Material flow path and extreme thermomechanical processing history during additive friction stir deposition

Abstract

With significant interest from the aerospace and defense industry, additive friction stir deposition (AFSD) is an emerging solid-state metal additive process that can result in fully-dense material with fine equiaxed grains and forged-like mechanical properties. Vital to quality control, the thermal and deformation history of the feed-rod governs the resulting microstructure and properties; however, such history has remained elusive due to experimental constraint. By leveraging both theoretical and experimental efforts, here we develop an experimentally-informed modeling framework that unravels the three-dimensional material flow path and quantifies the thermal and deformation history by tracing the temperature and effective strain rate along the flow path. We show that the feed material undergoes extreme thermomechanical processing during AFSD. Take an Al-Mg-Si alloy for example: the temperature is maintained high within a narrow range during AFSD (e.g., 75–80 % of the melting temperature); the peak strain rate is on the order of 101–102 s−1; the total accumulated strain is on the order of 102. Within the feed-rod, the edge and intermediate voxels are more rapidly deformed than the center voxel, but the values of the Zener-Hollomon parameter and flow stress are comparable among all voxels. This suggests a uniform grain structure distribution after continuous dynamic recrystallization, which is validated experimentally via microstructure mapping.