An alloy melting crystallizer casting system and design method for improving ingot quality

Abstract

This invention relates to an alloy melting and crystallizing casting system and design method for improving ingot quality. The system includes a gating system and a water-cooled crystallizer. The gating system comprises a curved section and a horizontal section, with the horizontal section positioned above the molten ingot pool formed inside the water-cooled crystallizer. The horizontal section is tangentially positioned along the circumference of the molten ingot pool, and the distance L between the central axis of the horizontal section's outlet and the center of the molten ingot pool is greater than 0 and less than the radius R of the molten ingot pool. This invention replaces the traditional method of vertically injecting the melt from the edge of the water-cooled crystallizer with a tangential horizontal injection at a distance L from the center of the molten ingot pool, significantly improving ingot quality. The casting system design method of this invention involves screening different candidate designs based on finite element simulation to obtain the optimal design.

Description

Technical Field

[0001] This invention relates to the field of metal casting, specifically to an alloy melting and crystallizing casting system and design method for improving ingot quality. Background Technology

[0002] The quality of alloy ingots such as titanium alloys and aluminum alloys has a decisive impact on the production of subsequent products and the quality of the final product. However, some defects generated during the smelting process in the existing technology are difficult to eliminate in the subsequent processing.

[0003] Currently, electron beam cold hearth furnaces generally use edge casting to inject the molten metal into the crystallizer, while semi-continuous casting of aluminum alloys typically uses center casting. However, in actual production, it has been found that electron beam melting of round ingots often presents numerous surface problems on the casting side, including surface wrinkles, cold shuts, and even molten metal flow issues. These problems severely affect the yield and quality of subsequent processing. Center casting in semi-continuous casting results in an excessively deep molten pool and a large solid-liquid interface area, leading to severe macroscopic segregation. Multiple melting comparisons and analyses indicate that this problem may be related to how the molten metal enters the crystallizer during the ingot melting process, i.e., the design of the gating system. In existing technologies, the gating system gating is vertically set. For example, patent CN118703810A discloses a crystallizer with several overflow ports and a method for establishing a visualization model during the melting of TC4 titanium alloy in an electron beam cold hearth furnace. The crystallizer inlet is vertically set, meaning the molten metal flows in vertically.

[0004] Through multiple melting comparisons and analyses, it is evident that the pouring method of the molten metal significantly affects the molten pool depth and solidification interface. A reasonable solidification interface should help reduce component segregation and improve compositional uniformity. Numerous studies have shown that a shallower molten pool within the crystallizer is beneficial for improving the surface quality and reducing internal segregation of the cast billet. Currently, methods to improve molten pool depth and solidification interface typically focus on reducing casting speed, i.e., altering the molten pool temperature by changing the electron beam scanning pattern and power. However, during the electron beam cold hearth furnace melting of titanium alloys, elements with high saturated vapor pressure, such as Al, are prone to volatilization in a vacuum environment, thus affecting the alloy element composition. Reducing casting speed and increasing the molten pool surface temperature both exacerbate element volatilization. In the semi-continuous casting system for aluminum alloys, the segregation problem of large-scale ingots has long been a major challenge for high-strength aluminum alloys, severely restricting important applications such as aerospace. Experimental comparative analysis reveals that this is mainly due to the lack of stirring during alloy solidification, resulting in an unfavorable molten pool shape, affecting the solidification process and exacerbating segregation. The current design of the molten metal flowing vertically into the crystallizer does not meet these requirements. Summary of the Invention

[0005] The purpose of this invention is to provide an alloy melting crystallizer casting system and design method for improving ingot quality. It changes the traditional method of vertically injecting the melt from the edge of the water-cooled crystallizer to tangential horizontal injection at a distance L from the center of the ingot pool, which can significantly improve ingot quality.

[0006] The objective of this invention is achieved through the following technical solution:

[0007] An alloy melting and crystallizing casting system for improving ingot quality includes a gating system and a water-cooled crystallizer. The gating system includes a curved section and a horizontal section, and the horizontal section is located above the ingot pool formed inside the water-cooled crystallizer. The horizontal section is arranged tangentially along the circumference of the ingot pool, and the distance L between the central axis of the horizontal section and the center of the ingot pool is greater than 0 and less than the radius R of the ingot pool.

[0008] L = 1 / 2 × R.

[0009] An electron beam heat source is provided above both the gating channel and the ingot molten pool.

[0010] A design method for an alloy melting crystallizer casting system for improving ingot quality, comprising the following steps:

[0011] Step 1: Establish a three-dimensional geometric fluid domain model in the software based on the dimensions of the water-cooled crystallizer;

[0012] Step 2: Generate a hexahedral mesh for the three-dimensional geometric fluid domain model established in Step 1;

[0013] Step 3: Add alloy and water-cooled crystallizer properties to the software solver, and set reasonable boundary conditions according to the production process parameters;

[0014] Step 4: Use energy, turbulence, and solidification / melting models in the solver, specifically by establishing the conservation equations for the mass, momentum, and energy of the molten metal for simulation, where:

[0015] I. The mass conservation equation is:

[0016]

[0017] In the above formula (1), The unit velocity is represented by ρ, density is represented by t, and time is represented by t.

[0018] These are the unit vectors for the x, y, and z coordinate axes, respectively.

[0019] II. The momentum conservation equation is:

[0020]

[0021] In equation (2) above, Let p be the stress tensor and p be the hydrostatic pressure. It is the acceleration due to gravity. For thermal-solute buoyancy, For the momentum sinking in the mushy region, where:

[0022]

[0023] In equation (3) above, β T Where T is the coefficient of thermal expansion, and T is the temperature. liq Where β is the liquidus temperature. c，i Y is the coefficient of thermal expansion. i，liq Y0 represents the local average concentration of solute element i in the liquid phase, and is the initial mass fraction of solute i.

[0024]

[0025] In equation (4) above, β is the liquid volume fraction, and ε is a constant, taken as 0.01. A is the solid velocity caused by the pulling speed. mush This is the constant for the pasty region, with a value of 100000.

[0026] III. The energy conservation equation is:

[0027]

[0028] In equation (5) above, H is enthalpy and k is thermal conductivity. S represents the energy of heat conduction, where S is the source term and H is the enthalpy of the material, which is the apparent enthalpy h. e The sum of latent heat ΔH:

[0029] H = h e +ΔH (6);

[0030]

[0031] In equation (7) above, h ref For reference enthalpy, T ref For reference temperature, c p Specific heat.

[0032] IV. Solidification and melting are performed using the enthalpy-porosity equation:

[0033] Enthalpy-porosity is based on the energy conservation equation, defining β as the amount of liquid fraction, representing the fraction of the mesh cell volume in liquid form, and associated with each mesh cell in the neighborhood;

[0034] The liquid fraction β is defined as:

[0035]

[0036] In equation (8) above, T solidus T is the solidus temperature. liquidus This is the liquidus temperature;

[0037] Substitute the liquid fraction β into the above formula (4), and then use software simulation to obtain the temperature field, flow field and molten pool profile of the ingot molten pool under different gating injection methods;

[0038] Step 5: Compare the temperature field, flow field and pool profile of the ingot molten pool obtained in Step 4 for different gating injection methods, and determine the gating injection direction and position. The gating injection direction is selected as the horizontal injection direction of the horizontal part of the gating after comparison. The distance between the central axis of the horizontal part of the gating and the center of the ingot molten pool (5) is L = 1 / 2 × R.

[0039] Step five is as follows:

[0040] Step 5.1: Initialize the model, set the time step and number of iterations, and start the calculation;

[0041] Step 5.2: When the calculation reaches a steady state, the temperature field, flow field, and molten pool profile of the ingot are obtained based on the calculation.

[0042] The advantages and positive effects of this invention are as follows:

[0043] 1. This invention changes the traditional method of vertically injecting melt from the edge of the water-cooled crystallizer to horizontal tangential injection at a distance L from the center of the ingot pool, which can significantly improve the quality of the ingot.

[0044] 2. Based on the characteristics of the ingot melt, this invention establishes energy, turbulence, solidification and melting models in the solver for finite element simulation analysis. By comparing the temperature field, flow field, and molten pool profile of different injection methods, the casting system design of this invention is selected. The simulation analysis shows that the overall temperature distribution of the ingot molten pool is roughly symmetrical during casting, and the overall molten pool is approximately symmetrical with a significantly shallower depth compared to the edge-injection method. This shallow molten pool casting condition can shorten the ingot cooling time during the ingot melting process, reduce the volatilization of volatile elements, reduce the overall compositional segregation of the ingot, and improve the surface quality. At the same time, the melt forms a counterclockwise flow at the top of the ingot, which will bring a certain stirring effect to the molten pool, thus contributing to the improvement of the compositional uniformity of the molten pool. Attached Figure Description

[0045] Figure 1 This is a schematic diagram of the system of the present invention.

[0046] Figure 2 for Figure 1 Top view of a water-cooled crystallizer

[0047] Figure 3 This is a flowchart of the design method of the present invention.

[0048] Figure 4 The design method of this invention is aimed at Figure 1 A three-dimensional geometric model and boundary condition diagram of the structure.

[0049] Figure 5 for Figure 4 The temperature field contour map obtained by the model through simulation is shown in the figure.

[0050] Figure 6 for Figure 4 The molten pool profile obtained through simulation in the model.

[0051] Figure 7 for Figure 4 The flow field diagram at the top of the ingot obtained through simulation in the model.

[0052] Figure 8 for Figure 4 The longitudinal profile flow field diagram obtained by the model through simulation.

[0053] Figure 9 A schematic diagram of defects in ingots produced using existing technology.

[0054] Among them, 1 is the material processing zone, 2 is the refining zone, 3 is the gating system, 301 is the curved part of the gating system, 302 is the horizontal part of the gating system, 4 is the electron beam heat source, 5 is the ingot molten pool, 6 is the water-cooled crystallizer, 7 is the vertical pouring port, and 8 is the heat exchange interface between the crystallizer and the side wall of the ingot. Detailed Implementation

[0055] The invention will now be described in further detail with reference to the accompanying drawings.

[0056] like Figures 1-2 As shown, the system of the present invention includes a gating system 3 and a water-cooled crystallizer 6, wherein the gating system 3 includes a curved section 301 and a horizontal section 302, and the horizontal section 302 is located above the ingot molten pool 5 formed inside the water-cooled crystallizer 6, as shown. Figure 2As shown, the horizontal section 302 of the gating system is tangentially arranged along the circumference of the molten ingot pool 5, and the distance L between the central axis of the horizontal section 302 and the center of the molten ingot pool 5 is greater than 0 and less than the radius R of the molten ingot pool 5. When the system of this invention is working, the feed (sponge titanium or alloy return material) is first melted and preliminarily refined in the melting zone 1. Then, the melt flows into the refining zone 2 for further refining to eliminate any high- or low-density inclusions that may be mixed in the raw material. The melt then flows from the refining zone 2 into the gating system 3. When the melt passes through the curved section 301 of the gating system, its speed increases due to gravity and it enters the horizontal section 302, where it becomes a horizontal flow. Because the horizontal section 302 is close to the molten ingot pool 5 and is tangentially arranged along the circumference of the molten ingot pool 5, this... Figure 2 As shown, the molten material flows approximately horizontally along the circumference into the ingot pool 5 and propels it to rotate within the water-cooled crystallizer 6. Once the molten material solidifies into an ingot within the water-cooled crystallizer 6, the ingot pulling mechanism pulls the ingot from the bottom of the crystallizer into the ingot chamber. Furthermore, an electron beam heat source 4 is located above the material processing zone 1 for melting the raw materials, above the refining zone 2 for refining, above the gating system 3 for maintaining the gating system temperature, and above the ingot pool 5 for maintaining the pool temperature. The material processing zone 1, refining zone 2, electron beam heat source 4, water-cooled crystallizer 6, and ingot pulling mechanism are all technologies known in the art.

[0057] The method of the present invention uses software simulation to determine the optimal value of the distance L between the axis of the horizontal part 302 of the gating system and the center of the ingot pool 5, because this value affects the rotation and stirring of the melt in the water-cooled crystallizer 6.

[0058] The design method of the present invention includes the following steps:

[0059] Step 1: In the software, establish a three-dimensional geometric fluid domain model based on the dimensions of the water-cooled crystallizer (6 units), where, for example... Figure 4 As shown in model (a), the water-cooled crystallizer 6 model in the prior art usually adopts a vertical pouring port 7, for example, see patent CN118703810A, while... Figure 4 As shown in model (b) above, the present invention is based on Figure 1 The gating system is designed such that the horizontal section 302 of the gating system is set horizontally.

[0060] Step 2: Divide the three-dimensional geometric fluid domain model established in Step 1 into a hexahedral mesh, which is a well-known technique in this field.

[0061] Step 3: Add the alloy and water-cooled crystallizer 6 properties to the software solver, and set reasonable boundary conditions according to the production process parameters. This is a well-known technique in the field.

[0062] Step 4: Use energy, turbulence, and solidification / melting models in the solver, specifically by establishing the conservation equations for the mass, momentum, and energy of the molten metal for simulation, where:

[0063] I. The mass conservation equation is:

[0064]

[0065] In the above formula (1), The unit velocity is represented by ρ, density is represented by t, and time is represented by t.

[0066] These are the unit vectors for the x, y, and z coordinate axes, respectively.

[0067] II. The momentum conservation equation is:

[0068]

[0069] In equation (2) above, Let p be the stress tensor and p be the hydrostatic pressure. It is the acceleration due to gravity. For thermal-solute buoyancy, For the momentum sinking in the mushy region, where:

[0070]

[0071] In equation (3) above, β T Where T is the coefficient of thermal expansion, and T is the temperature. liq Where β is the liquidus temperature. c，i Y is the coefficient of thermal expansion. i，liq Y0 represents the local average concentration of solute element i in the liquid phase, and is the initial mass fraction of solute i.

[0072]

[0073] In equation (4) above, β is the liquid volume fraction, and ε is a constant, taken as 0.01. A is the solid velocity caused by the pulling speed. mush This is the constant for the pasty region, with a value of 100000.

[0074] III. The energy conservation equation is:

[0075]

[0076] In equation (5) above, H is enthalpy and k is thermal conductivity. S represents the energy of heat conduction, where S is the source term and H is the enthalpy of the material, which is the apparent enthalpy h. e The sum of latent heat ΔH:

[0077] H = he +ΔH (6);

[0078]

[0079] In equation (7) above, h ref For reference enthalpy, T ref For reference temperature, c p Specific heat.

[0080] IV. Solidification and melting are performed using the enthalpy-porosity equation:

[0081] Enthalpy-porosity is based on the energy conservation equation, defining β as the amount of liquid fraction, representing the fraction of the mesh cell volume in liquid form, and associated with each mesh cell in the neighborhood;

[0082] The liquid fraction β is defined as:

[0083]

[0084] In equation (8) above, T solidus T is the solidus temperature. liquidus This is the liquidus temperature;

[0085] Substituting the liquid fraction β into the above formula (4), and then using software simulation calculations, the temperature field, flow field, and molten pool profile of the ingot molten pool 5 under different gating 3 injection methods are obtained.

[0086] Step 5: Compare the temperature field, flow field and molten pool profile of the ingot molten pool 5 obtained in Step 4 under different gating injection methods, and determine the injection direction and position of the gating 3. After comparison, the injection direction of the gating 3 is selected as the horizontal injection direction of the gating horizontal part 302. The distance between the central axis of the gating horizontal part 302 and the center of the ingot molten pool 5 is preferably L = 1 / 2 × R.

[0087] This step is specifically as follows:

[0088] Step 5.1: Initialize the model, set the time step and number of iterations, and start the calculation;

[0089] Step 5.2: When the calculation reaches steady state, compare and analyze the advantages and disadvantages of different casting methods based on the calculated temperature field, flow field, molten pool profile, etc., such as... Figure 5 As shown in Figures 6, 7, and 8. The software used in this embodiment is Fluent software.

[0090] The design concept of this invention is as follows: Considering the limitations of experimental research in characterization and testing, which makes it impossible to observe the temperature distribution of the ingot, the internal flow of the molten pool, and the contour morphology of the molten pool during the smelting process, this invention makes full use of the advantages of material calculation and simulation methods. Through software simulation, a casting system and casting method that can improve the quality of the ingot are designed. That is, the traditional method of vertically injecting the melt from the edge of the crystallizer is changed to horizontally injecting the melt along the tangential direction of about 1 / 2R of the circumference of the round ingot on the upper surface of the ingot.

[0091] The following application example further illustrates the invention.

[0092] like Figure 9 As shown, existing ingots exhibit numerous surface problems on the pouring side, including surface wrinkles, cold shuts, and even molten flow. For a clearer comparison and analysis, as... Figures 4-8 As shown, this application example only simulates and compares the vertical pouring gate 7 in the prior art with the horizontal part 302 of the gating system of the present invention. The heat exchange interface 8 between the crystallizer and the ingot sidewall in both methods is as follows: Figure 4 As shown.

[0093] Depend on Figure 5 As shown, in both casting methods, the temperature distribution is such that the high-temperature melt enters from the inlet, and the temperature gradually decreases as it passes through the side wall and bottom of the crystallizer, until it solidifies. However, in (a), when the melt is injected from the edge of the ingot, the area of ​​the high-temperature region on the same side is significantly larger than that on the opposite side, resulting in an asymmetrical temperature distribution on both sides. When the casting method designed in this invention is used as shown in (b), and the melt is injected horizontally along the tangent direction at 1 / 2 radius of the ingot, the temperature distribution on both sides shows that the high-temperature region of the melt and the ingot on the opposite side of the casting position is slightly higher than that on the same side, but the difference is not significant. Compared with the significant deviation from symmetry in side casting, the overall temperature distribution is roughly symmetrical.

[0094] like Figure 6As shown, the molten pool profiles along the longitudinal section of the ingot are compared when the ingot reaches a steady state under two different casting methods: When the alloy melt is injected from the edge of the ingot, as shown in (a), the molten pool profile is deeper near the inlet and shallower further away from the inlet. Analysis shows that this asymmetry in depth is a key factor causing surface quality problems in the ingot. Due to fluctuations in melting conditions, the ingot edge near the inlet may not even have time to form a stable shell before being dragged away from the crystallizer by the ingot dragging device, causing leakage of incompletely solidified titanium alloy melt, resulting in problems such as nodules, cold shuts, and leakage. In addition, poor temperature field distribution can also cause segregation of alloy solutes, affecting the internal quality of the ingot. When the casting method designed in this invention is used, as shown in (b), when horizontally injected along the tangent direction at 1 / 2 radius of the ingot, the profile of the longitudinal section of the molten pool presents a low-arc crescent shape, with the molten pool on the opposite side of the inlet being slightly deeper than the inlet side. The overall symmetry is approximately symmetrical, and the depth of the molten pool is significantly shallower than that of the edge-injection casting method. This shallow molten pool casting condition can shorten the ingot cooling time during the ingot smelting process, reduce the volatilization of volatile elements, reduce overall component segregation of the ingot, and improve surface quality.

[0095] like Figures 7-8 As shown, the flow field along the longitudinal section of the ingot pool reaches steady state under two casting methods: When the high-temperature melt is injected from the edge, as shown in (a), most of the melt moves downward to the mushy region, and then flows further away, with the speed gradually decreasing, forming a stable molten pool. When the casting method designed in this invention is used, as shown in (b), when the high-temperature melt is injected horizontally from the upper surface of the ingot along the tangent direction of 1 / 2 radius circumference, the melt forms a counterclockwise flow on the liquid surface at the top of the ingot. The flow speed is related to the horizontal speed of the melt injection and the casting time. For uniform injection and ingot dragging, a steady state is gradually reached over time. This flow of melt will bring a certain stirring effect to the molten pool, thus contributing to the improvement of the uniformity of the molten pool composition.

[0096] The above application example uses the melting of TC4 titanium alloy as the implementation subject for detailed explanation. The pouring speed is 0.5 m / s, and the velocity directions are vertically downward along the axis of symmetry of the crystallizer and horizontally tangentially along 1 / 2R of the circumference inside the crystallizer. The pouring temperature is 2273 K, assuming that the temperature of the top surface of the ingot is constant at 2273 K, and the heat transfer coefficient between the water-cooled copper crucible and the surface of the ingot is 2000 W / m. 2 / K, ambient temperature 300K. This invention is not limited to TC4 alloy, but is also applicable to pure titanium and other titanium alloys, and is not limited to specific crystallizer sizes. Casting systems of other sizes can also be optimized using the simulation and experimental approach of this invention. This invention can also be applied to other alloys suitable for top-injection semi-continuous casting processes, such as aluminum alloys. For aluminum alloys, the injection speed can be adjusted by changing the height and size of the gating outlet to obtain a suitable molten pool rotation.

Claims

1. An alloy melting and crystallizing casting system for improving ingot quality, characterized in that: The system includes a gating system (3) and a water-cooled crystallizer (6). The gating system (3) includes a curved section (301) and a horizontal section (302). The horizontal section (302) is located above the ingot pool (5) formed inside the water-cooled crystallizer (6). The horizontal section (302) is arranged tangentially along the circumference of the ingot pool (5). The distance L between the central axis of the horizontal section (302) and the center of the ingot pool (5) is greater than 0 and less than the radius R of the ingot pool (5).

2. The alloy melting and crystallizing casting system for improving ingot quality according to claim 1, characterized in that: L = 1 / 2 × R.

3. The alloy melting and crystallizing casting system for improving ingot quality according to claim 1, characterized in that: An electron beam heat source (4) is provided above the gating channel (3) and above the ingot molten pool (5).

4. A design method for an alloy melting crystallizer casting system for improving ingot quality according to claim 2, characterized in that: Includes the following steps: Step 1: Establish a three-dimensional geometric fluid domain model in the software based on the dimensions of the water-cooled crystallizer (6); Step 2: Generate a hexahedral mesh for the three-dimensional geometric fluid domain model established in Step 1; Step 3: Add the alloy and water-cooled crystallizer (6) properties to the software solver, and set reasonable boundary conditions according to the production process parameters; Step 4: Use energy, turbulence, and solidification / melting models in the solver, specifically by establishing the conservation equations for the mass, momentum, and energy of the molten metal for simulation, where: I. The mass conservation equation is: In the above formula (1), The unit velocity is represented by ρ, density is represented by t, and time is represented by t. These are the unit vectors for the x, y, and z coordinate axes, respectively. II. The momentum conservation equation is: In equation (2) above, Let p be the stress tensor and p be the hydrostatic pressure. It is the acceleration due to gravity. For thermal-solute buoyancy, For the momentum sinking in the mushy region, where: In equation (3) above, β T Where T is the coefficient of thermal expansion, and T is the temperature. liq Where β is the liquidus temperature. c，i Y is the coefficient of thermal expansion. i，liq Y0 represents the local average concentration of solute element i in the liquid phase, and is the initial mass fraction of solute i. In equation (4) above, β is the liquid volume fraction, and ε is a constant, taken as 0.

01. A is the solid velocity caused by the pulling speed. mush This is the constant for the pasty region, with a value of 100000. III. The energy conservation equation is: In equation (5) above, H is enthalpy and k is thermal conductivity. S represents the energy of heat conduction, where S is the source term and H is the enthalpy of the material, which is the apparent enthalpy h. e The sum of latent heat ΔH: H=h e +ΔH (6); In equation (7) above, h ref For reference enthalpy, T ref For reference temperature, c p Specific heat. IV. Solidification and melting are performed using the enthalpy-porosity equation: Enthalpy-porosity is based on the energy conservation equation, defining β as the amount of liquid fraction, representing the fraction of the mesh cell volume in liquid form, and associated with each mesh cell in the neighborhood; The liquid fraction β is defined as: In equation (8) above, T solidus T is the solidus temperature. liquidus This is the liquidus temperature; Substitute the liquid fraction β into the above formula (4), and then use software simulation to obtain the temperature field, flow field and molten pool profile of the ingot molten pool (5) under different gating injection methods; Step 5: Compare the temperature field, flow field and pool profile of the ingot pool (5) with different gating injection methods obtained in Step 4, and determine the injection direction and position of the gating (3). The injection direction of the gating (3) is selected as the horizontal injection direction of the gating horizontal part (302) after comparison. The distance between the central axis of the gating horizontal part (302) and the center of the ingot pool (5) is L = 1 / 2 × R.

5. The design method for an alloy melting crystallizer casting system for improving ingot quality according to claim 4, characterized in that: Step five is as follows: Step 5.1: Initialize the model, set the time step and number of iterations, and start the calculation; Step 5.2: When the calculation reaches a steady state, the temperature field, flow field and molten pool profile of the ingot molten pool (5) are obtained from the calculation.

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