The velocity and temperature of steel droplets during electric arc spraying

The average dynamic and thermal behaviour of electric arc sprayed Fe-0.8wt.%C droplets has been investigated using a laser based time of flight velocimeter system (Laser2Focus-L2F) and a two-colour pyrometry system (In-flight Particle Pyrometer-IPP) respectively. Radial and axial variations of the spray velocity and temperature have been investigated, together with their dependence on atomising gas type, arc voltage, atomising gas pressure, and wire feed rate. Under all conditions, the average spray temperature was significantly above the steel liquidus temperature indicating most droplets were fully molten. Once projected away from the region of atomisation, the droplet spray typically cooled at ∼1 °C mm−1 and ∼105 °C s−1 and typical axial velocities were 100 m s−1. Because of asymmetry in the arc itself and the resulting differences in the melting behaviour of the wires, the spray exhibited asymmetric radial variations of both velocity and temperature about the spray cone axis. The presence of oxygen in the atomising gas had a significant effect in increasing spray temperature via exothermic oxidation of the steel droplets. Processing maps have been constructed in which the variation of average spray velocity and temperature with arc voltage, gas pressure and wire feed rate have been fitted to a series of simple polynomial expressions. These maps have been explained in terms of the heat and momentum transfer processes occurring during atomisation and subsequent droplet flight.