The present invention relates to a drill pipe expansion assembly (1) for expansion of a completion component (3) located at a fixed position in a well (4) having a longitudinal axis (5), the drill pipe expansion assembly comprising an expansion unit (6) which is moveable in relation to the completion component comprising a tubular element (7) comprising a first end (8) and a second end (10) and a wall (11) having an outer face (12) and an outlet (14) for ejection of pressurised fluid, and a first sealing element (15,15a) and a second sealing element (15, 15b) arranged on the outer face on each side of the outlet, and a positioning unit (20) comprising a body (21) and a first fixation element (22) movable between a projected position and a retracted position in relation to the body, the positioning unit being connected with the tubular element in order to maintain the expansion unit in a fixed position in relation to the completion component (3) during expansion. The present invention further relates to a downhole system.