Analysis of the mechanical damage of a solid-state recycled aluminium alloy under tensile loading
Résumé
Solid state recycling of aluminium chips by hot extrusion allows to produce dense profiles, but the former chips result in the presence of a meso-scale oxide network in the parts, referred to as prior chips boundaries. This study links the detailed characterization of this oxide network structure and the alloy microstructure with the damage evolution in the material during tensile loading. The chips flow, due to the die geometry, results in a prior chips boundary network made of planes mostly aligned along the extrusion direction. Depending on their thickness and number, these planes can have low impact on the microstructure, with even crystalline continuity on both sides, indicating perfect chips welding, or be correlated to grain boundaries. Tensile tests in the transverse and longitudinal directions confirm highly ductile plastic behavior (above 10 % of uniform engineering strain) and reveal a fracture comprised of delamination and ductility. Combining fracture surface analyses and damage nucleation and growth followed by X-ray tomography, this study reveals that pores, or cracks, develop in the prior chip boundaries planes. Their nucleation and growth is similar, in size, number and evolution with strain, to what is conventionally observed in non-recycled materials. They contribute, at most, to a loss of 1 %, or 1.5 MPa for an ultimate tensile strength of about 150 MPa, to the material’s mechanical resistance when the prior chip boundary planes are oriented parallel to the tensile axis, and thus have a very limited impact. However, it is shown that planes perpendicular to the tensile axis, mostly found in specimens with tensile direction perpendicular to the extrusion axis, lead to a significant decrease in ductility, up to the absence of post-necking behavior. This study therefore reveals the details of damage evolution in solid state recycled materials, proving, in favorably oriented cases, the absence of damage induced by the oxide network, but points out the major importance of its orientation.
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