Processing strategies to avoid abnormal grain growth during T4 heat treatment of AA2024 friction surfacing depositions

Friction surfacing (FS) is a friction stir-based solid-state layer deposition process, leading to deposits characterized by fine-grained microstructures. Precipitation-hardenable alloys typically exhibit hardness gradients. To enhance and homogenize the mechanical properties, a post-process heat treatment is required, which can result in unfavorable abnormal grain growth (AGG). This work explores processing strategies to avoid AGG during the post-process heat treatment: room temperature (RT), preheating FS, and FS combined with post-processing via hybrid friction diffusion bonding (HFDB) of AA2024. Preheating FS and HFDB post-processed conditions successfully preserve the refined grains during heat treatment. Grain stability has been shown to be influenced by the Zener pinning effect, which is affected by grain size and particle dispersion. The AGG of the RT FS condition has been correlated with the smallest grain size, the lowest secondary particle volume fraction, and secondary particle equivalent diameter. Secondary particle shapes and locations, and microstructure strain energy contribute to additional stability to preheating FS and HFDB post-processed conditions. The combination of RT FS with HFDB has demonstrated the highest material efficiency of the processes, joining efficiency of the deposits, and grain stability during heat treatment. This understanding enables the suppression and/or tailoring of AGG for FS deposits.