To join parts made of tungsten (W), molybdenum (Mo), various structures of stainless steels and nickel alloys (aircraft and instrument making), plasma cutter welding is used, where the plasma flow is the source of energy. The temperature of a plasma arc sometimes reaches 30,000°C, while the familiar electric arc has only 5000-7-000°C - the difference is quite significant. Practice has shown that this method turned out to be much more effective in many respects compared to classical gas and electric welding.
Plasma is understood as a fully or partially ionized gas consisting of charged electrons and ions, as well as electrically neutral molecules and atoms Source tutmet.ru
Plasma welding and cutting technology
The principle of plasma-arc welding technology is the powerful heating of the metals being processed by plasma, which in this case is an ionized gas obtained when an electric arc operates under high pressure. The burner used in such units is called a plasma torch - with its help you can cook and cut any metals shown in the periodic table. Also, a plasma torch can be used to cook and cut non-metals, if this is not prevented by the physical or chemical properties of the material, such as lack of adhesion, various reactions, etc.
Main features of plasma welding
The plasma torch allows you to cut and weld any metals and non-metals known in nature, unless there are serious fundamental physical or chemical obstacles to this Source proinstrumentinfo.ru
Let's consider the essential features that plasma-arc welding has. A jet of plasma is ejected into the working area from the plasma torch, but sometimes, if necessary, additional inert gas is also used to create a protective cloud, which creates optimal operating conditions for a chemically neutral environment. As a result, it turns out that all the thermal energy is transferred through the plasma jet to the weld pool, but only partially (in small quantities) reaches nearby areas. Steel is used for the body of the plasma torch, and copper is used for the anode. In this case, the copper electrode has a special cavity through which water flows for cooling, and between it and the cathode with a pressure of 2 to 5 atmospheres, working gas flows to feed the arc.
general information
Plasma is an ionized gas, also the fourth state of matter. Modern plasma welding machines are capable of producing temperatures from 5 to 10 thousand degrees Celsius. A device for plasma cutting and welding is often called a plasma torch. The plasma torch can form three types of arc: indirect, mixed or direct.
Before welding, the arc is “twisted” in a special way, since it is prone to decay. After “twisting”, a thin, stable arc is formed, since the gas is ionized very quickly. The end result is a very powerful arc, capable of generating energy at one point. It is at this point that the base metal melts.
A plasma arc can heat almost any metal, regardless of its thickness and composition. Heating to the melting point and even to the boiling point is possible. The only condition is that it is necessary to provide a sufficiently powerful arc.
Additionally, the welding zone is protected with another gas, inert. This can be nitrogen, argon or organic acetone vapor. The torch of the plasma torch is designed in such a way that it can supply shielding gas and plasma simultaneously. So the weld pool is protected while work is being carried out. The metal does not oxidize and the seams are of very high quality.
Video description
Gorynych - plasma welding and cutting machine
So, the gas in the plasma torch is heated by the plasma arc, which leads to its ionization. The volume of hot gas due to the property of thermal expansion increases from 50 to 100 times, which contributes to high-speed ejection from the nozzle. It turns out that kinetic and thermal energy are the main reasons for the appearance of a powerful energy flow in plasma welding. It should also be noted that the plasma torch usually uses DC torches.
There are several types of such units:
- the arc is between the consumable electrode and the weld pool;
- the arc is between the non-consumable electrode and the weld pool, and the plasma is ejected as a gas stream.
Note: Gases used to form plasma are argon (Ar), nitrogen (N2), oxygen (O2) or ordinary air.
Also, all welding of this type differs in current strength:
- small (microplasma) – 0.1-50 A;
- medium - 50-150 A;
- large from 150 A and above.
If the microplasma option allows you to avoid burn-throughs in the parts being welded, then units operating at high currents melt metal up to 8 mm thick in one pass without edge cutting, which makes it possible to cut workpieces without much difficulty. It is quite natural that at medium currents you will be able to both weld and cut metal.
About the process in brief
The energy source in this type of welding is a plasma flow generated by a torch made of a tungsten electrode, nozzles and pipes supplying gas and water to cool the workpieces. Such devices are called plasmatrons.
A gas (usually argon, oxygen, nitrogen or air) is introduced into the arc zone, heated and ionized. If a conventional electric arc reaches a temperature of 7000 degrees, then a plasma arc reaches 30,000 degrees. The thermal expansion of the gas, leading to an increase in its volume by 50-100 times, coupled with the kinetic energy of the particles, forms a plasma flow.
The most popular plasma welding machines in Russia
This is a generator of electric arc low-temperature plasma, obtained by heating the vapors of the working fluid to an ionization state and is intended for metals Source sevproekt.rf
MPK (multifunctional portable plasma complex) Gorynych is designed to generate plasmas from a liquid - water or a water-alcohol mixture, where the steam performs a protective function . Such units are produced with a current of 8,10 and 12 A and at the same time they are universal, that is, Gorynych can both cut and weld various parts, but that’s not all. By setting the required power, the device can be used as a blowtorch, a forge, and even a fire extinguisher if water is used as the liquid. The MPPC is quite light - the weight of the plasma torch with cable and hose does not exceed 5.4 kg, and to power it you need a regular socket ≈220±22 V and 50 A. The unit creates a straight polarity arc with an efficiency of at least 80%.
Being a new generation device, with improved quality, it is more than 2.5 times more economical and 5 times lighter than the plasmatrons used Source eduard-romanov.uaprom.net
If we talk about money, Multiplaz-15000 is the most profitable plasma welding machine among its analogues. In addition, such a unit can be called the lightest among similar ones, for example, the weight of the power source is 33 kg and the weight of the plasma torch together with a cable and a 9-meter hose is 5 kg. Power consumption is 15 kW at an input voltage of 380±38 V, with a network frequency of 50 A. Welding operates in the current range from 20 to 100 A, consumes 480 l/min of compressed air and its efficiency is 85% - this allows you to cut thick steel sheets up to 50 mm. Of course, Multiplaz-15000 is more suitable for industrial enterprises and auto repair shops, but it is also bought for domestic use.
By what characteristics to choose
For a high-quality and correct selection, you should consider those characteristics that will greatly affect the operation of the entire unit. This list includes:
- Power supplies. Here you have the opportunity to choose a tool with a transformer and inverter type of power supply.
- Methods of electrical connections. You can choose a device with contact and non-contact types.
- Types of ignitions. There are models on the market with contact-type ignitions, pneumatic and high-frequency.
- Types of cooling system. There are air or liquid cooling systems.
- Type of welding. You have the opportunity to choose a device that works using conventional and non-consumable electrodes.
- Type of control. There are automatic and manual controls.
- Indicators of service life.
- Price.
Of course, it is worth understanding that the choice of such a tool should be based on the purpose and scope of application.
Plasma spraying plants
Plasma spraying devices come in a wide variety of designs. We will consider them from the most “traditional” to the most “advanced”.
The most common devices are those with one cathode and one anode, and with the powder introduced outside a short nozzle, perpendicular to the flame axis.
The operating principle of such devices is shown in the diagram (Figure 28):
Rice. 28. The principle of plasma spraying.
As can be seen from the diagram, the short nozzle of the plasmatron is also an anode. The powder is introduced outside the nozzle perpendicular to the flame axis, in close proximity to the arc.
The most popular device of this type is the 3MB plasmatron from Sulzer Metco, which, with minor modifications, has been around for more than 40 years. Figure 29 shows the current models of this series with a maximum power of 40 kW.
Rice. 29. Plasmatron 3MB.
A slightly newer and more powerful (55 kW) single-cathode device is the F4 plasmatron, shown in Figure 30.
Rice. 30. Plasmatron F4.
The 9MB device is one of the most powerful single-cathode plasmatrons of the traditional type (80 kW at a current of 1000 A and a voltage of 80 V) also produced by Sulzer Metco (Figure 31):
Rice. 31. Plasmatron 9MB
Traditional single-cathode plasmatrons from other companies differ little from Sulzer Metco plasmatrons: they all operate at a relatively low gas flow rate, low (< 100 V) voltage and high (up to 1000 A) arc current. None of the traditional plasmatrons allows particles to reach the speed of sound.
The advantage of plasmatrons with a low gas flow rate is the ability to impart a very high temperature to the particles (> 4000°C) due to the relatively long time they remain in the hot zone of the flame next to the arc. Such high particle temperatures make it possible to melt almost any ceramic and metal materials.
The development of plasma spraying technology in the last twenty years has been moving along the path of increasing particle speed. To give the particles greater speed, it is necessary to increase the pressure of the plasma-forming gases in front of the nozzle, which automatically leads to an increase in gas flow and an increase in arc voltage.
A modern, powerful (up to 85 kW, current up to 379 A, voltage up to 223 V) device with one cathode and anode is the 100HE plasmatron of the American company Progressive Technologies Inc., which, thanks to the high pressure and flow rate of plasma-forming gases, allows particle velocities to be achieved - close to the speed of sound (Figure 32):
Rice. 32. Plasmatron 100HE.
Due to the high speed of the plasma-forming gas, the residence time of particles in the hot zone of the flame and, accordingly, their temperature decreases. To counteract this, it is necessary to increase the arc power and use a large amount of hydrogen in the plasma-forming gas, which, thanks to the process of dissociation-association of molecules, lengthens the hot zone of the flame. Thus, the 100HE plasmatron realizes the temperature of particles with a size of 20-30 microns above 2300°C at a speed of about 250 m/sec, which makes it possible to spray coatings of Cr3C2 - NiCr, Cr2O3 and Al2O3 with low porosity.
The second direction of development, in combination with an increase in gas consumption, is the division of one arc into three parts, which improves the stability and uniformity of the flame, reduces wear of the electrodes and increases the total flame power. A typical example of such a device is the latest TriplexProTM-210 plasmatron from Sulzer Metco with one anode and three cathodes, a maximum power of 100 kW (Figure 33):
Rice. 33. TriplexProTM plasmatron.
1 – rear part of the body; 2 – anode stack; 3 – front part of the body; 4 – insulator; 5 – union nut; 6 – three cathodes in a ceramic block; 7 – anode stack element; 8 – plasma channel; 9 – nozzle with three powder nozzles.
Sulzer Metco's Triplex technology entered the thermal spray industry in the 1990s. These devices have, in comparison with plasmatrons with a single arc, a significantly longer service life and stability of deposition results. For many commercial powders, Triplex plasmatrons can also improve spraying productivity and efficiency while maintaining coating quality.
GTV GmbH has released, bypassing the Sulzer Metco patent for three-cathode plasmatrons, the GTV Delta device with one cathode and three anodes, which, in principle, is a degraded compilation of TriplexPro (Figure 34):
Rice. 34. GTV Delta plasmatron.
The last, third direction of development is the abandonment of radial powder input in favor of a much more rational one - axial. The key design element of a plasmatron with axial powder injection, Convergens, was invented in 1994 by the American Lucian Bogdan Delcea.
Currently, there is only one similar device - the Axial III plasmatron, with a maximum power of 150 kW, produced by the Canadian company Mettech, which combines all three directions of development (high gas flow, three arcs and axial powder input). Plasma spraying units with the Axial III plasmatron are also manufactured and distributed by the German company Thermico GmbH.
Figures 35, 36 and 37 show the Axial III device itself and its design diagram:
Rice. 35. Plasmatron Axial III.
Rice. 36. View of the Axial III device from the nozzle side.
Rice. 37. Schematic diagram of Axial III.
All modern plasma spraying installations are automatic, that is, the control of current sources, water cooling system and gas flow is regulated by a CNC system with visualization and saving of recipes on a computer. For example, the Axial III plasmatron is supplied by Thermico GmbH complete with a computerized control system that independently ignites arcs and enters the operating mode, selects spraying recipes, and controls all the main parameters: the flow of three plasma-forming gases (argon, nitrogen and hydrogen) , arc currents, water cooling system parameters. The same automatic system also controls the powder feeder.
Special mention needs to be made about the Thermico powder feeder. This, the most “advanced” device in the world today, allows not only to constantly regulate the mass flow of the powder and the flow of the carrier gas (nitrogen or argon), but also allows the use of fine-grained powders with poor flowability, unsuitable, for example, for Sulzer Metco feeders.
The author has personally worked with the Axial III plasmatron for a long time and can say from his own experience that despite some design flaws, this plasmatron is the most advanced thermal spraying device, combining the advantages of high-speed spraying with a high temperature strictly reducing flame. The main advantage of Axial III is the axial input of powder.
Popular makes and models
The most popular manufacturers of plasma-type inverter welding mechanisms are the companies Gorynych, Plazarium and Multiplaz. Which is better is up to the end users to judge; you can follow the links in the text below to read reviews for specific models.
The main feature of the Gorynych company’s model is the fact that it uses water in its pure form or mixed with alcohol as a mixture used to form plasma.
This property becomes an obstacle to the formation of corrosion. Such a device can operate both from the mains and from a generator.
The principle of operation of the device ]Multiplaz[/anchor] is similar to the previous one. Another advantage of this inverter is its compact size, because the weight of its “younger models” (see the link for details) does not exceed six kilograms. In addition, such a welding machine, unlike its analogues, practically does not emit harmful substances during operation.
Plazarium does not develop such powerful devices as its competitors. Accordingly, prices for its products are much lower. However, a characteristic innovation of the models of this company is the presence on the devices of special sensors that help regulate the welding temperature in order to avoid breakdown due to overheating.
Purpose
Manufacturing of parts and products with functional surface properties, for example, wear resistance (except for parts experiencing impact-abrasive wear), anti-friction, corrosion resistance, heat resistance, cavitation resistance, erosion resistance, electrical insulation, resistance to fretting corrosion, decorative characteristics, etc. Use for restoration sizes of worn and defective surfaces, application of retention coatings in dentistry.
Pros and cons of a plasma welding machine
Experts have studied this instrument inside and out. Therefore, they identified several advantages, including:
- Fast and high-quality welding and cutting process.
- Compatible with many types of metals.
- Ensuring high precision work.
- Low danger.
- Quite easy to use.
And the list of disadvantages includes:
- Inability to work with metals that have a thickness of more than one hundred millimeters.
- Inability to connect two burners to one unit.
- High demands on the availability of powerful and stable electrical networks.
