Process responses during μFSW of AA6061-T6 under the influence of triple-spiral micro-grooves on shoulder end-surface

Triple-spiral micro-grooves on the tool-shoulder end-surface is a feature recently being used for improving material flow during micro-friction stir welding (mu FSW). However, its effect on process responses in comparison to the plain tool-shoulder end-surface lacks recognition, which has been targeted in the present work. An additional contribution to the knowledge base of mu FSW tools having such features is also made by investigating the effect of imparting different cross-sections (rectangular, circular, right-trapezoidal type I and right-trapezoidal type II) to the micro-grooves, on the process responses. Forces, temperature, and material flow pattern were mainly the process responses analyzed through experiment and FEA simulation of mu FSW in 0.5 mm thick AA6061-T6 sheets at a tool rotation of 1900 rpm and welding speed 500 mm/min. Furthermore, the welds' quality was assessed by its surface topography, microstructure, and tensile property. The physics of the interaction between shoulder end-surface and plasticized sheet material showed that besides the triple-spiral shape, it was the cross-section of micro-grooves (mainly the orientation angle and shape of micro-grooves' wall) that crucially determined the mechanical, thermal, and flow characteristics of the plasticized sheet material, ultimately affecting the process responses. The possibility of low-temperature welding with enhanced material mixing had been achieved with the micro-grooved tools. A higher amount of plastic deformation was also seen, with these tools, through X-ray tomographic images of the deformed copper foils placed transverse to the welding direction. Moreover, a remarkable improvement in the weld surface finish was obtained using the micro-grooved tools. Among the four micro-grooved tools, the right-trapezoidal type II cross-section showed the best performance by yielding moderate process loads along with the best weld surface finish and tensile property.