Dynamic evolution of powder stream convergence with powder feeding durations in direct energy deposition

During direct energy deposition (DED), changes in the powder feeding behavior significantly affect the deposition process. In this study, a high-speed photography method was developed to observe the dynamic evolution of the powder stream for various powder feeding durations, and the synchronous evolution of the inner channel characteristics of the nozzle were also measured. Combined with powder stream simulations, the contribution of the evolution of each channel characteristic on the powder divergence was discussed. The results show that as the powder feeding duration increased, the inner channel characteristics of the nozzle changed dynamically, which led to the powder stream divergence. For a powder feeding duration of 60 h, the nominal powder spot size increased by 41.4% at the plane 15 mm below the nozzle exit. As the powder feeding time increased, the inner wall roughness, inner channel diameter, and the exit size of the nozzle also increased owing to the scouring erosion from the high-speed powder particles. Subsequently, changes in the inner wall roughness and inner channel diameter of the nozzle tended to be smaller owing to the formation of a work hardening layer. It was also demonstrated that the changes in the inner wall roughness of the nozzle had a negligible effect on the powder stream divergence, an increase in the inner channel diameter of the nozzle extended the maximum divergence range of the particle trajectory at the nozzle exit, and the expansion of the nozzle exit size reduced the constraint causing the allowed trajectory range of the ejected powder particles to increase. The deposited height gradually decreased as the powder feeding duration increased, resulting in a 38.7% decrease after 60 h.