The structure and properties of welded joints formed from aluminum alloy 1550 by means of double-side friction welding with mixing are investigated. The optimum regimes of sample preparation for determining the microhardness and for macro-and micro-investigations of structure are determined experimentally. Different regions of a welded joint, e.g., the core of the weld, the thermomechanically affected zone, the heat-affected zone, the matrix metal, the entering side and side of withdrawal of the working tool, and the surfaces of first and second welding passes, are investigated.