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Detailed explanation of the principle and characteristics of plasma spray welding

2021-08-27

Principle of Plasma Spray Welding
Plasma spray welding is a process of surface cladding (surfacing) alloy using argon plasma arc as high temperature heat source and alloy powder as filler metal.
The basic process of plasma spray welding is to use a plasma arc torch (or spray gun, plasma arc generator) to ionize the gas between the cathode and the water-cooled copper nozzle, or between the cathode and the workpiece, to form an arc. The arc passes through a small aperture. The diameter of the arc column of the nozzle hole is limited. Under the action of the cold air wall of the compressed hole, thermal contraction effect, mechanical compression effect, and self-magnetic compression effect are generated, so that the arc column is forcibly compressed. This kind of arc is a "compression arc". It is called a plasma arc. After the arc is compressed, it will have a big change compared with the free arc. The highlight is that the diameter of the arc column becomes smaller, which promotes the arc column current density to increase significantly, and the gas ionization is sufficient, so the arc has high temperature, energy concentration, and arc stability. , Good controllability and other characteristics. The plasma arc generated by the plasma arc welding gun is divided into non-transferred arc (the arc established between the cathode and the nozzle) and the transferred arc (the arc established between the cathode and the workpiece). The main heat source of plasma spray welding is the transfer type plasma arc.
When combined arc surfacing is used, two independent DC arc welding machines are generally used as power sources to supply the non-transferred arc (referred to as "non-arc") and the transferred arc (referred to as "revolving arc") respectively. The negative poles of the two power supplies are connected in parallel and connected to the tungsten level (cathode) of the welding gun through a water cable. The positive pole of the non-arc power supply is connected to the nozzle of the welding gun through a water cable. The positive pole of the arc power supply is connected to the workpiece. The circulating cooling water is led to the welding gun through the water cable to cool the nozzle and electrode. The argon gas enters the welding gun through the solenoid valve and the flow regulator. After the non-arc power supply is turned on, the non-transferred arc is ignited by means of a high-frequency spark generated between the electrode and the nozzle. After the arc turning power is turned on, the arc is ignited with the help of the non-arc conductive path between the tungsten electrode and the workpiece. After arc ignition, the non-arc can be retained or cut off, and the heat of the arc is mainly used to generate a molten pool and melted alloy powder on the surface of the workpiece. The alloy powder is continuously supplied according to the required amount, and is fed into the welding torch by the powder feeding airflow, and blown into the electric arc. The powder is pre-heated in the arc column, falls into the molten pool in a molten or semi-melted state, fully melts in the molten pool, and discharges gas and floats out of molten slag. By adjusting the transfer arc current to control the molten alloy powder and the heat transferred to the workpiece, the alloy is fused with the surface of the workpiece. With the relative movement of the welding gun and the workpiece, the alloy molten pool gradually solidifies, and the required alloy surfacing layer is obtained on the workpiece.

 
Characteristics of Plasma Spray Welding
Plasma spray welding uses plasma arc as the heat source with concentrated heat and good controllability, and uses easy-to-melt, especially self-melting alloy powder as the filler metal. Compared with other surface surfacing methods, it has the following characteristics :
(1) Suitable for surfacing welding of high-alloy or composite materials that are easy to be made into powder but difficult to be made into wire.
(2) The alloy powder and its molten pool have a buffer effect on the arc, can effectively control the penetration depth, and the base material has a low dilution rate.
(3) The surfacing layer has uniform hardness and uniform structure, which is easy to avoid quality defects.
(4) The weld bead is formed smooth and beautiful, with a wide adjustable range of size and deposition rate, and a wide range of adapting to the size of the workpiece.
(5) With the advantage of easy melting of fine powder, micro-beam plasma arc can be used as the heat source to realize fine surfacing.
(6) The surfacing process is continuous, and it is easy to realize fully automated surfacing.


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