Synergistic effect of low heat input rotary friction welding and post-weld heat treatment on strength enhancement of AA6061-T6511 joints

This study elucidates how the synergistic combination of a low heat input rotary friction welding (LHI RFW) process and post-weld heat treatment (PWHT) governs the microstructure and mechanical properties of AA6061-T6511 joints. Conventional RFWed (CON RFW) joints suffer from severe thermal softening due to high-temperature dislocation annihilation and precipitate dissolution, which drastically reduces the joint efficiency. In contrast, the developed LHI technique lowers the peak interfacial temperature by approximately 100 °C compared to CON RFW. This significant thermal reduction effectively preserves a greater fraction of strengthening β'' precipitates and dislocation networks in the as-welded state, thereby enhancing the initial joint integrity. Subsequent rapid PWHT at 210 °C for 10 min triggers markedly different strengthening pathways. CON RFWed joints show minimal improvement because the strength loss from GP zone dissolution is not adequately compensated by very limited new precipitation. Conversely, in LHI joints, the retained dislocations actively accelerate precipitation kinetics via dislocation-assisted diffusion and nucleation during ageing. This effective microstructural coupling between preserved dislocations and optimized precipitate formation enables the joint efficiency approaching 98.6%. A quantitative strengthening model confirms that in LHI joints after PWHT, dislocation hardening remains the dominant mechanism, while a substantial increase in precipitation hardening works synergistically to drive the superior, near-complete recovery of the joint strength.