Study on Welding Deformation Correction of TC4 Titanium Alloy Panel Parts

Aiming at the problem that TC4 titanium alloy panel parts after welding have large internal residual stress and obvious panel deformation, which can’t meet the requirements of the parts, the viscoelastic plastic model of titanium alloy panel parts is established by finite element analysis, and the welding deformation-thermal alignment collaborative simulation environment is established by ANSYS, and the welding deformation and thermal alignment process are analyzed by numerical simulation. The law of shape correction when the correction temperature is 600 °C, 650 °C, 700 °C and the holding time is 3600s, 5400s and 7200s, respectively, and the calculation formula of springback rate is established to evaluate the effect of thermal shape correction, so as to optimize the heat treatment correction process. The results show that the correction temperature and holding time have obvious influence on the correction effect. When the correction temperature is 700 °C and holding time is 7200s, the correction effect is the best, and the springback rate is controlled below 25%, which meets the requirements of machining accuracy.


Introduction
Panels are widely used in the aviation field.For titanium alloy panels, welding is mostly used in the forming process.Due to the uneven temperature distribution around the welding seam during the welding process, the deformation of the components after welding is obvious, and the residual stress inside the components is obviously increased.Therefore, it is necessary to improve the machining accuracy of the components through the correction process [1][2][3][4] .Welding deformation correction methods mainly include: cold work correction, mechanical correction and thermal correction [5] .Sun Jie et al. studied the reverse bending correction of the whole structure, established the basic curvature relationship and the calculation relationship of the pressing displacement in the correction process, and discussed the threshold value of safe correction.Mechanical correction usually uses the method of rolling correction or stretching correction, but its application scope is limited [6] ; Because TC4 titanium alloy has the characteristics of low elastic modulus and serious springback [7] , thermal correction is more widely used in the correction process because of its characteristics of reducing residual stress and ensuring high dimensional accuracy of parts [8] .In recent years, with the further development of the research in the field of deformation correction, some new shape correction technologies have been gradually applied to engineering practice.With the further development of creep forming technology, the thermal shape correction technology based on this method has attracted much attention because of its advantages of low residual stress after correction, uniform distribution, small required shape correction load and high shape accuracy after correction [9][10][11][12] .However, there is little research on thermal alignment after TIG welding, and the alignment process depends on workers' experience and alignment test, which lacks theoretical basis.In this paper, by constructing a collaborative simulation environment of welding deformation-thermal alignment of titanium alloy panel parts, the influencing laws of various factors of thermal alignment are analyzed from the perspective of stress change and springback prediction, and the process parameters are optimized.

Model and materials
A. Model Building TC4 Titanium alloy siding parts are composed of siding, stringers and ribs, as shown in Figure 1, and the dimension parameters of each part are shown in Table 1.During the simulation analysis, due to the heat source loading near the weld, the grid is divided according to the way of dense grid near the weld and sparse grid at other positions, as shown in Figure 1.

C. Stress relaxation creep equation
Norton creep rate is an important law to study the steady-state evolution process of creep.Using MATLAB data analysis software, it is fitted into Norton model formula: The experimental material involved in this study is TC4 titanium alloy, which is applied to the wall panel parts and the hot shaping die respectively.

Simulation of welding-thermal correction
A. Simulation scheme formulation (1) Thermal correction should be carried out on the basis of welding deformation.To establish a welding deformation-thermal correction collaborative simulation environment, it is necessary to complete ANSYS welding deformation simulation first, realize the sequential movement of double ellipsoidal heat source and room temperature cooling by APDL parametric design, and extract the deformation results as shown in Figure 2, and design the correction mold according to the deformation as shown in Figure 3. (2) The annealing temperature needs to be increased to activate the high-temperature creep property of titanium alloy, that is, above 580℃.Therefore, in this study, the research parameters of the correction temperature are set at 600℃, 650℃ and 700℃, and the holding time is all 3600s.The temperature change curves are shown in Figure 4.In this study, the correction method of annealing heat treatment and cooling with the furnace was selected.The holding time was 3600s, 5400s and 7200s, and the correction temperature was 700℃.The temperature change curve is shown in Figure 5.The formula for calculating the rebound rate S1 by the radius method is: B. Simulation result analysis (1) The influence of the correction temperature on the correction accuracy of TC4 titanium alloy during heat treatment The evolution of displacement with time at different corrected temperatures is shown in Figure 7 and Table 3, the evolution of residual stress with time is shown in Table 4, and the creep curve is shown in Figure 7.  From the above evolution, it can be seen that at the beginning of thermal correction, due to the displacement constraint of the die, it entered the fast creep stage, and its displacement obviously decreased, but it did not decrease to zero.The main reason was that the workpiece and the processing table were not completely attached under the displacement constraint.With the continuous thermal correction, it entered the steady creep stage, and its displacement slowly decreased, and the elastic deformation inside the workpiece gradually turned into plastic deformation.For the residual stress in the workpiece, when the correction temperature gradually increases, the stress is released quickly, and when the holding temperature is reached, the stress release rate decreases.It can be seen from the data that, with the increase of correction temperature, the deformation decreases gradually, the stress release increases gradually, the creep strain increases, and the residual plastic deformation inside the workpiece decreases, but the creep rate decreases with the time of thermal correction.
The calculation results of rebound rate are shown in Table 5.Through the calculation results of workpiece displacement, stress evolution, creep change with time and workpiece springback rate, it can be seen that the thermal correction effect with the correction temperature of 700℃ is the best.
(2) The influence of holding time on the residual stress and the correction accuracy after correction.The evolution of displacement and creep strain with time for different holding time is shown in Figure 8 and Table 6, and the evolution of residual stress with time is shown in Table 7.The calculation results of rebound rate are shown in Table 8.According to the above data of displacement, residual stress, creep strain and rebound rate, with the extension of holding time, more elastic deformation inside the workpiece is transformed into plastic deformation under the action of creep, and the longer the holding time is, the better the correction effect of the workpiece is.

Heat treatment shaping test
In order to verify the simulation results of thermal shaping process optimization of TC4 titanium alloy, this section adopts the method of annealing heat treatment to perform thermal shaping treatment on the deformed workpiece after welding.The specific experimental scheme parameters are set as shown in Table 9.The annealing heat treatment equipment selected in this experiment is SRJX-8-13 box resistance furnace, and the temperature probe is inserted through the temperature measuring port in the middle of the furnace door to keep the temperature in the control furnace within the specified temperature range.The equipment is shown in Figure 9.The springback rate is measured and calculated by using the advantages of multi-direction measurement of three-coordinate measuring instrument, and the calculation results are shown in Table 10 From the measurement of deformation and the calculation of springback rate, it can be seen that the increase of workpiece correction temperature or the extension of holding time are beneficial to the transformation of residual elastic deformation inside the workpiece into plastic deformation, so that the residual elastic deformation inside the workpiece is less and the workpiece correction effect is better.

Conclusion
Based on the welding deformation-thermal alignment co-simulation and thermal alignment test, the process parameters are optimized by finite element simulation and measurement calculation of stress, strain and deformation.The main conclusions are as follows: (1) Through the simulation analysis and test before and after thermal shaping, it can be known that the measurement and calculation of springback rate is an important way to accurately analyze the effect of thermal shaping; (2) At the beginning of the thermal shaping process, the corrected displacement and the residual stress of the workpiece decreased rapidly under the action of the correcting die, and the loading of the die did not make the workpiece completely fit with the processing table.Instead, with the increase of temperature and the joint action of the die, the creep behavior slowly progressed, and the internal elastic deformation of the workpiece gradually changed into plastic deformation, so that the shape of the workpiece gradually recovered.The main reason for the incomplete recovery of the final shape was that there was still residual elasticity in the workpiece.
(3) Through the development of simulation and experiment, the comparative analysis of different process schemes shows that the improvement of correction temperature and the extension of holding time will be beneficial to the improvement of the thermal shaping effect.Among the thermal shaping schemes set in this paper, the thermal shaping effect is the best when the correction temperature is 700℃ and the holding time is 7200s.

Figure 4 .Figure 5 .
Figure 4. Temperature variation curves in different temperature correction processes

Figure 9 .
Figure 9. Box resistance furnace and Temperature measuring probe

Table 1 .
Dimension parameters of TC4 titanium alloy wallboard parts (mm)

Table 2 .
Heat source model parameters

Table 4 .
Evolution of residual stress (Pa)

Table 5 .
Calculation result of rebound rate

Table 7 .
Evolution of residual stress (Pa)

Table 8 .
Calculation result of rebound rate

Table 9 .
Test parameters