CN114985684A - A design method of metal mold low pressure casting gating system with overflow slag ladle - Google Patents

Abstract

The invention belongs to the technical field of low-pressure casting, and particularly discloses a design method of a metal type low-pressure casting gating system with an overflow slag ladle, which comprises the steps of preliminarily determining the position of the slag ladle according to a flow state result in a mold flow analysis and the design of a parting line of a casting; designing the thickness of a slag ladle neck and the size of a slag ladle according to the temperature result in the mold flow analysis and the wall thickness characteristic of a casting; according to the particle tracking result in the mold flow analysis, the specific shape of the slag ladle and the position of the exhaust plug in the slag ladle are optimized, the method is particularly suitable for large-scale complex thin-wall aluminum alloy castings such as large-scale aluminum alloy hollow integral auxiliary frames, battery boxes, motor shells and the like which are formed by metal mold low-pressure casting, and the entrainment gas generated in the metal mold aluminum alloy low-pressure casting and filling process and the oxide and the cold charge at the front end of molten metal are discharged into the slag ladle, so that the undercasting risk and the content of oxidized slag inclusion at the intersection position of the molten metal filling tail end and the molten metal in the castings are obviously reduced, and the quality and the qualification rate of the complex thin-wall large-scale castings are improved.

Description

A design method of metal mold low pressure casting gating system with overflow slag ladle

technical field

The invention belongs to the technical field of low-pressure casting, and in particular relates to a design method of a metal mold low-pressure casting gating system with overflow slag ladle, more particularly, a method for combining casting mold flow analysis results with structural features of castings. The overflow slag ladle is designed in the pouring system, and the position of the slag ladle, the size and shape of the slag ladle, and the position of the exhaust plug are designed and optimized, as well as the obtained pouring system.

Background technique

Driven by the pressure of energy conservation and emission reduction and the demand for performance improvement of new energy vehicles, the lightweight process of automobiles is accelerating. Among them, the lightweight of automobile chassis is a new blue ocean, and the battery pack, subframe, control arm, steering knuckle, etc. made of aluminum alloy The penetration rate continues to increase, and more and more thin-walled castings, hollow castings and large-scale castings made of aluminum alloys with high integration are introduced to the market.

The lightweight design of large-scale castings usually has the following characteristics: 1) The integrated structure of multiple parts is changed to an integral casting structure, which is large in size and complex in shape, and the technical requirements of different parts of the casting are obviously different; 2) The wall thickness of the casting is further reduced , designed according to the minimum limit thickness of the molding process; 3) In order to improve the lightweight effect, the casting is designed as a hollow structure.

CN114653924A discloses a low-pressure casting gating system for complex thin-walled parts feeding path, which adopts T-shaped runner, bottom-casting sprue, side-casting sprue, column-type sprue and slot-type sprue In order to solve the problems of shrinkage porosity and poor mechanical properties during the casting process of L-shaped aluminum alloy castings with complex geometry, thin wall thickness and large longitudinal height difference. CN105033223A discloses a casting system for metal aluminum ring low-pressure casting, including sprue, runner and inner gate, etc. The outer sidewall of the sand core in the area where each casting cavity is located is annularly evenly distributed with several convexities body. Through one module, the production cost is saved. However, the above low-pressure casting gating systems are not suitable for the preparation of low-pressure casting of large-scale, complex, thin-walled and lightweight castings. Due to the structural characteristics of large-scale, complex, thin-walled and lightweight castings, the casting hot spots are scattered, and multiple gates need to be set to feed the hot spots, which brings great challenges to the filling process of traditional low-pressure casting. Due to the long process and complex flow field during the filling process, more front-end cold materials, oxidized inclusions and entrained air will be generated, which seriously affects the quality of castings.

At present, the filter screen is usually placed at the position before the top of the riser pipe enters the casting cavity to reduce oxides and inclusions in the molten metal, but it has no effect on the entrainment and secondary oxidation slag inclusions formed after the molten metal passes through the filter screen. The traditional metal mold low-pressure casting process can no longer meet the production of large-scale complex thin-walled aluminum alloy castings such as high-performance hollow integral subframes and battery packs.

In order to reduce the problems caused by the cold material in the front section, oxidation inclusions and air entrapment, in the design of the casting gating system, the slag bag and the exhaust groove are often designed at the first impact, confluence or last forming part of the molten metal to avoid the problems. . However, the actual design of the casting process for large-scale components such as hollow integral subframes and battery packs is mainly judged by the experience of workers. The improvement and optimization are realized by trial and error, which has low efficiency, long cycle and relatively high cost. Lack of effective and reasonable process design methods.

SUMMARY OF THE INVENTION

In order to solve the above technical problems, the present invention provides a design method of a metal mold low-pressure casting gating system with an overflow slag ladle, so as to eliminate or reduce the entrainment and secondary oxidation during the filling process of large and complex castings in low-pressure casting. Slag inclusion, improve casting quality. The invention is particularly suitable for large-scale complex thin-walled aluminum alloy castings such as large-size aluminum alloy hollow integral subframes, battery boxes, and motor shells formed by metal mold low-pressure casting.

To achieve the above purpose, the complete technical scheme of the present invention includes:

A design method of a metal mold low-pressure casting gating system design method with an overflow slag bag, comprising the following steps;

Step 1: Perform mold flow analysis on the low-pressure casting process plan of the hollow monolithic subframe, and fill the mold from three risers on both sides and the center of the monolithic subframe. The mold flow calculation includes casting mold filling and particle tracking. , during the filling process, the aluminum liquid enters the mold through the liquid riser and fills the subframe body;

Step 2: According to the results of mold filling flow and particle tracking in the results of mold flow analysis, set an overflow slag bag on each side of the middle position of the front beam of the subframe;

Step 3: After selecting the position of the slag ladle, move the parting line to the upper part of the casting, modify the parting line from a straight shape to a slope shape, and design the first shape feature of the diversion effect on the casting, the first shape It is characterized by a diamond-shaped diversion groove set on the surface of the casting;

Step 4: Design the width, thickness and length of the slag bag neck. The radius of the long edge at the boundary between the slag bag neck and the casting is 1.3 times the thickness of the slag bag neck. times;

Step 5: The shape of the slag bag is designed to be an "8" shape composed of double cylinders along the flow direction of the molten metal, and the molten metal is guided to form two vortices in the slag bag;

Step 6: Perform the mold flow analysis again on the process plan after adding the slag bag. If the air contact time of the casting part is less than 0.3s, and the molten metal enters the slag bag to form a vortex, and the air pressure in the slag bag is less than 1100mbar, and the front end of the molten metal If there is no region where the temperature is lower than 15°C of the alloy liquidus before entering the slag ladle, it is considered qualified.

If the temperature of the front end of the molten metal exists in the area of the slag bag neck that is 15°C lower than the liquidus line of the alloy, the thickness of the slag bag neck should be increased, and the long edge fillet radius of the boundary between the slag bag neck and the casting, and the boundary between the slag bag neck and the slag bag should be modified accordingly. Fillet radius, and perform mold flow analysis again for the modified scheme until all indicators meet the requirements.

In the step 1, the casting material is cast aluminum A356 alloy, the pouring temperature is 720°C, the mold temperature is selected as 350°C, the liquid rising rate is set to 20 mbar/s, and the liquid rising rate in the cavity is set to 8 mbar/s.

In the step 4, the designed slag bag neck width is 40mm, the slag bag neck thickness is 2mm, and the length is 15mm.

The angle between the two sides of the diamond-shaped diversion groove of the overflow slag bag is 32° and 148° respectively, and the depth of the diversion groove: side length=1:3.7.

The diameter of a single circle of the "8"-shaped overflow slag bag is Φ45mm, the distance between the two circles is 50mm, the thickness of the overflow slag bag is 24mm, the side slope of the overflow slag bag is 12°, and the total volume of the overflow slag bag is 68.3cm³. A Φ12 mandrel is set in the middle of the "8"-shaped overflow slag bag, and a Φ12 exhaust plug is set at the center where the molten metal forms a vortex in the slag bag.

A method for designing an overflow slag bag in a low-pressure casting gating system of a metal mold aluminum alloy, comprising the following steps;

Step 1: Perform mold flow analysis on the low-pressure casting process plan of the split subframe, and fill the mold from a riser in the center of the split subframe. The mold flow calculation includes casting mold filling and particle tracking. During the mold filling process, aluminum The liquid enters the mold through the liquid riser and fills the subframe body;

Step 2: According to the results of mold filling flow and particle tracking in the mold flow analysis results, an overflow slag bag is set on each side of the welding area at the mold filling end of the split aluminum alloy subframe casting;

Step 3: Design the parting line of the position of the slag ladle on the upper part of the casting, and design the second shape feature of the diversion effect on the casting. Including the arc section located in front of the center, including the arc section for diversion, and the curved section connected to it on both sides of the arc section for buffering;

Step 4: Design the width, thickness and length of the slag bag neck. The radius of the long edge at the boundary between the slag bag neck and the casting is 1.6 times the thickness of the slag bag neck. times;

Step 5: Design the shape of the slag bag as a "9" shape along the flow direction of the molten metal to guide the molten metal to form a vortex in the slag bag;

Step 6: Perform the mold flow analysis again on the process plan after adding the slag bag. If the air contact time of the casting part is less than 0.3s, and the molten metal enters the slag bag to form a vortex, and the air pressure in the slag bag is less than 1100mbar, and the front end of the molten metal If there is no region where the temperature is lower than 15°C of the alloy liquidus before entering the slag ladle, it is considered qualified.

In the step 1, the casting material is cast aluminum A356 alloy, the pouring temperature is 715°C, the mold temperature is selected as 350°C, the liquid rising rate is set to 20 mbar/s, and the liquid rising rate in the cavity is set to 8 mbar/s.

In step 4, the designed slag bag neck width is 20mm; the slag bag neck thickness is 2mm; and the length is 15mm.

The described overflow slag bag of a low-pressure casting system for metal-aluminum alloy low-pressure casting is characterized in that the circular diameter of the "9"-shaped overflow slag bag is Φ38mm, the thickness of the overflow slag bag is 19mm, and the side slope of the overflow slag bag is 19mm. is 20°, the total volume of the overflow slag bag is 14.9cm³, and a Φ12 exhaust plug is set at the center where the molten metal forms a vortex in the slag bag.

The advantages of the present invention relative to the prior art are:

1) Eliminate or reduce the influence of air entrainment and secondary oxidation slag inclusion in low-pressure casting on the performance of castings, especially for complex castings with long processes and variable flow states and thin-walled castings with high performance requirements, especially suitable for use The large-sized aluminum alloy hollow integral subframe, battery box, motor shell and other large and complex thin-walled aluminum alloy castings formed by metal mold low pressure casting significantly reduce the under-casting at the end of the molten metal filling and the intersection of the molten metal in the casting. The risk and the content of oxidized slag inclusions improve the quality and pass rate of complex thin-walled and large-sized castings.

2) Provide a reference for the size and shape of the overflow slag bag design in the gating system, shorten the process design time, and reduce the iterative verification time of the subsequent overflow slag bag effect;

3) Use mold flow analysis and practical experience to establish specific result criteria to evaluate the overflow slag ladle effect of the pouring system before the official mold opening.

Compared with the prior art, the method of the present invention can effectively guide the technicians in the art to simultaneously design an effective overflow slag bag in the development process of large and complex aluminum alloy low-pressure casting, so as to eliminate or reduce the large and complex thin-walled aluminum alloy low-pressure casting. Parts entrained gas and secondary oxidation slag inclusions, improve the quality and performance of castings, shorten the development cycle, and improve product qualification rates. It has great economic and social value.

Description of drawings

1 is a schematic structural diagram of an integral aluminum alloy sub-frame used in Example 1 of the present invention.

FIG. 2 is a schematic diagram of the particle tracking results of the mold flow analysis in Example 1. FIG.

3 is a schematic diagram of the structure of the parting line at the overflow slag bag in Example 1.

FIG. 4 is a top view of the overflow slag bag in Example 1. FIG.

FIG. 5 is a front view of FIG. 4 .

6 is a schematic structural diagram of a split aluminum alloy sub-frame used in Example 2 of the present invention.

FIG. 7 is a top view of the overflow slag bag in Example 2. FIG.

In the picture: 1-Liter pipe, 2-Integral subframe body, 3-Subframe front beam, 4-Overflow slag bag, 5-Parting line, 6-First shape feature, 7-Second shape features.

Detailed ways

The technical solutions of the present invention will be further described in detail below with reference to the accompanying drawings of the present invention. Obviously, the described embodiments are only used as examples and are not used to limit the present application.

Example 1

The application of the invention in the design of the casting system of the integral aluminum alloy sub-frame casting of multi-gate low-pressure casting, the overflow slag bag is designed to collect the entrained air produced by the intersection of multiple strands of molten metal and the secondary oxidation slag inclusions at the front end of the molten metal, Improve casting quality.

In this embodiment and the accompanying drawings, unless otherwise specified, the parameters represented by the symbols used are interpreted as follows: W: slag wrap neck width, T: slag wrap neck thickness, L: slag wrap neck length, α: draft angle, R1: the boundary between the long side of the slag bag neck and the casting, R2: the boundary between the short side of the slag bag neck and the casting, and R3: the boundary between the slag bag neck and the slag bag.

As shown in FIG. 1 , it is a schematic diagram of low-pressure casting and pouring of the integral subframe of the present invention, including a liquid riser 1 , an integral subframe body 2 , a slag ladle, and the like.

Step 1: Perform mold flow analysis on the low-pressure casting process plan of the integral subframe. During the analysis process, the casting material is selected as cast aluminum A356 alloy, the initial pouring temperature is selected as 720 ℃, and the initial mold temperature is selected as 350 ℃. Filling is performed from a total of three liquid risers 1 from both sides and the center of the integral subframe, as shown in Figure 1; the liquid rising rate is set to 20mbar/s, and the liquid rising rate in the cavity is set to 8mbar/s. The mold flow calculation includes casting filling and particle tracking. During the filling process, the molten aluminum enters the mold through the liquid riser and fills the integral subframe body 2 .

Step 2: As shown in Figure 2, according to the results of mold flow analysis and particle tracking, it is determined that there are two turbulent flows of molten metal in the middle of the front beam 3 of the subframe. An overflow slag bag 4 is arranged on each side of the position to collect the cold material at the front end of the molten metal, turbulent entrained air and secondary oxides.

Step 3: After selecting the position of the slag ladle, move the parting line 5 to the upper part of the casting, and modify the parting line 5 from a straight shape to a sloped shape, as shown in Figure 3, the slag ladle is designed to be located in the final filling. Finished position to achieve the effect of slag collection and overflow discharge. And the first shape feature 6 of the diversion effect is designed on the casting to increase the tendency of the molten metal to flow into the overflow slag ladle. In this embodiment, the first shape feature 6 is a rhombus-shaped diversion groove set on the surface of the casting. The angles between the two sides of the rhombus diversion groove are 32° and 148° respectively, and the diversion groove is determined according to the flow conditions. The depth, after design, is chosen to make the depth of the diversion groove: side length = 1:3.7.

Step 4: Design the width and thickness of the slag neck: According to the range of the molten metal turbulence of the particle tracking track in the mold flow analysis, the width of the slag neck is designed to be 40mm; the thickness of the slag neck is initially designed to be the thinnest 2mm for the convenience of subsequent removal. ; In order to prevent the heat of the slag bag from affecting the solidification sequence of the casting, the initial design is based on the longest 15mm.

Design the junction shape of the slag neck and the casting: as shown in Figure 5, the long edge fillet R1 of the junction between the slag neck and the casting is designed to be 1.3*the thickness of the slag neck = 2.6. According to the particle tracking flow direction in the preliminary mold flow analysis, the fillet R2 at the interface between the short side of the slag bag and the casting is designed to be 10, so that the particle tracking trajectory can smoothly enter the slag bag and avoid vortices and cavities in the casting and the slag bag neck. The fillet R3 at the junction of the slag bag neck and the slag bag is designed to be 5.2*the thickness of the slag bag neck=10.4.

Step 5: The slag bag is located at the intersection of two strands of molten aluminum, and the shape of the slag bag is designed to be an "8" shape composed of double cylinders in accordance with the flow direction of the molten metal to guide the molten metal to form two vortices in the slag bag, as shown in Figure 4 , and set a Φ12 exhaust plug in each of the two vortex centers. The diameter of a single circle with the "8" shape of the slag bag is designed to be Φ45mm, the center distance of the two circles is designed to be 50mm, the thickness of the slag bag is designed to be 24mm, the side slope of the slag bag is designed to be 12°, and the total volume of the slag bag is 68.3cm³. A Φ12 mandrel is set in the middle of the 8" shape to facilitate the mold release of the slag bag.

Step 6: Perform mold flow analysis again on the process plan after adding the slag bag. After checking the analysis results, it is found that the air contact time of the casting part is less than 0.3s, indicating that the oxides at the front end of the molten metal are effectively discharged into the slag bag during the filling process. The flow state results show that the molten metal represented by the particle tracking trajectory smoothly enters the slag bag to form a vortex. The air pressure results show that the air pressure in the slag bag is less than 1100mbar, and the exhaust plug in the slag bag has a good exhaust effect. However, the filling temperature shows that the temperature of the front end of the molten metal is in the region of 592 °C in the slag ladle neck, which is about 20 °C lower than the liquidus of the A356 alloy, and there is a risk of insufficient pouring.

In order to improve the risk of insufficient pouring, the thickness of the slag bag neck was increased to 3mm. Due to the change in the thickness of the slag bag neck, the corresponding long edge fillet R1 at the junction between the slag bag neck and the casting was changed to 3, and the slag bag neck and the slag bag The boundary fillet R2 Modified to 10.8. After the 3D digital model of the modified scheme is analyzed again, it is found that all the indicators meet the requirements.

The integral aluminum alloy subframe casting described in Example 1 has good internal quality at the position of the front beam, and meets the requirements of grades 0 to 2 in the ASTM E155-2015 "Radiographic Reference Films for Testing Aluminum and Magnesium Castings" in the technical casting requirements. . After T6 heat treatment, the tensile strength, yield strength and elongation were tested by a tensile testing machine, reaching 310MPa, 240MPa, and 10.9%, respectively, meeting the technical requirements of the product. The subframe assembly has passed the bench fatigue test verification.

Example 2

The invention is applied in the design of the casting system of the low-pressure casting split type aluminum alloy subframe casting, and the overflow slag bag is designed to collect the entrained air at the end of the filling and the secondary oxidation slag inclusions at the front end of the molten metal, so as to improve the casting quality. In this case, the filling end is the welding area between the subframe casting and the extruded aluminum profile. In order to ensure the strength of the weld and avoid blistering and blackening of the weld, the internal quality of this area is extremely high.

As shown in Figure 6, it is a schematic diagram of low-pressure casting and pouring of the split sub-frame of the present invention, which specifically includes:

Step 1: Perform mold flow analysis on the low-pressure casting process plan of the split subframe. During the analysis process, the casting material is selected as cast aluminum A356 alloy, the initial pouring temperature is selected as 715 ° C, and the initial mold temperature is selected as 350 ° C. The liquid rising rate in liquid pipe 1 is set to 20 mbar/s, and the liquid rising rate in the cavity is set to 8 mbar/s. Mold flow calculations include casting filling and particle tracking.

Step 2: According to the mold flow analysis results and the particle tracking results, there is metal turbulence and oxide accumulation in the welding area of the mold filling end of the split aluminum alloy sub-frame casting. Each side is provided with an overflow slag bag 4 to collect the cold material at the front end of the molten metal, turbulent entrained air and secondary oxides.

Step 3: Design the parting line of the position of the slag ladle on the upper part of the casting, and design the second shape feature 7 of the diversion effect on the casting to increase the tendency of the molten metal to flow into the overflow slag ladle, as shown in Figure 7, in this implementation In an example, the second shape feature 7 is a gap located in the middle of the two overflow slag bales 4, and the gap includes an arc-shaped section located in front of the center for diverting the flow, and is connected to it on both sides of the arc-shaped section, respectively, Curve segment for buffering.

Step 4: Design the width and thickness of the slag neck: According to the range of the molten metal turbulence of the particle tracking trajectory in the mold flow analysis, the width of the slag neck is designed to be 20mm; the thickness of the slag neck is initially designed to be the thinnest 2mm in order to facilitate subsequent removal. ; In order to prevent the heat of the slag bag from affecting the solidification sequence of the casting, the initial design is based on the longest 15mm.

Design the junction shape of the slag neck and the casting: the long edge fillet R1 of the junction of the slag neck and the casting is 1.6*the thickness of the slag neck=3.2. According to the particle tracking flow direction in the preliminary mold flow analysis, the fillet R2 at the interface between the short side of the slag bag and the casting is designed to be 15, so that the particle tracking trajectory can smoothly enter the slag bag and avoid vortices and cavities in the casting and the slag bag neck.

Design the junction shape of the slag bag neck and the slag bag: fillet R3 according to 4.2*slag bag neck thickness=8.4.

Step 5: The slag bag is located at the end of the filling, and the shape of the slag bag is designed to be a "9" shape according to the flow direction of the molten metal to guide the molten metal to form a vortex in the slag bag, and a Φ12 vent plug is set in the center of the vortex. The diameter of the slag bag "9"-shaped circle is designed to be Φ38mm, the thickness of the slag bag is designed to be 19mm, the side slope of the slag bag is designed to be 20°, and the total volume of the slag bag is 14.9cm³.

Step 6: Perform mold flow analysis again on the process plan after adding the slag bag. After checking the analysis results, it is found that the filling temperature shows that there is no area at the front end of the molten metal that is 15°C lower than the liquidus of the A356 alloy, and there is no risk of insufficient pouring due to too low temperature. The oxide results show that the air contact time of the casting part is less than 0.3s, indicating that the oxide at the front end of the molten metal is effectively discharged into the slag bag during the filling process. The flow state results show that the molten metal represented by the particle tracking trajectory smoothly enters the slag bag to form a vortex. The air pressure results show that the air pressure in the slag bag is less than 1100mbar, and the exhaust plug in the slag bag has a good exhaust effect.

The internal quality of the welding area of the split aluminum alloy subframe casting described in Example 2 is good, and meets the requirements of grades 0 to 1 in the ASTM E155-2015 "Radiographic Reference Films for Detection of Aluminum Castings and Magnesium Castings" in the technical casting requirements. After T6 heat treatment, the tensile strength, yield strength and elongation of the aluminum alloy subframe castings were tested by a tensile testing machine, reaching 300MPa, 230MPa, and 11.3%, respectively. After the casting and the profile are connected by inert gas shielded welding, the appearance is fish scale pattern, without bubbles and blackening. The tensile strength, yield strength and elongation were tested by a tensile testing machine to reach 220MPa, 178MPa, and 7.9%, respectively, which were higher than 60% of the performance of the casting body and met the technical requirements of the product. The subframe assembly has passed the bench fatigue test verification.

The above are only preferred embodiments of the present invention and do not limit the present invention. Any simple modifications, changes and equivalent structural changes made to the above embodiments according to the technical essence of the present invention still belong to the technology of the present invention. within the scope of the program.

Claims (10)

1. a design method of a metal mold low pressure casting gating system with overflow slag bag, is characterized in that, comprises the steps: Step 1: Perform mold flow analysis on the low-pressure casting process plan of the hollow monolithic subframe, and fill the mold from three risers on both sides and the center of the monolithic subframe. The mold flow calculation includes casting mold filling and particle tracking. , during the filling process, the aluminum liquid enters the mold through the liquid riser and fills the subframe body; Step 2: According to the results of mold filling flow and particle tracking in the results of mold flow analysis, set an overflow slag bag on each side of the middle position of the front beam of the subframe; Step 3: After selecting the position of the slag ladle, move the parting line to the upper part of the casting, modify the parting line from a straight shape to a slope shape, and design the first shape feature of the diversion effect on the casting, the first shape It is characterized by a diamond-shaped diversion groove set on the surface of the casting; Step 4: Design the width, thickness and length of the slag bag neck. The radius of the long edge at the boundary between the slag bag neck and the casting is 1.3 times the thickness of the slag bag neck. times; Step 5: The shape of the slag bag is designed to be an "8" shape composed of double cylinders along the flow direction of the molten metal, and the molten metal is guided to form two vortices in the slag bag; Step 6: Perform the mold flow analysis again on the process plan after adding the slag bag. If the air contact time of the casting part is less than 0.3s, and the molten metal enters the slag bag to form a vortex, and the air pressure in the slag bag is less than 1100mbar, and the front end of the molten metal If there is no region where the temperature is lower than 15°C of the alloy liquidus before entering the slag ladle, it is considered qualified. 2. The design method of a metal mold low-pressure casting gating system with overflow slag ladle according to claim 1, characterized in that, if the temperature of the front end of the molten metal is lower than the alloy liquidus line 15 in the slag ladle neck In the region of ℃, the thickness of the slag bag neck is increased, and the long edge fillet radius of the interface between the slag bag neck and the casting and the fillet radius of the slag bag neck and the slag bag interface are modified accordingly. fulfil requirements. 3. The design method of a metal mold low pressure casting pouring system with overflow slag bag according to claim 1, wherein in the step 1, the casting material is cast aluminum A356 alloy, and the pouring temperature is 720 ℃ ℃, the mold temperature is selected as 350℃, the liquid rising rate is set to 20 mbar/s, and the liquid rising rate in the cavity is set to 8 mbar/s. 4. The method for designing a metal mold low-pressure casting gating system with overflow slag ladle according to claim 1, wherein in the step 4, the designed slag ladle neck width is 40mm; the slag ladle neck thickness is 2mm; length 15mm. 5. A metal mold low pressure casting gating system with overflow slag ladle designed according to the method of any one of claims 1-4, characterized in that, between the two sides of the rhombus diversion groove of the overflow slag ladle The included angles are 32° and 148°, respectively, and the depth of the diversion groove: side length = 1:3.7. 6. A metal mold low pressure casting gating system with overflow slag bag according to claim 5, characterized in that, the diameter of a single circle of the "8"-shaped overflow slag bag is Φ45mm, and the distance between the two circles is 50mm, The thickness of the overflow slag bag is 24mm, the side slope of the overflow slag bag is 12°, the total volume of the overflow slag bag is 68.3cm³, and a Φ12 mandrel is set in the middle of the "8"-shaped overflow slag bag. A Φ12 exhaust plug is set in the center of the swirl position. 7. A design method for a metal mold low pressure casting gating system with overflow slag bag, characterized in that, comprising the steps: Step 1: Perform mold flow analysis on the low-pressure casting process plan of the split subframe, and fill the mold from a riser in the center of the split subframe. The mold flow calculation includes casting mold filling and particle tracking. During the mold filling process, aluminum The liquid enters the mold through the liquid riser and fills the subframe body; Step 2: According to the results of mold filling flow and particle tracking in the results of mold flow analysis, set an overflow slag bag on each side of the welding area at the mold filling end of the split aluminum alloy subframe casting; Step 3: Design the parting line of the position of the slag ladle on the upper part of the casting, and design the second shape feature of the diversion effect on the casting. Including the arc section located in front of the center, including the arc section for diversion, and the curved section connected to it on both sides of the arc section for buffering; Step 4: Design the width, thickness and length of the slag bag neck. The radius of the long edge at the boundary between the slag bag neck and the casting is 1.6 times the thickness of the slag bag neck. times; Step 5: Design the shape of the slag bag as a "9" shape along the flow direction of the molten metal to guide the molten metal to form a vortex in the slag bag; Step 6: Perform mold flow analysis again on the process plan after adding the slag bag. If the air contact time of the casting part is less than 0.3s, and the molten metal enters the slag bag to form a vortex, and the air pressure in the slag bag is less than 1100mbar, and the front end of the molten metal If there is no region where the temperature is lower than 15°C of the alloy liquidus before entering the slag ladle, it is considered qualified. 8 . The design method of a metal mold low pressure casting pouring system with overflow slag bag according to claim 7 , wherein in the step 1, the casting material is cast aluminum A356 alloy, and the pouring temperature is 715 ℃. 9 . ℃, the mold temperature is selected as 350℃, the liquid rising rate is set to 20 mbar/s, and the liquid rising rate in the cavity is set to 8 mbar/s. 9. The method for designing a metal mold low pressure casting gating system with overflow slag ladle according to claim 8, wherein in step 4, the designed slag ladle neck width is 20mm; the slag ladle neck thickness is 2mm ; The length is 15mm. 10. A metal mold low pressure casting gating system with overflow slag bag obtained according to any one of claims 7-9, characterized in that the circle diameter of the "9"-shaped overflow slag bag is Φ38mm, and the overflow The thickness of the overflow slag bag is 19mm, the side slope of the overflow slag bag is 20°, and the total volume of the overflow slag bag is 14.9cm³. A Φ12 exhaust plug is set at the center of the vortex formed by the molten metal in the slag bag.

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