Electric Arc Spray
electric arc spraying, a thermal spray process in which an arc is struck between two consumable electrodes of a coating material. Compressed gas is used to atomize and propel the material to the substrate. (1)
The electric arc spray process utilizes metal in wire form. This process differs from other thermal spray processes in that there are no external heat sources as in any the combustion gas/flame spray processes. Heating and melting occur when two electrically opposed charged wires, comprising the spray material, are fed together in such a manner that a controlled arc occurs at their intersection. The molten metal is atomized and propelled onto the prepared workpiece by jets of compressed air or gas.
As early as 1914, Schoop in collaboration with Bauerlin (2), an electrical engineer, experimented with electrical heating for spraying. Initial attempts were unsuccessful as they attempted to tailor their spray apparatus on the lines of molten metal equipment rather than wire. One pole was a graphite crucible, loaded with the consumable, the other a carbon rod. An arc was struck between the crucible and the rod causing the metallic consumable to melt and flow through an orifice. On exiting, the molten metal was atomized by jets of compressed gas. Eventually, a device was built utilizing two wires, insulated from each other, made to advance and intersect at some point. Generally, the wires were given a difference of electrical potential of about 89 V that caused the wires to melt and; in the presence of a gas stream, spraying was produced. Later guns, developed by Schoop, do not radically differ from those used today.
The gun is relatively simple. Two guides direct the wires to an arcing point. Behind this point a nozzle directs a stream of high-pressure gas or air onto the arcing point where it atomizes the molten metal and carries it to the workpiece as in the graphic above. Typically, power settings of about 450 A can spray over 50 kg/hr (110 lb/hr). Electric arc spray systems are offered that feed wire by either an air or electrical motor. Some units push the wire to the gun while others pull the wire into the arc. Controls include volt and ampere meters and air regulators.
Electric arc spraying has the advantage of not requiring the use of oxygen and/or a combustible gas; it has demonstrated the ability to process metals at high spray rates; and is, in many cases, less expensive to operate than either plasma and/or wire flame spraying. "Pseudo" alloy coatings, or those constructed by simultaneously feeding two different materials, are readily fabricated. An example would be copper-tin coatings constructed by feeding pure copper and tin wires into the arc to produce a heterogeneous mixture of each in the coating. Also, the introduction of cored wires has enabled the deposition of complex alloys (such as MCrAlYs) as well as carbide-containing metal alloys that were only attainable using powdered materials as feedstock. Some materials produce "self-bonding" coatings that are sprayed in a "superheated" condition. The overheated, hot particles tend to weld to many surfaces thereby increasing the coatings' adhesive strength.
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