Failure Behavior of Laser Metal Deposited Additive Manufacturing Ti-6Al-4V: Effects of Stress State and Initial Defects.

To investigate the mechanical property and failure behavior of laser metal deposited additive manufacturing Ti-6Al-4V (LMD Ti64) in a wide range of stress states and strain rates, different types of specimens were tested at strain rates of 0.001-5000/s. Numerical simulations were conducted to collect the local fracture strain at the critical position where the failure happened for all specimens. By comparing with Ti64 alloy manufactured by different techniques, the failure behavior of LMD Ti64 alloy shows a stronger sensitivity to Lode angle parameter and strain rate. The role of initial defects in failure was discussed. It is found that high laser power and overlap ratio can improve the failure behavior by reducing the number of initial defects. The initial defects on the fracture surface at much higher strain rates were observed, indicating that the initial crack rather than initial void acts as the crack growth point leading to the final fracture at higher strain rates. The scanning electron microscope observation of the fracture surface shows that the failure mechanism of LMD Ti64 alloy varies from different stress states and strain rates. The failure mechanism is characterized by the shear fracture at the negative stress triaxiality, whereas the void growth fracture plays a dominant role in the failure mechanism of LMD Ti64 alloy at a high stress triaxiality on the quasi-static loading condition.