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INTRODUCTION In
1938, Lludwig Bergman and some colleagues were experimenting with ultrasonic Waves
and their effects on metal. He found that many metals could be combined by using
ultrasonic welding that could not be joined by any other method. It was also found
that any metal could be strengthened by subjecting the metal in its molten state
to ultrasonic vibrations. The ultrasonic effect upon the molten metal generates
a smaller grain size, giving the metal more strength. Ultrasonic welding combines
pressure and high frequency vibration motions to form a solid state bond. The
range ultrasonic frequency used in this welding is from 20kHz to 60kHz. This cool,
strong weld capable of joining such combination as aluminium to steel, aluminium
to tungsten, aluminium to molybdenum and nickel to brass. Ultrasonic welding has
also made it possible to join metals with vastly different melting temperatures,
making strong rigid joints. Thus many applications previously considered unweldable
can now revaluated.
WORKING
The circuit of UMW is as shown below.
It consists of an ultrasonic generator,
which generates a frequency of 20 kHz to 40 kHz from a supply of 220v/50Hz. The
converter transforms the high frequency electric energy produced by the generator
into mechanical energy. 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. The sonotrode or horn is the working tool of the ultrasonic
metal welding.
The pieces to be welded are clamped between the welding tip called sonotrode and
anvil. Both tip and anvil are faced with high-speed steel, since considerable
wear can occur at the contacting surface. The process of ultrasonic welding is
fairly simple. It begins when the parts those are to be welded, such as two multi-strand
copper wires for example, are placed together in the welding unit. The system
then compresses the wires together with a force of between 50 and several 100
pounds per square inch to form a close connection between the two pieces. Next,
the ultrasonic horn is used to vibrate the two pieces together at a rate of around
20000 or 40000 Hz, depending on the application.
The system that is used to scrub the pieces together consists of four major components.
The first of these is the anvil. This is simply a piece of the machine, usually
with a replaceable head, that holds one of the components still while the other
is rubbed against it. The "business end" of the ultrasonic system consists
of three major parts. The first of these is the ultrasonic transducer. This component
takes an electric signal from a power supply that is providing a 20 kHz AC (by
using an ultrasonic generator) signal and converts it to a mechanical motion at
the same frequency. The vibration that results is at a frequency that is appreciably
above the range of human hearing, hence the name ultrasonic.
Although
this motion is very strong, it has a very low amplitude or stroke length. This
is not suitable for welding. The next part of the system, appropriately called
the booster, increases the amplitude of the motion, at the cost of some of its
force. This motion
is then passed into the ultra sonic horn. This is the portion of the system that
actually vibrates the work piece. In addition to providing the interface between
the ultra sonic generator and the work piece, the horn also further amplifies
the amplitude of the motion, again reducing its force. Like the anvil, the horn
ends in a replaceable head.
Before
the interaction of the pieces at the interface can be explained, some basic molecular
physics must be reviewed. The first principal is that when two clean pieces of
metal are placed in intimate contact, they will begin to share electrons, thus
welding together, second, at atomic scale even surfaces those that look perfect
and smooth are very rough and impure. The majority of these impurities are in
the form of metal oxides that were produced when the bare metal was exposed to
the atmosphere. The second part of contamination is in the form of ordinary dirt
and oils. These impurities form a layer that prevents the electrons in the two
parts from passing between them, thus preventing them from welding together. In
addition, the rough surface prevents the metals from being in intimate contact,
which also prevents the exchange of electrons
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