A hot sealing process for titanium alloy vacuum consumable melting zero-shrinkage ingot

Abstract

This invention belongs to the field of titanium alloy metallurgical technology and relates to a hot capping process for zero-shrinkage-cavity titanium alloy vacuum consumable melting ingots. The process includes the following steps: First, a three-dimensional microstructure field numerical model of the ingot solidification is constructed. The evolution path of the baseline heat input during the hot capping stage is determined through simulation, and a multi-platform stepped decreasing current control strategy is formulated. During the hot capping process, the temperature at the top of the molten pool and the liquid level are monitored in real time, and the current control parameters are dynamically corrected through closed-loop feedback. When the temperature deviation and comprehensive deviation index at the top of the molten pool meet the preset requirements for several consecutive cycles, and the control strategy is completed, the hot capping is terminated, achieving zero shrinkage-cavity at the top of the ingot. This method achieves accurate modeling and intelligent closed-loop control of the hot capping process, effectively ensuring the quality of the ingot.

Description

Technical Field

[0001] This invention relates to the field of titanium alloy smelting technology, and in particular to a hot capping process for zero-shrinkage ingots produced by vacuum consumable melting of titanium alloys. Background Technology

[0002] Vacuum arc remelting (VAR) is currently the mainstream process for preparing titanium alloy ingots, widely used in aerospace and high-end equipment manufacturing. This process maintains arc heating in a vacuum environment, causing the consumable electrode to slowly melt. The molten metal drips into a water-cooled crystallizer and solidifies, significantly reducing inclusion content and improving purity and microstructure uniformity. However, in the later stages of melting, the molten pool level gradually decreases, the amount of residual metal diminishes, gas escape becomes difficult, and the accelerated solidification rate easily leads to the formation of shrinkage cavities at the top. Shrinkage cavities not only affect the density and mechanical properties of the ingot but also increase machining allowances, reducing material utilization.

[0003] To suppress such defects, traditional techniques typically employ a hot capping process, which involves manually reducing the melting rate and extending the molten pool holding time at the end of the melting process to achieve feeding and venting. However, this process is highly dependent on experience, with crude process control and a lack of clear quantitative standards for current adjustment, which can easily lead to discontinuous solidification at the top or insufficient density in certain areas.

[0004] Existing research has attempted to optimize the arc termination process. For example, Chinese patent application CN113881854A improves the top quality by controlling electrode quality and stabilizing arc current, but it does not achieve full-process modeling and feedback adjustment. Chinese patent application CN117845067A uses a dual-vacuum self-consuming arc melting process and a linear current reduction scheme to improve purity, but its parameter control is still based on human experience and lacks real-time perception and adaptive adjustment capabilities for the solidification state of the molten pool. Some other technologies use temperature or flow fields for numerical prediction, but most of them are static simulations and fail to directly reflect the dense state of the microstructure, resulting in lag and indirectness in the prediction and control of shrinkage cavities.

[0005] In summary, existing vacuum arc melting hot capping processes for titanium alloys generally suffer from the following problems: inability to accurately perceive the microstructure at the top; limited process control paths with delayed response; and decoupling between current regulation and molten pool behavior, leading to unstable arc termination quality. Based on these three issues, existing hot capping processes still struggle to meet the "zero shrinkage cavity, high density" target for aerospace-grade large-size ingots. Summary of the Invention

[0006] The purpose of this invention is to provide a hot capping process for titanium alloy vacuum self-consumable melting zero shrinkage ingots, which solves the problem that existing hot capping processes cannot meet the goal of "zero shrinkage and high density" for aerospace-grade large-size ingots.

[0007] This invention is achieved through the following technical solution: This invention discloses a hot capping process for titanium alloy vacuum consumable melting zero shrinkage ingots, comprising the following steps: Step 1) Based on the material property parameters of the target titanium alloy, the geometric structure parameters of the vacuum self-consuming electric arc melting furnace, and the initial conditions of the smelting process, a three-dimensional microstructure field numerical model of the ingot solidification process is constructed. Based on the three-dimensional microstructure field numerical model, the temperature field, melt flow, liquid-solid phase transformation and microstructure evolution behavior in the melting and casting area are simulated and analyzed offline to obtain the simulation results. Step 2) Generate the baseline thermal input evolution path for the thermal capping stage based on the simulation results, and formulate a multi-platform stepped decreasing current control strategy based on the baseline thermal input evolution path for the thermal capping stage as the baseline current control path for the thermal capping stage. Among them, the multi-platform stepped current control strategy includes multiple current control platform current control strategies, and each current control platform current control strategy includes current value, current deceleration rate and duration. Step 3) During the hot capping stage, the temperature at the top of the molten pool and the liquid level height of the molten pool are monitored in real time and the temperature deviation and liquid level height deviation are calculated respectively. The comprehensive deviation index of liquid level height is calculated based on the liquid level height deviation and the comprehensive deviation index of liquid level height. Step 4) When the temperature deviation or liquid level deviation at a certain time point is not within the preset deviation range, the current value, current deceleration rate and duration in the current control strategy of the current control platform corresponding to that time point are dynamically corrected using a closed-loop feedback control method. Step 5) When the temperature deviation and comprehensive deviation index at the top of the molten pool simultaneously meet the preset requirements within multiple consecutive sampling cycles, and the multi-platform stepped decreasing current control strategy is executed to the last current control platform and reaches the preset duration, the current control of the hot sealing stage is terminated. Then, non-destructive testing is used to confirm that there are no shrinkage defects at the top of the ingot, and the top area of ​​the ingot is determined to meet the zero shrinkage quality requirement.

[0008] Furthermore, in step 1), the three-dimensional tissue field numerical model includes an energy conservation sub-model, a momentum conservation sub-model, a mass conservation sub-model, and a microstructure and pore evolution sub-model. The energy conservation sub-model is based on the equivalent heat capacity method and is used to describe the heat conduction, convection heat transfer and liquid-solid phase change latent heat release behavior in the melting and casting area. The momentum conservation sub-model is used to describe the flow behavior of the melt under the action of temperature gradient and gravity, and introduces the mushy region damping effect in the liquid-solid coexistence region. The mass conservation sub-model is used to describe the changes in liquid volume fraction and the feeding behavior caused by phase transition during solidification. The microstructure and porosity evolution sub-model is used to describe the characteristics of grain nucleation, growth, and porosity evolution, in order to evaluate the trend of microstructure density changes under different heat input conditions.

[0009] Furthermore, the energy conservation governing equation of the energy conservation sub-model is as follows: ; in, Represents the density of titanium alloys. Represents the specific heat capacity at constant pressure. Represents temperature. Represents smelting time. Represents the melt flow velocity vector; The gradient symbol represents the rate of change of a physical quantity in space. Represents thermal conductivity; Represents latent heat of fusion; Represents the solid volume fraction, obtained through linear interpolation between the liquid and solid phase lines.

[0010] Furthermore, the governing equations of the momentum conservation sub-model are expressed as follows: ; in, Represents the density of titanium alloys. Represents the melt flow velocity vector; Represents smelting time; The gradient symbol represents the rate of change of a physical quantity in space. Represents pressure; Represents dynamic viscosity; Represents the gravitational acceleration vector; This represents the permeability function of the pasty region.

[0011] Furthermore, the governing equations of the mass conservation sub-model are expressed as follows: ; in, Represents smelting time; Represents the density of the liquid phase; Represents the liquid phase volume fraction, and is related to the solid phase volume fraction. satisfy ; This represents the volume average velocity of the liquid phase within the melt. Represents the phase change mass source term; The gradient symbol represents the rate of change of a physical quantity in space.

[0012] Furthermore, the microstructure and pore evolution sub-model includes a continuous heterogeneous nucleation model, a grain growth model, and a pore evolution model; The expression for the continuous heterogeneous nucleation model is: ; in, , Represents localized supercooling; Represents the maximum nucleation density; Represents the average activated subcooling; The standard deviation represents the local supercooling. The liquidus temperature of titanium alloys; Representing titanium alloys at the current time The actual local temperature; Represents the nucleation rate; The expression for the grain growth model is as follows: ; in, Represents the total supercooling at the front edge of the interface. Represents the Gibbs-Thomson coefficient. Represents the radius of the dendrite tip. Represents the slope of the liquidus line. Represents the initial composition concentration of the titanium alloy. Represents the equilibrium distribution coefficient. This represents the diffusion coefficient of the solute in the liquid phase. Represents the steady-state growth rate of the dendrite tip; The expression for the pore evolution model is as follows: ; in, Represents the current time porosity; Represents the initial porosity; Activation energy representing the change in porosity; Represents the radius of the dendrite tip; Represents temperature; Represents the stress sensitivity coefficient; Represents stress; A nonlinear coefficient representing the rate of change of porosity; Represents the rate of change of porosity; Represents constants related to phase transition processes; This represents the rate at which the phase transition process affects the porosity evolution; Represents an exponential function; This represents the balanced distribution coefficient.

[0013] Furthermore, in step 2), the multi-platform stepped decreasing current control strategy is obtained by discretizing the continuous current decreasing function and divided into multiple current control platforms. Assuming the thermal capping stage is divided into N current control platforms, the current change process can be represented as a piecewise function: ; in, This represents the starting time point of the second current control platform, and so on. Representing the N The start time point of a current control platform; Represents the initial current value. This represents the current value of the second current control platform, and so on. Representing the N The current value of each current control platform; This represents the total solidification time.

[0014] Furthermore, in step 3), the temperature deviation is calculated as follows: ; in, represent The moment is located at the radial position from the center of the molten pool Temperature deviation at the location; represent At any time in radial position The measured temperature at the top of the molten pool; Represents the reference current control path Time, radial position The target temperature value at that location; The calculation method for liquid level deviation is as follows: ; in, represent Time, located in radial position The deviation in liquid level height at that location; represent Time, in radial position The measured height of the molten pool; Represents the reference current control path Time, radial position The target liquid level height at the location; Representing the Radial coordinates of each liquid level measurement location; The comprehensive deviation index of liquid level height is calculated based on the liquid level height deviation, and the expression is as follows: ; in, represent The comprehensive deviation index of liquid level height at any given time is used to reflect the overall degree of deviation of the molten pool liquid level in the radial direction; Represents the radial position The number of liquid level measurement points at the location; The comprehensive deviation index is calculated based on the combined deviation index of temperature deviation and liquid level height, and the expression is as follows: ; in, E ( k ) represents the first The comprehensive deviation index corresponding to each sampling period; Representing the Temperature deviation in the central region at the top of the molten pool within each sampling period; Representing the Comprehensive deviation of liquid level height within each sampling period; , The weighting coefficients represent the relative importance of temperature deviation and liquid level deviation in control decisions. This represents the sampling period number.

[0015] Furthermore, in step 4), the closed-loop feedback control method adopts a control method combining proportional, integral, and derivative functions, and the control output is expressed as: ; in, Representing the Feedback adjustment amount per sampling period; , , These represent the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively. It represents the cumulative amount of the comprehensive deviation index, used to eliminate steady-state error; This represents the time interval between adjacent sampling periods; Representing the -1 sampling period corresponds to the comprehensive deviation index.

[0016] Furthermore, the specific expressions for the current value, current reduction rate, and duration of each current control platform in the stepped decreasing current control strategy are as follows: ; ; ; in, Representing the N The current control platform in the first Current value within each sampling period; Representing the N The current control platform in the first Current deceleration rate within each sampling period; Representing the N The current control platform in the first The duration within each sampling period; These represent the upper and lower limits of the permissible current value, respectively. These represent the upper and lower limits of the allowable current deceleration rate, respectively. These represent the upper and lower limits of the allowable duration of the current control platform, respectively; clip(·) represents the clipping function.

[0017] Compared with the prior art, the present invention has the following beneficial technical effects: This invention provides a hot capping process for zero-shrinkage-cavity titanium alloy vacuum consumable melting ingots. By constructing a three-dimensional microstructure field numerical model of the ingot solidification process, offline simulation analysis is performed on the temperature field, melt flow, liquid-solid phase transformation, and microstructure evolution behavior within the melting and casting region. This provides a physical basis for formulating current control strategies during the hot capping stage, avoiding the uncertainties caused by indirect inference of shrinkage risk based solely on temperature field or empirical parameters in existing technologies. The simulation results based on the three-dimensional microstructure field numerical model generate a baseline heat input evolution path for the hot capping stage, and a multi-platform stepped decreasing current control strategy is designed accordingly. Compared to the single linear decreasing current method commonly used in existing technologies, this approach better matches the heat release characteristics of the molten pool in the final stage, which is beneficial for maintaining a stable solidification state in the top region of the molten pool. Real-time monitoring of the molten pool surface temperature and liquid level is introduced during the hot capping stage. Based on the monitoring data, a deviation parameter is constructed, and closed-loop feedback control is implemented for the current value, current reduction rate, and duration in the multi-platform stepped decreasing current control strategy. This reduces process deviations caused by manual judgment or operating condition fluctuations, improving the repeatability and stability of the hot capping process. When the temperature deviation and comprehensive deviation index at the top of the molten pool simultaneously meet preset requirements within multiple consecutive sampling cycles, and the multi-platform stepped decreasing current control strategy reaches the last current control platform and achieves the preset duration, the current control of the hot capping stage is terminated. This ensures that the top region of the molten pool maintains continuous feeding conditions at termination, effectively reducing the probability of macroscopic shrinkage cavities and improving the microstructure density of the top region of the ingot. Using the method of this invention for vacuum consumable arc melting can improve the quality stability of the top region of titanium alloy ingots without adding additional hardware structures, reducing the risk of rework and scrap. It is suitable for industrial applications with high requirements for ingot microstructure density and internal quality consistency. Attached Figure Description

[0018] Figure 1This is a flowchart of a hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to the present invention; Figure 2 Microstructure field of longitudinal section of titanium alloy ingot calculated by numerical simulation; Figure 3 The zero-shrinkage titanium alloy ingot smelted in Example 1; Figure 4 The titanium alloy ingot containing shrinkage defects was smelted for Comparative Example 1. Detailed Implementation

[0019] To make the objectives, technical solutions, and advantages of the present invention clearer, the following detailed description is provided in conjunction with the accompanying drawings and embodiments. It should be understood that the specific embodiments described herein are merely illustrative of the invention and are not intended to limit the invention; that is, the described embodiments are only a part of the embodiments of the present invention, and not all of them.

[0020] The detailed description of the embodiments of the present invention provided in the following figures is not intended to limit the scope of the claimed invention, but merely to illustrate one selected embodiment of the invention. All other embodiments obtained by those skilled in the art based on the figures and embodiments of the present invention without inventive effort are within the scope of protection of the present invention.

[0021] like Figure 1 As shown, this invention discloses a hot capping process for zero-shrinkage ingots produced by vacuum consumable melting of titanium alloys, comprising the following steps: Step 1) Based on the material property parameters of the target titanium alloy, the geometric structure parameters of the vacuum self-consuming electric arc melting furnace, and the initial conditions of the smelting process, a three-dimensional microstructure field numerical model of the ingot solidification process is constructed. Based on the three-dimensional microstructure field numerical model, the temperature field, melt flow, liquid-solid phase transformation and microstructure evolution behavior in the melting and casting area are simulated and analyzed offline to obtain the simulation results. Step 2) Generate the baseline thermal input evolution path for the thermal capping stage based on the simulation results, and formulate a multi-platform stepped decreasing current control strategy based on the baseline thermal input evolution path for the thermal capping stage as the baseline current control path for the thermal capping stage. Among them, the multi-platform stepped current control strategy includes multiple current control platform current control strategies, and each current control platform current control strategy includes current value, current deceleration rate and duration. Step 3) During the hot capping stage, the temperature at the top of the molten pool and the liquid level height of the molten pool are monitored in real time and the temperature deviation and liquid level height deviation are calculated respectively. The comprehensive deviation index of liquid level height is calculated based on the liquid level height deviation and the comprehensive deviation index of liquid level height. Step 4) When the temperature deviation or liquid level deviation at a certain time point is not within the preset deviation range, the current value, current deceleration rate and duration in the current control strategy of the current control platform corresponding to that time point are dynamically corrected using a closed-loop feedback control method. Step 5) When the temperature deviation and comprehensive deviation index at the top of the molten pool simultaneously meet the preset requirements within multiple consecutive sampling cycles, and the multi-platform stepped decreasing current control strategy is executed to the last current control platform and reaches the preset duration, the current control of the hot sealing stage is terminated. Then, non-destructive testing is used to confirm that there are no shrinkage defects at the top of the ingot, and the top area of ​​the ingot is determined to meet the zero shrinkage quality requirement.

[0022] The present invention discloses a hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot, which specifically includes the following steps: 1. Construct a three-dimensional microstructure field numerical model of the ingot solidification process. To enable controllable adjustment of the microstructure density in the top region of the ingot during the vacuum arc melting of titanium alloys, and to provide a modeling basis for the subsequent zero-shrinkage hot capping process, this invention constructs a three-dimensional microstructure field numerical model of the ingot solidification process based on numerical simulation methods.

[0023] In the process of constructing the three-dimensional microstructure field numerical model, the material property parameters of the target titanium alloy, the geometric structure parameters of the vacuum self-consuming arc melting furnace, and the corresponding initial conditions of the smelting process are input into the three-dimensional numerical simulation platform, which is implemented using the commercial numerical simulation software ProCAST.

[0024] The material properties of the titanium alloy include, but are not limited to, thermal conductivity, specific heat capacity, density, coefficient of thermal expansion, latent heat of phase transformation, thermal diffusivity, Young's modulus, Poisson's ratio, and dynamic viscosity. These material properties are obtained by combining experimental test data with theoretical models. The geometric parameters of the vacuum self-consuming arc melting furnace are determined based on the actual industrial equipment dimensions. The initial conditions of the smelting process include initial current, voltage, melting rate, and vacuum degree.

[0025] The three-dimensional tissue field numerical model includes an energy conservation sub-model, a momentum conservation sub-model, a mass conservation sub-model, and a microstructure and pore evolution sub-model.

[0026] After importing the geometric model, the three-dimensional microstructure field numerical model is meshed for subsequent numerical solutions of the solidification process. 1) Energy conservation sub-model An energy conservation sub-model is established based on the equivalent heat capacity method to realize the heat conduction and convection heat transfer and the release of latent heat during the liquid-solid transition in the vacuum self-consuming arc melting process. The energy conservation governing equation is written as follows: ; in, Represents the density of titanium alloys. Represents the specific heat capacity at constant pressure. Represents temperature. Represents smelting time. Represents the melt flow velocity vector; The gradient symbol represents the rate of change of a physical quantity in space. Represents thermal conductivity; Represents latent heat of fusion; Represents the solid volume fraction, obtained through linear interpolation between the liquid and solid phase lines.

[0027] In the numerical simulation process, a physical model of transient heat transfer and solidification coupling is used in the three-dimensional numerical simulation platform, and the latent heat release in the liquid-solid phase change process is uniformly processed by the equivalent heat capacity method. At the same time, the melt flow calculation results are introduced as the velocity field input in the convection term, so that the energy equation reflects both conduction and convection heat transfer effects.

[0028] By solving the above energy conservation sub-model, the evolution law of temperature field in the melting and casting area with time and space can be obtained, providing a physical basis for subsequent analysis of the molten pool morphology and determination of the evolution path of the reference heat input in the hot capping stage.

[0029] 2) Momentum conservation sub-model To describe the flow behavior of the melt in the casting region during vacuum consumable arc melting and its influence on the stability of the molten pool morphology, the three-dimensional microstructure field numerical model further introduces a momentum conservation sub-model to characterize the velocity field distribution and evolution characteristics of the melt under heat input.

[0030] The momentum conservation sub-model comprehensively considers the natural convection effect, gravity, melt viscous shear effect, and damping effect of the paste region in the liquid-solid coexistence area caused by temperature and composition gradients. Its governing equation is expressed as: ; in, Represents pressure, Represents dynamic viscosity. Represents the gravitational acceleration vector. This represents the permeability function of the pasty region.

[0031] In the numerical simulation process, a melt flow calculation model is enabled in the three-dimensional numerical simulation platform, and the momentum conservation sub-model and the energy conservation sub-model are coupled. At the same time, a gravity field is introduced to characterize the natural convection effect, and a mushy region damping model related to the solid fraction is used in the liquid-solid coexistence region to modulate the melt flow, thereby reflecting the physical characteristics of gradually restricted flow during solidification.

[0032] By solving the above momentum conservation sub-model, the evolution law of the melt velocity field and flow pattern in the casting area can be obtained. This can be used to analyze the stability of the molten pool morphology and the formation of melt feeding channels under different heat input conditions, and provide a physical basis for the flow behavior in order to rationally formulate the current reduction strategy in the hot capping stage.

[0033] 3) Mass conservation sub-model To describe the mass transfer and feeding behavior of liquid metal in the casting region during solidification in vacuum consumable arc melting, a three-dimensional microstructure field numerical model is introduced, which is based on the law of mass conservation and is used to characterize the influence of changes in liquid volume fraction and phase transition processes on local mass distribution.

[0034] The governing equations of the mass conservation sub-model are expressed as follows: ; in, Represents smelting time. Represents the density of the liquid phase. Represents the liquid phase volume fraction, and is related to the solid phase volume fraction. satisfy ; This represents the volume average velocity of the liquid phase within the melt. This represents the phase change quality source term.

[0035] In the numerical simulation process, a physical model coupling flow and continuity is enabled in the three-dimensional numerical simulation platform, allowing the mass conservation relationship to be solved simultaneously with the melt flow calculation; the liquid volume fraction is automatically calculated using the enthalpy-temperature relationship. With solid volume fraction The change in volume fraction caused by the phase transition is incorporated into the continuous solution process.

[0036] Furthermore, by incorporating a mushy region damping model within the liquid-solid coexistence region, the liquid phase flow is modulated, and shrinkage and compensation descriptions are introduced, so that the compensation behavior caused by volume shrinkage during solidification can be reflected through the mass conservation relationship.

[0037] By solving the above mass conservation sub-model, the mass transfer and feeding characteristics of liquid metal in the casting region during solidification can be obtained, providing a physical basis for mass conservation in analyzing whether the top region of the ingot has a continuous feeding channel and assessing the risk of shrinkage cavity formation.

[0038] 4) Sub-model of microstructure and pore evolution The microstructure and pore evolution sub-models include a continuous heterogeneous nucleation model, a grain growth model, and a pore evolution model.

[0039] The continuous heterogeneous nucleation model employs a continuous heterogeneous nucleation mechanism, where the activation supercooling of all potential nucleation sites follows a Gaussian distribution, and its nucleation rate... Controlled by the following formula: ; , Represents localized supercooling; Represents the maximum nucleation density; Represents the average activated subcooling; The standard deviation represents the local supercooling. The liquidus temperature of titanium alloys; Representing titanium alloys at the current time The local actual temperature.

[0040] The grain growth model, based on the steady-state solute distribution at the dendrite tip, comprehensively considers three mechanisms: thermal undercooling, curvature undercooling, and solute undercooling, revealing the grain growth behavior. Its basic expression is as follows: ; in, Represents the total supercooling at the front edge of the interface. Represents the Gibbs-Thomson coefficient. Represents the radius of the dendrite tip. Represents the slope of the liquidus (K / wt.%). Represents the initial composition concentration of the titanium alloy. Represents the equilibrium distribution coefficient. This represents the diffusion coefficient of the solute in the liquid phase. This represents the steady-state growth rate of the dendrite tip.

[0041] The evolution of porosity in the pore evolution model is described by the following formula: ; in, Represents the current time porosity; Represents the initial porosity; Activation energy representing the change in porosity; Represents the radius of the dendrite tip; Represents temperature; Represents the stress sensitivity coefficient; Represents stress; A nonlinear coefficient representing the rate of change of porosity; Represents the rate of change of porosity; Represents constants related to phase transition processes; This represents the rate at which the phase transition process affects the porosity evolution; Represents an exponential function; This represents the balanced distribution coefficient.

[0042] In the numerical simulation process, physical models related to microstructure evolution are enabled in the three-dimensional numerical simulation platform. Based on the continuous heterogeneous nucleation model, grain growth model and pore evolution model, the temperature field, flow field and composition field are coupled and solved to achieve a unified description of the macroscopic heat-fluid-mass transport process and the microstructure evolution behavior.

[0043] The above-mentioned microstructure and pore evolution sub-model can be used to obtain the grain size, microstructure continuity and pore evolution characteristics during the solidification process of the ingot, providing microscopic physical support for the realization of the microstructure density control target in the hot capping stage control strategy.

[0044] 2. Development of a stepped current reduction strategy during hot capping Based on the simulation results of the three-dimensional tissue field numerical model, a reference heat input evolution path for the thermal capping stage is generated, and a multi-platform stepped decreasing current control strategy is formulated based on the reference heat input evolution path for the thermal capping stage as the reference current control path for the thermal capping stage.

[0045] Based on the simulation results of the three-dimensional microstructure field numerical model, this invention formulates a multi-platform stepped decreasing current control strategy for the solidification characteristics of the hot capping stage of vacuum self-consuming arc melting, as the benchmark execution process path for the hot capping stage.

[0046] The multi-platform stepped decreasing current control strategy adjusts the arc input power in stages by setting up multiple current control platforms to form a controlled decreasing heat input process during the thermal capping stage. This multi-platform stepped decreasing current control strategy is used to define the initial reference trajectory of current change during the thermal capping stage and serves as the benchmark control path for subsequent real-time monitoring and closed-loop feedback regulation.

[0047] 1) Setting the reference current decrease function The change in the reference current during the hot capping stage can be expressed as a time function: ; in, Represents the current time The current value; Represents the initial current value; Represents the total solidification time; The exponential coefficient representing the decrease in current determines the rate of current decrease.

[0048] 2) Construction of a stepped current control platform To enhance the engineering feasibility of the multi-platform stepped current reduction control strategy, the reference current reduction process is further discretized into multiple current control platforms. Assuming the hot capping stage is divided into N current control platforms, the current change process can be represented as a piecewise function: ; in, This represents the starting time point of the second current control platform, and so on. Representing the N The start time point of a current control platform; Represents the initial current value. This represents the current value of the second current control platform, and so on. Representing the N The current value of each current control platform.

[0049] 3) Reference definition of current deceleration rate Between adjacent current control platforms, the current deceleration rate is defined as: ; in, Represents the rate of decrease in current; Representing the N The current value of each current control platform; Representing the N -1 Current value of a current control platform; Representing the The start time point of a current control platform; Representing the N -1 The start time point of a current control platform.

[0050] 4) Explanation of the relationship with the closed-loop control mechanism The aforementioned multi-platform stepped decreasing current control strategy is used to define the reference current control path during the hot capping stage. In the actual melting process, this current decreasing rate is not fixed, but rather dynamically corrects the current value, current decreasing rate, and duration in the current control strategy of the current control platform by combining real-time monitoring data and a closed-loop feedback control mechanism, in order to suppress deviations in the molten pool state and ensure the stability of the hot capping process.

[0051] 3. Real-time monitoring and data feedback collection mechanism To ensure that the multi-platform stepped decreasing current control strategy can be accurately executed during the hot capping stage and to promptly identify deviations in the molten pool state, this invention constructs a real-time monitoring and data feedback acquisition mechanism to continuously sense the thermal and geometric state of the molten pool and provide basic data for closed-loop feedback control.

[0052] The real-time monitoring and data feedback acquisition mechanism mainly includes the following two types of key physical parameters: (1) Real-time monitoring of the temperature at the top of the molten pool and calculation of temperature deviation Temperature is an important parameter reflecting the thermal stability of the molten pool and changes in solidification behavior. This invention uses a non-contact infrared thermometer to monitor the temperature at the top of the molten pool in real time. The measurement accuracy of the non-contact infrared thermometer is no less than ±2 ℃, and it can operate stably in high-temperature and strong radiation environments.

[0053] To comprehensively reflect the temperature change characteristics of the central region at the top of the molten pool and in the radial direction, the present invention defines the calculation method for temperature deviation as follows: ; in, represent The moment is located at the radial position from the center of the molten pool Temperature deviation at the location; represent At any time in radial position The measured temperature at the top of the molten pool; Reference current control path corresponding Time, radial position The target temperature value at that location.

[0054] By measuring temperature deviation Through real-time calculation and analysis, the control system can determine whether there is local overheating at the top of the molten pool, excessively rapid overall cooling, or abnormal radial temperature distribution, thereby providing a basis for current regulation and cooling control.

[0055] (2) Real-time monitoring and multi-point deviation assessment of molten pool level The molten pool level reflects changes in the feeding process and the overall geometric state of the liquid metal, and is an important parameter for assessing the stability of the smelting process. This invention employs a non-contact molten pool level measurement device for real-time monitoring of the molten pool level, preferably a laser displacement sensor, with a measurement accuracy of no less than ±1 mm.

[0056] During the liquid level monitoring process, multiple discrete measurement locations are selected along the radial direction from the top of the molten pool. Collect data from each location separately. Calculate the liquid level at time t and the corresponding liquid level deviation: ; in, represent Time, located in radial position The deviation in liquid level height at that location; represent Time, in radial position The measured height of the molten pool; Represents the reference current control path Time, radial position The target liquid level height at the location; Representing the The radial coordinates of each liquid level measurement location.

[0057] To comprehensively characterize the overall morphological changes of the molten pool surface, this invention further defines a comprehensive deviation index for the molten pool height: ; in, represent The comprehensive deviation index of liquid level height at any given time is used to reflect the overall degree of deviation of the molten pool liquid level in the radial direction; Represents the radial position The number of liquid level measurement points at the location.

[0058] Through the Through real-time calculations, the control system can identify trends such as insufficient overall liquid level replenishment, increased central depression, or abnormal liquid surface morphology.

[0059] (3) Data acquisition cycle and feedback data output Temperature deviation and liquid level deviation data are collected by the data acquisition system according to a preset sampling period and uploaded to the control system. The default sampling period is 60 seconds, but it can be shortened to 30 seconds when an accelerated rate of change in the molten pool state or an abnormal trend is detected.

[0060] The above temperature deviation Overall deviation with liquid level As a key feedback data output, it provides a real-time and reliable data foundation for subsequent closed-loop feedback control mechanisms.

[0061] 4. Closed-loop feedback control mechanism and process correction strategy Based on the real-time monitoring and data feedback acquisition mechanism, this invention constructs a closed-loop feedback control mechanism to dynamically correct the current reduction strategy and related process parameters during the hot capping stage, so as to suppress the deviation of the molten pool state and maintain the stability of the smelting process.

[0062] The control system uses a preset reference current control path as the target state and evaluates the degree of deviation between the current molten pool state and the target state based on real-time temperature deviation and liquid level height deviation data.

[0063] (1) Construction of comprehensive deviation index To uniformly characterize the impact of various types of deviations on process control, this invention defines a comprehensive deviation index: ; in, E ( k ) represents the first The comprehensive deviation index corresponding to each sampling period; Representing the Temperature deviation in the central region at the top of the molten pool within each sampling period; Representing the Comprehensive deviation of liquid level height within each sampling period; , The weighting coefficient represents the relative importance of temperature deviation and liquid level deviation in control decisions. Its value can be set or calibrated according to material characteristics and process stage. This represents the sampling period number.

[0064] (2) Closed-loop feedback regulation strategy Based on comprehensive deviation index The control system adopts a closed-loop feedback regulation method to dynamically correct the current and cooling-related process parameters.

[0065] Preferably, the closed-loop feedback control method adopts a control method combining proportional, integral, and derivative functions, and its control output can be expressed as: ; in, Representing the Feedback adjustment amount per sampling period; , , These represent the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively. It represents the cumulative amount of the comprehensive deviation index, used to eliminate steady-state error; This represents the time interval between adjacent sampling periods; Representing the -1 sampling period corresponds to the comprehensive deviation index.

[0066] (3) Constraint update of process parameters Adjust the amount based on feedback. The control system updates each process parameter in a limited manner to avoid over-adjustment.

[0067] The specific expressions for the current value, current reduction rate, and duration of each current control platform in the stepped decreasing current control strategy are as follows: ; ; ; in, Representing the N The current control platform in the first Current value within each sampling period; Representing the N The current control platform in the first Current deceleration rate within each sampling period; Representing the N The current control platform in the first The duration within each sampling period; These represent the upper and lower limits of the permissible current value, respectively. These represent the upper and lower limits of the allowable current deceleration rate, respectively; These represent the upper and lower limits of the allowed duration of the circuit control platform, respectively; clip(·) represents the clipping function, used to limit the updated parameters within the allowed range.

[0068] (4) Execution and Update The above process parameters are updated collaboratively by the controller and the database system, and are refreshed in real time according to the preset sampling period, thus forming a stable and reliable closed-loop feedback control system.

[0069] 5. Termination Criteria and Zero Shrinkage Cavity Quality Judgment Mechanism To ensure that the hot capping stage terminates at the appropriate time and to guarantee that the top area of ​​the ingot meets the quality requirements of zero shrinkage and dense structure, this invention constructs a termination criterion and a zero shrinkage quality judgment mechanism to determine the timing of the end of the melting process and the final forming quality.

[0070] (1) Termination criteria for the hot capping stage During the hot capping phase, the control system continuously receives and analyzes the real-time monitoring data and closed-loop feedback control results. The hot capping phase is considered complete and the stepped decreasing current control process terminates when the following termination conditions are simultaneously met: a) Temperature deviation in the central region at the top of the molten pool It remains within the preset allowable range for multiple consecutive sampling periods and no longer shows a continuous upward or rapid downward trend; b) Overall deviation of liquid level height If the molten pool surface morphology is less than the preset threshold for multiple consecutive sampling periods and its rate of change is lower than the set limit, it indicates that the molten pool surface morphology tends to be stable. c) The stepped decreasing current control strategy has been executed to the last current control platform and has reached the preset duration.

[0071] in: Representing the Temperature deviation in the central region at the top of the molten pool within each sampling period; Representing the The comprehensive deviation of liquid level height within each sampling period; the preset allowable range, preset threshold and preset duration are set or calibrated based on material characteristics, ingot specifications and process experience.

[0072] When the above conditions are met simultaneously, the control system issues a termination command, ending the current control of the hot capping stage and entering the natural cooling or subsequent process stage.

[0073] (2) Zero shrinkage cavity quality judgment mechanism After the hot capping stage is completed, this invention determines the forming quality of the top region of the ingot. The zero shrinkage cavity quality determination mechanism is based on the process status data at the end of the hot capping stage, and its determination logic is as follows: a) Temperature deviation at the top of the molten pool within the preset time window before the termination of the hot capping stage. Overall deviation with liquid level All remained within the corresponding threshold range, and no mutations occurred; b) During the closed-loop feedback control process, the current parameter did not trigger the limit constraint state of the limiting function, indicating that no drastic process disturbance occurred; c) The molten pool level did not show any obvious collapse or abnormal downward trend at the termination time and within several sampling periods before and after it.

[0074] When the above conditions are met, the control system determines that the top area of ​​the ingot has been adequately fed during solidification, the risk of shrinkage is effectively suppressed, and the top area of ​​the ingot meets the zero shrinkage quality requirement.

[0075] (3) Output and recording of judgment results The termination criterion triggering conditions and the zero shrinkage quality judgment results are recorded by the control system and stored together with the corresponding current control parameters, temperature deviation data and liquid level deviation data for process traceability, quality assessment and subsequent process parameter optimization.

[0076] The features and performance of the present invention will be further described in detail below with reference to embodiments.

[0077] Example 1: Hot capping process based on Φ820 mm TC4 titanium alloy ingot 1. Simulation of microstructure and generation of ideal solidification progress curve This embodiment takes a TC4 titanium alloy ingot with a specification of Φ820 mm as the object and uses an industrial vacuum self-consuming arc melting equipment to actually implement and verify the process control process in the hot capping stage.

[0078] Before implementing the hot capping process, a three-dimensional microstructure field numerical model of the ingot solidification process is first constructed based on the material thermophysical parameters of TC4 titanium alloy, the geometric parameters of the corresponding vacuum consumable arc furnace, and the predetermined initial conditions of the smelting process. This model is used to simulate the temperature field distribution, melt flow behavior, liquid-solid phase transformation process, and microstructure evolution behavior in the molten pool area during the hot capping stage.

[0079] Thermophysical properties (such as density, Young's modulus, thermal conductivity, specific heat capacity, viscosity, etc.) were obtained through experimental measurements and software correction, and vary with temperature. Specifically, the density of the titanium alloy is 4359.69 kg / m³ at 311.99 K. 3 The concentration dropped to 3922.49 kg / m³ at 2003.15 K. 3 The Young's modulus decreased from 104599.8 MPa to 120 MPa, the thermal conductivity increased from 8.24 W / (m·℃) to 34.60 W / (m·℃), the specific heat capacity increased from 0.52 J / (g·K) to 1.517 J / (g·K), and the viscosity decreased from 2.4847 centipoise to 1.5747 centipoise.

[0080] The calculation of the three-dimensional microstructure field numerical model adopts the finite volume method, and the energy conservation sub-model, momentum conservation sub-model, mass conservation sub-model and microstructure and pore evolution sub-model are coupled and solved. Combined with actual working condition parameters such as electrode diameter Φ800mm, ingot structure length-to-diameter ratio of about 1.3, hot capping initiation current of about 25 kA, and melting rate of about 20 kg / min, the behavior of the molten pool during the hot capping stage is simulated.

[0081] like Figure 2 As shown, the microstructure distribution of the longitudinal section of the ingot is obtained through a three-dimensional microstructure field numerical model.

[0082] After the three-dimensional microstructure field numerical model was solved, the calculation results were not directly used. Instead, the simulated surface temperature distribution and geometric morphology of the molten pool were checked for engineering consistency. Specifically, the simulated molten pool profile and temperature gradient variation trend were compared with the furnace front records of historically stable smelting batches of ingots of the same specification. After confirming that the range of molten pool depth variation and the evolution trend of liquid surface morphology were within a reasonable engineering range, the simulation results were used as a reference for generating the control path in the hot capping stage. During the initial verification process, for cases where the local high-temperature zone was too large, the heat transfer boundary conditions and some thermophysical parameters of the high-temperature section were corrected a limited number of times until the output of the three-dimensional microstructure field numerical model matched engineering experience.

[0083] Based on this, a reference current control path for the hot capping stage is generated, including a reference temperature path and a reference liquid level path, which are used for subsequent real-time monitoring of deviation construction and closed-loop feedback control.

[0084] 2. Stepped Decreasing Current Control Scheme This embodiment employs a five-stage stepped decreasing current control strategy to ensure stable solidification in the top region of the molten pool and prevent shrinkage cavities. The specific current control scheme is as follows: At the start of the process, the parameters of the first current control platform are: the current is maintained at 25 kA for 20 minutes to ensure that the metal at the top of the molten pool is fully melted and reaches temperature equilibrium.

[0085] The parameters for the second current control platform are: the current is reduced to 24 kA and maintained for 20 minutes. At this time, the temperature of the molten pool remains stable to prevent the top area from solidifying prematurely and to ensure that the metal in the molten pool is heated fully and evenly.

[0086] The parameters for the third current control platform are: the current is reduced to 18 kA for 30 minutes, and the cooling water flow rate is adjusted to 250 L / min to ensure that the cooling rate of the molten pool is moderate, to prevent premature solidification, and to meet the feeding requirements of the top area.

[0087] The fourth current control platform parameters are: maintain the current at 18 kA for 30 minutes to ensure a stable molten metal surface and prevent uneven solidification caused by excessively rapid cooling. The focus of this stage is to ensure stable flow of molten metal within the pool and to facilitate the final replenishment of the top region.

[0088] The parameters for the fifth current control platform are as follows: the current is reduced to 5 kA and maintained at this current level for 120 minutes to ensure that the final solidification of the top region of the molten pool proceeds smoothly and to avoid shrinkage defects. The current reduction rate during this stage is 0.5 kA / min. By gradually reducing the current, uniform solidification of the molten metal in the top region is ensured, and metal densification is promoted.

[0089] 3. Real-time monitoring and deviation construction process During the hot capping stage, a non-contact temperature measuring device is used to monitor the temperature at the top of the molten pool in real time, and a non-contact displacement measuring device is used to monitor the liquid level in the molten pool in real time, with a sampling period set to 60 seconds. To balance the overall thermal state of the molten pool with the ability to identify local anomalies, this embodiment selects a center point and multiple radial measuring points at the top of the molten pool for simultaneous data acquisition.

[0090] (1) Principles and engineering basis for the layout of measuring points For temperature monitoring, this embodiment selects the center point (r=0) at the top of the molten pool and two radial measuring points (r=120 mm and r=260 mm). The center point reflects the overall heat input level at the top of the molten pool; the medium-radius measuring point monitors the radial temperature gradient change outward from the center; and the outer measuring point identifies whether there is premature cooling or abnormal solidification trend in the edge region. This combination of measuring points can effectively assess the radial consistency of the temperature field at the top of the molten pool without significantly increasing measurement complexity.

[0091] For liquid level monitoring, four measuring points were selected radially along the top of the molten pool: r1=0, r2=150 mm, r3=300 mm, and r4=420 mm, corresponding to a total of 4 measuring points. This point arrangement is used to simultaneously characterize the overall feeding status of the central region of the liquid surface and the changes in liquid surface morphology near the edge, avoiding misjudgments of local collapse or overall tilting trends caused by relying solely on single-point measurements. The locations of these measuring points were determined based on furnace-front observation experience with ingots of the same specifications, and can cover the area most sensitive to liquid level changes during the hot capping stage.

[0092] (2) Construction of temperature deviation, threshold setting and sample data The permissible threshold for temperature deviation is not arbitrarily set, but determined based on the statistical fluctuation range of the hot capping stage in historically stable smelting batches. Through retrospective analysis of temperature data from multiple hot capping processes of ingots of the same specification, it was found that when the temperature deviation at the top center is controlled within ±5 ℃, the molten pool state generally remains stable, and no risk of top shrinkage cavities caused by temperature anomalies was observed. Therefore, in this embodiment, ±5 ℃ is used as the reference permissible range for temperature deviation.

[0093] Considering that fluctuations in electric arc radiation and changes in surface emissivity may introduce short-term measurement noise, this embodiment uses a "3-point moving average + abrupt change elimination" method to determine the effective value of temperature data: when the temperature abrupt change in adjacent sampling periods exceeds 20°C and immediately drops back in the next period, it is determined to be transient interference and replaced by the neighborhood mean.

[0094] Taking the monitoring data around the 38th minute as an example (center point r=0): ; ; ; Meanwhile, the radial measurement data were checked, and the temperature deviations at r=120 m and r=260 mm were approximately +48 ℃ and +35 ℃, respectively, indicating that this stage was an overall overheating deviation rather than a single-point anomaly.

[0095] (3) Liquid level height deviation, threshold source and comprehensive deviation example The threshold for liquid level deviation is also based on engineering experience and historical data analysis. Statistical analysis of liquid level changes from multiple stable smelting records revealed that when the comprehensive liquid level deviation index... When the weld pool thickness remains below 2.0 mm for an extended period, the molten pool feeding pattern is typically quite uniform; however, when... When multiple consecutive sampling cycles exceed approximately 2.0 mm, there is often a tendency for the center of the liquid surface to sink or for insufficient replenishment. Therefore, in this embodiment, 2.0 mm is used as the reference threshold for the overall deviation of the liquid surface height.

[0096] Considering the surface fluctuations of the molten pool and the interference of optical reflection, this embodiment uses median filtering and combines radial consistency constraints to process the liquid level data: when a certain measuring point has a jump of more than 3 mm in a single cycle and adjacent measuring points do not show consistent changes, it is determined to be a measurement interference and median replacement is used.

[0097] Taking the liquid level monitoring data around the 82nd minute as an example: Minute 81:

[0098]

[0099]

[0100]

[0101] E was calculated H =1.34 mm.

[0102] Minute 82:

[0103]

[0104]

[0105]

[0106] E was calculated H =2.14 mm.

[0107] Minute 83:

[0108]

[0109]

[0110]

[0111] E was calculated H =2.24 mm.

[0112] (4) Data output and recording Throughout the entire hot sealing phase, temperature and liquid level data are continuously collected and stored synchronously at 60-second intervals. Based on the approximately 160-minute hot sealing process in this embodiment, the system cumulatively records approximately 160 sets of temperature data and approximately 160 sets of liquid level data, and outputs the data. , and As input data for closed-loop feedback control and termination criteria.

[0113] 4. Closed-loop feedback control and process correction During the hot capping stage, closed-loop feedback control is used to suppress deviations in the molten pool state during the execution of the reference stepped decreasing current path. The control system does not immediately execute adjustments based on a single measurement result, but rather triggers process parameter corrections after comprehensively judging the continuity and trend of the deviation.

[0114] (1) Setting the feedback trigger criteria In this embodiment, when one of the following conditions is met, the control system determines that the molten pool state has entered the interventionable range and enters the feedback evaluation process: Temperature deviation If the value exceeds the preset allowable range for two or more consecutive sampling periods, and the trend of change does not show a natural decline; The liquid level deviation exceeds the reference threshold for two or more consecutive sampling periods, or its rate of change continues to increase; Temperature deviation and liquid level deviation both show a trend of deviation in the same direction.

[0115] The above criteria are used to avoid unnecessary process adjustments caused by transient noise or short-term disturbances.

[0116] (2) Priority and regulation strategy of feedback regulation After confirming the existence of a valid deviation, the control system adjusts the process parameters in the order of "time first, then speed, and then amplitude": 1. Prioritize adjusting the duration of the current control platform to compensate for the cumulative deviation of heat input or compensation state in the preceding stage; 2. When simply adjusting the duration cannot suppress the deviation trend, a limited correction is made to the current deceleration rate; 3. Only when the deviation persists and cannot be eliminated by time and rate adjustment should the current value of the current control platform be slightly adjusted.

[0117] The above adjustment sequence is based on experience in furnace engineering and can reduce the risk of arc instability caused by frequent changes in current amplitude.

[0118] (3) Confirmation and callback mechanism after feedback adjustment After each feedback adjustment is completed, the control system keeps the current process parameters unchanged and runs at least one observation window to evaluate the response of the molten pool state to the adjustment measures.

[0119] When monitoring data shows that the temperature deviation or liquid level deviation shows a downward trend for two or more consecutive sampling cycles, and no reverse fluctuation occurs, the adjustment is deemed effective, and the subsequent control platform continues to be executed.

[0120] If the deviation does not improve significantly within the observation window, or shows a reverse amplification trend, the next level of adjustment strategy is entered. When two consecutive adjustments still cannot suppress the deviation, the control system marks the current stage as a high-sensitivity range and enhances the overall stability by extending the duration of the final stage current control platform.

[0121] (4) Description of typical feedback correction process In this embodiment, the thermal capping stage triggers three effective feedback adjustments.

[0122] First feedback (around 38 minutes): The temperature deviation at the top center remained consistently higher than the reference path for two consecutive sampling periods. The system then extended the duration of the primary platform to reduce the heat input rate, and subsequently, the temperature was observed to gradually decrease and stabilize.

[0123] Second feedback (around 82 minutes): If the overall deviation of the liquid level exceeds the threshold for two consecutive cycles, the control system will prioritize adjusting the current deceleration rate and observe the liquid level response while keeping the current control platform parameters unchanged. Once the replenishment state is confirmed to have recovered, the system will continue to execute.

[0124] Third feedback (around 104 minutes): In the later stages of hot sealing, the temperature and liquid level deviation show a coupled amplification trend. The system determines that it is necessary to enhance the stability of the final solidification stage and achieve deviation convergence by extending the duration of the final stage current control platform.

[0125] Throughout the entire closed-loop feedback control process, all adjustments are completed under automatic analysis and confirmation logic, without the need for real-time manual intervention.

[0126] 5. Termination Criteria and Zero Shrinkage Confirmation At the end of the hot capping stage, the control system needs to determine a reasonable time to stop the arc, ensuring that the top region of the molten pool has been sufficiently fed back. To avoid insufficient top feeding due to premature arc cessation, or coarsening of the microstructure and increased energy consumption due to excessive delay, this embodiment uses a termination criterion based on the stability of the molten pool state to determine the end time of the hot capping stage.

[0127] (1) Principles for constructing termination criteria In this embodiment, the termination criterion for the thermal capping stage does not rely on a single physical quantity, but is based on the simultaneous convergence and stability confirmation of the combined deviation of temperature deviation and liquid level height. Specifically, it includes the following conditions: The temperature deviation in the central region at the top of the molten pool remained within the preset allowable range for multiple consecutive sampling periods, and its trend did not show a continuous increase or a rapid decrease. Comprehensive Deviation Index If the liquid level is less than the reference threshold for multiple consecutive sampling periods and its rate of change is lower than the preset limit, it indicates that the liquid surface morphology has become stable. During the period when the above conditions are met, the stepped decreasing current control strategy has been executed up to the final control platform, and the current value remains within the low power stable range.

[0128] The aforementioned termination criteria are used to ensure that the top region of the molten pool has continuous and stable feeding conditions before entering the final solidification stage.

[0129] (2) Confirmation of operation and decision-making logic before termination When the control system detects for the first time that the termination criterion is met, it does not immediately trigger the arc-stopping operation, but instead enters the termination confirmation phase. During this termination confirmation phase, the control system maintains the operation of the final-stage current control platform and continues to monitor the combined deviation of temperature deviation and liquid level height according to the predetermined sampling period.

[0130] In this embodiment, the termination confirmation phase covers at least one complete confirmation window to verify whether the key deviation indicators remain stable without further adjustments. When the combined deviation of temperature deviation and liquid level height does not show a rebound or abnormal fluctuation trend within multiple consecutive sampling periods, the control system determines that the molten pool state has entered the safe termination range.

[0131] The verification process is used to avoid misjudgment of arc termination due to short-term stability illusions or measurement noise.

[0132] (3) Connection between arc cessation and subsequent cooling After the termination confirmation is completed, the control system automatically issues an arc-stop command, terminating the current input during the hot capping stage. After the arc is stopped, the ingot enters the controlled cooling stage, with the cooling process smoothly transitioning to the hot capping stage to avoid thermal stress concentration caused by sudden temperature changes.

[0133] In this embodiment, after the arc is stopped, the ingot continues to cool under predetermined cooling conditions and is kept for a sufficient time to complete the final solidification and microstructure stabilization process of the top region.

[0134] (4) Process confirmation and result verification of zero shrinkage state After the hot capping stage is completed, this embodiment comprehensively confirms the feeding status of the top region of the ingot based on process status data. Specifically, in the confirmation window before termination: Both the temperature deviation and the combined deviation of the liquid level height remained within the corresponding threshold range, and no sudden changes occurred; No limit or abnormal intervention state was triggered during the closed-loop feedback control process; The liquid level changed gradually, and no central collapse or abnormal descent was observed.

[0135] The above process characteristics indicate that the feeding channel in the top region of the ingot remains continuous during solidification, and the risk of shrinkage cavity formation has been effectively suppressed.

[0136] After the ingot has cooled and been demolded, the top area undergoes quality verification, including non-destructive testing and microstructure analysis. For example... Figure 3 As shown, the test results indicate that no macroscopic shrinkage cavities or concentrated pore defects were found in the top area of ​​the ingot, and the microstructure is continuous and dense, which is consistent with the results of the process state determination during the hot capping stage.

[0137] (5) Summary of project implementation results Based on the termination criteria trigger records, process monitoring data, and final quality inspection results of the hot capping stage, it is confirmed that the hot capping control method adopted in this embodiment can achieve stable control of the density and feeding state of the top region during the melting process of Φ820 mm TC4 titanium alloy ingots, thus achieving the quality target of zero shrinkage cavities.

[0138] Comparative Example 1 This embodiment uses a Φ860 mm TC4 titanium alloy ingot as the object and adopts a traditional experience-based hot capping process control method. The specific steps are as follows: S1. Process parameter settings: Based on years of experience of the operators, the melting current was set to decrease linearly and gradually. A fixed time and current range were used during the arc-ending stage. No real-time monitoring equipment was used, and the condition of the molten pool was judged solely by manual visual inspection and experience.

[0139] S2. Process execution procedure: The smelting current slowly decreases from a stable state to a low current. During this period, the cooling water flow rate is manually adjusted, but the adjustment range and timing are mainly based on operator experience, lacking systematic parameter optimization and dynamic feedback control.

[0140] S3, Molten Pool Status Judgment: Based on the operator's observation of the molten pool level and changes in current and voltage during the smelting process, the solidification process and stability of the molten pool are judged, but it is impossible to obtain the internal temperature or microstructure of the molten pool in real time and accurately.

[0141] S4. Process completion determination: The arc termination time and current reduction end rely on experience presets, and there is no closed-loop monitoring to ensure that the molten pool reaches the ideal solidification state, which easily leads to uneven solidification and shrinkage cavities.

[0142] S5. Result Evaluation: Metallographic and non-destructive testing of the ingots revealed that, for example Figure 4 As shown, there is a serious shrinkage defect in the top area, with a shrinkage depth of about 350 mm, which causes a lot of material loss and makes subsequent processing difficult, seriously affecting product quality and performance.

[0143] This invention is applicable to vacuum consumable arc melting of TC4, TC11, TC19 and other titanium alloy ingots with diameters ranging from 200mm to 1000mm.

[0144] Finally, it should be noted that the above embodiments are only used to illustrate the technical solutions of the present invention and not to limit it. Although the present invention has been described in detail with reference to the above embodiments, those skilled in the art should understand that modifications or equivalent substitutions can still be made to the specific implementation of the present invention. Any modifications or equivalent substitutions that do not depart from the spirit and scope of the present invention should be covered within the protection scope of the present invention.

Claims

1. A hot capping process for zero-shrinkage ingots from vacuum consumable melting of titanium alloys, characterized in that, Includes the following steps: Step 1) Based on the material property parameters of the target titanium alloy, the geometric structure parameters of the vacuum self-consuming electric arc melting furnace, and the initial conditions of the smelting process, a three-dimensional microstructure field numerical model of the ingot solidification process is constructed. Based on the three-dimensional microstructure field numerical model, the temperature field, melt flow, liquid-solid phase transformation and microstructure evolution behavior in the melting and casting area are simulated and analyzed offline to obtain the simulation results. Step 2) Generate the baseline thermal input evolution path for the thermal capping stage based on the simulation results, and formulate a multi-platform stepped decreasing current control strategy based on the baseline thermal input evolution path for the thermal capping stage as the baseline current control path for the thermal capping stage. Among them, the multi-platform stepped current control strategy includes multiple current control platform current control strategies, and each current control platform current control strategy includes current value, current deceleration rate and duration. Step 3) During the hot capping stage, the temperature at the top of the molten pool and the liquid level height of the molten pool are monitored in real time and the temperature deviation and liquid level height deviation are calculated respectively. The comprehensive deviation index of liquid level height is calculated based on the liquid level height deviation and the comprehensive deviation index of liquid level height. Step 4) When the temperature deviation or liquid level deviation at a certain time point is not within the preset deviation range, the current value, current deceleration rate and duration in the current control strategy of the current control platform corresponding to that time point are dynamically corrected using a closed-loop feedback control method. Step 5) When the temperature deviation and comprehensive deviation index at the top of the molten pool simultaneously meet the preset requirements within multiple consecutive sampling cycles, and the multi-platform stepped decreasing current control strategy is executed to the last current control platform and reaches the preset duration, the current control of the hot sealing stage is terminated. Then, non-destructive testing is used to confirm that there are no shrinkage defects at the top of the ingot, and the top area of ​​the ingot is determined to meet the zero shrinkage quality requirement.

2. The hot capping process for a titanium alloy vacuum self-consumable melting zero-shrinkage ingot according to claim 1, characterized in that, In step 1), the three-dimensional tissue field numerical model includes an energy conservation sub-model, a momentum conservation sub-model, a mass conservation sub-model, and a microstructure and pore evolution sub-model. The energy conservation sub-model is based on the equivalent heat capacity method and is used to describe the heat conduction, convection heat transfer and liquid-solid phase change latent heat release behavior in the melting and casting area. The momentum conservation sub-model is used to describe the flow behavior of the melt under the action of temperature gradient and gravity, and introduces the mushy region damping effect in the liquid-solid coexistence region. The mass conservation sub-model is used to describe the changes in liquid volume fraction and the feeding behavior caused by phase transition during solidification. The microstructure and porosity evolution sub-model is used to describe the characteristics of grain nucleation, growth, and porosity evolution, in order to evaluate the trend of microstructure density changes under different heat input conditions.

3. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 2, characterized in that, The energy conservation governing equation of the energy conservation sub-model is as follows: ； in, Represents the density of titanium alloys. Represents the specific heat capacity at constant pressure. Represents temperature. Represents smelting time. Represents the melt flow velocity vector; The gradient symbol represents the rate of change of a physical quantity in space. Represents thermal conductivity; Represents latent heat of fusion; Represents the solid volume fraction, obtained through linear interpolation between the liquid and solid phase lines.

4. The hot capping process for a titanium alloy vacuum self-consumable melting zero-shrinkage ingot according to claim 2, characterized in that, The governing equations of the momentum conservation sub-model are expressed as follows: ； in, Represents the density of titanium alloys. Represents the melt flow velocity vector; Represents smelting time; The gradient symbol represents the rate of change of a physical quantity in space. Represents pressure; Represents dynamic viscosity; Represents the gravitational acceleration vector; This represents the permeability function of the pasty region.

5. The hot capping process for a titanium alloy vacuum self-consumable melting zero-shrinkage ingot according to claim 2, characterized in that, The governing equations of the mass conservation sub-model are expressed as follows: ； in, Represents smelting time; Represents the density of the liquid phase; Represents the liquid phase volume fraction, and is related to the solid phase volume fraction. satisfy ; This represents the volume average velocity of the liquid phase within the melt. Represents the phase change mass source term; The gradient symbol represents the rate of change of a physical quantity in space.

6. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 2, characterized in that, The microstructure and porosity evolution sub-models include a continuous heterogeneous nucleation model, a grain growth model, and a porosity evolution model; The expression for the continuous heterogeneous nucleation model is: ； in, , Represents localized supercooling; Represents the maximum nucleation density; Represents the average activated subcooling; The standard deviation represents the local supercooling. The liquidus temperature of titanium alloys; Representing titanium alloys at the current time The actual local temperature; Represents the nucleation rate; The expression for the grain growth model is as follows: ； in, Represents the total supercooling at the front edge of the interface. Represents the Gibbs-Thomson coefficient. Represents the radius of the dendrite tip. Represents the slope of the liquidus line. Represents the initial composition concentration of the titanium alloy. Represents the equilibrium distribution coefficient. This represents the diffusion coefficient of the solute in the liquid phase. Represents the steady-state growth rate of the dendrite tip; The expression for the pore evolution model is as follows: ； in, Represents the current time porosity; Represents the initial porosity; Activation energy representing the change in porosity; Represents the radius of the dendrite tip; Represents temperature; Represents the stress sensitivity coefficient; Represents stress; A nonlinear coefficient representing the rate of change of porosity; Represents the rate of change of porosity; Represents constants related to phase transition processes; This represents the rate at which the phase transition process affects the porosity evolution; Represents an exponential function; This represents the balanced distribution coefficient.

7. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 1, characterized in that, In step 2), the multi-platform stepped decreasing current control strategy is obtained by discretizing the continuous current decreasing function and divided into multiple current control platforms. Assuming the thermal capping stage is divided into N current control platforms, the current change process can be represented as a piecewise function: ； in, This represents the starting time point of the second current control platform, and so on. Representing the N The start time point of a current control platform; Represents the initial current value. This represents the current value of the second current control platform, and so on. Representing the N The current value of each current control platform; This represents the total solidification time.

8. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 1, characterized in that, In step 3), the temperature deviation is calculated as follows: ； in, represent The moment is located at the radial position from the center of the molten pool Temperature deviation at the location; represent At any time in radial position The measured temperature at the top of the molten pool; Represents the reference current control path Time, radial position The target temperature value at that location; The calculation method for liquid level deviation is as follows: ； in, represent Time, located in radial position The deviation in liquid level height at that location; represent Time, in radial position The measured height of the molten pool; Represents the reference current control path Time, radial position The target liquid level height at the location; Representing the Radial coordinates of each liquid level measurement location; The comprehensive deviation index of liquid level height is calculated based on the liquid level height deviation, and the expression is as follows: ； in, represent The comprehensive deviation index of liquid level height at any given time is used to reflect the overall degree of deviation of the molten pool liquid level in the radial direction; Represents the radial position The number of liquid level measurement points at the location; The comprehensive deviation index is calculated based on the combined deviation index of temperature deviation and liquid level height, and the expression is as follows: ； in, E ( k ) represents the first The comprehensive deviation index corresponding to each sampling period; Representing the Temperature deviation in the central region at the top of the molten pool within each sampling period; Representing the Comprehensive deviation of liquid level height within each sampling period; , The weighting coefficients represent the relative importance of temperature deviation and liquid level deviation in control decisions. This represents the sampling period number.

9. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 8, characterized in that, In step 4), the closed-loop feedback control method adopts a control method combining proportional, integral, and derivative functions, and the control output is expressed as: ； in, Representing the Feedback adjustment amount for each sampling period; , , These represent the proportional adjustment coefficient, integral adjustment coefficient, and derivative adjustment coefficient, respectively. It represents the cumulative amount of the comprehensive deviation index, used to eliminate steady-state error; This represents the time interval between adjacent sampling periods; Representing the -1 sampling period corresponds to the comprehensive deviation index.

10. The hot capping process for a titanium alloy vacuum consumable melting zero-shrinkage ingot according to claim 1, characterized in that, The specific expressions for the current value, current reduction rate, and duration of each current control platform in the stepped decreasing current control strategy are as follows: ； ； ； in, Representing the N The current control platform in the first Current value within each sampling period; Representing the N The current control platform in the first Current deceleration rate within each sampling period; Representing the N The current control platform in the first The duration within each sampling period; These represent the upper and lower limits of the permissible current value, respectively. These represent the upper and lower limits of the allowable current deceleration rate, respectively. These represent the upper and lower limits of the allowed duration of the current control platform, respectively; clip(·) represents the clipping function.

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