Residual stress and long fatigue crack growth behaviour of laser powder bed fused Ti6Al4V: Role of build direction

Laser powder bed fused (LPBF) Ti6Al4V alloy has a great prospect to manufacture complex aircraft structures. The anisotropic fatigue crack growth behaviour and the residual stress effects are two main concerns on the damage tolerance design of aircraft structures, and should be studied further. In this paper, the compact tension (CT) specimens in four off-axis build directions (15°, 30°, 60° and 75°) were designed and manufactured. Residual stresses of the as-built and the stress relieved samples were measured. Fatigue crack paths and fracture surfaces were analyzed and compared. It was shown that the residual stress along the loading direction in the 15° as-built sample was about 3.5 times larger than that perpendicular to the loading direction, while that of the 75° as-built sample was opposite. The crack deflection angle of the 60° as-built sample was the largest, and it was about 22.4°. Fatigue crack growth rate in the as-built sample fluctuated. After post-treatment, the anisotropy of residual stress state and fatigue crack growth behaviour decreased.