# Numerical Simulation Study on Welding Process of Upper Frame of Hydropower Unit
## 5 Results and Discussion  
For the convenience of viewing the results, a local coordinate system was established, with the X-direction perpendicular to a certain large rib plate, the Y-direction parallel to the bottom weld of the large rib plate, and the Z-direction representing the thickness direction of the upper frame. The upper frame is a symmetrical structure, and the deformation results in the X and Y directions are similar, so only the welding deformation in the X-direction of the local coordinate system is shown. The overall welding deformation prediction results of the upper frame of the hydropower unit are shown in Figure 24.  
Figure 23: Finite element meshing of the upper frame. Figure 24a shows the overall welding deformation of the upper frame in Scheme 1. As depicted in the figure, the deformation of the upper frame is relatively symmetrical, with the maximum deformation occurring at the middle of the outer edge of the upper ring, reaching a maximum value of 3.153 mm. This area is located between the two arms, where the constraint from the large rib plates is weakest, resulting in the greatest deformation. The deformation in the middle of the tank wall is relatively small, mainly due to its distance from the welds, resulting in minimal influence from welding. Figure 24b shows the welding deformation in the X-direction of the upper frame in Scheme 1. The maximum deformation occurs at the middle edge between the two belly plates, with a maximum deformation value of 1.959 mm in the positive X direction and 2.028 mm in the negative X-direction. The deformation on the side welded later is smaller, mainly because it is constrained by the weld on the side welded first. Figure 24c shows the deformation in the Z-direction of the upper frame. The maximum deformation in the Z-positive direction occurs at the bottom wing plate, measuring 1.779 mm, while the maximum deformation in the Z-negative direction occurs at the middle of the outer edge of the upper ring, measuring 3.149 mm. Significant deformation is also observed in the area between the two rib plates. Comparing the overall deformation of the upper frame under three different welding sequences, it can be observed that the overall deformation trends of the three schemes are consistent. The areas with significant welding deformation are the outer edges between the two rib plates, the edges of the web plates, and the upper and lower wing plates. The maximum deformations produced by the three schemes in each direction are shown in Table 3. From the table, it can be seen that there is almost no difference in the deformations in the X and Y directions among the three schemes, and the change in welding sequence in the central area has little effect on the two side arms. The maximum  
Figure 24: Simulation results of welding deformation on the upper frame of the hydropower unit. (**a**) Overall welding deformation of Scheme 1; (**b**) welding deformation in X-direction; (**c**) welding deformation in Z-direction; (**d**) overall welding deformation of Scheme 2; and (**e**) overall welding deformation of Scheme 3.  
overall deformation for Scheme 2 is 2.188 mm, which is approximately 30.6% lower than the maximum deformation of 3.153 mm produced by the currently most commonly used welding sequence, Scheme 1. According to Figure 25, the Z-directional deformations of the upper ring outer path 1, upper ring inner path 2, lower ring outer path 3, and lower ring inner path 4 were extracted to further analyze the influence of different welding sequences. The deformations of different path schemes are shown in Figure 26.  
Figure 26a shows the Z-direction deformation on the outer side of the upper ring. From the graph, it can be observed that the post-welding deformation in Scheme 2 is the smallest, with a maximum value of 1.648 mm. This is a reduction of 1.501 mm compared to Schemes 1 and 3, which had a maximum deformation of 3.149 mm. This is because in Scheme 2, the inner T-shaped welds of the upper ring were welded first.