Characterization of Joint Quality in Ultrasonic Welding of Battery Tabs

Abstract

Manufacturing of lithium-ion battery packs for electric or hybrid electric vehicles requires a significant amount of joining such as welding to meet desired power and capacity needs. However, conventional fusion welding processes such as resistance spot welding and laser welding face difficulties in joining multiple sheets of highly conductive, dissimilar materials with large weld areas. Ultrasonic metal welding overcomes these difficulties by using its inherent advantages derived from its solid-state process characteristics. Although ultrasonic metal welding is well-qualified for battery manufacturing, there is a lack of scientific quality guidelines for implementing ultrasonic welding in volume production. In order to establish such quality guidelines, this paper first identifies a number of critical weld attributes that determine the quality of welds by experimentally characterizing the weld formation over time. Samples of different weld quality were cross-sectioned and characterized with optical microscopy, scanning electronic microscopy (SEM), and hardness measurements in order to identify the relationship between physical weld attributes and weld performance. A novel microstructural classification method for the weld region of an ultrasonic metal weld is introduced to complete the weld quality characterization. The methodology provided in this paper links process parameters to weld performance through physical weld attributes.

Looking Deeper into the Galaxy (Note 7)

“…Figure 5 shows SEM images of the anode and cathode welded tab regions that transport current (charge) to the battery electrodes. This is typically used because conventional fusion welding processes (such as resistance spot welding and laser welding) are faced with challenges in joining multiple sheets of highly conductive, dissimilar materials over large areas [30]. The fusing together of cell current collectors and tabs achieved by ultrasonic welding uses high-frequency energy ≥20 kHz to generate oscillating shears at the interface between a sonotrode and metal sheets.…”

“…Evident within this type of welding is material flow, whereby excessive plastic deformation can thin and negatively affect the weld quality [30]. This can result in pieces of material from the thermomechanically affected zone (TMAZ), becoming detached and the weld bond being discontinuous.…”

“…We did however detect areas of unbonded weld regions and one of the assessment criteria of weld quality is correlated to the joint characteristics. An ultrasonic metal weld with good weld quality should have dense and continuous interfacial bonds [30].…”

“…ultrasonic metal weld with good weld quality should have dense and continuous interfacial bonds [30]. ultrasonic metal weld with good weld quality should have dense and continuous interfacial bonds [30]. The quality and reliability of Li-ion battery tabs is of significant importance especially as batteries are becoming increasingly energetic.…”

Joining of dissimilar materials

“…The oscillating shears generated by the relative movements create a solid-state bond between metals [290]. This process overcomes the difficulties of multiple sheets of dissimilar materials by using its inherent advantages derived from the solid-state process characteristics [128,129]. In addition, unlike resistance welding and laser welding, the temperature in the ultrasonic welding process does not exceed the melting point of the metal workpiece, eliminating undesirable compounds, phases and metallurgical defects that commonly exist in most other fusion welds [11,117].…”