Numerically evaluating the effect of pin geometries on interlayer material mixing and thermo-mechanical characteristics during additive friction stir deposition (AFSD)

Additive Friction Stir Deposition (AFSD) is an additive manufacturing technique used to fabricate large-sized components layer-by-layer in a solid state, below the melting temperature. Poor interlayer mixing between subsequent layers results in a lack of mechanical interlocking, leading to low-strength components. The role of pin design plays a crucial role in fabricating high-strength components, requiring a comprehensive understanding of thermo-mechanical behaviour and flow pattern in the deposition zone. In this study, five different pin geometries are presented and investigated using a three-dimensional (3D) computational fluid dynamics (CFD) model. A user-defined function (UDF) was used to calculate strain- and temperature-dependent viscosity. To obtain the optimum pin configuration, various parameters, including mixing index, velocity vectors, thermal gradient, flow patterns, required torque, and required transverse forces, have been evaluated at a constant deposition speed of 300 mm/min and a deposition rate of 9 kg/hr. Moreover, four different tool rotational speeds (380, 640, 840, and 1050 RPM) and six diameter ratios (1.75, 2, 2.25, 2.5, 2.75, and 3) were used to evaluate their effect on inter-layer mixing and material flow behaviour. The numerical results obtained in this study are validated by the available literature and show good agreement. A pin profile with three flat faces (3FP) showed a 90.9% enhancement in vertical material flow velocity. This resulted in better interlayer mixing, with a mixing index of 35.5%, compared to the circular pin (CP), which was approximately four times lower (6.7%). Furthermore, better mixing was achieved with 1050 RPM and a diameter ratio of 2. The torque and transverse force required by the tool with a 3FP pin profile were, respectively, 7.23% and 17% lower than those of a tool with a circular pin (CP). This paper suggests that a triangular (3FP) pin profile with a diameter ratio of 2 and a rotational speed of 1050 RPM yields better interlayer mixing and reduced power consumption during the AFSD process.