Precipitation in a heat-treatable aluminum alloy cooled at different rates

A series of cylindrical bars from an extruded heat-treatable type 6063 aluminum alloy were solubilized for 4 h at 520 degreesC and were cooled to room temperature at different rates. Samples cut from these bars were aged at 130, 180 and 230 degreesC for periods of time ranging from 20 min to 96 h. The characteristic behavior of precipitation-hardened alloys was observed, and it was found that the peak hardness at a given aging temperature was reduced in samples cooled at rates below 10 degreesC/s. Selected samples were studied by X-ray diffraction (XRD) and transmission electron microscopy (TEM). No precipitation was observed to occur in unaged samples cooled at rates of around 60 degreesC/s; the matrix of these samples was that of the face centered cubic type with a lattice parameter of 4.055 Angstrom, which is slightly bigger than that reported for pure aluminum. In contrast, a series of small precipitates were observed in unaged samples cooled at a slower rate. Changes in lattice spacing in aged and unaged samples confirmed precipitation when the cooling rate was reduced. (C) 2001 Elsevier Science Inc. All rights reserved.