Expensive is not necessarily good, and cheap is definitely not that product.
The nut intermediate frequency welding machine you used in Sunal has no spark or splash at all, which only shows that this welding machine can't work. How to weld firmly without sparking? Sparks and splashes will produce a lot of welding slag. Only welding slag can be used to repair the defects of nut threads, and the effect will naturally be good.
Suggestion: If Sunal nut spot welding machine is used, there is no spark and splash, you can increase the current, or extend the welding time, or apply some dirty oil on the nut before welding, so that the phenomenon of spark and splash will be more obvious.