Influence of the amount of CuAl2 intermetallide on the microhardness of the welded joint

Abstract

Regularities of the formation of the structure and properties of the transition zone of the welded joint of copper-aluminum tubes into the socket, performed by resistance welding, are considered. The change in the element-by-element composition of the sections of a dissimilar welded joint obtained by solid-liquid transformations of the tube metal is determined. The content of mass fractions of components of a natural-layered composite material (CM) obtained in telescopic welded joints of Al-Cu pipes by resistance-butt welding was: Al (63–66)% + Cu (29–34)%. At the surface of the copper tube they were: Al ~(15–17)% + Cu ~(74–82)%, on the side of the composite interlayer, respectively – Al ~(45–48)% + Cu ~(46–48)%.A composite intermediate layer was revealed in a welded joint with aluminum fusion and diffusion copper joint witha eutectic intermediate composite layer of complex composition. It contains a naturally aged aluminum-based solid solution, a small amount of a secondary phase, a eutectic, alumina and an intermetallic compound (CuAl2), and from the metal side of the copper tube, an intermetallic compound, a supersaturated solid solution, martensite, alumina, and a copper-based interstitial phase. The length of the interface in the Cu – CuAl2 section is ~6...8 ?m, the length of the interface in the CuAl2 – Al section is ~2...6 ?m. The chemical composition and microhardness of the metal have been determined over the thickness of the surfaces of fusion and diffusion welding (brazing) – the connection of the heat exchanger tubes. The range of values of the microhardness of the fusion surface is НV100 ~370…2200 MPa. The reasons for the detachment of the aluminum tube and the phase of penetration along the boundaries with the copper tube in operation are indicated.