Joining of Metal-Thermoplastic-Tube-Joints by Hydraulic Expansion

To encounter current issues regarding climate change, the hybridization of structures with lighter, often dissimilar, materials is an essential cornerstone of lightweight design. The different mechanical behavior of these materials results in challenges in terms of joining. This paper utilizes the joining process by hydraulic expansion to manufacture tube-to-tube joints of aluminum alloy AA6060 T66 and thermoplastic polycarbonate (Lexan) at room temperature. In contrast to metals, elastic and plastic strains coexist in thermoplastics from the beginning of deformation. Based on the theory of linear elasticity, an equation was derived to calculate the fluid pressure that expands the polycarbonate up to a strain value where plastic strains start to increase significantly in comparison to elastic strains. Tensile tests of the joined tubes revealed that the transferable tensile load increased approximately exponentially with increasing plastic deformation of the polycarbonate. With ongoing plastic deformation, micro-cracks appeared and merged within the thermoplastic. The appearance of these so-called crazes had no negative influence on the transferable load within the range of applied fluid pressure.