Fatigue crack growth in additive manufactured titanium: residual stress control and life evaluation method development

This paper presents fatigue crack growth behaviour in titanium alloy Ti-6Al-4V built by the Wire + Arc Additive Manufacture (WAAM®) process. Process induced residual stress and stress relief by cold working were measured by neutron diffraction and contour methods. Residual stress retained in the compact tension test specimens was evaluated by the finite element method based on the measured stresses in the WAAM wall. Fatigue crack growth rate in as-built and stress relieved conditions are discussed with respect of the effects of material build orientation, residual stress, and microstructure characteristics. Key conclusions are: (a) residual stresses arising from the WAAM process can be controlled and reduced significantly by cold working. Residual stress retained in compact tension specimens is low, resulting in low residual stress intensity factor. (b) Microstructure affects fatigue crack growth rate in twodifferent material’s build orientations. (c) Fatigue crack growth rate in WAAM Ti-6Al-4V is lower than that in traditional wrought plate, with and without the residual stress relief. Therefore, WAAM is a viable additive manufacture process to produce aerospace titanium alloys for damage tolerance design.