Similar and dissimilar welds of ultrafine grained aluminium obtained by friction stir welding

Samples from commercially pure aluminium were subjected to various number of passes of Incremental Equal Channel Angular Pressing (I-ECAP) and subsequently welded using Friction Stir Welding (FSW). Similar and dissimilar welds were obtained and investigated in terms of their microstructure and mechanical properties. In the case of similar weld from coarse grained aluminium in the stir zone a decreased average grain size was obtained, which resulted in enhanced microhardness. In the case of samples after I-ECAP, the ultrafine grained regime has not been preserved, which caused a drop in microhardness in the stir zone. Nevertheless, obtained results were still higher than those for coarse grained sample. In all welds the average grain size of 2.1–3.7 μm was obtained. No correlation between the microstructure of base material and stir zone has been found. Tensile tests revealed, that the localization of deformation was obtained in each weld in the area of the biggest average grain size. For dissimilar welds from deformed and undeformed samples a gradient change in microstructure and microhardness was obtained on the cross-section of the welds.