The thermal conductivity of a powder bed is a crucial property to guarantee the stability of the additive powder bed fusion (PBF) processes [1]. For PBF for metallic materials where an electron beam (PBF-EB/M) is used, the thermal conductivity controls the interaction between the electron beam and the powder material, the heat transfer during the whole process, the cooling rate and, consequently, the final component microstructure. However, powders generally have a lower conductivity than bulk material, which is extremely poor in some cases. Because of that, a sintering step is realised to promote the formation of connections between the particles, usually called necks. The necking allows for the desired conductivity level to be achieved. [2]. After the initial necking, the sintering continues because of the production of subsequent layers and the high temperature in the working chamber. Despite the commonly adopted sintering step, this process is usually uncontrolled and calibrated only via a trial-and-error approach. Therefore, the actual value of the thermal conductivity of the powder remains unknown. This work presents a novel numerical framework to evaluate the sintering among the particles and the powder bed's thermal conductivity. A phase field model combined with a novel definition of the thermal load was adopted to investigate the sintering of the powder particles at the conditions of the PBF-EB process, including cooling. The proposed modelling approach was validated against experimental data obtained by designing an ad-hoc experiment setup when processing Ti6Al4V material powder by PBF-EB. With the information about neck formation and evolution, an innovative numerical approach was designed to evaluate the thermal conductivity of the powder bed. This approach proposed a new definition of the hydraulic tortuosity factor, which is used in this case to describe the powder bed's geometric complexity. The model was validated against experimental thermal conductivity data for Ti6Al4V processed with a PBF-EB system available in the literature.