Helicopter fuselage thin-wall structure welding deformation optimization method based on reverse engineering and finite element method

Abstract

The invention discloses a helicopter fuselage thin-wall structure welding deformation optimization method based on reverse engineering and a finite element method, belongs to the technical field of welding numerical simulation, and is suitable for welding deformation and dimensional deviation control of helicopter thin-wall weldments with more welding seams and complex structures. According to the method, a material model and a heat source model are simulated and checked through a welding process optimization experiment of a test piece, and a welding simulation model of a machine body thin-wall structure under a reference process is established based on a thermal-elastic-plastic finite element method; the method comprises the steps that a simulation model is established, actual welding deformation of a fuselage thin-wall structure is measured through the three-dimensional scanning and reverse reconstruction technology to verify the simulation model, then virtual welding procedure optimization is conducted, the optimal procedure is obtained, and finally the deformation optimization effect of the optimal procedure is verified again through the three-dimensional scanning and reverse reconstruction technology. The welding deformation optimization method is applied to the helicopter fuselage airflow channel structure, the 50% deformation optimization effect is achieved, and the feasibility of the method is verified.

Description

Technical Field

[0001] A method for optimizing the welding deformation of helicopter thin-walled structures based on reverse engineering and the finite element method proposed by the present invention belongs to the technical field of welding numerical simulation and is applicable to the control of welding deformation and dimensional deviation of helicopter thin-walled welded parts with a large number of welds and complex structures. Background Art

[0002] With the development of manufacturing technology, newly developed helicopter models require extremely high dimensional accuracy of welded components during the production process. The welded components of helicopters are concentrated in the fuselage, and due to considerations of hydrodynamic design, manufacturing feasibility, and weight reduction, they are usually designed as thin-walled structures with complex geometric shapes and composed of multiple sheet metal parts welded together. After experiencing multiple welding thermal processes, the fuselage thin-walled structure often has large welding deformations. If not controlled, it will affect the geometric accuracy after the helicopter fuselage is assembled, and further reduce the flight performance indicators designed during the helicopter development. For the thin-walled structure of the helicopter fuselage with a large number of welds and complex structures, carrying out welding process optimization is an important means to control welding deformation and improve dimensional accuracy. Using the trial-and-error method based on experiments for such welding process optimization has the disadvantages of long cycle and high cost. In contrast, the welding thermo-elasto-plastic simulation method based on the finite element method can not only reproduce the actual welding process of the welded parts, predict the distribution characteristics of welding temperature and stress deformation, but also has the advantages of short cycle and low cost, and has gradually become an important method for researching welding process optimization and controlling welding deformation. However, when using the welding thermo-elasto-plastic simulation method to carry out welding process optimization, it is necessary to strictly verify the reference process. Only by carrying out process optimization design and simulation based on the verified reference process, the obtained results can have credibility and engineering application value.

[0003] At present, for the verification of welding thermo-elasto-plastic simulation results, most domestic and foreign scholars have carried out research from aspects such as weld appearance, welding thermal cycle, and welding residual stress. Since the optimization process of weld forming quality cannot be achieved by welding thermo-elasto-plastic simulation, necessary weld forming quality optimization experiments need to be carried out before simulation to obtain the welding process parameters with the best weld forming, such as welding current, welding voltage, and welding speed, etc., for calculating the welding heat input of the finite element model. Therefore, the verification of weld appearance is an essential means for the verification of welding thermo-elasto-plastic simulation and is also the preferred verification method for domestic and foreign scholars. This method obtains the actual size and appearance of the weld with the best forming through metallographic experiments, conducts simulations based on the actual welding parameters, and adjusts the parameters of the welding heat source model to verify whether the simulated welding molten pool is close to the actual weld, so as to ensure that the simulated component and the actual component have similar weld zones and heat affected zones. However, the accurate verification of the welding thermo-elasto-plastic finite element simulation results cannot be achieved only through the weld appearance, because only the melting area information is available in the weld appearance verification and the peak temperature information is not available; the peak temperature affects the heating rate and cooling rate of the molten pool, and also affects the heat conduction process from the molten pool to the base metal; these affected factors will further affect the stress and deformation behaviors of the component. Therefore, to achieve the accurate verification of the welding thermo-elasto-plastic finite element simulation results, it is necessary to further verify the welding thermal cycle or welding residual stress or welding deformation on the basis of the weld appearance verification. Given the dependence relationship among welding temperature, stress, and deformation, verifying temperature or stress, or deformation has the same effect. Some domestic scholars have achieved the comprehensive verification of weld appearance and welding thermal cycle or weld appearance and welding residual stress.

[0004] However, when verifying the welding thermal cycle curve by the thermocouple method, the peak temperature of the molten pool usually cannot be measured. When the thermocouple wire is spot-welded too close to the molten pool, it is easy to block the welding torch, and the measurement accuracy depends on the spot-welding quality of the thermocouple wire. When verifying the welding thermal cycle curve by the infrared temperature measurement method, it is greatly affected by factors such as ambient temperature and surface emissivity of the object, and the transmission of infrared rays will be blocked in the welding fume environment, thus affecting the measurement accuracy. When verifying the welding residual stress by the blind hole method, generally only the residual stress on the surface and near the surface can be measured, and the measurement results are greatly affected by the drilling process and calculation model. Drilling the welded part may cause the release of residual stress and affect the measurement accuracy. When verifying the welding residual stress by X-ray diffraction, generally only the stress of a few micrometers to dozens of micrometers on the surface can be measured, and the measurement accuracy is greatly affected by the surface roughness. In addition, whether verifying the thermal cycle or the welding residual stress, only the results at a single or a few positions can be obtained, and the temperature or stress distribution results of the entire component cannot be obtained. Therefore, there are inevitable accuracy problems in verifying the welding thermo-elastoplastic finite element model of the fuselage thin-walled structure from the perspectives of thermal cycle and welding residual stress. Reverse engineering technology based on 3D scanning can achieve high-precision reverse reconstruction of the geometric shape of the processed and formed product components, and is often applied in fields such as cultural relic restoration, mold development, automotive parts design, and aerospace parts restoration. Therefore, it is expected to use reverse engineering technology to perform high-precision reverse reconstruction of the geometric shape of the components before and after welding, and then obtain the results of the welding deformation distribution, and verify the welding thermo-elastoplastic simulation results from the perspective of deformation. Summary of the Invention

[0005] In view of the problems and ideas described in the background art, the present invention proposes a method for optimizing the welding deformation of the fuselage thin-walled structure of a helicopter based on reverse engineering and the finite element method. The specific process is as Figure 1 shown and includes the following steps:

[0006] Step 1: Carry out an optimization experiment on the process parameters of the weld formation quality of the test-piece-level components, and conduct weld quality evaluation and obtain weld morphology data;

[0007] Step 2: Establish a simulation material model of the fuselage thin-walled structure, establish a welding heat source model of the fuselage thin-walled structure, and check the heat source morphology parameters;

[0008] Step 3: Based on the thermo-elastoplastic finite element method, establish a welding simulation model of the fuselage thin-walled structure, and carry out welding process simulation and result analysis under the benchmark process;

[0009] Step 4: Measure the actual welding deformation distribution of the fuselage thin-walled structure through 3D scanning and reverse reconstruction technology, and verify the welding deformation simulation results under the benchmark process;

[0010] Step 5: Optimize the virtual welding process based on the verified benchmark process simulation model, compare the welding residual stress and deformation obtained by simulation, and obtain the optimal process;

[0011] Step 6: Carry out verification welding of the fuselage thin-walled structure based on the optimal process, and obtain the welding deformation distribution of the fuselage thin-walled structure under the optimized process through 3D scanning and reverse reconstruction technology to verify the deformation optimization effect.

[0012] ● Among them, the test piece-level component in Step 1 is a flat component with the same material and thickness as the fuselage thin-walled structure, and the size is determined according to the welding process design standard in the "Welding Specification for Aerospace Application Occasions"; the process parameter optimization experiment is based on the process window adopted in actual production, designs multiple groups of process parameters, conducts trial welding, and obtains a parameter combination with the weld macro morphology and mechanical properties meeting the requirements; the weld quality assessment includes the assessment of weld surface forming defects - burn-through, lack of fusion, weld internal defects - porosity and cracks, as well as the assessment of weld mechanical properties - tensile strength and Vickers hardness. The acquisition of weld morphology data includes the measurement of weld width and penetration and the photographing of the weld cross-section profile.

[0013] ● Among them, the material model established in Step 2 includes mechanical property parameters varying with temperature - density, Young's modulus, Poisson's ratio, yield strength, coefficient of thermal expansion, and thermophysical property parameters varying with temperature - specific heat capacity, thermal conductivity; the welding heat source model types of the fuselage thin-walled structure are divided into two types. For TIG welding and MIG welding methods, the welding heat source model is a double-ellipsoid heat source model, and the heat source morphology parameters to be checked include the front axis length, rear axis length, half-width, and depth of the heat source model. For electron beam welding and laser welding methods, the welding heat source model is a Gaussian rotational body heat source model, and the heat source morphology parameters to be checked include the height and opening end face radius of the heat source model.

[0014] ● Among them, the thermo-elasto-plastic method in Step 3 includes two major control equations - the heat conduction equation and the equilibrium equation. The welding simulation model of the fuselage thin-walled structure includes the transition grid model of the fuselage thin-walled structure, weld position and welding direction information, initial and thermodynamic boundary conditions, welding condition definition, and welding operation information; the transition grid model is based on the geometric model of the fuselage thin-walled structure. First, geometric simplification is carried out to remove chamfer and hole features, and then dense and sparse transition grids are divided with the weld as the center. Dense grids are divided for the weld and the heat-affected zone, and sparse grids are divided at positions far from the weld. A 3:1 transition is adopted between the dense and sparse grids; the initial and thermodynamic boundary conditions include the initial temperature, ambient temperature, surface heat dissipation coefficient, and thermal radiation coefficient during component welding; the position - area and force of the fixture constraint; the welding condition is the initial and boundary conditions when welding a certain weld; the welding operation is the welding sequence of multiple welds. The benchmark process is the process of the fuselage thin-walled structure before welding process optimization; the welding process simulation and result analysis include the distribution and peak values of welding stress and deformation.

[0015] Heat conduction equation:

[0016]

[0017] Equilibrium equation:

[0018]

[0019] In the equation, λ t and C h are the thermal conductivity and specific heat capacity matrices, T is the temperature, Q is the internal heat, Q i is the heat of inelastic deformation, Q f is the frictional heat, K(T, μ, t) is the stiffness matrix, μ is the nodal displacement, D and M are the damping and mass matrices, F is the external force, F t is the thermal force. The thermo-elasto-plastic coupling simulation is achieved through the heat of inelastic deformation Q i , the frictional heat Q f and the stiffness matrix K, the thermal force F t .

[0020] ● Among them, the three-dimensional scanning in step 4 is laser non-contact scanning, and a point cloud model of the outer surface contour morphology of the component is obtained; reverse reconstruction is performed on the point cloud model, and the specific process includes removing miscellaneous points, noise points, uniform sampling, encapsulation, repair, and surface patch fitting of the point cloud model, and finally a surface patch model of the component is obtained. To obtain the actual welding deformation distribution of the thin-walled structure of the fuselage, the thin-walled structure of the fuselage needs to be scanned three-dimensionally and reverse-reconstructed twice before welding and after all welds are welded, and surface patch models before and after welding are obtained. Finally, three-dimensional comparison is performed based on these two surface patch models, and the obtained size deviation distribution result is the actual welding deformation distribution result of the thin-walled structure of the fuselage. Verifying the welding deformation simulation results of the thin-walled structure of the fuselage under the benchmark process includes maximum deformation verification, deformation trend verification, and distribution area verification of local large-deformation regions.

[0021] ● Among them, the virtual welding process optimization in step 5 needs to design different welding sequences according to the weld distribution and the number of welds. The process with small welding peak residual stress, small maximum welding deformation, or small distribution area of local large-deformation regions is the best welding process.

[0022] ● Among them, the method for obtaining the welding deformation distribution result of the thin-walled structure of the fuselage based on the best process in step 6 is the same as that in step 4, and the evaluation of the deformation optimization effect depends on the maximum welding deformation or the distribution area of local large-deformation regions before and after process optimization.

[0023] Advantages of the present invention: The proposed method for optimizing the welding deformation of the thin-walled structure of a helicopter fuselage based on reverse engineering and the finite element method can reproduce the entire welding process of the thin-walled structure of a helicopter fuselage with a large number of welds and a complex structure and conduct comprehensive and accurate verification at the component level. Based on a more credible and engineering application-valued virtual welding model of sheet metal components at the component level, the welding process of the thin-walled structure of the fuselage is optimized to obtain the best process, which can not only reduce the experimental cost but also significantly improve the production efficiency. It is also of great significance for controlling the welding deformation of the thin-walled structure of the helicopter fuselage, improving the dimensional accuracy, and ensuring the flight reliability of the helicopter. Description of the Drawings

[0024] Figure 1 Optimization process of the welding deformation of the thin-walled structure of a helicopter fuselage based on reverse engineering and the finite element method;

[0025] Figure 2 Experimental results of optimizing the welding process parameters of the test piece of the air flow channel structure of the helicopter fuselage;

[0026] Figure 3 Welding simulation material model of the air flow channel structure of the helicopter fuselage;

[0027] Figure 4 Verification results of the welding heat source model of the air flow channel structure of the helicopter fuselage;

[0028] Figure 5 Welding simulation transition mesh model of the air flow channel structure of the helicopter fuselage;

[0029] Figure 6 Simulation results of the residual deformation of the barrel mouth and the bottom edge of the air flow channel structure under different welding process schemes; Detailed Implementation Modes

[0030] The present invention will be further described based on examples below. The drawings provided here are for illustrative purposes only.

[0031] Taking the air flow channel structure of the helicopter fuselage as the research object, the optimization of the welding deformation of the thin-walled structure of the helicopter fuselage based on reverse engineering and the finite element method is carried out. The material of this component is 5A02 aluminum alloy, the welding method is TIG welding, and there are a total of 6 welds.

[0032] Step 1: First, conduct an optimization experiment on the welding process parameters of the test piece-level component. The size of the test piece is 300mm×125mm×1.2mm (length×width×thickness). Use a WSME-315R AC / DC welding power source, adopt a mixed gas of 80%Ar + 20%CO2 as the protective gas for the experiment, with a gas flow rate of 6L / min. Based on the process window (welding current range: 40 - 50A) adopted by the actual production line, design an experimental scheme with different welding current parameters. The specific welding parameter configuration is shown in Table 1. The macroscopic morphology obtained from the welding process parameter optimization experiment is as Figure 2 shown. The test results of the joint mechanical properties are shown in Table 2. Based on the macroscopic morphology of the joints and the test results of the mechanical properties of the above-mentioned various schemes, conduct a comprehensive comparative analysis, and select the best process parameters as a welding current of 46A and other corresponding parameters in Table 1. The subsequent finite element modeling and process optimization will all adopt the process parameters listed below.

[0033] Table 1 TIG welding parameters for 5A02 aluminum alloy flat plates

[0034]

[0035] Table 2 Test results of the mechanical properties of TIG butt welding

[0036]

[0037] Step 2: The chemical composition of the 5A02 aluminum alloy material, which is the structural material of the helicopter air flow channel, is shown in Table 3. Based on this chemical composition, calculate the material property parameters and combine the results of literature review to obtain the simulation material model of 5A02 as Figure 3 shown. According to the best process parameters obtained from the process optimization experiment, conduct a finite element simulation of the welding thermal process at the test piece level. Select the double ellipsoid heat source model, and adjust the front axis length, rear axis length, half width, and depth parameters of the heat source model to make the simulated molten pool cross-section consistent with the measured weld morphology, as Figure 4 shown, so as to obtain the verified heat source parameters.

[0038] Table 3 Composition and proportion of 5A02 aluminum alloy

[0039]

[0040] Step 3: Geometrically simplify the geometric model of the helicopter air flow channel structure, remove the chamfer and hole features, and then perform transition mesh division according to the simplified geometric model. Divide fine meshes near the circumferential weld, transverse weld, and 4 longitudinal welds and their heat-affected zones, divide sparse meshes in the areas far from the welds, and divide fine meshes between the dense and sparse meshes, as Figure 5As shown, the final grid model of the air flow channel structure is obtained, with a total of 178,623 elements, 322,423 nodes, a minimum grid size of 1.0 mm * 0.6 mm * 0.8 mm, a maximum grid size of 9.0 mm * 9.0 mm * 1.2 mm, and the width of the heat affected zone is 23.0 mm. Referring to the actual production conditions of the air flow channel structure, the initial conditions and boundary conditions in the finite element model are set. The initial temperature is set to 25 °C, the emissivity of radiative heat dissipation in the thermal boundary conditions is set to 0.005, the heat convection coefficient is set to 30 W / (m 2 ·K), the heat source parameters in the heat source boundary conditions are set to the parameters verified in Step 2. A fixed displacement boundary condition is applied inside the helicopter air flow channel structure. The working conditions of each weld are set as radiative heat dissipation boundary, convective heat dissipation boundary, heat source boundary, and tooling boundary. The numbers and welding directions of six welds are marked. The entire welding operation is set to weld in the order of Weld 1 - 6. Finally, a complete welding finite element model is established. After submitting the calculation, the simulation results are obtained, and the maximum deformation, deformation trend, and distribution characteristics of the local large deformation area of the component are analyzed. It is found that the residual deformation of the structure is mainly the warping deformation at the bottom edge, with a maximum of 7.6 mm, located at the protruding end. The roundness of the barrel opening of the air flow channel structure is greatly affected, and the deformation amplitude ranges from 2.3 - 3.0 mm.

[0041] Step 4: Conduct a three-dimensional laser scanning experiment on the helicopter air flow channel structure. First, calibrate the accuracy of the scanning equipment and paste the marking points in sequence. The calibration error is 0.040817 pixels. Connect the three-dimensional laser scanning system (three-dimensional laser scanner, computer, power supply system), select the laser scanning mode, set the scanning accuracy to 0.1 mm. After the scanner starts working, it emits cross lasers and conducts three-dimensional scans on the air flow channel structure before and after welding to obtain the corresponding point cloud models. Then, perform reverse reconstruction processing on them in sequence, including removing miscellaneous points, noise points, unified sampling, encapsulation, repair, and surface patch fitting. Finally, the surface patch models of the air flow channel structure before and after welding are obtained. Finally, perform fitting alignment and three-dimensional comparison to obtain the actual welding deformation distribution results of the helicopter air flow channel structure. The results show that the measured welding deformation and the simulated welding deformation trend are consistent, and the maximum deformations are similar. The measured maximum deformation is 7.1 mm, also located at the protruding end of the bottom edge, showing warping deformation. The difference between the simulated and measured deformations is 6.3%, and the deformation amounts and deformation trends in the large deformation areas of the two are highly similar, mainly distributed at the free end protruding from the bottom and the lower part of the barrel opening.

[0042] Step 5: Weld the finite element model according to the verified air flow channel structure, keep the process parameters of TIG welding unchanged, and carry out process optimization. Six process plans are designed based on the benchmark process plan (Plan A), as shown in Table 5. Simulate the welding deformation under the process, extract the deformation result nephogram, and find that the areas with larger deformation are basically located at the edge of the barrel mouth and the protruding position of the bottom edge of the air flow channel structure; extract the residual deformation path curves of the circumferential path of the barrel mouth (Path 1) and the circumferential path of the protruding end of the bottom edge (Path 2) under the six plans, as Figure 6 shown. It is found that the maximum residual deformation is between 7.3 - 7.6 mm, and the deformations of Plans A - F are 7.56, 7.51, 7.53, 7.34, 7.54, and 7.34 mm respectively. Among them, the deformation control of Plan D is the most significant, with a reduction of 0.218 mm.

[0043] Table 5 Design of TIG welding process optimization plan for air flow channel structure

[0044]

[0045] Step 6: Based on the best welding process explored in Step 5, carry out the actual optimized process welding of the air flow channel structure, and obtain the welding deformation distribution of the air flow channel structure under the optimized process welding conditions by using the same process as in Step 4. The results show that the actual maximum deformation also appears at the free end of the bottom, and the maximum welding deformation is 6.1 mm, which is significantly improved compared with the maximum welding deformation of 7.1 mm before optimization. Secondly, at the edge of the barrel mouth and the protruding position of the bottom edge, where the deformation is relatively large, the welding deformation is optimized from 4.4 mm and 6.1 mm to 2.2 mm and 3.2 mm, and the deformation optimization effect reaches 50%, indicating that the optimized process obtained through virtual welding process optimization can effectively improve the welding deformation of the helicopter air flow channel structure.

[0046] The protection scope of the present invention includes but is not limited to the above embodiments. The protection scope of the present invention is subject to the claims, and any substitutions, deformations, and improvements that are easily conceivable by those skilled in the art to this technology fall within the protection scope of the present invention.

Claims

1. A method for optimizing welding deformation of helicopter fuselage thin-walled structure based on reverse engineering and finite element method, characterized in that The following steps are involved: Step 1: Conduct an experiment to optimize the process parameters of weld formation quality of specimen-level components, evaluate weld quality and obtain weld morphology data; the material and thickness of the specimen-level components are consistent with the thin-walled structure of the fuselage; the weld morphology data includes weld width, weld depth and weld cross-sectional profile; Step 2: Establish a simulation material model of the thin-walled structure of the fuselage, establish a welding heat source model of the thin-walled structure of the fuselage and verify the heat source morphology parameters; Step 3: Based on the thermo-elastic-plastic finite element method, a welding simulation model of the fuselage thin-wall structure is established to simulate the welding process and analyze the results under the benchmark process; the welding simulation model of the fuselage thin-wall structure includes the transition mesh model of the fuselage thin-wall structure, weld position and welding direction information, initial and thermodynamic boundary conditions, welding condition definition and welding operation information; Step 4: The actual welding deformation distribution of the fuselage thin-walled structure is measured by 3D scanning and reverse reconstruction technology, and the welding deformation simulation results under the benchmark process are verified; wherein, the 3D scanning is a non-contact laser scanning, which can obtain a point cloud model of the outer surface contour of the component; the point cloud model is reversely reconstructed, including clutter removal, noise point removal, unified sampling, packaging, repair, and surface patch fitting, and finally the surface patch model of the component is obtained; the actual welding deformation of the fuselage thin-walled structure needs to be obtained by performing two 3D scanning and reverse reconstruction on the fuselage thin-walled structure before welding and after all welds are welded, respectively, to obtain the surface patch models before and after welding, and then perform 3D comparison; Step 5: Optimize the virtual welding process based on the verified benchmark process simulation model, compare the simulated welding residual stress and deformation, and obtain the optimal process; Step 6: Carry out verification welding of the thin-walled structure of the fuselage based on the optimal process. The welding deformation distribution of the thin-walled structure of the fuselage under the optimized process is obtained through 3D scanning and reverse reconstruction technology to verify the deformation optimization effect.

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