Eliminating microstructural defects and improving the mechanical and fatigue properties of an Al-Mg-Si alloy extrusion by optimising homogenisation

Coarse intermetallic secondary phases generated during conventional homogenisation treatment severely deteriorate the surface qualities and fatigue properties of Al-Mg-Si alloy extrusions. In this work, microstructural defects related to the coarse intermetallic secondary phases and their external manifestation (i.e., a micro surface defect) were characterised in detail by scanning electron microscopy and X-ray diffraction. The corresponding mechanisms were analysed as well. Based on the mechanisms, an optimised homogenisation (578 degrees C for 18 h) was performed to improve the microstructural quality and remove the surface defects of the extrusion profile. Moreover, the tensile and fatigue properties before and after optimising the experimental profiles were compared. The results showed that the microstructural defects were substantially a result of the insufficient beta-AlFeSi to alpha-AlFeSi phase transformation during conventional homogenisation at 560 degrees C for 12 h. The micro surface defects resulted from the de-bonding of coarse lamellar beta-AlFeSi particles with sharp edges from the Al matrix, when stored in a slightly corrosive environment. Meanwhile, after the optimised homogenisation treatment, the microstructural defects were mostly eliminated; consequently, the surface defects were removed. Moreover, the elongation and ultimate fatigue strength of optimised samples increased by approximately 50% and 26%, respectively, compared with those of conventional homogenised samples. These results can provide favourable guidance for industrial production of Al-Mg-Si profile extrusions.