Meaning
Intermetallic phases appear during the joining of dissimilar metals such as copper and aluminum in busbar welding. A specific crystalline structure known as cu4al3 forms as a result of diffusion at the interface when heat is applied too long or at too high a temperature. The brittle layer reduces the mechanical strength of the electrical connection and creates a point of potential failure under mechanical stress.
This metallurgical development is a primary concern for the longevity of battery interconnects.
Thermal Development
High temperatures during the welding process accelerate the migration of copper atoms into the aluminum lattice. The formation of cu4al3 occurs most frequently in laser welding or ultrasonic bonding when the energy input is not precisely controlled. Monitoring the melt pool helps prevent the growth of this specific phase.
Connection Performance
Electrical conductivity drops as the thickness of the intermetallic layer increases. Because cu4al3 has a much higher resistivity than the parent metals, it creates a localized heat source during battery discharge. Excessive heat leads to a feedback loop that further degrades the junction.
Mechanical Strain
Fracture toughness is notably lower in the intermetallic zone than in the surrounding bulk material. Vibration and thermal expansion cause cracks to propagate through the cu4al3 layer. Reliable packs require joints where this phase is kept below a microscopic thickness.
