Influence of grain growth direction on fatigue crack growth behavior of laser directed energy deposited Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy

The solidification microstructure, which grows and aligns along the deposition direction in laser directed energy deposition (L-DED), leads to directional differences in mechanical properties, particularly in fatigue crack growth (FCG) behavior. Understanding the relationship between the microstructure and FCG behavior is critical for structural safety of L-DED aircraft components. This study employed a rotationally sampling strategy to achieve the specimens with varying angular deviations between the grain growth direction and the maximum principal stress plane. The influence of grain growth direction on the FCG behavior of L-DED Ti-6.5Al-3.5Mo-1.5Zr-0.3Si alloy were discussed by characterizing the spatial orientation of solidification microstructure, the plastic zone at crack tip, and the crystallographic orientation of grain boundaries. Experiment results demonstrate that the grain growth direction effects crack path selection during FCG, which subsequently leads to differences in FCG rate. When the grain growth direction aligns closely with the maximum principal stress plane, cracks tend to grow along grain boundaries. This intergranular growth requires lower energy consumption and exhibits reduced crack growth resistance, resulting in higher FCG rate. Conversely, when significant mis-direction exists between these directions, transgranular growth becomes dominant. This growth mode demands higher energy expenditure and greater crack growth resistance, consequently leading to lower FCG rate. These findings elucidate the effect of solidification microstructures on the FCG via grain growth directionality and provide guidance for enhancing titanium alloy structural safety of L-DED aircraft component.