Achieving brittle-intermetallic-free and high-conductivity aluminum/copper joints using nickel-phosphorus coatings

Mechanical degradation due to brittle intermetallic compounds (IMCs) formed at the faying interface is a predominant deficiency in dissimilar metal joints. In copper/aluminum (Cu/Al) joints, additional defects (such as partially-bonded interfaces, porosity and cracks) lead to further weakened strength and lowered electrical conductivity. In this study, nickel‑phosphorus (Ni-P) coatings are deposited on Al to address these issues. With the aid of Ni-P coatings, the detrimental Cu-Al IMC is eliminated, a donut-shaped weld with a partially-bonded interface is evolved into an hourglass-shaped weld with a fully-bonded interface, while the porosity and cracks are inhibited. Numerical simulations indicate that, during the welding without Ni-P coating, the Al oxide aggravates the inhomogeneity of heat generation at the Cu/Al interface, promoting the formation of donut-shaped weld and defects. Microstructural characterization suggests that the Ni-P coatings obstruct the Cu-Al interdiffusion which results in CuAl₂-free interfaces, while the amorphous Ni-P convert into eutectic microstructure composed of nanocrystalline Ni and fine Ni₃P grains through a solid-state transformation. Using the Ni-P coatings, joints gain an improvement of 140% in lap-shear peak load and a 25-fold increase in lap-shear maximum elongation, as well as an 84% reduction in electrical resistance.