# Numerical Simulation Study on Welding Process of Upper Frame of Hydropower Unit
## 1 Introduction  
The hydropower unit is the core mechanical device for hydropower energy development and utilization, and its manufacturing quality is directly related to the efficiency of hydropower energy extraction and conversion. The upper frame is the key structural component of the hydropower unit, which plays the role of supporting and guiding the shifting of the hydropower unit. The upper frame is mainly formed by welding, and welding residual stress and deformation will inevitably occur after welding, and large residual stress and deformation will reduce its structural strength and stability, thus affecting the overall performance of the unit [1]. The upper frame structure is shown in Figure 1.  
Therefore, it is necessary to study the welding process of the upper frame in order to reduce the residual stress and deformation and improve the service life and operation safety of the unit. With the development of computer technology and finite element theory, numerical simulation can accurately simulate the welding process, thus reducing the test cost and improving the welding quality [2]. Therefore, this paper adopts numerical  
Figure 1: The structure of the upper frame. simulation methods to study the welding process of the upper frame of a hydropower unit and predict its welding deformation. Currently, the thermal elastic-plastic method (TEPM) and inherent strain method are the two main methods in the field of welding numerical simulation. Professor [PERSON] [3] of Osaka University in Japan was the first to propose the TEP finite element method (FEM) and applied it to two-dimensional and three-dimensional finite element models to simulate the stress-strain field during the welding process. Since then, with the improvement of computer performance and the emergence of professional simulation software, researchers have conducted more in-depth research based on the TEP-FEM and have achieved abundant research results. [PERSON] et al. [4] simulated the welding deformation of fillet welds using the TEPM and verified its effectiveness through experiments. [PERSON] et al. [5] used the TEPM to simulate multi-pass welding, taking into account heat convection and radiation, and obtained relatively accurate results. [PERSON] et al. [6] used the TEPM to simulate the laser-MIG hybrid welding process. They compared the weld pool shape, residual stress, and deformation obtained from the simulation and tests, which were in good consistency. [PERSON] et al. [7] established a three-dimensional finite element model based on the TEPM to study the effects of different level shapes on the residual stresses of dissimilar metal welding and carried out the residual stress detection using the blind hole method, which showed that the simulation results were in good agreement with the test. At the same time, numerous scholars have conducted extensive research on the effects of the welding process on both the temperature and stress-deformation fields. [PERSON] et al. [8] investigated the impact of preheating temperature on welding residual stress in P91 steel and discovered that preheating to 200 \({}^{\circ}\)C can effectively reduce both residual stress and deformation. [PERSON] et al. [9] developed a three-dimensional TEP finite element model and found that external constraints have a greater effect on the final residual stress and deformation of fillet welds than the welding sequence. [PERSON] et al. [10] investigated the effects of plate thickness, welding sequence, and leg size on residual stress and deformation in cross-shaped welded structures using numerical simulation. The results were found to be in close agreement with the experimental data. [PERSON] et al. [11] used ABAQUS to simulate the welding process of an aluminum alloy bicycle frame, optimized the welding process parameters, and verified the effectiveness of the welding process through experiments. The TEPM can obtain relatively accurate calculation results of temperature and stress-deformation fields.