| An investigation of the effects of welding on the mechanical and fatigue crack growth (FCG) properties, as well as the microstructure of the Al-Li-Cu alloy 2095 (heat treated to the T8 temper condition) was undertaken. The material concerned is a light weight, ultra-high strength aluminum alloy targeted for cryogenic and other aerospace applications. FCG tests were performed in Laboratory air on specimens prepared from the base metal (BM), heat affected zone (HAZ), and weld metal (WM). Microstructural characterization was performed using optical microscopy, transmission electron microscopy (TEM), and high resolution electron microscopy. Electron probe microanalysis was used to study segregation of solutes while the fatigue fractured surfaces were studied on the scanning electron microscope (SEM). The texture was deduced from the TEM foils and the residual stress distribution was examined by the X-ray diffraction technique. The microstructure of the BM exhibited partially recrystallized grains, a sub-grain structure, precipitation of T{dollar}sb1{dollar} (Al{dollar}sb2{dollar}CuLi), {dollar}deltaspprime{dollar} (Al{dollar}sb3{dollar}Li), {dollar}Thetaspprime{dollar} (Al{dollar}sb2{dollar}Cu), {dollar}Theta{dollar} (Al{dollar}sb2{dollar}Cu), and a new needle-like phase on {dollar}{lcub}{dollar}110{dollar}{rcub}{dollar} planes. The HAZ showed recrystallization, grain growth, dissolution of T{dollar}sb1{dollar}, reprecipitation of {dollar}deltaspprime{dollar} and of formation {dollar}delta{dollar} (AlLi). In the WM, complex intermetallics were precipitated in the interdendritic regions. WM TEM specimens prepared by ion milling showed a marked precipitation of a T{dollar}sb1{dollar}-like phase. A series of fine pores lined the fusion line (FL). Tensile failure of the weldments occurred consistently at the FL--a factor attributable to the FL pores. The hardness was lowest in the weld metal and showed maxima and minima in the HAZ. The BM had a crystallographic texture dominated by the {dollar}{lcub}{dollar}112{dollar}{rcub}langle{dollar}111{dollar}rangle{dollar} type, a feature which accounted for the 22{dollar}spcirc{dollar} deviation in the fatigue crack profile and the crystallographic shear micromechanism of FCG observed in this deviated region. A high compressive residual stress was determined at the notch tips of the WM specimens, leading to a lower FCG rate. FCG in the HAZ showed a marked dependence on {dollar}Delta{dollar}K and was influenced by the proximity of the HAZ to the FL.; Mathematical relationships between the weld thermal cycle and the HAZ hardness, and a conceptual model, relating the crack profile to the orientation of the close packed planes in a textured material, were developed. |
