MICROSTRUCTURE AND PROPERTIES OF Al-Li-Cu-Mg-Zr (8090) HEAVY SECTION FORGINGS

Abstract

The microstructure and properties of heavy section forgings of the 8090 Al-Li alloy were investigated including the as-cast, homogenized, forged, and heat-treated conditions. The ingots, 305x965x2000-3600 mm in size, were cast by Alcan. Homogenization involved 24 to 48 h soaks at 545°C. Ingots were hand-forged by HDAF Ltd. to sizes up to 356x356x1524 mm, then solution-treated at 530°C for 6 h and water quenched. The material was aged at various times at temperatures of 150, 170, and 190°C. Microstructures were examined by TEM, CBED, SEM, AES, and optical microscopy. Mechanical properties were characterized by tensile, fracture toughness, and stress corrosion tests. A strong correlation was observed between grain boundary precipitation of a icosohedral (I) phase and certain mechanical properties. The I-phase has been tentatively identified as Al6CuLi3, historically called T2. When the I-phase was predominant, the fracture toughness, SCC, and tensile ductility were invariably low. Factors identified as promoting the formation of I-phase were cooling rates from the solution treatment temperature slower than -10°C s-1, increased aging time, and increased aging temperatures in the range 150 to 190°C. Aging conditions which minimized the formation of the grain boundary I-phase and, consequently, improved the mechanical properties were determined. Three distinct constituent phases were found in cast ingots having only slightly different chemistries. Remnants of these constituent phases were present in every subsequent stage of thermal processing. One, AlLiSi, was discovered to promote surface pitting and to substantially lower the stress corrosion cracking resistance. This phase's reactivity with seawater appears to promote dissolution of the adjacent matrix. Material heat-treated to suppress the formation of the I-phase, but high in silicon, revealed low SCC resistance. In summary, many factors including composition, casting practice, metal-working, and heat-treatment, determine the mechanical properties of 8090 Al-Li in heavy section forgings, and which range from unacceptable to acceptable for high performance aerospace structures.