# Numerical Simulation Study on Welding Process of Upper Frame of Hydropower Unit
## 4 Simulation Analysis of Welding Deformation of Upper Frame Based on ISM
### Simulation of Welding Deformation for the Upper Frame of Hydropower Unit
#### 4.2.2 Welding Sequence Planning for the Upper Frame of Hydropower Unit  
Due to the large number of welds in the upper frame, the welds are described first in order to facilitate the description of the welding sequence. The upper frame of a certain model hydropower unit studied in this paper is mainly assembled and welded from components such as the upper ring, lower ring, brace ring, small rib plates, large rib plates, oil tank bottom plate, oil tank wall, upper wing plates, lower wing plates, and web plates. Except for the oil tank bottom plate with a thickness of 30 mm, the thickness of other components is 20 mm. The overall diameter is 3400 mm, and the height is 840 mm, as shown in Figure 22.  
Before the overall welding of the upper frame, the upper rings were already integrated with the upper wing plates, and the lower rings were integrated with the lower wing plates to ensure a certain strength during frame positioning assembly. Therefore, in this study of deformation prediction, they were considered as wholes, with the main focus on investigating the influence of the welding sequence of T-shaped welds on the overall deformation.  
The entire upper frame has a total of 136 T-shaped welds: the brace ring is located between the lower ring and the oil tank bottom plate, and due to the presence of small rib plates, it needs to be divided into 8 segments during welding, resulting in a total of 32 welds inside and outside; each of the 4 small rib plates has two T-shaped fillet welds and four T-shaped welds, totaling 24 welds; the large rib plates, along with the lower ring, brace ring, oil tank wall, upper and lower wing plates, and web plate, all have welds. Each large rib plate has six T-shaped fillet welds and ten T-shaped welds, totaling sixty-four welds; the oil tank wall is located between the upper ring and the oil tank bottom plate, and due to the presence of the large rib plate, it needs to be divided into four segments during welding, resulting in a total of sixteen T-shaped welds.  
To reduce the number of workpiece flips during welding and improve efficiency, it is necessary to weld the bottom welds and fillet welds first, then flip the assembly after welding, and finally weld the upper welds. According to the welding process requirements for the upper frame, the fillet welds of the small and large rib plates need to be completed first, followed by the T-shaped welds. Based on this, the most common welding sequence currently used is as follows: complete the welding of the bottom large rib plates first, followed by the small rib plates; weld the inner T-shaped welds of the brace ring first,  
Figure 22: Upper frame structure.  
followed by the outer T-shaped welds; weld the outer T-shaped welds of the oil tank wall first, followed by the inner T-shaped welds.  
Due to the large number of welds involved in the upper frame, the overall welding deformation simulation is complex, and it is not feasible to simulate all scenarios. Therefore, the most commonly used welding sequence is taken as scheme 1, and two additional welding schemes are designed for the inner and outer welds of the brace rings, the tank wall, and the T-shaped welds of the small and large rib plates. The criterion for evaluating the superiority and inferiority of the schemes is to minimize the welding deformation.  
Scheme 2 is as follows: first complete the welding of the bottom small rib plates, followed by the large rib plates; weld the outer T-shaped welds of the brace rings first, followed by the inner T-shaped welds; and weld the inner T-shaped welds of the upper ring first, followed by the outer T-shaped welds.  
Scheme 3 is as follows: alternate welding of small and large rib plates at the bottom; both the brace rings and upper ring are welded with outer T-shaped welds first, followed by the inner T-shaped welds.