For stacked pouch cells, the ultrasonic welding process is an essential and critical step. Ensuring the welding strength between welded samples (current collector foils and tab pieces) is a very important research topic.

A schematic diagram of the ultrasonic welding machine is shown below:

1. Welding Machine Principle

In simple terms, ultrasonic welding applies energy generated by high-frequency vibration to the surface of the workpieces to be welded, thereby bonding metal parts together.

The principle of ultrasonic metal welding is as follows: the high-frequency voltage generated by the ultrasonic generator is converted into high-frequency mechanical vibration by the transducer. The mechanical vibration is then adjusted in amplitude by the booster and transmitted to the horn. Under pressure, the horn and the anvil clamp the upper and lower workpieces.

After ultrasonic activation, tens of thousands of high-frequency vibration waves per second are transmitted to the surfaces of the two metal workpieces. The welding pressure causes friction between metal surfaces, forming a molecular-level fusion, thereby achieving welding.

2. Welding Process Parameters

For ultrasonic welding, the interaction between different parameters is very important. It can be summarized by the following formula:

E = P × T = (F × A × f) × T

• E: Energy (J)
• P: Power (W)
• T: Time (s)
• F: Pressure (N/kPa)
• A: Amplitude (μm)
• f: Frequency (Hz)

Parameter Definitions:

1. Welding Modes

• Energy Mode: Ultrasonic energy is applied until the preset welding energy is reached. This is the most commonly used mode in lithium battery manufacturing.
• Height Mode: Ultrasonic energy is applied until the welding components reach a preset position.
• Time Mode: Ultrasonic energy is applied until the preset welding time is reached.

2. Peak Power

The maximum power required to keep the ultrasonic system operating during the welding cycle.

3. Power

A function of pressure and amplitude. The system monitors the power required to maintain movement and generates a power curve. The displayed value is peak power.

4. Amplitude

The vibration distance of the welding area under pressure. It determines the welding depth.

5. Pressure

The mechanical pressure applied during welding. Welding strength first increases and then decreases as pressure increases.

• Too low pressure → insufficient friction → weak bonding
• Too high pressure → excessive penetration → reduced interface movement → weaker bonding

6. Frequency

The number of complete vibrations per second.

3. Welding Actions

During ultrasonic welding, parameters such as welding distance, pressure, and amplitude/power play a critical role in welding quality.

1. Welding Distance

From start to holding stage, the gap between horn and anvil gradually decreases.

2. Welding Pressure

Pressure increases from zero after contact until reaching the preset value, then decreases after welding.

3. Welding Amplitude/Power

Once pressure is reached, ultrasonic energy is applied until welding ends.

4. Parameter Settings

To better understand the welding process, we break down the ultrasonic welding machine’s operation (from the start to the end of ultrasonic welding) into the relevant actions, welding actions, and parameter settings (the horizontal lines correspond to the process parameters for two types of welding machines).

1. Delay Time

Time from machine start to ultrasonic activation (typically ~650 ms). Too short may trigger ultrasound before contact.

2. Holding Time

Time from ultrasonic end to horn lift. Usually set to 0 if no sticking occurs.

3. Secondary Ultrasonic Delay Time

Controls delay before second ultrasonic action. Helps reduce sticking and remove debris.

4. Lag Ultrasonic Delay Time

Time between ultrasonic end and delayed ultrasonic start.

5. Secondary Ultrasonic Duration / Lag Duration

Duration of secondary ultrasonic energy transmission.

5. Welding Process Experiment Summary

1. Insufficient Welding Strength – Solutions

• Increase welding energy (note: strength increases then decreases with energy)
• Increase amplitude (higher power, shorter time, improved strength)
• Adjust pressure (find optimal value, not linear relationship)
• Check material compatibility or horn pattern mismatch

2. Workpiece Sticking to Horn – Solutions

• Clean tab surface with alcohol wipes
• Clean horn/anvil to remove metal debris
• Optimize parameters (reduce pressure or increase amplitude)
• Enable secondary ultrasonic function to reduce sticking