A friction stir welding defect simulation method and system

Abstract

The present application relates to the technical field of welding numerical simulation, in particular to a friction stir welding defect simulation method and system. The friction stir welding defect simulation method of the present application, when setting the velocity boundary condition of the welding process, establishes a local coordinate system consistent with the actual tilt angle of the stir head, decomposes the velocity vector of the stir head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stir head; finite element analysis is performed on the three-dimensional geometric model of the grid to obtain material flow and welding defect analysis results. By setting a local coordinate system for the stir head, aligning it with the actual tilt direction of the tool, decoupling the rotation and translation motion of the stir head through vector decomposition, decomposing the velocity in the global coordinate system into the component of the local coordinate system to match the actual tilt angle, the degree of restoration of the real working condition is improved, and the accuracy of the model calculation is improved.

Description

Technical Field

[0001] This invention relates to the field of welding numerical simulation technology, and in particular to a method and system for simulating defects in friction stir welding. Background Technology

[0002] Numerical simulation, based on theoretical analysis, applies the concept of the finite element method to perform numerical calculations on practical problems and display the results graphically. Combining numerical simulation with experimental testing can greatly improve research efficiency. In recent years, numerical simulation technology has developed significantly worldwide and has gradually become an important tool guiding welding process research.

[0003] Friction stir welding (FSW) is a novel solid-state welding technology with a series of characteristics such as low defects, low deformation, and high joint quality. Therefore, it is widely used in the welding of aluminum extruded profile boxes for battery packs.

[0004] In existing coupled Euler-Lagrange (CEL) methods for simulating friction stir welding, some researchers neglect the welding tilt angle, while others, although setting the welding tilt angle, keep the stirring head stationary and control the material flow. This results in low computational efficiency and accuracy, hindering effective guidance for selecting welding process parameters and optimizing welding tool design. Summary of the Invention

[0005] The purpose of this invention is to provide a method and system for simulating defects in friction stir welding, so as to improve the computational efficiency and accuracy of the model and provide effective guidance for the selection of welding process parameters and the optimization design of welding tools.

[0006] To address the aforementioned technical problems, this invention provides a method for simulating defects in friction stir welding.

[0007] The simulation method for defects in friction stir welding in this application includes:

[0008] Establish a three-dimensional geometric model of the workpiece, stirring head, and Eulerian domain;

[0009] The three-dimensional geometric model is meshed;

[0010] Assign temperature-dependent thermomechanical physical parameters and constitutive models to the workpiece;

[0011] Set the analysis step;

[0012] Establish a heat source model and set the contact conditions between the workpiece and the stirring head;

[0013] Set the thermal boundary conditions for the welding process;

[0014] Set the speed boundary conditions for the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head.

[0015] Finite element analysis was performed on the meshed three-dimensional geometric model to obtain the analysis results of material flow and welding defects.

[0016] Furthermore, the velocity in the global coordinate system of the stirring head includes the pressing velocity and the welding velocity.

[0017] Furthermore, the Euler domain includes a material region and a vacancy region, with the workpiece located in the material region and the vacancy region used to accommodate material flowing upwards during the welding process.

[0018] Furthermore, tracer particles were placed at different thicknesses of the workpiece before performing the finite element analysis;

[0019] During the finite element analysis, the displacement of the tracer particles is tracked and the element volume fraction of the set area is calculated to determine the relationship between the formation cause of welding defects and the material flow state.

[0020] Furthermore, the step of setting tracer particles at different thicknesses of the workpiece includes:

[0021] Tracer particles are evenly spaced at different thickness layers in the cross-section of the workpiece and cover the workpiece in the thickness direction.

[0022] Further, tracking the displacement of the tracer particle includes:

[0023] Record the displacement of the tracer particles in the X, Y, and Z directions over time, and plot the flow trajectory.

[0024] Furthermore, the defined area includes an empty area, a dynamic flow zone around the stirring head, and a thickness layer containing the tracer particles.

[0025] Furthermore, when the element volume fraction of the set area is within the first set range, the welding defect is a hole or a tunnel defect; when the element volume fraction of the set area is within the second set range, the welding defect is an incomplete penetration or an edge non-fusion defect; the first set range is smaller than the second set range.

[0026] Furthermore, when setting the contact conditions between the workpiece and the stirring head, the coefficient of friction is made to vary with temperature.

[0027] This application also provides a simulation system for defects in friction stir welding, including:

[0028] The 3D geometry modeling module is used to create 3D geometric models of the workpiece, stirring head, and Eulerian domain.

[0029] A mesh generation module is used to divide the three-dimensional geometric model into a mesh.

[0030] The parameter assignment module is used to assign temperature-varying thermomechanical physical parameters and constitutive models to the workpiece;

[0031] The heat source model building module is used to build a heat source model and set the contact conditions between the workpiece and the stirring head;

[0032] The thermal boundary condition setting module is used to set the thermal boundary conditions for the welding process;

[0033] The speed boundary condition setting module is used to set the speed boundary conditions of the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head.

[0034] The analysis module is used to perform finite element analysis on the meshed three-dimensional geometric model and output the analysis results of material flow and welding defects.

[0035] Compared with the prior art, the present invention has at least the following beneficial effects:

[0036] The technical solution of this application solves the technical problem of defect simulation in friction stir welding by a series of steps, including establishing a three-dimensional geometric model, importing it into finite element software, assigning thermomechanical physical parameters and constitutive models, setting analysis steps, setting boundary conditions, establishing a local coordinate system, setting contact relationships and friction coefficients, and performing finite element analysis. Specifically, by setting a local coordinate system for the stirring head and aligning it with the actual tilt direction of the tool, the tilt angle of the stirring head is simulated. Vector decomposition decouples the rotation and translational motions of the stirring head, decomposing the velocity in the global coordinate system into components in the local coordinate system to match the actual tilt angle, thereby improving the fidelity to the real working condition and thus enhancing the accuracy of the model calculation. These technical features work together, and through precise model establishment and parameter setting, the coupled analysis of multiple factors such as temperature, displacement, and contact relationships during the welding process is achieved, effectively simulating the formation process of welding defects and providing a theoretical basis for the optimization of welding technology. Attached Figure Description

[0037] Figure 1 This is a flowchart illustrating an embodiment of the friction stir welding defect simulation method of the present invention;

[0038] Figure 2 This is a schematic diagram of a stirring head model in one embodiment of the friction stir welding defect simulation method of the present invention;

[0039] Figure 3 This is a schematic diagram of the Eulerian domain model in one embodiment of the friction stir welding defect simulation method of the present invention;

[0040] Figure 4 This is a diagram showing the relationship between equivalent shear stress and contact pressure in a modified Coulomb's friction law, as presented in one embodiment of the friction stir welding defect simulation method of the present invention.

[0041] Figure 5 This is a schematic diagram of the workpiece boundary conditions in one embodiment of the friction stir welding defect simulation method of the present invention;

[0042] Figure 6 This is a schematic diagram of the decomposition of the boundary velocity vector of the stirring head in one embodiment of the simulation method for friction stir welding defects of the present invention;

[0043] Figure 7 This is a schematic diagram of heat dissipation of a three-dimensional geometric model in one embodiment of the simulation method for friction stir welding defects of the present invention;

[0044] Figure 8 This is a schematic diagram of the arrangement of tracer particles in one embodiment of the friction stir welding defect simulation method of the present invention;

[0045] Figure 9 This is a schematic diagram of the arrangement of tracer particles from another perspective in one embodiment of the friction stir welding defect simulation method of the present invention;

[0046] Figure 10 This is an example of the simulation method for simulating defects in friction stir welding of the present invention, showing the simulated plastic strain distribution and the comparison between the defects and metallographic test results of the cross-section of the friction stir welded joint at a rotation speed of 500 rpm.

[0047] Figure 11 This is a comparison diagram of simulated plastic strain distribution and defects with metallographic test results of a friction stir welding joint cross-section at a rotation speed of 1000 rpm, as shown in one embodiment of the friction stir welding defect simulation method of the present invention.

[0048] Figure 12 This is a spatial position diagram of tracer particles at 2.25 seconds during a 500 rpm welding process, as shown in one embodiment of the friction stir welding defect simulation method of the present invention.

[0049] Figure 13 This is a spatial position diagram of tracer particles at 3.75 seconds during a 500 rpm welding process, as shown in one embodiment of the friction stir welding defect simulation method of the present invention.

[0050] Figure 14This is a spatial position diagram of tracer particles at 4.5 seconds during a 500 rpm welding process, as shown in one embodiment of the friction stir welding defect simulation method of the present invention.

[0051] Figure 15 This is a spatial position diagram of tracer particles at 5.25 seconds during a 500 rpm welding process, as shown in one embodiment of the friction stir welding defect simulation method of the present invention.

[0052] Figure 16 This is a spatial position diagram of tracer particles at 6 seconds during a welding process at 500 rpm, as shown in one embodiment of the friction stir welding defect simulation method of the present invention. Detailed Implementation

[0053] The simulation method and system for friction stir welding defects of the present invention will be described below with reference to schematic diagrams, which illustrate preferred embodiments of the invention. It should be understood that those skilled in the art can modify the invention described herein while still achieving its advantageous effects. Therefore, the following description should be understood as being of general knowledge to those skilled in the art and is not intended to limit the invention.

[0054] The component designations used in this document, such as "first" and "second," are merely for distinguishing the described objects and do not have any sequential or technical meaning. The terms "connection" and "linkage" used in this application, unless otherwise specified, include both direct and indirect connections (linkages). In the description of this invention, it should be understood that the terms "upper," "lower," "front," "rear," "left," "right," "vertical," "horizontal," "top," "bottom," "inner," "outer," "clockwise," and "counterclockwise," etc., indicate the orientation or positional relationship based on the orientation or positional relationship shown in the accompanying drawings. They are used solely for the convenience of describing the invention and for simplifying the description, and do not indicate or imply that the device or element referred to must have a specific orientation, or be constructed and operated in a specific orientation. Therefore, they should not be construed as limitations on the invention.

[0055] In this invention, unless otherwise explicitly specified and limited, "above" or "below" the second feature can mean that the first feature is in direct contact with the second feature, or that the first feature is in indirect contact with the second feature through an intermediate medium. Furthermore, "above," "over," and "on top" of the second feature can mean that the first feature is directly above or diagonally above the second feature, or simply that the first feature is at a higher horizontal level than the second feature. "Below," "below," and "under" the second feature can mean that the first feature is directly below or diagonally below the second feature, or simply that the first feature is at a lower horizontal level than the second feature.

[0056] The invention is described more specifically by way of example in the following paragraphs with reference to the accompanying drawings. The advantages and features of the invention will become clearer from the following description and claims. It should be noted that the drawings are in a very simplified form and use non-precise proportions, and are only used to facilitate and clarify the illustration of the embodiments of the invention.

[0057] The following is in conjunction with the instruction manual appendix. Figure 1 To be continued Figure 16 The present invention introduces the simulation method for defects in friction stir welding.

[0058] The inventors discovered that in existing technologies for simulating friction stir welding, some researchers neglect the welding tilt angle, while others, although setting the welding tilt angle, keep the stirring head stationary and control the material flow. However, this does not conform to the actual stirring process and leads to an increase in the volume of the Eulerian domain containing the material, resulting in low computational efficiency and accuracy.

[0059] Further analysis by the inventors revealed that during simulation analysis, the motion of the stirring head can only be defined in the global coordinate system, such as moving directly along the Z-axis. If the stirring head is tilted, its downward pressure and welding direction will be inconsistent with the global coordinate system axis. Directly applying tilting motion to the stirring head in the global coordinate system will cause the velocity direction to mismatch with the mesh, resulting in an incorrect motion direction, which in turn makes subsequent simulation analysis impossible. Therefore, it is difficult to make the stirring head have both a welding tilt angle and simulate the motion of real conditions during simulation.

[0060] To address the aforementioned problems, this application provides a method for simulating defects in friction stir welding, which simulates the welding tilt angle by setting up a local coordinate system and velocity vector decomposition. Specifically, the method for simulating defects in friction stir welding in this application is as follows: Figure 1 As shown, it includes the following steps:

[0061] S100: Establish a three-dimensional geometric model of the workpiece, stirring head, and Eulerian domain;

[0062] S200: Mesh the three-dimensional geometric model;

[0063] S300: Assigns temperature-dependent thermomechanical physical parameters and constitutive models to the workpiece;

[0064] S400: Set the analysis step;

[0065] S500: Establish a heat source model and set the contact conditions between the workpiece and the stirring head;

[0066] S600: Sets the thermal boundary conditions for the welding process;

[0067] S700: Set the velocity boundary conditions for the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head.

[0068] S800: Perform finite element analysis on the meshed three-dimensional geometric model to obtain the analysis results of material flow and welding defects.

[0069] By setting a local coordinate system for the stirring head and aligning it with the actual tilt direction of the tool, the tilt angle of the stirring head is simulated. The rotation and translation motion of the stirring head are decoupled through vector decomposition, and the velocity in the global coordinate system is decomposed into components in the local coordinate system to match the actual tilt angle, thereby improving the fidelity to the real working condition and thus improving the accuracy of the model calculation.

[0070] In one embodiment, in step S200, the velocity in the global coordinate system of the stirring head includes the pressing velocity and the welding velocity.

[0071] Specifically, the downward pressure speed refers to the speed at which the stirring head moves in the direction perpendicular to the workpiece surface, while the welding speed refers to the speed at which the stirring head moves along the welding path. This clear distinction and definition of the two speeds helps to more accurately decompose the velocity vector of the stirring head from its global coordinate system into the local coordinate system when establishing the local coordinate system, thereby matching the actual tilt angle of the stirring head.

[0072] In one embodiment, in step S100, the Euler domain includes a material region and a vacancy region, the workpiece is located in the material region, and the vacancy region is used to accommodate material flowing upward during the welding process.

[0073] Specifically, the division of the Eulerian domain allows the material region and the vacancy region to simulate the material flow during the workpiece and welding process, respectively. The material region is mainly used to simulate the initial state of the workpiece and the material distribution during welding, while the vacancy region is used to accommodate the upward flow of material during welding, thus more accurately simulating the material flow behavior during welding.

[0074] In one embodiment, tracer particles are placed at different thicknesses of the workpiece before finite element analysis is performed;

[0075] During the finite element analysis, the displacement of the tracer particles is tracked and the element volume fraction of the set area is calculated to determine the relationship between the formation cause of welding defects and the material flow state.

[0076] Specifically, to improve accuracy when setting tracer particles, they are evenly spaced at different thicknesses across the workpiece cross-section, covering the workpiece in the thickness direction. This allows for effective tracking of the tracer particle displacement during finite element analysis, enabling the analysis of the relationship between material flow and the causes of welding defects. This setup allows for more accurate simulation and prediction of material flow behavior during welding, providing reliable data support for optimizing the welding process.

[0077] The tracer particles can be set up by uniformly distributing them at predetermined intervals across the cross-section of the workpiece in each thickness layer. These tracer particles can be tiny markers whose positions are precisely recorded and tracked during finite element analysis. In the thickness direction, the distribution of tracer particles should cover the entire thickness of the workpiece to ensure that the flow behavior of the material at different thickness layers can be captured during the analysis.

[0078] As a preferred embodiment, the set area may include an empty area, a dynamic flow zone around the stirring head, and a thickness layer containing the tracer particles.

[0079] The dynamic flow zone around the stirring head is typically located within a radius of 3 mm near the tip and sidewall of the stirring head.

[0080] By analyzing the elemental volume fraction in these regions, the relationship between the formation of welding defects and the material flow state can be further determined. For example, when the elemental volume fraction in a set region is within a first set range, the welding defect may be a hole or a tunnel defect; when the elemental volume fraction is within a second set range, the welding defect may be an incomplete penetration or an edge fusion defect. Furthermore, the first set range is smaller than the second set range. This method improves the accuracy and efficiency of welding defect detection through quantification.

[0081] For example, the first set range is greater than 0 and less than 0.5, and the second set range is greater than or equal to 0.5 and less than 0.8. The smaller the element volume fraction, the worse the material filling at the current location, and the more serious the welding defect. In other embodiments, the first set range and the second set range can also be set to other ranges as needed.

[0082] As a preferred implementation, the tracer particles can be precisely arranged using computer-aided design software to ensure that their position and distribution in the workpiece meet the preset uniform spacing requirements.

[0083] Because the vacancy layer allows material to flow freely and avoids mesh distortion interfering with the displacement data of the tracer particles, the layered tracer particles can capture the true flow trajectory of the material. Combined with element volume fraction analysis, the filling status of different areas of the workpiece can be quantified, thereby accurately locating defects.

[0084] Preferably, when tracking the displacement of the tracer particles, the displacement of the tracer particles in the X, Y, and Z directions is recorded over time to plot the flow trajectory. For example, the displacement of the tracer particles can be realized using a particle tracking module in finite element software, which can record the position changes of the particles in real time and generate corresponding displacement data. Furthermore, this displacement data can be used to plot the flow trajectory diagram, thereby visually displaying the flow state of the material during the welding process.

[0085] In one embodiment, finite element analysis time is optimized by mass scaling in order to reduce finite element analysis time.

[0086] Mass scaling is a technique used in finite element analysis to accelerate computation. By adjusting the mass distribution of the model, computation time can be reduced without significantly affecting the accuracy of the results. Specifically, mass scaling can be achieved by adjusting the mass matrix of the model, which can be done by changing the material density or adjusting the time step. For example, in the simulation of defects in friction stir welding, the computation process can be accelerated by increasing the material density or decreasing the time step, thereby reducing the time required for finite element analysis.

[0087] As a preferred implementation, mass scaling can be achieved by setting a mass scaling factor in the finite element software. The purpose of mass scaling is to reduce the settling time increment and improve computational efficiency by introducing an imaginary density. During mass scaling, thermal properties are adjusted to maintain the thermal time constant, making it suitable for velocity-dependent parameters.

[0088] In one embodiment, the coefficient of friction is varied with temperature when setting the contact conditions between the workpiece and the stirring head.

[0089] Specifically, firstly, based on the temperature variation range during welding, the variation curve or functional relationship of the friction coefficient is determined. For example, the friction coefficient can be set as a linear function, exponential function, or other nonlinear function of temperature to reflect the dynamic characteristics of the friction coefficient changing with temperature during actual welding. Secondly, the change in the friction coefficient can be calibrated using experimental data or theoretical models to ensure its accuracy under different temperature conditions. Furthermore, the setting of the friction coefficient can also be combined with the material property module in finite element analysis software, allowing for dynamic adjustment of the friction coefficient by inputting temperature-related friction coefficient data.

[0090] The setting of friction coefficient variation with temperature can be achieved through the following steps: First, define the relationship between friction coefficient and temperature in the finite element analysis software, for example, through table input or function expression. Second, during the welding process, monitor the temperature change of the workpiece in real time, and dynamically adjust the friction coefficient according to the preset relationship between friction coefficient and temperature. Furthermore, the dynamic adjustment of the friction coefficient can be achieved through the field variable function in the finite element analysis software, where temperature is the field variable and the friction coefficient is a function of temperature. Therefore, the dynamic change of the friction coefficient can more accurately simulate the change of friction coefficient during actual welding, thereby improving the accuracy and reliability of welding simulation.

[0091] The technical solutions in the embodiments of this application will be described more clearly and completely below with reference to the accompanying drawings.

[0092] Taking the finite element analysis of the friction stir welding process of 6061-T6 aluminum alloy liquid-cooled base plate as an example, the simulation method for friction stir welding defects includes the following steps:

[0093] S1: Establish a three-dimensional geometric model of the 6061-T6 aluminum alloy workpiece, stirring head, and Euler domain. Import the established three-dimensional geometric model into Abaqus finite element analysis software and mesh it.

[0094] The stirring head was meshed using four-node rigid body elements (C3D4T), with a total mesh size of 51598. To improve computational efficiency, it was constrained as a rigid body, such as... Figure 2 As shown.

[0095] The welding workpiece is a three-dimensional Euler body with Euler domain dimensions of 80mm × 70mm × 6mm, meshed using thermally coupled Euler elements (EC3D8RT). The central welding region is meshed with a 0.5mm mesh, while the outer regions are meshed with a 1mm mesh, for a total of 172,800 meshes. Furthermore, the Euler domain is divided into two parts to define a vacancy region (red) and a material region (blue). The vacancy region is not assigned material properties; its purpose is to accommodate material flowing upwards during the downward pressure of the stirring head. The thicknesses of the vacancy region and the material region are 1mm and 5mm, respectively. Figure 3 As shown.

[0096] S2: Assign the thermomechanical physical parameters and constitutive model that vary with temperature to the workpiece, establish the heat source model, and set the contact conditions between the workpiece and the stirring head.

[0097] Among them, since complex material flow occurs during friction stir welding, the accurate determination of flow stress is of great significance to the accuracy of numerical simulation. The Johnson-Cook plastic material flow model is a temperature, strain, and strain rate related viscoplastic model. Therefore, this application adopts the Johnson-Cook plastic material flow model as the constitutive model, and the flow stress is obtained by the following equation (1).

[0098]

[0099] In the formula:

[0100] In the constitutive model of 6061 aluminum alloy, the material parameters are A = 324, B = 114, C = 0.002, n = 0.42, and m = 1.34. For equivalent plastic strain; T melt T is the melting point of the material. ref The ambient temperature.

[0101] Heat is generated during friction stir welding. The heat mainly comes from the friction between the stirring head and the workpiece, as well as the plastic deformation of the workpiece. The heat generated by friction between the stirring head and the workpiece can be calculated by formula (2):

[0102]

[0103] In the formula:

[0104] q f Surface heat flux density; τ is the shear rate; τ is the shear stress generated by friction, which is expressed by formula (3) according to the classical Coulomb friction law:

[0105] τ=μP (3)

[0106] In the formula:

[0107] μ is the coefficient of friction; P is the normal pressure between the tool head and the workpiece at the contact surface.

[0108] At low speeds, classical Coulomb friction yields accurate output results. However, at high speeds, the output results may be inaccurate due to the increased dynamic effects of the stirring head. Therefore, this application employs a modified Coulomb friction law. If the contact shear stress is less than the maximum shear stress, viscosity exists. If the contact shear stress is greater than the maximum shear stress, sliding occurs. This application considers the plastic deformation behavior of the welded materials at the contact interface, where the contact interface shear stress (equivalent shear stress) reaches the maximum shear stress τ. max No further additions will be made, such as Figure 3 As shown.

[0109] The heat generated by plastic deformation is calculated using formula (4):

[0110]

[0111] In the formula: q pl η is the volumetric heat flux density; η is the rate coefficient that determines the conversion of energy from plastic deformation to temperature; it is taken as 0.9 in the software; σ fs For stress; This represents the plastic strain rate.

[0112] The calculation of heat generation during friction stir welding depends primarily on selecting an accurate coefficient of friction. This application uses a coefficient of friction that varies with temperature, as shown in Table 2.

[0113] Table 2

[0114]

[0115] S3: Set up three analysis steps: pressing, preheating, and welding, and couple temperature and displacement in each analysis step, and check the field output data.

[0116] Assemble the model. During assembly, the bottom of the stirring needle is fixed to the surface of the plate by moving and rotating it. Set a suitable tilt angle; in this embodiment, the tilt angle between the axis of the stirring head and the vertical direction is set to 2.5°. Set three analysis steps: pressing, preheating, and welding. Select "Dynamics, Temperature-Displacement, Explicit" for the step, and check the required data options in the field output.

[0117] The pressing process involves the stirring head rotating around its axis and moving vertically downwards; the preheating process involves the stirring head rotating around its axis and remaining stationary; and the welding process involves the stirring head rotating around its axis and moving horizontally to the left.

[0118] S4: Set the boundary conditions for the welding process.

[0119] The boundary conditions include velocity boundary conditions and thermal boundary conditions.

[0120] Regarding velocity boundary conditions, in the friction stir welding process, a normal velocity is set to restrict material flow, constraining all degrees of freedom on the bottom and side surfaces of the workpiece, while the top surface remains free. Figure 5 As shown.

[0121] Because the stirring head has an inclination angle in actual working conditions, a local coordinate system Rp is set at its reference point. During the pressing phase, the pressing speed along the y-axis of the global coordinate system and the rotational speed around the x-axis of the local coordinate system are given; during the preheating phase, the pressing speed along the y-axis is set to 0; during the welding phase, the welding speed along the z-axis of the global coordinate system and the rotational speed around the x-axis of the local coordinate system need to be given. In Abaqus software, only the same coordinate system can be used when setting the same boundary condition. Therefore, the pressing and welding speeds in the global coordinate system are transformed into the local coordinate system using vector decomposition, such as... Figure 6 As shown.

[0122] In this application, during the pressing process, the tilt angle between the axis of the stirring head and the vertical direction is set to 2.5°, then the rotational speed VRx (local) = VR1 (local) = 83.76 rad / s;

[0123] Regarding the downward pressure speed, the VY (global coordinate system) is transformed into two local coordinate system components, specifically:

[0124] Vx(local) = V1(local) = VY(global) * cos(2.5°) = 1 mm / s * cos(2.5°) = 0.999 mm / s; Vy(local) = V2(local) = -VY(global) * sin(2.5°) = -1 mm / s * sin(2.5°) = -0.0436 mm / s.

[0125] Similarly, during the welding process:

[0126] Vx (local) = V1 (local) = VZ (global) * sin(2.5°) = 4.99 mm / s * sin(2.5°) = 0.2175 mm / s;

[0127] Vy (local) = V2 (local) = VZ (global) * cos(2.5°) = 4.99 mm / s * cos(2.5°) = 4.995 mm / s.

[0128] Regarding thermal boundary conditions, during friction stir welding, thermal convection and thermal radiation occur simultaneously on the upper and side surfaces of the workpiece, requiring the application of thermal boundary conditions at the corresponding locations. Since radiation heat transfer is complex, it is typically converted into convective heat transfer. Therefore, the total heat flux density of convection and radiation on the upper and side surfaces of the workpiece is given by equation (5):

[0129] Q total =h total (T a -T) (5)

[0130] Where: h total The overall heat transfer coefficient is taken as a constant of 150 W / (m²·K), T aT represents the ambient temperature, while T represents the surface temperature of the object.

[0131] The heat transfer between the workpiece bottom surface and the pad is closely related to various factors, including the inherent properties of the workpiece, the temperature and pressure of the contact surface, and the contact state. In practice, it is difficult for the workpiece bottom surface to completely adhere to the pad, and heat conduction, heat radiation, and heat convection coexist, making the working conditions quite complex. Therefore, the thermal boundary condition at the bottom of the workpiece is simplified to a convective heat transfer expression, with a heat transfer coefficient of 1000 W / (m²·K). Figure 7 This is a schematic diagram of the model's heat dissipation.

[0132] S5: Place tracer particles at different thicknesses of the workpiece.

[0133] To track the material flow trajectory, tracer particles are placed at different thicknesses on the workpiece. Specifically, the selected tracer particle plane is located on the cross-section of the workpiece during the steady-state welding stage, and is distributed as follows: Figure 8 and Figure 9 As shown, firstly, a set of 5 nodes spaced 0.5mm apart was set along the cross-sectional direction of the weld, covering the layer thickness direction of the workpiece. Secondly, the node displacements were output by modifying the inp file.

[0134] S6: Optimize finite element analysis time through mass scaling.

[0135] When simulating the FSW process, mass scaling techniques can be used to optimize simulation time. The purpose of mass scaling is to reduce the settling time increment and improve computational efficiency by introducing a hypothetical density. In this method, thermal properties are adjusted to maintain the thermal time constant, making it suitable for velocity-dependent parameters. First, the overall computation time is determined by the minimum settling time increment Δt; a larger Δt value results in faster computation. The calculation formula is as follows:

[0136]

[0137] In the formula, L e Minimum cell length; C d Let L be the expansion wave velocity of the material; E be the Young's modulus of the material; and ρ be the density of the material. If the mesh size is already determined, then L... e Since Δt is constant, the material density needs to be increased to increase Δt.

[0138] Navis's equation is used to describe the thermoelasticity of materials, as shown in the following equation:

[0139]

[0140] In the formula: u is the displacement vector; E is the linear strain tensor; α is the coefficient of thermal expansion; ρ is the density. λ and μ are the Ram coefficients. When the density ρ is replaced by the artificial density, the artificial density ρ* = kmρ (km>1), and a correction for specific heat capacity is introduced for compensation. In this application, km is taken as 1×10⁵.

[0141] S7: Perform finite element analysis on a three-dimensional thermo-mechanical coupling model of the friction stir welding process, and output the analysis results of material flow and welding defects.

[0142] After the three-dimensional thermo-mechanical coupling model is established, it is submitted for calculation and post-processing analysis. The output results of material flow and welding defect analysis are as follows: Figure 10 , Figure 11 As shown.

[0143] Figure 10 and Figure 11 Simulated plastic strain distribution and defects in the cross-section of friction stir welded joints at speeds of 500 rpm and 1000 rpm were compared with metallographic experiments. Figure 10 As shown in the attached figure (a), when a lower stirring head speed is used, uneven plastic deformation occurs during welding, leading to insufficient mixing of the softened metal during the welding stage and the formation of voids. Both the simulated defects and the defects in the metallographic test results are located near the tip of the stirring pin on the weld advancement side, and their sizes are almost identical. Figure 11 As shown, when the rotation speed is increased to 1000 rpm, the plastic deformation and heat generation in the welding area are enhanced, the fluidity of the softened material is improved, and the pore defects are completely eliminated. Figure 11 As shown in the attached figure (b), the plastic deformation in the simulation results is more uniform and no defects are generated, which is completely consistent with the experimental results.

[0144] Figures 12-16 This demonstrates the spatial position changes of tracer particles at different welding times during a 500 rpm welding process. Three viewing angles were used for observation at each welding moment: a top-down view, a 3D view, and a frontal view. Due to the extremely complex material flow during FSW, with different flow states between the upper and lower layers, and between the advancing and returning sides, the top-down view was divided into four parts: upper returning side, upper advancing side, lower advancing side, and lower returning side, corresponding to the four regions ①, ②, ③, and ④ of the frontal view, respectively. Red arrows 1-9 point to the tracer particles. Figure 13-16 The four corresponding areas from the top-down perspective and Figure 12 Regions ①, ②, ③, and ④ are the same.

[0145] When t = 2.25s (e.g.) Figure 12As shown in the diagram, once the stirring head encounters a layer of tracer particles, it causes the particles' spatial positions to change. Because the shoulder presses downwards into the workpiece a certain distance, some tracer particles near the outer edge of the shoulder begin to move upwards, forming flash, as shown by red arrow 1. The spatial positions of tracer particles in other layers remain unchanged.

[0146] When t = 3.75s (e.g.) Figure 13 As shown, the leading edge of the stirring needle begins to contact the tracer particle layer. More particles from the first layer are propelled upwards by the shoulder and flow towards the front of the stirring head, forming a flash at the outer edge of the shoulder. The particles on the upper advancing side begin to migrate upwards under the influence of the stirring needle and flow towards the returning side, as shown by red arrow 2.

[0147] When t = 4.5s (e.g.) Figure 14 As shown, the shoulder and leading edge of the stirring needle come into contact with all the tracer particle layers, and the material flow becomes increasingly intense. The upper return-side particles spiral upwards and backwards along the direction of the stirring needle's rotation, then are deposited downwards by the forging action of the inclined shoulder, distributing in the region behind the stirring needle; some particles even reach the advancing-side region, as shown by red arrow 3. Meanwhile, the upper advancing-side particles follow the stirring needle upwards in a spiral motion, migrating a greater distance and distributing near the edge of the stirring needle. Compared to the upper particles, the flow capacity of the lower tracer particles is significantly reduced. The lower advancing-side particles follow the stirring needle spiral upwards to the front of the stirring needle, then migrate to the return-side region, while the lower return-side particles show almost no flow, as shown by red arrows 3 and 4.

[0148] When t = 5.25s (e.g.) Figure 15 As shown in the diagram, the contact area between the stirring needle and the tracer particle layer further increases. More upper-layer return-side tracer particles follow the rotation direction of the stirring needle and spiral upwards backwards. Some upper-layer return-side particles continue to migrate and flow to the position shown by red arrow 5. Most upper-layer forward-side particles follow the stirring needle for several revolutions and are evenly distributed near the edge of the stirring needle. Compared to the welding time of 4.5s, the migration distance of the lower-layer particles has increased significantly at this time. Some lower-layer return-side particles spiral upwards backwards, as shown by red arrow 6. Similarly, lower-layer forward-side particles move to the front of the stirring needle and then spiral upwards backwards to the return-side region, with a migration distance greater than that of lower-layer return-side particles, as shown by red arrow 7.

[0149] When t = 6s (e.g.) Figure 16As shown in the diagram, the trailing edge of the stirring needle begins to leave the monitoring plane where the tracer particles are located. Upper-layer return-side particles begin to accumulate behind the stirring needle, with a few particles remaining in the forward-side region, while most of the upper-layer forward-side particles flow back to the forward-side region. Lower-layer return-side particles continue to flow, about to reach the weld centerline behind the stirring needle, as shown by red arrow 8. Lower-layer forward-side particles continue to flow, with most remaining in the return-side region and a small portion distributed near the weld centerline, as shown by red arrow 9.

[0150] Throughout the process of the stirring head passing through the tracer particle region, material near the shoulder flows preferentially, followed by material near the stirring pin, but the flow is most vigorous, especially the upper advancing side material near the stirring pin. Horizontally, the advancing side material moves significantly more vigorously than the returning side material, particularly some upper advancing side particles that flow around the stirring pin several times before settling behind it. The advancing and returning side material flows exhibit cross-flow and mixed characteristics. After the stirring head passes through the tracer particle region, the particles are distributed near the weld centerline behind the stirring head, filling the gap behind the stirring head to form the weld. Vertically, the returning side material flow process involves most of the material initially flowing slightly upwards to the rear of the stirring pin, then flowing downwards and depositing. The advancing side material flow occurs in two forms: the upper advancing side material initially flows slightly downwards to the front of the stirring pin, then flows upwards to the rear of the stirring pin and deposits downwards; the lower advancing side material follows a similar flow pattern to the returning side material. The advancing and returning side materials complement each other under mutual suction and compression, forming a complementary "crossing" and "interlocking" interface.

[0151] In summary, the technical solution of this application solves the technical problem of defect simulation in friction stir welding by a series of steps, including establishing a three-dimensional geometric model, importing it into finite element software, assigning thermomechanical physical parameters and constitutive models, setting analysis steps, setting boundary conditions, establishing a local coordinate system, setting contact relationships and friction coefficients, and performing finite element analysis. Specifically, by setting a local coordinate system for the stirring head and aligning it with the actual tilt direction of the tool, the tilt angle of the stirring head is simulated. Vector decomposition decouples the rotation and translational motions of the stirring head, decomposing the velocity in the global coordinate system into components in the local coordinate system to match the actual tilt angle, thereby improving the fidelity to the real working conditions and thus enhancing the accuracy of model calculations. These technical features work together, and through precise model establishment and parameter settings, the coupled analysis of multiple factors such as temperature, displacement, and contact relationships during the welding process is achieved, effectively simulating the formation process of welding defects and providing a theoretical basis for the optimization of welding processes.

[0152] This application also provides a simulation system for defects in friction stir welding, including:

[0153] The 3D geometry modeling module is used to create 3D geometric models of the workpiece, stirring head, and Eulerian domain.

[0154] A mesh generation module is used to divide the three-dimensional geometric model into a mesh.

[0155] The parameter assignment module is used to assign temperature-varying thermomechanical physical parameters and constitutive models to the workpiece;

[0156] The heat source model building module is used to build a heat source model and set the contact conditions between the workpiece and the stirring head;

[0157] The thermal boundary condition setting module is used to set the thermal boundary conditions for the welding process;

[0158] The speed boundary condition setting module is used to set the speed boundary conditions of the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head.

[0159] The analysis module is used to perform finite element analysis on the meshed three-dimensional geometric model and output the analysis results of material flow and welding defects.

[0160] This application presents a simulation system for defects in friction stir welding. Through a series of steps, including establishing a three-dimensional geometric model, importing it into finite element software, assigning thermomechanical physical parameters and constitutive models, setting analysis steps, setting boundary conditions, establishing a local coordinate system, setting contact relationships and friction coefficients, and performing finite element analysis, the system solves the technical problem of defect simulation in friction stir welding. Specifically, by setting a local coordinate system for the stirring head and aligning it with the actual tilt direction of the tool, the tilt angle of the stirring head is simulated. Vector decomposition decouples the rotation and translational motions of the stirring head, decomposing the velocity in the global coordinate system into components in the local coordinate system to match the actual tilt angle, thereby improving the fidelity to the real working conditions and thus enhancing the accuracy of the model calculations. These technical features work together, and through precise model establishment and parameter settings, the coupled analysis of multiple factors such as temperature, displacement, and contact relationships during the welding process is achieved, effectively simulating the formation process of welding defects and providing a theoretical basis for the optimization of welding processes.

[0161] Obviously, those skilled in the art can make various modifications and variations to this invention without departing from its spirit and scope. Therefore, if these modifications and variations fall within the scope of the claims of this invention and their equivalents, this invention also intends to include these modifications and variations.

Claims

1. A method for simulating defects in friction stir welding, characterized in that, include: Establish a three-dimensional geometric model of the workpiece, stirring head, and Eulerian domain; The three-dimensional geometric model is meshed; Assign temperature-dependent thermomechanical physical parameters and constitutive models to the workpiece; Set the analysis step; Establish a heat source model and set the contact conditions between the workpiece and the stirring head; Set the thermal boundary conditions for the welding process; Set the speed boundary conditions for the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head. Finite element analysis was performed on the meshed three-dimensional geometric model to obtain the analysis results of material flow and welding defects.

2. The method for simulating defects in friction stir welding according to claim 1, characterized in that, The velocities in the global coordinate system of the stirring head include the downward pressure velocity and the welding velocity.

3. The method for simulating defects in friction stir welding according to claim 1, characterized in that, The Euler domain includes a material region and a void region, with the workpiece located in the material region and the void region used to accommodate the material flowing upwards during the welding process.

4. The method for simulating defects in friction stir welding according to claim 3, characterized in that, Before performing finite element analysis, tracer particles are placed at different thicknesses of the workpiece. During the finite element analysis, the displacement of the tracer particles is tracked and the element volume fraction of the set area is calculated to determine the relationship between the formation cause of welding defects and the material flow state.

5. The method for simulating defects in friction stir welding according to claim 4, characterized in that, The step of setting tracer particles at different thicknesses of the workpiece includes: Tracer particles are evenly spaced at different thickness layers in the cross-section of the workpiece and cover the workpiece in the thickness direction.

6. The method for simulating defects in friction stir welding according to claim 4, characterized in that, The tracking of the displacement of the tracer particle includes: Record the displacement of the tracer particles in the X, Y, and Z directions over time, and plot the flow trajectory.

7. The method for simulating defects in friction stir welding according to claim 4, characterized in that, The defined area includes an empty area, a dynamic flow zone around the stirring head, and a thickness layer containing the tracer particles.

8. The method for simulating defects in friction stir welding according to claim 4, characterized in that, When the element volume fraction of the set area is within a first set range, the welding defect is a hole or a tunnel defect; when the element volume fraction of the set area is within a second set range, the welding defect is an incomplete penetration or an edge non-fusion defect; the first set range is smaller than the second set range.

9. The method for simulating defects in friction stir welding according to claim 1, characterized in that, When setting the contact conditions between the workpiece and the stirring head, the coefficient of friction is made to vary with temperature.

10. A simulation system for defects in friction stir welding, characterized in that, include: The 3D geometry modeling module is used to create 3D geometric models of the workpiece, stirring head, and Eulerian domain. A mesh generation module is used to divide the three-dimensional geometric model into a mesh. The parameter assignment module is used to assign temperature-varying thermomechanical physical parameters and constitutive models to the workpiece; The heat source model building module is used to build a heat source model and set the contact conditions between the workpiece and the stirring head; The thermal boundary condition setting module is used to set the thermal boundary conditions for the welding process; The speed boundary condition setting module is used to set the speed boundary conditions of the welding process, establish a local coordinate system consistent with the actual tilt angle of the stirring head, and decompose the velocity vector of the stirring head in the global coordinate system into the local coordinate system to match the actual tilt angle of the stirring head. The analysis module is used to perform finite element analysis on the meshed three-dimensional geometric model and output the analysis results of material flow and welding defects.

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