Effect mechanism of Ni-P coating on the structure and properties of Cu/Al joints

To investigate the influence of Ni-P coating on the interfacial reaction of Cu/Al dissimilar metal brazing, two brazing filler metals, Zn98Al and BAl67CuSi, were first used to braze 3003 aluminum alloy and T2 copper with or without Ni-P coating. Four different brazing joints were obtained. The Cu side interfacial microstructure, joint shear strength, joint fracture morphology, joint microhardness and joint bending morphology were systematically studied. Additionally, the joints without Ni-P coating were also used for comparison. The results showed that the interfacial structure of the Cu/Zn98Al/Al brazed joint was Cu/diffusion layer+8.8 μm thickness Cu_3.2Zn_4.2Al_0.7. After the preparation of the Ni-P coating on the T2 copper surface, the interfacial structure transformed into Ni-P coating+1.5 μm thickness Al₃Ni, which leaded to the higher strength and toughness. However, the interfacial structure of the Cu/BAl67CuSi/Al brazed joint changed from diffusion layer+15 μm thickness CuAl₂ to Ni-P coating+1.8 μm thickness Cu₃NiAl₆, which leaded to the higher toughness but the lower strength. The effect mechanism of Ni-P coating on the interfacial reaction of Cu/Al joint was clarified. The mechanical properties test results were consistent with the microstructure transition of the Cu side interface of the joint. Finally, the interfacial reaction model of the Cu/Al joint was established, and the influence mechanism of Ni-P coating on the interfacial structure and mechanical properties of the Cu/Al joint was illustrated.