In an expansion assembly (1), an expandable bottom plug (10) is arranged below a primary expander (6) for expanding a tubular element (2). The expandable bottom plug (10) includes an expandable tubular clad element with a sealing section for sealing the expanded clad element to a lower portion of the expanded tubular element (2), and an anchoring section for anchoring the expanded clad element to the lower portion of the expanded tubular element. A secondary expander (22) is provided for radially expanding the clad element in the lower portion of the expanded tubular element by axial movement of the secondary expander through the clad element.