Abstract <p>The article presents the results of a study on the influence of friction stir welding modes and subsequent heat treatment on the physical and mechanical properties of the deformable heat-resistant nickel alloy VZh175-ID. The study examines the processes of rotational (inertial) friction welding with and without preheating (grinding). Metallographic studies show a significant difference in the mechanisms of alloy hardening during heat treatment of samples obtained by rotational friction welding. The samples showed a deformation texture and elongation of particles in the direction of rotation. In the base material zone, hardening occurs through the mechanism of precipitation of strengthening intermetallic compounds ranging in size from 0.5 to 10 μm with a total grain size of 3 to 24 μm. In the zone of thermomechanical impact, grain coarsening was observed, and the separated γ'-phase particles were irregularly rounded in shape with the formation of carbides. Mechanical tests were performed. In a welded joint, hardening occurs through grain refinement, with grain size reduced by 2–3 times relative to the base material. The particles of the primary, secondary, and tertiary γ'-phase that are released retain their proportions relative to grain size, as in the base material. The hardening of two different mechanisms is equivalent, but due to grain growth in the modified zone, additional technological measures must be taken, since the mismatch in grain size leads to embrittlement and can serve as a source of crack initiation.</p>