EUROPEAN CONGRESS AND EXHIBITION ON ADVANCED MATERIALS AND PROCESSES - EUROMAT 2021

Materials with ultrafine grained (UFG) structure exhibit significant improvement in mechanical properties due to reduced grain size below 1 µm. To obtain such materials in a bulk form severe plastic deformation processes are used. Nevertheless, high stored energy and highly deformed structure result in reduced thermal stability, which causes a rapid grain growth at elevated temperature. This phenomenon is particularly unfavorable in the case of welding, which is one of elementary processes in the industry. Therefore, UFG materials require advanced joining techniques in order to preserve refined microstructure and their properties. In the present study, friction stir welding has been chosen as a joining technique and its possibilities for welding UFG commercially pure aluminium and Al-Mg-Si alloy have been investigated. The main emphasis has been put on a microstructure evolution in the weld zones. For this purpose SEM/EBSD and TEM were used. Together with microhardness measurements and tensile tests it allowed to characterize obtained welds. The results revealed a grain growth in the stir zone to the value of about 3-6 µm, with a 1 µm for base material. It caused a drop of mechanical properties in comparison to UFG base materials but an increase in comparison to annealed aluminium. In order to inhibit a grain growth in a stir zone the influence of the addition of Al203 nanoparticles has been also investigated and its influence on mechanical properties.