Friction extrusion describes the processing of metallic materials by inducing severe plastic deformation via frictional heating and shear strain. Rotational motion between die and feedstock is the key feature defining the potential of the process to generate consolidated extrudates with refined, homogenized microstructure. In this study, the effect of volumetric core defects on the material flow and properties of the extrudate is investigated, by processing from Al-Cu billets with a centric bore. Optical as well as scanning electron microscopy, X-ray microtomography and micro-hardness measurements are applied. Different material flow patterns and defect closure mechanisms are identified in correlation with the defect volume and the potential of controlling material flow via geometrical feedstock modification is discussed.