Achieving brittle-intermetallic-free and high-conductivity aluminum/copper joints using nickel-phosphorus coatings
Nannan Chen, Hongliang Wang, Pawan Veeresh, Jingjing Li, Jay Oswald, Xi Liang, S. Wagner, Ryan C. Sekol, Vic Liu, Ke Wang, Thomas A. Perry, James G. Schroth
Abstract
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.