Temperature, mass and velocity of droplets in wire-arc directed energy deposition using a bypass hybrid plasma arc

The droplet state (temperature, mass and velocity) has a very important influence on the shape accuracy and material quality of the manufacturing components in wire-arc directed energy deposition (DED). The heat input of the wire end and base metal can be controlled freely by independently adjusting the main and bypass currents under a bypass hybrid plasma arc (BHPA). This approach offers great potential for regulating the base metal thermal state and the droplet state from the wire. However, the modulation capability and mechanism for the droplet state under BHPA are still unclear, which hinders the widespread application of this process. Therefore, in an innovative approach, an auxiliary magnetic field is utilized to deflect the hybrid arc tail flame based on the solid copper calorimeter, effectively avoiding interference. The droplet temperature can be measured using a solid calorimeter, while droplet mass and velocity can be determined using a high-speed camera and subsequent image processing. Based on this, the regulation rules of the wire feeding speed, main current and bypass current on the droplet state are studied. The results reveal that the droplet temperature ranges between 1138 and 1726 ℃ under BHPA, the droplet velocity varies from 0.02 to 1.75 m/s, and the droplet mass spans from 1.4 to 159 mg. Second, the heat flux at the wire end was quantified via numerical simulation. A positive correlation is identified between the droplet temperature and the heat flux at the wire end for unit mass transfer. The utilization efficiency of the droplet temperature for heat at the wire end increases with increasing X-coordinate. Subsequently, an analysis of droplet acceleration was performed to determine the distribution of the plasma flow force, revealing a Gaussian distribution with a characteristic radius of 2 mm. The contribution of the main current to the plasma flow force is approximately three times that of the bypass current. Finally, the distribution along the X-axis and the mathematical model of the droplet state were studied. X = -2 mm is a characteristic point of the distribution of the droplet state. This work reveals the regulation law of the droplet state under BHPA, and lays a foundation for further systematic study of the control mechanism of deposition layer morphology under this process. [Display omitted] •In BHPA, the droplet temperature can be regulated between 1138 and 1726 ℃.•The heat utilization efficiency is between 0.49 and 0.83.•T