Effect of heat treatment environment on Li depletion and on mechanical properties in Al-Li alloy sheets

Abstract

The loss of lithium during solution heat treatment of various commercial Al-Li alloy sheets as a function of the heat treatment environment is investigated. The thickness of the soft layer formed due to solutionizing at 530°C for 30 min in laboratory air, argon and salt bath environments is determined by microhardness measurements. Li loss from the surfaces is interpreted with the help of normalized microhardness profiles. Results showed that prior Li-depleted layer does not hinder further loss of Li from the surfaces and the heat treatment environment has relatively little effect on the loss of Li. The loss of tensile strength due to the Li depletion in the peak-aged condition is found to be more pronounced in 1 mm thick sheet than in 1.5 mm or 2 mm thick sheets. Possible reasons for loss of strength in 8090 series alloys after the heat treatment are discussed. Explanations for the observed ductile fracture features in Li-depleted layers is offered.