Abstract
The study aims at investigating the influence of grain size on microhardness across the thickness in the stir zone of 20-mm-thick AA5456-H112 plates joined by floating-bobbin-tool friction stir welding at various feed rates and spindle speeds. Contrary to the conventional friction stir welding, in all the joined samples, the grain size at the bottom layer was coarser than that in the top and middle layers of the stir zone, and the microhardness values were always higher in the top layer of the stir zone. It was found that the microhardness at the top, middle, and bottom layers of the stir zone, as well as the size and compaction of the intermetallic particles in the stirred region, is lower at higher spindle speed and slower feed rate. However, under these conditions, the recrystallized grain size is increased in all three layers, and the intermetallic particles become fragmented and dispersed, as predicted by the Hall–Petch relationship. The Hall–Petch equation between microhardness and grain size in the stirred region indicated that zones with finer grain size have a higher microhardness, whereas a decrease in microhardness occurs with increasing grain size.
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