WO2010126135A1

WO2010126135A1 - Method for producing salt core for casting

Info

Publication number: WO2010126135A1
Application number: PCT/JP2010/057690
Authority: WO
Inventors: 浩一安斎; 及川　勝成; 養司山田
Original assignee: 国立大学法人東北大学; ヤマハ発動機株式会社
Priority date: 2009-05-01
Filing date: 2010-04-30
Publication date: 2010-11-04
Also published as: JP5419549B2; US20120048502A1; JP2010279951A; EP2425910A1

Abstract

First, a mixed salt is heated to produce a melt (S101). Next, a mold for core molding is heated to within a temperature range of 0.52 x Tm to 0.7 x Tm (S102). It should be noted that Tm is the liquidus temperature of the mixed salt expressed as an absolute temperature (K). Then the above-mentioned melt is poured under pressure into the mold heated as described above (S103). The melt is hardened inside the mold to form the salt core for casting (S104).

Description

Manufacturing method of salt core for casting

The present invention relates to a method for producing a salt core for casting having water solubility.

Aluminum alloy die-casting is widely applied mainly in the automobile industry due to features such as light weight, high strength, high productivity, and high dimensional accuracy, and its production volume is increasing. In recent years, there has been a demand for adaptation to parts having more complicated shapes. For example, application to a water jacket for water cooling such as a cylinder block of an internal combustion engine is required. However, there is a drawback that it is difficult to mold such a hollow structure or a cast product having an undercut shape.

For such hollow structures and undercut molding, a collapsible core that can be removed after casting is essential. However, the core used for die casting requires a high-strength core that can withstand the impact of the molten metal injected at high speed and high pressure. Specifically, a gate speed of 38.7 m / s, which is the recommended value for NADCA (The North American Die Casting Association), or 25.4 m / s to 40.6 m / s recommended in the NADCA PQ2 manual. It is important to endure the impact (see Non-Patent Documents 1 and 2). Moreover, in order to crush the cast hole which is likely to be generated on the surface of the heat insulating core due to the blowhole, it is a condition required for the die casting core not to be deformed to an injection pressure of 75 Mpa or more.

However, as is well known, in general sand cores, strength and disintegration are contradictory properties, so emphasis is placed on optimizing strength and disintegration by adjusting the type and amount of binder. (See Non-Patent Documents 3 and 4).

On the other hand, the application of salt is also being studied. The salt is water-soluble and can be easily removed by high-speed high-pressure running water. In addition, a core using a salt (salt core) hardly deteriorates removability even when the strength is increased. For these reasons, the salt core is considered suitable for the die casting process. By the way, a salt is a material having brittleness similar to that of ceramics, and it is known that structure control such as high densification and crystal grain refinement is effective for increasing the strength (Non-patent Documents 5 and 5). 6). Recently, there is a report example of a salt core whose dispersion is strengthened by mullite (see Non-Patent Document 7).

In addition, the inventors have systematically studied the mechanical properties and solidification structure of melt-formed salt for the purpose of applying the salt core to aluminum die casting (see Non-Patent Documents 8 to 12). For example, aluminum borate whiskers have been shown to be effective for strengthening dispersion of salt cores using alkali chlorides (see Non-Patent Document 8). Further, it has been clarified that a mixed salt of alkali chloride and alkali carbonate exhibits a very high strength of 20-30 MPa without a reinforcing material such as whisker (see Non-Patent Document 10). In addition, the comparison between the calculated phase diagram and the solidified structure clarifies the conditions for the high-strength material in the KCl—NaCl—K ₂ CO ₃ —Na ₂ CO ₃ multi-component mixed salt (see Non-Patent Document 11). . Although these high-strength mixed salts have been produced by the gravity casting method, improvement in the dimensional accuracy and production efficiency of the core can be expected by casting by the die casting method.

However, the salt core formed by the die casting method has a problem in that it has a large variation in strength and has not yet been fully put into practical use.

The present invention has been made to solve the above-described problems, and the practical strength of a salt core for casting formed by a die casting method by melting a salt such as sodium is more stable. It aims to be obtained.

A method for producing a casting salt core according to the present invention is a method for producing a casting salt core made of a molten salt containing a salt of sodium, wherein the molten salt is formed by heating the mixed salt. If the liquidus temperature of the mixed salt is Tm in absolute temperature (K), the core molding die is heated to a temperature range higher than 0.52 × Tm and lower than 0.7 × Tm. 2 steps, a third step of pressing the molten metal into the heated mold, and a fourth step of solidifying the molten metal inside the mold to form a casting salt core.

In the above casting salt core manufacturing method, the mold may be heated to a temperature range of 225 ° C to 250 ° C. The mixed salt may have a composition of Na ⁺ : K ⁺ = 70 mol%: 30 mol%, Cl ⁻ : CO ₃ ²⁻ = 46.2 mol%: 53.8 mol%.

As described above, according to the present invention, when the liquidus temperature of the mixed salt is Tm in absolute temperature (K), the core molding die is higher than 0.52 × Tm and higher than 0.7 × Tm. Because it is heated to a low temperature range, and molten salt of mixed salt is press-fitted into this heated mold, it is for water-soluble casting made of a cast product of a salt die casting method that melts a salt such as sodium. The practical strength of the salt core can be obtained more stably.

FIG. 1 is a flowchart for explaining a method for producing a salt core for casting according to an embodiment of the present invention. FIG. 2 is a photograph showing a state of a molded product for obtaining a test piece produced in an experiment in the embodiment of the present invention. FIG. 3 is a characteristic diagram showing an example of a typical pressure change in the runner part and the test piece part during the die casting process. FIG. 4 is a configuration diagram showing the shape of a test piece prepared in an experiment in the embodiment of the present invention. FIG. 5 is a correlation diagram showing the relationship between average bending strength and mold temperature. FIG. 6 is a characteristic diagram in which average bending strengths of samples having mold temperatures of 175 ° C. and 250 ° C. are plotted against injection pressure. FIG. 7 is a characteristic diagram showing the relationship between the hardness of the test piece and the mold temperature. FIG. 8 shows the result of visual observation of the surface defect of the test piece by the dye penetrating flaw detection method. FIG. 9 is a photograph showing an SEM image of the bottom surface of the test piece. FIG. 10 is a photograph showing SEM images near the sample surface and near the inner center of the cross section perpendicular to the longitudinal direction. FIG. 11 is a Weibull plot showing the relationship between the cumulative failure probability and the bending stress in the bending test of the sample of the present embodiment. FIG. 12 is a perspective view showing the configuration of a closed deck type cylinder block made of aluminum alloy die casting. FIG. 13 is a correlation diagram showing the relationship between the average bending strength and the Weibull coefficient of each sample. FIG. 14 is a photograph showing the result of visual observation of the surface defect of the test piece of composition A by the dye penetrant flaw detection method. FIG. 15 is a photograph showing the result of visual observation of the surface defect of the test piece of composition B by the dye penetrant flaw detection method. FIG. 16 is a photograph showing the result of visual observation of the surface defect of the test piece of composition C by the dye penetrant flaw detection method. FIG. 17 is a photograph showing a result of visual observation of a surface defect of a test piece of composition D by a dye penetrant flaw detection method.

Hereinafter, embodiments of the present invention will be described with reference to the drawings. FIG. 1 is a flowchart for explaining a method for producing a salt core for casting according to an embodiment of the present invention. First, in step S101, the mixed salt is heated to produce a molten metal. Next, in step S102, the core molding die is heated to a temperature range higher than 0.52 × Tm and lower than 0.7 × Tm. Tm represents the liquidus temperature of the mixed salt as an absolute temperature (K). Next, in step S103, the molten metal is pressed into the mold heated as described above. Next, in step S104, the molten salt is solidified inside the mold to form a casting salt core.

Here, the mixed salt is, for example, a mixture in which Na ₂ CO ₃ is 50 mol%, NaCl is 20 mol%, and KCl is 30 mol%. When the composition of this mixed salt is analyzed, the composition ratio of ions is Na ⁺ : K ⁺ = 80 mol%: 20 mol%, Cl ⁻ : CO ₃ ²⁻ = 50 mol%: 50 mol%. In this case, the mold may be heated to a temperature range of 225 ° C to 250 ° C. According to the present embodiment thus configured, variations in strength of the obtained salt core are reduced, and higher strength can be obtained.

The following is a detailed explanation based on experimental examples.

In the following, KCl-NaCl-Na ₂ CO ₃ mixed salt is cast and formed by die casting, and the influence of the mold temperature and the injection pressure of the molten metal on the bending strength is investigated. The results of trial production of salt cores for blocks are shown.

[experimental method]
[Preparation of sample]
Using 99.5% pure NaCl, KCl, Na ₂ CO ₃ as raw materials, dissolution was performed in an alumina crucible using a resistance heating furnace. The atmosphere during melting was air, and the melting temperature was 688 ° C. The dissolved sample composition is 50 mol% Na ₂ CO ₃ -20 mol% NaCl-30 mol% KCl, which is in the composition region reported to be high in strength. According to the calculation phase diagram (see Non-Patent Document 13), the primary crystal is a carbonate with a high sodium ion concentration, and a eutectic reaction between carbonate and chloride occurs below the eutectic temperature. The liquidus temperature, eutectic start temperature, and eutectic end temperature were calculated to be 638 ° C, 574 ° C, and 573 ° C, respectively. Moreover, the superheat degree at the time of melt | dissolution is about 50 degreeC (688 degreeC).

The die-casting machine used a cold chamber type with a clamping force of 110 tons, and was cast with a mold capable of taking two rectangular test pieces as shown in FIG. The casting conditions were four conditions of mold temperature of 175 ° C., 200 ° C., 225 ° C., and 250 ° C., and injection pressure was three conditions of 39.2 MPa, 58.8 MPa, and 78.4 MPa. It was. Molding was performed in order from the lowest mold temperature and injection speed. Other molding conditions were a sleeve diameter of 50 mm, an injection speed of 34 mm / second, and a filling rate of about 60%. Due to the low-speed filling, the problem of air entrainment is considered to be small, and the two-stage injection that is performed by general die casting is not performed.

Also, in order to remove the chill layer (breaking chill) generated on the sleeve surface, a core (partition plate) is inserted after the sleeve (pouring gate). When the broken chill generated in the sleeve enters the mold, it causes a large variation in the bending strength of the molded product. In order to suppress the penetration of the broken chill into the mold, a core protruding from the peripheral end of the gate is inserted into the gate of the sleeve. By inserting the core, variation in the bending strength of the molded product can be suppressed.

The mold temperature was controlled by inserting a thermocouple into each of the upper mold and the lower mold between the two test pieces and measuring the temperature. The sleeve temperature was also controlled to be the same as the mold temperature.

FIG. 3 shows an example of a typical pressure change in the runner part and the test piece part during the die casting process. After the molten salt is completely filled, the pressure applied by the plunger reaches a maximum value during holding and gradually decreases. At this time, the maximum values of the runner part and the test piece part are the same, and it is considered that the plunger pressure is sufficiently transmitted to the test piece part during solidification.

[Test method]
Next, the test method will be described. The die cast material cast as described above is cut at the gate portion to obtain a test piece having the shape shown in FIG. The mechanical strength of the test piece thus obtained is evaluated by a four-point bending test. The crosshead speed of the bending tester was 1 mm / min, and the fulcrum diameter was 4 mm. From the maximum load F [N] when the test piece breaks while applying force, the bending strength σ is obtained from the following equation (1).

In the formula (1), L ₁ is the lower fulcrum interval, which is 50 mm, and L ₂ is the upper fulcrum interval, which is 10 mm. W is the width of the test piece, 20 mm, and T is the height of the test piece, 18 mm. About 10 to 20 test pieces were produced under each of the casting conditions described above, and the average bending strength was calculated under each condition.

In addition, a part of the bending test piece was cut, and the surface of the cut surface and the microstructure were observed. When observing the microstructure, the cross section was dry-polished with # 4000 water-resistant abrasive paper, ultrasonically cleaned in acetone, then vacuum carbon deposited and observed with a scanning electron microscope (SEM). Moreover, the hardness test was done by micro Vickers. The test conditions were a load of 4.9 N and a load holding time of 30 seconds. The measurement was performed 10 times each in the vicinity of the surface and in the center, and the average value of each measurement result was obtained. Further, the surface defects were visually observed by a dye penetrant flaw detection method.

[result]
[Bending test]
Next, the results of the bending test will be described. FIG. 5 is a correlation diagram showing the relationship between average bending strength and mold temperature. In Fig. 5, error bars indicate standard deviation. When the mold temperature is 175 ° C. to 225 ° C. at each injection pressure, the average bending strength tends to increase as the mold temperature increases. For materials cast at a mold temperature of 225 ° C. and 250 ° C., although the bending strength is somewhat large and small, it is not a significant difference but can be said to be almost equivalent. When the mold temperatures are 225 ° C. and 250 ° C., the average bending strength exceeds 25 MPa at each injection pressure, and shows a high value almost equivalent to that of the gravity cast material (see Non-Patent Document 9).

FIG. 6 is a characteristic diagram in which the average bending strength of samples with mold temperatures of 175 ° C. and 250 ° C. is plotted against injection pressure. Although there is some variation, it is considered as an error range, and it can be said that there is no significant effect on the bending strength within this injection pressure range. The same was true for mold temperatures of 200 ° C and 225 ° C.

[Hardness test]
Next, the results of the hardness test will be described. As described above, there was almost no influence of the injection pressure on the bending strength. For this reason, the Vickers hardness was investigated mainly on samples with an injection pressure of 78.4 MPa. FIG. 7 is a characteristic diagram showing the relationship between the hardness of the test piece and the mold temperature. In the vicinity of the sample surface (black circles), there is some variation, but the hardness does not greatly depend on the mold temperature and is almost the same value. On the other hand, at the center of the sample (black triangle), the lowest mold temperature is 175 ° C., which is the highest. When the mold temperature is 175 ° C., there is no significant change in hardness on the surface and inside. In contrast, at other mold temperatures, the surface was clearly harder than the center of the sample.

[Surface and microstructure observation]
Next, the observation results of the surface and microstructure will be described. Regarding the dye penetrant flaw detection test and the structure observation by SEM, a sample with an injection pressure of 78.4 MPa was mainly investigated. FIG. 8 shows the result of visual observation of the surface defect of the test piece by the dye penetrating flaw detection method. In the figure, the colored part is a cracked part. At the lowest mold temperature of 175 ° C., many large cracks are confirmed on the surface as shown in FIG. In the sample having a mold temperature of 200 ° C. shown in FIG. 8B, large cracks are observed although not as low as 175 ° C. On the other hand, when the mold temperatures are 225 ° C. and 250 ° C., as shown in FIG. 8C and FIG. 8D, clear cracks are hardly observed on the surface. During solidification of the molten salt, the solidified shell that is initially formed on the mold surface tends to shrink during solidification, but because it is under pressure, it cannot shrink and tensile stress is generated on the surface. When the mold temperature is low, the solidified shell cannot be deformed, and it is considered that surface cracks are generated by tensile stress.

FIG. 9 shows an SEM image of the bottom surface of the test piece. At the lowest mold temperature of 175 ° C., as shown in FIG. 9A, a pattern like a hot water bath was observed, and microcracks were generated in the vicinity of this pattern. When the mold temperature is increased to 200 ° C., 225 ° C., and 250 ° C., the pattern like a hot water bath is remarkably reduced as compared with the sample of 175 ° C. In addition, the number of microcracks decreased as the mold temperature increased. In addition, there were areas where micropits of several microns in size were seen on the surface. This micropit is presumed to have been caused by deliquescence. These surface defects are thought to greatly affect the bending strength. The pattern like a hot water bath is thought to be due to the rapid cooling effect of the surface during solidification.

FIG. 10 shows SEM images near the sample surface and near the center of the cross section perpendicular to the longitudinal direction. When the mold temperature is 175 ° C., there is almost no change in the microstructure at the surface and the center. On the other hand, when the mold temperatures are 200 ° C., 225 ° C., and 250 ° C., coarse primary crystal dendrites are clearly observed as compared with the surface. In addition, the surface texture does not change greatly depending on the mold temperature. The structure near the surface is considered to be a chill layer in which a solidified shell is rapidly formed on the mold surface in the early stage of solidification. The higher the mold temperature, the more slowly the inside solidifies, and it is thought that the dendrite was coarsened. Moreover, it can be said that the tendency of hardness is consistent with the tendency of the coarseness of the structure.

[Discussion]
From the result of the experiment described above, it was confirmed that the average bending strength increases as the mold temperature increases. It was also suggested that the change in bending strength was related to surface cracks. In order to examine the change in bending strength in more detail, an analysis of bending strength is attempted using Weibull statistics used in the field of brittle materials (see Non-Patent Document 12).

In order to improve the accuracy of analysis, the number of bending tests was increased to 20 times. The analysis was performed under two conditions: (Condition 1) injection pressure 74.8 MPa, mold temperature 250 ° C. and (Condition 2) injection pressure 74.8 MPa, mold temperature 200 ° C. The former is an example where the mold temperature is high and the average bending strength is high, and the latter is an example where the mold temperature is low and the average bending strength is low.

The 2-parameter parameter Weibull distribution function used for brittle materials is given by the following equation (2) (see Non-Patent Document 12).

In equation (2), F (σ) is the cumulative failure probability, V _E is the effective volume, σ ₀ is the scale parameter, σ _f is the bending stress, and m is the shape parameter. The larger the shape parameter m, the smaller the variation in strength and the higher the reliability. In general ceramics, m takes a value of about 5 to 20. An average rank method was used to obtain F (σ) from the experimental data σ _i . The cumulative failure probability F (σ _i ) corresponding to each data is given by the following equation (3) (see Non-Patent Document 13).

In equation (3), N is the number of data and i is the rank of the data.

FIG. 11 shows a Weibull plot of cumulative failure probability and bending stress in the above-described sample bending test. Under condition 1 (black square) indicating high strength, the Weibull plot consists of one straight line, and it is considered that the sample was destroyed from one type of destruction source. Further, as a result of analyzing the Weibull plot by the least square method, a high value of m = 8.49 was obtained. As an example of the m value of the salt, there is a report that m = 5.34 (see Non-Patent Document 4) in a bending test of a NaCl sintered material, but it is considered that the reliability is high compared to this.

On the other hand, in condition 2 (white circle) indicating low strength, the Weibull plot is refracted in the vicinity of 16 MPa, which clearly indicates that the sample was broken from a plurality of breakage sources. Assuming that there are two types of destruction sources, the least square method analysis was performed by dividing into regions I and II as shown in FIG. The m values of Region I and Region II were 1.04 and 6.75, respectively. Region I shows that m is remarkably small and the reliability is low. On the other hand, the region II has a relatively large m value and is considered to have a relatively high reliability.

Generally, in brittle materials such as ceramics, strength is governed by defects and fracture toughness values, and it is known that the relationship between them is given by the following equation (4).

In equation (4), K _IC is the fracture toughness value, σ _c is the stress at the time of fracture, and c is the depth of the crack. Since tensile stress is applied to the surface in the bending test, the bending strength is considered to be governed by the fracture toughness value and defects near the surface. The fracture toughness value K _IC is a value determined by the elastic constant of the crack tip and crack propagation energy. Since the microstructure and hardness of the surface are not greatly affected by the mold temperature, it is assumed that K _IC does not change greatly at the mold temperature. Therefore, surface defects are considered to be the dominant cause of changes in bending strength due to mold temperature.

Under condition 2, as shown in FIG. 8, it has been observed that deep cracks of several millimeters or more are irregularly generated by a dye penetration test. This is considered to be a destruction source of the region I having low reliability and strength. The region II having a high shape parameter m value has a high bending strength and a smaller depth of cracks serving as a fracture source. As such a fracture source, a microcrack observed by SEM can be considered. The depth of this microcrack is at most several tens of μm. Therefore, in region II, when there is no large crack between the fulcrums clearly observed in the dye penetration test, it is considered that the microcrack becomes a fracture source and breaks.
Even when the bending strength is high (Condition 1), micro cracks are observed although the number is small, and the observed micro cracks are considered to be the fracture source of the bending test. It is expected that the bending strength increases as the microcracks become smaller. Therefore, it will be important to study the relationship between microcracks and mold temperature in more detail in the future.

[Application to cylinder boolock core]
The present invention is intended for application to a closed deck cylinder block made of aluminum alloy die casting. For example, a single cylinder cylinder block as shown in FIG. The water jacket portion has an undercut shape. As a core for forming such a water jacket portion, as shown in FIG. 12 (b), a core for a water jacket portion of a single cylinder cylinder block was successfully produced by the die casting method of the present invention. ing. The surface is smooth and free of visible defects, and is expected to be suitable for application to aluminum alloy die-cast products.

[Conclusion]
A die core for aluminum alloy die casting (salt core for casting) is manufactured, and the effects of injection pressure, mold temperature on bending strength and Vickers hardness are examined, and surface defects and microstructure are observed. I got a conclusion like this.

(1) The bending strength increases as the mold temperature increases. On the other hand, the bending strength does not change significantly within the range of injection pressures used in this study.
(2) When the mold temperature is low, a large crack is observed on the surface by the dye penetration test, which is considered to be a cause of remarkably reducing the bending strength. On the other hand, surface defects and microcracks were observed when the mold temperature was lower than the surface observation by SEM, and decreased when the mold temperature increased. These are considered to be the starting points of fracture during the bending test. During solidification of the molten salt, the solidified shell that is initially formed on the mold surface tends to shrink during solidification, but because it is under pressure, it cannot shrink and tensile stress is generated on the surface. When the mold temperature is low, the solidified shell cannot be deformed, and it is considered that surface cracks are generated by tensile stress.
(3) As a result of analyzing the results of the bending test by Weibull statistics, it became clear that not only the bending strength increases but also the reliability of the bending strength increases as the mold temperature increases.
(4) Based on the above knowledge, we succeeded in producing a core for a water jacket part of a single cylinder closed deck type cylinder block.

Here, the increase in the strength of the salt core due to the mold temperature described above heats the mixed salt liquidus temperature Tm (absolute temperature K) to a temperature range higher than 0.52 × Tm and lower than 0.7 × Tm. It is thought that it is obtained by this. For example, as shown in the above-mentioned example, in the case of a mixed salt in which Na ₂ CO ₃ is 50 mol%, NaCl is 20 mol%, and KCl is 30 mol%, a practical bending temperature is in the range of 225 to 250 ° C. Strength can be obtained. The liquidus temperature Tm of this mixed salt is 911 K (638 ° C.), the temperature of 0.52 × Tm is about 474 K (201 ° C.), and the temperature of 0.7 × Tm is about 638 K (365 ° C.). ). This range includes “range of 225 to 250 ° C.”.

Next, the results of experiments on a casting core in which the composition ratio of the mixed salt is changed will be described.

[sample]
First, the sample will be described. A sample having a composition with the following ion composition ratio is prepared.

Composition A: Na ⁺ : K ⁺ = 70 mol%: 30 mol%, Cl ⁻ : CO ₃ ²⁻ = 46.2 mol%: 53.8 mol%

Composition B: Na ⁺ : K ⁺ = 80 mol%: 20 mol%, Cl ⁻ : CO ₃ ²⁻ = 45 mol%: 55 mol%

Composition C: Na ⁺ : K ⁺ = 80 mol%: 20 mol%, Cl ⁻ : CO ₃ ²⁻ = 50 mol%: 50 mol%

Composition D: Na ⁺ : K ⁺ = 100 mol%: 0 mol%, Cl ⁻ : CO ₃ ²⁻ = 50 mol%: 50 mol%

[Preparation of sample]
Similarly to the above, each sample is prepared by a die casting method from a mixed salt of compositions A, B, C, and D. In this production, the injection pressure and the mold temperature are each changed.

[Test method]
As described above, a sample of each composition prepared under each condition is tested.

[result]
[Bending test]
A bending test is performed as described above. Table 1 shows the results of a bending test performed on samples with different injection pressures for each composition. In this case, the mold temperature is 250 ° C. In addition, Table 2 below shows the results of bending tests on samples with different mold temperatures for each composition. In this case, the injection pressure is 78.4 MPa. In addition, the result of each sample in Tables 1 and 2 is an average bending strength (MPa).

As shown in Table 1, the higher the injection pressure, the higher the strength. Moreover, as shown in Table 2, in any composition, the higher the mold temperature, the higher the strength. Further, composition A satisfies the minimum requirement (15 MPa) of the strength as the core for aluminum die casting, and the latter half of 20 MPa or more in all the mold temperature ranges of 200 to 265 ° C. tested. Composition B satisfies the above-mentioned minimum requirement (15 MPa) at 200 ° C. and 225 ° C. or more excluding 175 ° C., and the latter half of 20 MPa or more. Composition C satisfies the above-mentioned minimum requirements of 225 ° C. and 250 ° C. or more excluding 175 ° C. and 200 ° C. and 20 MPa or more. Composition D satisfies the above-mentioned minimum requirements at 225 ° C. and 250 ° C. or more excluding 175 ° C. and 200 ° C. Composition C and Composition D are slightly below the minimum requirement at 200 ° C. From these results, it is considered that each composition satisfies the minimum requirement (15 MPa) when the mold temperature is 201 ° C. or higher.

Next, Table 3 shows the analysis results of bending strength by Weibull statistics for each composition. The numerical values in Table 3 are Weibull coefficients.

As shown in Table 3, it can be seen that the higher the mold temperature, the larger the Weibull coefficient. Composition A has a low Weibull coefficient at a mold temperature of 265 ° C. This is presumably because the mold temperature was high, seizure with the mold occurred, and defects were generated on the surface of the formed sample.

Here, the composition A of 225 ° C. and 250 ° C. has a large Weibull coefficient and high reliability, and there is no surface defect. The composition A at 200 ° C. has two Weibull coefficients and is small, so it is not reliable.

200 ° C. of composition B has a Weibull coefficient of m = 5.85, which is smaller than the Weibull coefficients of composition A at 225 ° C. and 250 ° C. Further, the composition B at 175 ° C. has two Weibull coefficients and is not reliable.

The composition C has a Weibull coefficient of m = 8.59, which is smaller than that of the composition A, but the reliability is considerably good. The composition C of 200 ° C. has two Weibull coefficients and is not reliable.

Compositions 250 ° C and 200 ° C have two Weibull coefficients and are not reliable. The composition D at 225 ° C. has a Weibull coefficient of m = 4.56, which is smaller than that of the composition A, and is inferior in reliability. At 175 ° C. of composition D, the Weibull coefficient is as small as m = 1.81.

Next, FIG. 13 shows the relationship between the average bending strength (Table 2) and the Weibull coefficient (Table 3) of each sample. As the salt core material created by the die casting method, the composition at the upper right of the graph is considered to be the most suitable (region surrounded by an ellipse). The composition on the upper right is composition A, and on average strength and Weibull coefficient, composition A, casting pressure of 78.4 MPa, mold temperatures of 250 ° C. and 225 ° C. can be said to be optimal compositions and conditions.

As described above, it can be seen that in all compositions, higher strength and reliability are obtained as the mold temperature is higher. However, in the case of a mold temperature of 265 ° C., it can be seen that high strength cannot be obtained due to seizure with the mold. Moreover, in the composition range described above, particularly high strength and reliability are obtained in the composition A.

[Penetration testing]
Next, the result of the dyeing | penetration penetration killing test in the sample of each composition is demonstrated. FIG. 14, FIG. 15, FIG. 16, and FIG. 17 show the results of visual observation of the surface defects of the test pieces having the compositions A, B, C, and D by the dye penetrant flaw detection method. In the figure, the colored part is a cracked part. The number (temperature) in the figure indicates the mold temperature. In addition, Table 4 below shows a rough degree (evaluation result) of the defect amount based on the observation results of FIGS. 14 to 17 for the respective compositions A, B, C, and D.

As can be seen from FIGS. 14 to 17, it can be seen that crack cracks on the surface decrease as the mold temperature increases in all compositions. In composition A, many surface cracks are observed when the mold temperature is 200 ° C., and almost no surface cracks are observed when the temperature is 250 ° C. or higher. Similarly, the composition B has a mold temperature of 200 ° C. or higher, the composition C has a mold temperature of 225 ° C. or higher, and almost no surface cracks are observed. On the other hand, with respect to the D composition, defects remain even at a mold temperature of 250 ° C., and it is expected that a higher mold temperature is required to eliminate all surface defects. From these results, it can be seen that surface defects decrease when the mold temperature is increased. This reason is considered to be because the tensile stress generated when the mold temperature is low is relaxed as described above.

Also, depending on the composition, it is considered that there is a mold temperature threshold at which many surface cracks occur. As shown in Table 4, there seems to be a correlation between the amount of defects (surface cracks) and the solidification interval. Table 4 shows that the mold temperature at which the surface crack almost disappears is a temperature in which the composition conditions are lower in the order of B, C, A, and D, and this can be said to be the order of difficulty of cracking. Furthermore, this order is considered to be consistent with the rank order of the coagulation interval.

Claims

In a method for producing a salt core for casting formed by a molten salt comprising a mixed salt containing a salt of sodium,
A first step of heating the mixed salt to form a molten metal;
When the liquidus temperature of the mixed salt is Tm in absolute temperature (K), the second step of heating the core molding die to a temperature range higher than 0.52 × Tm and lower than 0.7 × Tm; ,
A third step of pressing the molten metal into the heated mold;
And a fourth step of forming a casting salt core by solidifying the molten metal inside the mold.
In the manufacturing method of the salt core for casting of Claim 1,
The method for producing a salt core for casting, wherein the mold is heated to a temperature range of 225 ° C to 250 ° C.
In the manufacturing method of the salt core for casting of Claim 1 or 2,
The mixed salt has a composition of Na + : K + = 70 mol%: 30 mol% and Cl − : CO 3 2− = 46.2 mol%: 53.8 mol%. Manufacturing method.