Analysis of microstructure and hardness of a double-layered AA7075 build developed by friction stir additive manufacturing

In this work, a double-layered AA7075 build was formed using the friction stir additive manufacturing technique, and the microstructural evolution along the build and hardness were investigated. Build cross-section and microstructural evolution were conducted using optical macroscopy, electron backscattered diffraction, and transmission electron microscopy, while hardness measurement was conducted using a Vickers hardness tester. Solid-state deformation resulted in a defect-free build with a basin-shaped cross-section. Recrystallized microstructures were observed along the build due to dynamic recrystallization, with grain sizes measuring 2.32, 2.70, and 2.82 μm at the top, center, and bottom, respectively. Maximum hardness of 146.3 HV was observed at the top of the build, and the values dropped at the overlapped and bottom zones due to grain growth caused by excess thermal cycle. Enhancement in hardness is attributed to strain hardening, grain boundary strengthening, and Orowan strengthening mechanisms.