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Fatigue life of superplastically formed Ti-6Al-4V

: Busch, W.-B.; Kunze, H.-D.

Deutsche Gesellschaft für Materialkunde e.V. -DGM-, Oberursel:
Titanium. Science and Technology. Vol.2
Oberursel, 1985
pp.725-732 : Abb.
International Conference on Titanium <5, 1984, München>
Conference Paper
Fraunhofer IFAM ()
crack initiation; microstructure; strain rate sensitivity; superplasticity; Ti6Al4V; Woehler-diagram; yield strength

Superplasticity as a phenomenon has been well known for a number of years. Especially the titanium alloy which is often used in aircraft constructions, has been investigated in several countries. Modern investigations concerning superplastically formed titanium parts are carried out to test materials behaviour under real conditions. The mechanical properties of the materials are influenced by the forming techniques as well as by thermal and mechanical loads. They cause changes in texture, residual stresses, grain size and volume fraction of the different phases. Superplastically formed samples show significantly coarser grains than undeformed ones. Moreover, microporosity may exist, which acting as notches leads to failure under cyclic loading. In special cases, variations in texture are supposed to be found during superplastic flow. These influences are widely unknown with regard to fatigue life of superplasticly formed samples. The knowledge of Wöhler diagrams is of great importance, especially for aircraft applications. An investigation program was set up to examine the dependence of forming temperature, strain rate and deformation ratio on the fatigue life of the alloy. As parameters, three different temperatures, three strain rates and three deformation ratios were chosen for superplastic forming. Then fatigue tests were carried out and presented in the form of Wöhler diagrams. A significant loss in fatigue strength was observed.