Analysis of extruded pins manufactured by friction stir forming for multi-material joining purposes

Nowadays the application of multi-material parts has become a standard in several sectors, such as in transportation where the use of dissimilar material contributes to reduce the weight of structural components, as well as to decrease fuel consumption and CO2 emissions. The scientific literature proposes various methods for combining dissimilar materials, but when there are too incompatible, the process becomes a real challenge. To this aim and to propose an alternative joining technique, which does not require the use of additional external parts, the authors investigated a method based on the Friction Stir Forming (FSF) technology. FSF is a process, where a rotating tool moves on a metal sheet pushing the material through the holes of a forming die positioned under the worked sheet. In the current study, the authors aimed at improving the quality of the aluminium extruded pins, which leads to the improvement of the connection with composite materials. To this aim, a specific equipment has been manufactured and installed on a general purpose milling machine and an experimental plan has been designed. Both process (i.e. spindle speed, forming velocity) and geometrical parameters (i.e. hole diameter and the distance between the holes) have been considered. A deep analysis has been carried out on the extruded aluminium pins in terms of internal structure. Specifically, an X-ray micro-tomographic analysis has been performed to investigate, qualitatively and quantitatively, the distribution of the material within the manufactured pins. The void content and the final height of the pins have been measured aiming at understanding how these outputs can be affected by the investigated parameters.