- Welding technology overview
- Select consumables
- Preparation for work
- Welding modes
- Spatial position of the seam
- Related videos:
- Root, filling and cosmetic joints
Welding of the MIG-MAG type quite deservedly bears the title of an amateur way of joining metal structures; nevertheless, it is a separate technology, which has its own rules for performing work and requirements for the qualifications of a welder. All this – in our review on semi-automatic welding.
Welding technology overview
MIG / MAG is welding with metal wires (M – Metal) in inert (IG – Inert Gas) or active shielding (AG – Active Gas) gas. The wire feeds automatically, and the feed speed is adjusted according to the effective welding current. This is one of the main features of a semiautomatic machine, which is due to an important technological difference from welding with coated electrodes: instead of stabilizing the current strength, the power source provides a constant voltage.
As with other types of welding, the essence of MIG / MAG is the transfer of metal from the filler to the weld pool, but the process itself is very specific, which must be understood in order to master the necessary work skills. There are two mechanisms for metal transfer:
The first: when the wire touches the part to be welded, a short circuit occurs, the wire heats up, flows into the weld pool, partially evaporating, thereby supporting a short-term ignition of the arc, due to which the entire area of the weld pool is heated. After the edge of the wire has melted, the circuit is opened, then the metal touches the part again and everything is repeated again with a frequency of about 20-30 times per second. This mechanism is called cyclic closure..
Second welding mechanism – spray transfer. If the wire is fed at a sufficiently low speed at high voltage, and the gaseous medium contributes to the continuous burning of the arc, the additive will not touch the part itself, but will melt in small drops and fall into the crater of the weld pool, as well as onto the area adjacent to it. With this method of welding, a much more intense heating of a wide zone occurs, which is indispensable when filling large seams and welding thick workpieces. A characteristic difference is the clicking and at the same time hissing sound of an arc instead of a rhythmic crackle when cycling.
When welding with a semiautomatic device, three types of consumables are used, without the correct choice of which correct operation is impossible. The main consumable is a metal filler wire with a protective anti-corrosion coating. The wire may differ in alloy material, diameter and size of the coil, which is selected in accordance with the capabilities of the device. The additive material should be as close as possible to the composition of the parts to be welded..
For most structural steels, wire grades starting with SV-08G, according to foreign marking – ER70S, are optimal. For stainless steels, use wire SV-04X19, SV-07X25, SV-10X16, where the numbers in the second part of the designation indicate the content of nickel and chromium. Here it is necessary to be vigilant: the heterogeneity of the materials of the part and the seam dramatically worsens the strength characteristics of the joint. Wire grades D16, VAD23, AK4 are suitable for welding aluminum – it should also be selected in accordance with the presence of additional additives in the parts to be welded. Also, flux-cored wire can be used for welding, which does not require a shielding gas environment, but at the same time is more expensive.
Wire of all types can have a diameter of 0.6 mm and more with a pitch of 0.2 mm. The thinnest additive is used for spot welding of parts with a thickness of less than 1.5 mm, 0.8 mm – up to 4 mm, 1 mm – up to 6 mm. Amateurs rarely use wire over 1 mm, the reason for this is simple: a welding current of more than 200 A is required, which is typical only for professional welding machines.
The second consumable is shielding gas. For welding non-ferrous metals, in particular aluminum, an inert gas argon is used, and with a significant thickness of workpieces – a mixture of argon with helium. The disadvantage of argon is that in its environment the arc is maintained very actively, which does not allow cooking by cyclic closure, and this is the mode recommended for ferrous metals. Because of this, when welding structural and stainless steel, a gas mixture is used in which argon is from 75% by weight, the rest is carbon dioxide. For any grade of welding wire, the preferred gas mixture is indicated.
The third consumable material is collector tips. First, they should be changed depending on the diameter of the wire used. Secondly, as it works, the tip wears out and the diameter of the hole in it increases, which causes overheating of the torch and malfunctions in the operation of the welding machine, up to a complete jam of the wire.
Preparation for work
Before starting work, it is necessary to install a spool of wire on the pin of the brake drum and adjust the clamping screw, achieving the complete disappearance of radial play and inertia during rotation. After that, you should reposition the guide roller to the corresponding wire diameter, wipe the feed mechanism from dirt, lower the pressure roller and adjust the pressure so that the wire does not slip, but does not pinch too much.
The end of the wire must be guided into the rear socket of the plug of the connected torch. The hose must be fully extended. After pressing the push button, the wire must be passed along the welding sleeve, having previously unscrewed the current collector tip. When the wire is 3–5 cm out of the torch, the tip should be put back in place, the nozzle should be put on and the wire should be cut flush with the edge of the torch. It remains only to turn the valve on the cylinder all the way, adjust the pressure at the outlet of the reducer and the gas flow rate, after which you can start welding.
Before welding, parts must be carefully cleaned and sharpened so that there is no gap of more than 1 mm at the joint. The edges of parts 20-25 mm from the welding area must be free from paint, dirt, rust and even from a darkened layer of oxides, for a semi-automatic machine this is critical.
Even professionals, before welding any structure or part, perform several test seams on materials of the same type. The purpose of the tests is to establish optimal settings for stable arc burning and temperature conditions corresponding to the thickness of the parts to be welded..
There are two regulators for setting the welding mode: V – voltage and A – wire feed speed and corresponding welding current. There is also a toggle switch for switching the speed mode: with a wire of 0.8 mm or less, you should cook at an increased speed, a thicker one at a reduced one. Some machines have a third regulator of the inductive current component, it is designed to adjust the profile of the weld.
A test setting is performed directly near the device, while the controls should initially be turned to the extreme left position. Once the arc has been struck, the feed rate and voltage must be gradually increased to achieve the correct welding condition. For a seam depth of up to 2 mm and a width of up to 4 mm, it is optimal to weld in a loop. The speed must be increased until the rare clicks are replaced by a stable crackle with a frequency of about 20 Hz. If at the same time gaps are heard, the voltage should be slightly increased, if the metal is heavily splashed – reduce.
In cases where excess metal accumulates in the weld pool, the feed rate should be reduced or the torch movement should be accelerated, but only if the temperature allows it. To increase the melting rate, the torch nozzle must be guided closer to the part. If it is required to apply a filling seam with a thickness of more than 2 mm or a width of 5–7 mm, the welding is carried out by spraying, for which the voltage must be raised almost to the maximum. At the same time, the feed rate increases from zero until the moment when the machine starts to cook in a cyclic mode, and then decreases to an acceptable convenience in guiding the weld in accordance with its width and depth.
The inductance setting is done after the welding mode is stable. An increase in inductance leads to an increase in the arc temperature, due to which the bead of the seam spreads more and becomes flatter, however, only the top layer of the material warms up. At low inductance, heating is carried out deep into the seam, but the bead is more convex. The inductance must be adjusted so that the edges of the roller melt and gently mate with the adjacent surfaces.
Spatial position of the seam
The advantages of semi-automatic welding are most pronounced when welding bulk structures, where it is necessary to periodically change the position of the seam. At the same time, minimal adjustments are made to the settings; in most cases, additional manipulations are not required. Nevertheless, the sewing technique has specific differences..
Beginners should learn the basics of semiautomatic welding in the lower seam position. The torch is held at a 60 ° angle with the wire pointing towards the seam. It is extremely important to maintain a constant distance from the surface to the nozzle of about 5–10 mm. By lifting the torch vertically, the temperature can be increased and the flatness of the roller can be adjusted without changing the machine settings. The torch guiding speed should be constant and at the same time correlated with the wire feed speed so that the wire is always in the front part of the crater and the same thickness is maintained along the entire length of the weld, while the molten metal should solidify in wavy growths 10-15 mm from the weld pool.
After mastering the seams in the lower position, you can go to the horizontal one. The torch is held at 45 ° to the surface and directed upwards at an angle of 15–20 ° from the vertical to compensate for the fluidity of the metal. The burner is guided forward from itself, the wire is held on the leading edge of the crater. It is recommended to slightly lower the wire feed and seam tracking speed for better control over the weld pool..
Vertical seams are made using a similar welding technique, but the nozzle must be held parallel to the seam at an angle of 45 ° to the surface, the wire is guided to the center of the welding crater. The movement of the burner is carried out from top to bottom, it must be fast enough to overtake the falling drop of metal. Ceiling joints are not much more difficult to cook, but much less convenient. The burner must be guided with the nose forward, the wire is directed to the front tap of the bath and the joint in front of it. The ceiling joint is thin enough to prevent molten metal from flowing down.
Root, filling and cosmetic joints
In conclusion, we should talk about the difference in welding with a semiautomatic device of parts of different thicknesses. The practical maximum of the seam thickness when working with amateur devices is 1.5 … 2.5 mm with a wire thickness of up to 0.6-1 mm. Deeper welds should be done in several steps.
Parts with a thickness of 1.5 mm or less are not welded with a continuous seam in order to prevent warping of the metal when heated. The seam consists of dots with a diameter of 3-4 mm, which corresponds to holding the button on the torch for about 1 second, located in increments of 10 to 25 mm. Parts up to 4 mm are welded on both sides: first, they are positioned with a gap of about 0.5 mm and a conventional weld is made from the front side in a cyclic mode. After that, the voltage is slightly raised and the penetration is performed from the inside out, leaving a thin, gentle bead of the seam.
Welding parts with a thickness of 6 mm or more requires thorough preparation: first, the edges are sharpened for a snug fit, then a steep bevel at 30 ° is removed from the front side, leaving a straight edge section from 1 to 2 mm at the bottom of the seam. Before welding, the parts must be reliably immobilized with the formation of a gap of 0.5-2 mm, depending on the thickness, for which they can be temporarily interconnected with plates on the edge from the back side.
The first stage is the implementation of the root seam. It is brewed in a cyclic closure mode, ensuring that the seam fills the bottom of the joint to the level where the chamfer begins. Then the apparatus is switched to the spray welding mode and the seam is filled to the full depth in several passes. In this case, it is imperative to ensure that the edges of the joint are thoroughly warmed up, as evidenced by the discoloration on the metal surface in the adjacent area. After the completion of the root joint and between the passes of the filling joint, the bottom of the joint must be cleaned with a metal brush, or better with an abrasive disc..
When the joint between the parts is filled almost flush with the surface, it is covered with a cosmetic seam. In this case, the feed rate is slightly reduced, and welding is carried out with a wide front – about 8–15 mm, depending on the thickness of the part. In this case, the wire is led from side to side along the front edge of the weld pool, the vibration speed should be so high that both edges are kept warm, while the beads of metal on the seam are small enough. In this case, the burner is led towards itself, the spout is directed towards the front of the bath. The edges of the cosmetic seam must be well melted to adhere to the metal surface, the height of the bead should be no more than 1.5–2 mm.