8. Heavy-Duty Construction
Ultrasonic soldering is okay for electrical and electronic circuits but that won't do when working with larger, heavier materials. These types of bonds require greater strength, not just conductivity. When the Tenth Doctor built that pulley system in New Earth , it needed much more than a mere solder joint for a solder joint wouldn't hold his and Rose's weight. A stronger bond, such as a weld, was required for those components. in Daleks in Manhattan/Evolution of the Daleks the Tenth Doctor was breaking the welds of the Dalekanium on top of the Empire State Building until he dropped his screwdriver. Unlike the ultrasonic soldering, ultrasonic welding joins metallic parts in a strong bond. It excites the molecules on the surface of each component by agitating the surface molecules and with that heat, the molecules bond to each other. And once cooled there is remains a welded bond between the two pieces of metal.