Influence of the polarity of the arc on the melting of the electrodes. Influence of arc burning conditions on the electrode melting process
5.1 Purpose of work
Study of the influence of welding mode parameters on the process of electrode melting, familiarization with the method of experimental determination of the characteristics of electrode melting.
Theoretical introduction
The heat introduced by the welding arc into the electrode is spent on heating and melting the electrode rod and the electrode coating. The process of melting the electrode rod and the transition of the molten metal into the weld pool depends on a number of factors: the magnitude, type and polarity of the current, the composition of the electrode coating and the rod, the position weld in space, etc. The properties of the electrode, which characterize the productivity of its melting, are evaluated by the melting coefficient α p, determined by the formula
where g p is the mass of molten metal, g;
I - welding current, A;
t is the electrode melting time.
During welding, losses of liquid metal are observed due to its oxidation by air and through slag, as well as as a result of evaporation and splashing outside the weld pool. Waste and spatter losses are estimated by the loss factor
Waste loss and splashing fluctuate quite widely depending on various factors. For manual arc welding, the melting factor, depending on the specific brand of the electrode, is 8-15 g / A h, the loss factor is 5-30%; for automatic submerged arc welding - α p = 13-23 g / Ah, ψ = 2-4%.
An increase in welding current leads to an increase in the temperature of the arc column and the intensity of electrode melting and, as a result, to an increase in α р. At high current densities, the transition of metal drops from the electrode to the weld can be of a jet nature, which reduces spatter losses.
When welding with reverse polarity, the melting performance is significantly higher than when welding with alternating current and with direct polarity. This is explained by the fact that 2-3 times more heat is released at the anode than at the cathode due to the bombardment of the anode by fast electrons, while energy is spent on their emission at the cathode.
The values of α р and ψ are influenced by the type of electrode and the composition of the rod, which determines the composition of the atmosphere of the arc column and, as a consequence, the effective ionization potential. In turn, a change in the effective ionization potential leads to a change in the temperature of the arc column in accordance with the empirical formula applicable to manual arc welding
T = 800U eff (5.3)
An increase in the temperature of the arc column leads to an increase in the amount of gases formed, increases their pressure in the electrode metal drop, and, ultimately, can lead to increased spatter.
The coefficient α p significantly depends on the heating temperature of the electrode rod. Heating the electrode rod with Joule heat accelerates its melting in an arc discharge and α p increases, while the value of ψ practically does not change. In automatic and semi-automatic welding, to increase α p, welding with an increased wire stickout (the distance between the current-carrying mouthpiece and the product) is widely used. An increase in overhang leads to an increase in the resistance of the wire and, as a consequence, an increase in its heating temperature. In manual arc welding, the variability of α p during the burning of the electrode rod can lead to a violation of the mode of formation of the seam, therefore, the maximum current strength for each diameter of an electrode of a particular brand is strictly limited. The uniformity of the melting of the electrode is facilitated by an increase in the thickness of the electrode coating, tk. it does not conduct current, is not heated by Joule heat, and cools the electrode rod.
Equipment and materials
1. Stations for manual arc welding on direct and alternating currents, equipped with devices for measuring the welding current.
2. Technical scales with weight.
3. Stopwatch.
4. Caliper and ruler.
5. Welding electrodes MP-3 Æ4 mm.
6. Mild steel plates.
Work procedure
1. Clean, mark and weigh the plates to be surfacing.
2. Prepare the electrodes, mark, determine the diameter and initial length of the electrode rod.
3. For each brand of electrode, determine the mass of l linear centimeter of the electrode rod, which is equal to the mass of the electrode rod cleaned from coating, divided by its length.
4. Weld the bead onto the plate using an electrode with direct current of reverse polarity. In the process of surfacing, record the arc burning time and current strength (recommended current strength for all variants of experiments is 120-200 A) with subsequent entry in Table 5.1.
5. After surfacing, cool, dry, clean from slag and weigh the plate. Determine the mass of deposited metal and enter the result in Table 5.1.
6. Measure the length of the part of the electrode remaining after surfacing and calculate the mass of the molten metal, followed by entering it in Table 5.1.
7. Calculate the characteristics of the melting of the electrode, followed by entering in table 5.1.
8. Repeat the experiment according to claim 4 with the changed values of the current strength 2 times.
9. Repeat the experiment according to claim 4 for straight polarity and alternating current.
Arc welding compared to gas welding, has some special features. This is a higher, up to 5000 ° C, temperature of the arc itself, which exceeds the melting temperatures of all existing metals, and a wide variety of types and types of welding, and, accordingly, the methods and purposes of its application. Electric arc welding differs in the degree of mechanization, the type of current, the type of arc and the properties of the welding electrode, as well as other parameters. In this article, I would like to consider some of the nuances of electric arc welding, depending on the polarity of the welding electrodes.
Types of welding.
According to the type of current used, two types of arc welding are distinguished:
- welding with an electric arc powered by alternating current;
- welding with an electric arc powered by direct current.
In turn, welding using direct current is of two types:
- direct polarity welding;
- reverse polarity welding.
Consider the features of each type of DC welding in more detail.
Welding current direct polarity.
Under direct polarity welding, it is customary to understand welding, during which a positive charge is applied to the welded part (product) from a welding rectifier, that is, welding cable connects the structure to be welded to the plus terminal of the welding machine. A negative charge is applied to the electrode through an electrode holder connected by a cable to the negative terminal.
Since the temperature at the positive pole (anode) is always much higher than at the negative pole (cathode), it is recommended to use a direct polarity current when it is necessary to cut metal structures and weld thick-walled parts, as well as in other cases when it is necessary to achieve a large heat release, which is exactly and is characteristic feature this type of connection.
Reverse polarity welding.
To carry out welding with reverse polarity current, the connection should be carried out in the opposite way: apply a negative charge to the workpiece from the "minus" terminal, and a positive charge from the "plus" terminal to the electrode.
This polarity of the welding electrodes provides the situation opposite to direct connection - more heat is generated on the electrode, and the heating of the part is relatively reduced. This allows for a more "delicate" welding and reduces the likelihood of part burn through. Accordingly, reverse polarity welding is recommended when it is necessary to weld thin sheets of metal, stainless steel, alloy steel, other steels and alloys that are sensitive to overheating.
Also, submerged arc welding and gas shielded welding are generally done using reverse polarity connections.
General aspects.
Regardless of which particular polarity of welding electrodes is used, there are several common points:
- unlike welding with alternating current, when using direct current, a more "neat" weld with less spatter of metal is obtained, since there is no frequent change in the polarity of the supplied current;
- since the anode and cathode are heated differently, among other things, if a consumable electrode is used, the amount of metal transferred from the consumable electrode to the workpiece depends on the connection method;
- in order to avoid damage to the welded part at the point of connection of the cable with a positive or negative charge due to the occurrence of microdischarges, it is recommended to use a clamping clamp for a more reliable connection.
In conclusion, I would like to note that in this article only some points regarding welding using an electric arc are disclosed. In practice, this topic is much broader, and the variety of types of electric welding allows it to be used in almost any, sometimes unique conditions and technical situations.
Mechanized arc welding with a consumable electrode in a protective gas environment is a kind of electric arc welding in which the electrode wire is fed automatically with constant speed, and the welding torch is moved along the seam manually. In this case, the arc, the stick-out of the electrode wire, the pool of molten metal and its solidifying part are protected from the effects of ambient air by a shielding gas supplied to the welding zone.
The main components of this welding process are:
Power source that provides the arc with electrical energy;
- a feeding mechanism that feeds an electrode wire into the arc at a constant speed, which is melted by the heat of the arc;
- protective gas.
The arc burns between the workpiece and the consumable electrode wire, which is continuously fed into the arc and which serves as filler metal. The arc melts the edges of the parts and the wire, the metal of which passes to the product into the resulting weld pool, where the metal of the electrode wire is mixed with the metal of the product (that is, the base metal). As the arc moves, the molten (liquid) metal of the weld pool solidifies (that is, crystallizes), forming a weld that connects the edges of the parts. Welding is performed with direct current of reverse polarity, when the positive terminal of the power source is connected to the burner, and the negative terminal is connected to the product. Sometimes direct polarity of the welding current is also used.
Welding rectifiers are used as a power source, which must have a rigid or gently dipping external current-voltage characteristic. This characteristic provides automatic restoration of the set arc length in case of its violation, for example, due to the fluctuations of the welder's hand (this is the so-called self-regulation of the arc length). In more detail, power sources for MIG / MAG welding are described in the article.
As a consumable electrode, an electrode wire of a solid section and a tubular section can be used. A tubular wire is filled inside with a powder of alloying, slag and gas-forming substances. Such a wire is called flux-cored wire, and the welding process in which it is used is flux-cored wire welding.
There is a fairly wide selection of welding electrode wires for welding in shielding gases, differing in chemical composition and diameter. The choice of the chemical composition of the electrode wire depends on the material of the product and, to some extent, on the type of shielding gas used. The chemical composition of the electrode wire should be close to the chemical composition of the base metal. The diameter of the electrode wire depends on the thickness of the base metal, type welded joint and welding positions.
The main purpose of the shielding gas is to prevent direct contact of the ambient air with the metal of the weld pool, stick out of the electrode and the arc. Shielding gas affects the stability of the arc, the shape of the weld, the depth of penetration and the strength characteristics of the weld metal. More detailed information about shielding gases, as well as about welding wires, is given in the article.
Varieties of the MIG / MAG welding process
In Europe, gas-shielded consumable electrode welding is short title MIG/MAG (MIG/MAG). MIG stands for "Metal Inert Gas". With this type of process, an inert (inactive) gas is used, i.e. one that does not chemically react with the metal of the weld pool, such as argon or helium. As a rule, when welding in pure inert gas, despite the good protection of the welding zone from the effects of ambient air, the formation of the weld deteriorates and the arc becomes unstable. These shortcomings can be avoided if mixtures of inert gases with small additions (up to 1 - 2%) of such active gases as oxygen or carbon dioxide (CO 2) are used.
MAG stands for Metal Active Gas. This type of welding in shielding gases includes welding in mixtures of inert gases with oxygen or carbon dioxide, the content of which is 5 - 30%. With such an oxygen or carbon dioxide content, the mixture becomes active, i.e. it affects the course of physicochemical processes in the arc and weld pool. Welding of low-carbon steels can be carried out in pure carbon dioxide (CO 2) environment. In some cases, the use of pure carbon dioxide provides better shape penetration and reduces the tendency to pore formation.
Since with this welding method, the electrode wire is fed automatically, and the welding torch moves along the seam manually, this welding method is called mechanized, and the welding installation is called a mechanized machine (semiautomatic welding machine). However, gas-shielded welding can also be carried out automatically when mobile carts or mobile welding heads are used.
Areas of use
MIG or MAG welding processes are suitable for welding all common metals such as non-alloyed and low-alloyed steels, stainless steels, aluminum and some other non-ferrous metals. Moreover, this welding process can be used in all spatial positions. Due to its many advantages, MIG/MAG welding is widely used in many industries.
Welding machine for MIG/MAG welding
It consists of:
Power source of the welding arc;
- electrode wire feed mechanism;
- welding torch;
- control panel of the apparatus (combined with a power source and sometimes with an electrode wire feeder).
Typical appearance welding mechanized machine for welding MIG / MAG
Source of power is designed to provide the welding arc with electrical energy that ensures its functioning as a heat source. Depending on the characteristics of a particular welding process, the power source must have certain characteristics (the required form of the external current-voltage characteristic - I–V characteristic, inductance, a certain value of open-circuit voltage and short-circuit current, the required ranges of welding current and arc voltage, etc.). For MIG / MAG welding, DC power sources (rectifiers or generators) with a hard (sloping) VVC are used. The range of welding currents that power supplies for mechanized welding machines provide is 50 - 500 A. But, as a rule, modes in the range of 100 - 300 A are used. For more information about arc welding power sources, see Power sources for arc welding
Wire feed mechanism is designed to feed consumable electrode wire into the arc at a given speed. The main components of the electrode wire feed mechanism are shown in the figure below.
Through the connector for connecting the welding torch and the feed mechanism, the electrode wire and shielding gas are supplied to the welding zone, and the "Start - Stop" button on the torch is connected to the control circuit of the feed mechanism. The connector shown in the figure below is a standard Euro connector. In practice, other types of connectors may also be encountered.
An obligatory element of the control panel for the feed mechanism is the electrode wire feed speed controller. Sometimes, for the convenience of adjusting the parameters of the welding mode, especially in the case of using portable feeders, an arc voltage regulator can also be placed on this console, as in the case shown in the figure.
For mechanized welding with a consumable electrode in shielding gases (MIG / MAG), two types of feeders are used:
With 2 roller drive;
- with 4 roller drive.
In the pictures below on the left one of the 2 roller drives of the feed mechanism is shown (the upper roller is the pressure roller). Drives of this type are used for broaching only steel wire solid section. The same figure on the right shows an example of a 4-roll drive feed mechanism, which is recommended for drawing flux-cored wires and wires made of soft materials (aluminum, magnesium, copper), as it provides stable wire feeding with less pressing pressure on the pressure rollers, which prevents wire breakage.
In modern drives of the feed mechanism, as a rule, rollers of a special design are used - with a drive gear. Thus, after the pressure roller is pressed against the drive roller and their gears are engaged, the transmission of the pulling force from the feed drive to the electrode wire is carried out through both rollers.
The profile of the feed rollers (i.e. the shape of the surface or groove) depends on the material and construction of the filler wire. For solid steel wire, pinch rollers are flat-surfaced or knurled and V-groove, and drive rollers V-groove and sometimes knurled.
For wires made of soft materials (aluminum, magnesium, copper), rollers with a U-shaped or V-shaped smooth groove are used. Notched rollers should not be used, as they cause the formation of small chips that clog the guide channel in the burner.
For flux-cored wire, rolls with a V-shaped smooth groove (in 4-roller drives of the feed mechanism) or with a V-shaped groove with a notch are used.
The rollers vary in groove depth depending on the wire diameter. The nominal diameter of the electrode wire for this roller is indicated on its side surface.
Feed mechanisms are made of several types:
- in a single case with a power source (for compactness)
- placed on the power source (for high power devices)
- portable (to expand the service area of welding)
The wire feed mechanism can also be built into the torch. In this case, the electrode wire is pushed through the hose by a standard feed mechanism and simultaneously pulled out of it by the torch mechanism. This system ("push-pull") allows the use of burners with significantly longer hoses.
In some feed mechanisms, the wire reel is placed outside. This makes it easier to replace it. This is important for cases where, due to the intensive mode of operation, the wire in the reel quickly runs out.
The bobbin braking device provided in the feed mechanisms prevents its spontaneous unwinding.
Device control panel designed to regulate the electrode wire feed speed and open circuit voltage (arc voltage), programming the welding cycle (gas pre-purge time, gas purge time after welding current is turned off, "soft start" parameters, etc.), setting the parameters of the pulsed welding mode , settings for synergic control of the welding process and for other functions.
The control panel of a mechanized welding machine with a separate electrode wire feeder can be divided; some of the controls are located on the front panel of the power source (this is, first of all, the power button, the arc voltage regulator, etc.), and the other part is on the front panel of the feed mechanism (for example, the electrode wire feed speed regulator).
Some controls (primarily arc voltage and wire feed speed), as well as indicators of welding mode parameters, can be placed on the handle of the welding torch.
The photo below shows some types of remote controls (from simple to complex).
- designed to direct the electrode wire into the arc zone, supply welding current to it, supply shielding gas and control the welding process.
Typically, MIG/MAG welding torches are naturally air-cooled. However, torches with forced water cooling of the power cable in the torch hose and the welding torch head up to the gas nozzle are also used for welding at higher modes.
At one end of the torch hose there is a connector for connection to the feed mechanism. Through the connector for connecting the welding torch and the feed mechanism, the electrode wire and shielding gas are supplied to the welding zone, the welding current is supplied to the arc, and the "Start - Stop" button on the torch is connected to the control circuit of the feed mechanism. The hose itself has a spiral through which the welding wire, welding (power) cable, gas hose and control cable are fed.
The other end of the hose is connected to the handle of the welding torch, in the head of which there is:
Diffuser with holes for protective gas;
- current-carrying tip;
- gas nozzle.
Current-carrying tips are designed to supply welding current to the electrode wire. They come in a variety of designs and are made from copper-based alloys. Tips must be selected in accordance with the diameter of the electrode wire used.
Depending on the design of the welding torch, the gas nozzles also have different shape and sizes.
On the handle of the welding torch there is a start-stop button. On some modern types of welding torches, some controls (first of all, arc voltage and electrode wire feed speed), as well as indicators of welding mode parameters, can also be placed there.
Gas flow meters
In welding installations, float and throttle type gas flow meters are used. Float type flowmeters or rotameters consist of a glass tube with an internal conical bore. The tube is located vertically with the wide end up. A float is placed inside the tube, which moves freely in it. The gas is brought to the lower end of the tube and removed from the upper one. When passing through the tube, the gas lifts the float until the gap between the float and the tube wall reaches such a value at which the pressure of the gas jet balances the weight of the float. The greater the gas flow, the higher the float rises.
Each flow meter is equipped with an individual calibration chart, which shows the relationship between the scale divisions on the tube and the air flow. The flow rates measured by the rotameter are changed by changing the weight of the float, making it from ebonite, duralumin, corrosion-resistant steel or other materials.
The throttling type flow meter is designed on the principle of changing the pressure drop in the chamber before and after the throttling orifice with a small orifice. When gas passes through a small hole, a different pressure is established before and after the diaphragm, depending on the gas flow rate. The flow rate is judged from this pressure drop. An individual schedule is built for each flow meter and gas. The flow rate measurement limits are changed by changing the diameter of the hole in the diaphragm. The U-30 and DZD-1-59M reducer flow meters are built on this principle, which allow measuring gas flow in the range of 2.5-55 l / min.
Gas dryers
Gas dryers are used when using wet CO 2 . dehumidifiers are high and low pressure. The high pressure dryer is installed upstream of the reduction gear. The dryer is small and requires frequent replacement of the desiccant. The low pressure dryer is of considerable size, it is installed after the reduction gear, it does not require frequent replacement of the desiccant. Such a dryer is simultaneously a gas receiver and improves the uniformity of the gas supply. Silica gel and alumogel are used as a desiccant, less often blue vitriol and calcium chloride. Silica gel and copper sulphate, saturated with moisture, can be restored by calcining at a temperature of 250-300°C.
The carbon dioxide gas heater is an electrical device and is designed to heat carbon dioxide in order to protect gas channels from freezing. It is installed in front of the reduction gear. For safety reasons, gas heaters are usually made with a low voltage power supply of 20 ... 36 V and, as a rule, are connected to the corresponding socket of the welding machine power source. To avoid overheating gas reducer it should be separated from the heater by a transition tube at least 100 mm long.
gas valve
The gas valve is used to conserve shielding gas. It is advisable to install the valve as close as possible to the welding torch. At present, electromagnetic gas valves are most widely used. In semi-automatic devices, gas valves built into the handle of the holder are used. The gas valve must be turned on in such a way that the supply of shielding gas is provided before or simultaneously with the ignition of the arc, as well as its supply after the arc breaks until the weld crater is completely solidified. It is desirable to be able to also turn on the gas supply without starting welding, which is necessary when setting up the welding installation.
Gas mixers designed to obtain mixtures of gases in the case when it is not possible to use a pre-prepared mixture of the desired composition.
Types of metal transfer in MIG/MAG welding
The MIG/MAG welding process, being a consumable electrode process, is characterized by the transfer of the electrode metal through the arc into the weld pool. The transfer of metal is carried out by means of drops of molten electrode metal formed at the end of the electrode wire. Their size and frequency of transition to the weld pool depend on the material and diameter of the electrode wire, the type of shielding gas, the polarity and value of the welding current, arc voltage and other factors. The nature of the transfer of the electrode metal determines, in particular, the stability of the welding process, spatter level, geometric parameters, appearance and quality of the weld.
In MIG/MAG welding, metal transfer occurs mainly in two forms. In the first form, the drop touches the surface of the weld pool even before separation from the end of the electrode, forming a short circuit and causing extinction of the arc, which is why this type of transfer is called transfer with short circuits. Usually, metal transfer with short circuits takes place at low welding conditions, i.e. low welding current and low arc voltage (a short arc ensures that the drop touches the surface of the pool before it separates from the end of the electrode).
Due to the low welding conditions, as well as the fact that the arc does not burn for part of the time, the heat input into the base metal during welding with short circuits is limited. This feature of the short circuit welding process makes it most suitable for welding thin sheet metal. A small weld pool and a short arc that limits excessive droplet growth provide easy process control and allow welding in all spatial positions, including overhead and vertical, as shown in this picture.
When short circuit welding is used on thicker joints, undercuts and lack of penetration may occur.
In the second form, the drop separates from the electrode end without touching the surface of the weld pool and, therefore, this type of transfer is called transfer without short circuits. The latter form of metal transfer is subdivided into coarse droplet transfer and fine droplet transfer.
Large-drop metal transfer occurs when welding is carried out at high arc voltages (excluding short circuits) and medium welding currents. It is generally characterized by an irregular transition of large drops of molten electrode metal (greater than the diameter of the electrode) and a low transfer rate (from 1 to 10 drops per second). Due to the fact that gravity plays a crucial role in this type of metal transfer, welding is limited to the down position only.
When welding in a vertical position, some drops may fall down, bypassing the weld pool (as you can see in this picture on the last frame).
The weld pool is large and therefore difficult to control, with a tendency to run down when welding in a vertical position or fall out when welding in an overhead position, which also precludes welding in these spatial positions. These shortcomings, as well as the uneven formation of the weld, make it undesirable to use this type of metal transfer in MIG/MAG welding.
Small-drop metal transfer is characterized by identical drops of small sizes (close to the diameter of the electrode), which are separated from the end of the electrode with a high frequency.
This type of transfer usually occurs when welding with reverse polarity in an argon-based shielding mixture and at high arc voltages and welding currents. Due to the fact that this type of transfer requires the use of high welding current, resulting in high heat input and a large weld pool, it can only be used in the down position and is not suitable for welding thin sheet metal. It is used for welding and filling metal grooves of large thicknesses (usually more than 3 mm thick), primarily in the welding of heavy metal structures and in shipbuilding. The main characteristics of the welding process with fine droplet transfer are: high arc stability, practical absence spatter, moderate formation of welding fumes, good wetting of the weld edges and high penetration, smooth and uniform surface of the weld, the possibility of welding at elevated modes and high deposition rate. Due to these advantages, droplet transfer of metal is always desirable where its application is possible, however, it requires strict selection and maintenance of welding process parameters.
When welding MAG in a CO 2 environment, only one type of transfer is possible - with short circuits.
Pulse transfer of electrode metal
In one of the varieties of MIG / MAG welding, current pulses are used that control the transition of electrode metal drops in such a way that small-drop metal transfer is carried out at medium welding currents (Iav) below the critical value. With this method of controlling metal transfer, the current is forced to change between two levels, called the base current (Ib) and the pulse current (Ii). The level of the base current, which is approximately equal to 50 ... 80 A, is selected from the condition of sufficiency to ensure the maintenance of the arc with a slight effect on the melting of the electrode. The function of the pulse current, which exceeds the critical current (the level of current at which large-drop metal transfer turns into small-drop), is the melting of the electrode end, the formation of a drop of a certain size and the separation of this drop from the end of the electrode by the action of electromagnetic force (Pinch effect). The sum of the pulse durations (ti) and the base (tb) determines the current ripple period, and its reciprocal value gives the ripple frequency. The repetition rate of current pulses, their amplitude and duration determine the released arc energy, and, consequently, the rate of electrode melting.
The pulse arc welding process combines the advantages of a short circuit welding process (such as low heat input and the ability to weld in all spatial positions) and a fine transfer welding process (no spatter and good weld metal formation).
During one current pulse, from one to several drops can be formed and transferred to the weld pool. The optimal metal transfer is when only one drop of electrode metal is formed and transferred for each current pulse, as shown in the figure below. For its implementation, careful adjustment of the IDS welding parameters is necessary, which in modern power sources is carried out automatically on the basis of synergistic control.
MIG/MAG welding parameters
The MIG/MAG shielded gas welding mode parameters include:
Welding current (or electrode wire feed speed);
- arc voltage (or arc length);
- welding current polarity;
- welding speed;
- the length of the stick out of the electrode wire;
- tilt of the electrode (torch);
- welding position;
- electrode diameter;
- the composition of the protective gas;
- shielding gas consumption.
Influence of current polarity on MIG/MAG welding process
The polarity of the welding current significantly affects the nature of the MIG / MAG welding process. So, when using reverse polarity, the welding process is characterized by the following features:
Increased heat input to the product;
- deeper penetration;
- lower efficiency of electrode melting;
- a large selection of implemented types of transfer - metal, allowing you to choose the optimal one (with short circuits, large-drop, small-drop, jet, IDS ...).
While when welding in direct polarity, there is:
Reduced heat input to the product;
- less deep penetration;
- high efficiency of electrode melting;
- the nature of the transfer of the electrode metal is extremely unfavorable (large-drop with low regularity).
 Increased heat input to the product |
 Reduced heat input to the product |
Qualitative comparative analysis features of MIG / MAG welding on reverse and straight polarity
Differences in the properties of the arc with direct and reverse polarity are associated with the difference in the release of heat from the arc at the cathode and anode during consumable electrode welding; more heat is generated at the cathode than at the anode. The following is an approximate amount of heat generation in different parts of the arc in relation to MIG / MAG welding (as the product of the voltage drop in the corresponding region of the arc and the welding current):
In the cathode region: 14 V x 100 A = 1.4 kW over a length of ≈ 0.0001 mm;
In the arc column: 5 V x 100 A = 0.5 kW for a length of ≈ 5 mm;
In the anode region: 2.5 V x 100 A = 0.25 kW over a length of ≈ 0.001 mm.
The difference in heat release in the anode and cathode regions determines a deeper penetration of the base metal at reverse polarity, a higher rate of electrode melting at straight polarity, as well as an unfavorable metal transfer observed at straight polarity, when the drop tends to be repelled in the opposite direction from the weld pool . The latter is the result of an increased reaction force. The reaction force arises as a result of the reactive action on the drop of the jet of metal vapor emanating from the active spot, i.e. the area of the drop surface with the highest temperature. The reaction force prevents the separation of the drop from the end of the electrode, and being significant, it can cause metal transfer with a characteristic repulsion of the drops away from the arc, accompanied by a large spatter of the metal. The action of this force is an order of magnitude lower on reverse polarity (when the electrode is an anode) than on a straight line (when the electrode is a cathode).
In the summary diagram below areas of recommended combinations of arc voltage and welding current for welds of various types and different spatial positions are shown.
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Effect of torch position and welding technique on weld formation.
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Advantages and disadvantages
The main advantages of the MIG/MAG welding process are high productivity and high quality weld. High productivity is explained by the absence of time losses for changing the electrode, and also by the fact that this method allows the use of high welding current.
Another advantage of this welding method is the low heat input, especially when welding with a short arc (when welding with short circuits), which makes this method most suitable for welding thin sheet metal, as well as for welding in all spatial positions.
These advantages make MIG/MAG welding particularly well suited for robotic welding.
The disadvantages of this process compared to coated electrode welding include the following:
Equipment is more complex and more expensive;
- it is more difficult to weld in hard-to-reach places, since the torch, as a rule, is larger than the electrode holder and must be close to the welding zone, which is not always possible;
- more complex relationship between welding parameters;
- higher demands are placed on the preparation and cleaning of the edges;
- stronger radiation from the arc.
MIG/MAG welding with flux-cored wire
Flux cored wire welding can be performed on the same equipment as solid wire welding. Abbreviated name of this process, adopted abroad - FCAW (Flux Cored Arc Welding).
Cored wire is a tube of unalloyed steel filled with powder (flux). The design of some types of flux-cored wires is shown below.
Each type of flux cored wire has its own flux composition. Through the flux, it is possible to change the characteristics of the arc and the transfer of the electrode metal, as well as the metallurgical features of the formation of the weld. Thanks to this, it was possible to overcome some of the disadvantages inherent in the MAG welding process with solid wire. For example, flux-cored wire makes it possible to introduce alloying elements through the flux into the weld metal, which cannot be done in the case of using solid wire, due to the deterioration in the nature of drawing.
Typically, gas shielding in FCAW welding is provided by gas supplied from outside (Gas-shielded FCAW - FCAW-G). However, wires have been developed in which a sufficient amount of shielding gas is produced by the decomposition of the flux upon heating; this is the so-called self-shielded flux-cored wire welding process (Self-shielded FCAW - FCAW-S).
In fact, flux-cored welding is just a special type of gas-shielded welding process. Therefore, it is characterized by the same features as other gas-shielded welding processes, since it also needs effective gas shielding of the welding zone. For example, the requirement to maintain a minimum distance between the gas nozzle and the workpiece is also valid for FCAW welding. Measures must be taken against drafts from open doors and windows, as they can blow the shielding gas to the side. The same applies to air flows from ventilation systems and even from air-cooled welding systems.
Functions of Flux Cored Wire Core
The composition of the flux is developed according to the field of application of the flux-cored wire. The main function of the flux is to clean the weld metal from gases such as oxygen and nitrogen, which have a negative effect on the mechanical properties of the weld. In order to reduce the content of oxygen and nitrogen in the weld metal, silicon and manganese are added to the flux of the wire, which are deoxidizers, and also improve mechanical properties metal. Elements such as calcium, potassium and sodium are introduced into the flux in order to impart properties to the slag that improve the protection of the molten metal from exposure to atmospheric air during metal crystallization.
In addition, the slag provides:
Formation of the weld surface of the required profile;
- retention of a bath of molten metal during welding in vertical and overhead positions;
- reduction of the rate of cooling of the metal of the weld pool.
In addition, potassium and sodium contribute to a softer (stable) arc and reduce spatter.
alloying elements. Alloying the weld metal through a flux-cored wire is more preferable than alloying the weld metal through a solid wire (it is technically easier to introduce alloying components into the core of a flux-cored wire than to make a solid wire from an alloyed metal). The following alloying elements are commonly used: molybdenum, chromium, nickel, carbon, manganese, etc. The addition of these elements to the weld metal increases its strength and ductility, and at the same time, the yield strength, and also improves the weldability of the metal.
The composition of the flux determines whether the cored wire will be rutile or basic type (as is the case with coated electrodes).
Cored wires with a high content of metal powder (metal-cord) are also used. The flux of this type of flux-cored wire contains a large amount of iron powder, as well as additives of silicon and manganese, which are usually found in solid wires. Some wires also contain up to 2% nickel, which increases toughness at low temperatures.
Wires of the metal-cord type are used for welding butt and fillet welds in all spatial positions. They provide high deposition performance. The weld has a smooth surface and is free of slag, meaning that multiple passes can be made without first cleaning the previous bead.
Areas of use
Currently, flux-cored wire welding is used where coated electrodes were previously used, for example, in shipbuilding and other branches of heavy engineering in relation to thicknesses of more than 1.5 mm of products made of ordinary low-carbon, heat-resistant, corrosion-resistant and stainless steels.
Advantages of flux-cored welding
Welding with flux-cored wire is characterized by the following advantages:
The use of this welding method is beneficial from an economic point of view. It provides high speeds welding and long arcing intervals without interruptions (since there is no need for frequent change of electrodes);
- at the same time, there are practically no losses of the electrode wire;
- the method provides acceptable quality when welding metals characterized by low weldability;
- flux-cored wires of the main type are less sensitive to contamination of the base metal and provide a tight weld with a low tendency to crack;
- welding can be performed in all spatial positions;
- the arc and the weld pool are clearly visible;
- after welding, the seam requires only minor processing;
- the probability of formation of dangerous defects in the weld is lower compared to solid wire welding.
Disadvantages of the FCAW Welding Process
Some of the disadvantages of flux cored welding are listed below:
This welding method is very sensitive to drafts (open doors and windows), air currents from ventilation systems and even from air cooling systems of welding installations;
- additional costs for the construction of a shelter for the welding place when working outdoors;
- in case of insufficient knowledge of the welder of the features of the process and the relationship between the parameters of the mode, such serious defects in the weld as insufficient penetration are possible;
- large capital expenditures on equipment are required;
- when welding with flux-cored wire, especially self-shielded, a relatively large amount of smoke is released.
Electric arc welding can be carried out using equipment that generates direct or alternating current. If work on alternating current has no nuances in the matter of the correct connection of the mass and the electrode holder, then when welding on direct current, the polarity of the welding electrodes is of great importance.
General concepts
Depending on which pole of the welding machine is connected to the holder, the type and features of the welding mode are determined:
- Welding in direct polarity involves connecting the positive pole to the workpieces to be joined (mass), and the negative pole to the electrode holder.
- To perform work with reverse polarity, the poles are reversed (plus for the holder, minus for ground).
Regardless of which electrode polarity is used, DC welding has common features compared to using AC voltage:
Welding with straight polarity
With this method of connecting the electrodes, the workpiece is subjected to greater heating, and not the electrode.. This mode is characterized by the release of much more heat.
Therefore, straight polarity welding is recommended for the following operations:
- Cutting metal with any type of electrodes.
- Welding workpieces of considerable thickness.
- Working with metals having a higher melting point.
It is in these cases that it is necessary to heat the workpieces to more high temperatures, for the performance of these works requires a significant heat release.
Reverse polarity welding
In this case, the electrode is subjected to greater heating, so a smaller amount of thermal energy is transferred to the workpiece.
Due to this, reverse polarity electrodes allow you to work in a softer (delicate) mode.
This is true in many cases, for example, welding of stainless or thin sheet steel, alloys that are sensitive to heat.
Also, such a connection is used for work in a shielding gas environment or under a flux.
Determining the required polarity
There is a lot of controversy about how to determine the polarity of the electrodes during welding, with each side giving seemingly correct arguments. Opponents of the above version refer to textbooks on welding technology published in the middle of the last century, believing that the information indicated in them is the most correct.
But it is worth considering that since then there has been a significant improvement in welding equipment and consumables. Therefore, relying on recommendations regarding outdated technologies is still not worth it. The choice of polarity described above is considered the most correct.
There is another group of welders who believe that it is better (or rather more convenient) to perform any work exclusively on reverse polarity. This is primarily due to the fact that in this mode the electrodes stick less and there is no risk of burning through the metal. But the advent of inverter welding technology has solved this problem.
It is worth paying attention to the type of electrodes. There are brands that can only be used with direct or reverse polarity, violating the manufacturer's recommendations can not only complicate the welding process, but also make it impossible in principle.
To date, manufacturers already offer electrodes that can operate at any voltage and different polarity.
The correct choice of the polarity of the electrode connection helps to simplify the welding process and improve the quality of the weld.
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