A water-soluble salt core (WSSC) strengthened by reinforcing particles, including bauxite powder, glass fiber powder, and sericite powder, was fabricated by gravity-casting process. The surface quality, bending strength, water solubility, humidity resistance, and shrinkage rate of WSSC were investigated, and the synergistic effect between the different reinforcements on the bending strength was analyzed. Scanning electron microscope (SEM) was used to study the micromorphology of WSSC. The results indicate that the binary composite WSSC after being strengthened has excellent comprehensive performance, the bending strength increases by more than 1.4 times with the maximum value of 47.89 ± 0.83 MPa whose 24-h hygroscopic coefficient is lower than 0.18%, the water solubility rate is higher than 163.97 kg/(min m3) in still water at 80 °C, and the shrinkage rate is dramatically lower than that without any reinforced materials; in addition, there are no obvious casting defects on the core surface. The microscopic analysis demonstrates that the homogeneous distribution of the reinforcements in the matrix consumes more energy during the crack propagation procedure and the grain refinement of WSSC is also observed, above which is the main reason for the improvement of the bending strength. Furthermore, the practical casting test of the complex soluble salt core prepared by pressure core making was used for zinc alloy die casting.
Zinc alloy die casting has features of low melting point, energy saving, excellent surface finish, high strength, outstanding corrosion resistance, and ductility and hence is widely used in applications such as automotive, electronics, instrumentation, and other industries [1, 2]. However, most of zinc alloy die castings often possess complex inner cavities or hollow structures; therefore, they make the casting components and the foundry cores become very complicated, and the interior pores of the casting parts are difficult to clean, which require an expendable core removable after casting.
Unfortunately, the rising demands on the complexity of castings cannot be met in the die-casting technology with the existing simple metal cores. Consequently, the interest in new manufacturing processes of the soluble cores is growing [3–6]. Notably, the density of zinc alloy is about 2 to 2.6 times compared with magnesium or aluminum alloys, which makes enormous demands on the cores to have greater impact resistance of the fluid melt during service .
Traditionally, the channels and undercuts are shaped by water-soluble sand core using casting method, but this core has insufficient strength and poor stability and dimensional accuracy [2, 7–11]. Meanwhile, the cleaning process may have a greater corrosion of the casting by using soluble ceramic cores [12–16].
The water-soluble salt core leads to many benefits, for example, high strength, excellent casting removability, good humidity resistance, and recyclable use [17–22]. Therefore, the water-soluble salt core is regarded as an attractive candidate for the water-soluble core material fabricated for die castings. Herein, the preparation of the watersoluble salt core with excellent comprehensive performance has great meaning for industrial production application and represents the highest application prospect.
The water-soluble salt core in aluminum or magnesium alloy die casting has been successfully applied [23–26]. However, zinc alloy die casting with the water-soluble salt core is rarely reported. The binary system of KNO3- 20 mol% KCl water-soluble salt core, whose liquidus temperature is adequate to be used in zinc alloy die castings, was formed by melting gravity-pouring process in this paper.
The properties of water-soluble salt cores strengthened by reinforcements were compared with those without any reinforcements. The morphology of the fractures and their structures were investigated by scanning electron microscope (SEM); in addition, the strengthening mechanism of the water-soluble salt core was determined and discussed preliminarily.
Furthermore, the feasibility of the water-soluble salt core when strengthened was verified by the practice of pouring experiments.
2 Material and experimental details
2.1 Materials and equipment
Two pure salts, including high-melting salt potassium chloride (KCl) and low-melting salt potassium nitrate (KNO3), were used in this work, whose purity was over 99 mass%. By mixing the two salts, the binary system of KNO3-20 mol% KCl was obtained. The average particle size of industrial bauxite powder was 325 mesh, and the average particle size of industrial glass fiber powder was 75 μm with a diameter of 5~13 μm. The last reinforced material was layered sericite powder with a size of 325 mesh. The acetone was of analytical purity. The water-soluble salt core specimens with a dimension of 173.36 mm × 22.36 mm × 22.36 mm were fabricated by the permanent mold-casting method. Both ends of the semicircular design of the stainless steel mold were convenient for mold release. The salt mixture was melted in a pure corundum crucible with an electric resistance heater. Other test equipment included the type SWY hydraulic strength-testing machine, the type JA5003N electronic balance with the accuracy of 0.001 g, digital vernier caliper, the type DHG-9077A drying oven, the type HH-1 thermostat water bath cauldron, the type KQ-50E ultrasonic cleaner, the type SQ-80 sample-cutting machine, and Quanta 200 environmental scanning electron microscope.
2.2.1 Pretreatment of the salt mixture
Firstly, the salt mixture with a predetermined amount was weighed by the electronic balance. Secondly, the mixture was poured into a mortar and then mixed uniformly until there were no obvious clusters of small pellets. Finally, the salt mixture was put into the drying oven with a preset temperature of 105 °C and kept for 4 h, cooling down to ambient conditions with the furnace afterwards. Then, the preparation of the salt mixture was finished.
2.2.2 Preparation of water soluble salt core The as-prepared salt mixture was put into the corundum crucible to melt in the resistance furnace. After the salt mixture was completely melted, the aluminum oxide mixing bar was used to stir the melted salt mixture manually, which made the melt mix uniformly. Stirring was stopped when the temperature of the molten superheated at 15 °C. The molten salt was directly poured into the permanent metal mold with a preheating temperature of 130–160 °C under gravity. After about 30 s, the salt sample was taken out, naturally cooling down to the room temperature. Finally, the water-soluble salt core sample was fabricated.
2.2.3 Property test
The surface quality of the salt core samples, which were cut out of the riser, was observed, and the sizes were measured by a digital vernier caliper. Bending strength of the specimen after solidification forming was tested by the type SWY hydraulic strength-testing machine. A portion of the bending specimen, with a size of 30 mm × 22.36 mm × 22.36 mm, was exposed in constant-humidity bottle (the relative humidity was 98– 100%), and moisture absorption rate (ψ) can be calculated by the equation of ψ = (M1 − M0)/M0 × 100%, where M0 is the original weight and M1 is the weight of the specimen after being put into a humidistat (relative humidity at 98–100%) for a certain period of time. Water-soluble rate (K) can be calculated by the equation of K = m/(V × t), where m is the weight of a sample (sample with a size of 30 mm × 22.36 mm × 22.36 mm was immersed in 80 °C still water), V is the volume of the sample, and t is the dissolution time of the sample in water. Each testing and calculating result was the average value of five measurements in every experiment. After the bending test, the salt core specimens were cut, rough-grinded with the grade no. 500 SiC waterproof papers, then grinded more carefully with the grade nos. 1000 and 2500 SiC waterproof papers, and subsequently cleaned with ultrasonic waves in acetone. After that, the polished sections were coated with carbon powder for microstructure observation microanalysis.
3 Results and discussion
3.1 The surface quality of the water-soluble salt core
Figure 1a, b shows the surface quality of the cast sample with the composition of pure salt core, in which three typical and severe casting defects were found as indicated by the arrows. One was the fold on the core surface, and the low thermal conductivity of the salt was the most probable cause. Using the metal surface deformation curvature formula (Eq. 1 below)  for reference, the thermal conductivity had a significant effect on curvature; therefore, pure salt cores had distinct variations in curvature which would result in the formation of folds. The defect of folds could influence the surface accuracy of the castings. On the other hand, it would bring about the naissance of the potential cracks, causing the low strength of the core.
For the above formula, α is the linear expansion coefficient of the metal, h is the thermal conduction coefficient between the mold and the liquid metal, T0 is the pouring temperature of the metal, Tm is the mold temperature, and λ is the thermal conductivity of the metal. Clear cracks were observed on the core skin as indicated by the arrow in Fig. 1a. In this study, the strength of pure salt cores which were inclined to produce cracks was very low. These cracks were generated during the solidification process because of the generation of thermal stress in the specimen and shrinkage of the core. The third defect was the unevenness formed on the surface. The shrinkage of pure salt cores was greatly affected by liquid-solid state changes during the solidification process; in addition, the internal and external solidification times had been much more varied. In other words, this defect was due to thermal deformation caused by the temperature differences inside and outside of the initially solidified shell [19, 22]. Compared with the pure salt core, there were no obvious abovementioned defects of the binary composition salt core fabricated by KNO3-20 mol% KCl as shown in Fig.1c, d. Therefore, further study about the core materials based on KNO3-20 mol% KCl is demonstrated in the following sections.
3.2 Effect of the reinforcing particles on the bending strength of WSSC
3.2.1 Effect of the mass fraction of three different reinforcing particles
Figure 2 shows the bending strength comparison of the watersoluble salt core strengthened by three different reinforcing particles; the mass fraction of reinforcing particles increases from 0 to 30 mass%, respectively. As illustrated in Fig. 2, the bending strength of water-soluble salt core samples increases monotonously with the increased mass fraction of bauxite powder, and the bending strength can reach a maximum of 40.88 MPa with the mass fraction of 30 mass%. While the bending strength of the specimens strengthened by glass fiber
owder or sericite powder firstly increases and then decreases, the maximum peak values are 35.08 and 39.26 MPa with the addition of 20 mass%, respectively. When the proportion of glass fiber powder or sericite powder continues to increase to 30 mass%, the bending strength of samples decreases rapidly. It is also noted that the strength of the soluble salt core with reinforcements is higher than that without reinforcements. The strength of the salt samples is determined by the kind of reinforcements, size, shape and orientation, and mass fraction in the matrix. The water-soluble composite salt core using bauxite powder as the major ingredient and others as auxiliary ingredient to study the synergistic effect between the different reinforcements is investigated in the following sections, and the total mass fraction remains at 30 mass%. Also, taking into consideration the liquidity of the WSSC, the reason why the mass fraction of reinforcements does not exceed 30 mass% is because excessive additions will deteriorate the melt liquidity, as well as reduce the collapsibility and removability of the salt core. If the content of the reinforcing particles is very low, the strengthening effect is not apparent. The symbol of “Max” in the figures represents the maximum value of the bending strength, and an error bar indicates a standard deviation, the same as below. 3.2.2 Effect of the addition amount of sericite powder or glass fiber powder inter-coupling with bauxite powder Figure 3 is a correlation diagram showing the relationship between the bending strength and the addition amount of particles (glass fiber powder or sericite powder) inter-coupling with bauxite powder. Figure 3 reveals that when the addition amount of inter-reinforcing powder increases, the bending strength of core samples firstly increases and then decreases. The maximum bending strength values of soluble salt core materials are 46.02 and 47.89 MPa when the addition value of inter-reinforcing particles is 15 mass%, respectively.
The strength values decrease dramatically when the addition amount ranges from 20 to 30 mass%. In their respective curves, the strength of core materials at the addition amount between 10 and 15 mass% has a slight difference, but this difference is insignificant, so the values are almost equal. In addition, there is a slight variation for two solid lines with a certain range between 0 and 20 mass%; this variation can be regarded as an error range. In summary, there is a good synergistic effect on the water-soluble salt core when the addition amount of the inter-reinforcing particles is moderate.
3.3 Water-soluble rate and hygroscopic rate of WSSC
As is well-known, the salt is water-soluble and can be readily removed by hot, running water, with the result that the salt also can absorb moisture from the air and the moisture will deteriorate the strength of the soluble salt core, generally. However, reinforcements are not soluble in water and scarcely absorb moisture in air. Thereby, the water-soluble rate can be used to characterize the solubility of the water-soluble salt core in water, and the hygroscopic rate can be used to indicate the applicability of the soluble salt core. Figure 4a is a graph plotting the water-soluble rate of samples strengthened by three different particles with the mass fraction from 0 to 30 mass%. Figure 4a shows that when the proportion of reinforcements increase, the water-soluble rate decreases accordingly.
As depicted in Fig. 4b, it is obvious that the reinforcing particles can greatly improve the humidity resistance of the salt cores, which well matches that of the test result of the water-soluble rate. This is because reinforcing particles uniformly distribute in the matrix of the salt core, form skeleton, reduce the moisture absorption area, impede the diffusion of water molecular, and reduce the hydration of salt. Nevertheless, the salt cores do not significantly lose the removability even when strengthened by reinforcing particles,and the water solubility rate of all casting samples is higher than 163.97 kg/(min m3 ) in still water with 80 °C in this research.
3.4 The casting shrinkage of water-soluble salt core
Figure 5a indicates the shrinkage of the soluble salt core
strengthened by three particles, where the linear and the volume shrinkages are connected by the dashed line and
3.4 The casting shrinkage of water-soluble salt core
Figure 5a indicates the shrinkage of the soluble salt core strengthened by three particles, where the linear and the volume shrinkages are connected by the dashed line and the solid line, respectively, the same as in Fig. 5b. The line or volume shrinkage of the core samples decreases with the increased addition of particles in Fig. 5a. When the mass fraction of sericite powder exceeds to 30 mass%, the line and volume shrinkages are 1.19 and 4.14%, respectively. As shown in Fig. 5b, there is a slight variation for each dashed or solid line in the whole range when strengthened by reinforcing particles, and the shrinkage rate of all data is lower than that of the unreinforced. It is evident that the reinforcement that adds into melt can largely improve the shrinkage property of cores. Therefore, it is helpful to guarantee the dimension precision of salt castings. On the other hand, smaller shrinkage rate can avoid crack forming on the core skin during solidification process.
3.5 The microanalysis of water-soluble salt core
Figures 6 and 7 show the fracture surface micrograph and the morphology of the solidified structure of the water-soluble salt core, respectively.
Figure 6a reveals that the micrograph of the fracture is smooth, and there are small microholes as the arrow direction indicates within the solidification structure. Figure 6b, c suggests that the surface of the fracture is coarse, compact, and almost without defects; fibers and layered particles can be found on the fracture surface. Figure 6b shows pullout holes and fiber bridges clearly. On the one hand, the bending strength of the core material is improved due to the large external force required for pullout effect of fibers ; on the other hand, bridging fibers produce the closure force on the matrix, which consumes more energy. The layer particles are well-embedded in the matrix in Fig.6c; in addition, chip breakage, matrix fracture, and the pullout effect of the chip are beneficial for enhancing the strength of the cores. Furthermore, the homogeneous distribution of the bauxite particles in the matrix makes the deflection of the crack direction, which is also advantageous to the improvement of the strength
. However, Fig. 6d shows the phenomenon of agglomerations of fibers as indicated by the circle; it suggests that the excess of the fiber particles is weakly bound to the matrix, resulting in a decrease in strength. Similarly, too much layered sericite powder perhaps could split the matrix.
Figure 7 shows the solidified structure morphology of the water-soluble salt core. It is obvious that a lot of micropits and microcracks can be found clearly in Fig. 7a. From Fig. 7b, the abovementioned defects are improved significantly; what is more, there are finer crystal grains.
The reason is that the reinforcements are evenly distributed in molten salt and the presence of the reinforcements acts as a foreign crystal nucleus, inhibits the growth of columnar crystals, above which results in downsizing of crystal particles. In general, the finer grains contribute to the increase in strength. Thus, the melt addition treatment of reinforcements is a simple and effective way to strengthen the dispersion of the water-soluble salt core.
3.6 Practical casting test
The soluble salt core material with 15 mass% bauxite powder and 15 mass% glass fiber powder was chosen to carry out the practical casting tests. Figure 8 shows that the salt core and the zinc alloy casting after the core were leached out in the laboratory. Figure 9 shows the water-soluble salt core and the zinc alloy die casting of one component fabricated by highpressure die casting in the factory. The results of the test show that the salt core still maintains the initial shape after pouring is completed, which indicates that the salt core can shield the impact of the liquid metal and can be used for the forming of die castings with intracavity. In addition, the casting surface contacting with the core is smooth and the core could be easily self-collapsing and water-soluble.
In this study, the performance of water-soluble salt core was investigated. The micrograph of cast specimens was analyzed, and the strengthening mechanism of the water-soluble salt core was determined and discussed preliminarily. The experimental results could be summarized as follows.
- The water-soluble composite salt core strengthened by reinforcing particles exhibited a more excellent comprehensive performance. In addition, there were basically no obvious casting defects on the core skin.
- The strength of the water-soluble salt core strengthened by 15 mass% bauxite powder and 15 mass% glass fiber powder or 15 mass% bauxite powder and 15 mass% sericite powder exceeded over 45 MPa. The volume shrinkage rates were 5.01 and 4.52%, respectively. Furthermore, the water-soluble composite salt core possessed excellent casting removability and humidity resistance.
- The microanalysis indicates that the presence of the homogeneous distribution of reinforcing particles in the matrix consumes more energy during the crack propagation procedure. The reinforcements act as a foreign crystal nucleus and increase the number of nuclei, resulting in refining the grains.
- The complex soluble salt core was prepared with 15 mass% bauxite powder and 15 mass% glass fiber powder, which was used for zinc alloy die casting. The casting surface was smooth, and the core could be easily selfcollapsing and water-soluble after pouring.
Water-soluble salt core . Reinforcing particles .
Zinc alloy . Mechanical properties . Strengthening mechanism
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This work was jointly supported by grants from the National Nature Science Foundation of China (Nos. 51375187 and 51775204), the project funded by the China Postdoctoral Science Foundation, the research project of the State Key Laboratory of Materials Processing and Die & Mould Technology, and the Analytical and Testing Center, HUST.
The International Journal of Advanced Manufacturing Technology volume 95, pages505–512 (2018)Cite this article