A detailed analysis of metal flow through a porthole die to produce a rectangular hollow aluminium profile was performed by means of three-dimensional FE simulation using DEFORM 3D. It was aimed at revealing the flow patterns of a medium-strength aluminium alloy 7020 through a porthole die and gaining an insight into the formation of longitudinal weld seams inside the welding chamber during extrusion. In the case of extruding a rectangular hollow profile through a porthole die with four ports, two neighbouring ports were different from each other. Using an FE model including these two ports, different flow patterns of two individual metal streams were revealed. The 3D FE simulation also showed how two unequal metal streams contacted each other and became bonded in the welding chamber under a certain hydrostatic pressure and at a certain temperature, before the metal flew through the die bearing. The difference in velocity between the metal streams led to uneven flow at the die bearing and thus a wavy extrusion nose.