Effect of homogenization temperature on microstructure and mechanical properties of Al-Mg-Si alloy containing low-melting point elements

To evaluate the effect of homogenization conditions on the possible loss of low-melting-point Pb and Bi to the surface in the free-cutting AA6026 alloy, three homogenization regimes were applied: 480 degrees C/12 h, 530 degrees C/12 h, and 550 degrees C/6 h. The microstructural characterization by optical, scanning, and transmission electron microscopy coupled with EDS analysis, macroanalysis of chemical composition by ICP-AES as well tensile tests at room and 500 degrees C, and Charpy impact test were employed to evaluate the different homogenization regimes. It was found that the choice of homogenization temperature had no significant effect on the level of loss of low-melting point elements. The optimal homogenization regime appeared to be 550 degrees C/ 6 h as it led to almost complete beta-AlFeSi -> alpha-AlFe(Mn)Si transformation and beta-Mg2Si dissolution resulting in improved mechanical properties. The presence of the liquid phase did not lead to catastrophic failure due to the liquid metal embrittlement of the aluminum matrix, but the wetting of Fe,Mn - bearing constituents by molten Pb led to decohesion of the constituents. The morphological change to globular alpha-AlFe(Mn)Si decreased the surface area and interconnectivity of the microconstituents, which improved the hot ductility. During cooling after homogenization at T > 500 degrees C, the Q-phase precipitated, pointing up the potential quench sensitivity of the alloy. However, precipitation of the Q-phase laths in dispersoid-free zones reduced strain localization and improved room temperature ductility and impact toughness