Since the invention of FSW many publications mentioned the particular stirred zone features (onion ring like structure) of FSW joints, which is caused by material flow and deformation and subsequent recrystallization. Beside this artefact there are sometimes line peculiarities in the joint that do not follow the onion ring type structure. Especially AlMgSi (6xxx type) alloys seem to be preferred to create these particle rows which prove to be micro meter sized fragments during high resolution, metallurgical investigation aligned in the bulk material. First investigations using SEM and EDX indicated that these phases might be aluminium and magnesium oxides. However the amount of particles that can be found is unexpected high compared to the known natural oxide layer thickness on standard Al materials (before joint by FSW). Additionally premature joint failures are sometimes linked with this very specific weld seam contamination. A fundamental investigation had been performed using and characterising ultra clean Al (99.999 percent), standard Al (99.5 percent) and an AlSiMgCu alloy (AA 6056). Three different surface preparations were applied prior FSW processing combined with 2 different FSW tool geometries. Detailed metallurgical and scanning electron examination were conducted to create a basic understanding of origin and interaction of the oxide contaminations that were detected in the welds. One focus was put to the question whether the oxide particles and their amount are determined prior to welding or an "in situ" generation of oxides during welding might occur.
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