Investigation of fatigue behavior of laser powder bed fusion Ti-6Al-4V: Roles of heat treatment and microstructure

High cycle fatigue (HCF) performance of additive manufactured (AM) titanium products is of great significance for its structural and functional applications. However, tying fatigue performance to the complex microstructures in AM titanium alloys is challenging. The work here carried out a thorough investigation into the influence of microstructures (particularly the grain boundary alpha-phase (GB-alpha) with varied morphologies) on the fatigue performance of laser powder bed fusion (LPBF) Ti-6Al-4V (Ti-64). Results showed that the improvement in the high-cycle fatigue life of LPBF Ti-64 could be achieved by the formation of low aspect ratio a lath and discontinuous GB-alpha via optimized post-fabrication heat treatment. Discontinuous GB-alpha could fully accommodate the deformation, improving the fatigue crack propagation resistance. Moreover, a lath with a low aspect ratio could lead to less strain accumulation on the interface between adjacent a lath, and thereby inhibit the crack initiation at these interfaces. This study enhances the understanding of how LPBF-induced complex microstructures influence fatigue behavior, and provides a pathway for the improvement of fatigue performance of additive manufactured titanium alloy.