Effects of defect type on failure modes of laser powder bed fusion Ti–6Al–4V alloy under impact dynamic loading

Laser powder bed fusion Ti–6Al–4V alloy exhibits great potential for aerospace applications, and there have already been many studies of the effects of the print parameters on defects in this alloy. However, there has been little research on the effects of defect type on failure modes under impact dynamic loading. To clarify the relationship between defect type and failure mode, in this study, specimens are printed with three main defect types: gas-entrapped pores, keyhole porosity, and lack-of-fusion defects. Eight material states with controlled defect populations are engineered by modulating laser energy density and characterized through x-ray computed tomography. A decision-tree classification algorithm with the maximum axis dimension, the sphericity, and the aspect ratio is constructed. Dynamic compression impact tests are performed over a wide strain rate range from 0.001 to 3000 s−1. Failure modes and mechanical behavior are studied. It is shown that keyhole pores exhibit progressive closure with predictable deformation, limiting the stress drop to less than 15% at 3000 s−1 with 2.358% porosity. By contrast, angular lack-of-fusion defects trigger stochastic crack propagation through geometric stress concentration, inducing higher stress oscillations. On the basis of these mechanisms, a yield strength model incorporating strain rate sensitivity, porosity, and defect-specific morphology is developed and validated.