An Analysis of the Effect of Ambient Temperature Aging on Al-Li Airframes

Abstract

Aluminium Lithium (Al-Li) alloys exhibit behaviours in mechanical properties which separate them from conventional precipitation hardening aluminium (Al) alloys. One such behaviour is an increase in strength and a reduction in ductility which occurs as a result of exposure to relatively low temperatures (between 70°C to 100°C). This work sets out to establish a representative exposure temperature for internal helicopter main lift frame structure that is subjected to ‘hot’ environments. This representative temperature, determined to be 53.1°C, was investigated to understand whether exposure at this temperature for times ranging from 0 to 1000 hours had any effect on the mechanical properties of Al-Li 8090 T852 main lift frame forgings. The material for this project was obtained from in-service military aircraft that had been operating for a minimum of a decade in a number of theatres. This material provided a unique opportunity to compare and contrast the elemental chemical, microstructural and mechanical properties of the in-service material against the original design specification and against material that had been exposed to 53.1°C for between 0 to 1000 hours. A comprehensive mechanical testing regime has shown that, after 1000 hours exposure at 53.1°C, the main lift frame material exhibited a 36% reduction in ductility accompanied by a 7.8% in UTS and a 13.2% increase in 0.2% proof stress. Furthermore, a repeatable pattern of an increase and reduction in strength of the material over the first 72 hours of exposure to 53.1°C has been established and investigated. A detailed study of the main strengthening precipitate δ’ (Al3Li), using transmission electron microscopy, differential scanning calorimetry and comparison with the current literature was carried out. This has shown that the initial strengthening mechanism (from 0 to 24 hours) is likely to be the result of an increase in volume fraction of Guinier-Preston zones or potentially δ’ precursors, which form from available solute in the material. This is likely to have occurred due to decomposition of the solid solution, which in turn is a result of low temperature exposure during operational service. However, after 36 hours, a significant decrease in strength indicates that this formation is unstable. A second strengthening mechanism, (48 hours onwards) is likely to be the formation of very finely dispersed δ’ precipitates (2-4 nm in diameter) and is well documented as being the reason for the increase in strength and loss of ductility in Al-Li alloys that are exposed to these modest in-service temperatures.