Numerical and Experimental Study on the Effect of Tool Pin Profile of Friction Stir Additive Manufactured AA6061/AA7075 Joints

The current study deals with the effect of different tool pin profiles (square (SQ), tapered cylindrical (TAC), and tapered cylindrical threaded (TCT)) on the local heat generation, material flow velocity, and strain rate of AA6061/AA7075 joints fabricated by the friction stir additive manufacturing (FSAM) process. COMSOL multiphysics software that utilizes a 3D-coupled thermomechanical modelling is used to compare the local properties at the tool-workpiece interface in the stir zone (SZ). The numerical study reveals appreciable material mixing, flow velocity, temperature generation, and strain rate in the case of the TCT pin profile. The experimental study also shows noticeable material mixing and interfacial bonding for AA6061/AA7075 joints fabricated using a TCT pin profile. Interfacial microstructural properties were evaluated using an optical microscope (OM) and electron backscattered diffraction (EBSD) analysis. EBSD analysis reveals the presence of fine-grain microstructural features in the SZ of the AA6061/AA7075 joints for the TCT pin profile. Thread pitch on the tool pin profile allows sound intermixing of dissimilar aluminium alloys in the SZ that promotes the generation of high heat input leading to continuous dynamic recrystallization (CDRX) in the region. The presence of a higher fraction of high-angle grain boundaries and finer grain size in the SZ leads to higher microhardness in this region.