CN103624237B - A kind of magnesium alloy Anti-gravity casting device and method thereof - Google Patents

Abstract

The invention discloses a magnesium alloy anti-gravity casting device, which comprises: a melting furnace 2 and a crucible 18 are built in a lower tank 19, and the upper part of the lower tank 19 is isolated from the outside by a sealing cover plate 16; 2 and the sealing cover plate 16, the lower tank 19 is connected to the first protective gas pipeline 3; the middle and lower part of the lower tank 19 is connected to the pressurizing device 1; the negative pressure gas protection box 11 is connected to the second protection gas pipeline 3; Gas pipeline 8 is connected with negative pressure pipeline 9. The invention also discloses a casting method using the device. A magnesium alloy anti-gravity casting device disclosed in the present invention can produce large, complex and thin-walled magnesium alloy castings. The surface quality and safety of the castings are high and meet production requirements. The device is simple to manufacture and is suitable for large-scale production of magnesium alloys. efficient.

Description

A kind of magnesium alloy anti-gravity casting device and method thereof

technical field

The invention relates to the technical field of casting, in particular to a magnesium alloy anti-gravity casting device and a method thereof.

Background technique

Magnesium alloy has low density and excellent performance. As an important structural material, it is widely used in aviation, aerospace, national defense, automobile, electronic communication and other fields. At present, the urgent need for weight reduction in my country's aviation, aerospace, and national defense fields provides opportunities and challenges for the development and application of new materials and processes for magnesium alloys. At present, one of the main factors limiting the wide application of magnesium alloy castings is the difficulty in casting magnesium alloy castings, especially for large, complex, thin-walled magnesium alloy castings, it is currently difficult to produce castings that meet the needs. One of the difficulties in casting is that there is no suitable casting. Magnesium alloy casting production equipment, because of the characteristics of magnesium alloy itself, its casting is relatively difficult, and the casting process is prone to combustion or oxidation, which has become a major bottleneck restricting the production of magnesium alloy castings.

Die casting is currently the most common forming process for magnesium alloys. More than 90% of magnesium alloy castings are die castings, but die casting can only form castings with small size and simple structure, and due to the existence of pores, castings cannot be heat treated or welded. Deeper machining is also not possible.

Shanghai Jiaotong University has developed a precision low-pressure casting molding process for large magnesium alloy castings, using double melting furnaces and pressure converters to ensure the high purity of magnesium liquid; using a new crucible sealing technology to seal the crucible (low melting point alloy sealing method, improve protection Compression pressure; use cold iron, gas cooling, etc. to achieve the solidification sequence required by the casting; use protective gas pressurization technology, that is, add a new type of R152 protective gas in dry compressed air, and use coating transfer technology to ensure the non-processed surface of the casting. Surface finish. But the technology is more complex and not easy to operate.

For magnesium alloy gypsum precision casting, the Fifth Research Institute of China Ordnance Industry invented a ceramic gypsum composite casting mold to solve the oxidation and combustion problems of magnesium alloy filling process. In addition, there are reports that flame retardants are added to the gypsum base powder. Or vacuum pouring, take out the dried gypsum mold, clean the impurities in the mold cavity, connect the vacuum pump to the vacuum pump, seal the gate, and start pouring after vacuuming with the vacuum pump. Exhaust pipes are placed on the cavity, but the above studies are related to the improvement of the plaster type, and the process is relatively complicated, so it is not suitable for mass industrial production.

Contents of the invention

(1) Technical problems to be solved

The technical problem to be solved by the present invention is: to provide a magnesium alloy anti-gravity casting device to overcome the technical problem in the prior art that the magnesium alloy is prone to combustion or oxidation during the pouring process; and to provide a casting device using the device method.

(2) Technical solutions

In order to solve the above technical problems, on the one hand, the present invention provides a magnesium alloy anti-gravity casting device, which includes: a melting furnace 2 and a crucible 18 are built in the lower tank 19, and the upper part of the lower tank 19 adopts a sealing cover plate 16 Isolated from the outside; between the melting furnace 2 and the sealing cover plate 16, the lower tank 19 is connected to the first protective gas pipeline 3; the middle and lower part of the lower tank 19 is connected to the pressurizing device 1; the The crucible 18 is built with a riser 17, the lower end of the riser 17 has a certain distance from the bottom of the crucible 18, and the riser 17 is connected to the surface of the magnesium alloy melt; The port part passes through the sealing cover plate 16 and is aligned with the gate of the plaster mold 14 placed directly on the sealing cover plate 16; the plaster casting mold 14 has a cavity 15 on which asbestos 13 for sealing, and the asbestos 13 is covered with an asbestos pressing plate 7;

The negative pressure gas protection box 11 is connected with the second protection gas pipeline 8 and the negative pressure pipeline 9; the negative pressure gas protection box 11 is directly placed on the plaster mold 14, and the bottom of the negative pressure gas protection box 11 The end is fixedly connected with the plaster mold 14, and the joint is sealed with a sealing silicone ring 5; the negative pressure gas protection box 11 is fixedly connected with the sealing cover 16 and locked by the locking mechanism 4.

Preferably, a spring 12 and an adjustment stud 10 are also provided in the negative pressure gas protection box 11, and the asbestos pressing plate 7 fastens the asbestos 13 and the plaster mold 14 through the spring 12 and the adjustment stud 10 connect.

Preferably, there are four springs 12 and adjusting studs 10, which are evenly distributed.

Preferably, a guide plate (21) with holes is welded on the middle and upper part of the inner wall of the negative pressure gas protection box (11), and the spring (12) passes through the guide plate (21).

Preferably, asbestos is used to seal the contact surfaces between the riser pipe 17 and the sealing cover plate 16 .

Preferably, the first protective gas pipeline 3 and the second protective gas pipeline 8 are respectively connected to the first protective gas injection device and the second protective gas injection device; or the first protective gas pipeline 3 and the second protective gas pipeline 8 is connected to the same protective gas injection device, and the protective gas injection device is controlled by a control system. The control system is provided with a first control valve on the first protective gas pipeline 3, and a first control valve is provided on the second protective gas pipeline 8. There is a second control valve.

Preferably, the negative pressure gas protection box 11 and the sealing cover plate 16 are connected by studs 6 .

Preferably, the second protective gas pipeline 8 and the negative pressure pipeline 9 are arranged on the upper part or the side wall of the negative pressure gas protective box 11 .

Preferably, the protective gas is a mixed gas of 1-10% sulfur hexafluoride and 90-99% argon.

On the other hand, the present invention also provides a casting method utilizing the above-mentioned device, the method comprising the following steps:

Step 1. Put the magnesium alloy into the melting furnace to melt and remove slag at 730°C-820°C, and heat the gypsum mold at 200°C-400°C;

Step 2. Put the molten magnesium alloy after smelting and deslagging into the crucible, seal the lower tank with a sealing cover, seal the contact surface between the riser and the sealing cover with asbestos, and lock the The sealing cover plate is locked with the lower tank;

Step 3. Place the heated plaster mold directly on the sealing cover, align the gate with the upper port of the riser, and use asbestos to seal the plaster mold, and then turn the negative pressure The gas protection box is directly placed on the plaster mold and fixedly connected with the sealing cover;

Step 4, using a mixed gas of 1-10% sulfur hexafluoride and 90-99% argon at a pressure of 1-5KPa to pressurize the lower tank through the first protective gas pipeline for 1-3 minutes;

Step 5. Vacuumize the negative pressure gas protection box through the negative pressure pipeline located on the upper part or side wall of the negative pressure gas protection box, and the vacuum degree reaches -20～-30KPa; then immediately put 1-10% six The mixed gas of sulfur fluoride and 90-99% argon is injected into the negative pressure gas protection box through the second protective gas pipeline, and the vacuum degree reaches -10～-20KPa; adjust the adjusting stud, and align the adjusting stud with the spring through the guide plate ;

Step 6: Pressurizing the surface of the magnesium alloy melt with an air pressure of 10-80 KPa by means of a pressurizing device, so that the magnesium alloy melt enters the mold cavity from bottom to top along the riser pipe, and implements low-pressure casting.

(3) Beneficial effects

A kind of magnesium alloy anti-gravity casting device and method provided by the present invention has the following advantages:

1. Vacuumize the negative pressure gas protection box through the negative pressure pipeline, and then inject the protective gas through the second protective gas channel, and the protective gas enters the gypsum casting mold to discharge the air in the cavity, preventing the magnesium alloy melt from contacting the gypsum The reaction combustion of the casting mold avoids defects such as surface oxidation, slag inclusion and cold shut of the casting. Surface quality and internal organizational structure. The negative pressure gas protection box has an independent structure, is simple and easy to operate, and can realize mass production of thin-walled complex magnesium alloy parts, and is convenient for trial production or small-batch production of single large thin-walled magnesium alloy parts.

2. Inject protective gas into the lower tank through the first protective gas pipeline to prevent the magnesium alloy melt from burning, and apply an external force to the lower tank through the pressurizing device to make the magnesium alloy melt overcome its own gravity, viscous force and pressure in the cavity. pressure, so that the magnesium alloy melt moves from bottom to top along the riser pipe, realizes anti-gravity pouring, and ensures that the magnesium alloy melt flows in the cavity at a stable speed, which can prevent the cold shut phenomenon of the casting and prevent the casting from forming Oxidizes slag inclusion defects, thereby facilitating the formation of thin-walled parts.

Therefore, adopting the casting device of the present invention, the production of magnesium alloy castings has high safety, and is suitable for producing large, complex and thin-walled magnesium alloy castings. The surface of the castings has no oxidation, slag inclusions, and cold shut defects, and the internal structure is dense, and the performance is good, which meets the production requirements; The device is simple to manufacture, easy to popularize, suitable for large-scale production of magnesium alloys, and has high production efficiency.

Description of drawings

Fig. 1 is the structural diagram of a kind of magnesium alloy anti-gravity casting device of the present invention;

Fig. 2 is the structure and installation diagram of the negative pressure gas protection box of the present invention;

Fig. 3 is an effect diagram of a part of a magnesium alloy casting being oxidized.

Detailed ways

The specific implementation manners of the present invention will be further described in detail below in conjunction with the accompanying drawings and the embodiments thereof. The following examples are only used to illustrate the present invention, but not to limit the scope of the present invention.

Embodiment one:

As shown in FIG. 1 , this embodiment describes a magnesium alloy anti-gravity casting device, which includes: a melting furnace 2 and a crucible 18 built in a lower tank 19 , and the upper part of the lower tank 19 is isolated from the outside by a sealing cover 16 . In order to prevent the magnesium alloy melt from burning with oxygen, between the melting furnace 2 and the sealing cover plate 16 , the lower tank 19 is connected to the first protective gas pipeline 3 . In this way, the mixed gas of 1-10% sulfur hexafluoride and 90-99% argon injected through the first shielding gas pipeline 3, the sulfur hexafluoride with a larger proportion can float on the magnesium alloy melt, and react with the magnesium alloy The melt reacts to form a surface film with different protective effects on the surface of the magnesium alloy melt. The surface film is dense, which can not only isolate the magnesium alloy melt from the air, prevent the reaction from continuing, but also prevent the magnesium alloy melt from forming. evaporation. The middle and lower part of the lower tank 19 is connected with the pressurizing device 1, which can ensure that the protective gas on the surface of the magnesium alloy melt can be avoided to be blown away during the pressure application process, and the possibility of further burning of the magnesium alloy melt can be prevented. A riser tube 17 is built in the crucible 18, the lower end of the riser tube 17 has a certain distance from the bottom of the crucible 18, and the inside of the riser tube 17 communicates with the surface of the magnesium alloy melt. When the pressure device 1 is used to pressurize the lower tank 19, the applied external force overcomes the gravity of the magnesium alloy melt itself, the viscous force and the pressure in the cavity 15, so that the magnesium alloy melt moves from bottom to top along the riser pipe 17. Movement, to achieve anti-gravity pouring. The anti-gravity pouring process is a controllable process. According to the wall thickness, height, weight and complexity of the casting, the applied external force can be adjusted to provide stable liquid lifting pressure, mold filling pressure and crust growth for the magnesium alloy melt. Compression pressure and crystallization boost pressure, to overcome the relatively large resistance in the cavity 15 due to the complex structure of the casting, to ensure that the magnesium alloy melt flows in the cavity 15 at a steady speed, which can prevent the cold shut phenomenon of the casting, and It can prevent the formation of oxidized slag defects in castings, which is beneficial to the formation of thin-walled parts. The upper port portion of the riser pipe 17 passes through the sealing cover plate 16 and is aligned with the gate of the plaster mold 14 . The gypsum casting mold 14 is directly placed on the sealing cover plate 16, and the top of the gypsum casting mold 14 adopts asbestos 13 to seal, and the asbestos pressing plate 7 is covered on the asbestos 13.

The top or side wall of the negative pressure gas protection box 11 is connected with the second protection gas pipeline 8 and the negative pressure pipeline 9 . The negative pressure gas protection box 11 is directly placed on the gypsum mold 14, and the two ends of the bottom of the negative pressure gas protection box 11 are fixed with the gypsum mold 14 by countersunk screws, and the joint is sealed with a sealing silicone ring 5, which reduces the weight. It is easy to disassemble and reduces the cost. The negative pressure gas protection box 11 is fixedly connected with the sealing cover plate 16 through studs, and locked by the locking mechanism 4, which can improve the stability and sealing performance of the device.

Through the negative pressure pipeline 9, the negative pressure gas protection box 11 is evacuated, and the gas inside the plaster mold 14 is drawn away by the asbestos 13 and the asbestos pressure plate 7, and the negative pressure pipeline 9 is closed; then through the second protective gas pipeline 8 to Negative pressure gas protection box 11 injects protection gas, makes the air pressure inside negative pressure gas protection box 11 rise, but still keeps in negative pressure state. Under negative pressure, the protective gas enters the mold cavity 15 through the asbestos pressure plate 7 and asbestos 13, and isolates the gypsum mold 14 from the magnesium alloy melt, preventing the magnesium alloy from reacting with the gypsum mold 14, and preventing the surface of the casting from being oxidized , thereby improving the quality of castings.

In order to ensure that the protective gas can enter smoothly into the gypsum casting mold 14 through the asbestos 13, the air in the cavity 15 is discharged to prevent partial oxidation of the casting, as shown in Figure 2, so in the negative pressure gas protection box 11, there is also a The spring 12 and the adjusting stud 10 , the asbestos pressing plate 7 securely connects the asbestos 13 and the plaster mold 14 through the spring 12 and the adjusting stud 10 , as shown in FIG. 2 . By adjusting the stud 10 and the spring 12, the tightness of the asbestos pressing plate 7 against the gypsum mold 14 is adjusted to achieve the purpose of regulating the air permeability of the gypsum mold 14.

In order to ensure the uniform penetration of the protective gas into the cavity 15, to ensure the stability of the gas in the cavity 15, and to stabilize the flow of the magnesium alloy melt in the cavity 15, it is beneficial to control the rising pressure and pressure of the magnesium alloy melt during the pouring process. The process parameters such as liquid rising speed can improve the surface quality of castings. This method can be provided with 1 or more springs 12 and adjusting studs 10, preferably 4, and are evenly distributed.

When adjusting the adjustment stud 10, in order to ensure that the adjustment stud 10 is aligned with the spring 12, so that the asbestos pressure plate 7 is evenly stressed, a guide plate 21 with holes is welded on the middle and upper part of the inner wall of the negative pressure gas protection box 11. The spring 12 passes through the guide plate 21 .

In the above embodiment, asbestos is used to seal the upper and lower surfaces of the liquid riser 17 and the sealing cover 16 to prevent the outside air from entering and burning the magnesium alloy melt.

In the above embodiment, the first protective gas pipeline 3 and the second protective gas pipeline 8 are respectively connected to the first protective gas injection device and the second protective gas injection device; they can also be connected to the same protective gas injection device, and the protective gas injection device It is controlled by a control system. The control system is provided with a first control valve on the first protective gas pipeline 3 and a second control valve on the second protective gas pipeline 8 . When the protective gas is injected into the lower tank 19, the second control valve is closed and the first control valve is opened; otherwise, when the protective gas is injected into the negative pressure gas protection box 11, the first control valve is closed and the second control valve is opened.

Embodiment two:

This embodiment describes a casting method using the device described in the above embodiments, the method includes the following steps:

Step 1. Put the magnesium alloy into a melting furnace at 730° C. to 820° C. for smelting and removing slag, and at the same time, heat the gypsum mold at 200° C. to 400° C. ;

Step 2. Put the molten magnesium alloy after smelting and deslagging into the crucible, seal the lower tank with a sealing cover, seal the contact surface between the riser and the sealing cover with asbestos, and lock the The sealing cover plate is locked with the lower tank.

Step 3. Place the heated plaster mold directly on the sealing cover, align the gate with the upper port of the riser, and use asbestos to seal the plaster mold, and then turn the negative pressure The gas protection box is directly placed on the plaster mold and fixedly connected with the sealing cover plate.

Step 4, using a mixed gas of 1-10% sulfur hexafluoride and 90-99% argon at a pressure of 1-5KPa to pressurize the lower tank through the first protective gas pipeline for 1-3 minutes.

Step 5. Vacuumize the negative pressure gas protection box through the negative pressure pipeline located on the upper part or side wall of the negative pressure gas protection box, and the vacuum degree reaches -20～-30KPa; then immediately put 1-10% six The mixed gas of sulfur fluoride and 90-99% argon is injected into the negative pressure gas protection box through the second protective gas pipeline, and the vacuum degree reaches -10～-20KPa; adjust the adjusting stud, and align the adjusting stud with the spring through the guide plate ;

Step 6: Pressurizing the surface of the magnesium alloy melt with an air pressure of 10-80 KPa by means of a pressurizing device, so that the magnesium alloy melt enters the mold cavity from bottom to top along the riser pipe, and implements low-pressure casting.

The preparation of a complex thin-walled gypsum-type magnesium alloy with an average wall thickness of 1.5mm is introduced in detail below:

1. Raw material selection: magnesium alloy is ZM3, and the gypsum-type composition ratio (mass fraction) is: 15% to 20% of quartz powder, 6% to 8% of talcum powder, 60% to 85% of α hemihydrate gypsum, 18%-25% MgSO4, plus water and other materials, the mixed gas ratio is 5% sulfur hexafluoride and 95% argon gas.

2. After weighing the raw materials according to the ratio in step 1, melt the magnesium alloy, remove the slag and wait for pouring. The temperature of the alloy melt is 790°C.

3. Preheat the gypsum type to 400°C, close the box and cover the negative pressure gas protection box.

4. Use a mixed gas of 5% sulfur hexafluoride and 95% argon at a pressure of 5KPa to pressurize the lower tank 19 for 2 minutes.

5. Vacuumize the negative pressure gas protection box 11, and the vacuum degree is -30KPa, then inject a mixed gas of 5% sulfur hexafluoride and 95% argon, so that the vacuum degree of the negative pressure gas protection box 11 reaches -20KPa, and then Perform pressure casting according to the process parameters in step 6.

6. The pouring process parameters are: liquid raising speed 30m/s, liquid raising pressure 5KPa, filling speed 30m/s, filling pressure 30KPa, crusting time 5s, crusting boost pressure 5KPa, crystallization time 150s, crystallization boost Pressure 5KPa, resistance coefficient 1.0.

A mold of 12 gypsum-type magnesium alloy test bars produced according to the above-mentioned process steps has a tensile property of 190 MPa and an elongation of 2%.

The following is a detailed introduction to the preparation of a complex thin-walled gypsum cast magnesium alloy with an average wall thickness of 2.4mm. The gypsum cast magnesium alloy has a high degree of complexity, with reinforcing ribs on the outside and bosses on the inside, without machining allowance , specifically prepared as follows:

1. Raw material selection: magnesium alloy is ZM5, and the gypsum-type composition ratio (mass fraction) is: 15% to 20% of quartz powder, 6% to 8% of talcum powder, 60% to 85% of α hemihydrate gypsum, 18%-25% MgSO4, plus water and other materials, the mixed gas ratio is 5% sulfur hexafluoride and 95% argon gas.

2. After weighing the raw materials according to the ratio in step 1, melt the magnesium alloy, remove the slag and wait for pouring. The temperature of the alloy melt is 820°C.

3. Preheat the gypsum type to 380°C, close the case and cover the tank.

4. Pressurize the lower tank 19 for 3 minutes with a mixed gas of 5% sulfur hexafluoride and 95% argon at a pressure of 3KPa.

5. Vacuumize the negative pressure gas protection box 11, and the vacuum degree is -20KPa, then inject a mixed gas of 5% sulfur hexafluoride and 95% argon, so that the vacuum degree of the negative pressure gas protection box 11 reaches -10KPa, and then Perform pressure casting according to the process parameters in step 6.

6. The pouring process parameters are: liquid lifting speed 45m/s, liquid lifting pressure 8KPa, filling speed 50m/s, filling pressure 40KPa, crusting time 5s, crusting boost pressure 8KPa, crystallization time 250s, crystallization boost Pressure 5KPa, resistance coefficient 1.5.

According to the one-piece plaster-type thin-walled casting produced by the above-mentioned process steps, the average wall thickness of the casting is 2.4mm, the maximum diameter is 480mm, the height is 600mm, and the surface finish is 3.6.

The above is only a preferred embodiment of the present invention, it should be pointed out that for those of ordinary skill in the art, without departing from the technical principle of the present invention, some improvements and modifications can also be made, these improvements and modifications It should also be regarded as the protection scope of the present invention.

Claims (9)

1. a magnesium alloy Anti-gravity casting device, is characterized in that, this device comprises:

Lower tank (19) is built-in with melting furnace (2) and crucible (18), and the top of described lower tank (19) adopts seal cover board (16) and external isolation; Between described melting furnace (2) and described seal cover board (16), described lower tank (19) is connected with the first protective gas pipeline (3); The middle and lower part of described lower tank (19) is connected with pressue device (1); Described crucible (18) is built-in with stalk (17), certain distance is arranged at the lower end of described stalk (17) and described crucible (18) bottom, and described stalk (17) forms with magnesium alloy molten surface and is communicated with; Described stalk (17) upper end oral area, through described seal cover board (16), is aimed at the cast gate of the gypsum mould (14) be placed directly on described seal cover board (16); With die cavity (15) in described gypsum mould (14), adopt asbestos (13) to seal above it, described asbestos (13) are stamped asbestos pressing plate (7) above; Described gypsum mould (14) is upper, the top of die cavity (15) offers the through hole being connected to asbestos;

Negative-pressure gas guard box (11) is connected with the second protective gas pipeline (8) and negative-pressure pipeline (9); Described negative-pressure gas guard box (11) is placed directly on described gypsum mould (14), the two ends, bottom of described negative-pressure gas guard box (11) are fixedly connected with described gypsum mould (14), and its junction adopts sealed silicon cushion rubber (5) sealing; Described negative-pressure gas guard box (11) is fixedly connected with described seal cover board (16), and by retaining mechanism (4) locking;

Also be provided with spring (12) in described negative-pressure gas guard box (11) and regulate stud (10), described asbestos pressing plate (7) is by described spring (12) and regulate stud (10) described asbestos (13) and gypsum mould (14) to be fastenedly connected.

2. device according to claim 1, is characterized in that, described spring (12) and adjustment stud (10) are provided with 4, are evenly distributed.

3. device according to claim 2; it is characterized in that; be welded with guide plate (21) with holes in the middle and upper part of described negative-pressure gas guard box (11) inwall, described spring (12) is through described guide plate (21).

4. device according to claim 1, is characterized in that, described stalk (17) and described seal cover board (16) contact surface all adopt asbestos to seal.

5. the device according to any one of Claims 1 to 4, it is characterized in that, described first protective gas pipeline (3) is connected with the first protective gas injection device and the second protective gas injection device respectively with the second protective gas pipeline (8); Or described first protective gas pipeline (3) is connected with same protective gas injection device with the second protective gas pipeline (8); this protective gas injection device is controlled by control system; described control system is provided with the first control valve at described first protective gas pipeline (3), is provided with the second control valve at described second protective gas pipeline (8).

6. device according to claim 1, is characterized in that, described negative-pressure gas guard box (11) and described seal cover board (16) adopt stud (6) to be connected.

7. device according to claim 1, is characterized in that, described second protective gas pipeline (8) and negative-pressure pipeline (9) are located at top or the sidewall of described negative-pressure gas guard box (11).

8. device according to claim 1, is characterized in that, described protective gas is the mist of 1-10% sulfur hexafluoride and 90-99% argon gas.

9. utilize the device described in any one of claim 1-8 to carry out a method of casting, it is characterized in that, the method comprises the steps:

Step 1, magnesium alloy is put into melting furnace carry out melting slagging-off at 730 DEG C ~ 820 DEG C, gypsum mould is heated at 200 DEG C ~ 400 DEG C simultaneously;

Step 2, the magnesium alloy liquation after melting slagging-off is put into crucible, and with the lower tank of seal cover board sealing, with the contact surface of stalk described in asbestos seal and described seal cover board, by retaining mechanism, described seal cover board and described lower tank are locked;

Step 3, heating after gypsum mould be placed directly on described seal cover board, the upper end oral area of described stalk aimed at by cast gate, and adopt asbestos to seal described gypsum mould, then negative-pressure gas guard box is placed directly on described gypsum mould, and is fixedly connected with described seal cover board;

Step 4, be that 1-5KPa carries out suppressing for 1 ~ 3 minute to lower tank by the first protective gas pipeline with the mist of 1-10% sulfur hexafluoride and 90-99% argon gas, air pressure;

Step 5, by being positioned at the described top of negative-pressure gas guard box or the negative-pressure pipeline of sidewall vacuumizes described negative-pressure gas guard box, vacuum reaches-20 ~-30KPa; Then immediately the mist of 1-10% sulfur hexafluoride and 90-99% argon gas is injected negative-pressure gas guard box by the second protective gas pipeline, vacuum reaches-10 ~-20KPa; Regulate stud, make adjustment stud alignment springs by guide plate;

Step 6, adopt the air pressure of 10 ~ 80KPa to exert pressure to described magnesium alloy molten surface by pressue device, make described magnesium alloy liquation from bottom to top enter die cavity along described stalk, carry out low-pressure casting.