The Heat Treatment of Al-Cu-Li Alloys Ensured the Stability of Structure and Properties at Long Low Temperature Exposure

Abstract

The structural transformations and properties changes of sheets from Al-Cu-Li alloys with two levels of Li content (alloy I with 1.7 wt. % and alloy II with 1.5 %) during long low temperature exposure (LLTE) at 85 °C, 1000 h and 90 °C, 4000 h have been investigated. Three-stage ageing ensured the strengthening of alloy II mainly at the expense of T 1 -phase precipitates with the favorable phases morphology on the grain boundaries and in their bodies. The alloy II hot rolled sheets have the high level of properties, including Impact Behavior Energy before and after LLTE The absence of structural changing during LLTE were confirmed by DSC and TEM data. The alloy I sheets, aged on offered regime, had some changes in properties, however they were no more than 5 %. 1. Introduction. The Al-Cu-Li alloys have a unique combination of high strength, low density, weldability, satisfactory damage tolerance characteristics in conditions of static and fatigue loading [1, 2] A lack of these alloys is the reduced plasticity and inherent to the majority of aluminium alloys with lithium insufficient stability of the structure and properties in conditions of long low temperature exposure (LLTE) [3]. The cause of this are the changes in quasiequilibrium solubility at ageing temperature and LLTE temperature and the fine dispersed δ'-precipitates formation from the nonequilibrium solid solution which has stayed supersaturated after ageing [4, 5], and also formation of GPI zones containing Al and Cu [6]. For increase of thermal stability in the previous work [5] the two-stage ageing 120°C, 20 h + 170°C, 12 h with slow cooling from temperature 170°C up to temperature LLTE was offered. This mode has ensured the sufficient stability for the alloy containing 1.75 weight % of lithium in conditions LLTE 85°C, 1000 hours. In the present work it was necessary to check up, as far as the structure and properties of the alloy will stable after the more rigid test - 90°C. 4000 hours. Assume that the reason of structure instability during LLTE is Li solid solution supersaturation [4, 5], for increase of stability it is reasonable to decrease Li content in alloy. Therefore in the given work the alloy with the content of 1.5 weight % lithium was investigated also. The optimum ageing regime was developed and the phase composition, precipitates morphology, complex of properties and their stability during LLTE were investigated. 2. Experimental. The study was carried out on the 10 and 7 mm hot rolled sheets of two Al-Li-Cu (type 1460) alloys with 1.75 wt. Li % (alloy 1) and 1.5 % (alloy II). The hot rolling temperature was 440-470°C. Sheets were solutionised at 530°C, 1.5 h and quenched in water. The time between quenching and stretching was less than 3 h. The residual deformation degree at the stretching was 3 %. After quenching sheets have unrecrystallized structure. The average fiber thickness was 15 μm. The tensile properties, KCU, K e0 , FCPR, Impact Behavior Energy were determined. The changes in the sheets structure were studied by TEM, X-Ray phase analysis, DSC techniques. The alloy thermal stability estimation during LLTE (85°C, 1000 h or 90°C, 4000 h) is difficult due to the limited researches time. There was a necessity to substitute these regimes for more brief high temperature imitative ones.