Metallographic Index-Based Quantification of the Homogenization State in Extrudable Aluminum Alloys

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.

Microstructural Evolution in a 6060 Extrudable Al-Alloy: Integrated Modeling and Experimental Validation

“…This was also confirmed by EDX analysis discussed below. No eutectic particles were observed, and the Fe-bearing intermetallics found presented morphological changes typical in homogenized materials including rounding of particle edges, pinching of elongated particles and segmentation into smaller particles to a lesser extent [30]. SEM micrographs of representative microstructural features in the homogenized material can be found in Figure 13.…”

“…For subsequent homogenization simulations, a typical heating cycle for batch homogenization of 6060 Al-alloys was used, during which the material is gradually heated and held to a temperature up to 580 • C over the course of several hours. The elimination of elemental segregation in the interdendritic space, the transformation of β-AlFeSi to the more favorable α-AlFeSi and the dissolution of undesired intermetallic compounds and mixtures [30] was studied. The segregation profile and intermetallic phase fractions provided by Scheil-Gulliver solidification simulations were used as initial conditions.…”

“…Metallographic images of the as-cast material at 1/2 of the billet radius, using optical microscopy are presented in Figure 10. Intermetallic compounds can be found in the interdendritic space and on grain boundaries, with a blocky, acicular or rod-shaped morphology, characteristic of β-AlFeSi (Al 9 Fe 2 Si 2 ) particles which are known to limit extrudability [5,30]. Irregularly shaped particles, rounded particles and eutectics were also found to a lesser extent.…”

Homogenisation Efficiency Assessed with Microstructure Analysis and Hardness Measurements in the EN AW 2011 Aluminium Alloy

“…However, these alloys exhibit low extrusion rates, require high extrusion pressure, and are therefore assigned a low extrudability index [1,2]. As ingots are cast, non-equilibrium solidification usually results in micro-sized eutectic phases with a relatively low melting point [3][4][5] that are likely to melt and cause irreversible damage during thermomechanical treatments at high temperatures [6]. The low-temperature annealing is less effective in eliminating eutectics and micro-segregation of dissolved elements due to insufficient solid solubility and limited diffusivity of alloying elements.…”

Modeling of Microsegregation and Homogenization of 6xxx Al-Alloys Including Precipitation and Strengthening During Homogenization Cooling

“…The α-AlFeSi phase undergoes morphological changes, which involve rounding of edges, pinching, and necklace formation. These changes have been described in [11,12,13] while a quantitative description of the progress of the homogenization, based on microstructural indices, such as the aspect ratio and circularity, was recently presented in [14]. Secondary precipitation has been followed with high-temperature flow stress measurements in [15].…”