High cycle fatigue and fatigue crack growth rate in additive manufactured titanium alloys

The Wire + Arc Additive Manufacture (WAAM) process can produce large metal parts in the metre scale, at much higher deposition rate and more efficient material usage compared to the powder bed fusion additive manufacturing (AM) processes. WAAM process also offers lead time reduction and much lower buy-to-fly ratio compared to traditional process methods, e.g. forgings. Research is much needed in the areas of fatigue and fracture performance for qualification and certification of additive manufactured aircraft components. In this study, specimens made of WAAM Ti-6Al-4V alloy were tested and analysed focusing on two key areas of structural integrity and durability: (1) High cycle fatigue and effect of defects: crack initiation at porosity defects was investigated via fatigue and interrupted fatigue-tomography testing performed on specimens with porosity defects purposely embedded in the specimen gauge section. Key findings are as follows. Presence of porosity did not affect the tensile strengths, however both ductility and fatigue strength were significantly reduced. Fatigue life could not be correlated by the applied stress, e.g. in terms of the S-N curves, owing to the different pore sizes. Using the fracture mechanics approach and Murakami's stress intensity factor equation for pores, good correlation was found between the fatigue life and stress intensity factor range of the crack initiating defects. Predictive methods for fatigue strength reduction were developed taking account of the defect size, location, and distribution. (2) Fatigue crack growth rate: effect of heterogeneous microstructure was investigated via two different material deposition methods and testing two crack orientations. Fatigue crack growth rates were measured for damage tolerance design considerations. Unique microstructure features and their effect on the property anisotropy are discussed.