Understanding what ultrasonic energy is and how it is generated and controlled is fine. However, a fair question now is - so what? Why use it when so many other techniques exist such as resistance welding, laser welding, oven brazing and soldering? The answer is that ultrasonic metal welding does in one single operation what these other processes need several steps to accomplish. It also eliminates consumables, does not require pre weld preparation or post weld cleaning, uses very little energy (1/30 that of a fusion welder), uses no hazardous chemicals, generates no noxious fumes and can be very precisely controlled and monitored to ensure consistent high quality results.
Finally, it is a low temperature process. Typically, the frictional heat generated does not raise the temperature of the parts being welded beyond approximately one third of their melt temperature. Since little heat is generated, cooling water for tooling is not required and there is no melting or annealing of parts being joined. Operators can often handle parts immediately after welding.
This low temperature characteristic is very important when welding Copper to Aluminum. Fusion welding creates a nonconducting, brittle, intermetallic compound that reduces the ductility of the Copper/Aluminum bond. As the molten metals mix and approach an alloy of 12% Copper3 an alloy is formed that is too brittle for most applications. Since the ultrasonic welding process does not cause melting, these intermetallics and brittle alloys are not formed when joining these materials.4
The metallurgical makeup of the copper wire used in the manufacture of electrical harnesses is a very difficult one to resistance spot weld due to the presence of electrolytic tough pitch copper often present in the wires to be joined. When heated in a reducing atmosphere, the oxygen content in the copper may cause embrittlement and therefore threaten the integrity of the connection. Furthermore, the copper wire used in wire harnesses is very conductive and extraordinarily high electrical currents are needed to generate sufficient heat in the wire to effect a melt. Oxides and contaminants on the wires as well as electrode wear will drastically change the resistance of the assembly and the amount of heat produced. When too much heat is generated during a resistance weld cycle, the assembly can be damaged, making it possible for an extreme variation from part to part depending on the amount of oxides or contaminants that were present on the wires before welding.5
Other temperature sensitive materials benefit as well. A good example is Beryllium Copper spring material which can be partially annealed by fusion joining processes but retains its spring properties when ultrasonically welded.
