Thermal coupling simulation method for composite manufacturing of laser additive and friction stir processing

Abstract

The invention discloses a thermal coupling simulation method for laser additive manufacturing and friction stir processing composite manufacturing, and belongs to the technical field of laser additive manufacturing and friction stir processing. Based on abaqus simulation software, secondary development is carried out to establish a thermal-mechanical coupling numerical simulation model in the process of processing the TC4 titanium alloy through laser metal deposition and friction stir, and then temperature and residual stress are predicted. Finally, it can be known through simulation that after the additive deposition process, most areas of the deposition layer show residual tensile stress, and residual compressive stress is shown in the edge area of the deposition layer; the average residual stress of the deposition layer after composite manufacturing is reduced by 43.2%, and the average residual stress in the composite manufacturing process measured by experiments is reduced by 37.9%. And the reliability and the accuracy of the thermal coupling model are explained. Through the thermal-mechanical coupling model, the distribution rule and the evolution mechanism of the temperature and the stress in the part equal-material-increasing composite manufacturing process can be predicted, and the stress regulation and the performance of the part in actual machining can be improved.

Description

Technical Field

[0001] The present invention relates to the technical field of laser additive manufacturing and friction stir processing, and in particular to a thermal coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing. Background Art

[0002] In the current part processing process, the laser metal deposition manufacturing process is that the material undergoes periodic, non-steady-state, long-term thermal cycle loads and the rapid solidification constraints of the moving molten pool. This manufacturing method will cause a large residual stress in the formed component. The friction stir processing process can produce plastic deformation in the material, which helps to eliminate metallurgical defects generated by laser metal deposition or other processing processes, and can also reduce residual stress.

[0003] However, some people have experimentally demonstrated the stress distribution of additive and friction stir processing composite manufacturing, but the experiment only measured the surface stress distribution and could not accurately analyze the thermal stress in the composite manufacturing process and the residual stress inside the deposited layer after processing.

[0004] Therefore, the prior art still needs to be improved and enhanced. Summary of the invention

[0005] In view of the above-mentioned deficiencies in the prior art, the purpose of the present invention is to provide a thermal-mechanical coupling simulation method for laser additive manufacturing and friction stir processing composite manufacturing, aiming to solve the problem that the experimental model in the prior art is unable to accurately analyze the thermal stress in the composite manufacturing process and the residual stress inside the deposited layer after processing.

[0006] In order to achieve the above object, the present invention adopts the following technical solutions:

[0007] A thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing, comprising:

[0008] Step S1, first, use the double ellipsoid heat source parameters to build a metal deposition heat load model, establish the TC4 alloy thermophysical parameters, then superimpose the results of the temperature field calculation model of the same load step to obtain the total strain, and construct a thermoplastic matrix; based on the problem of thermoelastic-plastic mechanics, establish a thermoplastic geometric model; finally, solve the stress by substituting the provided thermal load into the geometric equation.

[0009] Step S2, based on the "energy cycle inheritance method", the laser metal deposition residual stress is loaded and the result is obtained as the initial condition. The energy cycle inheritance is to assign stress values ​​in the three directions of x / y / z to each node of the finite element model grid.

[0010] Step S3, finally, construct the stir friction heat load based on the composite heat source parameters of the Gaussian surface heat source and the Gaussian cone heat source, calculate the force load of the stir friction equivalently loaded by the down pressure and shear force during the stir friction processing, establish the thermoplastic geometric equation and the equilibrium differential equation, and solve the composite manufacturing stress.

[0011] Furthermore, in S1, the heat source in the direction of laser metal deposition is divided into two asymmetric ellipses in front and back for description, with the center point of the heat source heating as the boundary. Due to the influence of the laser processing speed, the heating area in front of the laser is smaller than that behind the laser, resulting in the front and rear heating areas being asymmetric ellipsoidal shapes. The heat source formula of the double ellipsoid heat source is as follows:

[0012]

[0013]

[0014] Where: a r 、a f , b, c represent the semi-axis lengths of the double ellipsoid heat source in different directions; f1 and f2 are the absorptivity of the front and rear hemispheres; Q is the laser power.

[0015] Furthermore, in S1, the thermoplastic geometric model is as follows: the substrate size is 80 mm × 50 mm × 8 mm, the deposition layer size is 50 mm × 20.78 mm × 2 mm, the first deposition size is 50 mm × 3.5 mm × 1 mm, and there are 20 deposition layers in total. After the process parameters are optimized, the overlap rate is selected as 45%, the scanning strategy adopts a zigzag path, the deposition layer grid size is 0.5 × 0.5 × 0.5 mm, and the substrate is a gradient grid of 0.5-2 mm.

[0016] Furthermore, in step S1, the temperature evolution of the deposition layer and the substrate is mainly analyzed, and the temperature field temperature control equation of the same load step is:

[0017]

[0018] Where: ρ, λ and C p is the density, thermal conductivity and specific heat of TC4 alloy; T is the temperature function; t is the simulation time; ΔH is the latent heat of phase change, q is the laser energy per unit volume;

[0019] The heat conduction process follows Fourier's law, and the basic equation is:

[0020]

[0021] Where: q is the heat flux, k is the thermal conductivity, is the temperature gradient;

[0022] Thermal convection and thermal radiation are boundary conditions, and their governing equations are:

[0023]

[0024] Where: q1 is the heat convection density, q2 is the heat radiation density, h c is the heat transfer coefficient, T is the ambient temperature function, T0 is the ambient temperature, which is 22℃, is the emissivity, which is taken as 0.8 in the calculation, and x is the Stefan-Boltzman constant.

[0025] Furthermore, the temperature field distribution results are imported into the mechanical analysis model as the load of stress analysis to realize the coupled calculation of temperature field and stress field. The stress-strain equation is:

[0026] ε=D- 1 σ+ε th

[0027] Where: ε is the total strain vector; D is the elastic matrix; σ is the stress vector; ε th is the thermal strain vector. Since the substrate is placed horizontally on the workbench for deposition experiments, a fixed constraint must be added to the bottom surface of the substrate before performing stress field analysis;

[0028] The thermal strain equation is calculated based on the temperature field distribution and the thermal expansion coefficient of the material:

[0029]

[0030] Where: T r is the ambient temperature; α is the coefficient of thermal expansion;

[0031] According to the residual stress characteristics of laser metal deposition and the basic theory of plastic mechanics, the Mises yield criterion is used to obtain the equivalent stress calculation formula:

[0032]

[0033] Furthermore, in S3, based on the heat generation characteristics of the friction stir processing, the heat generation of the friction stir shoulder is usually modeled as a Gaussian surface heat source. The maximum heat flux density of the Gaussian surface heat source decreases exponentially from the center to the outside. The Gaussian plane heat flux expression is:

[0034]

[0035] Where: K is the absorption rate of TC4 powder to laser; ω is the laser radius; r is the distance from the starting point to the center of the light source, r 2 =x 2 +y 2 .

[0036] Gaussian cone heat flow expression:

[0037]

[0038] Where: r e and r i are the maximum and minimum radii of the cone, z e and z i are the maximum and minimum values ​​of the cone in the Z direction respectively.

[0039] Furthermore, the equivalent heat source in step S3 is:

[0040] Gaussian surface heat source:

[0041]

[0042] Gaussian cone heat source:

[0043]

[0044] Furthermore, the equivalent friction heat in step S3 is:

[0045] In the friction stir processing, the maximum and minimum radii of the stirring needle are r2 and r3 respectively, and the width of the stirring needle ring is d. r , an infinitesimal cross section with a radius of r0, the tool angular velocity is represented by ω, the friction coefficient is represented by μ, and the downward force during the FSP process is represented by P, then the width is d r The friction df on the infinitesimal cross section can be expressed as:

[0046] df=ωF=ωPr0dθdr(r3 <r0<r2)

[0047] The relative displacement S of the stirring head per unit time is calculated by the following formula:

[0048] S=ωr0

[0049] The expression of unit friction heat power dq is:

[0050] dq=sdf=μPωr0 2 dθdr

[0051] The friction heat Q between the stirring shoulder and the deposited layer surface can be calculated by integrating dq over the ring:

[0052]

[0053] The length of the stirring needle is recorded as h, and the friction heat q between it and the side of the deposit layer can be derived as:

[0054]

[0055] Furthermore, the medium effective load in step S3 is:

[0056] The mechanical action exerted by the stirring head has an important influence on the distribution of stress in the FSP process. Considering the downward force and torque of the tool, the downward force is the surface force evenly distributed in the shoulder area, and the torque is the tangential force of the stirring needle on the contact surface. The tangential force is obtained by integrating the surface force in the contact area in the forward direction of the stirring needle. The action of force is realized in the friction stir processing model using the Dload subroutine. The magnitude of each force is expressed as:

[0057]

[0058] Where: P is pressure, F is the pressure applied by the stirring head, F x 、F y 、F z is the tangential force applied to the model, F c is the friction force per unit volume, and x and y are the coordinates of the integration point.

[0059] The technical solution adopted by the present invention has the following beneficial effects:

[0060] In the thermomechanical coupling numerical simulation method of laser additive manufacturing and friction stir processing composite manufacturing proposed in the present invention: Abaqus numerical simulation software is used, and the residual stress obtained by laser additive manufacturing is used as the initial condition through the field energy transfer method to solve the final stress distribution in the friction stir processing composite numerical model. Accurate simulation can reveal the evolution mechanism and distribution law of temperature and stress over time in the additive manufacturing process. It can simply, quickly and accurately realize stress analysis, reduce experimental costs, and further improve the prediction reliability of parameter optimization. BRIEF DESCRIPTION OF THE DRAWINGS

[0061] Attached Figure 1 A heat source model diagram of laser metal deposition in an embodiment of the present invention;

[0062] Attached Figure 2 A diagram showing a heat source model of friction stir processing in an embodiment of the present invention;

[0063] Attached Figure 3 This is a load model diagram of friction stir processing in an embodiment of the present invention;

[0064] Attached Figure 4 A model diagram and a mesh division diagram of laser metal deposition in an embodiment of the present invention;

[0065] Attached Figure 5 A diagram of a stress transfer model in an embodiment of the present invention;

[0066] Attached Figure 6Graphs showing residual stresses in different directions of laser metal deposition in an embodiment of the present invention;

[0067] Attached Figure 7 It is a residual stress diagram in different directions of the friction stir processing composite manufacturing in the embodiment of the present invention;

[0068] Attached Figure 8 A comparison diagram of residual stresses of laser additive manufacturing and friction stir processing composite manufacturing in an embodiment of the present invention;

[0069] Attached Fig. 9 This is a comparison diagram of residual stress measurements after the experiment of laser additive manufacturing and friction stir processing composite manufacturing in an embodiment of the present invention;

[0070] Attached Fig.10 Flow chart of the thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing in an embodiment of the present invention. DETAILED DESCRIPTION

[0071] In order to make the purpose, technical solution and effect of the present invention clearer and more specific, the present invention is further described in detail with reference to the accompanying drawings and examples. It should be understood that the specific embodiments described herein are only used to explain the present invention and are not used to limit the present invention.

[0072] A thermal-mechanical coupling simulation method for laser additive manufacturing and friction stir processing composite manufacturing, the process is as follows Fig.10 As shown, including:

[0073] Step S1, first, use the double ellipsoid heat source parameters to build a metal deposition heat load model, establish the TC4 alloy thermophysical parameters, then superimpose the results of the temperature field calculation model of the same load step to obtain the total strain and construct a thermoplastic matrix; based on the problem of thermoelastic-plastic mechanics, establish a thermoelastic-plastic geometric model; finally, solve the stress by substituting the provided thermal load into the geometric equation.

[0074] Step S2, based on the "energy cycle inheritance method", the laser metal deposition residual stress is loaded and the result is obtained as the initial condition. The energy cycle inheritance is to assign stress values ​​in the three directions of x / y / z to each node of the finite element model grid.

[0075] Step S3, finally, construct the stir friction heat load based on the composite heat source parameters of the Gaussian surface heat source and the Gaussian cone heat source, calculate the force load of the stir friction equivalently loaded by the down pressure and shear force during the stir friction processing, establish the thermoplastic geometric equation and the equilibrium differential equation, and solve the composite manufacturing stress.

[0076] Specifically, in this embodiment, the geometric shape of the ellipsoid heat source model is close to the shape of the molten pool during laser metal deposition, similar to a semi-oval. Figure 1As shown in the figure, the heat source in the direction of laser metal deposition is divided into two asymmetric ellipses in front and behind, with the center point of the heat source heating as the boundary. This is because the heating area in front of the laser is smaller than that behind the laser due to the influence of the laser processing speed, resulting in the front and rear heating areas being asymmetric ellipses.

[0077] Heat flow expression of the front half ellipsoid:

[0078]

[0079] Heat flow expression of the rear half ellipsoid:

[0080]

[0081] Where: a r 、a f , b, c represent the semi-axis lengths of the double ellipsoid heat source in different directions; f1 and f2 are the absorptivity of the front and rear hemispheres; Q is the laser power.

[0082] Based on the heat generation characteristics of stir friction processing, the heat generation of stir friction shoulder is usually modeled as a Gaussian surface heat source. The maximum heat flux density of the Gaussian surface heat source decreases exponentially from the center to the outside. The specific shape is as follows Figure 2 As shown; the heat generated by the stirring needle is modeled as a Gaussian cone heat source. The cone heat source model is a body heat source. Its heat flux density in the radial direction decreases exponentially from the center to the outside, while in the depth direction it is a certain rotating body heat source. The specific shape is as follows Figure 2 shown.

[0083] Gaussian plane heat flow expression:

[0084]

[0085] Where: K is the absorption rate of TC4 powder to laser; ω is the laser radius; r is the distance from the starting point to the center of the light source, r 2 =x 2 +y 2 .

[0086] Gaussian cone heat flow expression:

[0087]

[0088] Where: r e and r i are the maximum and minimum radii of the cone, z e and z i are the maximum and minimum values ​​of the cone in the Z direction respectively.

[0089] This paper mainly analyzes the temperature evolution of the deposited layer and the substrate, so the "thermal-solid" coupling model is adopted. The temperature control equation of this model is:

[0090]

[0091] Where: ρ, λ and C p are the density, thermal conductivity and specific heat of TC4 alloy; T is the temperature function; t is the simulation time; ΔH is the latent heat of phase change, and q is the laser energy per unit volume.

[0092] The heat conduction process follows Fourier's law, and the basic equation is:

[0093]

[0094] Where: q is the heat flux, k is the thermal conductivity, is the temperature gradient.

[0095] Thermal convection and thermal radiation are boundary conditions, and their governing equations are:

[0096]

[0097] Where: q1 is the heat convection density, q2 is the heat radiation density, h c is the heat transfer coefficient, T is the ambient temperature function, T0 is the ambient temperature, which is 22℃, is the emissivity, which is taken as 0.8 in the calculation, and x is the Stefan-Boltzman constant.

[0098] The temperature field distribution results are imported into the mechanical analysis model as the load of stress analysis to realize the coupled calculation of temperature field and stress field. The stress-strain equation is:

[0099] ε=D -1 σ+ε th

[0100] Where: ε is the total strain vector; D is the elastic matrix; σ is the stress vector; ε th is the thermal strain vector. Since the substrate is placed horizontally on the workbench for deposition experiments, a fixed constraint must be added to the bottom surface of the substrate before performing stress field analysis.

[0101] The thermal strain equation is calculated based on the temperature field distribution and the thermal expansion coefficient of the material:

[0102]

[0103] Where: T r is the ambient temperature; α is the coefficient of thermal expansion:

[0104] According to the residual stress characteristics of laser metal deposition and the basic theory of plastic mechanics, the Mises yield criterion is used to obtain the equivalent stress calculation formula:

[0105]

[0106] Unlike traditional welding, FSP relies on the friction heat generated between the stirring head shoulder, the stirring needle and the base material as the main heat source, so that the friction heat softens the base material, and the high-speed rotation of the tool causes the base material to flow violently and enter a plastic state. In addition, plastic deformation will generate a certain amount of heat. Since the heat of plastic deformation of the material has a very small effect on the entire welding process, it is not considered when calculating the heat generation to simplify the analysis.

[0107] Figure 3 The schematic diagram of the contact friction heat generation between the shaft shoulder, the stirring needle and the deposited layer is shown in Figure 1. The shaft shoulder radius is r1, and the maximum and minimum radii of the stirring needle are r2 and r3 respectively. r , an infinitesimal cross section with a radius of r0. The tool angular velocity is represented by ω, the friction coefficient is represented by μ, and the downward force during the FSP process is represented by P, then the width is d r The friction df on the infinitesimal cross section can be expressed as:

[0108] df=ωF=ωPr0dθdr(r3<r0<r2)

[0109] The relative displacement S of the stirring head per unit time is calculated by the following formula:

[0110] S=ωr0

[0111] The expression of unit friction heat power dq is:

[0112] dq=sdf=μPωr0 2 dθdr

[0113] The friction heat Q between the stirring shoulder and the deposited layer surface can be calculated by integrating dq over the ring:

[0114]

[0115] The length of the stirring needle is recorded as h, and the friction heat q between it and the side of the deposit layer can be derived as:

[0116]

[0117] The friction stir processing model is simplified, and the heat generation of the shoulder is regarded as a Gaussian surface heat source whose heat flux increases linearly with the increase of the shoulder radius, and the heat generation of the stirring needle is regarded as a Gaussian cone heat source with uniform distribution. In addition, 75% of the total heat is distributed on the contact surface between the shoulder and the deposited layer surface, and the remaining 25% is dispersed in the volume range of the contact of the side of the stirring needle. In order to load the heat source into the FSP model, a moving heat source is applied using the Dflux subroutine, and the composite heat source heat flux Q model is as follows:

[0118] Q(total)=0.75*q(r)+0.25*Qr)

[0119] The mechanical action exerted by the stirring head has an important influence on the distribution of stress during the FSP process. In this paper, the downward force and torque of the tool are considered. The downward force is considered to be a surface force uniformly distributed in the shoulder area, and the torque is considered to be a tangential force of the stirring needle on the contact surface. The tangential force is obtained by integrating the surface force in the contact area in the forward direction of the stirring needle. The action of force is implemented in the friction stir processing model using the Dload subroutine, and the magnitude of each force is expressed as:

[0120]

[0121] Where: P is pressure, F is the pressure applied by the stirring head, F x 、F y 、F z is the tangential force applied to the model, F c is the friction force per unit volume, and x and y are the coordinates of the integration point.

[0122] In the actual process, the support plate and the fixture are used to support and fix the weldment, so the thermal boundary conditions and mechanical boundary conditions should be considered. Since the contact area between the support plate, the fixture and the weldment is large, the heat loss caused by the support plate and the fixture should be fully considered in the simulation. Here, the contact heat transfer coefficient is 100W / (m 2 ·C), the convective heat transfer coefficient between titanium alloy and air is determined to be 20W / (m 2 In order to describe the strong convection caused by the protective gas during titanium alloy processing, the convection heat transfer coefficient of the processing area is set to 80W / (m 2 C). Assume the emissivity is 0.8.

[0123] During the laser metal deposition process, the material undergoes rapid melting and solidification as well as multiple thermal cycles. Therefore, the deposited parts will experience dynamic non-uniform temperature changes. During the non-uniform temperature change process, large internal stress will be generated inside the deposited layer, causing it to deform or even crack. This study established a finite element model based on the life-death unit technology of ABAQUS finite element analysis. Figure 4As shown in a. The heat source model was established using the parametric design language (FORTRAN). The command was submitted to the ABAQUS job to build a thermal-mechanical coupling model and solve it. In order to ensure the authenticity of the simulation process, the "birth-death unit" technology was used to simulate the deposition process of the part. First, the substrate and the deposited layer were modeled. Then, in the analysis step before loading the heat source, all the deposited layer units were "killed". The mesh refinement in the finite element model affects the accuracy of the simulation analysis results. Finer mesh division will make the calculation results closer to the actual value. Refined meshes are used in the deposited layer and the substrate area near the deposited layer, while the mesh coarseness of the substrate area far away from the deposited layer is increased. The scanning strategy adopts a zigzag path, taking into account the overlap and remelting between deposition paths. After the process parameters are optimized, the overlap rate is selected as 45%, and the scanning direction is kept consistent in each layer. The geometric mesh and scanning path of the finite element model are shown in the figure. Figure 4 As shown in b. The substrate size is 80mm×50mm×8mm, the deposition layer size is 50mm×20.78mm×2mm, the first deposition size is 50mm×3.5mm×1mm, there are 2 layers and 20 passes in total, the deposition layer grid size is 0.5×0.5×0.5mm, and the substrate is a 0.5-2mm gradient grid. The bottom of the substrate is fixed. The simulated process parameters are determined according to the actual processing parameters: scanning speed 10mm / s, laser power 1600W, spot diameter 4mm.

[0124] After laser metal deposition, there is a large stress concentration problem. Based on the finite element model after laser metal deposition, friction stir processing is performed, such as Figure 5 As shown in (a), the entire FSP process can be divided into four stages. In the first downward pressure stage, the high-speed rotating stirring needle is inserted into the deposited layer to a specific depth. Then, entering the second dwelling stage, the stirring head continues to rotate at a high speed while keeping the depth unchanged, which is conducive to the further softening of the metal material. In the third processing stage, the stirring head begins to move along the processing direction during the rotation process. The material in contact with the stirring head on the deposited layer is softened, then flows along the rotation direction, and solidifies after the processing is completed. Finally, in the rising cooling stage, the stirring needle is pulled out of the deposited layer to form a keyhole, and the stirring friction part is cooled to room temperature. The entire stirring friction processing path is linear. In order to transfer the stress of the additive process to the isomaterial process, the geometric mesh of the stirring friction processing finite element model is consistent with the mesh of the laser metal deposition model. Based on the "energy cycle inheritance mechanism", the residual stress generated by laser metal deposition is used as the initial stress condition. Energy cycle inheritance is to assign stress values ​​in the three directions of x / y / z to each node of the finite element model mesh, such as Figure 5(b) is shown. The dimensions of the stirring head are: the diameter of the shaft shoulder is 15 mm, and the bottom has a stirring needle with a length of 2 mm, a minimum diameter of 4 mm, and a maximum diameter of 8 mm. The simulated process parameters are determined based on the actual processing parameters: the travel speed is 50 mm / min, the processing speed is 100 r / min, and the downward pressure is 0.05 mm.

[0125] When the numerical simulation calculation of laser metal deposition is completed, Figure 6 This is the distribution cloud diagram of VonMises equivalent stress, σx, σy and σz when the laser metal deposition is cooled to room temperature. After cooling to room temperature, the residual stress of the workpiece is mainly concentrated in the deposition layer and the area where it is combined with the substrate, while the residual stress in the rest of the substrate is relatively low. The stress value of the deposition layer along the X-axis direction (scanning direction) is relatively large, with a maximum of about 891MPa for σx, and a maximum of about 538MPa for σy along the Y-axis direction (overlap direction), while the stress value along the Z-axis direction (deposition direction) is the smallest, with a maximum of 270MPa for σz.

[0126] After the friction stir composite manufacturing simulation calculation is completed, the residual stress distribution after the friction stir composite processing is explored. Figure 7 It is the distribution cloud diagram of Von Mises equivalent stress, σx, σy and σz after the friction stir processing is cooled to room temperature. It can be seen from the figure that the residual stress in the stirred area after the processing is completed and cooled is significantly lower than the residual stress in the unstirred area.

[0127] After the laser metal deposition and friction stir processing are completed, the z-axis path of the model center and the y-axis path of the upper surface are selected to extract the residual stress curve. Figure 8 As shown, under composite manufacturing, the average residual stress of the substrate and the deposited layer along the direction of the deposited layer is reduced by 27.7%, and the average residual stress at the deposited layer is reduced by 70.7%; under composite manufacturing, the average residual stress at the upper surface of the deposited layer is reduced by 41.1%.

[0128] In this paper, the Bragg equation 2dsinθ=nλ and the generalized Hooke's law σ φ = X-ray diffraction of KM The residual stress is measured by Fig. 9 As shown in a, the average residual stress of the measurement point obtained by simulation is 692.25MPa, while the residual stress obtained by experiment is 634.75MPa, with an error of 8.3%. Therefore, the additive model is effective and feasible; Fig. 9 As shown in (b), the average residual stress of the measurement point obtained by simulation is 257.03MPa, while the residual stress obtained by experiment is 230.54MPa, with an error of 10.3%. Therefore, the friction stir composite manufacturing model is effective and feasible.

[0129] Those skilled in the art will readily appreciate other embodiments of the present invention after considering the specification and practicing the schemes disclosed herein. The present invention is intended to cover any variations, uses or adaptations of the present invention that follow the general principles of the present invention and include common knowledge or customary techniques in the art that are not disclosed in this disclosure. The description and examples are to be considered exemplary only, and the true scope and spirit of the present invention are indicated by the claims.

Claims

1. A thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing, characterized in that: include: Step S1, first, a metal deposition heat load model is constructed using double ellipsoid heat source parameters to establish the thermophysical parameters of TC4 alloy, and then the results of the temperature field calculation model of the same load step are superimposed to obtain the total strain and construct a thermoplastic matrix; Based on the problem of thermoelastic-plastic mechanics, a thermoelastic-plastic geometric model is established; finally, the stress is solved by substituting the provided thermal load into the geometric equation; Step S2, based on the "energy cycle inheritance method", the residual stress of laser metal deposition is loaded, and the result is obtained as the initial condition. The energy cycle inheritance is to assign stress values ​​in the three directions of x / y / z to each node of the finite element model grid; Step S3, finally, construct the stir friction heat load based on the composite heat source parameters of the Gaussian surface heat source and the Gaussian cone heat source, calculate the force load of the stir friction equivalently loaded by the down pressure and shear force during the stir friction processing, establish the thermoplastic geometric equation and the equilibrium differential equation, and solve the composite manufacturing stress.

2. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: In S1, the heat source in the direction of laser metal deposition is divided into two asymmetric ellipses in front and back, with the center point of the heat source heating as the boundary. Due to the influence of the laser processing speed, the heating area in front of the laser is smaller than that behind the laser, resulting in the front and rear heating areas being asymmetric ellipsoidal shapes. The heat source formula of the double ellipsoid heat source is as follows: Where: a r 、a f , b, c represent the semi-axis lengths of the double ellipsoid heat source in different directions; f1 and f2 are the absorptivity of the front and rear hemispheres; Q is the laser power.

3. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: In S1, the thermoplastic geometric model is as follows: the substrate size is 80 mm × 50 mm × 8 mm, the deposition layer size is 50 mm × 20.78 mm × 2 mm, the first deposition size is 50 mm × 3.5 mm × 1 mm, and there are 2 layers and 20 passes in total. After the process parameters are optimized, the overlap rate is selected as 45%, the scanning strategy adopts a zigzag path, the deposition layer grid size is 0.5 × 0.5 × 0.5 mm, and the substrate is a gradient grid of 0.5-2 mm.

4. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: In step S1, the temperature evolution of the deposition layer and the substrate is mainly analyzed, and the temperature field control equation of the same load step is: Where: ρ, λ and C p is the density, thermal conductivity and specific heat of TC4 alloy; T is the temperature function; t is the simulation time; ΔH is the latent heat of phase change, q is the laser energy per unit volume; The heat conduction process follows Fourier's law, and the basic equation is: Where: q is the heat flux, k is the thermal conductivity, is the temperature gradient; Thermal convection and thermal radiation are boundary conditions, and their governing equations are: Where: q1 is the heat convection density, q2 is the heat radiation density, h c is the heat transfer coefficient, T is the ambient temperature function, T0 is the ambient temperature, which is 22℃, is the emissivity, which is taken as 0.8 in the calculation, and x is the Stefan-Boltzman constant.

5. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 4, characterized in that: The temperature field distribution results are imported into the mechanical analysis model as the load of stress analysis to realize the coupled calculation of temperature field and stress field. The stress-strain equation is: e=D -1 s+e th Where: ε is the total strain vector; D is the elastic matrix; σ is the stress vector; ε th is the thermal strain vector. Since the substrate is placed horizontally on the workbench for deposition experiments, a fixed constraint must be added to the bottom surface of the substrate before performing stress field analysis; The thermal strain equation is calculated based on the temperature field distribution and the thermal expansion coefficient of the material: Where: T r is the ambient temperature; α is the thermal expansion coefficient; According to the residual stress characteristics of laser metal deposition and the basic theory of plastic mechanics, the Mises yield criterion is used to obtain the equivalent stress calculation formula:

6. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: In S3, based on the heat generation characteristics of stir friction processing, the heat generation of the stir friction shoulder is usually modeled as a Gaussian surface heat source. The maximum heat flux density of the Gaussian surface heat source decreases exponentially from the center to the outside. The Gaussian plane heat flux expression is: Where: K is the absorption rate of TC4 powder to laser; ω is the laser radius; r is the distance from the starting point to the center of the light source, r 2 =x 2 +y 2 ; Gaussian cone heat flow expression: Where: r e and r i are the maximum and minimum radii of the cone, z e and z i are the maximum and minimum values ​​of the cone in the Z direction respectively.

7. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: The equivalent heat source in step S3 is: Gaussian surface heat source: Gaussian cone heat source:

8. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: The equivalent friction heat in step S3 is: In the friction stir processing, the maximum and minimum radii of the stirring needle are r2 and r3 respectively, and the width of the stirring needle ring is d. r , an infinitesimal cross section with a radius of r0, the tool angular velocity is represented by ω, the friction coefficient is represented by μ, and the downward force during the FSP process is represented by P, then the width is d r The friction df on the infinitesimal cross section can be expressed as: df=ωF=ωPr0dθdr(r3<r0<r2) The relative displacement S of the stirring head per unit time is calculated by the following formula: S=ωr0 The expression of unit friction heat power dq is: dq=sdf=μPωr0 2 dθdr The friction heat Q between the stirring shoulder and the deposited layer surface can be calculated by integrating dq over the ring: The length of the stirring needle is recorded as h, and the friction heat q between it and the side of the deposit layer can be derived as:

9. The thermal-mechanical coupling simulation method for composite manufacturing of laser additive manufacturing and friction stir processing according to claim 1, characterized in that: The medium effective load in step S3 is: The mechanical action exerted by the stirring head has an important influence on the distribution of stress in the FSP process. Considering the downward force and torque of the tool, the downward force is the surface force evenly distributed in the shoulder area, and the torque is the tangential force of the stirring needle on the contact surface. The tangential force is obtained by integrating the surface force in the contact area in the forward direction of the stirring needle. The action of force is realized in the friction stir processing model using the Dload subroutine. The magnitude of each force is expressed as: Where: P is pressure, F is the pressure applied by the stirring head, F x 、F y 、F z is the tangential force applied to the model, F c is the friction force per unit volume, and x and y are the coordinates of the integration point.

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