Arc Initiation for the Electromagnetic Powder Deposition Gun

The Electromagnetic Powder Deposition (EPD) process converts pulsed electrical energy into kinetic and thermal energy to accelerate and heat powder material to conditions suitable for bonding. A high pressure plasma armature is electromagnetically accelerated using a railgun. A supersonic pressure wave is created when the armature accelerates through and "snowplows" the ambient gas ahead of it. The gas column is heated, compressed, and accelerated to the entrainment section of the gun, where some of the thermal and kinetic energy is transferred to an injected stream of powder material. The acceleration burst is repeated rapidly to supply the required deposition rate and to achieve steady thermal conditions. Development of a starter plasma which is reliable at ambient pressure was a major programmatic task. Generation of a low pressure linear arc required to form a planar armature during the pulsed event was investigated. Several geometries (pointto-point breakdown, rail-to-rail breakdown, and confined glow discharge) were explored using different voltage sources (de, 60Hz ac, 150 MHz rf, pulsed). Satisfactory operation of the confined glow discharge approach at atmospheric pressure was achieved using rf excitation. Results of testing under the various scenarios are presented and critiqued. THE GOAL OF THE EPD PROGRAM is to develop coating and build-up processes which exceed the performance of existing technologies. Improvements are expected in the areas of materials compatibility, bond strength, coating strength and density. A plasma armature railgun technique [1] studied two decades ago has been selected for accelerating powder material to velocities in excess of those achieved by existing thermal spray processes. The railgun consists of two metallic rails with insulating sidewalls separating them (Figure 1). A low mass plasma armature, initiated in the breech, is used to commute the gun current between the rails. In this configuration, current flow generates a magnetic field within the bore, behind the armature. The current carrying ions and electrons of the plasma armature are acted on by the magnetic field, accelerating the armature forward. As it is accelerated, it collisionally interacts with the ambient gas in the bore and accelerates the gas to the same velocity. This mechanism of momentum transfer is referred to as the snowplow. Essentially, the arc armature acts like a piston, pushing the gas in front of it as it moves down the gun bore. The gas eventually passes the powder injection point, exerting viscous drag and accelerating the powder particles to a velocity of 2 krn/s. High velocity is the most important attribute of electromagnetic powder deposition (EPD) as a coating process. Another unusual aspect of EPD, shared with some detonation processes, is the repetitive pulse mode of operation (-30Hz). While substrate heating is reduced by thermal relaxation between pulses, some heating of the powder will occur via interaction with the plasma and the gas column heated by a shock front propagated through the gas. Since EPD will be used at atmospheric pressures, oxidation of deposition surfaces may occur. To avoid oxidation, the process will employ an inert gas, argon, for the purpose of substrate shielding as well as for plasma seeding and powder feeding. To behave as a piston, the armature must fill the entire transverse section of the bore. For maximum efficiency, the ideal shape of the armature would be a plane or "sheet". A major challenge of the process, and the focus of this paper, is the initiation of such an armature. Historically, plasma railguns have been used to accelerate gases. Unlike the EPD process, however, the plasma initiator took the form of an exploding fuse. The pulse power sources driving these wire or foil fuses operated with risetimes of only a few microseconds so that the fuse material would be thoroughly vaporized. In the EPD process, because a longer acceleration pulse is needed, the risetime is also considerably longer due to the characteristics of the power supply. The power source will provide a 20 jlS ramp-up, after which a constant 160 kA current is delivered for 100 jlS. Another important reason for