Abstract
In the present article, mechanical and tribological behaviour of multi-pass friction stir processed (M-FSPed) Al–Zn–Mg–Cu alloy were investigated. Three overlaps (100%) of multi-pass friction stir processing were carried out which led to homogenization and refinement of the as-cast microstructure, elimination of porosity and uniform distribution of fragmented fine precipitates in the fine grain aluminium matrix. The mechanical properties of FSPed alloy were investigated in terms of tensile behaviour which includes measurement of 0.2% offset yield strength (YS), ultimate tensile strength (UTS), and ductility. Microstructure analysis was performed using optical microscopy and scanning electron microscopy to differentiate the various zones and dispersion of precipitates after FSP. Micrograph of FSPed sample shows the defect free stir zone and the average grain size decreases with the increasing number of passes. About 30% increase in YS and 34% increase in UTS was observed after third pass FSP as compared to as cast alloy. The improvement in the tensile strength of third pass FSP as compared to as-cast alloy may be attributed to the collective effect of grain refinement, shear lag mechanism, and Orowan strengthening. Fractograph examination of tensile tested alloy before and after FSP confirmed the transformation of cleavage fracture to ductile fracture. Wear resistance and mean coefficient of friction increased as the number of FSP passes increased.
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