Ultrasonic Welding

Ultrasonic welding is represented as a friction welding method, where oxides and other contaminants present on the material surfaces are broken up and also the components to be welded are brought together under simultaneous pressure. Molecular bonding, just like the conventional cold-press welding, then takes place. Ultrasonic welding is the conversion of high frequency electrical energy into high frequency mechanical energy. In ultrasonic welding spot welds in thin steels are produced by the local application of high frequency vibrating energy to work pieces held together under pressure. The work pieces are clamped together under a moderate static force applied normal to their face and oscillating shear stresses of ultrasonic frequencies (1 KHz to 40 KHz) with a power ranging of 700 to 6000 watts are applied parallel to the interface. The vibrating probe called “a sonotrode” induces lateral vibrations and slip between the surfaces fracturing the brittle oxide layers and softening the asperities because of localized heating. The combined effects of pressure and vibrations cause movement of metal molecule bringing about a sound weld.

The bonding is achieved in solid state without application of external heat, filler rod or high pressure. There is also no need for any thorough cleaning before welding because all contaminants, oxides, moisture etc are removed by the vibrating motion.

Ultrasonic Welding Equipment:

The ultrasonic vibrating unit consists of following main components:

1. Frequency converter,
2. Booster,
3. Horn or sonotrode,
4. Pneumatic Press /Actuator,
5. Ultrasonic power supply, and
6. holding fixture

This converts 50 Hz – 60 Hz line power into high frequency electrical power and a transducer which changes the high frequency electrical power into ultrasonic vibratory motion that is transmitted to the joint. The weld is completed in 0.5 to 1.5 seconds.

Ultrasonic welding of plastics:

Plastics are typically engineered materials consisting of polymers. Polymers are shaped by polymerisation that may be a chemical action during which two or more molecules are combined to make a larger molecule. Polymers are often classified as either thermosets or thermoplastics. Thermosets aren’t appropriate for ultrasonic assembly because they degrade when subjected to intense heat. Thermoplastics on the opposite hand soften when heated and cool when hardened and are thus ideally fitted for ultrasonic assembly.

Materials for Ultrasonic Welding of Plastics:

Most of the thermoplastic materials can be ultrasonic weldable. Teflon with low coefficient of friction and high melting temperature is impossible to weld using this process. 

Welding Temperature Achieved:

Ultrasonic welding produces a localized temperature rise from the combined effects of elastic hysteresis, interfacial slip and plastic deformation. The weld interfaces reach roughly 1/3 the temperatures required to melt the metals. Since the temperature doesn’t reach the melting point of the material, the physical properties of the welded material are preserved. As the ultrasonic welding method is an exothermic reaction, as welding time will increases so does weld temperature.

 

The ultrasonic welding process has the advantage that since no bulk heating of the work pieces is involved and there is no danger of any mechanical or metallurgical bad effects. Although metals have up to 2.5 mm thick have been welded by this process. It is used mostly for welding foils. This process is suitable only for thermoplastics with the exception of thermosetting resins and Teflons. The process can be used on a variety of metals including the refractory metals. Even dissimilar metals can be welded because there is no fusion. The process can also be used on temperature sensitive materials because temperature rise is limited.