Effect of friction stir processing on the damage resistance of 6xxx series aluminium alloys
(2016) EMMC15 — Location: Brussels (7.September.2016)
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Hannard, Florent 
- Abstract
- Damage evolution in ductile metals is characterized by the nucleation, growth and coalescence of small internal voids. In 6XXX aluminium alloys, the void population generally nucleates by the decohesion or fracture of the iron rich intermetallic particles. Previous studies [1] 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, the presence of an initial porosity and of particle clusters reduce the fracture strain by favoring 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, to fragment the intermetallic particles and to distribute these particles more homogeneously in order 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 microscopy. 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 broken tensile samples and on in situ tensile tests in X-ray tomography. 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 spatial distribution has been observed and quantified. Tensile tests have confirmed an increase of the fracture strain of the material while conjunctly rendering isotropy in fracture strain. A cellular automaton model, involving a high number of particles with distribution of position, sizes and void nucleation stress is developed to predict fracture [1]. The model treats local interaction between neighbouring cavities in a simplified way and captures cluster effects on the coalescence process. The model parameters are extracted from the detailed microstructure analysis and predicts the improved fracture strain. [1] F. Hannard, T. Pardoen, E. Maire, C. Le Bourlot, R. Mokso, A. Simar, Characterization and micromechanical modelling of microstructural heterogeneity effects on ductile fracture of 6xxx aluminium alloys, Acta Materialia, Volume 103, 15 January 2016, Pages 558-572
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Hannard, F., Simar, A., Pardoen, T., & Maire, E. (2016). Effect of friction stir processing on the damage resistance of 6xxx series aluminium alloys. EMMC15, Brussels. https://hdl.handle.net/2078.5/181599