UltraSonic Machining

problems related to heat treatments. The joining mechanism is solid state diffusion, leading to higher

mechanical properties with respect to other techniques.

Components

The principal components for the USM and USW are:

 The electronic oscillator, which converts the alternate current (50 - 60 Hz) into electric energy into

ultrasonic frequency (typically 20 - 40 kHz). A peculiarity of the generator is the ability to keep the

working parameters constant during the process, especially frequency and amplitude.

 The transducer, that can be divided into piezoelectric and magnetostrictive.

o Piezoelectric. 1

The piezoelectric effect is a reversible process: piezoelectric materials exhibiting the direct

2 3

effect also exhibit the reverse piezoelectric effect . Max 1 kW of power is achieved and there

is no need for chilling systems. However, a precise frequency is required to excite the

material. Generally, a system contains more than one PZT crystal in order to work on more

1 There are both natural crystals and synthetic ceramics.

2 The internal generation of electrical charge resulting from an applied mechanical force.

3 Of course, the internal generation of a mechanical strain resulting from an applied electrical field.

Pag. | 2 materials. The advantages of PZT are the relative low power, high frequency, small mass;

they are energy-efficient but with a limited duration.

o Magnetostrictive.

Made by a nucleus N, formed by a packing of metal laminae (alloys Fe-Ni or Fe-Co), and by a

winding A crossed by alternating current (at the same frequency as the contraction of the

laminate) completed by a tool holder P for the assembly of the sonotrode C, which will be

inserted the tool H. Just modifying the electromagnetic energy it is possible to control the

obtainable frequency, and higher power up to 20 kW are possible.

The two nuclei T are with windings A polarize the main magnetic field allowing sinusoidal

displacement of higher amplitude, thus low frequencies can be achieved. The relative large

mass can be a drawback, on the other hand.

Pag. | 3

In the following table a comparison between the transducers is provided.

Piezoelectric Magnetostrictive

More efficient Less efficient (+ passages energy)

Hysteresis: the properties and performance No hysteresis

degrade and decay over time

Fasteners may lose the connection due to Once welded, no disconnection occurs due to

vibration vibrations

These materials are sensitive to mechanical Co-Fe alloys are highly sensitive to mechanical

shocks (brittle crytals) shocks, Ni-alloys are less sensitive

Low weight of the active parts: susceptible to High weight of the active parts: not susceptible

varying loads to variable loads

Duration: limited (then high cost of machining) Duration: high (low hysteresis) 4

High temperature resistance: null, PZT do not High temperature resistance: good

withstand high T

 The booster, which is a mechanical amplitude transformer between the transducer and the

sonotrode. It increases or reduces the amplitude of the mechanical vibrations coming from the