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 °C and were cooled to room temperature at different rates. Samples cut from these bars were aged at 130, 180 and 230 °C 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 °C/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 °C/s; the matrix of these samples was that of the face centered cubic type with a lattice parameter of 4.055 إ, 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.