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Plasma arc spraying principle
Plasma arc spraying is a thermal spraying method that uses plasma flame (non-transferred plasma arc) as a heat source to heat and accelerate the powder spray material and spray it onto the surface of the workpiece to form a spray layer. The principle of plasma arc spraying is shown in Figure Schematic diagram of plasma arc spraying principle. When the power is turned on, a high-frequency electric spark is generated between the tungsten tip and the nozzle, and the plasma arc is ignited. After the continuously fed working gas passes through the arc, it becomes a high-temperature plasma flame ejected from the nozzle. The spray powder floats in the powder-feeding airflow, is sent into the plasma flame, quickly reaches a molten state, and is sprayed onto the surface of the workpiece at high speed to form a spray coating. During the spraying process, the workpiece is not connected to the power supply, so a molten pool will not form on the surface of the workpiece, and the temperature can be maintained at a low temperature (below 200°C) without deformation or change of the original quenching structure.
Characteristics of plasma arc spraying
Plasma arc spraying has no deformation of parts and does not change the heat treatment properties of the base metal; there are many types of coatings, and spray coatings with various properties can be obtained; the process is stable and the coating quality is high; the size of parts is not limited and the machining allowance is small; Spraying can be used to enhance features such as the surface of ordinary workpiece parts.
Plasma arc spraying equipment
Plasma arc spraying equipment mainly consists of a spray gun, power supply, powder feeder, cooling water supply system gas system, control system, etc. The arrangement between them is shown in Figure Schematic diagram of plasma arc spraying equipment layout. The power supply adopts DC power supply, and the rated power mainly has three specifications: 40 kW, 50 kW and 80 kW. The performance requirements for plasma arc spraying power supplies are basically the same as those for arc welding power supplies.
The spray gun is the core device of plasma arc spraying equipment and is essentially a plasma generator. According to different uses, plasma arc spray guns can be divided into two categories: outer cylindrical spray guns and inner hole spray guns. The outer cylindrical spray gun is mainly used for spraying on the outer surface of parts, and can also be used for spraying on the surface of shallow inner holes with larger diameters; Hole spray guns are used for spraying the surface of deeper inner holes. Commonly used powder feeding methods include powder feeding inside the gun and powder feeding outside the gun. When feeding powder into the gun, the heating efficiency of the powder is high, but the melted powder easily accumulates at the nozzle outlet and blocks the nozzle. Feeding powder outside the gun can avoid clogging the gun, but the heating efficiency of the powder is relatively low.
Plasma arc spraying engineering examples
The sealing cover of the large brake drum of heavy-duty vehicles is made of wear-resistant cast iron. Its parts are shown in Figure Large brake drum sealing cover parts diagram. This part works in conjunction with the sealing ring. Due to the high relative movement speed between the two, the wear serious. If the welding process is used for repair, such thin-walled parts are easily deformed and scrapped, so plasma arc spraying is used for repair.
Coating selection
Select nickel-aluminum composite powder as the bonding bottom material, with a particle size of -160~+240 mesh; select NiO4 powder as the working layer material, with a particle size of -140~+300 mesh.
Spraying process
Before spraying, place the workpiece in the furnace for heating or use a flame to bake it repeatedly. The temperature should not be greater than 250 degrees Celsius and the heating time should be 2.5 hours. After the oil stains seep out of the surface of the workpiece, use cleaning agent to clean it.
Cut 0.3 mm downward in the radius direction of the surface to be sprayed on the part, and remove the wear layer and fatigue layer on the surface of the workpiece.
Use No. 20 to 30 corundum sand (Al2O3) for sandblasting, then use compressed air to blow the surface of the workpiece clean, and spray immediately.
Spray.
After spraying, it is processed by turning to the specified size.