Role of microstructure on the fatigue crack propagation behavior of a friction stir welded Ti-6Al-4V

Fatigue crack propagation behavior of a friction stir welded Ti-6Al-4V alloy was investigated with focus on the role of residual stresses and inhomogeneous microstructure. It was found that, in the weld affected areas, significant microstructural evolution occurred with α grain refinement, development of a bi-modal structure and increase in β phase volume fraction. Residual stresses obtained through the XRD method were low, about 5% of the yield strength, in all regions of the weld. Fatigue crack propagation rates obtained through miniature CT specimens with initial notches in the base metal (BM), stirred zone (SZ), interfacial zone (IZ) and heat affected zone (HAZ) were higher in the SZ and the IZ. It was found, based on the coplanar maximum strain energy release rate theory, that the nominally linear crack propagation paths in the fine-grained structure of the SZ and IZ result in higher FCP rates than those of the torturous cracks in the BM. Considerations were also made with reference to the role of the increased β volume fraction on the compliance of the SZ and IZ specimens.