Studies On The Interfacial Microstructures And Mechanical Properties Of Thin-Wall Cu/Al Tube

The electric resistance pressure welding technique invented by both Ocean University of China and Qingdao Haiqing Machinery General Factory involves many items of innovative technology and solves the welding problems of thin-wall Cu/Al tube, such as joint embrittlement and poor sealing. The interfacial microstructures and mechanical properties of the electric resistance pressure welded Cu/Al thin walled tube were studied in this paper. Metallographic microscope and scanning electron microscope (SEM) were used to observe and analyze the interfacial microstructures. The appearance and the component of the microstructures were researched on microcosmic angle. Cu/Al binary phase diagram, the theory of metal crystallography and the welding technology were used to analyze the factors of formation. The peel strength and tensile strength of different diameter and wall thickness Cu/Al joints were tested by the universal tension equipment. The objective is to analyze the possible influences of microstructures on the mechanical properties and explore the relevant welding standards. The results show that:1 A certain interdiffusion of aluminum and copper takes place in the welding interface of the Cu/Al thin walled tube and the diffusion from Cu side is deeper than the opposite direction. The welding interface region of the Cu/Al tube is about 1-10μm, which is consist of columnar crystals, lamellar crystals and granular crystals. There are some defects existing in the wider welding interface, such as pore and diffusion hole. The average ratio of aluminum atom and copper atom of interface is 3 to 2. There may be eutectic organization(α+θ) with two-phase alternate growth and intermetallic compounds like CuAl2 and CuAl.2 The organizations of columnar crystals, lamellar crystals and granular crystals in the interface are related to the temperature gradient, composition undercooling, welding parameters and the non-equilibrium crystallization. In the positive temperature gradient, the interface tends to planar growth, which is prone to lamella. When there exists a small constitutional supercooling in the positive temperature gradient, the interfacial plane is easily broken and produces the cellular organization. In the negative temperature gradient, the interface processes seriously and is easy to generate dendritic organization (columnar structure). With the current increasing or welded time prolonging, the weld width increases, the number of phases and interface layers is bigger and the weldability is poorer. Intermetallic compound CuAl is generated by peritectic reaction and transformation. CuAl2 is generated by the peritectic reaction. Eutectic organization(α+θ) is generated by the eutectic reaction.3 The connection mechanism of the electric resistance pressure welded Cu/Al thin walled tube:Under the action of pressure, hardened interfacial layer of copper and aluminum is torn, so soft aluminum is squeezed into cracks of copper to contact with each other. In terms of heat resistance, the surface atoms are activated and formed activation center in order to generate atom bond or metal bond. Then various microstructures are generated by diffusion or crystallization, which lead to the formation of the boundary layer with a certain diffusional depth. In other words, it is comprised of physical contact phase of copper and aluminum, contact activation phase and diffusion phase.4 Mechanical tests on large number of specimens:Peel test indicates that, the range of peel strength of samples with the same diameter is basically according to normal distribution. Peel strength increases on the whole with the increasing of wall thickness. Statistical analysis of large amounts of data combined with wall thickness obtains a qualified standard of peel strength:σ=38.347(?)-12.561. Peel strength is proportional to the remaining strip width and diameter of aluminum tube. Specimens are ductile fracture from the macro-morphology of the specimen fractured. Tensile test shows that, specimens fracture is aluminum fracture and joint's tensile strength is greater than the parent aluminum's. The tensile strength of aluminum tube don't reduce after welding, which is consistent with the parent metal 1060A1 and 3003Al basically. The weldability of electric resistance pressure welded thin-wall Cu/Al tube is good.