THE EFFECT OF SUPERPLASTIC DEFORMATION ON THE TENSILE AND FATIGUE PROPERTIES OF Al-Li (8090) ALLOY

Abstract

Superplastic deformation of the new generation of aerospace aluminium-lithium alloys has generated considerable interest in the aerospace industry not only with the potential savings in component weight and manufacturing costs but also for development of novel designs. Even though many papers have addressed the superplastic deformation characteristics of these alloys it is essential for their exploitation to determine and to understand the effect of the forming process on their mechanical properties. This paper presents results on the effect of superplastic strain and post form heat treatment on the tensile and fatigue performance of the Al-Li-Cu-Mg-Zr (8090) alloy with a low and high copper content of 1-1.2 wt% and 1.56 wt% respectively. Both alloys were superplastically formed biaxially under an imposed back pressure to prevent intergranular cavitation. For the low copper containing alloys the tensile properties decreased with superplastic strain and were independent of quench rate. In contrast the tensile properties of the high copper containing alloy were quench rate sensitive. The as-formed and aged strength were initially lower than those determined for the low copper alloy although both the 0.2 % proof stress and the tensile strength increased with superplastic strain. Incorporation of a solution heat treatment with a cold water quench prior to ageing resulted in a strength increase of 50 MPa over that determined for the low copper alloy and the tensile properties decreased with superplastic strain. The reduction in strength with superplastic strain is related to changes in the ageing kinetics resulting from dynamic recrystallisation and grain growth, which was similar for both alloys. The higher strength of the high copper alloy following re-solution heat treatment is due to enhanced homogeneous S phase precipitation. The S phase precipitation was reduced in the as-formed and aged material due to precipitation of a copper rich phase during the slower air cool, resulting in the lower strength. The fatigue performance, under sinusoidal loading and stress ratio (R) = 0.1, was similar for both alloys and the endurance limit was slightly reduced with increased superplastic strain. The results are compared to those for the aluminium-lithium alloy 8091, which has a higher copper content and with those for the superplastic aluminium alloys Supral 220 and 7475E. The implication of the results for manufacturing practice is also discussed.