Ultrasonic Welding

 Ultrasonic welding is a solid-state welding process in which two workpieces are bonded as a result of a pressure exerted to welded parts combined with the application of high frequency acoustic vibration. It uses the principle of converting high frequency electrical energy into high frequency mechanical energy. This mechanical energy is a vertical motion excess of 15000 cycles/second. Ultrasonic vibration causes friction between parts which results in a closer contact between two surfaces with simultaneous local heating of the contact area. Interatomic bonds formed under these conditions provide strong joint.

The components to be joined are held together under pressure and subjected to vibrations, usually at a frequency of 15 kHz to 60 kHz. The vibrations produced by a welding sonotrode or sonometer or horn are used to soften or melt the thermoplastic material at the joint line. Welding time is less than 3 seconds. The welding can proceed with or without the application of external heat. Thickness of the welded parts is limited by the power of the ultrasonic generator.

1. Ultrasonic Welding Equipment

Ultrasonic welding equipment consists of a machine press, generator, converter or transducer, booster, sonotrode or horn and component support tooling. A schematic of an ultrasonic welding machine is shown in Figure 2.34.

(i) Generator:

The generator converts electrical power from the single-phase mains to the correct frequency and voltage for the transducer to convert into mechanical vibrations. The microprocessor unit controls the welding cycle and feeds back the key welding information to the user, via the user interface. The user interface also allows the operator to enter the required welding parameters.

(ii) Machine press:

The machine stand is designed to hold the welding system or stack and it applies the force necessary for welding. It consists of a base-plate to hold the tooling jig and a pneumatic cylinder to apply the force. The machine has a pressure gauge and regulator for adjustment of the welding force. There is also a flow control valve to allow the adjustment of the speed at which the welding head approaches the component being welded.

(iii) Welding stack:

It is a part of the machine that provides the ultrasonic mechanical vibrations. It is generally a three-part unit consisting of transducer, booster and welding horn which are mounted on the welding press at the centre-point of the booster section. The stack is a tuned resonator similar to a musical instrument tuning fork. In order to function, the resonant frequency of the tuned welding stack must closely match the frequency of the electrical signal from the generator (within 30 Hz).

(iv) Transducer:

The transducer is also known as converter which converts the electrical energy from the generator to the mechanical vibrations used for the welding process. Transducers are made of piezo-electric materials.

A piezo-electric material increases its length when current flows through a coil surrounding the device: Examples of such materials are quartz, tourmaline and Rochelle salt.

Transducer consists of a number of piezo-electric ceramic discs sandwiched between two metal blocks, usually titanium. Between each of the discs, there is a thin metal plate which forms the electrode. As the sinusoidal electrical signal is fed to the transducer via the electrodes, the discs expand and contract, and produce an axial and peak-to-peak movement of 15 to 20 μm.

(v) Booster:

The booster section of the welding stack serves two purposes. It is primarily to amplify the mechanical vibrations produced at the tip of the transducer and transfer them to the welding horn. Its secondary purpose is to provide a location for mounting the stack on the welding press. The booster expands and contracts as the transducer apply ultrasonic energy. 

(vi) Welding horn:

The welding horn is the element of the welding stack that supplies energy to the component being welded. The tip of the welding horn delivers the ultrasonic energy to the component being welded. The horn is specially designed to have the correct sonic properties and it transmits pressure to welded surfaces and it vibrates to make the weld. It can be of cylindrical shape, bar shape or more complex shapes according to the parts to be welded. The horn can be made of steel alloy, aluminium alloy or titanium.

(vii) Support tooling:

Finally, the base of the machine press supports the tooling which holds the components during welding operation. The support tooling is designed to prevent the movement of a lower component while the ultrasound is applied. It is machined to match the contours of the component surface intimately.

2. Advantages, Limitations and Applications of Ultrasonic Welding

Advantages:

1. Dissimilar metals may be joined.

2. High quality weld is obtained.

3. The process is excellent for joining thin sheets to thick sheets.

4. Local plastic deformation and mechanical mixing result into sound welds.

5. Since no bulk heating of the workpieces is involved, there is no danger of any mechanical or metallurgical bad effects.

6. The process can be integrated into automated production lines.

7. Moderate operator skill level is enough.

8. The process is fast, economical and it can be automated.

9. It ensures the increased flexibility and versatility.

10. It produces high strength joints.

Limitations:

1. Only small and thin parts may be welded.

2. Workpieces and equipment components may fatigue at reciprocating loads provided by ultrasonic vibration.

3. Workpieces may bond to the anvil.

4. It is not suitable for ductile materials since they yield under the stresses.

5. It needs specially designed joints.

6. Ultrasonic vibrations can damage electrical components.

7. Tooling costs for fixtures are high.

Applications:

I. Ultrasonic welding is used mainly for bonding small workpieces in electronics, manufacturing communication devices, medical tools and watches.

2. It is used in health care industries due to clean welds.

3. It is used in sealing and packaging, aircraft, missiles and fabrication of nuclear components.

4. Ultrasonic welding is used in the automotive industry to fabricate headlamp parts, dashboards, buttons and switches, fuel filter, fluid vessels, seat-belt locks, electronic key fobs, lamp assemblies and air ducts.

5. Ultrasonic welding is also used to make medical parts such as filters, catheters, medical garment and masks.

6. It is used in welding of tubes to sheets in solar panels.