Abstract
Friction stir processing (FSP) of cast and mill annealed Ti-6Al-4V was studied to determine whether this thermomechanical processing technique is feasible for local microstructural modification to improve fatigue properties for aerospace engine applications. The ability to produce a friction stir processed surface layer is thought to provide a microstructure that is more resistant to fatigue crack initiation than the as-cast microstructure of the bulk. FSP resulted in a microstructure of either very fine equiaxed α or a colony microstructure with refined prior β grains when processing occurred below the β transus or above the β transus, respectively. The final microstructure observed in the stir zone was related to the processing conditions and was independent of the initial cast or wrought microstructures. Electron backscatter diffraction was used to assess the presence of microtexture in the stir zone. There was very little microtexture observed in sub-transus processed material. A moderate strength α phase texture was observed in the cast and wrought materials processed above the β transus. Fatigue tests performed on 4-point bend samples revealed a dramatic increase in fatigue life after FSP in both mill annealed (presented here) and cast + hot isostatic pressed materials (previously presented) although only a limited number of samples were available. Substantial tool wear is an issue and tungsten contamination is readily observed when FSP temperatures were below the β transus. This could be a concern in high temperature applications or for applications requiring postprocessing heat treatment.
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Pilchak, A.L., Juhas, M.C. & Williams, J.C. A Comparison of Friction Stir Processing of Mill Annealed and Investment Cast Ti-6Al-4V. Weld World 52, 60–68 (2008). https://doi.org/10.1007/BF03266670
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DOI: https://doi.org/10.1007/BF03266670