Die-casting forming method for integrated die-casting automobile rear floor

Abstract

The invention discloses a die-casting forming method for an integrated die-casting automobile rear floor, and relates to a forming method for a die-casting automobile rear floor. The technical problems that the wall thickness of eight symmetrically-arranged boss structures in an existing integrated die-casting automobile rear floor exceeds 10 cm, cooling lag exists in the position during solidification, an isolated liquid phase area is easily formed, and shrinkage porosity and shrinkage cavities are formed are solved. According to the method, firstly, the optimal technological parameters are determined through simulation, then die-casting automobile rear bottom plate forming is conducted, eight local loading feeding openings are formed in total, the local extrusion specific pressure reaches 120-150 MPa, local extrusion can be achieved on all hot spot areas of a casting and the thick and large part of a product, the product density is improved, and defects are avoided. According to the method, local forced feeding can be effectively achieved by conducting local pressure supplementing on the boss position, the effect of eliminating serious shrinkage porosity and shrinkage cavity defects of the hot spot position through forced feeding is quite remarkable, and the method is quite beneficial to improving the overall mechanical property of the floor after integrated die casting.

Description

Technical Field

[0001] The invention relates to a method for forming a die-casting rear floor of an automobile. Background Art

[0002] Large-scale integrated die-casting technology has obvious advantages over traditional manufacturing processes in terms of production cost, production efficiency and safety performance. The development of integrated die-casting technology will help new energy vehicles become more energy-saving and environmentally friendly means of transportation. In the integrated die-casting process, due to the extremely large size of the casting and the mold, the complex process, and the uneven distribution of wall thickness, there are thick and large heat nodes in the floor after integration, and the heat transfer is slow and difficult to cool. The wall thickness of the 8 symmetrically arranged boss structures (4 large and 4 small) exceeds 10 cm. This structure causes a lag in cooling at this position during solidification, which is easy to form an isolated liquid phase area and shrinkage cavities. Summary of the invention

[0003] The present invention aims to solve the technical problem that the wall thickness of the eight symmetrically arranged boss structures in the existing integrated die-cast automobile rear floor exceeds 10 cm, and there is a lag in cooling at this position during solidification, which easily forms an isolated liquid phase area and shrinkage cavities, and provides a method for die-casting an integrated die-cast automobile rear floor.

[0004] The method for die-casting the integrated die-cast automobile rear floor of the present invention is carried out according to the following steps:

[0005] Step 1: Merge the castings and perform finite element modeling: Merge the integrated rear floor casting, pouring system and overflow system, convert the model into X_t format and import it into the Visual-Mesh module in ProCAST, and then use the Repair, Assembly and Intersection functions to repair the model and obtain a three-dimensional model;

[0006] Step 2: Model pre-processing: The three-dimensional model obtained in step 1 is sequentially subjected to gravity direction setting, material setting, temperature setting, heat transfer coefficient setting, boundary condition setting, cooling condition setting and simulation time setting;

[0007] In Gravity / Virtual Mold, set the gravity direction to be parallel to the rear base plate;

[0008] The mass fraction composition of the material is: Si: 8-11%, Mg: 0.1-0.4%, Mn: 0.4-0.8%, V: 0.1-0.4%, Zr: 0.1-0.4%, Zn: 0.1-0.4%, Fe: 0.0-0.2%, and the rest is Al;

[0009] The selected aluminum liquid casting temperature is 670℃~710℃, and the mold temperature is set to 160~200℃;

[0010] The heat transfer coefficient is 2000W / (m·℃);

[0011] Boundary condition setting: the movable mold is in contact with the fixed mold, and the pressure chamber is in contact with the fixed mold;

[0012] Cooling conditions: The heat transfer between the mold and the casting is conducted, the heat transfer between the mold and the air is radiated, and the temperature inside the mold is controlled by cooling water channels;

[0013] Simulation time: filling time is 1s;

[0014] Step 3: The die-casting process is simulated based on ProCAST simulation software: the die-casting parameters are consistent with those in actual production; the die-casting parameters include maximum injection speed, slow speed deviation, mold closing stroke, injection pressure, slow injection speed of the ram, fast injection speed of the ram and acceleration;

[0015] In the integrated die casting filling process, the liquid filling speed is extremely fast, and the molten metal is regarded as an incompressible fluid. Its flow obeys the conservation of mass and momentum, and the two-equation turbulence model is used to describe the turbulence.

[0016] During the solidification process, temperature release includes three ways: heat conduction, convection and radiation;

[0017] Heat transfer occurs when the metal liquid contacts the metal mold, which is described by Fourier's law heat conduction differential equation:

[0018]

[0019] There is always convection heat transfer between the inside of the molten metal, between the molten metal and the mold, and between the mold and the outside air environment, which can be described by Newton's cooling law:

[0020] q=α(T f -T w )

[0021] Thermal radiation transfers energy through electromagnetic waves. Heat is transferred between castings, molds and the atmosphere through thermal radiation, which can be described by the law of air thermal radiation:

[0022]

[0023] Where ρ is the fluid density (kg / m 3 );

[0024] q——heat transfer flux (W / (m 2 K));

[0025] c p ——Specific heat capacity at constant pressure (J / kgk);

[0026] Q——heat source term;

[0027] α——convective heat transfer coefficient;

[0028] T f ——Fluid characteristic temperature (℃);

[0029] T w ——solid boundary temperature (℃);

[0030] T s ——absolute surface temperature (℃);

[0031] ε——radiation temperature (℃);

[0032] σ0——Stefen-Boltzman constant;

[0033] Step 4: Mesh the mold and casting: Due to the large size of the integrated floor, automatic meshing with small meshes may increase the number of meshes; while automatic meshing with large meshes may not ensure the quality of the meshes, so the casting needs to be divided into blocks, where the mesh size of the casting is 4mm×4mm×4mm, the mesh size of the pouring system is 15mm×15mm×15mm, and the mesh size of the overflow system is 6mm×6mm×6mm, so as to greatly reduce the number of meshes and shorten the calculation time; the casting has a huge size, complex shape, and thin wall thickness, so it is impossible to fully automatically mesh, and the mesh needs to be manually divided at the wheel cover, longitudinal beam and rear floor platform;

[0034] Step 5: According to the above parameter settings, pour the molten metal into the pressure chamber and then stand for 2 seconds. The molten metal contacts the surface of the pressure chamber to cause pre-crystallization. Then, the metal is pressed into the mold cavity for filling and solidification. When the molten aluminum enters the mold cavity 2 seconds later, the shrinkage piston of the local loading and compensating device starts to work, and its specific pressure at the boss position is 120MPa-150MPa.

[0035] The local loading and compensating pressure device is composed of an oil inlet, a piston cylinder, a fixing bolt, a sealing ring and a compensating piston; the oil inlet and the compensating piston are respectively arranged at both ends of the piston cylinder, the compensating piston and the piston cylinder are slidably connected, a sealing ring is arranged at one end of the piston cylinder located at the compensating piston, and 4 fixing bolts are evenly arranged on the outside of the piston rod; a total of 8 local loading and compensating pressure devices are arranged, which are respectively fixed on the 8 bosses of the rear bottom plate movable mold by fixing bolts, and the compensating piston is perpendicular to the boss surface and extends into the boss; the sealing ring prevents the leakage of hydraulic oil in the piston cylinder while maintaining the vacuum degree of the mold cavity; the oil inlet is connected to the external high-pressure oil pipe, and the size of the compensating pressure and the moving speed are controlled by the hydraulic servo motor;

[0036] A total of 29 groups of comparative tests were conducted to examine the effects of eliminating shrinkage defects under different combinations of pouring temperature, mold temperature, slow injection speed of the ram, and fast injection speed of the ram, and select the best parameters;

[0037] Step 6: Die-cast the rear floor of the automobile according to the optimal parameters obtained from the numerical simulation results in step 5. The specific steps are as follows:

[0038] a. The rear bottom plate die-casting machine adopts a horizontal die-casting machine. The rear bottom plate is divided into two parts: a movable die and a fixed die. The fixed die and the movable die are in contact to form a cavity for the rear bottom plate of the car. The pressure chamber is connected to the fixed die to send the aluminum liquid into the cavity. The heating device of the mold and the cooling water pipe in the solidification process are distributed in the movable die and the fixed die;

[0039] b. First, the die-casting machine is used to move the movable mold and the fixed mold to close the mold, and after closing the mold, the mold cavity is vacuumed;

[0040] c. To prevent the metal liquid from forming waves after pouring into the pressure chamber, which may cause the flowing metal liquid to be mixed with air, the metal liquid is poured into the pressure chamber and then left to stand for 2 seconds, and the metal liquid gradually stabilizes;

[0041] d. During the die-casting process, the punch moves forward at a multi-stage speed, including a low-speed and slow pressure stage, an acceleration stage, and a high-speed pressurization stage. In the low-speed and slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate the vacuum. When the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber and accelerate uniformly.

[0042] e. In the acceleration stage, the punch speed increases rapidly, and the kinetic energy is converted into thermal energy, which improves the fluidity of the molten metal to facilitate subsequent filling of the cavity. The acceleration is 400m / s 2 ;

[0043] f. High-speed pressurization stage: the molten metal fills the cavity. After integration, the floor is a complex thin-walled casting, which requires a higher injection speed. In this stage, the speed after the acceleration in step e runs at a constant speed until the filling is completed. The pressure needs to be maintained continuously. The pressure in the cavity is 35MPa.

[0044] g. 2 seconds after the filling is completed, the shrinkage piston of the local loading and compensating pressure device is used to compulsorily compensate the boss;

[0045] h. After the casting solidifies, the local loading and pressure-compensating device releases pressure and exits, and the movable mold is moved and opened through the horizontal die-casting machine to take out the formed part.

[0046] The present invention first determines the optimal process parameters through simulation, and then performs die-casting to form the rear floor of the automobile. A total of 8 local loading and shrinkage-feeding ports are configured, and the local extrusion pressure ratio reaches 120-150MPa. Local extrusion can be achieved in all hot spots of the casting and thick parts of the product, thereby improving the density of the product and avoiding defects.

[0047] The present invention can effectively realize local forced shrinkage compensation by locally compensating the boss position. The forced shrinkage compensation has a significant effect on eliminating serious shrinkage and shrinkage cavity defects at the hot node position, which is very beneficial to improving the overall mechanical properties of the floor after integrated die-casting. BRIEF DESCRIPTION OF THE DRAWINGS

[0048] Figure 1 This is a three-dimensional model diagram in step 1 of experiment 1;

[0049] Figure 2 This is a model diagram of the mold used in Experiment 1;

[0050] Figure 3 It is a schematic diagram of the local loading and compensating pressure device in step 5 of test 1;

[0051] Figure 4 This is a schematic diagram of the structure of the integrated die-cast automobile rear floor in Experiment 1;

[0052] Figure 5 Schematic diagram of the feeding piston position in step 5 of test 1;

[0053] Figure 6 This is a data graph showing the effect of different parameters on shrinkage porosity volume in step 5 of experiment 1;

[0054] Figure 7 A frontal photo of the rear floor of a partially loaded car for test 1;

[0055] Figure 8 A photo of the back of the rear floor of a partially loaded car for test one;

[0056] Fig. 9 This is the OM diagram of the local loading part, i.e., the boss loading in Test 1;

[0057] Fig.10 This is the OM diagram of test 1 at the non-locally loaded feeding position. DETAILED DESCRIPTION

[0058] Specific implementation method 1: This implementation method is a method for die-casting an integrated die-cast automobile rear floor, which is specifically carried out in the following steps:

[0059] Step 1: Merge the castings and perform finite element modeling: Merge the integrated rear floor casting, pouring system and overflow system, convert the model into X_t format and import it into the Visual-Mesh module in ProCAST, and then use the Repair, Assembly and Intersection functions to repair the model and obtain a three-dimensional model;

[0060] Step 2: Model pre-processing: The three-dimensional model obtained in step 1 is sequentially subjected to gravity direction setting, material setting, temperature setting, heat transfer coefficient setting, boundary condition setting, cooling condition setting and simulation time setting;

[0061] Step 3: The die-casting process is simulated based on ProCAST simulation software: the die-casting parameters are consistent with those in actual production; the die-casting parameters include maximum injection speed, slow speed deviation, mold closing stroke, injection pressure, slow injection speed of the ram, fast injection speed of the ram and acceleration;

[0062] In the integrated die casting filling process, the liquid filling speed is extremely fast, and the molten metal is regarded as an incompressible fluid. Its flow obeys the conservation of mass and momentum, and the two-equation turbulence model is used to describe the turbulence.

[0063] During the solidification process, temperature release includes three ways: heat conduction, convection and radiation;

[0064] Heat transfer occurs when the metal liquid contacts the metal mold, which is described by Fourier's law heat conduction differential equation:

[0065]

[0066] There is always convection heat transfer between the inside of the molten metal, between the molten metal and the mold, and between the mold and the outside air environment, which can be described by Newton's cooling law:

[0067] q=α(T f -T w )

[0068] Thermal radiation transfers energy through electromagnetic waves. Heat is transferred between castings, molds and the atmosphere through thermal radiation, which can be described by the law of air thermal radiation:

[0069]

[0070] Where ρ is the fluid density (kg / m 3 );

[0071] q——heat transfer flux (W / (m 2 K));

[0072] c p ——Specific heat capacity at constant pressure (J / kgk);

[0073] Q——heat source term;

[0074] α——convective heat transfer coefficient;

[0075] T f ——Fluid characteristic temperature (℃);

[0076] T w ——solid boundary temperature (℃);

[0077] T s ——absolute surface temperature (℃);

[0078] ε——radiation temperature (℃);

[0079] σ0——Stefen-Boltzman constant;

[0080] Step 4: Mesh the mold and casting: Due to the large size of the integrated floor, automatic meshing with small meshes may increase the number of meshes; while automatic meshing with large meshes may not ensure the quality of the meshes, so the casting needs to be divided into blocks, where the mesh size of the casting is 4mm×4mm×4mm, the mesh size of the pouring system is 15mm×15mm×15mm, and the mesh size of the overflow system is 6mm×6mm×6mm, so as to greatly reduce the number of meshes and shorten the calculation time; the casting has a huge size, complex shape, and thin wall thickness, so it is impossible to fully automatically mesh, and the mesh needs to be manually divided at the wheel cover, longitudinal beam and rear floor platform;

[0081] Step 5: According to the above parameter settings, pour the molten metal into the pressure chamber and then stand for 2 seconds. The molten metal contacts the surface of the pressure chamber to cause pre-crystallization. Then, the metal is pressed into the mold cavity for filling and solidification. When the molten aluminum enters the mold cavity 2 seconds later, the shrinkage piston of the local loading and compensating device starts to work, and its specific pressure at the boss position is 120MPa-150MPa.

[0082] The local loading and compensating pressure device is composed of an oil inlet, a piston cylinder, a fixing bolt, a sealing ring and a compensating piston; the oil inlet and the compensating piston are respectively arranged at both ends of the piston cylinder, the compensating piston and the piston cylinder are slidably connected, a sealing ring is arranged at one end of the piston cylinder located at the compensating piston, and 4 fixing bolts are evenly arranged on the outside of the piston rod; a total of 8 local loading and compensating pressure devices are arranged, which are respectively fixed on the 8 bosses of the rear bottom plate movable mold by fixing bolts, and the compensating piston is perpendicular to the boss surface and extends into the boss; the sealing ring prevents the leakage of hydraulic oil in the piston cylinder while maintaining the vacuum degree of the mold cavity; the oil inlet is connected to the external high-pressure oil pipe, and the size of the compensating pressure and the moving speed are controlled by the hydraulic servo motor;

[0083] A total of 29 groups of comparative tests were conducted to examine the effects of eliminating shrinkage defects under different combinations of pouring temperature, mold temperature, slow injection speed of the ram, and fast injection speed of the ram, and select the best parameters;

[0084] Step 6: Die-cast the rear floor of the automobile according to the optimal parameters obtained from the numerical simulation results in step 5. The specific steps are as follows:

[0085] a. The rear bottom plate die-casting machine adopts a horizontal die-casting machine. The rear bottom plate is divided into two parts: a movable die and a fixed die. The fixed die and the movable die are in contact to form a cavity for the rear bottom plate of the car. The pressure chamber is connected to the fixed die to send the aluminum liquid into the cavity. The heating device of the mold and the cooling water pipe in the solidification process are distributed in the movable die and the fixed die;

[0086] b. First, the die-casting machine is used to move the movable mold and the fixed mold to close the mold, and after closing the mold, the mold cavity is vacuumed;

[0087] c. To prevent the metal liquid from forming waves after pouring into the pressure chamber, which may cause the flowing metal liquid to be mixed with air, the metal liquid is poured into the pressure chamber and then left to stand for 2 seconds, and the metal liquid gradually stabilizes;

[0088] d. During the die-casting process, the punch moves forward at a multi-stage speed, including a low-speed and slow pressure stage, an acceleration stage, and a high-speed pressurization stage. In the low-speed and slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate the vacuum. When the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber and accelerate uniformly.

[0089] e. In the acceleration stage, the punch speed increases rapidly, and the kinetic energy is converted into thermal energy, which improves the fluidity of the molten metal to facilitate subsequent filling of the cavity. The acceleration is 400m / s 2 ;

[0090] f. High-speed pressurization stage: the molten metal fills the cavity. After integration, the floor is a complex thin-walled casting, which requires a higher injection speed. In this stage, the speed after the acceleration in step e runs at a constant speed until the filling is completed. The pressure needs to be maintained continuously. The pressure in the cavity is 35MPa.

[0091] g. 2 seconds after the filling is completed, the shrinkage piston of the local loading and compensating pressure device is used to compulsorily compensate the boss;

[0092] h. After the casting solidifies, the local loading and pressure-compensating device releases pressure and exits, and the movable mold is moved and opened through the horizontal die-casting machine to take out the formed part.

[0093] Specific implementation method 2: This implementation method is different from the specific implementation method 1 in that:

[0094] In step 2, set the gravity direction in Gravity / Virtual Mold to be parallel to the rear base plate;

[0095] The mass fraction composition of the material is: Si: 8-11%, Mg: 0.1-0.4%, Mn: 0.4-0.8%, V: 0.1-0.4%, Zr: 0.1-0.4%, Zn: 0.1-0.4%, Fe: 0.0-0.2%, and the rest is Al;

[0096] The selected aluminum liquid casting temperature is 670℃~710℃, and the mold temperature is set to 160~200℃;

[0097] The heat transfer coefficient is 2000W / (m·℃);

[0098] Boundary condition setting: the movable mold is in contact with the fixed mold, and the pressure chamber is in contact with the fixed mold;

[0099] Cooling conditions: The heat transfer between the mold and the casting is conducted, the heat transfer between the mold and the air is radiated, and the temperature inside the mold is controlled by cooling water channels;

[0100] Simulation time: The filling time is 1 s. Others are the same as the first embodiment.

[0101] Specific implementation method three: This implementation method is different from specific implementation methods one or two in that: in step three, the maximum injection speed is 12m / s, the slow speed deviation is 0.02m / s, and the mold closing stroke is 2600mm; the integrated rear floor is a complex large thin-walled part, the filling flow resistance is large, and the metal liquid flow is far beyond the conventional casting. The selected injection pressure range is 100-120MPa; the slow injection speed of the pressure head is 0.2-0.6m / s, the fast injection speed of the pressure head is 4-6m / s, and the acceleration is 400m / s 2 The rest is the same as the first or second embodiment.

[0102] Specific implementation method 4: This implementation method is different from any one of the specific implementation methods 1 to 3 in that the continuity equation of the two-equation turbulence model described in step 3 is:

[0103]

[0104]

[0105]

[0106]

[0107] Where ρ is the fluid density (kg / m 3 );

[0108] t——time(s);

[0109] v x 、v y 、v z ——Velocity components in x, y, and z directions at time t (m / s);

[0110] P——Surface force on the microelement, unit is (Pa);

[0111] F is the force per unit mass;

[0112] F x 、F y 、F z ——components of the unit mass force in the x, y, and z directions (N);

[0113] μ——dynamic viscosity (N·s / m 2 );

[0114] c——Specific heat of fluid (J / (kg·K));

[0115] T——fluid temperature (℃);

[0116] λ——thermal conductivity of fluid (W / (m·℃));

[0117] S——includes various source terms. The rest is the same as one of the specific implementation modes 1 to 3.

[0118] Specific implementation method 5: This implementation method is different from specific implementation method 4 in that: in step 6b, a horizontal die-casting machine is first used to move the movable mold and the fixed mold to close the mold, the maximum shooting speed is 12m / s, the slow speed deviation is 0.02m / s, the mold closing stroke is 2600mm, and after closing the mold, the mold cavity is vacuumed, and the air volume is 50mbar. Others are the same as specific implementation method 4.

[0119] Specific implementation method 6: This implementation method is different from specific implementation method 5 in that: in step 6d, during the die casting process, the punch moves forward at a multi-stage speed, including a low-speed slow pressure stage, an acceleration stage, and a high-speed pressurization stage; in the low-speed slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate; when the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber. The injection punch is located between 0 and 950 mm, the speed is 0.2 to 0.6 m / s, and the acceleration is uniform. The rest is the same as specific implementation method 5.

[0120] Specific implementation method 7: This implementation method is different from specific implementation method 6 in that: in step 6e, the punch speed increases rapidly during the acceleration phase, and the acceleration is 400m / s 2, kinetic energy is converted into thermal energy, improving the fluidity of the molten metal so as to fill the cavity later; the injection punch starts from the 950mm position at a speed of 4 to 6m / s. The rest is the same as the sixth embodiment.

[0121] Specific embodiment eight: This embodiment differs from specific embodiment seven in that: in step six f, during the high-speed pressurization stage, the metal liquid fills the cavity. The integrated floor is a complex thin-walled casting, which requires a higher injection speed. During this stage, the speed is maintained at 4m / s to 6m / s until the filling is completed. The pressure needs to be maintained continuously, and the pressure in the cavity is 35MPa. The rest is the same as specific embodiment seven.

[0122] Specific embodiment 9: This embodiment is different from specific embodiment 8 in that: in step 6g, 2s after the filling is completed, the boss is forced to be compressed by a compression piston, and the specific pressure of the forced compression is 125MPa-150MPa. The rest is the same as specific embodiment 8.

[0123] Specific embodiment 10: This embodiment differs from specific embodiment 9 in that the specific pressure of forced feeding in step 6g is 125MPa-150MPa and maintained for 30s. The rest is the same as specific embodiment 9.

[0124] The present invention is verified by the following tests:

[0125] Test 1: This test is a method for die-casting an integrated die-cast automobile rear floor, which is specifically carried out in the following steps:

[0126] The dimensions of the rear floor to be formed are 1842mm×1549mm×741mm, the weight exceeds 56kg, and the volume reaches 38158936mm 3 , the projected area reaches 2853258mm 2 The average wall thickness is 3mm, the minimum wall thickness is about 2mm, and the maximum thickness exceeds 10cm (boss), such as Figure 4 As shown;

[0127] Step 1: Merge the castings and perform finite element modeling: Merge the integrated rear floor casting, pouring system, and overflow system, and convert the model into X_t format and import it into the Visual-Mesh module in ProCAST. Then use the Repair, Assembly, and Intersection functions to repair the model and obtain a three-dimensional model (such as Figure 1 shown);

[0128] Step 2: Model pre-processing: The three-dimensional model obtained in step 1 is sequentially subjected to gravity direction setting, material setting, temperature setting, heat transfer coefficient setting, boundary condition setting, cooling condition setting and simulation time setting;

[0129] In Gravity / Virtual Mold, set the gravity direction to be parallel to the rear base plate;

[0130] The mass fraction composition of the material is: Si: 10%, Mg: 0.3%, Mn: 0.6%, V: 0.3%, Zr: 0.3%, Zn: 0.13%, Fe: 0.1%, and the rest is Al;

[0131] The selected aluminum liquid casting temperature is 670℃~710℃, and the mold temperature is set to 160~200℃;

[0132] The heat transfer coefficient is 2000W / (m·℃);

[0133] Boundary condition setting: the movable mold is in contact with the fixed mold, and the pressure chamber is in contact with the fixed mold;

[0134] Cooling conditions: The heat transfer between the mold and the casting is conducted, the heat transfer between the mold and the air is radiated, and the temperature inside the mold is controlled by cooling water channels;

[0135] Simulation time: filling time is 1s;

[0136] Step 3: The die-casting process is simulated based on ProCAST simulation software, and the die-casting parameters are consistent with the actual production; the maximum injection speed reaches 12m / s, the slow speed deviation is 0.02m / s, and the mold closing stroke is 2600mm; the integrated rear floor is a complex large thin-walled part with large filling flow resistance, and the metal liquid flow is far beyond conventional castings. The selected injection pressure range is 100MPa~120MPa; the slow injection speed of the pressure head is 0.2~0.6m / s, the fast injection speed of the pressure head is 4~6m / s, and the acceleration is 400m / s 2 ;

[0137] In the integrated die casting filling process, the liquid filling speed is extremely fast. The molten metal is regarded as an incompressible fluid, and its flow obeys the conservation of mass and momentum. The two-equation turbulence model is used to describe the turbulence, and its continuity equation is:

[0138]

[0139]

[0140]

[0141]

[0142] Where ρ is the fluid density (kg / m 3 );

[0143] t——time(s);

[0144] v x 、v y 、v z ——Velocity components in x, y, and z directions at time t (m / s);

[0145] P——Surface force on the microelement, unit is (Pa);

[0146] F is the force per unit mass;

[0147] F x 、F y 、F z ——components of the unit mass force in the x, y, and z directions (N);

[0148] μ——dynamic viscosity (N·s / m 2 );

[0149] c——Specific heat of fluid (J / (kg·K));

[0150] T——fluid temperature (℃);

[0151] λ——thermal conductivity of fluid (W / (m·℃));

[0152] S——contains various source terms;

[0153] During the solidification process, temperature release includes three ways: heat conduction, convection and radiation;

[0154] Heat transfer occurs when the metal liquid contacts the metal mold, which is described by Fourier's law heat conduction differential equation:

[0155]

[0156] There is always convection heat transfer between the inside of the molten metal, between the molten metal and the mold, and between the mold and the outside air environment, which can be described by Newton's cooling law:

[0157] q=α(T f -T w )

[0158] Thermal radiation transfers energy through electromagnetic waves. Heat is transferred between castings, molds and the atmosphere through thermal radiation, which can be described by the law of air thermal radiation:

[0159]

[0160] Where ρ is the fluid density (kg / m 3 );

[0161] q——heat transfer flux (W / (m 2 K));

[0162] cp ——Specific heat capacity at constant pressure (J / kgk);

[0163] Q——heat source term;

[0164] α——convective heat transfer coefficient;

[0165] T f ——Fluid characteristic temperature (℃);

[0166] T w ——solid boundary temperature (℃);

[0167] T s ——absolute surface temperature (℃);

[0168] ε——radiation temperature (℃);

[0169] σ0——Stefen-Boltzman constant;

[0170] Step 4: Mesh the mold and casting: Due to the large size of the integrated floor, automatic meshing with small meshes may increase the number of meshes; automatic meshing with large meshes may not ensure the quality of the meshes, so the casting needs to be divided into blocks, where the mesh size of the casting is 4mm×4mm×4mm, the mesh size of the pouring system is 15mm×15mm×15mm, and the mesh size of the overflow system is 6mm×6mm×6mm, so as to greatly reduce the number of meshes and shorten the calculation time; the casting has a huge size, complex shape, and thin wall thickness, so it is impossible to fully automatically mesh, and the mesh needs to be manually divided at the wheel cover, longitudinal beam and rear floor platform, and the final number of meshes is 8 million;

[0171] Step 5: According to the above parameter settings, pour the molten metal into the pressure chamber and then stand for 2 seconds. The molten metal contacts the surface of the pressure chamber to cause pre-crystallization. Then, the metal is pressed into the mold cavity for filling and solidification. When the molten aluminum enters the mold cavity 2 seconds later, the shrinkage piston of the local loading and compensating device starts to work, and its specific pressure at the boss position is 120MPa-150MPa. Figure 2 For the mold diagram;

[0172] The local loading and compensating pressure device is as follows: Figure 3As shown, it is specifically composed of an oil inlet 1-1, a piston cylinder 1, a fixing bolt 2, a sealing ring 3 and a feeding piston 4; the oil inlet 1-1 and the feeding piston 4 are respectively arranged at both ends of the piston cylinder 1, the feeding piston 4 and the piston cylinder 1 are slidably connected, a sealing ring 3 is arranged at one end of the piston cylinder 1 located at the feeding piston 4, and 4 fixing bolts 2 are evenly arranged on the outside of the piston rod 4; a total of 8 local loading and compensating pressure devices are arranged, which are respectively fixed on 8 bosses of the rear bottom plate movable mold by fixing bolts 2, and the feeding piston 4 is perpendicular to the boss surface and extends into the boss (as shown in FIG. Figure 5 The sealing ring 3 prevents leakage of hydraulic oil in the piston cylinder 1 while maintaining the vacuum degree of the mold cavity; the oil inlet 1 is connected to the external high-pressure oil pipe, and the size of the shrinkage pressure and the moving speed are controlled by the hydraulic servo motor;

[0173] A total of 29 groups of comparative tests were conducted, as shown in Table 1, to investigate the effects of eliminating shrinkage defects under different combinations of pouring temperature, mold temperature, slow injection speed of the ram, and fast injection speed of the ram. The simulation results are shown in Figure 6 , the best parameters were selected: the pouring temperature was 694°C, the mold temperature was 186°C, the fast injection speed of the ram was 5.033m / s, and the slow injection speed of the ram was 0.397m / s;

[0174] Table 1

[0175]

[0176] Step 6: Die-cast the rear floor of the automobile according to the optimal parameters obtained from the numerical simulation results in step 5. The specific steps are as follows:

[0177] a. The rear floor die-casting machine adopts a horizontal die-casting machine. The rear floor is divided into two parts: a movable die and a fixed die. The fixed die and the movable die are in contact to form a cavity for the rear floor of the car. The pressure chamber is connected to the fixed die to send the aluminum liquid into the cavity. The mold heating device and the cooling water pipe in the solidification process are distributed in the movable die and the fixed die. The rear floor mold uses the latest mold temperature balance technology, and different lining molds are designed in blocks. Each area is individually temperature controlled. A total of 15 oil-heated mold temperature controllers and 30 water temperature controllers are set for graded control to ensure that the mold temperature is 186°C.

[0178] b. First, the die-casting machine is used to move the movable mold and the fixed mold to close the mold. The maximum shooting speed reaches 12m / s, the slow speed deviation is 0.02m / s, and the mold closing stroke is 2600mm. After closing the mold, the mold cavity is evacuated to a vacuum degree of 50mbar;

[0179] c. The casting temperature is 694°C. To prevent the metal liquid from forming waves after pouring into the pressure chamber, which may cause the flowing metal liquid to be mixed with air, the metal liquid is poured into the pressure chamber and then left to stand for 2 seconds, and the metal liquid gradually stabilizes;

[0180] d. During the die-casting process, the punch moves forward at a multi-stage speed, including a low-speed and slow pressure stage, an acceleration stage, and a high-speed pressurization stage. In the low-speed and slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate the vacuum. When the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber. The injection punch is located between 0 and 950 mm, with a speed of 0.397 m / s and uniform acceleration.

[0181] e. The punch starts from the 950mm position. During the acceleration phase, the punch speed increases rapidly to 5.033m / s, and the acceleration is 400m / s. 2 , kinetic energy is converted into thermal energy, which improves the fluidity of the molten metal for subsequent filling of the cavity;

[0182] f. High-speed pressurization stage: the molten metal fills the cavity. The integrated floor is a complex thin-walled casting, which requires a higher injection speed. This stage runs at a uniform speed of 5.033m / s until the filling is completed. The pressure needs to be maintained continuously. The pressure in the cavity is 35MPa.

[0183] g. 2 seconds after the filling is completed, the boss is forced to be compressed by the compression piston of the local loading and compensating pressure device, and the specific pressure of the forced compression is 125MPa and maintained for 30s;

[0184] h. After the casting solidifies, the local loading and pressure-compensating device releases pressure and exits, and the movable mold is moved and opened through the horizontal die-casting machine to take out the formed part.

[0185] like Figure 6 As shown in the figure, the numerical fluctuations of the pouring temperature, mold temperature, fast injection speed and slow injection speed on shrinkage porosity are: 139cm 3 、79cm 3 、66cm 3 and 52cm 3 The influence of different die-casting process parameters on the shrinkage volume of the integrated floor is as follows: pouring temperature > mold temperature > fast injection speed > slow injection speed, thus obtaining the optimal forming process parameters.

[0186] Figure 7 A front view of the rear floor of a partially loaded car for a test. Figure 8 Back side photo of the rear floor of a partially loaded car for a test. Figure 7 and Figure 8 It shows that under reasonable forming parameters, the surface of the rear floor of the automobile is smooth and free of defects, and the macroscopic morphology of the local loading position on the back is smooth and free of defects.

[0187] Fig. 9This is the OM diagram of the local loading part in test 1, i.e., the boss loading. Fig. 9 It shows that at the local pressure-compensating position, the critical nucleation radius is smaller than that at other positions under the action of additional pressure, and the solidification driving force increases. At this time, the grains are small and dense, and have good roundness, which is similar to the spherulites formed in the semi-solid state.

[0188] Fig.10 For experiment 1, the OM diagram at the position without local loading and shrinkage feeding, the microstructure is mainly dendrite structure, which is not conducive to the improvement of mechanical properties.

Claims

1. A method for die-casting an integrated die-casting automobile rear floor, characterized in that The method for die-casting the integrated die-casting automobile rear floor is carried out in the following steps: Step 1: Merge the castings and perform finite element modeling: Merge the integrated rear floor casting, pouring system and overflow system, convert the model into X_t format and import it into the Visual-Mesh module in ProCAST, and then use the Repair, Assembly and Intersection functions to repair the model and obtain a three-dimensional model; Step 2: Model pre-processing: The three-dimensional model obtained in step 1 is sequentially subjected to gravity direction setting, material setting, temperature setting, heat transfer coefficient setting, boundary condition setting, cooling condition setting and simulation time setting; Step 3: The die-casting process is simulated based on ProCAST simulation software: the die-casting parameters are consistent with those in actual production; the die-casting parameters include maximum injection speed, slow speed deviation, mold closing stroke, injection pressure, slow injection speed of the ram, fast injection speed of the ram and acceleration; In the integrated die casting filling process, the liquid filling speed is extremely fast, and the molten metal is regarded as an incompressible fluid. Its flow obeys the conservation of mass and momentum, and the two-equation turbulence model is used to describe the turbulence. During the solidification process, temperature release includes three ways: heat conduction, convection and radiation; Heat transfer occurs when the metal liquid contacts the metal mold, which is described by Fourier's law heat conduction differential equation: There is always convection heat transfer between the inside of the molten metal, between the molten metal and the mold, and between the mold and the outside air environment, which can be described by Newton's cooling law: q=α(T f -T w ) Thermal radiation transfers energy through electromagnetic waves. Heat is transferred between castings, molds and the atmosphere through thermal radiation, which can be described by the law of air thermal radiation: Where ρ is the fluid density (kg / m 3 ); q——heat transfer flux (W / (m 2 K)); c p ——Specific heat capacity at constant pressure (J / kgk); Q——heat source term; α——convective heat transfer coefficient; T f ——Fluid characteristic temperature (℃); T w ——solid boundary temperature (℃); T s ——absolute surface temperature (℃); ε——radiation temperature (℃); σ0——Stefen-Boltzman constant; Step 4: Mesh the mold and casting: Since the size of the integrated floor is large, the use of small meshes for automatic division may increase the number of meshes; while the use of large meshes for automatic division may not ensure the quality of the meshes. Therefore, the casting needs to be divided into blocks, where the mesh size of the casting part is 4mm×4mm×4mm, the mesh size of the pouring system is 15mm×15mm×15mm, and the mesh size of the overflow system is 6mm×6mm×6mm, so as to greatly reduce the number of meshes and shorten the calculation time. The castings are huge in size, complex in shape, and thin in wall thickness, so they cannot be fully meshed automatically. Manual meshing is required at the wheel arches, longitudinal beams, and rear floor platforms. Step 5: According to the above parameter settings, pour the molten metal into the pressure chamber and then stand for 2 seconds. The molten metal contacts the surface of the pressure chamber to cause pre-crystallization. Then, the metal is pressed into the mold cavity for filling and solidification. When the molten aluminum enters the mold cavity 2 seconds later, the shrinkage piston of the local loading and compensating device starts to work, and its specific pressure at the boss position is 120MPa-150MPa. The local loading and compensating pressure device is composed of an oil inlet, a piston cylinder, a fixing bolt, a sealing ring and a compensating piston; the oil inlet and the compensating piston are respectively arranged at both ends of the piston cylinder, the compensating piston and the piston cylinder are slidably connected, a sealing ring is arranged at one end of the piston cylinder located at the compensating piston, and 4 fixing bolts are evenly arranged on the outside of the piston rod; a total of 8 local loading and compensating pressure devices are arranged, which are respectively fixed on the 8 bosses of the rear bottom plate movable mold by fixing bolts, and the compensating piston is perpendicular to the boss surface and extends into the boss; the sealing ring prevents the leakage of hydraulic oil in the piston cylinder while maintaining the vacuum degree of the mold cavity; the oil inlet is connected to the external high-pressure oil pipe, and the size of the compensating pressure and the moving speed are controlled by the hydraulic servo motor; A total of 29 groups of comparative tests were conducted to examine the effects of eliminating shrinkage defects under different combinations of pouring temperature, mold temperature, slow injection speed of the ram, and fast injection speed of the ram, and select the best parameters; Step 6: Die-cast the rear floor of the automobile according to the optimal parameters obtained from the numerical simulation results in step 5. The specific steps are as follows: a. The rear bottom plate die-casting machine adopts a horizontal die-casting machine. The rear bottom plate is divided into two parts: a movable die and a fixed die. The fixed die and the movable die are in contact to form a cavity for the rear bottom plate of the car. The pressure chamber is connected to the fixed die to send the aluminum liquid into the cavity. The heating device of the mold and the cooling water pipe in the solidification process are distributed in the movable die and the fixed die; b. First, the die-casting machine is used to move the movable mold and the fixed mold to close the mold, and after closing the mold, the mold cavity is vacuumed; c. To prevent the metal liquid from forming waves after pouring into the pressure chamber, which may cause the flowing metal liquid to be mixed with air, the metal liquid is poured into the pressure chamber and then left to stand for 2 seconds, and the metal liquid gradually stabilizes; d. During the die-casting process, the punch moves forward at a multi-stage speed, including a low-speed and slow pressure stage, an acceleration stage, and a high-speed pressurization stage. In the low-speed and slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate the vacuum. When the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber and accelerate uniformly. e. In the acceleration stage, the punch speed increases rapidly, and the kinetic energy is converted into thermal energy, which improves the fluidity of the molten metal to facilitate subsequent filling of the cavity. The acceleration is 400m / s 2 ; f. High-speed pressurization stage: the molten metal fills the cavity. After integration, the floor is a complex thin-walled casting, which requires a higher injection speed. In this stage, the speed after the acceleration in step e runs at a constant speed until the filling is completed. The pressure needs to be maintained continuously. The pressure in the cavity is 35MPa. g. 2 seconds after the filling is completed, the shrinkage piston of the local loading and compensating pressure device is used to compulsorily compensate the boss; h. After the casting solidifies, the local loading and compensating pressure device releases the pressure and exits, and the movable mold is moved and opened by the horizontal die-casting machine to take out the formed part.

2. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 2, set the gravity direction in Gravity / Virtual Mold to be parallel to the rear base plate; The mass fraction composition of the material is: Si: 8-11%, Mg: 0.1-0.4%, Mn: 0.4-0.8%, V: 0.1-0.4%, Zr: 0.1-0.4%, Zn: 0.1-0.4%, Fe: 0.0-0.2%, and the rest is Al; The selected aluminum liquid casting temperature is 670℃~710℃, and the mold temperature is set to 160~200℃; The heat transfer coefficient is 2000W / (m·℃); Boundary condition setting: the movable mold is in contact with the fixed mold, and the pressure chamber is in contact with the fixed mold; Cooling conditions: The heat transfer between the mold and the casting is conducted, the heat transfer between the mold and the air is radiated, and the temperature inside the mold is controlled by cooling water channels; Simulation time: The filling time is 1s.

3. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 3, the maximum injection speed is 12m / s, the slow speed deviation is 0.02m / s, and the mold closing stroke is 2600mm; the integrated rear floor is a complex large thin-walled part with large filling flow resistance. The metal liquid flow is far beyond the conventional casting. The selected injection pressure range is 100-120MPa; the slow injection speed of the pressure head is 0.2-0.6m / s, the fast injection speed of the pressure head is 4-6m / s, and the acceleration is 400m / s 2 .

4. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that The continuity equation for the two-equation turbulence model described in step 3 is: Where ρ is the fluid density (kg / m 3 ); t——time(s); v x 、v y 、v z ——Velocity components in x, y, and z directions at time t (m / s); P——Surface force on the microelement, unit is (Pa); F is the force per unit mass; F x 、F y 、F z ——components of the unit mass force in the x, y, and z directions (N); μ——dynamic viscosity (N·s / m 2 ); c——Specific heat of fluid (J / (kg·K)); T——fluid temperature (℃); λ——thermal conductivity of fluid (W / (m·℃)); S——contains various source terms.

5. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 6b, a horizontal die-casting machine is first used to move the movable mold and the fixed mold for mold closing. The maximum shooting speed is 12m / s, the slow speed deviation is 0.02m / s, the mold closing stroke is 2600mm, and the mold cavity is vacuumed after mold closing, and the air density is 50mbar.

6. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that Step 6d: During the die-casting process, the punch moves forward at a multi-stage speed, including a low-speed and slow pressure stage, an acceleration stage, and a high-speed pressurization stage; in the low-speed and slow pressure stage, the punch advances at a low speed and seals the pouring port, and then begins to evacuate the vacuum; when the molten metal is concentrated at the inner gate, the low-speed injection stage is completed. In this process, the molten metal in the pressure chamber needs to be fully filled, and the vacuum system is better used to remove the gas in the pressure chamber. The injection punch is located between 0 and 950 mm, with a speed of 0.2 to 0.6 m / s and uniform acceleration.

7. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 6e, the punch speed increases rapidly during the acceleration phase, with an acceleration of 400 m / s 2 , kinetic energy is converted into thermal energy, which improves the fluidity of the molten metal for subsequent filling of the cavity; the injection punch starts from the 950mm position at a speed of 4 to 6m / s.

8. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 6f, during the high-speed pressurization stage, the molten metal fills the cavity. After integration, the floor is a complex thin-walled casting, which requires a higher injection speed. During this stage, the speed is maintained at 4m / s to 6m / s until the filling is completed. The pressure needs to be maintained continuously. The pressure in the cavity is 35MPa.

9. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 6g, 2 seconds after the filling is completed, the boss is forced to be compressed by a compression piston, and the specific pressure of the forced compression is 125MPa to 150MPa.

10. The method for die-casting an integrated die-cast automobile rear floor according to claim 1, characterized in that In step 6g, the specific pressure of forced shrinkage is 125MPa-150MPa and maintained for 30s.

Images