Abstract
Extrudability of aluminum alloys of the 6xxx series is highly dependent on the microstructure of the homogenized billets. It is therefore very important to characterize quantitatively the state of homogenization of the as-cast billets. The quantification of the homogenization state was based on the measurement of specific microstructural indices, which describe the size and shape of the intermetallics and indicate the state of homogenization. The indices evaluated were the following: aspect ratio (AR), which is the ratio of the maximum to the minimum diameter of the particles, feret (F), which is the maximum caliper length, and circularity (C), which is a measure of how closely a particle resembles a circle in a 2D metallographic section. The method included extensive metallographic work and the measurement of a large number of particles, including a statistical analysis, in order to investigate the effect of homogenization time. Among the indices examined, the circularity index exhibited the most consistent variation with homogenization time. The lowest value of the circularity index coincided with the metallographic observation for necklace formation. Shorter homogenization times resulted in intermediate homogenization stages involving rounding of edges or particle pinching. The results indicated that the index-based quantification of the homogenization state could provide a credible method for the selection of homogenization process parameters towards enhanced extrudability.
Highlights
The process chain of extrudable Al-alloys of the 6xxx series involves direct-chill casting followed by a homogenization cycle, prior to hot extrusion
The above effects are partially removed by the homogenization treatment, which includes the removal of elemental microsegregation, removal of non-equilibrium low-melting eutectics, the transformation of β-AlFeSi to α-AlFeSi and the spheroidization of the remaining undissolved intermetallics [1]
The aim of the present paper is the quantification of the homogenization state by means of quantitative metallography, in order to describe the morphological evolution of the intermetallic phases
Summary
The process chain of extrudable Al-alloys of the 6xxx series involves direct-chill casting followed by a homogenization cycle, prior to hot extrusion. The as-cast billets contain several inhomogeneities, such as elemental microsegregation, grain boundary segregation, and formation of low-melting eutectics as well as the formation of iron intermetallics. The presence of intermetallic phases, in particular, which possess sharp edges, can impair the deformability of 6xxx extrudable alloys especially when located in the grain boundary regions [1,2,3,4]. The above effects are partially removed by the homogenization treatment, which includes the removal of elemental microsegregation, removal of non-equilibrium low-melting eutectics, the transformation of β-AlFeSi to α-AlFeSi and the spheroidization of the remaining undissolved intermetallics [1]. The effect of Metals 2016, 6, 121; doi:10.3390/met6050121 www.mdpi.com/journal/metals
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