Studies On The Interfacial Microstructure And Corrosion Resistance Of Electric Resistance Pressure Welded Thin-wall Cu/Al Tube

Copper plays important roles in refrigeration industry because of its good heat conduction, good conductivity and higher corrosion resistance. However, in recent years , it has been an urgent demand to find copper substitute material owing to the price rising of copper and lack of copper resources. Aluminum ,with low density ,high heat conductivity and good electrical conductivity, can become an ideal replacement material of copper. However, copper cannot be replaced in some pipelines and how to join Cu tubes with Al tubes is to be solved. The Cu-Al welding belongs to dissimilar metal welding and it would produce brittle compounds, which affect the performance of Cu-Al joint. Therefore, the welding problem would hinder the application of Cu/Al tubes.The electric resistance pressure welding technique developed by both Qingdao Haiqing Machinery General Factory and Ocean University of China involves many items of innovative technology and solves the welding problem of thin-wall Cu/Al Tube. In this paper, the interfacial microstructure and characteristics of Cu/Al tubes welded by the electric resistance pressure welding are studied by SEM, TEM and other instruments. The objective is to know the interfacial microstructure and its formation cause, to analyze the possible influences of microstructure on the performance and find out the relation between the performance and the welding technology, and consequently to optimize the welding process and improve the quality of Cu-Al joint. In addition, neutral salt spray test is carried on to study the corrosion resistance of Cu/Al tubes. The results show that:1,The interfacial bonding strength of Cu-Al joint in 5mm range near the welding seam of outer surface is high and the joint is hard to tear, but the interfacial bonding strength out of the range is low and the joint is easy to tear with pore and other defects. A certain interdiffusion of aluminum and copper takes place in the welding interface of the Cu/Al tubes and the diffusion from Cu side into Al side is deeper than the opposite direction. The narrow interface consists of Al solid solution in Cu, CuAl thin layer, CuAl₂ columnar crystal layer and Cu solid solution in Al (from Cu side to Al side). The wide interface consists of Al solid solution in Cu, CuAl thin layer, CuAl₂ columnar crystal layer,α-Al/CuAl₂ eutectic layer and Cu solid solution in Al (from Cu side to Al side).2,Formation of the interfacial morphology may be related to the influence of temperature gradient and composition undercooling and formation of CuAl₂, CuAl phase andα-Al/CuAl₂ eutectic structure may be related to peritectic reaction and eutectic reaction. The welding technology is possible relevant to the grain refinement of the columnar crystals and the eutectic and refined columnar crystals can improve the joint strength with a thinner layer. Formation of the eutectic structure should be avoided because of its great brittleness and process parameters should be improved to decrease the eutectic formation and optimize the quality of Cu-Al joint.3,The neutral salt spray test indicates that, under marine atmospheric environment, pitting occurred on the aluminum tube surface and green and white corrosion products appeared on the copper surface. White corrosion products appeared in the welding seam and the Al side of the welding seam suffered from serious corrosion and even presented powdering phenomenon. The results of EDS indicates that the corrosion products may contain CuCl₂·3Cu(OH)₂, the composite membrane with CuCl₂·3Cu(OH)₂ and Al(OH)₃ and the mixture of Al(OH)₃ and Na₂O·xAl₂O₃.