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The booster The booster serves as an amplitude transformer for the required amplitude range as well as a general stabilizer for the oscillations of the transducer system. Amplitude magnification or reduction is achieved by certain design features or the geometrical shape of the booster (figures 23, 24).  Figure 23: Three boosters with different transformation ratios Plane and lapped coupling areas
Figure 24: Different types of booster The resonance frequency of the booster has to match the nominal frequency (working frequency) of the generator. The coupling surfaces to the converter and to the sonotrode have to be completely level and lapped so as to prevent a loss of energy during its transfer. If this is not achieved, an undesirable temperature rise, annoying noise and damage of the device may occur. The connection to the converter and the sonotrode is assured through high-tensile threaded pins and by tightening with a defined torque. High-tensile aluminum and titanium are the preferred materials for the construction of the booster.
The sonotrode Unlike sonotrodes for ultrasonic plastics welding, those used for ultrasonic metal welding devices are not made from aluminum or titanium, but are made exclusively from tool steel. The harding process requires that special procedures be observed. The manufacture of reliable steel sonotrodes used to be a serious obstacle when ultrasonic metal welding was still in its infancy. Steel has to expand and contract 20,000 times per second without heating up too much, tearing or even breaking. One of the considerations which dates back to the early days is the idea of designing the working area of the sonotrode with a replaceable tip as a separate part. Three-piece sonotrode In other words, the sonotrode did not consist of one single piece but of three pieces, including the fastening bolt, all with very different masses. Strongly tightened, this bracing constituted a feasible compromise when low amounts of energy and low amplitude were involved.
Figure 25: Sonotrodes for different applications The uncoordinated oscillations sooner or later led to destruction of either the tip, the bolt or the sonotrode itself. This problem has meanwhile been overcome, and all leading manufacturers have enough experience to make sophisticated sonotrodes in one solid piece (figure 25). The sonotrode must be designed to suit the requirements of a solid weld and of the geometrical shape of the parts to be welded. The work surfaces of the sonotrode are usually rough or structured to permit setting the workpiece in high-frequency acceleration without slip (relative motion) (figure 26). Change of the booster amplitude Connected to the booster, the sonotrode accommodates the amplitude of the booster and transforms it throughout its geometrical shape. The working amplitude achieved in this way acts as a welding parameter. The amplitude is usually adjustable electronically, with the help of the ultrasonic generator, or even by exchanging the booster for another one with a different transformation ratio. The lifetime of a sonotrode depends on various factors. In general, the lifetime is determined by the difference of material hardness between sonotrode (hardened steel) and coupled workpiece (i.e. nonferrous metal), as well as by the welding time itself. While the difference in material hardness between sonotrode and workpiece usually promotes a long lifetime of the sonotrode, advantageous machine parameters should be selected to further reduce the welding time of the process and thereby increase the life of the tool. 250,000 to 500,000 welds If a sonotrode is used which has been well-tuned to the transducer system and the generator, the tool life for nonferrous metals will amount to between 250,000 to 500,000 welds. The worn surface or serration on the working surfaces can be reconditioned. After reconditioning, the lifetime of the sonotrode starts over again. At this point, it has to be mentioned that the resonant frequency may change slightly as a consequence of reconditioning.  Figure 26: Example of a sonotrode with rough work surface A fine-tuning of the sonotrode' s frequency to the resonant frequency of the generator (working frequency) may be required. All work should be carried out by the manufacturer, who will also check the overall quality of the sonotrode and of the coupling surfaces. The costs involved are negligible.
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The decisive factors are the inlet and outlet diameters of this part. In addition, the booster's nodal point offers the possibility of fixing the transducer system.
With high energies and/or high amplitudes, as often is the case with ultrasonic metal welding, the three different masses often produced a disharmony in the oscillation pattern of the sonotrode.
