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Plasma
Arc
Spraying - From the surface to be sprayed, all dust, oil and other foreign matter must be removed. After that, the surface should be roughened by (steel) grit blasting or acid etching. In plasma arc spraying, a non transferred*, direct current, high intensity arc is struck between the gun body, which acts as an (Cu) anode and a thoriated tungsten cathode (electrode).
An inert gas, for example, argon is passed through this are, where it is heated to the plasma state** (20,000 to 30,000°F) and accelerated to supersonic seeds. Argon, heated by the are, escapes out of the nozzle (i.e. copper anode) as a hot, brightly shining flame having a length of about 3-7 cm.
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Powdered metal is metered into the plasma stream which reaches the molten stage much more quickly than in flame spraying. (The powdered material melts in the air and not in the gun because of the high temperatures of the plasma arc). The powdered metal is then carried to the workpiece surface in a molten state by the high velocity of the plasma jet.
Equipment Required
1. Plasma spray gun
In a plasma spray gun, an arc is contained within a water cooled tube into which gas is injected. The arc path is between a tungsten cathode in a water cooled holder and a hollow water cooled copper anode.
The gas picks up energy from the arc and issues from the device in a configuration resembling an open welding flame.
Spray guns have a handling capacity of about 40 kilowatts. D.C. is used at 50-500 volts and 80-800 amperes. Several nozzle configurations can be used to accommodate different plasma gases and to spray different types of powders.
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2. Power supply
Selenium rectifier or silicon rectifier sets of 100% duty cycle-constant current type arc used as (DC) power units.
3. Gas supply
In, a plasma spray system, gases arc used for following purposes :
(i) To form plasma (such as Nitrogen and Argon)
(ii) As auxiliary gas, mixed in small quantities (5-10%) with the gas forming plasma (such as Hydrogen and Helium).
(iii) As a powder carrying gas.
The powder carrier gas and the plasma generating gas have a common source.
4. Powder metal supply
The plasma torches are usually of a type that permit only powder materials to be used. The grain size of the powders is 40 to 50 mesh to 5 micron size. Powder feed mechanisms are of two types:
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(i) An aspirator type feeder that is composed of powder hopper which contains a gas nozzle that aspirates powder adjacent to the jet into the discharge line.
(ii) A mechanical metering type feeder that utilizes the metering action of a screw to deliver precise, measurable amounts of powder to a mixing chamber, where the powder is introduced into the carrier gas stream.
5. Control console. It controls
(i) Plasma gas flow rate,
(ii) Arc current,
(iii) Starting and stopping functions, and
(iv) Powder feed unit.
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Some data related to plasma arc spraying
Arc current. . . . . . 80 to 800 amps
Voltage. . . . . . 50 to 500 volts
Wattage of the unit for spray coating. . . . . . 28 to 40 kW
Powder metal size. . . . . . 40 to 50 mesh to 5 micron size
For water cooling of nozzle, electrode and gun body. . . . . . 22 to 45 kg of water pressure with a flow of 14to 18 litres per min.
Distance of gun from work. . . . . . 50 to 150 mm.
Preheating of base materials to reduce stresses in the coating. . . . . . 93 to 150°C
Output of plasma torch. . . . . . 2 to 10 kg of spraying material/ hour.
Coating bond strength. . . . . . 350 to 700 kg/cm2
Hardness. . . . . . up to 75 Rc.
Sprayable Materials
(i) Metals: AI, Cu, W, Ni, Cr, Mo, Ta
(ii) Refractory hard metals: Titanium carbide, tungsten carbide, chromium carbide, etc.
(iii) Cenl1ets: Nickel-Magnesia, Aluminium Oxide-Nickel, Zirconium Oxide-Nickel, etc.
(iv) Oxides: Cerium oxide, Zirconium oxide, Titanium oxide, Chromium oxide.
(v) Plastics
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