JPH0420699B2 - - Google Patents

Description

Detailed Description of the Invention (Field of Industrial Application) The present invention relates to a cooling method used in lost wax casting of magnesium alloys by the ceramic shell molding method, and particularly suitable for casting thin-walled magnesium alloy castings. It is something.

(Prior Art) In the lost wax casting of magnesium alloys, the solid mold method has conventionally been mainly used. In this method, a frame is placed around the vanishing model, and a slurry made of water and a refractory using calcium sulfate (CaSO ₄ , gypsum) as a binder is filled in the frame, and after the slurry hardens, the vanishing model is placed. By causing the metal to disappear, a cavity in the shape of the product is created inside the mold, and after further firing, the molten magnesium alloy is cast into this cavity.

However, the solid mold method has a problem in that the heat capacity increases due to the back-up refractories, which slows down the cooling rate of the molten metal after casting, causing crystal grains to grow large and reducing mechanical strength. There was also the problem of poor productivity due to the long cooling time.

Therefore, it is conceivable to use the ceramic shell molding method for casting this magnesium alloy. This method involves repeating the process of applying slurry and refractory to a disappearing model several times to achieve the required thickness, and then firing the disappearing model to create a ceramic shell mold. According to this method, rapid cooling is possible because the heat capacity of the mold is smaller than that of the solid mold method.

However, when such a rapid cooling method is applied to casting magnesium alloys, cracks are likely to occur in the thin walled parts of the mold or product during cooling, especially in the case of thin castings or castings with uneven wall thickness. However, there were problems in that it was often impossible to produce sound castings, and the yield was extremely poor. This is because the amount of contraction during solidification of the metal is different from the amount of contraction of the mold, which creates stress in the thin walled parts, and if the mold is too hard or lacks elasticity, this stress increases and is thought to cause cracks. . This tendency is remarkable in magnesium alloys, where the volume change due to phase change during solidification is large and the strength immediately after solidification before age hardening is low, making it extremely difficult to mass-produce sound castings at a high yield. .

(Problems to be Solved by the Invention) The present invention was made in view of the above circumstances, and is intended to mass-produce healthy magnesium alloy castings with a high yield and high productivity without causing cracks in the mold or cracks in the product during solidification. The purpose of the present invention is to provide a cooling method in lost wax casting that is suitable for casting thin-walled magnesium alloys.

(Means for Solving the Problems) According to the present invention, this object is achieved by casting a molten magnesium alloy into a ceramic shell mold heated to a temperature slightly lower than that temperature under an inert gas atmosphere, after which a solid solution is generated. Cooling fluid is injected into the mold from the outside to forcibly cool the mold from the outside to a predetermined temperature higher than the solid solution temperature range, and then the mold is slowly cooled in a temperature-controlled holding furnace to a temperature lower than the solid solution temperature range. This is achieved by a cooling method in lost wax casting of magnesium alloy.

(Example) FIG. 1 is a process explanatory diagram of an example of the present invention, and in this example, a magnesium alloy with the alloy name AZ91 is used. This alloy AZ91 has the following composition:

A1 8.1~9.3% Zn 0.4~1.0% Mn 0.13~0.5% Si 0.3% or less, Cu 0.10% or less, Ni 0.01% or less, Mg balance Firing furnace 12 made in two stages, upper and lower
and a holding furnace 14. The firing furnace 12 is heated by a plurality of gas burners 16. The holding furnace 14 sucks a portion of the high-temperature air of the firing furnace 12 and outside air into the holding furnace 14 using exhaust negative pressure, so as to maintain a desired temperature distribution inside the holding furnace 14 .

Reference numeral 18 denotes a ceramic shell mold, which is placed on a pallet (not shown) and slowly transported within the firing furnace 12 by a transport device.

20 is a casting device, which includes a sealable pouring chamber 2;
2 and a ladle 24. Pouring spout 2 of ladle 24
6 can be sealed with a lid, and the bottom of the ladle 24 is provided with a valve port 30 whose opening and closing are controlled by a valve rod 28. The pressure inside the pouring chamber 22 is reduced by a vacuum pump 36 via a cooling device 32 and an on-off valve 34 . 38 is inert gas such as sulfur hexafluoride (SF ₆ ), carbon monoxide (CO), and bad sulfuric acid gas (SO ₂ ).
A gas cylinder containing reducing gas such as
It is connected to the pouring chamber 22 via a pressure reducing valve 40 and an on-off valve 42 . Further, 44 is an on-off valve for introducing the atmosphere into the pouring chamber 22.

Reference numeral 48 denotes a quenching device, which for example uses a spot cooler to apply cooling air to the mold 18 after pouring the metal to forcibly cool it down.

The casting procedure for the magnesium alloy is as follows. The shell mold 18 is fired in the firing furnace 12 and maintained at a predetermined temperature (approximately 700°C).
On the other hand, an alloy is melted in advance at a predetermined temperature (approximately 800°C) in a melting furnace (not shown), and its surface is covered with refining flux to provide fire prevention measures. This molten metal is quickly poured into a ladle 2.
4, its surface is covered with a small amount of flux, and the pouring port 26 is closed with a lid. When the temperature of the molten metal in the ladle 24 falls to the specified temperature (approximately 740 to 700°C), the mold maintained at the specified temperature in the firing furnace 12 is moved into the pouring chamber 22, and the sprue is placed under the valve port 30. will be placed in At this time, a strainer is set in this sprue. After closing and sealing the mold loading/unloading opening of the pouring chamber 22 with a cover plate, the vacuum pump 36 is activated and the valve 34 is opened to bring the inside of the pouring chamber 22 to the specified pressure (approximately -650 mmHg based on atmospheric pressure). Reduce the pressure to
When the pressure is reduced to this specified pressure, the pump 36 is stopped and the valve 3
4 is closed, and at the same time, the valve 42 is opened to send the reduced pressure reducing gas from the cylinder 38 to the pouring chamber 22. Then, the degree of vacuum in the pouring chamber 22 decreases (approaches atmospheric pressure), and when the pressure reaches the specified pressure (approximately -550 mmHg), the valve stem 28 is raised to open the valve port 30 and the mold 1 is immediately removed.
Pour the molten metal into Step 8. At this time, the inside of the pouring chamber 22 is -
It is desirable to complete casting by the time the temperature reaches 450mmHg. After the casting is completed, the valve 42 is closed to stop the gas supply, and the valve 44 is opened to introduce the atmosphere. The series of operations from transferring the molten metal from the melting furnace to the ladle 24 to opening the valve 44 and introducing the atmosphere is preferably completed within about 2 minutes.

Next, the lid plate of the pouring chamber 22 is opened, the sprue is covered with flux, and the molten metal is transferred to the quenching device 48.
Rapidly cool the mold by blowing cooling air onto the outside of the mold for 2 minutes. Figure 2 shows the main components of this alloy.
This is a phase diagram of an alloy of Mg and Al, and its solidus line a
The temperature range (approximately 570℃, A in Figure 2) is higher than the temperature range of the α solid solution (C in the figure) sandwiched between and the solubility line b.
This quenching device 48 rapidly cools the sample to a point). After that, the mold 18 is transferred to the holding furnace 14, where it is heated to 200 to 300℃.
(point B in Figure 2) for sufficient time (60 to 90
1 minute) and slowly cool down. FIG. 3 is a cooling characteristic diagram showing the relationship between cooling time (horizontal axis, logarithmic scale) and temperature (vertical axis).

In this way, since the alloy is rapidly cooled to a temperature higher than the solid solution temperature range (C in FIG. 2), the time required for the cooling process can be shortened and the casting efficiency can be improved. Also, in the solid solution temperature range C, the amount of alloy shrinkage is large;
Since the slow cooling is performed here, the stress caused by shrinkage is sufficiently absorbed between the mold and the mold, so that the stress does not become excessive.
Cracks do not occur even in thin parts.

(Effects of the Invention) As described above, the present invention sprays cooling fluid to the outside of the mold immediately after pouring the magnesium alloy in an inert gas atmosphere to rapidly cool it, and then slowly cools it in the solid solution temperature range. No stress is applied,
There is no cracking in the castings due to interference with the mold, and healthy castings can be produced at a high yield. Additionally, since the molten metal is rapidly cooled after pouring, the time required for the entire cooling process can be shortened, improving productivity and making it suitable for mass production.
Furthermore, this rapid cooling enables the refinement of the metal structure,
At the same time, it is possible to improve the physical and mechanical properties of the casting.

[Brief explanation of drawings]

Fig. 1 is a process explanatory diagram of one embodiment of the present invention;
The figure is a phase diagram of the alloy, and Figure 3 is a cooling characteristic diagram. 12... Holding furnace, 48... Rapid cooling device, C... Solid solution temperature range.

Claims (1)

[Claims] 1. After casting the molten magnesium alloy into a ceramic shell mold heated to a temperature slightly lower than that temperature under an inert gas atmosphere, a cooling fluid is injected into the mold from the outside until the temperature reaches a predetermined temperature higher than the temperature at which a solid solution is generated. A cooling method in lost wax casting of a magnesium alloy, which is characterized in that the cooling is forcibly quenched from the outside of the mold, and then slowly cooled in a temperature-controlled holding furnace to a temperature lower than the solid solution temperature range.