Understanding in-process responses in multi-layer friction stir additive manufacturing: Temperature, viscosity, tool torque, and mechanical properties

Abstract

Most melt-based processes greatly inhibit additive manufacturing of high-strength aluminium alloys due to porosity, cracking, and distortion. Friction stir additive manufacturing (FSAM) greatly enhances the printability of such alloys by avoiding melting. However, the repeated heating and cooling during the multilayer fabrication severely degrades the structure and properties of the final build. The effects of thermal cycles, peak temperatures, and cooling rates that substantially degrade the properties are not well reported in the literature. The processing conditions that control the complex viscoplastic flow of the material and the in-process force responses on tool important in order to understand the influence of the possible defects in the final build need to be better reported. Therefore, a systematic numerical and experimental study is conducted to quantitatively understand the spatial and temporal evolution of the build properties, bead profiles, tool torque, and traverse force for the first time in the FSAM literature. The results, which have been rigorously tested and verified, show that the peak temperature, cooling rate, bead profile, tool torque and traverse force were more sensitive to the print height, followed by traverse speed and tool rotation speed. However, the degradation of mechanical properties was found to be least affected by the higher traverse speeds as a result of the lower peak temperatures and the duration of thermal exposure. The numerically computed results corroborated well with the corresponding experimentally measured results, and the results from the independent literature, further enhancing the reliability of our findings. Further, the direct correlation between process variables and the final build properties via in-process responses could substantially reduce the existing trial-and-error approaches in the manufacturing of aluminum alloy structures through the FSAM route.