Working principle, influencing factors and ultrasonic effect of ultrasonic welding machine

As a key link in the manufacturing process of lithium battery, the welding process is applied to the connection of aluminum/copper positive and negative collectors, electrode plates, battery packaging and other positions of lithium battery. Any welding joint defect will significantly affect the consistency of lithium battery performance. Therefore, it is necessary to understand the ultrasonic welding process.

1. Principle of ultrasonic welding

In the process of ultrasonic welding, the transducer converts high-frequency electrical signals into ultrasonic vibration signals. The high-frequency vibration is transmitted to the surface of the metal to be welded through the welding tool head. The interface metal oxide film is broken under a certain pressure and the intense friction of ultrasonic vibration. The interface clean metal contacts and under the combined effect of friction and ultrasonic softening, further produces plastic flow and diffusion, so that the connection area gradually increases and finally forms a reliable connection.

The formation of welded joint needs to go through two stages: transition stage and stability stage.

The transition stage is a short process to remove the facial mask and oxide on the surface of the weldment, and the stability stage is a process in which the interface produces mutual diffusion and makes mutual diffusion stable. In the transition stage, the oxide film on the surface of the weldment is broken due to strong friction. At this time, friction is the main heat source. The increase of the workpiece temperature reduces the yield strength of the workpiece material, which is conducive to the breaking of the oxide film on the workpiece surface and plastic deformation, and plays an important role in the formation of joints.

In the stable stage, the friction is weakened after the metal contact surface becomes smooth, and the heat is gathered at the welding interface due to plastic deformation. The heat in this process is generated by the plastic deformation process of the workpiece, and the pressure exerted by the tool head causes the force between the interface atoms to form a metal connection process.

Plastic deformation of welding area

The connection interface between the workpiece and the workpiece only exists below the indentation groove. The sum of the connection lengths under all grooves of the joint connection interface is the sum of the joint connection lengths, which is called the effective connection length, and is also a measure of plastic deformation.

2. Study on the technological parameters of ultrasonic metal welding

The main technological parameters of ultrasonic metal welding process include welding pressure, welding energy/time, tool head amplitude and tool, tooth pattern and size of the head, etc.

(1) Effect of pressure

The welding pressure has a significant effect on the quality of welded joints, and the strength of welded joints increases first and then decreases with the increase of pressure. Welding pressure will change the sliding resistance of the welding interface. A small welding pressure will lead to a small sliding resistance of the interface, so that the energy generated by friction is not enough to make the interface form an effective connection; The excessive welding pressure causes the tool head to press down too deeply, and the metal of the welding interface occludes each other, which affects the relative movement of the interface, hinders the further connection of the interface metal, and leads to the poor mechanical properties of the welding joint. Therefore, the appropriate welding pressure parameters are decisive to the welding quality.

(2) Impact of time

The welding time directly affects the energy input in the welding process and has a direct impact on the welding effect. The welding time is too short, the input energy is insufficient, and it is difficult to form an effective solder joint due to insufficient friction; With the increase of welding time, the temperature increases due to mutual friction, the workpiece material begins to soften, the oxide film on the interface of the welding area is damaged and plastic deformation occurs, which can form a good connection; When the welding time is further extended, the welding head is easy to form deep traces on the surface of the workpiece, which will adversely affect the welding effect. In addition, too long welding time is easy to cause the bonding between the welding head and the workpiece to be welded;

(3) Effect of amplitude

The amplitude of the vibration system formed between workpieces in the ultrasonic welding process directly affects the instantaneous velocity of the workpiece interface vibration, and ultimately affects the friction heat generation and plastic deformation, affecting the welding quality.

(4) Effect of welding head

The welding head is a key component of ultrasonic metal welding. During the welding process, the welding head should grasp the weldee under pressure, so that the mechanical vibration generated by the ultrasonic welding machine can be transmitted to the interface of the weldee to form a solid phase connection. Different welding head area will lead to different distribution of welding pressure in the welding process, that is, different stresses on the connection interface will lead to different friction in the welding process, so that the heat generated by friction in the welding process will be different, which will lead to different workpiece temperatures in the welding process, and ultimately affect the joint quality. The tooth depth of the welding head pattern determines the difficulty of embedding the welding head pattern into the workpiece surface, directly affects the indentation depth of the workpiece surface, indirectly affects the workpiece temperature during welding, and affects the joint quality. Therefore, the shape and size of welding head play a very important role in joint quality.

For the same welding head area, the plastic deformation of rectangular welding head is stronger than that of circular welding head; When the shape of the welding head is the same, the welding head with large area can make the plastic deformation of the welding zone more intense.

When the welding head area is the same, the round welding head is easier to extrude the workpiece material under the welding head, forming deeper indentation; When the shape of the welding head is the same, the welding head with small area makes the pressure in the contact area of the workpiece surface larger, thus forming deeper indentation.

3. Welding quality monitoring

(1) Destructive testing

The quality of ultrasonic welding can be determined directly by testing the tensile strength of the welding area. When the welding is faulty or over welded, the tensile strength will be very low.

(2) Infrared test

Different welding process parameters will lead to changes in the total welding energy supplied by the welder to the welded parts, which will inevitably lead to different friction effects in the welding process, resulting in changes in the heat generated in the welding process. Then the temperature of the workpiece in the welding process will also change. The temperature of the welding head workpiece contact zone can effectively reflect the joint strength, and the joint quality can be predicted by measuring the temperature of the workpiece in the welding process. However, the higher the temperature of the contact zone is, the better. For each material to be welded, there is a critical temperature value. When the workpiece temperature is lower than the critical temperature, the higher the temperature is, the higher the joint strength is; When the workpiece temperature is higher than the critical temperature, the joint strength will weaken.

(3) Energy feedback

Different welding parameters and different welding effects require different energy, which can be judged by detecting the welding energy in the welding process.

Ultrasonic effect of ultrasonic welding machine

Ultrasonic effect

When the ultrasonic wave propagates in the medium, the interaction between the ultrasonic wave and the medium causes physical and chemical changes in the medium, resulting in a series of mechanical, thermal, electromagnetic and chemical ultrasonic effects, including the following four effects:

Mechanical effect

The mechanical action of ultrasound can promote the emulsification of liquid, the liquefaction of gel and the dispersion of solid. When standing waves are formed in ultrasonic fluid medium, the tiny particles suspended in the fluid are condensed at the wave nodes due to mechanical force, forming periodic accumulation in space. When ultrasonic waves propagate in piezoelectric materials and magnetostrictive materials, the induced polarization and magnetization are caused by the mechanical action of ultrasonic waves (see dielectric physics and magnetostriction).

Cavitation

When ultrasonic wave acts on liquid, a large number of small bubbles can be produced. One reason is that the local tensile stress in the liquid forms negative pressure. The reduction of pressure makes the gas originally dissolved in the liquid supersaturated, and then escapes from the liquid to become small bubbles. Another reason is that the strong tensile stress "tears" the liquid into a cavity, which is called cavitation. The cavity is filled with liquid vapor or another gas dissolved in liquid, or even vacuum. Small bubbles formed due to cavitation will move, grow up or burst suddenly with the vibration of surrounding medium. When burst, the surrounding liquid suddenly rushes into the bubble to generate high temperature, high pressure and shock wave. The internal friction associated with cavitation can form electric charge, and generate luminescence phenomenon due to discharge in the bubble. The technology of ultrasonic treatment in liquid is mostly related to cavitation.

heat effect

Because of the high frequency and large energy of ultrasonic, it can produce significant thermal effect when absorbed by the medium.

Chemical effect

The action of ultrasound can promote or accelerate certain chemical reactions. For example, pure distilled water generates hydrogen peroxide after ultrasonic treatment; Nitric acid is generated from nitrogen dissolved water after ultrasonic treatment; The aqueous solution of dye will change color or fade after ultrasonic treatment. The occurrence of these phenomena is always accompanied by cavitation. Ultrasound can also accelerate the hydrolysis, decomposition and polymerization of many chemicals. Ultrasound also has obvious effects on photochemical and electrochemical processes. After ultrasonic treatment of aqueous solutions of various amino acids and other organic substances, the characteristic absorption spectral bands disappear and show uniform general absorption, which indicates that the molecular structure has changed due to cavitation.

Application scope and characteristics of ultrasonic metal welding machine?

The application scope of ultrasonic metal welding machine is: ultrasonic welding for different processes of lithium battery electrode sheet nickel, aluminum, copper, aluminum nickel composite tape and other materials. What are the characteristics? As follows:

(1) The two objects to be welded are overlapped and joined into a solid form by ultrasonic vibration pressure. The joint time is short, and the joint part does not produce casting structure (rough surface) defects.

(2) Compared with resistance welding, ultrasonic welding has long service life, less time for mold repair and replacement, and is easy to realize automation.

(3) Ultrasonic welding can be carried out between the same metal and different metals, which consumes much less energy than electric welding

(4) Compared with other pressure welding, ultrasonic welding requires less pressure and less than 10% deformation, while cold pressure welding requires 40% - 90% deformation of the workpiece.

(5) Ultrasonic welding does not require pretreatment of the surface to be welded and post-treatment after welding as other welding does.

(6) No external factors such as flux, metal filler and external heating are required for ultrasonic welding.

(7) Ultrasonic welding can reduce the temperature effect of materials to a low level (the temperature of the welding area does not exceed 50% of the absolute melting temperature of the metal to be welded), so that the metal structure will not change, so it is very suitable for welding applications in the electronic field.