NDE Methods for Determination of Thermal History and Mechanical Properties of Al-Li Alloys

Abstract

The search for lighter alloys which would reduce fuel consumption and hence the cost of air transportation has recently been focussed on the aluminum-lithium system. The addition of lithium to aluminum increases the yield strength by typically an order of magnitude, and with the further addition of a few tenths of a percent of zirconium, which increases the ductility, alloys of aluminum, lithium, copper and magnesium can be made to meet the necessary specifications of low density, high strength and high ductility for aircraft applications.Production of these alloys requires a precise heat treatment schedule in which the alloy is quenched from the high temperature solid solution phase (a) and then aged at an elevated temperature to produce the desired mechanical properties. We have investigated the relationship between the thermal history (i.e., quench rate, aging temperature and aging time) and various resulting material properties. In this way, it is anticipated that identification of certain measurable parameters, such as eddy current response and elastic modulus, which are sensitive to thermal treatment, will be possible. The measurement techniques, themselves, should ideally be easily utilized for noncontact NDE of material emerging from a production line.Binary Al-Li alloys have been used with compositions of 8 and 12 atomic percent lithium. The specimens were chill cast and then mechanically processed to homogenize the grain size throughout the material. After solid-solutioning, the alloys were then aged at between 150°C and 275°C for different lengths of time to produce the o' phase, Al3Li, which emerges as a fine precipitate within the aluminum matrix, and causes an increase in yield strength. Measurements were made of hardness and eddy current response to enable us to determine details of the thermal history in a similar way to the work of Chihoski [1,2]. OBJECTIVESThe long term objective is to develop fieldable noncontact NDE methods, using, for example, eddy current and EMAT inspection systems, for determination of thermal history and mechanical properties of aluminum-lithium alloys. The immediate objective is to investigate the 1395 effects of quench rate, annealing temperature and aging time on the eddy current response, hardness and elastic moduli, to determine whether such techniques are sensitive enough to changes in process variables for use as fieldable NDE methods. BACKGROUND

Search for NDE Methods to Characterize Thermal History and Mechanical Properties of Al-Li Alloys

“…Furthermore, it appears that hardness measurements are sensitive to the volume fractions, and probably the morphology, of the various precipitates formed. Thf' results obtai.ned so far (see also [1]) as well as by other authors on different aluminum alloys [2][3][4], this work will continue with additional results to be reported in future publications.…”