A high-fidelity simulation method for additive manufacturing melt pool vapor flow

Abstract

The application discloses a kind of high fidelity simulation simulation methods of additive manufacturing melt pool steam flow, belong to the technical field of simulation of additive manufacturing.The application establishes the space grid including printing material domain and protective gas domain, calibrates the unit at the interface of two domains and occurs evaporation as evaporation surface unit, determines evaporation mass loss flux according to its temperature;Determine the unit different from evaporation surface unit in protective gas domain as steam source unit;Evaporation mass loss flux is loaded on evaporation surface unit, and steam mass source flux corresponding thereto and momentum source flux from printing material domain to protective gas domain are loaded on steam source unit;Steam velocity field is obtained by solving two-phase flow control equation.The application makes steam jet steady development, direction is determined by interface geometry, and is suitable for laser powder bed melting and other additive manufacturing processes.

Description

Technical Field

[0001] This invention relates to the field of numerical simulation technology for additive manufacturing, and specifically to a high-fidelity simulation method for vapor flow in an additive manufacturing molten pool. Background Technology

[0002] In high-energy beam-based metal additive manufacturing, a high-energy-density laser beam rapidly melts metal powder, forming a molten pool. When the surface temperature of the molten pool reaches or exceeds the boiling point of the printing material, the liquid metal undergoes violent evaporation, resulting in continuous loss of mass and energy. The recoil pressure generated by evaporation acts on the free surface of the molten pool, driving downward deformation and even keyhole formation. Simultaneously, the evaporated metal vapor is ejected from the molten pool surface in the form of a high-speed jet, entraining surrounding protective gas and causing spatter and powder erosion, ultimately resulting in fluctuations in the density, surface quality, and mechanical properties of the formed part. Numerical simulation of the above vapor jetting process is an important way to understand the formation mechanism of process defects and select process parameters accordingly.

[0003] Most existing additive manufacturing molten pool vapor simulations are conducted within a gas-liquid two-phase flow framework: a mesh is generated in the space containing the printing material domain and the protective gas domain; the free surface of the molten pool is tracked using interface capture methods such as the fluid volume method; the cell containing the gas-liquid interface is identified according to the cell's temperature and phase state; then, the mass loss, energy loss, and evaporation back pressure caused by evaporation are applied to this cell; and the temperature and velocity fields are solved by simultaneously solving the mass, momentum, and energy conservation equations. Within this framework, the mass sink representing evaporation and the mass and momentum sources representing vapor injection are usually applied to the same interface cell; both appear simultaneously in the discrete equations of this cell, with their numerical contributions in opposite directions.

[0004] Therefore, there is still a need in this field for a simulation method that makes the location of the steam jet generation independent of the interface where evaporation occurs at a discrete level, so as to stably obtain the steam flow field without significantly increasing the computational cost, and make the jet direction naturally determined by the local geometry of the free surface of the molten pool. Summary of the Invention

[0005] This invention proposes a high-fidelity simulation method for vapor flow in molten pools during additive manufacturing, in order to solve the technical problems of flux loading divergence and vapor jet direction simulation distortion caused by spatial coupling between the evaporation surface and the vapor source in existing additive manufacturing vapor simulation methods.

[0006] To address the aforementioned technical problems, this invention provides a high-fidelity simulation method for molten pool vapor flow in additive manufacturing, comprising the following steps: Step S1: Establish a spatial grid containing a printing material domain occupied by printing material and a protective gas domain occupied by protective gas, and configure material properties and initial conditions for each cell of the spatial grid; Step S2: Identify the cells located at the boundary between the printing material domain and the protective gas domain in the spatial grid that are undergoing evaporation, and designate them as evaporation surface cells, and determine the evaporation mass loss flux based on the temperature of the evaporation surface cells; Step S3: Within the protective gas domain, identify a unit different from the evaporation surface unit as a vapor source unit; Step S4: Load the evaporation mass loss flux onto the evaporation surface unit, and load the steam mass source flux determined by the evaporation mass loss flux and the momentum source flux from the printing material domain to the protective gas domain onto the steam source unit; Step S5: Solve the two-phase flow control equations to obtain the steam velocity field.

[0007] Preferably, in step S3, the steam source unit and the evaporation surface unit are not adjacent.

[0008] Preferably, in step S3, the steam source unit and the evaporation surface unit are separated by at least one intermediate unit that does not contain printing material.

[0009] Preferably, in step S3, the gradient of the phase fraction field of the evaporation surface unit is calculated, and the direction of the obtained gradient is reverse normalized to obtain the unit normal vector of the evaporation surface unit pointing to the protective gas domain; starting from the evaporation surface unit, the unit is moved step by step along the direction of the unit normal vector with the smallest unit size of the spatial grid as the step size, and the number and phase fraction of the reached unit are obtained after each step. The unit with a phase fraction of 0 reached for the first time is recorded as the intermediate unit, and the stepping continues until the unit with a number different from the intermediate unit and a phase fraction of 0 is reached, and the reached unit is marked as the steam source unit.

[0010] Preferably, in step S2, the unit that simultaneously meets the criteria of having a unit temperature that reaches or exceeds the boiling point temperature of the printing material and a unit phase fraction that is greater than 0 and less than 1 is designated as the evaporation surface unit.

[0011] Preferably, in step S2, the evaporation energy loss is determined based on the temperature of the evaporation surface unit, and in step S4, the evaporation energy loss is applied to the evaporation surface unit.

[0012] Preferably, in step S2, the saturated vapor pressure of the evaporation surface unit, the evaporation mass loss flux, and the evaporation energy loss are determined by the following formula: ; ; ; in, The saturated vapor pressure is... Due to environmental pressures, The evaporation mass loss flux is mentioned above. The energy loss due to evaporation, The latent heat of vaporization of the printing material. The molar mass of the printing material is... Let be the ideal gas constant. The boiling point temperature of the printing material under the ambient pressure. The temperature of the evaporation surface unit.

[0013] Preferably, in step S4, the steam mass source flux is a positive mass source term in the mass conservation equation, and its value is equal to the evaporation mass loss flux; the evaporation mass loss flux is a negative mass source term in the mass conservation equation; the direction of the momentum source flux is the same as the direction of the unit normal vector of the evaporation surface unit pointing to the protective gas domain.

[0014] Preferably, the two-phase flow control equations include a mass conservation equation, a momentum conservation equation, and an energy conservation equation. The momentum conservation equation includes a momentum source term generated by the evaporation backpressure, which is applied to the evaporation surface unit and directed from the protective gas domain to the printing material domain. The expression is: ; In the formula, This is the saturated vapor pressure of the evaporation surface unit.

[0015] Preferably, the method further includes step S6: Store the temperature field, velocity field, pressure field, and phase fraction field of the current time step as the initial conditions for the next time step, and repeat steps S2 to S5 until all preset time steps are completed.

[0016] The beneficial effects of the present invention include at least the following: 1) By placing the mass sink representing evaporation and the mass source and momentum source representing steam injection in two different units, the discrete equation of any unit no longer contains source and sink terms with opposite directions. The steam source unit obtains net mass and momentum injection, and the steam jet can develop stably and be ejected from the surface of the molten pool.

[0017] 2) After the steam source unit and the evaporation surface unit of the present invention are separated, the high-speed fluid in the steam source unit no longer participates in the discrete solution of the momentum equation of the evaporation surface unit through the control body interface shared by the two. The recoil pressure on the evaporation surface unit is independently determined by the evaporation mass loss flux. The concave deformation of the free surface of the molten pool under the recoil pressure can be correctly captured. The concave shape is narrow and deep and can meet the keyhole closure condition.

[0018] 3) The steam mass source flux loaded on the steam source unit corresponds to the evaporation mass loss flux loaded on the evaporation surface unit, thus achieving mass and momentum conservation over the entire computational domain. After the latent heat of vaporization is deducted as energy loss in the evaporation surface unit, it is no longer injected into the gas phase in the steam source unit. Since the latent heat of vaporization of metal accounts for a very small proportion of the convective and thermal conduction heat of the molten pool, the effect of this treatment on the temperature and velocity fields can be ignored.

[0019] 4) The steam source unit of the present invention is determined by the free surface geometry at the evaporation surface unit. The initial direction of the steam jet changes with the local morphology of the free surface at that location. There is no need to manually specify the jet direction, and the asymmetric impact behavior can be reflected.

[0020] 5) The effects of this invention are achieved by relying on a fixed spatial grid, without the need to dynamically refine the grid during the calculation process, and without the need to introduce bidirectional coupling between particles and fluids for the solution. Therefore, it is beneficial to reduce the computational cost and is suitable for industrial-scale process simulation and parameter optimization. Attached Figure Description

[0021] Figure 1 This is a schematic diagram illustrating the division and initialization of the steam simulation calculation domain for the laser powder bed melting process in an embodiment of the present invention; Figure 2 This is a schematic diagram of the evaporation surface unit calibration of the present invention; Figure 3 This is a schematic diagram of the calibration results of the steam source unit of the present invention; Figure 4 This is a simulation result of steam jetting in a fully spatially decoupled laser powder bed melting process according to an embodiment of the present invention. Figure 5 This is a comparison chart of steam jet simulation results for three source-sink arrangement methods in the embodiments of the present invention; Figure 6 This is a comparison diagram of fluid volume loss between adjacent cell decoupling and complete decoupling in an embodiment of the present invention. Detailed Implementation

[0022] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the protection scope of the present invention.

[0023] First, the terms used in this invention will be explained. In this embodiment, the printing material domain refers to the area in the spatial grid occupied by the metal material to be formed, and the protective gas domain refers to the area occupied by the protective gas. The phase fraction refers to the volume fraction of the printing material in a cell, with a value of 1 indicating that the cell is filled with printing material and a value of 0 indicating that the cell does not contain printing material. The evaporation surface cell refers to the cell located at the boundary between the printing material domain and the protective gas domain and where evaporation occurs. The steam source cell refers to the cell located in the protective gas domain and used to apply steam mass source flux and momentum source flux. The intermediate cell refers to the cell located between the evaporation surface cell and the steam source cell and does not contain printing material.

[0024] like Figure 1 As shown in the figure, this embodiment of the invention provides a high-fidelity simulation method for vapor flow in an additive manufacturing molten pool, which includes the following steps.

[0025] Step S1: Establish a spatial mesh containing the printing material domain and the protective gas domain, and configure material properties and initial conditions for each cell of the spatial mesh.

[0026] Specifically, in this embodiment of the invention, the computational domain space is defined according to the processing range and simulation accuracy requirements of the additive manufacturing process, and the computational domain is divided into spatial grids. The computational domain includes a printing material domain and a protective gas domain, which correspond to the areas occupied by solid or liquid metal and the areas occupied by gas, respectively.

[0027] The composition of the printing material domain depends on the process: for powder bed fusion processes, the printing material domain includes the substrate and the pre-spread powder layer on it; for directional energy deposition processes, the printing material domain includes the substrate and the material already deposited on it. The fed powder or filament is introduced into the calculation as a mass inlet or initial phase fraction distribution. The protective gas domain is the area occupied by the protective gas, and its range is all areas in the calculation domain not occupied by the printing material.

[0028] The spatial mesh can be a uniform mesh of consistent size throughout, or a locally refined mesh can be used in the laser-affected area and near the expected molten pool, while a coarser mesh can be used in other areas, balancing computational accuracy and efficiency. In this embodiment, the element size in the refined area is determined by its ability to distinguish the local curvature of the molten pool's free surface, and its ratio to the element size in other areas is typically no greater than 1 / 2; in this embodiment, 1 / 4 is used. A structured orthogonal mesh is preferable for the spatial mesh, allowing for the successive determination of steam source elements along the normal direction according to element numbering.

[0029] Next, material properties and initial conditions are configured for each unit, and heat source process parameters are set. In this embodiment, material properties include, but are not limited to, the density, specific heat capacity, thermal conductivity, dynamic viscosity, boiling point temperature, latent heat of vaporization, and molar mass of the printing material and protective gas. Initial conditions include, but are not limited to, the initial temperature field, initial velocity field, initial pressure field, and initial phase fraction field. Heat source process parameters include, but are not limited to, heat source power, scanning speed, spot diameter, and energy distribution mode. The phase fraction of the unit in the printing material domain is initialized to 1, and the phase fraction of the unit in the protective gas domain is initialized to 0.

[0030] Step S2: Identify the cells located at the boundary between the printing material domain and the protective gas domain in the spatial grid that are undergoing evaporation as evaporation surface cells, and determine the evaporation mass loss flux based on the temperature of the evaporation surface cells.

[0031] After the spatial mesh is established, all cells within the computational domain are traversed at each time step. Cells meeting both criteria—that their temperature reaches or exceeds the boiling point of the printing material and their phase fraction is greater than 0 and less than 1—are labeled as evaporation surface cells. The former criterion ensures the cell has indeed entered the evaporation state, while the latter ensures the cell is located above the gas-liquid interface, not inside the molten pool or in a pure gas region. Failure to meet either criterion will result in cells that are not evaporating or cells outside the interface being mistakenly labeled as evaporation surface cells. The labeling results of the evaporation surface cells are as follows: Figure 2 As shown.

[0032] In another embodiment of the present invention, the saturated vapor pressure corresponding to the unit temperature can be calculated first according to the following formula (1). The evaporation surface unit is calibrated based on the saturated vapor pressure reaching or exceeding the ambient pressure and the unit phase fraction being greater than 0 and less than 1. Since the boiling point temperature itself is defined as the temperature when the saturated vapor pressure is equal to the ambient pressure, this criterion is physically equivalent to the aforementioned temperature criterion and is more direct when the ambient pressure deviates from a standard atmosphere.

[0033] For each calibrated evaporation surface unit, based on its temperature Determine the evaporation mass loss flux. Calculate the saturated vapor pressure at this temperature using the Clapeyron-Clausius equation: (1) Determining evaporation mass loss flux based on Hertz-Knudsen relation: (2) The evaporation energy loss is then determined from the evaporation mass loss flux: (3) In equations (1) to (3), This represents the saturated vapor pressure corresponding to the temperature of the evaporation surface unit, in Pa. Environmental pressure, measured in Pa; The latent heat of vaporization of the printing material is expressed in J / kg. The molar mass of the printing material is expressed in kg / mol. The ideal gas constant is taken as 8.314 J / (mol·K); The boiling point temperature of the printing material under ambient pressure, in K; The temperature of the evaporation surface unit is expressed in K. Evaporation mass loss flux, expressed in kg / (m²·s); The energy loss due to evaporation is expressed in W / m²; the coefficient 0.82 is the evaporation coefficient, which characterizes the physical effect of some vapor molecules returning to the liquid surface during the actual evaporation process.

[0034] Step S3: Identify a unit within the protective gas domain that is different from the evaporation surface unit, and use it as a vapor source unit.

[0035] After the evaporation surface unit is calibrated and its evaporation flux is obtained, a unit different from the evaporation surface unit is determined in the protective gas domain as the steam source unit.

[0036] As an optional implementation, the unit adjacent to the evaporation surface unit with a phase fraction of 0 can be taken as the steam source unit. This method is simple to implement, and the steam jet can also be ejected from the surface of the molten pool. Since the steam source unit and the evaporation surface unit share a control volume interface, the convective flux interpolation through this interface is simultaneously affected by the phase fraction and velocity field of the units on both sides of the interface. The mass source term in the pure gas unit will act numerically on the evaporation surface unit through this interface, participating in the solution of its phase fraction evolution. Therefore, the fluid volume conservation deviation gradually accumulates as the calculation progresses. Similarly, the high-speed fluid in the steam source unit participates in the discrete solution of the momentum equation of the evaporation surface unit through this interface, numerically forming an effect opposite to the direction of the evaporation recoil pressure, partially offsetting the driving force of the recoil pressure on the free surface.

[0037] In another embodiment of the invention, the steam source unit and the evaporation surface unit are not adjacent, with at least one intermediate unit without printing material between them. The steam source unit is not the first unit without printing material immediately adjacent to the evaporation surface unit, but rather the second or more unit without printing material located at least one unit size away. The physical basis for this is that, within the finite volume method discretization framework, the mass, momentum, and energy flux between adjacent control volumes are coupled through interpolation calculations at the control volume interface. If the steam momentum and mass sources are directly loaded onto the first unit without printing material immediately adjacent to the evaporation surface unit, the high momentum value of that unit will directly react on the evaporation surface unit through convection and diffusion terms, causing artificial numerical rigidity in the local mass and momentum equations. This weakens or even negates the actual effect of spatial decoupling and may still induce numerical oscillations. By setting at least one intermediate unit with a phase fraction of 0 as a buffer zone, the steam source unit is decoupled from the direct influence domain of the evaporation surface unit in spatial topology. The steam source term only affects the molten pool surface through the pressure gradient within the intermediate unit, without directly algebraically coupling with the sink term of the evaporation surface unit at the discrete equation level. This achieves complete decoupling of the source and sink, avoiding numerical oscillations caused by the source and sink being in the same or adjacent cells.

[0038] The location of the steam source unit is performed as follows: First, the gradient of the phase fraction field of the evaporation surface unit is calculated. Then, the direction of the obtained gradient is reverse-normalized to obtain the unit normal vector pointing from the evaporation surface unit to the protective gas domain. Subsequently, starting from the evaporation surface unit, the grid is stepped along the direction of the unit normal vector, with the smallest unit size of the spatial grid as the step size. After each step, the number and phase fraction of the reached unit are obtained. The unit with a phase fraction of 0 reached for the first time is recorded as the intermediate unit. The stepping continues until a unit with a different number than the intermediate unit and a phase fraction of 0 is reached. The reached unit is then labeled as the steam source unit. Figure 3 As shown.

[0039] As another alternative positioning method, the steam source unit can be selected from the center of the evaporating surface unit by offsetting a distance of not less than twice the unit size along the unit normal vector; or, within the neighborhood centered on the evaporating surface unit, the steam source unit can be selected from the non-coplanar units of the evaporating surface unit and whose phase fraction is equal to 0.

[0040] Step S4: Load the evaporation mass loss flux into the evaporation surface unit, and load the steam mass source flux corresponding to the evaporation mass loss flux, as well as the momentum source flux from the printing material domain to the protective gas domain, into the steam source unit.

[0041] Specifically, after the positions of the two types of units are determined, the evaporation mass loss flux is a negative mass source term in the mass conservation equation for the evaporation surface unit, and the evaporation energy loss is a negative energy source term in the energy conservation equation for the evaporation surface unit; the steam mass source flux is a positive mass source term in the steam source unit in the mass conservation equation, and its value is equal to that of the evaporation mass loss flux; the momentum source flux is a positive momentum source term in the momentum conservation equation for the steam source unit, and its direction is the same as the direction of the unit normal vector of the evaporation surface unit pointing to the protective gas domain. Therefore, the initial direction of the steam jet changes with the local morphology of the free surface.

[0042] Equation (2) gives the flux per unit area, while the source term in the finite volume method is applied per unit volume. Therefore, the actual loading is based on... Conversion, among which This represents the area of ​​the gas-liquid interface within the unit. This refers to the volume of the unit. When the dimensions of the evaporation surface unit and the steam source unit are inconsistent, the mass flow rate of both is applied based on their equality; that is, the volume term of the steam source unit is calculated according to its own volume. The same value is taken on both units, thus ensuring that the mass is conserved over the entire computational domain.

[0043] After the source and sink terms are loaded, the two-phase flow governing equations are solved to obtain the steam velocity field. The two-phase flow governing equations include mass conservation, momentum conservation, and energy conservation equations. The momentum conservation equation, in addition to the aforementioned momentum source terms, also includes a momentum source term generated by the evaporation backpressure. The evaporation backpressure is applied to the evaporation surface unit, directed from the protective gas domain to the printing material domain, and its value is calculated according to… Confirmed, among which This is the evaporation back pressure. The saturated vapor pressure corresponding to the unit temperature of the evaporation surface obtained according to equation (1) is 0.54, which reflects the physical process in which the momentum of about half of the molecules in the vapor generated by evaporation is transferred to the liquid surface.

[0044] Step S5: Solve the two-phase flow control equations to obtain the steam velocity field.

[0045] In this embodiment of the invention, the fluid volume method is used to track the evolution of the free surface of the molten pool under the impact of steam jets, that is, to solve the transport equations of the phase fraction in the computational domain and reconstruct the interface accordingly.

[0046] After solving one time step, the temperature field, velocity field, pressure field, and phase fraction field of the current time step are stored as the initial conditions for the next time step. The process of calibrating the evaporation surface elements, determining the steam source elements, loading the source and sink, and solving the equations is repeated until all preset time steps are completed. Since the evaporation surface elements and steam source elements are recalibrated according to the temperature field and phase fraction field at each time step, their positions are updated as the free surface of the molten pool evolves, without the need for manual intervention during the calculation process.

[0047] A specific embodiment of the method of the present invention is given below. This embodiment takes the laser powder bed melting process of nickel-titanium alloy as the object, and the computational domain is defined as a cuboid with a length of 5 mm, a width of 3 mm, and a height of 3 mm according to the typical processing range of this process. The unit size of the basic mesh is 0.02 mm; for the laser-affected area, that is, the range of 2 mm to 3 mm in the length direction, 1 mm to 2 mm in the width direction, and 0 to 0.5 mm in the height direction, the local density is reduced to a unit size of 0.005 mm. The printing material domain is set to nickel-titanium alloy, and the protective gas domain is set to argon.

[0048] The material properties of nickel-titanium alloy are as follows: density 6450 kg / m³ 3 Specific heat capacity 470 J / (kg·K), thermal conductivity 18 W / (m·K), dynamic viscosity 5.6 × 10⁻⁶ -3 Pa·s, liquidus temperature 1583 K, boiling point temperature Take 3000K, latent heat of vaporization Take 6.5 × 10 6 J / kg, molar mass Take 5.33 × 10 -2 Argon has the following material properties: density 1.78 kg / mol and surface tension 1.5 N / m. 3 Specific heat capacity 520 J / (kg·K), thermal conductivity 1.77 × 10⁻⁶ -2 W / (m·K), dynamic viscosity 2.23×10 -5 Pa·s. Environmental stress Take 1.013×10 5 Pa.

[0049] The heat source process parameters were set as follows: laser power 290W, scanning speed 960mm / s, spot diameter 0.1mm, and energy distribution mode Gaussian distribution. The initial temperature field was set to room temperature 300K, the initial velocity field was set to stationary, the initial pressure field was set to 1 atm, the phase fraction in the printing material domain was set to 1, and the phase fraction in the protective gas domain was set to 0.

[0050] The method of the present invention is executed according to the above parameters: at each time step, all units are traversed, and units with a temperature of 3000K or higher and a phase fraction between 0 and 1 are identified as evaporation surface units; the saturated vapor pressure, evaporation mass loss flux and evaporation energy loss of each evaporation surface unit are determined according to equations (1) to (3); the unit normal vector pointing to the protective gas domain is obtained for each evaporation surface unit, and the unit is stepped along the direction with a step size of 0.005mm. After skipping the first unit with a phase fraction of 0, the second unit with a phase fraction of 0 is taken as the steam source unit; negative mass source terms and negative energy source terms are loaded on the evaporation surface unit, and a recoil pressure pointing to the printing material domain along the normal direction is applied; positive mass source terms and momentum source terms pointing to the protective gas domain along the normal direction are loaded on the steam source unit; the two-phase flow control equations are solved simultaneously and the free surface is tracked by the fluid volume method. After storing each field quantity, the next time step is entered.

[0051] Calculation results show that steam is stably ejected from the surface of the molten pool and develops obliquely upwards, entraining surrounding argon to form a jet structure; the free surface of the molten pool produces a significant downward indentation under the action of recoil pressure, and the indentation morphology is narrow and deep, satisfying the keyhole closure condition. (2×10) -4 At time s, the fluid volume conservation deviation is 0.0227%.

[0052] The following examples illustrate the applicability of the method of the present invention to different printing materials and processes. This example uses a laser-directed energy deposition process for titanium alloys. The computational domain is defined as a cuboid with a length of 8 mm, a width of 4 mm, and a height of 4 mm, based on the typical processing range of this process. The basic mesh unit size is 0.025 mm, and the molten pool and laser-affected area are locally refined to 0.006 mm. The printing material domain is set to Ti-6Al-4V titanium alloy, whose boiling point temperature is... Take 3560K, latent heat of vaporization Take 8.8 × 10 6 J / kg, molar mass Take 4.5 × 10 -2 kg / mol; the protective gas domain is still set to argon. Process parameters are set as follows: laser power 1200W, scanning speed 600mm / s. The remaining steps are the same as in the previous embodiment: the evaporation surface elements are calibrated based on a temperature reaching or exceeding 3560K and a phase fraction between 0 and 1. The elements are then stepped along the unit normal vector with a step size of 0.006mm, and the second element with a phase fraction equal to 0 is taken as the steam source element. The source and sink are loaded separately and then solved simultaneously. The calculation results show that the steam jet is also stably ejected, the free surface of the molten pool undergoes concave deformation, and the fluid volume conservation deviation is on the same order of magnitude as in the previous embodiment.

[0053] To illustrate the necessity of separating the source and sink and the impact of the separation distance, two sets of comparative calculations were performed under the same computational domain, grid, material, and process parameters as in the previous embodiment. The three sets of calculations were completely identical except for the location of the steam source unit.

[0054] In the first set of comparisons, the steam mass flux and momentum flux are applied to the evaporation surface element itself, meaning the source and sink are located in the same element. The calculation results show that the mass, momentum, and energy source terms of this element approximately cancel each other out in the discrete equations. The maximum steam velocity is 8.21 m / s, and the steam fails to form an effective jet; no depression is generated on the free surface of the molten pool. Although the mass, momentum, and energy source terms of this element cancel each other out, the adjacent gas element is heated by the temperature of this element, and the heated gas generates velocity. Therefore, there is still a flow on the order of several meters per second in the computational domain. However, this flow is caused by the heating of adjacent gas, and the hot gas flow velocity is relatively low. It cannot form a steam jet ejected from the molten pool surface, nor can it drive the free surface of the molten pool to produce a concave deformation. Therefore, the physical processes of steam injection and keyhole formation cannot be reproduced.

[0055] The second set of comparisons used the aforementioned adjacent implementation method, where the steam source unit was the unit with a phase fraction of 0 immediately adjacent to the evaporation surface unit. Calculation results showed that steam could be ejected from the molten pool surface, but the fluid volume conservation deviation continued to accumulate as the calculation progressed, reaching 2 × 10⁻⁶. -4 Time s reached 0.222%; meanwhile, no obvious downward depression was observed on the free surface of the molten pool, and the molten pool exhibited a wide, shallow, and symmetrical distribution, without forming a keyhole structure. The steam jet simulation results for the above two sets of comparative schemes and the three source-sink arrangement methods of the embodiments of the present invention are as follows: Figure 5 As shown.

[0056] The following is a horizontal comparison of three sets of calculations. The nickel-titanium alloy example adopts a separated implementation method, that is, there is at least one intermediate unit between the steam source unit and the evaporation surface unit. The fluid volume conservation deviation at the same time is 0.0227%, and a narrow and deep concave deformation occurs on the free surface of the molten pool. The comparison of the three sets of calculations shows that placing the source and sink in different units is a prerequisite for stable steam ejection, and retaining at least one intermediate unit between them further eliminates the influence of flux interpolation on the phase fraction equation and momentum equation of the evaporation surface unit on the shared control volume interface, reducing the conservation deviation by about one order of magnitude and allowing for accurate prediction of the free surface morphology. The steam jet simulation results of the separated implementation method of the present invention are as follows: Figure 4 As shown. The change in fluid volume conservation deviation over time between the two sets of calculations for the separated implementation method and the adjacent implementation method is shown in the figure. Figure 6 As shown.

[0057] The technical features of the above embodiments can be combined arbitrarily. For the sake of brevity, not all possible combinations of the technical features in the above embodiments are described; only preferred embodiments of the present invention are illustrated. The descriptions are relatively specific and detailed, but they should not be construed as limiting the scope of the present invention. As long as the combination of these technical features does not contradict each other, it should be considered within the scope of this specification.

[0058] It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of this invention. Therefore, the scope of protection of this invention should be determined by the appended claims.

Claims

1. A high-fidelity simulation method for vapor flow in an additive manufacturing molten pool, characterized in that, Includes the following steps: Step S1: Establish a spatial grid containing a printing material domain occupied by printing material and a protective gas domain occupied by protective gas, and configure material properties and initial conditions for each cell of the spatial grid; Step S2: Identify the cells located at the boundary between the printing material domain and the protective gas domain in the spatial grid that are undergoing evaporation, and designate them as evaporation surface cells, and determine the evaporation mass loss flux based on the temperature of the evaporation surface cells; Step S3: Within the protective gas domain, identify a unit different from the evaporation surface unit as a vapor source unit; Step S4: Load the evaporation mass loss flux onto the evaporation surface unit, and load the steam mass source flux determined by the evaporation mass loss flux and the momentum source flux from the printing material domain to the protective gas domain onto the steam source unit; Step S5: Solve the two-phase flow control equations to obtain the steam velocity field.

2. The method according to claim 1, characterized in that, In step S3, the steam source unit and the evaporation surface unit are not adjacent.

3. The method according to claim 2, characterized in that, In step S3, at least one intermediate unit without printing material is spaced between the steam source unit and the evaporation surface unit.

4. The method according to claim 3, characterized in that, In step S3, the gradient of the phase fraction field of the evaporation surface unit is calculated, and the direction of the obtained gradient is reverse normalized to obtain the unit normal vector of the evaporation surface unit pointing to the protective gas domain. Starting from the evaporation surface unit, the unit normal vector is used as the step size of the smallest unit size of the spatial grid. After each step, the number and phase fraction of the reached unit are obtained. The unit with a phase fraction of 0 that is reached for the first time is recorded as the intermediate unit. The stepping continues until the unit with a number different from the intermediate unit and a phase fraction of 0 is reached. The reached unit is then labeled as the steam source unit.

5. The method according to claim 1, characterized in that, In step S2, the unit that simultaneously meets the criteria of having a unit temperature that reaches or exceeds the boiling point temperature of the printing material and a unit phase fraction that is greater than 0 and less than 1 is designated as the evaporation surface unit.

6. The method according to claim 1, characterized in that, In step S2, the evaporation energy loss is determined based on the temperature of the evaporation surface unit, and in step S4, the evaporation energy loss is applied to the evaporation surface unit.

7. The method according to claim 6, characterized in that, In step S2, the saturated vapor pressure of the evaporation surface unit, the evaporation mass loss flux, and the evaporation energy loss are determined by the following formula: ； ； ； in, The saturated vapor pressure is... Due to environmental pressures, The evaporation mass loss flux is mentioned above. The energy loss due to evaporation, The latent heat of vaporization of the printing material. The molar mass of the printing material is... Let be the ideal gas constant. The boiling point temperature of the printing material under the ambient pressure. The temperature of the evaporation surface unit.

8. The method according to claim 1, characterized in that, In step S4, the steam mass source flux is treated as a positive mass source term in the mass conservation equation, and its value is equal to that of the evaporation mass loss flux; the evaporation mass loss flux is treated as a negative mass source term in the mass conservation equation; the direction of the momentum source flux is the same as the direction of the unit normal vector of the evaporation surface unit pointing to the protective gas domain.

9. The method according to claim 7, characterized in that, The two-phase flow control equations include mass conservation equations, momentum conservation equations, and energy conservation equations. The momentum conservation equation includes a momentum source term generated by the evaporation backpressure, which is applied to the evaporation surface unit and directed from the protective gas domain to the printing material domain. The expression is: ； In the formula, This is the saturated vapor pressure of the evaporation surface unit.

10. The method according to claim 1, characterized in that, The method further includes step S6: Store the temperature field, velocity field, pressure field, and phase fraction field of the current time step as the initial conditions for the next time step, and repeat steps S2 to S5 until all preset time steps are completed.

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