Factors Affecting Ultrasonic Welding of Lithium Batteries

1. Pressure Factors
Welding pressure has a significant impact on the quality of the welded joint; specifically, the strength of the joint initially increases as pressure rises, but subsequently decreases. Welding pressure alters the sliding resistance at the welding interface; insufficient pressure results in low sliding resistance, meaning the energy generated by friction is insufficient to establish an effective bond at the interface. Conversely, excessive pressure causes the welding tool head to press down too deeply, leading to mechanical interlocking of the metals at the interface. This hinders relative motion between the surfaces, impedes further bonding of the interface metals, and ultimately degrades the mechanical properties of the welded joint. Therefore, selecting appropriate welding pressure parameters is critical to ensuring high welding quality.

2. Energy/Time Factors
Welding time directly governs the energy input during the welding process and has a direct bearing on the final welding outcome. If the welding duration is too short, the energy input is insufficient; without adequate friction, it becomes difficult to form an effective weld spot. As welding time increases, mutual friction generates heat, causing the workpiece material to soften; simultaneously, the oxide film at the interface within the welding zone ruptures, and plastic deformation occurs, facilitating the formation of a robust bond. However, if the welding time is extended further, the welding tool head may leave deep indentations on the workpiece surface, adversely affecting the quality of the weld. Furthermore, excessively long welding times increase the risk of the tool head adhering to the workpiece being welded.

3. Amplitude Factors
During the ultrasonic welding process, the workpieces collectively form a vibrating system. The amplitude of these vibrations directly influences the instantaneous velocity of the interface between the workpieces. This, in turn, affects the generation of frictional heat and the extent of plastic deformation, thereby impacting the overall quality of the weld.

4. Tool Head Factors
The welding tool head is a critical component in ultrasonic metal welding systems. During the welding process, the tool head-acting under pressure-must firmly grip the workpiece being welded. This ensures that the mechanical vibrations generated by the ultrasonic welder are effectively transmitted to the interface of the workpiece, thereby enabling the formation of a solid-state bond.
Variations in the surface area of ​​the tool head result in differing distributions of welding pressure across the interface. Consequently, the stress levels at the bonding interface vary, leading to differences in frictional forces. These variations in friction result in differing amounts of heat generation during the process, causing temperature fluctuations within the workpiece, and ultimately affecting the quality of the welded joint. When the weld tip area is constant, a rectangular weld tip induces a greater degree of plastic deformation than a circular one; conversely, when the weld tip shape is held constant, a larger tip area results in more intense plastic deformation within the weld zone.

With a constant weld tip area, a circular tip is more prone to extruding the workpiece material beneath it, thereby forming a deeper indentation; conversely, when the weld tip shape is held constant, a smaller tip area generates higher contact pressure on the workpiece surface, resulting in a deeper indentation.