Types of electric arc. Electric arc: description and characteristics
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Electric arc and its properties
An electric arc is a long-term electric discharge that occurs in the gas gap between two conductors - the electrode and the metal being welded at a significant current strength. Ionization continuously occurring under the action of a rapid flow of positive and negative ions and electrons in the arc air gap creates the necessary conditions for long-term stable burning of the welding arc.
Rice. 1. An electric arc between a metal electrode and the metal to be welded: a - arc diagram, b - arc voltage graph 4 mm long; 1 - electrode, 2 - flame halo, 3 - arc column, 4 - welded metal, 5 - anode spot, 6 - molten pool, 7 - crater, 8 - cathode spot; h is the penetration depth in the arc, A is the moment of ignition of the arc, B is the moment of stable combustion
The arc consists of a column, the base of which is located in a recess (crater) formed on the surface of the molten pool. The arc is surrounded by a halo of flame formed by vapors and gases coming from the arc column. The column has the shape of a cone and is the main part of the arc, since it concentrates the main amount of energy corresponding to the highest density passing through the arc electric current. Top part column, located on the electrode 1 (cathode), has a small diameter and forms a cathode spot 8. Through the cathode spot is emitted the largest number electrodes. The base of the cone of the arc column is located on the welded metal (anode) and forms an anode spot. The diameter of the anode spot at average values of the welding current is greater than the diameter of the cathode spot by about 1.5 ... 2 times.
For welding, direct and alternating current are used. When using direct current, the minus of the current source is connected to the electrode (straight polarity) or to the workpiece “” (reverse polarity). Reverse polarity is used in cases where it is necessary to reduce the release of heat on the workpiece being welded: when welding thin or low-melting metal, overheat-sensitive alloyed, stainless and high-carbon steels, as well as when using certain types of electrodes.
Emitting a large amount of heat and having a high temperature. the electric arc at the same time gives a very concentrated heating of the metal. Therefore, the metal during welding remains relatively slightly heated already at a distance of several centimeters from the welding arc.
By the action of the arc, the metal is melted to a certain depth h, called the penetration depth or penetration.
The arc is ignited when the electrode approaches the metal to be welded and short-circuits the welding circuit. Due to the high resistance at the point of contact of the electrode with the metal, the end of the electrode quickly heats up and begins to emit a stream of electrons. When the end of the electrode is quickly removed from the metal at a distance of 2 ... 4 mm, an electric arc occurs.
The voltage in the arc, that is, the voltage between the electrode and the base metal, depends mainly on its length. With the same current, the voltage in a short arc is lower than in a long one. This is due to the fact that with a long arc, the resistance of its gas gap is greater. An increase in resistance in an electrical circuit at a constant current strength requires an increase in the voltage in the circuit. The higher the resistance, the higher the voltage must be in order to ensure that the same current flows in the circuit.
The arc between the metal electrode and the metal burns at a voltage of 18 ... 28 V. To initiate an arc, a higher voltage is required than that necessary to maintain it normal combustion. This is explained by the fact that at the initial moment the air gap is not yet heated enough and it is necessary to give the electrons a high speed to uncouple the molecules and atoms of the air. This can only be achieved with a higher voltage at the moment of ignition of the arc.
The graph of the change in current I in the arc during its ignition and stable burning (Fig. 1, b) is called the static characteristic of the arc and corresponds to the steady burning of the arc. Point A characterizes the moment of ignition of the arc. The arc voltage V quickly drops along the AB curve to a normal value corresponding to a stable arc at point B. A further increase in current (to the right of point B) increases the heating of the electrode and the rate of its melting, but does not affect the stability of the arc.
An arc is called stable if it burns evenly, without arbitrary breaks that require re-ignition. If the arc burns unevenly, often breaks and goes out, then such an arc is called unstable. The stability of the arc depends on many factors, the main of which are the type of current, the composition of the electrode coating, the type of electrode, the polarity and length of the arc.
At alternating current the arc burns less steadily than with a constant one. This is explained by the fact that at the moment when the current n goes to zero, the ionization of the arc gap decreases and the arc can go out. To increase the stability of the alternating current arc, it is necessary to apply io-coatings to the metal electrode. Pairs of elements included in the coating increase the ionization of the arc gap and thereby contribute to stable arcing with alternating current.
The length of the arc is determined by the distance between the end of the electrode and the surface of the molten metal of the workpiece to be welded. Normally, the normal arc length should not exceed 3…4 mm for a steel electrode. Such an arc is called a short arc. A short arc burns steadily and ensures the normal course of the welding process. An arc longer than 6 mm is called a long arc. With it, the process of melting the metal of the electrode is uneven. Drops of metal flowing down from the end of the electrode in this case can be oxidized to a greater extent with oxygen and enriched with atmospheric nitrogen. The deposited metal is porous, the weld has uneven surface, and the arc burns unstable. With a long arc, welding productivity decreases, metal spatter and the number of places of lack of penetration or incomplete fusion of the deposited metal with the base metal increase.
The transfer of electrode metal to the workpiece during consumable electrode arc welding is complex process. After ignition of the arc (position /), a layer of molten metal is formed on the surface of the end of the electrode, which, under the action of gravity and surface tension, is collected in a drop (position //). Droplets can reach large sizes and block the arc column (position III), creating a short circuit of the welding circuit, after which the bridge formed from the liquid metal breaks, the arc reappears, and the drop formation process is repeated.
The size and number of drops passing through the arc per unit time depend on the polarity and current strength, chemical composition and the physical state of the electrode metal, coating composition and a number of other conditions. Large drops, reaching 3 ... 4 mm, are usually formed when welding with bare electrodes, small drops (up to 0.1 mm) - when welding with coated electrodes and high current strength. The fine-drop process ensures the stability of the arc burning and favors the conditions for the transfer of the molten metal of the electrode in the arc.
Rice. 2. Scheme of metal transfer from the electrode to the metal being welded
Rice. 3. Deflection of the electric arc by magnetic fields (a-g)
Gravity can help or hinder the transfer of droplets in the arc. In overhead and partly in vertical welding, the force of gravity of the drop counteracts its transfer to the product. But due to surface tension liquid bath metal is kept from flowing out when welding in overhead and vertical positions.
The passage of electric current through the elements of the welding circuit, including the workpiece being welded, creates a magnetic field, the strength of which depends on the strength of the welding current. The gas column of an electric arc is a flexible conductor of electric current, therefore it is subject to the action of the resulting magnetic field, which is formed in the welding circuit. Under normal conditions, the gas column of an arc openly burning in the atmosphere is located symmetrically to the electrode axis. Under the action of electromagnetic forces, the arc deviates from the axis of the electrode in the transverse or longitudinal direction, which outward signs similar to the displacement of an open flame at strong air currents. This phenomenon is called magnetic blowing.
Attaching the welding wire in close proximity to the arc sharply reduces its deviation, since its own circular magnetic field of the current has a uniform effect on the arc column. The supply of current to the product at a distance from the Arc will lead to its deviation due to the thickening of the lines of force of the circular magnetic field from the side of the conductor.
- Electric arc (voltaic arc, arc discharge) - physical phenomenon, one of the types of electrical discharge in a gas.
It was first described in 1802 by the Russian scientist V. Petrov in the book “News of galvanic-voltaic experiments by means of a huge battery, sometimes consisting of 4200 copper and zinc circles” (St. Petersburg, 1803). An electric arc is a special case of the fourth form of the state of matter - plasma - and consists of an ionized, electrically quasi-neutral gas. The presence of free electric charges ensures the conductivity of the electric arc.
An electric arc between two electrodes in air at atmospheric pressure is formed as follows:
When the voltage between the two electrodes increases to a certain level in the air, an electrical breakdown occurs between the electrodes. The electrical breakdown voltage depends on the distance between the electrodes and other factors. The ionization potential of the first electron of metal atoms is approximately 4.5 - 5 V, and the arcing voltage is twice that (9 - 10 V). It is required to expend energy on the exit of an electron from the metal atom of one electrode and on the ionization of the atom of the second electrode. The process leads to the formation of a plasma between the electrodes and the burning of an arc (for comparison: the minimum voltage for the formation of a spark discharge slightly exceeds the electron output potential - up to 6 V).
To initiate a breakdown at the available voltage, the electrodes are brought closer to each other. During a breakdown, a spark discharge usually occurs between the electrodes, pulse-closing electrical circuit.
Electrons in spark discharges ionize molecules in the air gap between the electrodes. With sufficient power of the voltage source in the air gap, a sufficient amount of plasma is formed for a significant drop in the breakdown voltage or resistance of the air gap. In this case, spark discharges turn into an arc discharge - a plasma cord between the electrodes, which is a plasma tunnel. The resulting arc is, in fact, a conductor and closes the electrical circuit between the electrodes. As a result, the average current increases even more, heating the arc up to 5000-50000 K. In this case, it is considered that the ignition of the arc is completed. After ignition sustainable burning arc is provided by thermionic emission from the cathode heated by current and ion bombardment.
The interaction of electrodes with arc plasma leads to their heating, partial melting, evaporation, oxidation and other types of corrosion.
After ignition, the arc can remain stable when the electrical contacts are separated up to a certain distance.
During the operation of high-voltage electrical installations, in which the appearance of an electric arc is inevitable, the fight against it is carried out using electromagnetic coils combined with arc chutes. Among other methods, the use of vacuum, air, SF6 and oil circuit breakers, as well as methods for diverting current to a live load that independently breaks the electrical circuit, are known.
Electric arc is an electrical discharge in gases. The gas itself is an insulator, there are no current carriers in it. When formed in a gas a large number electrically charged particles - free electrons with a negative sign of charge and positively and negatively charged ions, the gas begins to conduct current.
When the end of the electrode contacts the base metal, a large amount of heat is released, as a result of which the movement of free electrons is accelerated.
When the electrode is detached from the base metal in the interelectrode gap, the electrons collide with neutral gas atoms and ionize them, i.e. separated into ions different signs charge. As a result, the gas becomes electrically conductive. Types of emission (exit) of electrons from the surface of the end of the electrode:
- thermionic emission;
- field emission;
- photoelectronic emission;
- electron emission due to heavy ion fluxes.
The stable burning of the arc is influenced by the processes of formation (ionization) of free electrons and ions in the volume of the neutral gas of the electric arc. Consider the types of ionization in an electric discharge.
Ionization by collision. The movement of electrons is greatly accelerated by the action of electric field in the cathode region. They meet neutral gas atoms on their way, hit them and knock out electrons. Ionization by heating (thermal ionization). The formation of ions in a gas medium is observed at temperatures above 1750°C. Ionization by heating proceeds due to inelastic collisions of gas particles with a large margin kinetic energy. Ionization of radiation (photoionization). In this case, the ionization of gases in an electric arc causes an effect on the gas gap of the energy of light radiation. Ionization by radiation will occur if the energy of light quanta exceeds the energy required for ionization of gas particles.
Welding arc properties
Ignition of the welding arc begins from the moment the electrode touches the metal being welded, i.e. with short circuit.
On fig. 1 shows the sequence of processes during ignition of the welding arc.
Since the end of the electrode and the surface of the metal being welded have irregularities, the contact between them during a short circuit occurs at separate points (Fig. 1a).
Fig.1. Arc ignition sequence
a - short circuit; b - formation of a bridge from liquid metal; c - the occurrence of an arc
Therefore, the current density at the contact points reaches large values, the metal instantly melts, forming a bridge of liquid metal between the electrode and the metal being welded (Fig. 1b).
When the electrode is removed from the metal surface to a certain length, called the arc length L, the liquid bridge is stretched with a decrease in the cross section, then, at the moment the metal reaches the bridge, the boiling point evaporates and the bridge breaks (Fig. 1c).
A discharge gap is formed, which is filled with charged particles of metal vapors, electrode coatings and gases. This is how a welding arc appears, which is a luminous column of heated gas, consisting of electrons, ions and neutral atoms.
This state of the gas is called plasma, which is electrically neutral because it contains the same number of positive and negative particles.
The temperature of the arc column is higher than the temperature of the boiling point of the metal of the electrode and the workpiece, and the end of the electrode and the workpiece are separated from the arc column by intermediate gas layers, called the near-electrode regions of the arc, (Fig. 2).
Rice. 2. Scheme of the welding arc.
1 - electrodes; 2 - cathode spot; 3 - cathode region; 4 - arc column; 5 - anode region; 6 - anode spot; 7 - welding pool; 8 - welded part.
In cathode region 3, electrons are emitted from cathode spot 2 into arc column 4, where they ionize neutral atoms.
In the cathode region, at a length of fractions of a millimeter, a significant part of the arc voltage is concentrated, which is called the cathode voltage drop and reaches 10 ... 16 V.
In the anode region 5 near the anode spot 6 there is a sharp drop in voltage over the mean free path of an electron. This voltage drop is called the anode voltage drop, the value of which is 6 ... 8 V. In this area, the electrons sharply increase their speed of movement and are neutralized at the anode spot. The anode receives energy from the arc in the form of a stream of electrons and thermal radiation, so the temperature of the anode region is higher than the temperature of the cathode region, and a large amount of heat is released at the anode.
When welding with direct current of direct polarity, the temperature in various zones of the welding arc:
- in the middle of the arc column - about 6000°C;
- in the anode region - 2600°C;
- in the cathode region - 2400°C;
- in the weld pool - 1700 ... 2000 ° С.
When welding on alternating current, the distribution of heat of the arc and the temperature in the cathode and anode regions are approximately the same (the cathode region on the electrode).
An electric arc is a powerful, long-term electric discharge between energized electrodes in a highly ionized mixture of gases and vapors. Characterized high temperature gases and high current in the discharge zone.
The electrodes are connected to AC sources ( welding transformer) or direct current (welding generator or rectifier) with direct and reverse polarity.
When welding with direct current, the electrode connected to the positive pole is called the anode, and to the negative - the cathode. The gap between the electrodes is called the arc gap area or arc gap (Figure 3.4). The arc gap is usually divided into 3 characteristic regions:
- an anode region adjacent to the anode;
- cathode region;
- arc post.
Any arc ignition starts with a short circuit, i.e. from the short circuit of the electrode with the product. In this case, U d \u003d 0, and the current I max \u003d I short circuit. A cathode spot appears at the closure site, which is an indispensable (necessary) condition for the existence arc discharge. The resulting liquid metal, when the electrode is withdrawn, is stretched, overheated and the temperature reaches, up to the boiling point - the arc is excited (ignited).
The arc can be ignited without contact of the electrodes due to ionization, i.e. breakdown of a dielectric air (gas) gap due to voltage increase by oscillators (argon arc welding).
The arc gap is a dielectric medium that must be ionized.
For the existence of an arc discharge, U d \u003d 16 ÷ 60 V is sufficient. The passage of electric current through an air (arc) gap is possible only if there are electrons (elementary negative particles) and ions in it: positive (+) ions - all molecules and atoms of elements (lighter form metals Me); negative (-) ions - more easily form F, Cr, N 2, O 2 and other elements with electron affinity e.
Figure 3.4 - Scheme of burning the arc
The cathode region of the arc is a source of electrons that ionize gases in the arc gap. The electrons released from the cathode are accelerated by the electric field and move away from the cathode. At the same time, under the influence of this field, + ions are sent to the cathode:
U d \u003d U k + U c + U a;
The anode region has a much larger volume U a< U к.
Arc column - the main part of the arc gap is a mixture of electrons, + and - ions and neutral atoms (molecules). The arc column is neutral:
∑ charge neg. = ∑ charges of positive particles.
The energy to maintain a stationary arc comes from the power supply of the power supply.
Different temperatures, sizes of anode and cathode zones and a different amount of heat released - determines the existence of direct and reverse polarity when welding with direct current:
Q a > Q to; U a< U к.
- when a large amount of heat is required for heating the edges of large thicknesses of metal, direct polarity is used (for example, when surfacing);
- with thin-walled and non-overheating welded metals, reverse polarity (+ on the electrode).
Electric arc and its properties
Electric arc welding has received the greatest distribution in mechanical engineering. Let us consider in more detail the features of electric arc welding.
An electric arc is a continuous discharge of electric current between two electrodes, occurring in a gaseous medium. The electric arc used to weld metals is called a welding arc. Such an arc in most cases burns between the electrode and the workpiece, i.e. is a direct arc.
The arc of direct action of direct current, burning between the metal electrode (cathode) and the metal being welded (anode), has several clearly distinguishable areas (Fig. 2.3). The electrically conductive gas channel connecting the electrodes has the shape of a truncated cone or cylinder. Its properties on various distances from the electrodes are not the same. Thin layers of gas adjacent to the electrodes have a relatively low temperature. Depending on the polarity of the electrode to which they are adjacent, these layers are called cathodic. 2 and anode 4 arc areas.
The length of the cathode region lk is determined by the mean free path of neutral atoms and is
̃ about 10 -5 cm. The length of the anode region l a is determined by the free path of an electron and is approximately 10 -3 cm. Between the near-electrode regions is the most extended, high-temperature region of the discharge - the arc column l c 3.
On the surface of the cathode and anode, spots are formed, called, respectively, the cathode 1 and anode 5 spot, which are the bases of the arc column, through which the entire welding current passes. Electrode spots are distinguished by the brightness of their glow at their relatively low temperature (2600 ... 3200 K). The temperature in the arc column reaches 6000...8000 K.
Total arc length l d is equal to the sum of the lengths of all three of its regions (l d \u003d l a + l k) and for real conditions is 2 ... 6 mm.
The total voltage of the welding arc, respectively, is the sum of the voltage drops in certain areas arcs 
and is in the range from 20 to 40 V. The dependence of the voltage in the welding arc on its length is described by the equation ,
where a - the sum of voltage drops in the cathode and anode regions, V; l d- arc column length, mm; b- specific voltage drop in the arc, i.e. referred to 1 mm of arc column length, V/mm.
One of the main characteristics of an electric arc discharge is a static current-voltage characteristic - the dependence of the arc voltage at a constant arc length on the current in it (Fig. 2.4).
With an increase in the length of the arc, the voltage increases and the curve of the static current-voltage characteristic of the arc rises higher, while approximately retaining its shape (curves a, b, c). Three regions are distinguished on it: falling I, rigid (almost horizontal) II and increasing III. Depending on the arc burning conditions, one of the sections of the characteristic corresponds to it. In manual arc welding with coated electrodes, welding in shielding gases with a non-consumable electrode and submerged arc welding at a relatively low high densities current, the characteristic of the arc will initially be falling, and with increasing current, it will completely turn into a rigid one. At the same time, with an increase in the welding current, the cross section of the arc column and the area cross section anode and cathode spots. The current density and arc voltage remain constant.
When welding submerged arc and in shielding gases with a thin electrode wire at high current densities, the arc characteristic becomes increasing. This is explained by the fact that the diameters of the cathode and anode spots become equal to the diameter of the electrode and cannot increase any more. In the arc gap, complete ionization of gas molecules occurs and a further increase in the welding current can occur only due to an increase in the speed of movement of electrons and ions, i.e., due to an increase in the electric field strength. Therefore, to further increase the welding current, an increase in the arc voltage is required.
The welding arc is a powerful concentrated source of heat. Almost all Electric Energy, consumed by the arc, is converted into heat. Complete thermal power arcs Q \u003d I sv U d(J/s) depends on the strength of the welding current I St(A) and arc voltage U d(AT).
It should be noted that not all the heat of the arc is spent on heating and melting the metal. Part of it is uselessly spent on heating the surrounding air or protective gas, radiation, etc. In this regard, the effective thermal power of the arc q eff(J / s) (that part of the heat of the welding arc, which is introduced directly into the product) is determined by the following relationship: 
where η is the efficiency factor (COP) of the process of heating the product with a welding arc, determined empirically.
The coefficient η depends on the welding method, electrode material, composition of the coating or flux, and a number of other factors. For example, when welding with an open arc with a carbon or tungsten electrode, it averages 0.6; when welding with coated (high-quality) electrodes - about 0.75; in submerged arc welding - 0.8 or more.