Internal pores in DED Ti-6.5Al-2Zr-Mo-V alloy and their influence on crack initiation and fatigue life in the mid-life regime

The present work aims to investigate the mechanism of crack initiation induced by internal pores, which are inevitable in additive manufacturing (AM), and the influence of internal pores on the fatigue performance of directed energy deposited (DED)Ti-6.5Al-2Zr-Mo-V. After fatigue test under constant amplitude alternating stress at three stress levels, thirty-one pieces of DED Ti-6.5Al-2Zr-Mo-V specimens were found that cracks initiating from internal pores. Scanning electron microscope (SEM)and its accessories, such as energy dispersive spectrometry (EDS)and electron backscattered diffraction (EBSD), were used to analyze the characteristics of pore defects and clarify the mechanism of crack initiation. The results show that the specificity of the microstructure affected by the DED process and pore defects, such as segregation of Al and the existence of incomplete grain boundaries, are the main causes of crack initiation. Then, the crack initiation modes were divided into three types, and a classification model was established that can make the effect of pore defects on fatigue life clearer and more intuitive.