The influence of thermal treatment on the cast structure of 6xxx aluminum alloys

This study was performed to develop an understanding of the influence of thermal treatment on the cast and homogenized structure of 6xxx aluminum alloys prior to extrusion. The primary purpose of this investigation was to study the evolution of homogenized microstructure, with emphasis on Mg 2Si. The secondary purpose of this study was to understand the cast structure, since the evolution of Mg2Si should be considered in the larger context.; Homogenization treatment characterization was performed on 6&inches;-diameter logs at Keymark Corporation, Fonda, NY.; A 6105 aluminum alloy which was already homogenized at Werner Company, Greenville, PA, also contained large globular undissolved Mg2Si.; In 6063 aluminum alloy, homogenized samples were exposed to heat treatment at 590°C for over 1 week. An indication of dissolution for large Mg 2Si particles was observable, but it was not confirmed by further investigation which demonstrated the coarsening of Mg2Si in this alloy at 590°C for 1 week and 2 weeks, as well.; The response of large particles to temperatures of 590, 600, 610, 620 and 630°C was investigated. It was found that large particles disappeared at 620 and 630°C for 6063 and 600, 610, 620 and 630°C for 6105. The appearance of extrema in the Mg concentration profile implies the variation in chemical composition in the matrix. This was verified by an electron probe line scan study showing areas of local high concentration of Mg and Si in the matrix.; The coarsening of Mg2Si is a major element in "homogenizing" thermal treatment of 6xxx aluminum alloys. The particle coarsening kinetics agree with scaling laws suggested by Lifshitz, Slyozov and Wagner (LSW). That is, the linear relationship between R&d1;3-R&d1; 3i and time fits the experimental measurements reasonably well, showing the kinetics to be diffusion controlled. The particle coarsening observations are inconsistent with the classical phase diagram, and the classical phase diagram appears to be in error. A small portion of the deviation from the classical diagram may be attributable to Mg2Si particle radius effects. Departure from the classical diagram can also be explained by anomalies in kinetic associated with uphill diffusion. (Abstract shortened by UMI.)...