Revealing the bonding mechanisms between deposit and substrate of the friction rolling additive manufactured hybrid aluminum alloys

Friction rolling additive manufacturing (FRAM) is an emerging solid-state additive manufacturing technology for high-strength aluminum alloys that are prone to produce solidification defects using fusion-based additive manufacturing method. However, the mechanisms responsible for good bonding between the FRAM interlayers remain unclear. This study successfully reveals such mechanisms for FRAM-produced hybrid AA2319 and AA6061 aluminum alloys using 3D X-ray computed tomography, electron backscatter diffraction, and trans-mission electron microscopy. Three findings concerning the quality of bonding between adjacent layers in FRAM were obtained. Firstly, material plastic flow and macroscopic migration behavior occurred simultaneously along the longitudinal and transverse directions at the interlayer, forming a macroscopic mechanically interlocking nonplanar interface. The convex thread at the tool head increased the material flow in the transverse direction, facilitating material mixing in the adjacent layer. Secondly, severe plastic deformation resulted in significant grain refinement on both sides of the interface. The average grain size reduction rates for the sides of AA2319 and AA6061 were 98.3% and 95.9%, respectively. Thirdly, direct contact with no obvious oxides resulted in complete metallurgical interface bonding. These findings can aid in the elucidation of the basic physical process of FRAM and provide guidance for process parameter optimization and tool head design in the future.