ALUMINUM LITHIUM ALLOYS - PROCESSING AND PROPERTIES

Aluminum lithium (Al-Li) alloys are lighter and stiffer than conventional aluminum on a gage for gage basis and have the same general formability characteristics. Commercially available alloys also have mechanical properties generally comparable to the most widely used aluminums in helicopter usage, 7075-T6 sheet and thin-walled extrusions and 7075-T73 heavy plate and forgings. Sikorsky is developing a data base on Al-Li alloys for use on future aircraft and derivatives of current aircraft. Al-Li alloys are proposed for applications in place of conventional aluminums to save weight with minimal additional costs. Design criteria used for selection of Al-Li alloys and tempers are summarized. The most cost effective application is identified as a gage-for-gage substitution in derivatives of current designs. The physical metallurgy and development of commercial alloys are discussed. Microstructures, processing, and properties of available alloys and product forms are presented. Particular emphasis is placed on 2090, an Al-Li alloy produced by ingot metallurgy for replacement of 7075-T6 sheet and thin-walled extrusions, and IN905XL, a powder metallurgy Al-Li alloy for replacement of 7075-T73 heavy plate and forgings. Forming, chemical milling, and finishing characteristics differ somewhat from current aluminums. Static properties of 2090 and IN905XL are slightly lower than for 7075 but fatigue strength is superior. Initial evaluations also indicate that 2090 has a greater degree of anisotropy than 7075-T6. The relationships between microstructure, processing, and mechanical properties are outlined.