A thermal spraying process using an arc between two consumable electrodes of surfacing materials as a heat source and a compressed gas to atomize and propel the surfacing material to the substrate. See STANDARD WELDING TERMS. See also THERMAL SPRAYING.
The arc spray process is used to apply coatings of various materials which enhance, protect, or seal the workpiece. The process uses an arc between two wires (feedstock), and is sometimes referred to as wire arc spraying. The wires are kept insulated from each other and automatically advance to meet at a point within an atomizing gas stream. A potential difference of 18 to 40 volts applied across the wires initiates an arc as they converge, melting the tips of both wires. An atomizing gas, usually compressed air, is directed across the arc zone, shearing off molten droplets which form the atomized spray, and ejecting them from the arc at the rate of several thousand particlesper second. The velocity of the gas through the atomizing nozzle can be regulated to control deposit characteristics.
In comparison with wire flame spraying, the quantity of metal oxides is better controlled and spray ratzs are higher in wire arc spraying, so this process is often more economical.
Equipment-
The wire arc spray system can be operated from a control console or from the gun.
The control console will have the switches and regulators necessary for controlling and monitoring the operating circuits that power the gun and control the spray procedure, as follows:
(1) A solid-state direct current power source, usually the constant voltage type
(2) A dual wire feeding system
(3) A compressed gas supply with regulators and flowmeter built into the control assembly
(4) Arc spray gun and appropriate console switching
The wire control unit consists of two reel (or coil) holders, which are insulated from each other, and connected to the spray gun with flexible insulated wire guide tubes. Wire sizes range from 1.6 to 3.2 mm (1/16 to 1/8 in.). The wire arc spray process can deposit as little as 0.45 kghr (1 l b h ) . Factors controlling the rate of application are the current rating of the power source and the permissible wire feed rate to carry the available power.
Direct current constant potential power sources providing a voltage of 18 to 40 volts are normally used in this process. This permits operation over a wide range of metals and alloys. The arc gap and spray particle size increase with a rise in voltage. The voltage should be kept at the lowest possible level, consistent with good arc stability, to provide the smoothest coatings and maximum coating density.
Advantages and Limitations-
Compared to flame spraying, energy and labor costs are lower for arc spraying because of its higher deposition rate, lower maintenance, low gas costs, and higher deposition efficiencies.
One adverse effect of the high energy state of the atomized particles is their tendency to change composition through oxidation or vaporization, or both. These effects can be minimized by judicious wire selection.
The arc spray method is less versatile than flame or plasma methods, because powders and nonconductive materials cannot be used.
A technique called bond coat mode can be used in this process to achieve higher strength bonds with some materials; when the conditions of this mode are carried out, the following are ensured: (1) fine spray particle size, (2) minimum loss of alloy constituents, (3) concentrated spray pattern, and (4) high melting rate.
Arc Spraying Applications
Arc spray deposits can provide protection against many types of corrosive attack on iron and steel. Zinc, aluminum, and stainless steels can be used as surfacing materials. A thick layer of zinc or aluminum can protect steel against oxidation and provide a strong bond for an organic coating.
Safety
Local, state, and federal safety regulations should be investigated, and procedures must comply with them. The potential hazards involved in arc spraying operations are electrical shock, fire, gases, dust and fumes, arc radiation and noise. These potentials are not unique to thermal spraying; the general requirements for the protection of thermal spray operators are the same as for welders, set forth in ANSI 249.1, Safety in Welding, Cutting and Allied Processes; ANSI 287.1, Practices for Occupational and Educational Eye and Face Protection; ANSI 288.2, Practices for Respiratory Protection; ANSI 289.1, Safety Requirements for Industrial Head Protection. Also, CGA P- 1, Safe Handling of Compressed Gases.