Importance of local microstructure for damage tolerant light weight design of Ti–6Al–4V forgings

Forged parts made of titanium are generally used in aerospace industry, e.g. for engine mounts, pylon fitting and frame parts, housings, gear box components, engine disks and so on. To achieve damage tolerant together with light weight design of such parts the local microstructure resulting from thermomechanical treatment has to be considered. Therefore a new approach is introduced in this paper containing methodologies for the determination of local microstructural-based S/N-curves. They can be embedded in a lifetime prediction based on local stresses. Long crack growth and fatigue tests were performed on Ti–6Al–4V with different types of microstructures and distinct differences were found in respect of fatigue and crack propagation. It is shown that the microstructural parameters primary α-grain size and (α + β)-content dominate the fatigue behavior based on rotating bending S/N-curves whereas the long crack growth is mainly affected by primary α-grain size and connectivity of α-grains. In addition first results regarding characterization of crack initiation phase and short crack growth of mill-annealed Ti–6Al–4V are presented for clarification of the fatigue process in respect of microstructure.