Wire plasma spray gun for surfacing. Plasma surfacing with wire (rods). The main advantages of the method

Nowadays, microwave ovens are so widespread that it is impossible, at times, to imagine our life without this device.

A huge amount of information can be found on the Internet about the dangers and benefits of microwave ovens. Let's not “kill” valuable time on this topic. Let's conduct a real experiment with an expensive microwave oven from a well-known manufacturer for electromagnetic radiation. At the same time, we will demonstrate the work of the VIOLE shielding fabric (produced in Switzerland).

The level of the electromagnetic field emitted by the microwave oven will be determined in 2 ranges: 50 Hz (industrial frequency) and 2.4 GHz (magnetron operating frequency).

As measuring instruments, we will use the attorney P3-33M and the SOEKS EMR detector (the alarm is set for exceeding the EMF levels by SanPin).

The microwave is set to maximum power (900 W). After switching on, we measured the levels of electromagnetic fields around the microwave oven. The measurement results are shown below.

Microwave range: The maximum flux density of electromagnetic energy is 286 μW / cm2 in close proximity to the microwave glass (10 cm from the glass). When the antenna is located at a distance of 60 cm from the glass of the microwave oven, the maximum power flux density is 36-50 μW / cm2.

50 Hz range: the excess of the electric field from the permissible was observed at a distance of 50-70 cm from the microwave oven from the side of the glass. The excess of the magnetic field was observed from the side of the magnetron.

Extract from SanPin 2.2.4 / 2.1.8.055-96 "Electromagnetic radiation of radio frequency range (EMR RF)"

Maximum permissible values ​​of energy exposure

Note: in these Sanitary Standards and Rules, in all cases, when specifying frequency ranges, each range excludes the lower and includes the upper frequency limit.

Maximum permissible levels of energy flux density in the frequency range 300 MHz - 300 GHz, depending on the duration of exposure

Note: with a duration of exposure less than 0.2 hours, further increase in the intensity of exposure is not allowed.

Maximum permissible levels of RF EMR for the population, persons under the age of 18, and women in a state of pregnancy

Hence, we can conclude that when the microwave oven is operating, to ensure the safety of human health, it is necessary to be at a distance of more than 1-1.5 meters from it.

As part of this experiment, we used a shielding fabric that does not require grounding (fabric name - VIOLE). After the next measurements in various frequency ranges (microwave and 50 Hz), this fabric just surprised us. Electric fields have been extinguished more than a hundred times!

It differs from other methods in that the heating and melting of the coating material and the surface layer of the base is carried out by a plasma jet

Figure - Scheme of plasma surfacing with powder injection into the arc:

1 - tungsten electrode; 2 - indirect arc power source; 3 - inner nozzle; 4 - plasma jet of indirect action; 5 - outer nozzle; 6 - plasma jet of direct action; 7 - source of direct action

Surfacing wire, powder is fed into the surfacing zone, or, in a combined method, both powder and wire (for example, 75 ... 80% of Sv-08G2S wire and 20 ... 25% of PG-SRU self-fluxing powder are used to restore worn car parts at the Vitebsk Motor Repair Plant).

Argon is used as a plasma-forming gas. Replacing it (up to 90%) significantly reduces the cost of restoring parts.

Plasma torches can be of direct, indirect, combined action, single and multi-arc, direct and reverse polarity.

Self-fluxing diffusion-alloyed iron-based powders developed by the scientific school of prof. Panteleenko F.I. (BNTU). They make it possible to obtain deposited coatings with the required hardness, wear and corrosion resistance (hardness range from 20 to 65 HRC, and more).

This school has significant experience in the restoration of worn out parts (shafts, rods, spindles, thermowells, etc.) for the heat power industry, petrochemicals, and the pulp and paper industry of the CIS countries.

Plasma surfacing is one of the most productive, versatile and economical methods of applying coatings from tenths to several millimeters thick.

Advantages of plasma cladding over other methods:

• -minimum allowance for machining (0.4 ... 0.9 mm)
• -minimum depth of penetration of the base (0.3 ... 3.5 mm) and heat-affected zone (3 ... 6 mm)
• -minimum heat input into the base

Plasma surfacing is suitable for the restoration of large-sized parts of great length and diameter exceeding 20 mm from carbon and alloy steels (for example, crankshafts, pump shafts of paper machines, etc.)

Surfacing is carried out on a high-speed plasma surfacing unit (power supply UPS-301, plasmatron SIB-4, current strength 90 ... 170A, voltage 30 ... 35V, transverse plasma torch feed 1.5 ... 2 mm / rev, surfacing distance 8 ... 10 mm).

The most acceptable is a diffusion-alloyed self-fluxing powder based on PR-Steel 45 with a particle size distribution of 40 ... 160 microns. Powder consumption 35 g / min, thickness of the deposited layer per pass 0.5 ... 1.5 mm, hardness - required (range 20 ... 60 HRC).

Electromagnetic surfacing or MEU, consists in the fact that a ferromagnetic powder is fed into the gap between the pole piece and the part, which, under the influence of a magnetic field, is built in the gap in the form of chains. An electric field applied to the pole piece and the part causes the particles to heat up, melt them and fix them on the restored surface.

Various powders of ferroalloys, steels, cast irons and diffusion-alloyed iron-based powders are used.

The coatings are rough, specific (up to 0.6 mm thick), but very effective for hardening flat and cylindrical surfaces of agricultural machinery knives, restoring parts with low wear. The work begun in this direction by the BSTU scientists has been developed in recent years at the G. P.O. Sukhoi and BNTU.

Laser cladding in which a concentrated laser beam is used as a heat source. The laser makes it possible to weld coatings, to melt previously sprayed or applied in the form of a slip coatings. The exceptional locality of the beam and high energy density predetermine its predominant fields of application and the greatest efficiency in the restoration of small surfaces (5 ... 50 mm2) with local wear of 0.1 ... 1.0 mm.

Most often, laser cladding is used to restore camshaft cams, valve chamfers, axes of fine oil filters, etc.

Scientists from the Physicotechnical Institute of the National Academy of Sciences of Belarus and BNTU have achieved significant success in laser surfacing and hardening.

Electron Beam Cladding (ELN) consists in melting the filler material with an electron beam. In many ways, this method is similar to the laser cladding method (locality, efficiency)

In recent years, Russian scientists (Tomsk) and the school of prof. Gruzdeva V.A. (PSU, Novopolotsk) created a highly efficient electron-beam complex based on a plasma source of electrons. It is easy to maintain and does not require a high vacuum. ELN is 10 ... 15 times more productive than induction surfacing, it is applicable for surfacing of any materials.

Induction surfacing is based on the use of high-frequency currents to heat the metal of the part and the material being deposited. The part with the applied charge is introduced into the inductor of the HFC installation. The HFC, passing through the inductor circuit, excites Foucault currents in the surface layer of the part, which heats the surface of the part. From the heated surface, a more low-melting charge is heated and melted, forming a coating.

The charge (surfacing powder and flux) may not include flux if the powder is self-fluxing.

It should be noted that the use of inexpensive self-fluxing powders, including those developed at BNTU, self-shielded flux-cored wires, makes it possible to abandon the use of expensive shielding gases in many surfacing methods and thus significantly reduce the cost of the technology for applying protective coatings.

Scientists of the Joint Institute of Mechanical Engineering of the NAS of Belarus (AMI NAS of Belarus) have achieved significant success in the development of induction surfacing technology for the Belarusian industry.

Electrical contact welding consists in fixing the wire, powder, tape with powerful current pulses (7 ... 30 kA) when pressure is applied (1000 ... 1600 N). In this case, the material of the base and the applied coating (powder, tape) is melted at their border in the place of maximum electrical resistance (Fig. 2.5).

The advantages of electric contact welding in comparison with arc surfacing methods are:

• - higher (2 ... 3 times) productivity
• - lower (3 ... 4 times) material consumption due to reduction of losses for spraying and minimum allowance for machining
• -minimum heat input into the base and no deformations
• - no waste of alloying elements
• -simple and cost-effective

Figure 2.5 - Scheme of electrical contact welding of the tape:

1 and 3 - rollers; 2 - part to be restored; 4 - transformer; 5 _ contactor

The method is effective for restoring shaft journals, other loaded cylindrical surfaces, holes in liners and cylinder blocks and is being developed at the OIM NAS of Belarus

Plasma surfacing is an innovative method of applying special coatings with a high wear resistance to the surface of worn-out products. It is performed for the restoration of machine parts and mechanisms, as well as during their production.

1 Plasma cladding - general information about the technique and its advantages

A number of units and mechanisms of various devices and machines today operate in difficult conditions, requiring products to meet several requirements at once. Often they are required to withstand the influence of aggressive chemical environments and high temperatures, and at the same time maintain their high strength characteristics.

It is almost impossible to make such assemblies from any one metal or other material. And from a financial point of view, such a complex production process is impractical to implement.

It is much wiser and more profitable to produce such products from one, the most durable material, and then apply certain protective coatings on them - wear-resistant, heat-resistant, acid-resistant, and so on.

As such "protection" you can use non-metallic and metallic coatings, which differ in their composition from each other. Such spraying allows you to give products the required dielectric, thermal, physical and other characteristics. One of the most effective and at the same time universal modern methods of coating materials with a protective layer is considered to be spraying and surfacing with a plasma arc.

The essence of plasma application is quite simple. For coating, material is used in the form of wire or granular fine powder, which is fed into a plasma jet, where it is first heated and then melted. It is in the molten state that the protective material falls on the part being welded. At the same time, its continuous heating occurs.

The advantages of this technology are as follows:

• the plasma flow allows you to apply materials of different parameters, and in several layers (due to this, the metal can be processed with different coatings, each of which has its own protective features);
• the energy properties of the plasma arc can be regulated within wide limits, since it is considered the most flexible heat source;
• the plasma flow is characterized by a very high temperature, due to which it easily melts even those materials that are described by increased refractoriness;
• Geometrical parameters and shape of the part for surfacing do not limit the technical capabilities of the plasma method and do not reduce its effectiveness.

Based on this, we can conclude that neither vacuum, nor galvanic, nor any other version of spraying can be compared in its efficiency with plasma. It is most commonly used for:

• strengthening of products that are subjected to constant high loads;
• protection against wear and rusting of shut-off and control and shut-off elements (metal spraying with the help of plasma significantly increases their durability);
• protection against the negative effects of high temperatures, causing premature wear of products used by glass enterprises.

2 Technology of the described surfacing and its subtleties

Plasma metal surfacing is performed using two technologies:

• a bar, wire or tape is introduced into the jet (they perform the function of a filler material);
• a powder mixture is fed into the jet, which is captured and transferred to the surface of the product being welded by gas.

The plasma jet can have a different layout. According to this indicator, it is divided into three types:

• Closed stream. With its help, spraying, metallization and hardening of metal are most often performed. The arc in this case is characterized by a relatively low intensity of the flame flow, which is caused by a high level of heat transfer to the atmosphere. In the described arrangement, the anode is either the burner channel or its nozzle.
• Open jet. With this arrangement, the part heats up much more, the anode is the bar or the workpiece itself. An open jet is recommended for applying protective layers or for cutting material.
• Combined option. Designed specifically for plasma-powder cladding. With this option, two arcs are simultaneously ignited, and the anode is connected to the torch nozzle and to the weldment.

In any arrangement, the gases used to form the flame are oxygen, argon, air, helium, hydrogen, or nitrogen. Experts say that helium and argon provide the highest quality metal spraying and surfacing.

3 Combined plasma torch for surfacing

Plasma-powder surfacing at most modern enterprises is carried out precisely in combined units. In them, the metal filler powder is melted between the torch nozzle and the tungsten electrode. And at the time when the arc burns between the part and the electrode, heating of the surface of the welded product begins. Due to this, there is a high-quality and fast fusion of the base and filler metal.

The combined plasmatron provides a low content of the deposited base material, as well as the smallest depth of its penetration. It is these facts that are recognized as the main technological advantage of plasma jet surfacing.

The surface to be welded is protected from the harmful effects of ambient air with an inert gas. It enters the nozzle (external) of the unit and reliably protects the arc by surrounding it. The transporting gas with inert characteristics also supplies the powder mixture for the additive. It comes from a special feeder.

In general, a standard plasmatron of a combined action type, in which metal spraying and surfacing is performed, consists of the following parts:

• two power supplies (one feeds the "indirect" arc, the other - the "direct");
• mixture feeder;
• resistance (ballast);
• hole where gas is supplied;
• nozzle;
• oscillator;
• burner body;
• a pipe for supplying a gas carrying a powder composition.

4 Main features of metal surfacing using plasma technology

The maximum productivity of the plasmatron is observed when a wire conductor additive is used. The arc in this case burns between this wire (it is the anode) and the cathode of the unit. The described method slightly melts the base material. But it does not make it possible to perform a uniform and thin surfacing layer.

If a powder is used, spraying and surfacing make it possible to obtain the specified thin layer with the maximum indicators of wear resistance and heat resistance. Usually cobalt and nickel are the constituents of the powder mixture for surfacing. After using such powders, the surface of the part does not need to be additionally processed, since its protective layer does not have any defects.

Plasma spraying, in comparison with surfacing, is described by a higher speed of the plasma jet and a denser heat flux. This fact is due to the fact that when spraying, metals and compounds with a high level of refractoriness are most often used (borides, silicides, tantalum, carbides, tungsten, zirconium, magnesium and aluminum oxides).

We add that the surfacing method considered in the article in its technical characteristics (the range of operating voltages and currents, the consumption of inert gas, and so on) is not much different from. And specialists have mastered this type of welding operations to perfection today.

It is the most progressive method of restoring worn-out machine parts and applying wear-resistant coatings (alloys, powders, polymers, ...) to the working surface in the manufacture of parts.

Plasma is a high-temperature highly ionized gas consisting of molecules, atoms, ions, electrons, light quanta, etc.

In arc ionization, the gas is passed through the channel and an arc discharge is created, the thermal effect of which ionizes the gas, and the electric field creates a directed plasma jet. The gas can also ionize when exposed to a high frequency electric field. The gas is supplied at 23 atmospheres, an electric arc is excited with a force of 400-500 A and a voltage of 120-160 V. Ionized gas reaches a temperature of 10-18 thousand o C, and the flow rate - up to 15000 m / sec. The plasma jet is formed in special torches - plasmatrons. The cathode is a non-melting tungsten electrode.

Depending on the connection diagram of the anode, they are distinguished (see Figure 1):

1. An open plasma jet (the anode is a part or a rod). In this case, an increased heating of the part occurs. This scheme is used when cutting metal and for applying coatings.

2. Closed plasma jet (the anode is the nozzle or burner channel). Although the temperature of the compressed arc is 20 ... 30% higher in this case, the flow rate is lower, because heat transfer to the environment increases. The scheme is used for hardening, metallization and powder spraying.

3. Combined circuit (the anode is connected to the workpiece and to the burner nozzle). In this case, two arcs are lit. The scheme is used for powder surfacing.

Fig. 1. Scheme of plasma welding with an open and closed plasma jet.

Metal surfacing can be done in two ways:

1-jet of gas captures and delivers powder to the surface of the part;

2 - filler material in the form of wire, rod, tape is introduced into the plasma jet.

Argon, helium, nitrogen, oxygen, hydrogen and air can be used as plasma gases. Best welding results are obtained with argon.

The advantages of plasma cladding are:

1. High concentration of heat output and the possibility of a minimum width of the heat affected zone.

2. Possibility of obtaining the thickness of the deposited layer from 0.1 mm to several millimeters.

3. Possibility of fusing various wear-resistant materials (copper, plastic) on a steel part.

4. The ability to perform plasma hardening of the surface of the part.

5. Relatively high efficiency of the arc (0.2-0.45).

It is very effective to use a plasma jet for metal cutting, because the gas, due to its high speed, removes the molten metal very well, and because of the high temperature, it melts very quickly.

The installation (Fig. 2.) consists of power supplies, a choke, an oscillator, a plasma head, devices for feeding a powder or wire, a water circulation system, etc.

For power supplies, it is important to hold a constant product J U, since power determines the constancy of the plasma flow. Welding converters of the PSO - 500 type are used as power sources. The power is determined by the length of the column and the volume of the plasma jet. Power over 1000 kW can be realized.

Powder is fed by means of a special feeder, in which, vertically located, the rotor with blades feeds the powder into the gas stream. In the case of using a welding wire, its feeding is carried out in the same way as when surfacing under a layer of flux.

By oscillating the torch in the longitudinal plane with a frequency of 40-100 min -1 in one pass, a layer of deposited metal with a width of up to 50 mm is obtained. The burner has three nozzles: an internal one for plasma supply, a middle one for supplying powders and an external one for supplying shielding gas.

Fig. 2. Plasma powder deposition scheme.

When surfacing powders, a combined arc is realized, i.e., open and closed arcs will burn simultaneously. By adjusting the ballast resistances, it is possible to regulate the power fluxes for heating the powder and for heating and melting the metal of the part. It is possible to achieve minimal penetration of the base material, therefore there will be a slight thermal deformation of the part.

The surface of the part must be prepared for surfacing more thoroughly than with conventional electric arc or gas welding, because in this case, the connection occurs without a metallurgical process, therefore, foreign inclusions reduce the strength of the deposited layer. For this, the surface is machined (grooving, grinding, sandblasting, ...) and degreasing. The magnitude of the power of the electric arc is selected so that the part does not heat up much, and so that the base metal is on the verge of melting.

The technological process of applying coatings when melting both the filler material (rods, wires, tubes, rods, tapes, powders) and the surface layer of the metal surface being welded. Depending on the type of heating source, surfacing can be carried out using the heat of a gas flame (gas flame), an electric arc (electric arc in a protective gas environment, submerged arc, etc.), molten slag (electroslag), concentrated energy sources - a compressed arc (plasma), laser beam (laser) and other methods.

Appointment

Manufacturing of parts with wear- and corrosion-resistant surface properties, as well as restoration of the size of worn and defective parts operating under conditions of high dynamic, cyclic loads or subject to intense wear.

Choice of method

The choice and use of a specific surfacing method is determined by the production conditions, the number, shape and size of the parts to be welded, the permissible mixing of the deposited and base metal, technical and economic indicators, as well as the amount of wear. The choice of the type of coating material is made in accordance with the operating conditions of the parts. In many cases, the most effective use of powders as a filler material for surfacing parts is that they are technologically advanced in production and provide a wide range of chemical and phase composition of the coating.

Dignity

• application of coatings of considerable thickness;
• no restrictions on the size of the surfaces to be welded;
• obtaining the required dimensions of recoverable parts by applying a material of the same composition as the base metal;
• use not only to restore the size of worn out and defective parts, but also to repair products by healing defects (cavities, pores, cracks);
• low heat input into the base metal during plasma surfacing;
• repeated carrying out of the restoration process and, consequently, high repairability of the parts being welded;
• high performance;
• relative simplicity and small size of equipment, ease of process automation.

disadvantages

• the possibility of changing the properties of the deposited coating due to the transition of the elements of the base metal into it;
• changes in the chemical composition of the base and deposited metal due to oxidation and burnout of alloying elements in the heat-affected zone;
• the occurrence of increased deformations due to thermal effects;
• the formation of large tensile stresses in the surface layer of the part, reaching 500 MPa and a decrease in the characteristics of fatigue resistance;
• the possibility of structural changes in the base metal, in particular, the formation of a coarse-grained structure, new brittle phases;
• the possibility of cracks in the deposited metal and the heat-affected zone and, as a consequence, a limited choice of combinations of base and deposited metals;
• the presence of large allowances for machining, leading to significant losses of the weld metal and an increase in the labor intensity of the machining of the deposited layer;
• requirements for the preferred location of the surface to be welded in the lower position;
• use in some cases of preheating and slow cooling of the welded product, which increases the labor intensity and duration of the process;
• the difficulty of surfacing small items of complex shape.

Plasma surfacing

Plasma production technologies are called production technologies that use the effect of plasma (the fourth aggregate state of matter) on various materials for the purpose of manufacturing, maintenance, repair and / or operation of products. In plasma surfacing, the part and filler material are heated by electric arc plasma, which is generated by a direct arc compressed by a plasma-forming nozzle and a plasma-forming gas or by an indirect arc burning between the electrode and the plasma-forming nozzle (between the electrode and the filler wire) or by two arcs simultaneously.

Plasma powder surfacing

In plasma-powder surfacing, both a process using one direct arc and a two-arc PTA process (plasma transferred arc) are used, where a direct arc, burning between the electrode and the workpiece, and an indirect arc, burning between the electrode and the plasma nozzle (fig. 1). Due to the fact that traditionally the process of applying coatings using an indirect arc is called plasma spraying, and with the use of a direct arc - plasma cladding, the PTA process is called plasma cladding-spraying.

The process of plasma surfacing-spraying can be characterized as a method of applying powder coatings with a thickness of 0.5-4.0 mm with controlled input of heat into the powder and the product with a plasma torch with two burning arcs of direct and indirect action. An indirect (pilot, pilot) arc is used to melt the filler powder, and the main arc is used to melt the surface layer of the part and maintain the required temperature of the powder on the part. Separate regulation of the parameters of the main and indirect arc ensures effective powder melting with minimal heating of the part surface.

The main advantages of plasma deposition-spraying:

• minimal thermal effect on the base metal;
• minimum mixing of base and weld metal;
• high utilization rate of filler material;
• insignificant allowances for machining;
• minimal deformation of the weld-on part;
• uniformity of the height of the deposited layer;
• high process stability.

Table 1 shows the distinctive characteristics of plasma surfacing-spraying from the closest analogs. Thus, the coatings applied by plasma cladding using a direct arc ensure excessive melting of the base metal and its mixing with the filler material, while the coatings applied by plasma spraying are not porous and are limited to a thickness of about 1 mm (beyond which cracking is possible due to high internal stresses ).

Table 1. Basic properties of coatings applied by plasma methods

The type of plasmatrons for the plasma surfacing-spraying process is shown in Fig. 2.

Rice. 2. Plasma torches for plasma surfacing-spraying

Comparative characteristics of all production plasma technologies are given in table. 2 (the positive aspects of the processes are highlighted in gray cells, and the greatest advantages are marked in bold), and in Fig. 3 shows the options for their use.

Table 2. Characteristics of plasma technologies

Plasma cladding is most commonly used for coating valves of automotive and marine engines, various extruders and screws, fittings and other parts. The economic efficiency of plasma surfacing is determined by an increase in the durability of the deposited parts with a decrease in the consumption of used powder materials, the cost of their processing, and gas savings.

Rice. 3. Plasma cladding process

Link to books and articles

• Sosnin N.A., Ermakov S.A., Topolyansky P.A. Plasma technology. A guide for engineers. Publishing house of the Polytechnic University. SPb .: 2013 .-- 406 p.
• Topolyansky P.A., Topolyansky A.P. Advanced coating technologies - surfacing, spraying, deposition. RHYTHM: Repair. Innovation. Technologies. Modernization. 2011, No. 1 (59). - S. 28-33
• Ermakov S.A., Topolyansky P.A., Sosnin N.A. Assessment of the quality of the plasma surfacing process. Welding and diagnostics. 2015. No. 3. - P. 17-19
• Ermakov S.A., Topolyansky P.A., Sosnin N.A. Optimization of plasma powder surfacing with a double-arc plasmatron. Repair. Recovery. Modernization. 2014. No. 2. - P. 19-25