What is the usual method to connect to materials? We can use fasteners like nails, screw and bolts to hold two material together. But this method is suited for metals, wood or plastics. For clothes or fabrics, we stitch them together using thread, plastics we can glue together. Glue is a chemical agent used for bond. For metals, we use welding method. Melting the two metals to form a bond or fusion once the melted metal surface has cooled. The conventional process of welding involves an open torch flame or high intensity arc current to elevate the metal temperatures. We have discussed this in details in our earlier blogs, click here to check those.

An alternative to the conventional gas or electric arc welding is ultrasonic welding.

As the name suggests, ultrasonic means sound with high intensity. Hence, high intensity sound is the force used to bond two metals together.

Opera singers use high notes while singing. We can often see Opera singers demonstrating an act where the singer breaks a glass with his/her high-pitched note. This is an example that high-frequency sounds can result in material to fail or break. Is it possible to use the same high-frequency sound to bond materials together?

How does ultrasonic welding work?

The basic principle of welding is to heat the metals to be joined to a temperature near it's melting point. This principle stands correct with ultrasonic welding as well. You must be thinking, how can sound cause rise in temperature? Let's try an experiment. Rub or clap your hand together rapidly. You will notice that the temperature of your palm has risen. Why did this happen? When you rub your hand or clap them, there is friction between your palms, and this friction in turn raises the temperature of your plan. The same is applicable to metal. The friction can cause rapid increase in temperature.

High-frequency sound waves causes rapid vibrations within the material. The high frequency movement induces frictional heat generation.

Diagrammatic representation of the setup for ultrasonic welding — Setup for ultrasonic welding

The vibrations cause the materials to rub against each other and the friction raises the temperature at the surfaces in contact. This rapid frictional heat is what sets the conditions for the materials to bind together.

What is the setup of ultrasonic welding?

Diagram representing the assembly of ultrasonic welding machine — Ultrasonic welding machine

Generator

The generator is the power source of this operation. It converts the low-frequency electricity into high-frequency electricity.

Machine press

The setup for tools in ultrasonic welding are assembled on the machine press. The machine press is useful in configuration of the weld force. The tool jig handling and pneumatic cylinder are calibrated by valves on machine press.

Welding stack

This part of the ultrasonic welding setup which provides the ultrasonic mechanical vibrations. It is generally a three-part unit consisting of transducer, booster and welding horn, mounted on the welding press at the center of the booster section.

Transducer

The high frequency electricity is converted into mechanical vibration by transducer, which is also known as converter. The apparatus is made of a series of piezo-electric plates made of ceramic sandwiched between two metal blocks. There is a thin metal plate between each of the discs forming electrodes. The discs expand and contracts to produce axial movement i.e. vibrations.

The vibrations are of 15 to 20µm.

Booster

The booster is a part of the welding stack which amplifies the intensity of the mechanical vibrations produced by the transducer and transfer those to the welding horn. Along with the amplification of vibrations it serves a secondary purpose, to provide a location for mounting the stack on the welding press.

Booster is like a nozzle, concentrate and position the flow.

Welding horn

This element of the welding stack that supplies energy to the component being welded. The amplitude is typically between 30 and 120µm. The shape of the welding horn is important since stress, caused by the axial expansion and contraction of the horn, could lead to cracking in high amplitude applications.

The tip of the welding horn delivers the ultrasonic energy to the component being welded. The tip should be specifically designed to match the component. This ensures that maximum energy transfer between the horn and the component is achieved.

What is the process of ultrasonic welding?

The basic process of ultrasonic welding can be described by the following steps:

The parts which are to be welded are placed in the anvil or fixture of the ultrasonic welding machine.
The horn is brought to contact with the parts to be welded.
Pressure is applied to keep the horn in contact with the welded materials and to hold them together.
The horn delivers ultrasonic vibrations to heat up the materials. The vibrations move less than a millimeter either up-and-down or side-to-side.
The materials are welded together.
The horn gets retracted and the welded materials can be removed from the anvil.

Steps involved in ultrasonic welding — Steps involved in Ultrasonic welding

What actually happens during the welding process depends on the nature of the materials. In metals, the ultrasonic vibrations are delivered parallel to the plane of the materials. The frictional heat increases the temperature of the metal surfaces to about one third of the melting temperature, but does not melt the metals. Instead, the heat removes metal oxides and films from the surfaces. This allows the metal atoms to move between the two surfaces and form bonds that hold the metals together.

Let's take a look at the application of this process.

Plastics are bonded using ultrasonic welding and it is being used in making electronics, medical devices and car parts. Electrical components in computer circuit boards are connected through ultrasonic welding in micro chips. Even in electric motors and capacitors, the assembly is done via ultrasonic welding. The packaging industry also uses this method to make films, assemble tubes and blister packs. To some extent, automotive chassis made from aluminum are fabricated using ultrasonic welding.

More interestingly, one of the applications of ultrasonic welding is to bond shoes. Normally the sole and canvas part of the shoe are glued together or sewed, but it can also be done through ultrasonic welding. The plastic or polymer base of the shoe is fused with the fiber or canvas of the top cover.

History

A cost effective welding method was introduced in the 1940s. The technique, ultrasonic welding, used high-frequency sound waves and pressure to bond metals together and required less energy than conventional welding. Ultrasonic metal welding developed during the 1950s through the 1990s as the electronics used in the equipment became more sophisticated and computers could control the process. Since this time, the technique has been applied to plastics, where it has really become popular.

Summary

The frictional heat generated can raise the temperature significantly in a very short time.
The process is widely accepted in many applications ranging from automotive light clusters to consumer electronics products, such as mobile telephone casings.
Transducers are delicate devices and should be handled with care. Once the elements are broken, the transducer will not function.
The booster expands and contracts as the transducer applies the ultrasonic energy.
The tip of the welding horn delivers the ultrasonic energy to the component being welded. The tip should be specifically designed to match the component.
The factors that vary in ultrasonic welding are the frequency of the sound waves, the pressure applied to hold the materials together, and the time over which the ultrasound is applied.

Short Video

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Ultrasonic Welding