composite tubes;  hydroforming;  Residual contact pressure

According to the basic assumption of the composite pipe, the stress state during the hydraulic expansion process of the composite pipe is analyzed. Using the elastoplastic theory, the stress-strain relationship between the inner pipe and the outer pipe of the composite pipe is discussed, and the deformation coordination conditions of the circumferential strain of the inner and outer pipes of the composite pipe are used to calculate the correlation formula of the residual contact pressure Pc* between the hydraulic expansion force Pi and the inner and outer pipes of the composite pipe, and the working pressure range of the hydroforming fluid pressure Pi under different conditions is obtained according to the different residual contact pressures during the forming process of the composite pipe.