Electrodynamics of a hypervelocity surface conversion process using electromagnetic accelerators
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The U.S. Government and commercial entities are dependent on chemical plating and coating processes to replace worn or eroded material on damaged parts. Logistics Centers have been forced to consider replacement materials for repair operations due to tightening of government regulations on the use of toxic and hazardous materials. Existing state of the art thermal spray processes (HVOF, D-gun, Plasma Spray) are limited to powder velocities of about 1 km/s because they rely on the thermodynamic expansion of gases. Because these methods rely on the mechani- cal bonding of the powder to the substrate the coatings and their bonds are a frac- tion of the parent material strength. To make full use of this repair process the coating has to bond to substrate with near parent material strength and coating has to build with good strength layer to layer. A new thermal spray process using electromagnetic forces can accelerate powder particles to a final velocity in ex- cess of 2 km/s. At this velocity powder particles have sufficient kinetic energy to melt their own mass and an equivalent substrate mass on impact. The kinetic en- ergy of the process allows fusion bonding of greater strength than that created by low velocity processes as well as improved coating density. This dissertation describes the invention of the Electromagnetic Powder Deposition (EPD) process. The railgun process is combined with a gas dynamic mechanism, called a snowplow, to produce controllable bursts of gas with the speed and duration required to accelerate finite segments of dispersed powder to the conditions required for plating purposes. The physics of the railgun and snow- plow process are presented and proof of principle experiments is described. A method to start a planar arc in repetitive fashion had to be developed. An added complexity was that the source had to be stable at atmospheric pressure. A repeti- tive discharge power supply, instrumentation and control system had to be devel- oped. To industrialize the process it had to be demonstrated that the process could build multiple layers with good bond strength layer to layer.