Why are there bubbles at the welding points of the intermediate frequency spot welding machine?

Why are there bubbles at the welding points of the intermediate frequency spot welding machine? The formation of bubbles first requires the formation of a bubble core, which must meet two conditions: one is that the liquid metal has supersaturated gas, and the other is that it has the energy required for nucleation. Analysis and solutions to the problem of solder joint bubbles:

The supersaturation in liquid metal is relatively high, and the higher the supersaturation, the more unstable it becomes. Gas is more likely to precipitate and form bubbles. Therefore, the molten pool in welding has the necessary supersaturation conditions to form bubbles. Like the process of metal crystallization, bubble nucleation can also occur in two ways: spontaneous nucleation and non spontaneous nucleation. If a bubble core is formed, the bubble must overcome liquid pressure and perform expansion work

Due to the increase in surface energy caused by the formation of new phases, if a bubble core with a critical size is formed in a liquid, sufficient energy must be provided to form nuclear energy. Obviously, the higher the nucleation energy, the less likely it is to form a bubble core. Conversely, the easier it is to form a bubble core.