Al-Cu bimetallic sheets were friction stir welded by three different kinds of pin profiles (threaded cylindrical, threaded conical and pyramidal). Among the three pin geometries used, the threaded conical tool provided the best compromise between the different material flow regions and thereby the fewest defects and the highest weld strength were obtained with this tool. As the cylindrical tool has the lowest contact area, it resulted in insufficient material flow around the pin and tunnel defects appeared on the advancing side. A downward material flow is essential to avoid tunnel defects on the advancing side. It was not possible to obtain a weld efficiency of one hundred percent due to the existence of a micro-void analogous to kissing bond on the retreating side. This flaw is a result of a downward material flow, which is essential to obtain a defect-less weld on the advancing side. Torque values and temperatures during welding were directly related to the downward material flow. In samples without tunnel defects on the advancing side, higher values of torque and temperature were recorded. Graphical abstract
