Studies On Electroplating Fabrication And Interface Thermal Stability Of The Cu/Al Composites

Cu/Al electric power fittings are usually used for connecting the Cu terminals and the Al conductors in transformer substations and electric transmission lines of power generation stations. At present, there are several technologies can be employed to fabricate the Cu/Al composite fittings. However, more or less shortages exist for each technique, so development of new technologies and processes is still paid more attention. Meanwhile, on the condition of long-term service at high voltage and current, syructure aging of the Cu/Al composite fittings likely happen, resulting in decrease of the reliability, even failure, of the composite fittings. Therefore, resrarch on the interfacial thermal stability of the the composite fittings is another essential issue.In this dissertation, Cu/Al composite materials were prepared through secondly dipping zinc layer or electroless plating Ni-P layer and then electroplating the Cu layer on the Al sheets, respectively. Some key issues on effect of the alkaline corrosion degree of the Al surface on the dipped Zn layer and electroplated Cu layer, mechanism of the electroless Ni-P alloy plating on Al surfaces in the acidic and/or alkaline plating baths were studied. Moreover, during aging, the interfacial characteristics and resistivities of the Cu/Al composites of different moderate layers of Zn and Ni-P alloy were also investugated. Accordingly, the interfacial thermal stability of the Cu/Al composites was estimated.As produing the Cu/Al composites with the dipping Zn middle layer, qualities of the dipping Zn layer and the Cu electroplating layer depend on the alkaline corrosion degree of the Al sheets. As the NaOH solution decreased to 5g/L, corrosion of the Al sheets is suitable without large and deep pits on them. The Zn layer has a high coverage ratio more than 95% and the Cu electroplating layer is flat and completely continuous. As produing the Cu/Al composites of the Ni-P alloy middle layer,5μm thick Ni-P alloy layer is deposited on the Al sheets after plating 5 min in the alkaline bath and then 25 min in the acidic bath. The alloy layer is dense, even and of a homogenous grain size. The Cu layer on the alloy layer therefore has a high surface quality.The dipping Zn middle layer has a great restraining effect on the Cu/Al interfacial diffusion and reaction during thermal aging. After annealing at 473 K for time till 360 h, the Zn layer survives, no Cu/Al interfacial diffusion was detected. As the composites annealed at 573 K and above, the Zn layer dissolves into Cu and Al and lose the prohibiting effect on the Cu/Al interface diffusion and reaction gradually. The diffusion of the Cu atoms in Al occurs to form the CuAl2 IMC layer. Finally, the tri-layer reaction region of Cu9Al4/CuAl/CuAl2 from the Cu side to the Al side forms. It leads to dramatic increase of the electrical resistivity of the Cu/Al composites.The electroless plating Ni-P alloy layer is stable. It keeps unattacked, even after the thermal aging at 673K for 360h. The Cu and Al sides are separated by the Ni-P alloy layer completely, no Cu/Al interfacial diffusion and reaction takes place. The resistivity of the Cu/Al composites is approximately constant. Overall, the thermal stability of the Cu/Al composites is greatly promoted.