A method is provided for joining tubes. An outer tube and an inner tube are provided. An end of the inner tube is inserted into an end of the outer tube to form an overlapped region of tube wall and form a pressure tight joint between the tubes. The overlapped region of the tubes is heated. The tubes are placed in pair of dies having a die cavity adjacent to the outer tube. Pressure is introduced into the tubes to expand the overlapped region of the tubes outwardly into the die cavity and thereby form an interlocking joint between the overlapped region of the outer tube and the inner tube.