Process of Ultrasonic Welding:
Although the theoretical method of manufacturing an ultrasonic weld is uncomplicated, the interactions of the varied weld parameters are vital and may be understood. When manufacturing an ultrasonic weld, there are 3 primary variables that interact;
• TIME the period of applied ultrasonic vibration
• AMPLITUDE the longitudinal displacement of the vibration
• FORCE the compressive force applied perpendicular (normal) to the direction of vibration
Power needed initiating and maintaining vibration (motion) throughout the weld cycle will be defined as:
P = F x A
P = Power (watts)
F = Force (psi)
A = Amplitude (microns)
Force = (Surface Area of the Cylinder) X (Air Pressure) X (Mechanical Advantage)
Energy is calculated as:
E = P x T
E = Energy (joules)
P = Power (watts)
T = Time (seconds)
Thus the complete ‘Weld to Energy’ process would be defined as:
E = (F x A) x T
A well designed ultrasonic metal welding system can compensate for normal variations within the surface conditions of the metals by delivering the required energy value. This is often achieved by permitting time (T) to regulate to suit the condition of the materials and deliver the required energy.
How Ultrasonic Welding Works:
Step 1: The parts to be welded are placed into a locating holder
Step 2: The ultrasonic tool descends to apply a clamping pressure between the weld parts.
Step 3: The tool then vibrates at a frequency 1 – 40 KHz. (The weld parts are thus scrubbed together under pressure causing surface oils and oxides to be dispersed)
Step 4: The base metals are then mechanically mixed causing a metallurgical bond between the parts. The parts are immediately welded. There is no hold time or curing time.
In Ultrasonic welding electrical power supply is applied to a Transducer at a frequency of 50 to 60 Hz, into a high frequency electrical supply operating at 20, 30 or 40 KHz. Here transducer converts electrical energy into mechanical energy. This electrical energy is supplied to the converts, which converts to mechanical energy at ultrasonic frequencies.
The vibrating energy is then transmitted through the booster that will increase the amplitude of the acoustic wave. The acoustic waves are then transmitted to the horn. The horn is an acoustic tool that transfers the vibrating energy directly to the components being assembled, and it additionally applies a welding pressure. The vibrations are transmitted through the workpiece to the joint area. The parts are “scrubbed” together under pressure at 20000 cycles per second. Here the vibrating energy is converted to heat through friction this then softens or melts the thermoplastic, and joins the components together. As the atoms are combined between the components to be welded, a real metallurgical bond is made.
Ultrasonic Welding Temperature Requirement:
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.