The present study deals with the effect of Zn on the linear friction stir lap welding of Al to Cu or to brass with a pinless tool. Zn was considered either as a filler metal or as an element in solid solution in α-brass. The lap joint configuration with a groove machined at the top of the bottom plate was chosen in order to test for the first time the influence of liquid Zn at the interface in case of melting by avoiding its ejection. Besides, the use of a pin free tool to join three millimeters thick plates is challenging. The paper presents and discusses the macrostructure and microstructure of the various joints together with their mechanical properties. A strong emphasis is given to the lubricating action of a liquid phase on the material flow as well as on the mechanisms of phase transformations at the various interfaces. The present work reports an early investigation of the material flow in linear friction stir lap welding with a pinless tool. The lap shear tensile fracture behavior of the welds is related to their interface morphology and microstructure. In particular, the fairly good shear resistance of the brass-Zn-Al joint arises from the composite nature of its interface despite its flatness and high thickness.