The Friction Stir Welding (FSW) process has been modified by applying a variable force to the workpiece along the tool rotation axis. This results in a reciprocating motion of the welding tool, which in turn causes a change in the temperature and plastic deformation applied to the weld material. A new technique was referred as Impulse Friction Stir Welding (IFSW). Although the first study of IFSW has identified a 40% increase in the fatigue strength of the AA6082-IFSW joints compared to FSW, there have been no systematic studies on the metallurgical phenomena taking place in the IFSW weld. The correlation between the impulse parameters, microstructure, and resulting mechanical properties has not been evaluated. The current dissertation aims to establish a fundamental understanding of microstructural and mechanical performance evolution during IFSW. For this purpose, IFSW was performed at various impulse parameters on two IFSW machines implementing impulses in different control modes. The FSW joint was used for the comparison. Two heat-treatable aluminium alloys, AA6082-T6 and AA2024-T351, were selected for study due to their wide application. Metallurgical factors such as the weld formation, grain- and subgrain structure, constituent phases, and crystallographic texture were systematically evaluated and related to the mechanical properties of the joints. A major emphasis of this research was placed on the assessment of strengthening precipitates. A comprehensive overview of the precipitation state in the IFSW weld zones was given, and the transformation sequence leading to the final structure was reconstructed according to the thermal cycle.