JP2645600B2 - Manufacturing method of magnesium alloy casting - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION [Industrial Application Field] The present invention relates to a magnesium alloy (including pure magnesium, hereinafter the same) cast by a vanishing model casting method (hereinafter sometimes referred to as “full mold casting method”). The present invention relates to a method for producing a casting, and more particularly, to a method for producing a magnesium alloy casting by a vanishing model casting method using a specific molding sand.

[Prior art] In "Theory and practice of the latest full mold method" (1970, Vue A Ozeroff, two other authors), cast iron, cast steel, aluminum alloy, and the like are used as metals cast by the vanishing model casting method. Has been picked up. However, there is no description about magnesium alloy. Even today, there is no reference to casting magnesium alloy castings by the vanishing model casting method.

Conventionally, foundry sand used for sand casting of a magnesium alloy and the like has been mainly silica sand. However, unlike molten iron and steel alloys, molten magnesium alloys are easily oxidized and easily react with silica sand (main component SiO ₂ ). Therefore, antioxidants and flame retardants are used to prevent oxidation by air and silica sand. And the like must be added and kneaded to silica sand.

However, when silica sand is used as the casting sand for the mold of the vanishing model casting method, the addition of an antioxidant or the like to the silica sand deteriorates the permeability of the silica sand, and the specific gravity of the magnesium alloy melt may be very light, resulting in defects in the casting. It is easy to occur. It is also desirable not to add an antioxidant or the like to silica sand in order to save labor in the casting operation. Furthermore, when silica sand containing no ordinary binder is used as molding sand, the mold is relatively easily broken, and when collapsed in the molten metal into which the silica sand is cast, defects occur in the casting, and the silica sand reacts with the molten metal. As a result, heat is generated and the temperature of the molten metal rises sharply, causing the molten metal to be ejected from the mold.

[Problems to be Solved by the Invention] Since magnesium is active, when silica sand containing no antioxidant is used as the molding sand in the casting method by the vanishing model casting method, the molding sand is cast into the mold for some reason. When collapsed in the molten magnesium alloy, the silica sand violently reacts with the molten metal to generate a large amount of heat, which may cause the molten metal to erupt from the mold, which is dangerous. In order to prevent the molten metal from being ejected due to the above-described factors specific to the magnesium alloy, a method of adding an antioxidant, a flame retardant and the like to the molding sand as in the conventional sand casting method has been considered.
However, this method reduces the permeability of the foundry sand and causes casting defects such as porosity in the foundry. In addition, since it requires complicated work, it is a means that should not be used for labor saving. An object of the present invention is to develop a casting sand that does not react with a molten magnesium alloy, and an object of the present invention is to provide a method of manufacturing a magnesium alloy casting by a vanishing model casting method that overcomes the above difficulties.

[Means for Solving the Problems] According to the present invention, the object described above is as follows: (1) In a vanishing model casting method, a metal element whose oxide is not reduced by metal magnesium at a temperature of 1000 ° C. or less in a casting sand. A method for producing a magnesium alloy casting, characterized by using a carbonate.

That is, the carbonate of the metal element used in the production method of the present invention is such that the oxide of the metal element is not reduced by the metal magnesium at a temperature of 1000 ° C. or less, and specific examples include magnesite, dolomite, and limestone. And
The object of the present invention is suitably achieved by using such a carbonate of a metal element.

In the conventional casting method, the molten metal is cast into a cavity previously formed in the mold, but in the vanishing model casting method, there is no cavity in the mold as is also called a full mold (FULL MOULD) method. That is, the vaporizable model is buried in the molding sand, and the casting is made by directly injecting the molten metal into the mold without removing the model from the molding sand. Styrofoam is usually used for the vaporizable model, and its surface is coated with a mold to prevent seizure between the casting and sand, and is sufficiently dried.
When the molten metal is cast, the model evaporates and disappears due to the heat of the molten metal, and the molten metal alternately flows into the cavity formed therein to form a casting. The molds used in the vanishing model casting method are roughly classified into two types: a mold reinforced by adding a binder or a hardening agent to sand, and a mold made of sand without a binder. The latter mold is suitable for mass production of the same type of casting, as in today, because the molding and unmolding operations are easy to perform. However, since the strength of this type of mold is relatively weak, the collapsed mold sand may fall into the cast molten metal. At this time, the gas adhering to the sand and a small amount of water are released from the sand, and the water reacts exothermically with the molten metal. If silica sand is used as the molding sand, a large amount of heat is generated due to the chemical reaction SiO ₂ + 2Mg → Si + 2MgO + 70.4kcal / mole. The heat of the reaction causes the temperature of the molten metal to rise rapidly, causing the styrofoam to rapidly evaporate, and the gas pressure causes the molten magnesium alloy to be ejected from the mold. Further, when the temperature of the molten metal rises and reaches the boiling point, the vapor of the magnesium alloy also contributes to the above-mentioned molten metal ejection. Magnesium alloy melts are usually cast at 700 to 750 ° C, but as casting sand, they are not reduced by metallic magnesium at normal pressure and at temperatures below 1000 ° C. When an oxide of a metal element that is more thermodynamically stable than magnesium oxide) is used, the molten magnesium alloy is not oxidized by the molding sand to generate heat. As the foundry sand, in addition to the above-mentioned oxides of the metal elements, carbonates thereof can also be used. When heated, carbonates decompose into oxides and carbon dioxide. For example, magnesite is 600 ℃, dolomite is 750
℃, limestone pyrolyzes at 900 ℃. Since this reaction is an endothermic reaction and acts in a direction to lower the temperature of the molten metal, there is no actual harm even if the mold collapses and the sand comes into contact with the molten metal. For example, when 1.5 kg (approximately 1 in bulk) of 20 ° C. magnesite powder falls into 7 kg of pure magnesium melt at 750 ° C., the temperature is reduced to 600 ° C. or less due to decomposition endotherm. Carbon dioxide should oxidize magnesium thermodynamically, but practically, almost no oxidation is observed at such a molten metal temperature. Oxides satisfying the above conditions include:
For example, magnesia clinker, or carbonate,
The above-mentioned magnesite, dolomite, limestone and the like are mentioned, but it goes without saying that they are not limited to these.

Hereinafter, the present invention will be described in detail with reference to Examples and Comparative Examples.

[Examples] (Example 1) As shown in Fig. 1, the center of a styrofoam disk having a diameter of 400 mm and a thickness of 30 mm is vertically and horizontally 40 mm long x 150 mm long.
The styrofoam prisms are glued together with an adhesive, and the molds are applied to the surfaces of the prisms in a dry state to a thickness of about 0.5 mm and dried for one week. A mold having a brick gate placed on the prism was prepared. A weight of about 20 kg was placed on the foundry sand composed of the limestone powder. The water content of the limestone powder was about 0.02%. In this mold, magnesium alloy AZ91 (containing Be)
The molten metal was degassed at 750 ° C. by adding 0.5% ethane hexachloride, and then cast at the same temperature.

When the cast molten metal was solidified and the casting was removed from the sand and examined, the dimensions were reduced by the amount of shrinkage when the molten metal was solidified, but the appearance was good.

(Example 2) Almost the same experiment as in Example 1 was performed. However, immediately after casting, the sand (limestone powder) near the gate was hit once with a mallet to give an impact. At this time, it was observed that the molten metal overflowed from the gate and flowed down onto the sand, and gas was slightly emitted from the inside of the gate, but there was no ejection of the molten metal. When the casting was removed from the sand after solidification and examined, it was found that the casting contained sand.

(Example 3) and (Example 4) The same experiment as in Example 1 and Example 2 was performed using magnesia clinker (water content: 0.05%) having a particle size of 0.5 mm or less as the molding sand. The results were almost the same.

(Comparative Example 1) Example 1 was used as a casting sand of silica sand having a particle diameter of 0.5 mm or less.
The same mold as that described above was prepared. The water content of the quartz sand was 0.02%. A AZ91 (Be-containing) alloy melt at 750 ° C. subjected to the same degassing treatment as in Example 1 was cast into this mold. However, unlike the case of Example 2, no impact was applied by a mallet. About 5 seconds after casting, the molten metal began to squirt from the gate.
After the casting was solidified, it was taken out of the sand and examined.
Both disk parts were deformed into irregular shapes, turning into black hollow ceramics.

(Comparative Example 2) The following flame retardant was added to silica sand having a particle diameter of 0.5 mm or less so that the following addition ratio was obtained, and the mixture was kneaded by a kneading machine as casting sand to prepare the same mold as in Example 1. Was.

Diethylene glycol 2% Boric acid 2% Potassium borofluoride 1% Sulfur (powder) 0.5% The same type of magnesium alloy melt as used in Example 1 was cast into this mold, but there was no ejection of the melt. The disc portion of the cast casting was much thicker than the original vanishing model, and the appearance was defective.

[Effects of the Invention] The present invention provides a method for manufacturing a magnesium alloy casting by a vanishing model casting method, in which a molten metal is not spouted during casting and work can be performed safely. In addition, there is no need to add a flame retardant or the like to the foundry sand, so that labor can be saved.

[Brief description of the drawings]

FIG. 1 is a schematic view showing a mold used in Example 1 of the present specification. …… Cast frame,… Limestone powder (casting sand),… Gate,… Styrofoam square pillar,… Styrofoam disc, …… Weight.

Claims (1)

(57) [Claims] 1. A method for producing a magnesium alloy casting, wherein in the vanishing model casting method, a carbonate of a metal element whose oxide is not reduced by metal magnesium at a temperature of 1000 ° C. or less is used as casting sand.