# Numerical Simulation Study on Welding Process of Upper Frame of Hydropower Unit
## 3 Simulation Analysis of T-Joint Welding Based on TEPM
### Welding Process Impact Analysis and Optimization
#### 3.3.1 Analysis of the Influence of Welding Speed  
In order to solve the problem of insufficient penetration of the first and second welds, in this paper, the welding speed of the first and second welds was changed for simulation calculation while the heat input power was kept unchanged, and the subsequent welding speed was kept unchanged to analyze the impact on the overall welding results.  
The welding speeds selected in the simulation analysis were 5 mm/s, 5.5 mm/s, 6.67 mm/s, and 8 mm/s, where 6.67 mm/s was the previous welding test parameter, and the other conditions were consistent with the previous finite element model. The calculation results of the steady-state temperature field of the first weld at different welding speeds were intercepted, as shown in Figure 11.  
The size of the molten pool morphology obtained at different welding speeds is different, the smaller the welding speed, the larger the molten pool morphology and the wider the spread of isotherms. This is because the welding speed decreases, resulting in greater line energy. When the welding speed is 5 mm/s and 5.5 mm/s, the molten pool morphology can meet the engineering requirements; when the welding speed is 8 mm/s, there is a failure to melt through.  
Figure 11: The calculation results of the steady-state temperature field of the first weld. (**a**) v = 5 mm/s; (**b**) v = 5.5 mm/s; (**c**) v = 6.67 mm/s; and (**d**) v = 8 mm/s.  
Figure 12 shows the equivalent residual stress clouds diagram of the specimen cooled to 3600 s. The maximum value of equivalent residual stress is 682.78 MPa, 681.89 MPa, 684.03 MPa, and 690.89 MPa, respectively. There is no significant fluctuation in equivalent residual stress following changes in welding speed for the first two welds, because the heat input for subsequent weld passes is higher, and thus has a more significant impact on equivalent residual stress.  
Figure 13 shows the Z-displacement cloud diagram of the specimen cooled to 3600 s. Unlike changes in residual stress, variations in welding speed significantly affect the z-displacement of the specimen. This is because as the welding speed decreases, the heat input into the weld-ment increases, leading to more pronounced deformation in the Z-direction.  
In order to further compare the influence of several welding speeds on deformation, the Z-direction deformation of the bottom of the flange perpendicular to the middle line of the welding direction was extracted, as shown in Figure 14. The maximum Z-displacement caused by four different welding speeds is 1.33 mm, 1.28 mm, 1.18 mm, and 1.10 mm, respectively. When the heat input power remains unchanged, the welding deformation in the Z direction increases by more than 5% when the welding speed of the first two welds is reduced by 0.1 mm/s.  
In summary, the results show that satisfactory weld pool morphology can be obtained when the first two welds are welded at speeds of 5 mm/s and 5.5 mm/s. When welding is performed at a speed of 5.5 mm/s, post-welding deformation is minimal and production efficiency is high. Therefore, a welding speed of 5.5 mm/s is more appropriate for the first two welds.  
Figure 12: The calculation results of the equivalent residual stress. (**a**) v = 5 mm/s; (**b**) v = 5.5 mm/s; (**c**) v = 6.67 mm/s; and (**d**) v = 8 mm/s.