Ultrasonic welding is a variant of the friction welding process. In ultrasonic welding, the ultrasonic acoustic vibration is applied on the workpieces being welded to create frictional movement and frictional heat.
In this article, we will discuss the principle of ultrasonic welding, equipment, how it works, welding of plastics and other materials that can be welded by ultrasonic welding, advantages and limitations , application and safety considerations.
Principle of ultrasonic welding
You must have seen and experimented with a tuning fork in your high school days. What happens when you hit a tuning fork on a hard rubber? Correct, it vibrates as well as produces a humming sound. If the frequency of the tuning fork is 512 Hz, it means it vibrates 512 times per second. If you touch the vibrating fork ends to a light object (like a ping pong ball), the vibrating forks transmit the vibration to the ball, and the ball is pushed. And, if the ball is kept stationary, the vibrating fork transmits its vibration to the ball, and the ball starts vibrating. This basic idea is made use of in ultrasonic welding. However, the frequency of vibrations used is much higher.
If you see the dictionary, the term ultrasonic indicates sound waves that have a frequency above the maximum limit of human hearing, and the upper or maximum limit of hearing in average human adults is 15 to 17 kHz (kilohertz). The frequency used in ultrasonic welding is above 15 kHz (15,000 hertz (1 kHz = 1000 Hz)). You can define ultrasonic welding as a process of joining two parts (metal or thermoplastic) using the localized application of ultrasonic acoustic vibrations and pressure to generate frictional heat and bonding.
Ultrasonic welding equipment or machine
The ultrasonic equipment will have a provision to clamp the two workpieces under pressure, and the source of pressure can be a pneumatic or electric drive. The equipment has the following major parts:
Welding stack (Sonotrode system).
Fixture, required for holding the workpiece.
1. Generator –The work of a generator is to convert the single-phase electrical power from the mains to the required frequency and voltage for the stack (transducer) that further converts it into mechanical vibrations. The generator takes the input power supply (50 to 60 Hz) and delivers a high-power electric signal to the stack (matching the required resonance frequency).
2. Base – The base of the machine is supported by adjustable height legs. The base supports the fixture for holding the workpiece, pneumatic cylinder for applying static pressure, and sonotrode system. Some people call the base “anvil.”
The machine has a pressure gauge with a regulator, and the required clamping pressure can be set using this (the required clamping pressure changes from welding to welding). Few manufacturers have replaced the pneumatic cylinder with the electromagnetic force, suitable for welding small and delicate parts.
3. Welding stack (sonotrode system) – Welding stack provides ultrasonic mechanical vibrations to the workpieces. The welding stack consists of a transducer or converter(piezoelectric transducer),booster, and horn. The converter, booster, and horn are tuned to resonate (vibrate) at the same ultrasonic frequency (15 kHz and above).
Converter (piezoelectric transducer); converts the electrical signals received from the generator to mechanical vibrations using the piezoelectric effect. The transducer or converter does the work of converting electrical energy received from the generator to mechanical ultrasonic vibrations required for the welding process. The transducer has several piezoelectric ceramic discs inserted between two titanium metal blocks. There is a thin metal plate between each of the ceramic discs, and this is the electrode. When the generator feeds the electrical sinusoidal energy to the transducer through the electrodes, the ceramic discs expand and contract and produce an axial peak to the oscillatory movement of 10 to 20 microns (µm). Transducers are delicate parts and are to be handled with care.
Booster; the booster does the job of amplifying the mechanical vibrations produced by the transducer and transferring them to the welding horn. The booster expands and contracts due to the ultrasonic energy received from the transducer. The booster enhances the amplitude of the vibration mechanically. The booster also clamps the stack in the welding machine.
Horn; the welding horn transmits the mechanical vibrations to the welded workpieces, and its design and geometry are crucial for quality welding. Some welding horns have hardened tips to reduce the wear during welding. The geometric shape of the horn tip is made compatible with the shape of the parts being joined. The horn modifies the amplitude of the mechanical vibrations and applies it to the parts being welded. The horn is made from titanium or aluminum. The amplitude of the mechanical vibration of the horn ranges between 30 and 120 microns (µm). The shape of the horn should be able to take the stress caused by the axial expansion and contraction of the horn to avoid cracking during high amplitude applications. The tip of the horn transmits the ultrasonic energy to the workpieces, and hence the tip is designed to match the geometry of the workpieces (the profile of the horn tip is matched with the contour of the workpiece). Such an arrangement ensures maximum energy transfer between the horn and the workpieces.
4. Fixture and tooling – The fixture or tool mounted on the machine base clamps the lower workpiece and prevents its movement when the ultrasonic energy is applied to the top workpiece. Compact ultrasonic welding machines are available with a sonotrode system that can be held in hand, and the joining of parts can be done by manually spot welding.