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Ultrasonic-welding is a solid state process.

It achieves the joining of metals or plastics by locally applying vibratory energy to workpieces pressed together.

The energy of vibration produces a local relative parallel displacement at the interface between two metal abutting surfaces.

The motion breaks and disperses surface oxides and causes intimate metal to metal contact that results in a weld.

A similar process is used to weld plastic materials but with two essential differences.

In the case of plastics the direction of vibration is normal to the surfaces (instead of parallel).

And the vibrational energy is transformed in actual local heating that causes fusion.

See our website page on Welding of Plastics.

A short introduction to Ultrasonic-welding was published (2) in Issue #008 of Practical Welding Letter for March 2004.
Click on PWL#008 to read it.

Ultrasonic-welding was originally developed for applications requiring the avoidance of heating, essential for explosives, combustible gases or liquids.

It was later used to join overlapping sheets, strips or foils and wires in diverse industries as electronics, automotive, aerospace.

Suitable materials display sufficient ductility to allow local deformation of the abutting cleaned surfaces.

Ultrasonic-welding, like most of other solid state processes, permits the joining of dissimilar metals that cannot be successfully fusion welded together.

An Ultrasonic-welding system includes:

  • an electronic power supply that elevates the frequency of the electrical current [from that of the grid to that required by the process, typically from 15 to 40 kHz (kilo Hertz)].
  • a piezoelectric transducer that transforms electrical into mechanical energy.
  • an acoustic coupling device or booster that modifies the shape and magnitude of the vibrations, and
  • a weld tool called horn or sonotrode that transmits the oscillations to the materials to be welded, clamped together by pressure unto the stationary anvil.

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The vibration amplitude is around 20 microns, the weld time is usually less than one second.

Best results are obtained by applying sufficient power for short durations.

Longer times at insufficient power fail to produce acceptable welds.

Shape, ductility, thickness and cleanliness are most important for acceptable results.

The most usual implementation introduces spot welds of shape and size that depend on the tool used.

The spot spacing may vary with the application, from as wide as possible to satisfy strength requirement, to overlapping if sealing is needed.

Ultrasonic-welding Benefits and Limitations

The Benefits of Ultrasonic-welding are:

  • Room temperature welding
  • Low energy consumption
  • Thin sections weldable to thick plates
  • Welding of incompatible materials possible
  • No filler metal needed
  • Suitable to automation
  • Environmentally friendly

Limitations:

  • Lap joint configuration only
  • Weldable thickness depends on power available
  • Brittle materials not readily weldable
  • Only thin sections are weldable in certain materials

If welded material sticks to the sonotrode (weld tool), a low power ultrasonic pulse will release it.

In some cases repeating the full welding cycle on a thick piece of brass may be helpful.

Most metals are easily joined by Ultrasonic-welding. If the metal oxide is much harder than the metal (as for Aluminum) it breaks easily and does not interfere.

Copper oxide which is more soft and ductile is not removed by the process, so that it has to be taken away by thorough cleaning beforehand.

Different materials display different ultrasonic weldability, so that the practical thickness limits must be established for each material to be used in real applications.

The economic benefits of the process are generally favorable to its applications when the performed welds meet requirements.

Energy expense is much lower than for comparable joining with alternative methods and equipment (resistance- or laser spot-welding).

A derivative process, called ultrasonic microwelding, was adapted to the requirements of microelectronic industry.

It is used to deal with extremely thin wire and ribbon leads, generally with soft and conductive materials.

The differences are not in the physical process, but in parameters like frequency (higher), power, static load and dimensions (much lower).

This kind of equipment is generally automated for mass production.

See also the following Article.
Ultrasonic Spot Welding: A New Tool for Aluminum Joining
http://www.aws.org/w/s/wj/2005/02/026/
(Copy and paste the above line in your browser and Enter).

Watch the following Video from a commercial company

Ultrasonic Metal Welders Video

http://www.sonobondultrasonic.com/welders-bonders-metals-video-english.asp

* * *

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To reach a Guide to the Title collection of the most important Articles from Past Issues of Practical Welding Letter, click on Welding Topics.

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