The effectiveness and problems of plasma surfacing are extremely acute for material engineers. Thanks to this technology, it is possible not only to significantly increase the service life and reliability of highly loaded parts and assemblies, but to restore, it would seem, one hundred percent worn out and destroyed products.
The introduction of plasma surfacing in the process significantly increases the competitiveness of engineering products. The process is not fundamentally new and has been used for quite some time. But he is constantly improving and expanding technological capabilities.
General Provisions
Plasma is called ionized gas. It is reliably known that plasma can be obtained by various methods as a result of electrical, temperature or mechanical action on gas molecules. For its formation, it is necessary to tear off negatively charged electrons from positive atoms.
In some sources, one can find information that plasma is the fourth aggregate state of matter along with solid, liquid and gaseous. Ionized gas has a number of useful properties and is used in many branches of engineering: plasma surfacing of metals and alloys with the aim of restoring and hardening highly loaded products experiencing cyclic loads, ion-plasma nitriding in a glow discharge for diffusion saturation and hardening of surfaces of parts, for the implementation of chemical processes etching (used in electronics manufacturing technology).
Preparation for work
Before proceeding to surfacing, you need to configure the equipment. In accordance with the reference data, it is necessary to select and set the correct angle of inclination of the nozzle of the burner to the surface of the product, verify the distance from the end of the burner to the part (it should be 5 to 8 millimeters) and insert the wire (if the wire is surfacing).
If surfacing will be carried out by oscillating the nozzle in the transverse directions, it is necessary to set the head so that the weld is exactly in the middle between the extreme points of the amplitude of the oscillation of the head. It is also necessary to adjust the mechanism, which sets the frequency and magnitude of the oscillatory movements of the head.
Plasma arc technology
The surfacing process is quite simple and can be successfully performed by any experienced welder. However, it requires the performer maximum concentration and attention. Otherwise, you can easily spoil the workpiece.
A powerful arc discharge is used to ionize the working gas. The separation of negative electrons from positively charged atoms is due to the thermal effect of the electric arc on the jet of the working gas mixture. However, subject to a number of conditions, leakage is possible not only under the influence of thermal ionization, but also due to the influence of a powerful electric field.
Gas is supplied under a pressure of 20-25 atmospheres. For its ionization, a voltage of 120-160 volts with a current strength of about 500 amperes is necessary. Positively charged ions are captured by the magnetic field and rush to the cathode. The speed and kinetic energy of elementary particles is so great that when they collide with a metal, they are able to give it a huge temperature - from +10 ... + 18,000 degrees Celsius. In this case, ions move at a speed of up to 15 kilometers per second (!). Plasma surfacing installation is equipped with a special device called “plasmatron”. It is this node that is responsible for ionizing the gas and obtaining a directed flow of elementary particles.
The arc power should be such as to prevent melting of the base material. At the same time, the temperature of the product should be as high as possible in order to activate diffusion processes. Thus, the temperature should approach the liquidus line in the iron-cementite diagram.
Fine powder of a special composition or electrode wire is fed into a jet of high-temperature plasma in which the material is melted. In the liquid state, surfacing falls on the hardened surface.
Plasma spraying
In order to realize plasma spraying, it is necessary to significantly increase the plasma flow rate. This can be achieved by adjusting the voltage and current strength. Parameters are selected empirically.
Materials for plasma spraying are refractory metals and chemical compounds: tungsten, tantalum, titanium, borides, silicides, magnesium oxide and alumina.
The indisputable advantage of spraying compared to surfacing is the ability to obtain the thinnest layers, of the order of several micrometers.
This technology is used for hardening cutting turning and milling replaceable carbide inserts, as well as taps, drills, countersinks, reamers and other tools.
Obtaining an open plasma jet
In this case, the blank itself directly acts as the anode, on which the plasma surfacing of the material is carried out. An obvious drawback of this processing method is the heating of the surface and the entire volume of the part, which can lead to structural transformations and undesirable consequences: softening, increased fragility, and so on.
Closed plasma jet
In this case, the gas burner acts directly as the anode, or rather, its nozzle. This method is used for plasma-powder surfacing in order to restore and improve the operational characteristics of parts and components of machines. This technology has gained particular popularity in the field of agricultural engineering.
Advantages of plasma surfacing technology
One of the main advantages is the concentration of thermal energy in a small area, which reduces the influence of temperature on the initial structure of the material.
The process is well manageable. If desired, and the appropriate equipment settings, the overlay layer can vary from a few tenths of a millimeter to two millimeters. The ability to obtain a controlled layer is especially relevant at the moment, as it allows to significantly increase the economic efficiency of processing and to obtain optimal properties (hardness, corrosion resistance, wear resistance and many others) of the surfaces of steel products.
Another equally important advantage is the ability to carry out plasma welding and surfacing of a wide variety of materials: copper, brass, bronze, precious metals, as well as non-metals. Traditional welding methods are far from always able to do this.
Equipment for surfacing
Installation for plasma-powder surfacing includes a choke, an oscillator, a plasma torch and power sources. It should also be equipped with a device for automatically feeding granules of metal powder into the working area and a cooling system with constant water circulation.
Current sources for plasma surfacing must meet the stringent requirements of consistency and reliability. Welding transformers cope with this role as well as possible.
When surfacing powder materials on a metal surface, the so-called combined arc is used. At the same time, an open and closed plasma jet is used. By adjusting the power of these arcs, you can change the penetration depth of the workpiece. Under optimal conditions, warping of products will not occur. This is important in the manufacture of parts and components of precision engineering.
Material feeder
Metal powder is dosed with a special device and fed into the reflow zone. The mechanism, or principle of operation of the feeder, is as follows: the rotor blades push the powder into the gas stream, the particles are heated and adhere to the treated surface. Powder is supplied through a separate nozzle. In total, three nozzles are installed in the gas burner: for supplying plasma, for supplying working powder, and for shielding gas.
If you use wire, it is advisable to use a standard feed mechanism of the welding machine for submerged arc welding.
Surface preparation
Plasma surfacing and spraying of materials should be preceded by a thorough cleaning of the surface from grease stains and other contaminants. If in conventional welding it is permissible to carry out only rough, surface cleaning of the joints from rust and scale, then when working with gas plasma, the surface of the workpiece should be perfectly (as much as possible) clean, without foreign impurities. The thinnest oxide film is able to significantly weaken the adhesive interaction of the surfacing and the base metal.
In order to prepare the surface for surfacing, it is recommended to remove an insignificant surface layer of metal by means of machining by cutting and subsequent degreasing. If the dimensions of the part allow, it is recommended to flush and clean the surfaces in an ultrasonic bath.
Important features of metal surfacing
There are several options and methods for plasma surfacing. The use of wire as a material for surfacing significantly increases the productivity of the process compared to powders. This is because the electrode (wire) acts as an anode, which contributes to a much faster heating of the deposited material, and therefore allows you to adjust the processing modes in the direction of increase.
However, the quality of the coating and the adhesive properties are clearly on the side of the powder additives. The use of small metal particles makes it possible to obtain a uniform layer of any thickness on the surface.
Surfacing powder
The use of powder surfacing is preferable from the point of view of the quality of the obtained surfaces and wear resistance, therefore, powder mixtures are increasingly used in production. The traditional composition of the powder mixture is cobalt and nickel particles. The alloy of these metals has good mechanical properties. After processing with such a composition, the surface of the part remains perfectly smooth and there is no need for mechanical refinement and elimination of irregularities. The fraction of powder particles is only a few micrometers.