“…An onion ring shape structure is reported to be formed by particlerich bands, geometrical features and microstructural changes [19,21,47,48] This particular pattern has been originated by the successive shearing of the materials layers from the front to the back of the FSW tool. Its fine structure is produced by continuous dynamic recrystallization, because in contrast to the TMAZ, the heat and plastic strain have been enough to promote it [49][50][51]. Several authors reported a uniform distribution of fine and near spherical Si particles in the aluminum matrix of the stir zone for Al-high Si alloys as it is observed in the present work [18,20,23].…”