering time is 45 min, the salt core has a serious bending deformation, as shown in Fig. 12d and h. Therefore, the sintering time of 30 min is the relatively optimal parameter.

Fig. 12
figure 12

Images of the salt core samples with 20 wt.% bauxite powder under different sintering times: a, e 0 min (green body); b, f 15 min; c, g 30 min; and d, h 45 min

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The complex salt core samples were prepared using the optimized parameters, and the bauxite powder content and sintering time are 20 wt.% and 30 min, respectively, as shown in Fig. 13. It can be seen that the salt core samples after sintering have no obvious deformation, and no other defects are presented, indicating that it is feasible to manufacture a complex high-strength water-soluble salt core via ME technology.

Fig. 13
figure 13

Images of complex salt core samples fabricated by material extrusion with optimal parameters

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4 Conclusion

In this paper, we utilized the material extrusion (ME) method to prepare water-soluble salt core strengthening by bauxite powder. The bauxite powder content and sintering parameters have a significant effect on the performance of the salt cores. Increasing the content of bauxite powder is beneficial to refine the NaCl phase, and prolonging the sintering time facilitates the densification of the salt core. However, excessive bauxite powder and longtime sintering are not conducive to the formation of the salt cores. The salt cores with 20 wt.% bauxite powder sintered for 30 min at 630 ℃ possess an excellent comprehensive performance, which satisfies the requirements for manufacturing hollow composite structure castings. The complex water-soluble high-strength salt core samples with high-strength and well-shaped morphology are successfully fabricated under optimal parameters, indicating the ME is promising in the preparation of water-soluble salt cores.