Laser welding of dissimilar metals, particularly copper (Cu) and aluminum (Al) interconnects, presents unique challenges due to differences in their thermophysical properties. The process is critical in battery manufacturing, where these metals are commonly used for electrical connections. The primary issues include intermetallic compound (IMC) formation, brittleness, and electrical performance trade-offs. This article explores solutions for laser welding Cu-Al joints, focusing on IMC mitigation, process optimization, and microstructural analysis.
Intermetallic compounds form when Cu and Al are welded due to their mutual solubility and diffusion at high temperatures. The most common IMCs in Cu-Al systems are Al2Cu (θ-phase), AlCu (η2-phase), and Al4Cu9 (γ1-phase). These compounds are brittle and can compromise joint integrity. The thickness of the IMC layer is a critical factor; excessive growth leads to cracking under mechanical or thermal stress. Studies show that keeping the IMC layer below 10 µm is essential for maintaining joint strength. The formation kinetics depend on temperature, cooling rate, and composition gradients, making laser parameters crucial for control.
Brittleness mitigation strategies focus on reducing IMC formation or improving joint ductility. One approach is using interlayers, such as nickel (Ni) or silver (Ag), which act as diffusion barriers. Ni interlayers, for example, reduce direct Cu-Al interaction by forming Ni-Al or Ni-Cu compounds with better mechanical properties. Another strategy involves beam oscillation, where the laser follows a predefined path (e.g., circular or sinusoidal) to distribute heat more evenly. This technique minimizes localized overheating, reduces IMC thickness, and improves weld homogeneity. Beam oscillation also refines the microstructure by promoting finer grain structures, which enhance mechanical properties.
Electrical performance is a key consideration in battery interconnects. While IMCs increase resistivity, their impact depends on thickness and distribution. A thin, discontinuous IMC layer may have negligible effects, whereas a thick layer can significantly raise resistance. Process optimization aims to balance mechanical strength and electrical conductivity. For instance, lower heat input reduces IMC formation but may compromise weld penetration. Pulsed laser welding offers a compromise by providing controlled energy deposition, minimizing IMC growth while ensuring adequate joint formation.
Microstructural analysis reveals the relationship between welding parameters and joint quality. Scanning electron microscopy (SEM) and energy-dispersive X-ray spectroscopy (EDS) are commonly used to examine IMC distribution. For example, a study of Cu-Al welds showed that a linear weld without oscillation produced a 15 µm IMC layer with cracks, while an oscillated weld reduced the layer to 5 µm with no visible defects. X-ray diffraction (XRD) can identify specific IMC phases, aiding in process refinement. Electron backscatter diffraction (EBSD) provides insights into grain orientation and strain distribution, which correlate with mechanical performance.
Process parameters such as laser power, speed, and focal position significantly influence weld quality. High power and low speed increase heat input, exacerbating IMC formation. Conversely, low power may result in incomplete fusion. Optimal parameters depend on material thickness and joint configuration. For 0.5 mm Cu-Al sheets, a power range of 1.5–2.5 kW and a speed of 3–6 m/min are typical. Shielding gases like argon or helium prevent oxidation and stabilize the weld pool. Gas flow rates between 10–20 L/min are commonly used.
Post-weld heat treatment (PWHT) can alleviate residual stresses and homogenize the microstructure. However, excessive heating may promote additional IMC growth, necessitating careful temperature control. Aging studies indicate that annealing at 200–250°C for 30 minutes improves ductility without significantly increasing IMC thickness. Mechanical testing, including tensile shear and peel tests, validates joint performance. A well-optimized Cu-Al laser weld can achieve tensile strengths exceeding 80% of the base Al material.
Electrical resistance measurements are critical for battery applications. Four-point probe tests show that optimized laser welds exhibit resistivities close to bulk Al, with increases of less than 10%. This is acceptable for most battery interconnect applications, where mechanical reliability is equally important. Cross-sectional analysis confirms that resistance correlates with IMC thickness and continuity.
In summary, laser welding of Cu-Al interconnects requires careful control of IMC formation through interlayers, beam oscillation, and parameter optimization. Microstructural analysis confirms that these strategies improve joint integrity and electrical performance. The process is viable for battery manufacturing, provided that welding parameters are tailored to specific material and design requirements. Future work may explore advanced techniques like dual-beam welding or in-situ monitoring to further enhance reliability.