A STUDY OF THE MICROSTRUCTURE AND PROPERTIES OF DIE FORGINGS IN ALUMINIUM-LITHIUM ALLOYS 2091 AND 8090

This paper reports on some of the results from an examination of sample die forgings in aluminium-lithium alloys 2091 and 8090. Whilst direct comparison of properties of the two forgings is complicated by the differing configurations, useful data has, nevertheless, been generated which will provide useful background information for further programmes about to commence. The present study has revealed density reductions of ~ 8% and ~ 10% respectively for the 2091 and 8090 compositions, compared to current 2xxx series alloys. With regard to microstructure, the 2091 forging exhibited predominantly unrecrystallised grains which contained linear features postulated to originate from a mechanical/deformation twinning type of mechanism. In contrast, the 8090 component showed extensive recrystallisation. Microhardness measurements indicated lithium and magnesium depletion at surface locations while optical and electron microscopy suggested that apparent porosity bands on the polished microsections were due to the presence of the equilibrium δ-AlLi phase, particles of which fragment and are removed during metallographic preparation. Mechanical properties showed significant anisotropy in the 2091 forging and were attributed to texture effects. Highest strengths were obtained in the longitudinal direction, while the lowest appeared at ~ 30° and ~ 70° to the longitudinal direction : long transverse strengths were intermediate. The resulting failure modes are briefly discussed. In contrast, mechanical properties in the 8090 component exhibited a gradual increase in strength from longitudinal to long transverse directions, in keeping with the recrystallised nature of this forging