JPH0229416B2 - - Google Patents

Description

[Detailed description of the invention]

(Industrial Application Field) This invention relates to a method for manufacturing a precision casting mold,
More specifically, the present invention relates to a method of manufacturing a mold with high hot strength using the improved slurry. (Prior art) For example, when molding a heat-controlled unidirectional solidification or single crystal precision casting product, the mold is heated to a temperature above the melting point of the metal, for example around 1500°C, and held for about 10 minutes before pouring the molten metal. The molten metal is then injected and then cooled slowly while controlling the heat from outside the mold through the mold. In order to facilitate this thermal control, it is desirable that the mold has good thermal conductivity or has a thin wall. However, alumina, which is a refractory material used as a mold material, has low thermal conductivity, and if the wall thickness is made thin, the strength of the mold decreases and it is easily damaged, and accidents such as hot water leakage are likely to occur, so the wall thickness cannot be made very thin. Experience has shown that the wall thickness is at least 8 to 10 mm, which also makes it difficult to control heat during cooling. (Problem to be solved by the invention) The bending strength (hereinafter simply referred to as strength) of a mold made of alumina, which is generally used as a mold material, depends on its composition, but the so-called de-soldering process, in which the solder model is melted and removed, In the previous raw form, it is 30-50Kg/cm ² , and when heated, it reaches a maximum of 300-400Kg/cm ² at about 1100℃. However, when heated further to 1,200℃ or higher, the hot strength rapidly decreases.For example, when casting a single crystal cast product, if the preheating temperature of 1,500℃ is held for 10 minutes, it decreases to about 25-30Kg/ ^cm2. . Therefore, molten metal must be injected while the hot strength of the mold is quite low, making it impossible to reduce the wall thickness of the mold. In view of these circumstances, an object of the present invention is to provide a method for manufacturing a precision casting mold with high hot strength. (Means for solving the problem) In a method for manufacturing a precision casting mold in which a solder mold is coated with slurry and refractory particles to form a shell,
The present invention relates to a method for manufacturing a precision casting mold with high hot strength, which uses a slurry in which alumina powder and 0.1 to 4% by weight of tri-iron oxide powder are suspended in colloidal silica. By the way, the reason for the decrease in the hot strength of alumina molds is not clear, but it may be due to Na ₂ O and colloidal silica stabilizers contained in alumina as impurities.
It is said that local formation of a glass phase or liquid phase due to a solid phase reaction between Na ₂ O or alumina and silica as a binder is involved. However, according to the inventor's research, Na ₂ O and
No significant relationship was observed with the strength of the mold at 1500°C. On the other hand, as a result of investigating the solid phase reaction between alumina and colloidal silica using X-ray diffraction, it was found that at temperatures below 1300°C, silica is amorphous, so α
Although only alumina could be identified, it was found that silica transforms into cristobalite at 130°C, and that mullite is produced when heated at 1500°C for more than 1 hour. Therefore, when we heated an alumina mold bonded with colloidal silica at 1500℃ for 1 hour and measured its hot strength, we found that it was about 200Kg/ ^cm2 , which was a significant increase in strength. When heated, no mullite was produced in an amount that could be identified by X-ray diffraction, and the strength was about 30 kg/cm ² . That is, if an alumina mold bonded with colloidal silica is reacted and sintered at high temperature for a sufficient period of time, mullite will be generated and the hot strength of the mold can be improved. It would be of practical benefit if this could be achieved in a short period of time, so we conducted various experiments to find conditions that would make this possible. Identification by X-ray diffraction confirmed that the addition of iron trioxide was effective. In other words, as mentioned above, the production of mullite from alumina and colloidal silica requires 1500
It is necessary to heat the product for about 1 hour at 1500°C, but if triiron oxide is added to this, mullite is produced by heating at 1500°C for about 10 minutes. Next, an example will be described. Example 1 Rubber plates (70×
150 x 2 mm) were prepared, and using slurry F and 100# alumina from the slurry composition shown in Table 1 on each model, the first layer of the mold shell was formed by a conventional method. . Next, using slurry A shown in Table 1 for one shell and slurry B for the other shell, shells consisting of a total of seven layers were fabricated using alumina particles as refractory particles, that is, stucco materials, by a conventional method. After thoroughly drying the shell, it was mechanically separated from the rubber plate and cut into a piece of 15 mm wide and 70 mm long to be used as a test piece.

[Table] The green strength of the test piece is 35~45Kg/ ^cm2 , 1200~1500℃
Figure 1 shows the hot strength when heated and held for 10 minutes. For comparison, Fig. 1 shows the strength when held at 1500°C for 1 hour. The strength of the mold made with slurry A and alumina is
Approximately 200Kg/cm ² at 1200℃ to approximately 10 minutes at 1500℃
It decreased to 25Kg/cm ² . In this state, mullite could not be identified by X-ray diffraction. When this is heated to 1500° C. for 1 hour, mullite is produced as described above, and the strength increases to about 200 kg/cm ² . On the other hand, the mold made using Slurry B, in which iron dioxide was added as a mineralizing agent, was heated to 1400℃.
Although the strength was the same as that using slurry A, it increased to about 150 kg/cm ² after heating at 1500° C. for 10 minutes, and the formation of mullite was identified by X-ray diffraction.
After heating at 1500°C for 1 hour, the strength increased to approximately 250Kg/cm ² . The amount of triferric oxide added is 0.1 per alumina powder.
~4% by weight is appropriate, and if it is less than 0.1%, the temperature is 1500℃ x 10
Heating for 1 minute will not produce enough mullite and no increase in the strength of the mold can be expected. The amount of mullite produced needs to be approximately 5% or more, which can be identified by X-ray diffraction, but increasing the amount of tri-iron oxide in an attempt to increase the amount of mullite is undesirable because the refractory of the mold will decrease. The limit is preferably 4%, preferably 0.5 to 2%. The particle size is preferably about 300 mesh from the viewpoint of reactivity. The standard heating temperature is 1500℃, which is close to the injection temperature for single-crystal casting.If the injection temperature is high, it will be higher, but from the viewpoint of economy and workability, it is recommended to keep it at about 1570℃ or less. Good, also around 1450
If the temperature is below ℃, it is difficult to obtain a satisfactory cast product. The standard heating time is 10 minutes, and from the point of view of economy and workability, it is preferable to set the heating time to about 5 minutes or more and 15 minutes or less. The appropriate size of the refractory particles used with the slurry is approximately 0.5 to 1.5 mm in consideration of strength and air permeability. In this way, a slurry made by adding a predetermined amount of iron dioxide to colloidal silica and alumina powder are applied to a solder model to form a shell, and after de-soldering,
Just by heating at 1500℃ for 10 minutes,
The hot strength of the solder mold is 150, which is about 6 times the resistance strength of conventional products.
Since it is about Kg/cm ² , the mold, which conventionally required a wall thickness of about 10 mm, has sufficient strength even as a mold with a wall thickness of 4 to 5 mm. Therefore, it is easy to control heat during precision casting, directional solidification or casting of single crystal castings is facilitated, the yield of good products is increased, and the number of times of shell coating can be reduced. The effect is extremely large. Example 2 A chisel type model for single crystal casting containing two creep rapture test pieces with a diameter of 6 mm was used, and the mold was the same as that of Example 1.
In the same manner as above, coat the wall 6 times to obtain a wall thickness of 4-5 mm.
A mold was made, and after de-soldering, it was fired at 1100°C for 1 hour and cooled. The mold was then placed in a single crystal furnace and heated to 1500°C.
The temperature was raised in 40 minutes to 100%, held for 10 minutes, and then poured under normal single-crystal casting conditions. A satisfactory single-crystal precision cast product could be obtained without mold destruction or other accidents or defects. (Effect) We used a slurry of alumina powder and 0.1 to 4% by weight of iron dioxide powder suspended in colloidal silica, and heated it at 1450 to 1570°C for 5 to 15 minutes. It is possible to increase the mechanical strength and reduce the wall thickness, which makes it easier to control heat during precision casting, and because it is fired in a short time, it improves the workability and economy of mold production. It has good properties and is very practical.

[Brief explanation of drawings]

FIG. 1 is a graph showing the relationship between heating temperature time and hot strength of an alumina mold.

Claims (1)

[Claims] 1. In a method for manufacturing a precision casting mold in which a solder mold is coated with slurry and refractory particles to form a shell, alumina powder and 0.1 to 4% by weight of triiron dioxide powder are suspended in colloidal silica. A method for producing an alumina mold for precision casting with high hot strength, characterized by using slurry and heating at 1450 to 1570°C for 5 to 15 minutes.