Abstract: The extrusion-resistance welding was developed to join Cu-Al casing pipe. The welded joints have characteristics of high cleanliness and high peel strength,without assistants and filling material. This technology has been widely used in refrigeration industry for the welding of various sized pipes. The microstructure of the Cu-Al joint was analyzed by SEM and TEM. The results show that Cu-Al joints made by extrusion-resistance welding are mainly composed of solid solution and intermetallic compounds (IMCs) (α,Al₂Cu and Al₄Cu₉). The high temperature generated by the resistance heat and pressure leads to the formation of new phase. The pressure exerted by the mandril extrudes the brittle composition,and enlarges the contact area,which makes the columnar grain easily formed.