Method and system for dynamic evolution simulation of nitrogen element in laser powder bed fusion of hydrogen resistant steel

Abstract

The application relates to a method and system for simulating the dynamic evolution of nitrogen elements in a laser powder bed melting process of hydrogen-resistant steel. The method constructs a three-dimensional model of the powder bed by the discrete element method, adopts Gaussian, flat, ring and Bessel laser heat sources to simulate different energy input characteristics, establishes a heat transfer model following the conservation of mass, momentum and energy by the volume method, sets natural convection, thermal radiation boundary and back pressure and gas-liquid interface change, constructs a movement model of nitrogen elements between the liquid-solid two phases according to the macroscopic segregation and solute migration mechanism, introduces a critical nitrogen content threshold, simulates the gasification escape and bubble formation process of the nitrogen elements, and can detail the nitrogen element distribution / escape simulation results based on the molten pool temperature field and flow field, clarify the evolution law of the molten pool dynamic mode and the convective heat and mass transfer of the melt under the condition of laser time domain and space domain shaping, clarify the nitrogen element distribution / escape characteristics, and reveal the influence mechanism of the time domain and space domain laser shaping regulation on the formation of gas hole defects.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of dynamic simulation of molten pool under shaped laser, and more particularly to a method and system for dynamic evolution simulation of nitrogen element in laser powder bed fusion of hydrogen-resistant steel. BACKGROUND

[0002] Laser powder bed fusion (L-PBF) is a typical laser additive manufacturing process, which forms a molten pool by scanning a metal powder bed with a high-energy laser beam, and then accumulates layer by layer to realize the forming of a complex three-dimensional structure. Due to its high precision, strong near-net shaping capability, and high material utilization rate, laser powder bed fusion technology has been widely used in the manufacturing of key parts in the fields of aerospace, medical implants, nuclear power equipment, etc.

[0003] In the laser powder bed fusion process, the energy interaction between laser and powder is highly complex, involving laser energy absorption, powder melting, molten pool convection, gas escape, element migration and solidification shrinkage, etc. Among them, the temperature field and flow field of the molten pool directly determine the forming quality, microstructure and the generation and evolution of defects (such as pores, cracks, segregation, etc.), which are the core of research on simulation modeling and process optimization of additive manufacturing.

[0004] Currently, the high temperature and rapid cooling environment in the laser scanning process of laser powder bed fusion in the local area can cause solute elements (such as nitrogen elements) to migrate and gasify and escape, forming bubble defects, and thus affecting the density and mechanical properties of the material; the existing models generally ignore the synergistic effect between solute migration and bubble movement process, making it difficult to accurately predict the defect formation mechanism. SUMMARY

[0005] The present application provides a method and system for dynamic evolution simulation of nitrogen element in laser powder bed fusion of hydrogen-resistant steel, which can simulate the results of nitrogen element distribution / escape based on the temperature field and flow field of the molten pool, clarify the dynamic pattern of the molten pool and the evolution law of melt convection heat and mass transfer under the condition of laser time-domain and space-domain shaping, clarify the characteristics of nitrogen element distribution / escape, reveal the influence mechanism of time-domain and space-domain laser shaping regulation on the formation of pore defects, and establish a dynamic simulation process of temperature field-flow field-solute migration coupling for hydrogen-resistant steel materials.

[0006] In the first aspect, the present application provides a method for simulating the dynamic evolution of nitrogen elements in laser powder bed melting of hydrogen-resistant steel, the method comprising: using the discrete element method to perform free-fall stacking and leveling of hydrogen-resistant steel powder to establish a three-dimensional model of the powder bed; defining the energy input characteristics of the laser spot based on the laser heat flux distribution function, and establishing Gaussian, flat-top, annular and Bessel laser heat source models; using the volume method to establish a powder bed heat transfer model that obeys the laws of conservation of mass, conservation of momentum, and conservation of energy, and defines the natural convection boundary, thermal radiation boundary, recoil pressure and gas-liquid interface changes; based on the macro-segregation and solute migration mechanism, a movement model of nitrogen elements between liquid and solid phases in the molten pool under the action of laser is established and a critical nitrogen content threshold is introduced to simulate the evolution process of nitrogen gasification and escape to form nitrogen bubbles during the laser powder bed melting of hydrogen-resistant steel.

[0007] In an optional scheme of the first aspect, when establishing a three-dimensional model of a powder bed, the method includes: defining the powder particles as spherical solids with preset diameters and setting the substrate, scraper and side walls in the calculation domain as rigid planes; using a no-slip Hertz-Mindlin nonlinear spring damping model to describe the interaction force between the powder particles; free-falling and scraping the hydrogen-resistant steel powder and setting the calculation domain, laser heat source and hydrogen-resistant steel thermophysical properties to establish a three-dimensional model of the powder bed.

[0008] In an alternative embodiment of the first aspect, when describing the interaction force between powder particles, the method includes: the normal contact force between powder particles for: , where k represents the elastic coefficient between the two particles, is the overlap length, , is the center distance between particles, , is the relative velocity between particles, and are the velocities of the two contacting particles, is the normal damping coefficient; the tangential contact force between powder particles for: , is the tangential damping coefficient.

[0009] In an optional solution of the first aspect, when establishing the laser heat source model, the method includes: establishing a Gaussian laser heat source, which is determined by the following equation: ,in is the heat flux distribution function, is the heat flux at the center of the spot, is the radial relative coordinate, is the inflection point of the Gaussian distribution; a flat-top laser heat source is established, which is determined by the following equation: wherein is the heat flux, is the laser power, is the heat flux at the center of the beam spot, is the radius of the beam spot; the annular laser heat source and the Bessel laser heat source are both determined by the following equation: , is the heat flux distribution function, is the heat flux at the center of the beam spot, is the polar coordinate of a point on the beam spot, is the equation for setting the energy factor at the radial coordinate .

[0010] In an optional implementation of the first aspect, in establishing the powder bed heat transfer model, the method comprises: a mass conservation equation is: wherein, is the density, is the time, is the velocity of the fluid, is the mass added to the continuous phase; a momentum conservation equation is as follows: wherein, is the pressure received by the volume unit, is the gravitational acceleration, is the volume force, is the viscous stress tensor on the volume unit; an energy conservation equation is as follows: wherein, is the temperature, is the specific heat capacity, is the latent heat of phase change, is the laser heat source term.

[0011] In an optional implementation of the first aspect, the powder bed and the gas phase region adopt natural convection and thermal radiation in the convective heat exchange mode.

[0012] In an optional implementation of the first aspect, in defining the natural convection boundary, the thermal radiation boundary, the recoil pressure and the gas-liquid interface change, the method comprises: an expression of the natural convection boundary is: wherein, is the heat of convective heat exchange, is the convective heat exchange coefficient, is the boundary temperature, is the ambient temperature; the thermal radiation boundary follows the Stefan-Boltzmann law, and an expression is: wherein, is the equivalent emission coefficient, is the Stefan-Boltzmann constant, is the ambient temperature; the punch force is simulated by the saturated vapor pressure curve equation, the expression of which is: , , wherein, is the rebound pressure, is the initial strength of the rebound pressure, is the evaporation coefficient, is the pressure value on the saturated vapor pressure curve, is the temperature value on the saturated vapor pressure curve, is the actual temperature of the molten pool surface, is the latent heat of vaporization, is the specific heat ratio of steam, is the constant volume specific heat ratio of the whole; the equation of the change of the gas-liquid interface is: wherein, is the liquid volume fraction, representing the volume proportion of liquid in each grid; wherein, =0 represents that there is no liquid phase in the grid; when 0 <1, it represents that there is a liquid-gas interface in the grid; when =1, it represents that the grid is filled with liquid phase.

[0013] In an optional solution of the first aspect, when the movement model of nitrogen element between the liquid-solid two phases in the molten pool under the action of the laser is established, the method comprises: defining the equation of macrosegregation as:

[0014] wherein, and are alloy components in the liquid phase and the solid phase, are flow velocities of the liquid phase in x, y and z directions, is the flowable fractional volume, is the gas constant, and are mass diffusion coefficients in the liquid phase and the solid phase, , , are flowable fractional areas in x, y and z directions; is the mixture component, which is determined by controlling the average liquid phase and solid phase components in the volume, and the equation is:

[0015] , wherein, is the solid phase fraction, is the distribution coefficient.

[0016] In an optional solution of the first aspect, in simulating the evolution process of nitrogen bubbles formed by the gasification and escape of nitrogen element in the laser powder bed fusion process of hydrogen-resistant steel, the method comprises: assuming that each bubble particle is a sphere with a diameter d, the motion of the nitrogen bubble is controlled by the following equation:

[0017] wherein, and are the average velocity and density of the bubble respectively, is the acceleration of gravity, and are the fluid velocity and the pressure received, is the coefficient related to the resistance, is the mass of the particle, is the added fluid mass; for spherical bubble particles, it is assumed that the added mass is equal to half of the displacement fluid mass, which is: .

[0018] In the second aspect, the application provides a nitrogen element dynamic evolution simulation system for the laser powder bed fusion process of hydrogen-resistant steel, which uses the method according to any one of the first aspect, comprising: a powder bed model establishment module, which is used for adopting the discrete element method to perform free fall accumulation and scraping of hydrogen-resistant steel powder, and establishing a three-dimensional model of the powder bed; a heat source model establishment module, which is used for defining the energy input characteristics of the laser spot according to the laser heat flux distribution function, and establishing Gaussian, flat-top, ring-shaped and Bessel laser heat source models; a heat transfer model establishment module, which is used for adopting the volume method to establish a powder bed heat transfer model subject to the laws of conservation of mass, momentum and energy, and defining natural convection boundaries, thermal radiation boundaries, recoil pressure and gas-liquid interface changes; a nitrogen element dynamic evolution module, which is used for establishing a motion model of nitrogen element between liquid-solid two phases in the molten pool under the action of laser according to the macrosegregation and solute migration mechanism, and introducing a critical nitrogen content threshold, and simulating the evolution process of nitrogen bubbles formed by the gasification and escape of nitrogen element in the laser powder bed fusion process of hydrogen-resistant steel.

[0019] It should be understood that the above general description and the following detailed description are only exemplary and do not limit the application. BRIEF DESCRIPTION OF DRAWINGS

[0020] The accompanying drawings, which are incorporated herein and form part of the specification, illustrate one or more embodiments of the present application and, together with the description, serve to explain the principles of the application and to enable a person skilled in the relevant art to make and use the application.

[0021] Figure 1 is a flowchart of an exemplary nitrogen element dynamic evolution simulation method according to some embodiments of the application.

[0022] Figure 2is a schematic diagram of an exemplary physical property of particle contact according to some embodiments of the present application.

[0023] Figure 3 is a schematic diagram of an exemplary powder bed model according to some embodiments of the present application.

[0024] Figure 4 is a schematic diagram of an exemplary computational domain according to some embodiments of the present application.

[0025] Figure 5 is a schematic diagram of an exemplary laser spot profile and energy distribution of a laser model used in simulation according to some embodiments of the present application; where (a) is Gaussian laser, (b) is flat-top laser, (c) is ring laser, (d) is Bessel laser.

[0026] Figure 6 is a schematic diagram of an exemplary comparison of experimental results and simulation results according to some embodiments of the present application.

[0027] Figure 7 is a schematic diagram of an exemplary molten pool profile under shaped laser according to some embodiments of the present application; where (al) to (dl) are top views of the molten pool under Gaussian, flat-top, ring, and Bessel lasers, respectively; (a2) to (d2) are cross-sectional views of the molten pool in the scanning direction under Gaussian, flat-top, ring, and Bessel lasers, respectively.

[0028] Figure 8 is a schematic diagram of an exemplary cross-section of the molten pool perpendicular to the scanning direction under shaped laser according to some embodiments of the present application; where (a) is Gaussian laser, (b) is flat-top laser, (c) is ring laser, (d) is Bessel laser.

[0029] Figure 9 is a schematic diagram of an exemplary comparison of temperature field distribution characteristics under shaped laser according to some embodiments of the present application; where (a) is Gaussian laser, (b) is flat-top laser, (c) is ring laser, (d) is Bessel laser.

[0030] Figure 10 is a schematic diagram of an exemplary temperature field distribution of the molten pool under shaped laser according to some embodiments of the present application; where (al) to (dl) are top views of the molten pool under Gaussian, flat-top, ring, and Bessel lasers, respectively; (a2) to (d2) are cross-sectional views of the molten pool in the scanning direction under Gaussian, flat-top, ring, and Bessel lasers, respectively.

[0031] Figure 11 is a schematic diagram of an exemplary cooling rate distribution of the molten pool under shaped laser according to some embodiments of the present application; where (a) is Gaussian laser, (b) is flat-top laser, (c) is ring laser, (d) is Bessel laser.

[0032] Figure 12 is an exemplary shaped laser molten pool flow field distribution diagram according to some embodiments of the present application; wherein (a1) to (d1) are top views of molten channels under Gaussian, flat-top, ring, and Bessel lasers; (a2) to (d2) are cross-sectional views of the molten channels in the scanning direction under Gaussian, flat-top, ring, and Bessel lasers.

[0033] Figure 13 is an exemplary shaped laser molten pool temperature flow field analysis diagram according to some embodiments of the present application; wherein (a) is the temperature gradient distribution from the surface to the bottom of the molten pool under different beams, (b) is the highest temperature at the front end of the molten pool under different beams, and (c) is the surface melt flow rate at z=-50 μm at the front end of the molten pool under different beams.

[0034] Figure 14 is an exemplary laser molten pool nitrogen solubility distribution diagram according to some embodiments of the present application; wherein (a) is a Gaussian laser, (b) is a flat-top laser, (c) is a ring laser, and (d) is a Bessel laser.

[0035] Figure 15 is an exemplary comparative diagram of the change of nitrogen solubility at the front end of the molten pool under different lasers according to some embodiments of the present application.

[0036] Figure 16 is an exemplary diagram of the movement and distribution of nitrogen bubbles in the molten pool under shaped lasers according to some embodiments of the present application; wherein (a) is a Gaussian laser, (b) is a flat-top laser, (c) is a ring laser, and (d) is a Bessel laser.

[0037] Figure 17 is an exemplary diagram of the residual of nitrogen bubbles under shaped lasers according to some embodiments of the present application; wherein (a) is the number of residual nitrogen bubbles in the molten pool, and (b) is the residual volume of nitrogen bubbles in the molten pool.

[0038] Figure 18 is an exemplary diagram of the driving force of the movement of nitrogen bubbles in the molten pool under shaped lasers according to some embodiments of the present application; wherein (a) is a Gaussian laser, (b) is a flat-top laser, (c) is a ring laser, and (d) is a Bessel laser.

[0039] Figure 19 is a module connection diagram of an exemplary nitrogen element dynamic evolution simulation system according to some embodiments of the present application.

[0040] Figure 20 is a structural diagram of an exemplary electronic device according to some embodiments of the present application. DETAILED DESCRIPTION

[0041] Example embodiments will now be described more fully with reference to the accompanying drawings. However, example embodiments may be implemented in many forms and should not be construed as limited to the examples set forth herein; rather, the description of these embodiments is intended to make the present disclosure more comprehensive and complete and to fully convey the concepts of the example embodiments to those skilled in the art. The described features, structures, or characteristics may be combined in any suitable manner in one or more embodiments. In the following description, numerous specific details are provided to provide a deeper understanding of the embodiments of the present invention.

[0042] During the laser melting process of hydrogen-resistant steel, nitrogen (N) exhibits high chemical activity and diffusivity, and its behavior in the high-temperature molten pool significantly affects the alloy's microstructure and properties. On the one hand, as a strengthening element, the distribution and retention of nitrogen play a key role in the material's strength and ductility. On the other hand, excessive nitrogen in the molten pool can lead to nitrogen vaporization and escape, forming blister defects and, in severe cases, even holes or cracks, reducing density and fatigue life.

[0043] The dynamic evolution of nitrogen in the molten pool is affected by the coupling of multiple factors, including the characteristics of the laser heat source, powder spreading uniformity, molten pool flow behavior, and interface migration mechanism. Therefore, it is necessary to establish a coupling model from a multi-physics and multi-scale perspective to accurately simulate the transmission, precipitation, and escape behavior of nitrogen, so as to guide the parameter optimization and defect control of hydrogen-resistant steel in the L-PBF process.

[0044] At present, existing models generally do not consider the distribution coefficient, critical escape concentration and bubble formation mechanism of nitrogen during the liquid-solid phase transition process, making it difficult to predict the formation process and evolution trend of metallurgical defects such as pores. In addition, nitrogen forms bubbles after vaporization in the molten pool and moves with the fluid. Its behavior is affected by surface tension, buoyancy and viscosity. Existing technologies do not explicitly model and track the dynamic behavior of bubbles, making it difficult to provide a physical basis for defect suppression.

[0045] Therefore, reference Figure 1 As shown, Figure 1 The following is a flow chart of an exemplary method for simulating the dynamic evolution of nitrogen in the present application. To address the deficiencies in the prior art, the present application relates to a method for simulating the dynamic evolution of nitrogen in laser powder bed melting of hydrogen-resistant steel, the method comprising:

[0046] 101. The discrete element method is used to perform free-fall accumulation and leveling of hydrogen-resistant steel powder, and a three-dimensional model of the powder bed is established.

[0047] 102. The energy input characteristics of the laser spot are defined according to the laser heat flux distribution function, and Gaussian, flat-top, annular and Bessel laser heat source models are established.

[0048] 103. A volume method is used to establish a powder bed heat transfer model that complies with the law of conservation of mass, momentum and energy, and to define natural convection boundaries, thermal radiation boundaries, back pressure and gas-liquid interface changes.

[0049] 104. According to the macrosegregation and solute migration mechanism, a nitrogen element movement model between liquid-solid two phases in the molten pool under laser action is established, and a critical nitrogen content threshold is introduced to simulate the evolution process of nitrogen bubbles formed by nitrogen element gasification escape during the laser powder bed melting process of hydrogen-resistant steel.

[0050] Specifically, the establishment of the powder bed model is carried out in two steps: random falling and scraping of the powder. The discrete element method (DEM) is used to simulate the powder laying process, including collision between powder particles, collision between particles and substrate, and collision between particles and scraper. The powder particles are considered to be spherical solids with different diameters, and the substrate, scraper and side wall in the calculation domain are set as rigid planes. The Hertz-Mindlin nonlinear spring damping model is used to describe the interaction force between the powder and the powder, i.e. the normal contact force Figure 2 and the tangential shear force as shown in the following equation:

[0051] where k represents the elastic coefficient between two particles, and is the overlap length when two particles are in contact, i.e. the part whose center distance is less than the sum of the radii and , is the center distance between particles, is the direction of the line connecting the two particles, , is the relative velocity between particles, and is the velocity of the two contact particles, is the viscous resistance coefficient: is the normal damping coefficient, is the tangential damping coefficient; the total moment of each particle can be obtained by adding the equations of the above normal contact force and tangential shear force .

[0052] Specifically, the normal contact force equation is , which means that when particles come into contact and overlap, it is equivalent to a spring compression, and the elastic force tries to separate them, which is similar to Hooke's law: force is proportional to overlap length, and the direction is along the normal. ​​It represents the energy dissipation caused by the speed difference during contact, suppresses the bounce, and reflects the viscous resistance; in summary, the normal contact force There is both an elastic force to restore the particles to their pre-contact state and a damping force to suppress high-frequency oscillations.

[0053] Specifically, the tangential shear force The equation represents the viscous friction force caused by the tangential velocity between particles (contact surface sliding), which is opposite to the tangential sliding and acts to slow down the relative sliding.

[0054] The powder bed is built by allowing HR-2 hydrogen-resistant steel powder to fall freely onto a substrate and then leveling it with a scraper. The particle diameter distribution is shown in the table below:

[0055] Table: Particle diameter distribution

[0056]

[0057] The thickness of the powder bed was obtained by experimenters through corresponding experiments. It was determined based on the stable layer thickness during the actual printing process with a nominal layer thickness of 40μm and was set to 120μm. The powder bed schematic diagram is referenced. Figure 3 shown.

[0058] The size of the powder bed is set to 3000 μm in length (X direction), 1000 μm in width (Y direction), and 720 μm in height (Z direction). The coarse grid size is 12 μm, covering the entire computational domain. The fine grid size is 7 μm, with a length of 3000 μm, a width of 400 μm, and a height of 400 μm. The laser heat source is set to move along the X direction. The computational domain is higher than the powder bed, but the gas flow dynamics in the void area are not considered. The overall computational domain is as follows: Figure 4 shown.

[0059] The thermophysical properties of the HR-2 hydrogen-resistant steel used in this application were obtained by experimenters based on actual experiments and material thermophysical property calculation software, as shown in the following table:

[0060] Table: Physical properties of HR-2 hydrogen-resistant steel powder bed

[0061]

[0062] In some examples of this application, the Gaussian laser heat source used in this application is given by the equation: OK, among them is the heat flux distribution function, is the heat flux at the center of the spot, is the radial relative coordinate ( =0 is located at the center of the laser spot, =1) is located at the outer edge of the laser spot, It is the inflection point of the Gaussian distribution; the heat flux of the Gaussian laser heat source decreases exponentially from the center to the outside, with a strong center and weak edges.

[0063] The flat-top laser heat source used in this application is given by the equation: OK, among them is the heat flux, is the laser power, is the heat flux at the center of the beam spot, is the spot radius; the heat flux in the entire spot of the flat-top laser heat source is constant, and the heat energy is evenly distributed in the circular area Above the boundary, it is 0.

[0064] The ring laser heat source and Bessel laser heat source used in this application are both given by the following equations: Sure, is the heat flux distribution function, is the heat flux at the center of the beam spot, is the polar coordinate of the point on the spot, is the energy factor setting for the radial coordinate The energy of the annular laser heat source is mainly concentrated in the annular area (non-central area), and the energy at the center of the spot is low or zero; the energy distribution of the Bessel laser heat source is a concentric interference pattern, with non-attenuated intensity in the center and multiple annular peaks on the periphery.

[0065] Therefore, the intensity distribution of Gaussian, flat-top, annular and Bessel laser heat sources used in this application is specifically referred to Figure 5 As shown, Figure 5 (a) is Gaussian laser, Figure 5 (b) is a flat-top laser, Figure 5 (c) is a ring laser, Figure 5 (d) is Bessel laser.

[0066] In some examples of this application, during the simulation of melt flow, the volume of fluid (VOF) method is used to accurately calculate the temperature around the melt pool and capture the real-time movement of the page. In this process, the laws of conservation of mass, momentum, and energy are obeyed. Among them:

[0067] The mass conservation equation is: ,in, is the density, It's time, is the velocity of the fluid, The source is the mass added to the continuous phase. This mass conservation equation is the fundamental governing equation that describes how mass in a fluid system changes with time and space; is the local mass change term, which represents the change of density with time, i.e. the cumulative change of mass with time in the local region. If this term is positive, it means that the mass in this region is increasing. If this term is zero, it means that the density in this region does not change with time (steady state); is the mass flux term, which represents the divergence of the mass flow rate (i.e. mass flow flux) composed of density and velocity, i.e. the net outflow of mass from this region. If it is positive, it means that mass is leaving this region. If it is negative, it means that mass is entering this region. If , it means that mass is being injected from outside. If , it means that mass is being removed from the system.

[0068] The momentum conservation equation is as follows: ,

[0069] where, is the pressure experienced by the volume element, is the gravitational acceleration, is the body force, is the viscous stress tensor on the volume element. This momentum conservation equation is a fundamental equation describing the time variation of momentum in a continuous medium (such as a liquid, gas or plasma), and is one of the momentum expressions of the Navier-Stokes equations; is the local momentum change rate, which describes the local change of momentum with time in a unit volume; is the momentum convection term, which reflects the rate of momentum entering or leaving a volume element due to flow; is the pressure gradient force, which represents the driving force on a unit volume due to the pressure from high to low; is the internal friction force caused by viscous stress, which is the friction force in the fluid due to the velocity gradient, represents the influence of gravity on a unit volume.

[0070] The energy conservation equation is as follows: where, is the temperature, is the specific heat capacity, is the latent heat of phase change, is the laser heat source term. This energy conservation equation describes the time variation of thermal energy in a unit volume, which is the result of the combined action of sensible heat change, convection migration, heat conduction, phase change heat absorption and release, and external heat source input. There are differential forms of energy conservation laws under the actions of phase change, heat conduction, convection and heat source input, etc.; is the local change of temperature with time, which represents the rate of change of sensible heat storage due to temperature rise / decrease in a unit volume; is the heat convection term, which describes the rate at which heat is transported to other places due to fluid motion; is the local change of latent heat, representing the heat storage or release when the material is in the phase change stage; is the convective transport term of phase change heat, which describes the phase change heat being carried away due to the flow of molten metal; is the heat conduction term, reflecting the spatial diffusion of heat energy caused by temperature gradient, which is one of the main ways of heat transfer.

[0071] Among them, the convective heat transfer mode of the powder bed and the gas phase region is natural convection and thermal radiation.

[0072] In addition, the heat loss of the molten pool and the recoil pressure also need to be considered, specifically:

[0073] The expression of the natural convection boundary is: wherein, is the heat of convective heat transfer, is the convective heat transfer coefficient, is the boundary temperature, is the ambient temperature. The expression of this natural convection boundary describes the heat exchange law between the surface of an object and the surrounding fluid under the condition of natural convection (free / natural convection): when the surface temperature of the object is , the heat is transferred from the object to the environment (heat dissipation), at this time ; when the surface temperature of the object is : the heat is transferred from the environment to the object (heat absorption), at this time ; when There is no temperature difference, no heat convection transfer, at this time .

[0074] The thermal radiation boundary follows the Stefan-Boltzmann law, and its expression is: wherein, is the equivalent emission coefficient, is the Stefan-Boltzmann constant, is the ambient temperature. The expression of this thermal radiation boundary indicates that the heat transfer rate per unit area (heat flux) at the boundary is equal to the energy exchange rate between the surface and the environment through thermal radiation.

[0075] The recoil pressure is simulated by the saturated vapor pressure curve equation, and the expression is:

[0076] , , wherein, is the recoil pressure, is the initial strength of the recoil pressure, is the evaporation coefficient, P is the pressure value on the saturated vapor pressure curve, T is the temperature value on the saturated vapor pressure curve, T is the actual temperature of the molten pool surface; L is the latent heat of vaporization, The larger the value of dT / dP, the higher the energy required for evaporation, and the more sensitive the temperature is to evaporation; Cp is the specific heat ratio of the vapor, Cv is the specific heat ratio of the liquid. The expression of the recoil pressure is used to simulate the physical model of the recoil pressure caused by the evaporation of metal in the high-temperature laser molten pool, which describes that the laser heats the molten pool surface, the temperature rises, and when the temperature exceeds a certain threshold (such as near the boiling point of the metal), the surface metal begins to evaporate violently, and the high-speed vapor molecules produced by evaporation produce a recoil pressure on the molten pool surface; it uses an exponential function to amplify the sensitivity of temperature to evaporation pressure, i.e. when The exponential term tends to 0, There is only weak recoil; when The exponential term quickly becomes large, Rapidly rising, causing strong spatter.

[0077] The equation for the change of the gas-liquid interface is: wherein, f is the liquid volume fraction, representing the proportion of liquid in each grid; wherein, =0 represents that there is no liquid phase in the grid, and the grid is completely filled with gas; when 0 <1, it represents that there is a liquid-gas interface in the grid; when =1, it represents that the grid is completely filled with liquid.

[0078] In some examples of the present application, in order to simulate the evolution behavior of nitrogen element in the additive manufacturing process, a macrosegregation model is introduced to describe the evolution process of binary alloy composition under the action of phase change, liquid and solid phase diffusion and liquid metal convection, and the specific equation is as follows:

[0079] wherein, and are the alloy compositions in the liquid and solid phases, is the flow velocity of the liquid phase in the x, y, z directions, is the flowable fractional volume, is the gas constant, and are the mass diffusion coefficients in the liquid and solid phases, , , are the flowable fractional areas (channel area fractions) in the x, y, z directions; is the mixture fraction, which is determined by controlling the average liquid and solid fractions within a volume, and the equation is:

[0080] , where, is the solid fraction, is the partitioning coefficient.

[0081] This equation expresses the evolution of the composition of the elements in the liquid and solid phases during the solidification of the alloy, which is simultaneously affected by the time variation, convection, and diffusion coupling; specifically, the elements in the liquid phase migrate by convection as the melt pool flows, diffuse in the liquid and solid phases due to the concentration gradient, and the composition is redistributed at the solid-liquid interface by the partitioning coefficient , and the macrosegregation phenomenon is formed by the mutual influence between the multiple phases, which directly affects the uniformity of the microstructure and the performance of the material

[0082] Specifically, is the rate of change of the mixture concentration, is the change of the mixture concentration per unit volume with time, multiplied by the flowable volume (i.e., the liquid volume fraction);

[0083] is the convection of the alloy components in the liquid phase with the fluid velocity in the x, y, and z directions.

[0084] Specifically, is the diffusion of the components in the liquid phase, which is controlled by the diffusion coefficient ; is the diffusion of the components in the solid phase, which is controlled by ; where, , , is the effective diffusion channel area in the control direction.

[0085] Assuming that each bubble particle is a sphere with a diameter of d, the motion of the nitrogen bubbles is controlled by the following equation:

[0086]

[0087] where, and are the average velocity and density of the bubbles, is the acceleration of gravity, and are the fluid velocity and the pressure received, is a coefficient related to the resistance, is the mass of the particle, is the mass of the added fluid;

[0088] For spherical bubble particles, assuming that the added mass is equal to half the displaced fluid mass, we have:

[0089] .

[0090] In particular, the bubble momentum equation describes the motion of a single bubble in a fluid subjected to a variety of forces, such as pressure difference, gravity, drag, and inertial coupling.

[0091] Due to the limitation of the model, the bubble cannot be generated directly in the molten pool. The generation size, number, and position of the bubble are determined through experiments and simulations, and are inserted into the model to realize the generation and motion of the simulated bubble. The specific method is as follows:

[0092] 1. Measure the nitrogen loss rate in the actual additive manufacturing process .

[0093] 2. Determine the total mass M of the molten pool in the simulation and the total mass of nitrogen corresponding to the mass of the molten pool through preliminary simulation

[0094] .

[0095] 3. Calculate the total amount of nitrogen loss of the molten pool .

[0096] 4. Calculate the critical nucleation radius of the nitrogen bubble through the non-equilibrium solidification equation ; wherein:

[0097] The total free energy of bubble nucleation is composed of two parts in competition:

[0098] Surface energy (hinders nucleation): the interfacial energy between the bubble and the liquid: ;

[0099] Volume free energy (drives nucleation): under non-equilibrium conditions, the change in volume free energy includes two parts:

[0100] Gas-liquid phase change free energy: due to gas supersaturation, the energy released by bubble formation, ;

[0101] Non-equilibrium solidification correction term: the decrease in liquid chemical potential caused by supercooling, which contributes additional driving energy,

[0102] (the heat of fusion of the liquid is positive);

[0103] Total volume free energy change (driving term, negative value): .

[0104] Critical nucleation radius derivation:

[0105] ​Critical radius Total free energy The maximum point (nucleation barrier peak, bubbles beyond this radius will spontaneously grow), need to meet the total free energy derivative of radius is 0:

[0106] Total free energy expression: Derivative of r and let:

[0107] ;

[0108] The critical nucleation radius formula of the bubble under non-equilibrium solidification is obtained: ; Replace The final expansion is: .

[0109] 5, The volume of the critical nucleation bubble .

[0110] 6, The nitrogen element mass of the critical nucleation bubble .

[0111] 7, The number of critical nucleation bubbles .

[0112] 8, The area where the nitrogen element in the preliminary simulated molten pool is less than or equal to is the bubble generation area.

[0113] Wherein, The nitrogen element content of the powder before the experiment, The nitrogen element content of the powder after the experiment, M is the total mass of the melting channel in the simulation, The total mass of nitrogen element corresponding to the melting channel mass, , , The critical nucleation radius, volume and nitrogen element mass of the nitrogen bubble, The number of critical nucleation bubbles, The gas-liquid interfacial tension, The liquid molar volume, The liquid melting enthalpy, The equilibrium solidification temperature, S is the supersaturation, R is the gas constant, and T is the actual solidification temperature.

[0114] Therefore, based on the above content, the molten pool morphology and temperature and flow field distribution experiment under the shaping laser are carried out, specifically:

[0115] Based on the CFD (Computational Fluid Dynamics, CFD) model, we carried out additive manufacturing simulations under different shaping laser conditions (Gaussian, Top-hat, Ring, Bessel), and obtained the differences in molten pool morphology and temperature flow field distribution under different shaping laser conditions. Figure 5 As shown, the Gaussian spot diameter is 75μm, the flat-top laser diameter is 200μm, the inner ring diameter of the ring laser is 100μm and the outer ring diameter is 200μm, the Bessel laser core energy accounts for 90% and the diameter is 80μm, the ring energy accounts for 10%, the inner ring diameter is 10μm and the outer ring diameter is 200μm, and the energy distribution conforms to the zero-order Bessel equation.

[0116] To verify the accuracy of the analysis model of laser-powder interaction under shaping laser, single-pass cladding experiments under Gaussian laser and flat-top laser were carried out, and the results were verified with the simulation results. Figure 6 As shown, Figure 6 The left side is the simulation result. Figure 6 The experimental results are shown to the right. The results show that the error between the simulation and experimental results is less than 10%, proving the model's effectiveness. Under a Gaussian laser at 250 W and 70 mm / s, the experimental single-pass melt width was 95 μm and the melt depth was 209 μm. The simulation results showed a melt depth of 203 μm and a melt width of 103 μm.

[0117] Therefore, based on the above content, this application analyzes the difference in molten pool morphology under shaping laser, and numerically simulates the laser powder bed melting (L-PBF) process of single-pass cladding of HR-2 hydrogen-resistant steel based on the transient time scheme, focusing on the comparative analysis of the dynamic evolution behavior of the molten pool under the action of Gaussian laser and three shaping lasers: flat top, Bessel, and annular. Figure 7 and Figure 8 As shown, Figure 7 A schematic diagram of an exemplary shaped laser molten pool morphology according to some embodiments of the present application is shown, wherein: Figure 7 (a1) to (d1) are top views of the melting channel under Gaussian, flat-top, annular, and Bessel lasers. Figure 7 (a2) to (d2) are cross-sectional views along the scanning direction of the melt path under Gaussian, flat-top, annular, and Bessel lasers; Figure 8 FIG2 shows a schematic cross-sectional view of an exemplary shaping laser lower melt path perpendicular to the scanning direction in some embodiments of the present application, wherein: Figure 8 (a) is Gaussian laser, Figure 8 (b) is a flat-top laser, Figure 8 (c) is a ring laser, Figure 8 (d) is a Bessel laser. Figure 7 、Figure 8 The cross-sectional morphology cloud diagram of the melt pool can intuitively observe the differences in fluid dynamic characteristics of different beam modes: under the significant influence of the energy density distribution characteristics, each laser mode exhibits completely different melt pool morphological characteristics.

[0118] Numerical simulation results show that due to the highly concentrated energy distribution at the center of the Gaussian laser, a deep and steep melt pool profile is formed under the synergistic effect of recoil pressure and surface tension. The width-to-depth ratio of the melt pool is only 0.44, the depression depth at the front end of the melt pool reaches 247 μm, and the front of the melt pool shows a significant gradient change feature. In comparison, the three shaping lasers effectively improve the melt pool morphology through the energy spatial redistribution strategy, and the overall width-to-depth ratio is increased to above 1.5. Among them, the flat-top laser forms the shallowest melt pool morphology (width-to-depth ratio of 1.9) due to its uniform energy distribution characteristics. The Bessel laser achieves the maximum melt depth (width-to-depth ratio of 1.8) in the shaping laser mode through the central energy focusing mechanism. The ring laser, due to the edge energy enhancement effect, causes the melt pool depression area to expand to 1.3 times the spot diameter (melt width 256 μm), showing the largest width-to-depth ratio (2.3). This differentiated molten pool evolution behavior reveals the decisive role of laser energy distribution pattern on the fluid dynamics characteristics of the molten pool, and provides a theoretical basis for the optimized application of laser shaping technology in the L-PBF process.

[0119] Based on the above findings, this application investigates the heat-flow field distribution of the melt pool under shaped lasers. In laser additive manufacturing, the spatial distribution of laser energy density, a key process parameter, plays a decisive role in regulating the thermodynamic behavior of the melt pool. This study, using a multi-physics field coupled numerical simulation method, systematically reveals the dynamic evolution of the melt pool and its inherent thermodynamic mechanisms under four typical laser modes: Gaussian, flat-top, annular, and Bessel.

[0120] Numerical simulation results show that the differential characteristics of laser energy distribution directly dominate the morphology of the high temperature zone of the molten pool and the direction of heat conduction. Figure 9 The melt pool temperature field cloud diagram is shown in the figure, where Figure 9 (a) is Gaussian laser, Figure 9 (b) is a flat-top laser, Figure 9 (c) is a ring laser, Figure 9(d) is a Bessel laser. Under Gaussian laser conditions, energy is highly concentrated at the center of the spot, and the melt pool has a very small aspect ratio. The high-temperature zone of the melt pool is located on the leading wall of the melt pool. With flat-top lasers, energy is evenly distributed within the spot, with the high-temperature zone appearing as a circular region close to the spot diameter. With annular lasers, the high-temperature zone is distributed as an annular region consistent with the spot shape. With Bessel lasers, due to the simultaneous presence of a central region with concentrated energy distribution and an annular region with lower energy distribution, the annular high-temperature region exhibits localized temperature decay in the latter part of the scanning direction, weakened by the heat accumulation effect. This dynamic thermal distribution characteristic provides a new approach for the controllable forming of complex three-dimensional structures.

[0121] When the liquidus temperature is used as the threshold to define the high temperature zone, Figure 10 As shown, Figure 10 FIG. 1 shows an exemplary schematic diagram of temperature field distribution of a shaping laser molten pool according to some embodiments of the present application, wherein: Figure 10 (a1) to (d1) are top views of the melting channel under Gaussian, flat-top, annular, and Bessel lasers. Figure 10 (a2) to (d2) are cross-sectional views of the melt path scanning direction under Gaussian, flat-top, annular, and Bessel lasers. Figure 11 , Figure 11 FIG. 1 shows an exemplary schematic diagram of cooling rate distribution of a laser molten pool according to some embodiments of the present application; wherein, Figure 11 (a) is Gaussian laser, Figure 11 (b) is a flat-top laser, Figure 11 (c) is a ring laser, Figure 11 (d) is a Bessel laser. By combining Figure 11 Comparative analysis of the melt pool cooling rate simulation results shows that the melt pool morphology under the action of the shaping laser exhibits significantly different evolutionary characteristics than that under the action of the Gaussian laser: the melt pool formed by the shaping laser exhibits superior forming ductility in both lateral expansion (melt channel width increased to over 200μm) and longitudinal extension (tail length increased by 1.6-2.3 times). This phenomenon can be attributed to the thermal cycling characteristics of the shaping laser. Its solidification cooling rate is reduced by approximately 20%-30% compared to the Gaussian mode. This change in thermodynamic parameters extends the lifetime of the liquid metal in the melt pool. This extended time scale provides sufficient thermodynamic conditions for gas migration and solute element diffusion within the melt pool.

[0122] In order to better describe the changes and differences in the temperature flow field of the molten pool under the shaping laser, points were taken from the center line of the molten pool, from the powder bed surface (z=0) to the inside of the substrate (z=-0.03), and the changes in the temperature gradient of the molten pool when the laser was scanned were obtained; a probe was set 50 μm below the powder bed surface to obtain the changes in the molten pool flow velocity at the front end of the molten pool under different laser scans; and the changes in the peak temperature of the molten pool during the additive manufacturing simulation were extracted. The results are shown in Figure 2. Figure 12 andFigure 13 As shown, Figure 12 FIG. 1 shows an exemplary flow field distribution diagram of a shaping laser molten pool according to some embodiments of the present application, wherein: Figure 12 (a1) to (d1) are top views of the melting channel under Gaussian, flat-top, annular, and Bessel lasers. Figure 12 (a2) to (d2) are cross-sectional views along the scanning direction of the melt path under Gaussian, flat-top, annular, and Bessel lasers. Figure 13 An exemplary thermal flow field analysis diagram of a shaping laser molten pool according to some embodiments of the present application is shown, wherein: Figure 13 (a) is the temperature gradient distribution from the surface to the bottom of the molten pool under different beams. Figure 13 (b) is the maximum temperature of the front end of the molten pool under different beams. Figure 13 (c) is the surface melt flow rate at z=-50μm at the front end of the molten pool under different light beams.

[0123] The results show that Gaussian laser energy concentration exhibits a higher maximum melt temperature, a larger temperature gradient, a large melt high flow rate area, and the highest melt flow rate; the maximum temperature of the molten pool formed by the uniform flat-top laser energy distribution is lower, resulting in a lower temperature gradient and a lower melt flow rate; the maximum temperature of the ring laser molten pool is lower than that of the Gaussian laser, and higher than that of the flat-top laser, with a larger melt area, a lower temperature gradient, and a lower melt flow rate; the maximum temperature of the Bessel laser molten pool is relatively high, the temperature gradient is higher, and the melt flow rate is slightly higher than that of the flat-top and ring lasers.

[0124] Therefore, based on the above content, this application studies the distribution and escape of nitrogen under shaping laser. Under the action of laser heat, the powder at the front end of the molten pool undergoes a dynamic process of lattice dissociation and nitrogen desolvation: the nitrogen atoms originally dissolved in the metal matrix as interstitial atoms are released as the lattice structure collapses, some escape as nitrogen bubbles through the gas-liquid interface, and the remaining nitrogen migrates and redistributes with the flow of the melt. In order to quantitatively analyze this process, a motion model of solute diffusion-bubble migration of nitrogen was constructed, and numerical simulations were carried out on the dissolved state distribution of nitrogen and its bubble motion behavior under the action of shaping laser. Figure 14 、 Figure 15 and Figure 18 As shown, Figure 14 : shows an exemplary nitrogen solubility distribution diagram of a laser molten pool in some embodiments of the present application, wherein: Figure 14 (a) is Gaussian laser, Figure 14 (b) is a flat-top laser, Figure 14 (c) is a ring laser, Figure 14 (d) is Bessel laser; Figure 15 A schematic diagram showing a comparison of changes in nitrogen solubility at the front end of a molten pool under different laser conditions according to some embodiments of the present application is shown; Figure 18An exemplary schematic diagram of driving force of nitrogen bubble movement under shaped laser molten pool is shown, wherein, Figure 18 (a) is a Gaussian laser, Figure 18 (b) is a flat-top laser, Figure 18 (c) is a ring laser, Figure 18 (d) is a Bessel laser.

[0125] Nitrogen solubility distribution under shaped laser molten pool is based on Figure 14 solute distribution simulation results, combined with Figure 11 cooling rate distribution in the molten pool cooling process and Figure 18 melt flow in the molten pool, the dynamic migration process of nitrogen element presents multi-mechanism synergistic effect characteristics: in the high cooling rate area (cooling rate K / s) in the front of the molten pool, the nitrogen solubility along the laser scanning direction (X+) presents a gradient decreasing characteristic, which is caused by the phenomenon that the liquid phase nitrogen element cannot complete sufficient diffusion due to rapid solidification; in the rear of the molten pool, due to the temperature field relaxation effect (cooling rate decreases to K / s), the nitrogen element forms a zonal segregation zone at the solid-liquid interface, and the peak concentration is increased by more than 1.5 times compared with the matrix. Driven by the Marangoni convection, the solute upwelling channel is formed in the molten pool, which causes the nitrogen concentration on the upper surface to be increased by 34% compared with the concave area. However, the lower edge area is at the end of the convection vortex with small flow rate, and the solute rejection effect (nitrogen element distribution coefficient k=0.01-0.02) of the solidification front is superimposed, which leads to the decrease of solute transport efficiency in this area. When the solidification rate exceeds the solute diffusion rate, the nitrogen element cannot backfill the solute rejected by the solid phase through diffusion, and finally forms a solute depletion zone at the solid-liquid interface.

[0126] Based on Figure 14 quantitative comparative analysis of nitrogen solubility, Figure 15 the solute migration behavior of different laser modes presents significant differences: the Gaussian laser has the highest peak temperature and the largest temperature gradient in the molten pool, and the nitrogen concentration at the front end of the molten pool decreases sharply to (loss rate 21%) under the dominance of the Marangoni effect; the flat-top laser has the lowest temperature and the smallest temperature gradient in the molten pool, and the weak Marangoni convection makes the nitrogen loss rate at the front end controlled at 8.5%; the Bessel laser is affected by the central energy focusing, and the temperature gradient of the molten pool increases, which leads to the increase of the nitrogen loss rate to 15%, and the solute depletion zone is formed at the bottom of the molten pool; the nitrogen loss rate of the ring laser mode is 12%, but due to the low flow rate of the vortex at the bottom, the solute transport lag effect forms a depletion zone at the solidification interface.

[0127] Nitrogen element escape behavior under shaped laser, based on the nitrogen element loss phenomenon observed in the early experiment of laser composite metal additive manufacturing, the application establishes a quantitative correlation model between nitrogen element content attenuation and nitrogen bubble generation mechanism. Set the initial nitrogen element content as When the material experiences 15% nitrogen loss (corresponding to the critical nitrogen content threshold ), nitrogen bubbles with a critical size of 5-15 μm are precipitated during the solidification process of the molten pool, and the bubble dynamics behavior (including collision fusion, migration escape, etc. Physical process) is introduced, combined with the thermal-mechanical coupling field characteristics of the shaped laser, a numerical model of nitrogen bubble motion evolution driven by laser energy field is constructed, and the dynamic behavior of the bubble is revealed by multi-physical field coupling simulation. The motion and distribution of nitrogen bubbles in the molten pool at different time points under the shaped laser are as follows Figure 16 , the volume and number of bubbles remaining in the molten pool after the laser is as shown in Figure 17 , and Figure 17 shows a schematic diagram of the nitrogen bubble remaining under the shaped laser according to some embodiments of the application, wherein Figure 17 (a) is the number of nitrogen bubbles remaining in the molten pool, Figure 17 (b) is the residual volume of nitrogen bubbles in the molten pool.

[0128] The application reveals the influence law of different laser energy distribution on the dynamic behavior of nitrogen bubbles in HR-2 hydrogen-resistant steel additive manufacturing by numerical simulation, combined with the influence of melt flow behavior under shaped laser on bubble motion, and Figure 18 The following analysis results can be obtained:

[0129] Based on the analysis of the molten pool-bubble interaction mechanism under the action of multi-mode laser, different energy distribution characteristics have significant different regulation on the behavior of nitrogen bubbles.

[0130] The flat-top laser shaped molten pool has the largest width-depth ratio (1.9) and the shallowest melt depth (135 μm), the upward heat flow from the bottom of the front concave area to the surface dominates the melt motion mode, the relatively gentle temperature gradient and the lower flow rate (peak value 1.5 m / s) promote the collision and fusion of micron-sized bubbles, forming larger size bubble clusters with an average diameter of 18.5 μm, and prolonging the residence time on the surface of the molten pool makes 72% of the bubbles escape, finally realizing the lowest nitrogen bubble residual volume and the optimal number density.

[0131] In contrast, the deep penetration characteristics of the Gaussian laser (width-depth ratio 0.44) expand the bubble generation area to the middle and lower part of the molten pool (61% of the area with a depth of >100 μm), and the downward vortex structure formed inside produces a dynamic pressure gradient, which significantly inhibits the upward floating of the bubbles, resulting in the retention of small bubbles with a diameter of 5-10 μm at a high density.

[0132] The Bessel laser forms a reverse vortex similar to the Gaussian mode, but the central energy density is concentrated, causing the directional transport of bubbles to the core area of the molten pool (radius < 50 μm), resulting in the maximum residual volume and local aggregation phenomenon.

[0133] The annular laser reduces the driving force of the bubble downward migration by weakening the vortex intensity, and the volume and number of residual nitrogen bubbles are between the flat top and Gaussian modes.

[0134] In summary, the width-depth ratio of the flat-top laser molten pool is significantly improved compared to the Gaussian laser molten pool, from 0.44 of the Gaussian laser to 1.9 of the flat-top laser, and the temperature gradient is reduced to 42% of the Gaussian mode, and the Marangoni convection intensity driven by the surface tension gradient is reduced. The peak flow rate of the melt is reduced from 4 m / s of the Gaussian laser to 2.4 m / s, and the nitrogen element loss is significantly reduced compared to the Gaussian laser (from 21% to 8.5%). Under the dominant effect of buoyancy, the bubble escape driving force is enhanced, and finally the simulation results of the nitrogen bubble escape rate of 72% and the nitrogen element loss rate of 8.5% under the flat-top laser are achieved, verifying the significant advantages of the flat-top laser in low nitrogen loss and low defect forming.

[0135] Thus, in some embodiments of the present application, reference is made to Figure 19 as shown, Figure 19 A module connection diagram of an exemplary nitrogen element dynamic evolution simulation system according to some embodiments of the present application is shown. The present application also relates to a nitrogen element dynamic evolution simulation system for laser powder bed melting of hydrogen-resistant steel, comprising: a powder bed model establishment module 201 for adopting the discrete element method to perform free fall accumulation and flattening of hydrogen-resistant steel powder, and establishing a three-dimensional model of the powder bed; a heat source model establishment module 202 for defining the energy input characteristics of the laser spot according to the laser heat flux distribution function, and establishing laser heat source models of Gaussian, flat-top, annular and Bessel; a heat transfer model establishment module 203 for adopting the volume method to establish a powder bed heat transfer model subject to the laws of conservation of mass, momentum and energy, and defining natural convection boundaries, thermal radiation boundaries, recoil pressure and gas-liquid interface changes; a nitrogen element dynamic evolution module 204 for establishing a motion model of nitrogen elements in the liquid-solid two-phase under the action of laser according to the macrosegregation and solute migration mechanism, and introducing a critical nitrogen content threshold, and simulating the evolution process of nitrogen bubbles formed by the gasification and escape of nitrogen elements in the laser powder bed melting process of hydrogen-resistant steel.

[0136] In some embodiments, reference is made to Figure 20 as shown, Figure 20A connection diagram of a terminal device for implementing the embodiments of the present application is shown. The terminal device 3 comprises a memory 301 and a processor 302, and the memory 301 stores a computer program which can be run on the processor 302. The processor 302 implements the method in the above embodiments when running the computer program. The number of the memory 301 and the processor 302 can be one or more.

[0137] The terminal device 3 further comprises:

[0138] A communication interface 303 for communicating with external devices and transmitting data.

[0139] If the memory 301, the processor 302 and the communication interface 303 are independently implemented, the memory 301, the processor 302 and the communication interface 303 can be connected with each other through a bus and complete communication therebetween.

[0140] The bus can be an Industry Standard Architecture (ISA) bus, a Peripheral Component Interconnect (PCI) bus or an Extended Industry Standard Architecture (EISA) bus, etc. The bus can be divided into an address bus, a data bus, a control bus, etc. For the convenience of representation, Figure 20 Only one thick line is used in the figure, but it does not mean that there is only one bus or only one type of bus.

[0141] Optionally, in a specific implementation, if the memory 301, the processor 302 and the communication interface 303 are integrated on a chip, the memory 301, the processor 302 and the communication interface 303 can complete communication therebetween through an internal interface.

[0142] The embodiments of the present application provide a computer readable storage medium which stores a computer program, and the program is executed by the processor 302 to implement the method provided in the embodiments of the present application.

[0143] The embodiments of the present application further provide a chip which comprises the processor 302, and is used for calling and running instructions stored in the memory 301 to enable a communication device installed with the chip to execute the method provided in the embodiments of the present application.

[0144] The embodiment of the present application further provides a chip, comprising: an input interface, an output interface, a processor 302 and a memory 301, the input interface, the output interface, the processor 302 and the memory 301 are connected through internal connection paths, and the processor 302 is used for executing code in the memory 301, when the code is executed, the processor 302 is used for executing the method provided by the embodiment of the present application.

[0145] It should be understood that the processor 302 described above can be a central processing unit (CPU), and can also be other general-purpose processors, digital signal processors (DSP), application specific integrated circuits (ASIC), field programmable gate arrays (FPGA) or other programmable logic devices, discrete gate or transistor logic devices, discrete hardware components, etc. The general-purpose processor can be a microprocessor or any conventional processor, etc. It should be noted that the processor 302 can be a processor supporting an advanced RISC machine (ARM) architecture.

[0146] Further, the aforementioned memory 301 can include a read-only memory and a random access memory, and can also include a non-volatile random access memory. The memory 301 can be a volatile memory or a non-volatile memory, or can include both volatile and non-volatile memories. Among them, the non-volatile memory can include a read-only memory (ROM), a programmable read-only memory (PROM), an erasable programmable read-only memory (EPROM), an electrically erasable programmable read-only memory (EEPROM), or a flash memory. The volatile memory can include a random access memory (RAM) used as an external cache. By way of example, but not limitation, many forms of RAM can be used. For example, a static random access memory (SRAM), a dynamic random access memory (DRAM), a synchronous dynamic random access memory (SDRAM), a double data rate synchronous dynamic random access memory (DDR SDRAM), an enhanced synchronous dynamic random access memory (ESDRAM), a synchlink dynamic random access memory (SLDRAM), and a direct rambus random access memory (DRRAM) can be used.

[0147] Other embodiments of the application will be apparent to those skilled in the art from consideration of the specification and practice of the application disclosed herein. It is intended that the specification and examples be considered as exemplary only, with the true scope of the application being indicated by the following claims.

Claims

1. A method for simulating the dynamic evolution of nitrogen elements in a laser powder bed fusion process of hydrogen-resistant steel, characterized in that, The method comprises the following steps: The powder particles are defined as spherical solids with different diameters, and the substrate, the scraper and the sidewall in the calculation domain are set as rigid planes; The Hertz-Mindlin nonlinear spring damping model without sliding is used to describe the interaction force between the powder particles; The powder particles are subjected to free-fall accumulation and scraping, and the calculation domain, the laser heat source and the thermal physical parameters of the hydrogen-resistant steel are set to establish the three-dimensional model of the powder bed. The method comprises the following steps:

2. The method of claim 1, wherein, The method comprises the following steps: The method comprises the following steps: The convection heat exchange mode of the powder bed and the gas phase region adopts natural convection and thermal radiation. The method comprises the following steps:

3. The method of claim 2, wherein, The recoil pressure is simulated by a saturated vapor pressure curve equation, and the expression is as follows: Normal contact force between powder particles is: where k represents the elastic coefficient between two particles, is the overlap length, , is the center distance between particles, , is the relative velocity between particles, and is the velocity of two contact particles, is the normal damping coefficient; Tangential contact forces between powder particles are: , is the tangential damping coefficient.

4. The method according to claim 1 or 2, characterized in that, The method comprises the following steps: A Gaussian laser heat source is established, which is determined by the following equation: where is the heat flux distribution function, is the heat flux at the center of the spot, is the radial relative coordinate, is the inflection point position of the Gaussian distribution; A flat-top laser heat source is established, which is determined by the following equation: where is the heat flux, is the laser power, is the heat flux at the center of the beam spot, is the spot radius; A ring laser heat source and a Bessel laser heat source are established, both of which are determined by the following equations: , is a heat flux distribution function, is a heat flux at the center of the beam spot, is a polar coordinate of a point on the light spot, is an equation for setting an energy factor at a radial coordinate .

5. The method of claim 4, wherein, The equation of macrosegregation is defined as follows: The mass conservation equation is: wherein, is the density, is the time, is the velocity of the fluid, sources are the mass added to the continuous phase; The momentum conservation equation is as follows: wherein, is the pressure to which the volume element is subjected, is the gravitational acceleration, is the volume force, is the viscous stress tensor on the volume element; The energy conservation equation is as follows: where, is temperature, is specific heat capacity, is latent heat of phase change, is a laser heat source term.

6. The method of claim 5, wherein, The method comprises the following steps:

7. The method of claim 5, wherein, Each bubble particle is a sphere with a diameter of d, and the motion of the nitrogen bubble is controlled by the following equation: The expression for the natural convection boundary is: where, is the heat of convection exchange, is the convection exchange coefficient, is the boundary temperature, is the ambient temperature; The thermal radiation boundary follows the Stefan-Boltzmann law, expressed as: where in the formula, is the equivalent emission coefficient, is the Stefan-Boltzmann constant, is the ambient temperature; 10. A nitrogen element dynamic evolution simulation system for a laser powder bed melting process of hydrogen-resistant steel using the method according to any one of claims 1-9, comprising: , , wherein, is the recoil pressure, is the recoil pressure initial strength, is the evaporation coefficient, is the pressure value on the saturation vapor pressure curve, is the temperature value on the saturation vapor pressure curve, is the molten pool surface actual temperature, is the latent heat of vaporization, is the steam specific heat ratio, is the uniform constant volume specific heat ratio; The equation of the gas-liquid interface change is: wherein, is the liquid volume fraction, representing the volume proportion of liquid in each grid; wherein, =0 represents that there is no liquid phase in the grid; when 0 <1, it represents that there is a liquid-gas interface in the grid; when =1 represents that the grid is filled with liquid phase.

8. The method according to claim 5 or 7, characterized in that, a powder bed model establishment module, configured to use the discrete element method to perform free-fall accumulation and scraping of hydrogen-resistant steel powder, and establish a three-dimensional model of the powder bed; a heat source model establishment module, configured to define the energy input characteristics of the laser spot according to the laser heat flux distribution function, and establish laser heat source models of Gaussian, flat top, ring and Bessel; ， wherein and are alloy compositions in the liquid and solid phases, are the flow velocities of the liquid phase in the x, y, z directions, is the fraction volume that can flow, is the gas constant, and are the mass diffusion coefficients in the liquid and solid phases, , , are the flowable channel area fractions in the x, y, z directions; is the mixture component determined by controlling the average liquid and solid components within the volume, the equation being: , wherein, is the solid phase fraction, is the partition coefficient.

9. The method of claim 8, wherein, a heat transfer model establishment module, configured to use the volume method to establish a powder bed heat transfer model subject to the laws of mass conservation, momentum conservation and energy conservation, and define natural convection boundaries, thermal radiation boundaries, recoil pressure and gas-liquid interface changes; a nitrogen element dynamic evolution module, configured to establish a motion model of nitrogen elements between liquid and solid phases in a molten pool under laser action according to the macrosegregation and solute migration mechanism, introduce a critical nitrogen content threshold, and simulate the evolution process of nitrogen bubbles formed by the gasification and escape of nitrogen elements in the laser powder bed melting process of hydrogen-resistant steel. where, and are the average velocity and density of the bubbles, respectively, is the acceleration of gravity, and are the fluid velocity and the pressure experienced, is a coefficient related to the resistance, is the mass of the particle, is the mass of the fluid added; For spherical bubble particles, let the added mass equal half the displaced fluid mass, so that: . ​ ​ ​ ​ ​

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