Choosing the Right Ultrasonic Welding Equipment: A Comprehensive Guide

Choosing the Right Ultrasonic Welding Equipment A Comprehensive Guide

Composition of Ultrasonic Welding Equipment

Depending on the form of the joint in the workpiece, ultrasonic welding machines are divided into four types: spot welders, seam welders, ring welders, and line welders. An ultrasonic welding machine typically consists of an ultrasonic generator, an acoustic system (electric-acoustic transducer coupling device), a pressure mechanism, and a programmable device.

Also, there are ultrasonic welding machines specifically for welding plastics. The composition of a typical ultrasonic spot welder is shown in Figure 6-14.

Figure 6-14 Composition of Ultrasonic Spot Welder
  • 1 – Ultrasonic Generator
  • 2 – Transducer
  • 3 – Concentrator
  • 4 – Coupling Rod
  • 5 – Upper Sound Pole
  • 6 – Workpiece
  • 7 – Lower Sound Pole
  • 8 – Electromagnetic Pressure Device
  • 9 – Pressure Control Power Supply
  • 10 – Programmable Controller

Ultrasonic Generator

The ultrasonic generator is used to convert industrial frequency (50Hz) current into an oscillating current of 15~60kHz, which is then matched with the transducer through an output transformer.

Based on different power output elements, ultrasonic generators come in various circuit forms such as electron tube amplifiers, transistor amplifiers, thyristor inverters, and transistor inverters. Electron tube amplifier ultrasonic generators were the earliest designs and are reliable, stable, and powerful, but they are inefficient, with efficiencies only between 30% and 45%.

They have gradually been replaced by transistor amplifier ultrasonic generators. In recent years, transistor inverter ultrasonic generators have emerged, utilizing high-power CMOS devices and microcomputer control. They are compact, highly reliable, flexible in control, and have efficiencies above 95%.

To achieve optimal system performance and high-efficiency power output, the ultrasonic generator must be matched with the acoustic system.

Because mechanical loads can vary significantly during ultrasonic welding and the heat generated by the transducer elements can easily alter the material’s physical properties, fluctuations in the transducer’s temperature can cause resonance frequency changes, impacting welding quality.

Therefore, to ensure stable welding quality, an output automatic tracking device is generally installed inside the ultrasonic generator to maintain resonance and constant power output between the generator and the acoustic system.

Acoustic System

The acoustic system consists of a transducer, a concentrator, a coupling rod, and sound poles. Its primary function is to transmit elastic vibration energy to the workpiece for welding.

(1) Transducer

The transducer’s function is to convert the electromagnetic oscillation (electromagnetic energy) of the ultrasonic generator into mechanical vibration energy of the same frequency, serving as the mechanical vibration source of the welding machine. Two common types of transducers are magnetostrictive and piezoelectric transducers.

Magnetostrictive transducers work based on the magnetostrictive effect. This effect is a macroscopic synchronous expansion and contraction deformation phenomenon that occurs in the length direction of ferromagnetic materials when placed in an alternating magnetic field. Common ferromagnetic materials include nickel sheets and iron-aluminum alloys.

The magnetostrictive effect of the material is related to the alloy content and temperature. Magnetostrictive transducers are semi-permanent devices that operate stably and reliably, but their transduction efficiency is only 20%~40%. Except for some special applications, magnetostrictive transducers have been replaced by piezoelectric transducers.

Piezoelectric transducers work using the inverse piezoelectric effect of certain non-metallic piezoelectric crystals (such as quartz, lead zirconate, and lead zirconate titanate). When piezoelectric crystal materials are subjected to pressure or tension on a certain crystal face, charges will appear, which is known as the piezoelectric effect.

Conversely, when an alternating electric field is fed into the piezoelectric crystal in the direction of the piezoelectric axis, the crystal will undergo synchronous expansion and contraction in a certain direction, known as the inverse piezoelectric effect. The main advantages of piezoelectric transducers are high efficiency and ease of use, with a general efficiency of 80%~90%.

The disadvantage is that they are relatively fragile and have a shorter lifespan.

(2) Concentrator

The concentrator’s role is to transmit the high-frequency elastic vibration energy converted by the transducer to the workpiece. It coordinates the parameters of the transducer and the load. In addition, it also amplifies the output amplitude of the transducer and concentrates energy.

According to the requirements of the ultrasonic welding process, its amplitude value is generally between 5~40μm. However, the amplitude of a typical transducer is less than this value, so it is necessary to amplify the amplitude to reach the value required by the process.

The key to designing a concentrator is to ensure that its resonance frequency matches the vibration frequency of the transducer. Various types of conical rods can be used as concentrators, with common concentrator structural forms shown in Figure 6-15.

Figure 6-15 Structural Form of Accumulator

a) Conical shape
b) Exponential form
c) Stepped shape

Among them, the step-shaped concentrator has a larger amplification factor and is easy to process, but its resonance range is small and the stress concentration at the section abrupt change is the largest, so it is only suitable for small power ultrasonic welding machines.

The exponential-shaped concentrator operates stably and has a high structural strength, making it the most widely used in ultrasonic welders. The conical concentrator has a wide resonance frequency range, but the smallest amplification factor.

The concentrator works under fatigue conditions, and the design should focus on the structure’s strength, especially the connecting parts of each component of the acoustic system. The material used to manufacture the concentrator should have high fatigue resistance and low vibration loss.

Currently, commonly used materials include 45 steel, 30CrMnSi low alloy steel, high-speed tool steel, ultra-hard aluminum alloy, and titanium alloy, etc.

(3) Coupling Rod

The coupling rod is used for vibration energy transmission and coupling. It changes the longitudinal vibration output of the concentrator into bending vibration. The coupling rod is structurally very simple, usually a cylindrical rod, made from the same material as the concentrator, and the two are connected by brazing.

(4) Sound Poles

Sound poles are components that directly contact the workpiece and are divided into upper and lower sound poles. The structure of the sound pole is related to the type of welding machine.

For ultrasonic spot welders, the upper sound pole can be connected to the concentrator or coupling rod in various ways, and the end is made into a spherical surface with a radius of curvature 50~100 times the thickness of the workpiece. For example, for spot welders that can weld a 1mm workpiece, the radius of curvature of the end face of the upper sound pole can be selected as 75~80mm.

The lower sound pole is used to support the workpiece and bear the reaction force of the applied pressure. When designing, the anti-resonance state should be chosen, so that the vibration energy can be reflected on the surface of the lower sound pole to reduce energy loss. The upper and lower sound poles of ultrasonic seam welders are mostly a pair of rollers.

The shape of the upper sound pole of the welder used for welding plastic changes with the shape of the parts. However, regardless of the type of sound pole, the basic issue in design is the calculation of the natural frequency of the upper sound pole.

Pressurization Mechanism

The pressurization mechanism that applies static pressure to the welding site mainly includes hydraulic, pneumatic, electromagnetic pressurization, and self-weight pressurization.

Among them, high-power ultrasonic welders mostly use hydraulic methods, which have small impact forces; low-power ultrasonic welders mostly use electromagnetic pressurization or self-weight pressurization, which can match faster control programs. In actual use, the pressurization mechanism also includes the clamping mechanism of the workpiece.

Program Controller

The main function of the ultrasonic welding machine’s program control device is to control the ultrasonic welding process, such as pressure and pressure size control, welding time control, and maintaining pressure time control, etc. Currently, most welding program control devices use computers for program control.

The typical ultrasonic spot welding control program is shown in Figure 6-16. Before inputting ultrasonic waves to the workpiece, there must be a pre-pressurization time t1.

This not only prevents the tangential displacement of the workpiece caused by vibration, ensuring the size accuracy of the weld spot, but also avoids fatigue failure of the workpiece caused by the combination of dynamic pressure and vibration during the pressurization process.

During time period t3, static pressure F has been relieved, but the amplitude A of the ultrasonic wave continues to exist. There will be relative movement between the upper sound pole and the workpiece, which can effectively clear the possible adhesion phenomenon between the upper sound pole and the workpiece.

This adhesion phenomenon is likely to occur when welding aluminum, magnesium, and their alloys with ultrasonic waves.

Figure 6-16 Typical Ultrasonic Spot Welding Control Program
  • t1 – Pre-pressurization Time
  • t2 – Welding Time
  • t3 – Time to Eliminate Adhesion
  • t4 – Rest Time.

Models and Technical Parameters of Some Ultrasonic Welding Machines.

Figure 6-17 shows a photo of the typical appearance of an ultrasonic welding device. See Table 6-4 for the models and technical parameters of some domestic ultrasonic welding machines.

Figure 6-17: Photos of Typical Ultrasonic Welding Equipment
  • a) Miniature Ultrasonic Welding Machine
  • b) Computer-Aided Ultrasonic Welding Machine
  • c) Ultrasonic Welding Production Line

Table 6-4: Models and Technical Parameters of Some Ultrasonic Welding Machines

ModelFrequency
/kHz
Output Power
/W
Welding Time
/s
Pressure
/MPa
Welding Material Thickness or Area
/mm
Welding Joint ShapeWelding Form
CJJD-12022000.01~50.2~0.6Al 0.2~0.6StripSpot Welding
CSDH-A2022000.01~50.2~0.6Cu 1~14mm²Dot, StripSpot Welding
CSDH-B2022000.01~50.2~0.6Cu 1~14mm²Dot, StripSpot Welding (Computer Controlled)
CJGH-12022000.01~50.2~0.6Al 0.2~0.6DiscSeam Welding
NP1080201000Adjustable0.8Al 0.5~1.3DotSpot Welding
NP1680201500Adjustable0.8Cu 0.3~0.7DotSpot Welding
NP2080202000Adjustable0.8Ni 0.2~0.5DotSpot Welding
NP3080153000Adjustable0.8Ag 0.5~1.2DotSpot Welding
NP3680153600Adjustable0.8Cu 0.1~0.3                            Al 0.1~0.5DiscSeam Welding
Ubsj1020201000Adjustable0.8Al 0.1~0.3DotSpot Welding
Ubsj2020202000Adjustable0.8Al 0.2~0.6        
 Cu 0.3~0.5
DotSpot Welding
Ubsj2520202500Adjustable0.8Al 0.2~0.6DiscSeam Welding

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