In extrusion of aluminium alloys, using porthole dies, the billet metal is split into separate metal streams during flow into the ports. After that, the streams join by seam pressure welding in the weld chamber of the die and finally flow through the die exit emerging as the desired extrusion. Because of re-joining of the metal streams the extrusion will contain as many extrusion seam welds as there are bridges in the die. Dependent on the die design the metal will flow in such a way that a gas pocket may form behind the die bridge and remain stable here throughout the course of the extrusion process. If, at this stage of the process, the feed of metal into the region behind the bridge is increased the space behind the bridge may fill up completely and the gas pocket will disappear. In this work we have investigated how finite element modeling can be used (for a simple idealized case of 2-D extrusion welding) to understand the extrusion seam welding process, and to characterize what extrusion conditions will provide stable gas pocket formation behind a bridge, and when the pocket will disappear. The geometry of the welding chamber and the die opening was varied in a number of simulations to study this phenomenon using the FEM-code Deform 2D®. Based on these simulations we have been able to propose an “extrusion seam weld limit diagram (ESWLD)”. The ESWLD shows when extrusion will occur with a gas pocket present behind the bridge and when there is transfer to a state where the pocket disappears and the die fills up completely with metal. By curve-fitting the results can be presented by a mathematical model, i.e. by an equation allowing the reduction in extrusion at the point of transfer from a state of presence of the pocket to a state of complete die filling, to be computed from the dimensions of the porthole and the weld chamber, for a given billet size and a given bridge geometry. The model also shows which geometrical extrusion parameters of the die will favor complete filling of the space behind a die bridge instead of gas pocket formation there.