Mechanisms on inter-track void formation and phase transformation during laser powder bed fusion of Ti-6Al-4V

Abstract The build quality of laser Powder Bed Fusion (PBF) components largely depends on printing issues such as inter-track voids and undesired microstructure. In this work, a comprehensive phenomenological model was developed to compute the complex transport phenomena during laser PBF of Ti-6Al-4V. The transient temperature and velocity fields during single-track and multi-track laser PBF were computed considering the melting and solidification of the powder feedstocks. Critical metallurgical variables including the molten pool characteristics and thermal cycles were obtained. The model was validated by comparing the computed results against corresponding experimental data. The formation and evolution of inter-track voids in different heat input conditions were studied. Two types of inter-track voids were specified considering their formation mechanisms. The first type appeared in irregular elongated shapes and was caused by the incomplete melting of the powder feedstocks. The second type appeared in near-spherical shapes and originated from the bubbles trapped in the overlap zone of adjacent tracks. Cooling rates were obtained to interpret the metallurgical conditions for the solid-state phase transformations. The novel findings from this research are helpful to the understanding of the formation and mitigation of inter-track voids, and the assessment of phase transformations during laser PBF of titanium alloys.