- The essence of gas shielded welding
- Inverter, power, type of current
- Filler wire
- Preparation for work
- Semi-automatic welding technique
- Additional recommendations
What novice welder doesn’t dream of working with a semi-automatic MIG / MAG torch? But such equipment goes beyond the amateur price tag and it becomes scary that, as a result, the inverter will collect dust aimlessly. We hasten to reassure you: the technology of welding with a semi-automatic machine is quite simple.
The essence of gas shielded welding
Maintaining the temperature regime is considered a primordial problem when welding metals. If the heating is insufficient, then there will be no high-quality melting of the edges of the parts to be joined and their mixing with each other and the filler material. If the temperature rises, the metal begins to boil and evaporate, chemical reactions with atmospheric gases are activated. The situation is complicated by the fact that some metals and alloys begin to violently react chemically even at temperatures insufficient for the formation of a high-quality weld.
This problem is solved in different ways in different types of welding. The semi-automatic welding we are considering today, also called MIG / MAG, has two technological differences from other methods. The first is the supply of shielding gas directly to the melting zone, and the second is the adjustment of the feed rate of the filler material with the corresponding automatic adjustment of the welding current. The wire is fed mechanically using the broaching mechanism, while the correct ratio of speed and melting temperature ensures uniform filling of the seam and high productivity of welding.
Semi-automatic MIG / MAG welding scheme: 1 – welded material; 2 – welding bath; 3 – protective atmosphere; 4 – gas nozzle; 5 – contact tip; 6 – welding wire; 7 – arc
Active gases such as oxygen, nitrogen and hydrogen, or inert gases such as argon or helium, can be used as a protective medium. In practice, most welders use a 4: 1 mixture of argon and carbon dioxide, which is sufficient for most common tasks. It is necessary to vary the mixtures when welding specific materials such as brass, duralumin or high-alloy tool steels.
Despite the high cost of consumables (gas and wire), semi-automatic welding is best suited for beginners for two reasons. The first is the simplicity of making a weld, you just need to refer to the reference documentation to set the appropriate parameters of the inverter for a specific type of connection. The second plus is ergonomics: complete visual control over the state of the seam, no restrictions in spatial position and, most importantly, the ability to weld even very thin parts. Of the minuses, one can only name the attachment to the workplace, although when using gas cylinders of small capacity, mobility can be significantly increased.
Inverter, power, type of current
For semi-automatic welding, inverter and transformer converters of electric current with a built-in wire feeder are used. Due to the increased technical complexity, even the simplest machines of this category are comparable in price with semi-professional MMA machines for welding with coated electrodes..
The choice between transformer and inverter welding machine depends primarily on the conditions of use. Transformer devices have high reliability and resistance to loads due to the simplicity of the device. At the same time, there is an impressive list of disadvantages: low efficiency, sensitivity to supply voltage, interference in the supply network, low degree of stabilization of the welding current.
Transformer semiautomatic device
The operation of inverters is based on a multistage electronic conversion, the main elements of which are a small-sized pulse transformer and power switches that generate the nature of the current required for each step. Due to this, inverter devices are less sensitive to the quality of the supply voltage, their efficiency is higher due to the absence of energy losses for saturation of the massive magnetic core. To the pluses, you can add light weight and dimensions, the ability to fine-tune and a high degree of stabilization of the welding current. The main disadvantages are sensitivity to operating conditions: the ingress of dust and moisture inside is categorically unacceptable, while temperature fluctuations of 20-30 ° C lead to a change in the ratings of circuit components, due to which significant deviations in operating parameters are observed.
Inverter semiautomatic device
Nevertheless, it is inverter machines that are recommended for use by beginners who are mastering the basics of semi-automatic welding. As for the cost, there is a fundamental difference in devices in the price range of 15-25 thousand rubles. not visible. Power should be selected based on the expected thickness of the parts to be welded: up to 160 A of output current with a thickness of up to 4 mm and about 200 A with a thickness of 6–7 mm will be sufficient. It is also important to have additional functions, such as pulling the welding wire without energizing it, reversing the polarity, selecting the wire feed speed and displaying parameters. And, of course, don’t forget about brand trust..
There are two types of filler wire: conventional, requiring a shielding atmosphere for welding, and cored, containing flux. In the latter case, welding can be carried out without gas supply, however, this method can be recommended only to increase mobility in the case of non-critical connections. In general, most of the advantages of semi-automatic welding are manifested precisely when working with a shielding gas supply..
Welding wire differs according to three main criteria: alloy grade, diameter and coil weight and dimensions. In the last two respects, the choice depends entirely on the capabilities of the inverter and the size of the conductive torch tip. With the choice of a brand of welding wire, it is more and more difficult, here you have to refer to the reference documentation. The general rule is that in terms of composition, that is, in terms of the content of carbon and alloying components, the filler material should be as close as possible to the material of the welded product. In this case, the strength characteristics of the wire should slightly exceed the material of the part.
The diameter of the wire should also be selected based on the thickness of the metal to be welded. Wire 0.8 mm thick is suitable both for working with thin-walled products at a minimum feed speed, and for single-sided welding of parts up to 3 mm thick. A wire diameter of 1 mm is suitable for single-pass welding of parts up to 7–8 mm thick, however, this increases the welding current to 200 A. For more massive metal products, you can use a wire up to 1.6 mm, capable of welding parts up to 14–16 mm in depending on the type of connection, however, high-quality heating of the seam is possible only at currents of 300-350 A.
Copper-bonded welding wire ER70S-3 and SV08G2S is considered the most common in amateur welding. The main difference between them is in the content of silicon and deoxidizing elements. The second of these brands requires less attention to the preparation of parts: cleaning, removal of rust and oil contamination. However, if a high-quality connection of critical parts is required, it is better to choose a wire with a lower content of additives, which forms a seam of high uniformity, the surface of which is most suitable for applying paint and varnish coatings..
Preparation for work
Before proceeding with the first tests, you should prepare the equipment and parts. Before turning on the device in the network, the torch and the mass cable are installed. A reducer must be installed on the shielding gas cylinder and make sure that the pressure in the cylinder is higher than the residual pressure. After that, a hose is put on and clamped with a clamp on the outlet fitting of the cylinder, the opposite end of which is connected to the device. By opening the control valve, set the value on the flow reducer recommended by the manufacturer of the welding machine.
Before pulling the wire, make sure that there is no filler material in the torch wire bore from previous use. The spool is mounted on the unwinding rod so that the positioning of the pins and the bore holes match. The wire is passed through a rolling roller, the size of the groove in which corresponds to the diameter of the filler. After that, the pressure roller is installed in place, then with the help of the adjusting screw such a clamping force is set so that the wire does not slip in the groove. The wire must be pulled into the torch cord channel with the conductive tip removed. After that, a tip of the appropriate diameter must be screwed onto the burner and the nozzle must be replaced. After plugging into the network, the device is ready for use.
It remains only to prepare the details. Semi-automatic welding is extremely sensitive to surface quality. The parts to be welded must be brushed to a metallic shine along the entire width of the edge on which the seam will be laid. The question arises: why then do we need a wire with deoxidizing additives? The fact is that such a wire is very convenient for working with rolled metal products of primary use, which have been in conservation storage for a long time. In this case, residual rolling grease and small inclusions of rust do not significantly affect the quality of the seam. Beveling and chamfering is not required when welding parts up to 2–2.5 mm thick. Thicker metal parts must be prepared in accordance with GOST 5264-80 or 8713-79.
Semi-automatic welding technique
During operation, the torch must be held so that the wire is oriented to the plane of the parts to be welded at an angle of 45–60 °. The movement of the torch is carried out mainly from itself, that is, with the wire forward, guided by the position of the edges to be welded. In some situations, it is allowed to pull the burner towards you, for example, when laying a seam from a blind corner. The optimal distance from the edge of the nozzle to the part should be from 10 to 20 mm, depending on the welding mode.
The welding technique is extremely simple, however, it requires a preliminary adjustment of the machine on cuttings of the same material of equivalent thickness. The wire is brought close to the weld seam, after which you need to press the trigger on the torch handle. After the arc has been ignited, the torch moves smoothly forward, while small transverse vibrations occur, the total width of which corresponds to the thickness of the parts to be welded. Upon completion of the seam, it is necessary to move the burner in the opposite direction by 2-3 mm, wait half a second to fill the crater and release the trigger.
The setup of the device during test welding is carried out by changing the welding voltage and wire feed speed on the fly. In the latter case, the welding current also increases in proportion to the feed rate, this is the main characteristic difference of semi-automatic welding. The feed rate should be such that the welder has full control over the metal melting and the filling of the weld pool. If the wire feed is too slow, the arc will be interrupted continuously and a distinct rhythmic crackling sound will be heard. If the feed rate is too high, discoloration around the edges of the seam will be very contrasting, indicating that the metal is overheating..
Many modern machines have a welding current inductance setting. The essence of the adjustment is to impart additional energy to the electrons, which ensures a deeper heating of the weld. To weld thin parts in the lower position, the inductance must be zero, its increase is required only in those places where deep penetration is required and there is no way to keep the torch in one place for a long time. An example is a vertical seam from bottom to top, as well as general cases of welding parts with a thickness of 4 mm or more. In general, the influence of the inductive component of the welding current on the convenience of welding is difficult to describe; it is better to independently test welding in various modes.
Although semi-automatic welding does not require special tools and equipment to work in different spatial positions, each of them requires different voltage and wire feed speed settings. So, for vertical and overhead seams, the values of both parameters must be lower than when welding in the lower position. In the instructions for use of the semiautomatic welding machine, manufacturers indicate a table with reference parameters that is suitable for most situations. Minor adjustments may be made depending on steel grade and filler wire. In general, the higher the voltage, the wider the zone and the higher the heating rate of the metal, and the higher the wire feed speed, the less deep the penetration is performed and the wider the bead formed along the top of the seam.
Also, we must not forget that welding of very thin and very thick parts has significant differences in performance technique. Welding of metal with a thickness of less than 1 mm is carried out mainly pointwise with an interval of 4-7 mm, this method helps to avoid twisting of parts from overheating. Parts with a thickness of up to 4 mm inclusive are welded with a continuous single-pass seam, pre-positioning them with a gap equal to about half the metal thickness. Moreover, the more massive the parts, the more pronounced the temperature shrinkage of the seam. To compensate for this phenomenon, the gap is made expanding towards the end of the seam by 1-1.5 mm.
Parts with a thickness over 4 mm require beveling. In this case, the seam is filled with a multi-pass method. To increase the filling density in one pass, it is necessary to select the correct wire feed speed and, when welding, perform oscillatory movements, the direction of which corresponds to the type of connection. For example, in flat butt welding, the torch vibrates in the transverse direction, and in corner joints, in the longitudinal or circular direction. Very often, only the root of the seam is filled with semi-automatic welding, after which the connected parts are covered with a cosmetic passage using coated electrodes.