THE SCIENCE AND APPLICATIONS OF ACOUSTICS - H. H. Arnold ...

17.2 Industrial Applications of Ultrasound 489Figure 17.7. Ultrasound soldering iron.produce spot and seam welds in metals such as 304 and 321 stainless steel, aluminum,brass, copper, zirconium, titanium, gold, molybdenum-0.5% titanium, andplatinum. This type of welding can be applied to very small wires to seam weldingof metal plates up to 0.5 cm in thickness. Both similar and dissimilar metals canbe welded. While the formation of a thin molten film in the interface seems to bethe primary mechanism of bonding, there is some evidence of solid-state bondinginstigated by diffusion in the welding zone at low amplitudes and high clampingpressure.Bonding of thermoplastic materials 1 through ultrasound is even easier than formetals, since the necessary equipment is smaller and requires appreciably lesspower. Ultrasonic welding of thermoplastic materials has gained wide acceptance,particularly in situations where thick materials are joined together and the toxicityof adhesives must be avoided. In addition to bonding plastic parts, the same equipmentcan be used to insert metal parts in plastic pieces. A hole slightly smallerthan the metal is drilled or incorporated in a molding part and the metal is driveninto the hole ultrasonically. During the insertion the melted plastic encapsulatesthe metal piece and fills flutes, threads, undercuts, and so on.Ultrasound welding combines the ideal ingredients sought in modern manufacturing.The process is fast and clean, requires no consumables, does not callfor skilled operators, and lends itself to automation. It is used extensively in theautomotive industry of assembly of taillights dashboards, heater ducts, and othercomponents where plastics have replaced the traditional use of metal and glass.1 Plastics fall into two categories: thermoplastic and thermosetting. The former plastic type will soften atsufficiently elevated temperatures and reset upon cooling. Thermosetting plastics retain their rigidityat elevated temperatures.