EXTRUSION PROCESSING OF AN Al-Li-Cu-Mg-Zr ALLOY AA 2091

Abstract
This communication reports work covering the variation in structure and properties with extrusion processing of an Al-Li alloy, AA 2091. An homogenisation temperature was established and used as standard throughout the work. Extrusions were performed over a range of temperature compensated strain rates. After solution treatment, a series of ageing curves was obtained for both stretched (2%) and unstretched specimens at various ageing temperatures, thus establishing the ageing response with respect to underage, peak age and overage tempers. A natural ageing (at room temperature) curve was also produced after solution treating and tensile tests performed at one and two week intervals. The effect of introducing a 2% stretch prior to artificial ageing was investigated in terms of microstructural changes and variations in the mechanical properties (tensile and fracture properties) for peak aged and overaged tempers at different ageing temperatures. All the extrusions had fully recrystallised surface layers. Up to 190°C, ageing at a higher temperature led to lower strength levels but the response was faster, and stretching gave an increase in strength. At 210°C the strength was not only higher than 190°C but was maintained even up to 1000 hours although a pre-age stretch caused a drop in strength after about 200 hours. This is due to the more copious precipitation of homogeneous S phase at 210°C and a reduced amount if stretched prior to ageing because the precipitation of heterogeneous S phase reduces the amounts available for homogeneous precipitation. Very high strengths were obtained especially in the peak-aged specimens. For the peak-aged condition, the introduction of a pre-age stretch did not necessarily improve toughness. TEM revealed wide-spread heterogeneous S phase precipitation right up to the grain boundaries for the stretched specimens.