EFFECT OF HEAT-TREATMENT ENVIRONMENT ON LI DEPLETION AND ON MECHANICAL-PROPERTIES IN AL-LI ALLOY SHEETS

The loss of lithium during solution heat treatment of various commerci al Al-Li alloy sheets as a function of the heat treatment environment is investigated. The thickness of the soft layer formed due to solutio nizing at 530-degrees-C for 30 min in laboratory air, argon and salt b ath environments is determined by microhardness measurements. Li loss from the surfaces is interpreted with the help of normalized microhard ness profiles. Results showed that prior Li-depleted layer does not hi nder further loss of Li from the surfaces and the heat treatment envir onment has relatively little effect on the loss of Li. The loss of ten sile strength due to the Li depletion in the peak-aged condition is fo und to be more pronounced in 1 mm thick sheet than in 1.5 mm or 2 mm t hick sheets. Possible reasons for loss of strength in 8090 series allo ys after the heat treatment are discussed. Explanations for the observ ed ductile fracture features in Li-depleted layers is offered,