| Copper and aluminium are widely used in electric power industry. An electrical connection between copper and aluminium is needed to allow the uninterrupted passage of electrical current during generation, transmission, substation and distribution of electric power. The commonly used mechanical fastening with bolts and nuts provides for easy disassembly but leads to a small effective conducting area, hence increasing the contact resistance and decreasing the safety of service. Thus, flash welding and friction welding are characterized to provide full bonding between copper and aluminium. However, in service, frequent current surges, environment corrosion and mechanical force cause the increase of joint defects, such as voids, oxide inclusion and intermetallic compounds (IMCs). This, in turn, can seriously decrease the overall electrical stability and mechanical integrity of Cu-Al joint.IMCs cannot be avoided in Cu-Al joint and their presence detrimentally affects the properties of joint due to high electrical resistance and brittle characteristics. Transient liquid phase (TLP) diffusion bonding can produce excellent joint, free from intermetallic precipitates and voids, and identical in composition and microstructure to that of the base metal. It has been successfully applied in joining of dissimilar materials and other materials to be difficultly welded. Some researchers, however, found that TLP bonding without interlayer alloy cannot produce a high strength Cu-Al joint due to the presence of IMCs. In order to prolong the service life of flash welded Cu-Al joint and improve the strength of TLP bonded Cu-Al joint, a novel Al base interlayer alloy and bonding process were developed to TLP bond copper to aluminium. The growth behavior of IMCs as well as its effect on joint properties was also studied.The base metals were pure copper (T2), pure aluminium (1060) and aluminium alloy (5A02). The interlayer alloy was Al base foil with the thickness of 60～80μm. A TLP bonding apparatus was self-developed to bond copper plate to aluminium plate, pure aluminium pipe and aluminium alloy pipe with faying surface of 50mm×5mm, φ57mm×3.5mm and φ60mmx8mm respectively. The faying surface was machined to a Ra0.23～0.68μm and degreased in alcohol. Two parts to be welded were mounted on a loading device. After arranging the interlayer foil between two parts, they were pressed along axial direction and induction heated. A chromel/type K thermocouple was spot-welded onto the outside edges of the test part for temperature control. During induction heating, the faying area was covered by argon flux to prevent the oxidation. When the bonding was finished according to the designed bonding temperature, time and pressure, the joint was automatically cooled to room temperature. The mechanical properties and the electrical resistance of the joints were obtained on universal test machine and portable micro-ohmmeter, respectively. The joints were cross-sectioned and analyzed with scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX). The IMCs were characterized on fracture sample using XRD. A heat treatment of 350℃×500 hours was applied to study the effect of IMCs on joint properties of TLP bonding and flash welding.Based on TLP bonding fundamental and oxide film removal mechanism, a series of Al base interlayer alloy with Si 6 mass%-8 mass%,Cu 2.5～4 mass%, Mg 0～2.5 mass%, Ga 1 mass%, RE 0.05 mass% were designed using alloying method. The joint microstructure shows that Al-8Si-4Cu-2Mg-1Ga-0.05RE can effectively depress melting point and remove oxide film, then accomplish the TLP bonding process in air. The tensile strength and bending strength of the joint are superior to those of non-interlayer TLP bonded joint and equivalent to those of aluminium base metal. The conductivity is superior to those of aluminium base metal. Mg has the excellent oxide film removal effect, but excessive Mg will break the balance of interface and lead to micro coarse interface, decreasing the ductility and conductivity of the joint. Cu and Si can strongly depress the melting point and lead to transient liquid, removaling the residual oxide scale. Therefore, the ductility and conductivity of the joint are improved.According to the high temperature tensile strength of aluminium, a wide range of bonding pressures are designed to exclude the oxide film, reduce voids and clean the interface in the Cu-Al joint. The phases and the thickness of IMCs are unaffected by the bonding pressure in the rangen of 8-13MPa, and so do the tensile strength and the resistivity of the joint. Based on the thickness of IMCs, the relation between bonding temperature and time can be calculated by Fick diffusion law. With the increase of bonding time from 2s to 10s, the growth of Cu9Al4 becomes slow while the growth of CuAl2 becomes fast, resulting in some voids. The fracture of Cu-Al joint changes from ductility to brittleness, and both the tensile strength and conductivity of the joint are decreased. The phases of IMCs are unaffected by the bonding temperature in the range of 560℃～640℃, while the thickness and defects of the joint are changed with the bonding temperature. A eutectic structure with thick dentric IMCs and plenty of voids are formed due to non-equilibrum solidification at low bonding temperature. A thick interface IMCs layer is formed at high bonding temperature due to rapid diffusion of copper and aluminium. The voids are reduced with the bonding temperature increasing but some cracks are formed at elevated temperature. A good Cu-Al joint is produced at 600℃ for 2 seconds under a bonding pressure of 9MPa. The joint is free from voids and oxide scale, and with two thin IMCs layers, which are Cu9Al4 and CuAl2 with total thickness of 2μm. The joint fails in aluminium when tensile test and no failure occur when the joint is bent to 180°. The resistivity of joint is lower than that of aluminium base metal and reaches the theoretical value. The joint poperties are superior to that of flash welded joint and non-interlayer TLP bonded joint.Heat treatments show that the IMCs in Cu-Al joint change from original two layers (Cu9Al4/CuAl2) to three layers (Cu9Al4/CuAl/CuAl2) both in TLP bonding and flash welding, and Cu9Al4 changes to CuO during heat treatment of flash welded Cu-Al joint. Some kirkendall voids form during the growth of IMCs. The voids in TLP bonded joint form at the interface of CugAl4/CuAl, and the voids in flash welded joint form at the interface of Cu/CugAl4 and grow up to crack. The residual stress gradient causes the difference of voids location in both joints. The growth kinetic of IMCs can be presented by w=K·tn, n=0.37～0.78. The growth of CuAl2 and CuAl do not conform to the parabolic law (n≠0.5), while the growth of total IMCs and Cu9Al4 conform to the parabolic law (n=0.5). The growth rate of CuAl2 is similar to that of Cu9Al4, and the growth of CuAl is the slowest among three IMCs. The growth of IMCs in flash welded joint is faster than that in TLP bonded joint during heat treatment. There is a critical thickness (w0) of IMCs in Cu-Al joint:w< w0, the joint tensile strength and resistivity do not change with the thickness of IMCs. w>w0, the joint tensile strength and resistivity are decreased with increasing the thickness of IMCs according to a linear relationship.The initial and service performances of Cu-Al joint produced by TLP bonding are higher than that of flash welding. Low residual stress gradient in TLP bonded joint can controll the growth of IMCs and voids, therefore improve the sercvice performance. |
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