Effect of friction stir processing on the damage resistance of 6xxx series aluminium alloys

Hannard, Florent [UCL] Simar, Aude [UCL] Pardoen, Thomas [UCL] Maire, Eric [INSA-Lyon]

Damage evolution in ductile metals is characterized by the nucleation, growth and coalescence of small internal voids. In aluminium alloys, the void population generally nucleates by the decohesion or fracture of the iron rich intermetallic particles. Previous studies have shown that the nucleation stress increases when the size of the intermetallic particles decreases retarding the final fracture of the material in tension. Furthermore, initial porosities and particle clustering reduce the final fracture strain as they favor earlier void coalescence by internal void necking. Hence, friction stir processing (FSP) has been applied to a 6xxx series aluminium alloy in order to assess the ability of the process to eliminate initial porosity, fragment the intermetallic particles and distribute them more homogeneously to improve the fracture strain of the material. Detailed microstructural analysis of the intermetallics distribution has been carried out including 3D X-ray tomography and SEM micrographs. The mechanical properties have been investigated by tensile testing under various heat treatment conditions. In addition, a statistical study of void nucleation has been performed on 2D metallographic sections of interrupted tensile tests, as well as on 3D microtomography scan of fractured tensile samples. This analysis confirmed that the size of intermetallic particles is reduced by FSP and that void nucleation is delayed. Furthermore, elimination of initial porosity and homogenization of the intermetallics distribution has been observed and quantified. Tensile tests have confirmed that these microstructural changes result in improved fracture strain.