Bending ductility and through thickness anisotropy in 6xxx extruded profiles

Abstract

6xxx aluminium alloy extruded profiles are widely used in the automotive bumper systems due to light weight, high strength and good ductility. Bending ductility dominates the behavior of bumper systems to absorb energy in a collision. Extrusion process contributes corresponding large change to microstructure and properties of the material. The grain size variation, texture change through thickness and particle heterogeneous distribution result in the anisotropic properties. It is found that the anisotropy is observed significantly influences the bending ductility of the extruded profiles. In this work, two aluminium profiles, i.e. recrystallized 606385 and 600540 of 3.5mm thick are investigated to study the bending ductility through thickness. This thesis involves both experimental and simulative work. In the comprehensive experimental work, ageing heat treatment in the lab is a start point after acquiring the materials from the extrusion plant in Raufoss. Both materials were well characterized by EBSD, BSE and XRD to observe microstructure, to analyze particle distribution, and to measure textures through thickness. Mechanical properties for original thickness, surface layer and center layer in different directions along extrusion direction were also obtained by tensile tests. The profiles, which were milled 1mm from inner surface or outer surface, were tested by Daimler 3-points bending machine in SINTEF Manufacturing Raufoss in both extrusion and transverse direction, original and milled surface. The fractures and cracks of bending samples were analyzed by optical microscopy and SEM. The simulative work of the Daimler 3-points bending has been carried out on FEM software - LS-DYNA. The modified Johnson-Cook model with Voce hardening law, Cockram-Latham fracture criterion, and two yield criteria, isotropic Von-mises and anisotropic Yld2004-18p were applied into the calculation, and the fitting processes were introduced as well. Significant difference in bending ductility for original surface and mid-layer surface along different directions is resulting from the heterogeneity of grain size, particles, textures through thickness. This phenomenon is also analyzed by the FEM simulation by LS-DYNA.