Welding is a method of joining parts from a homogeneous material: plastic with plastic, metal with metal. When welding, the contacting surfaces melt or tightly contract. In the contact zone is the fusion of two materials into one. The result is a strong tight connection of the two surfaces.
Welding is a combination of parts made from the same material to form a single design.
Melting metal welding is used for a quality hermetic connection of critical parts: pipeline elements, car body (bus, plane), metal garage walls and gates, sports horizontal bar supports, connection of reinforcement inside a concrete wall, and much more. What types of welding use modern welding technology? How is metal welding performed correctly?
Types of welding of metal surfaces
Welding of metals can be carried out with the melting of the contact surfaces or with their compression. In this case, the welding processes are called:
- fusion welding (or melting);
- welding by plastic deformation.
Classification of the main types of welding.
Deformation bonding can be performed with or without preheating. Deforming surfaces without heating is called cold welding. With dense compression, atoms of different materials are at close range and form interatomic bonds. Surface connection occurs.
During fusion welding, the joining surfaces are locally heated and melted. Often, a third (filler) material is used, which melts and fills the gap between two metals. At the same time, interatomic bonds between the main material and the additive (molten electrode) are formed in the liquid melt. After cooling and solidification, a solid weld is formed.
Local heating of parts for welding can be carried out by electric current or burning gas. Accordingly, according to the method of local heating, welding is divided into two types:
- electric (including electroslag, electrofluid, laser);
Names are determined by the heat source used. Electricity can work both directly and indirectly. With direct use, electric energy heats the metal and filler electrode due to the passage of current through it or the occurrence of an arc. In indirect use, various energies obtained from the action of electricity work: the energy of molten slag through which current flows, the energy of electrons in an electric field, the laser beam that occurs when electricity is applied.
Classification types of electric welding.
Welding of metal surfaces can be performed in manual or automatic mode. Some types of welded joints are possible only with the use of automation (for example, electroslag or seam), others are available for manual welding devices.
Electric welding is represented by two methods:
- electric arc;
- electric contact.
Let us consider in more detail how the joining of surfaces occurs during arc and contact welding.Back to table of contents
Electric arc welding of metals and electrocontact
Back to table of contents
Electric arc work
This type of welding uses to heat the heat of an electric arc. The arc formed between metal surfaces is a plasma. The interaction of metal surfaces with plasma causes their heating and melting.
The principle of operation of electric arc welding.
Electric arc welding can be performed using a consumable electrode or its non-consumable type (graphite, coal, tungsten). The melting electrode is simultaneously the causative agent of the electric arc and the supplier of the filler metal. With a non-consumable electrode, a rod is used to excite the arc, which does not melt. The filler material is introduced into the welding zone separately. When the arc is burned, the additive melts and the edges of the parts form, and the liquid bath formed after solidification forms a seam.
In some technological processes, the connection of surfaces occurs without the filing of filler material, only by mixing the two base metals. So produce welding tungsten electrode.
If an electric arc does not burn freely, but is compressed by a plasma torch, and the plasma of an ionized gas is blown through it, then this type of welding is called plasma. The temperature and power of plasma welding is higher, since during the compression of the arc, a higher temperature of its burning is reached, which allows welding of refractory metals (niobium, molybdenum, tantalum). The plasma gas is also a protective medium for the metals being joined.Back to table of contents
Protection of molten metal and alloying by electrical contact
Scheme of electrocontact welding.
If during the arc burning the metal surfaces protect against oxidation by gas or vacuum, then such a joint is called welding in a protective environment. Protection is necessary for welding chemically active metals (zirconium, aluminum), critical parts made of alloyed alloys. Protection of welding with other substances is possible: flux, slag, cored wire. Accordingly, the welding methods used were given the names: submerged arc welding, electroslag welding, vacuum. All this is a variation of the electric arc method, using a different protective environment to prevent melt oxidation, changing its chemical composition and loss of properties of the welded joint.
Electrical welding uses heat generated at the point of contact between the two surfaces to be welded. In this way, spot welding is performed: the parts are pressed against each other until they touch at several points. The points of contact will be places of maximum resistance and maximum heating of the surface. Due to this heating, the metal elements are melted and joined at the points of contact.Back to table of contents
Electric arc welding technology
The principle of connection and operation of electric arc welding.
The technology of welding metal using an electric arc consists in a sequence of actions to organize the work of the welding machine and directly perform welding.
Preparation consists in installing a welding inverter, selecting electrodes and performing the necessary bevel of the edge (surface preparation).
After the welding machine is installed at the welding site, the contact wire with the help of a “crocodile” (connecting terminal design) is fixed on one of the contact metal surfaces. The welding machine is turned on and its current is set by the current regulator. The strength of the current is governed by the size of the electrode and the thickness of the parts to be welded. For an electrode with a diameter of 3 mm, the current should correspond to 80-100 A.
If the surface of the metal is painted or oxidized to form a rust layer, it must be scratched with a metal brush to ensure proper contact in the joint.
The type of connection of the contact surfaces is determined:
- butt joint
Types of welded joints and seams.
Let us consider in more detail the features of welding of various types of joints. The butt joint often requires preliminary preparation of the edges of the surfaces to be welded: bevels are made along their edges. V-shaped bevels are made along the edges of sheets with a thickness of 5 to 15 mm, X-shaped bevels - on sheets with a thickness of more than 15 mm. Removing the V-shaped edge at the junction of surfaces allows to obtain a recess, which is used for welding. X-shaped edges suggest the presence of grooves and the implementation of welds on both sides of the joint.
Corner and T-joints can also be made with bevel edges (with cutting surface) or without bevels and cutting (depending on the thickness of the welded section).
T-shaped and angular connections allow you to connect parts of different thickness. The position of the electrode should be more vertical to the surface, which has a greater thickness.Back to table of contents
Electrodes for welding: types and selection
The electrode for welding is a metal rod coated with a coating. The coating composition is designed to protect the weld metal from burnout during oxidation. The flux displaces oxygen from the molten metal, which prevents oxidation, and gives off protective gas, which also prevents oxidation. The composition of the coating includes the following components:
The scheme of the electrode for welding: 1 - rod; 2 - transition section; 3 - coating; 4 - pin end without coating; L is the length of the electrode; D is the diameter of the coating; d is the nominal diameter of the rod; l is the length of the stripped end
- ignition and combustion stabilizers (potassium, sodium, calcium);
- slag-forming protection (spar, silica);
- gas-forming (wood flour and starch);
- refining compounds (for the removal and binding of sulfur and phosphorus, impurities harmful to the welding of metal);
- alloying elements (if the seam requires special properties);
- binders (liquid glass).
Commercially available electrodes have a diameter of 2.5 to 12 mm; for manual welding, 3 mm electrodes are most widely used.
The choice of the diameter of the electrode is determined by the thickness of the surfaces being welded, the required penetration depth. There are tables that give the recommended values for the diameters of the electrodes depending on the thickness of the surfaces being melted. You need to know that a small reduction in the diameter of the electrode is possible, while increasing the time to perform the process. The electrode of smaller diameter makes it possible to better control the process, which is important for a novice welder. A thinner electrode can be moved more slowly, which is important in the learning process.Back to table of contents
Characteristics of arc welding: definition and meaning
Before welding, the optimum characteristics of the welding process are determined:
Table selection of current for welding.
- Current strength (adjustable on the welding machine). The current is determined by the diameter of the electrode and the material of its coating, the location of the seam (vertically or horizontally), the thickness of the material. The thicker the material, the greater the current required to heat the penetration. Insufficient current does not completely melt the cross-section of the seam, as a result of lack of penetration. Too much current will lead to an excessively fast melting of the electrode when the base metal is still not melted. The recommended current value is indicated on the electrode package.
- Current properties (polarity and gender). Most welding devices use direct current, it is converted from the current by the rectifier built into the apparatus. At a constant current, the electron flow moves in one (specified by polarity) direction. Welding polarity determines the direction of flow of electrons. The existing polarities are expressed in the connection of the electrode and the part:
- a straight line - a detail to "+", and an electrode to "-";
- the reverse is the detail to “-”, the electrode to “+”. Due to the movement of electrons from the “minus” to the “plus”, more heat is generated at the positive pole of the “+” than at the negative “-”. Therefore, the positive pole is placed on the element that requires more significant heating: cast iron, steel 5 mm thick or more. Thus, direct polarity provides deep penetration. When connecting thin-walled parts and sheets, reverse polarity is applied.
- Arc voltage (or arc length) is the distance maintained between the end of the electrode and the metal surface. For an electrode with a diameter of 3 mm, the recommended arc length is 3.5 mm.
How arc welding is performed: technology
Back to table of contents
Start of welding: arc ignition sequence
Methods of ignition welding arc.
To create an arc, a new electrode is inserted into the clamp and tapped on a hard surface to remove the coating on its working end. Under the slag is a metal additive, the slag itself serves as insulation and closes the additive from ignition. After that, the electrode rod is brought closer to the metal surface at the minimum possible distance, 3-5 mm, avoiding contact. In this case, the electrode is held at an angle to the surface of the metal being welded. The technology of metal welding with an electrode regulates the angle of inclination of the electrode in the amount of 60-70ºC. Visually, this angle is perceived as almost vertical, with a slight bias.
To ignite the arc, the electrode is struck on the surface of the metal, like the ignition of a match on a box of sulfur.
If the electrode is too close to the metal surface to be welded, sticking and short-circuit will occur. For those who start cooking, the electrode sticks frequently. With the acquisition of the skill of the correct location of the electrode above the metal, support the optimal sticking distance should not occur. An adherent electrode can be torn off by tilting it in the other direction or turning off the welding machine.
If the electrode sticks too often, it is possible that the current is not high enough, it must be increased.
At the optimum correct distance of the electrode from the welding point (about 3 mm), an arc is formed with a temperature of about 5000-6000ºC. After ignition of the arc, the electrode can be slightly raised from the working surface by a few millimeters.Back to table of contents
Electrode transfer and weld pool
The scheme of the weld pool.
When the electrode and the base material melt, a weld pool is formed (a pool of molten metal).
The electrode and the arc together with the welded bath (molten metal zone) smoothly move along the connection line. The speed of movement of the electrode is determined by the rate of melting of the metal and the change in its color. The rapid movement of the electrode is carried out when working with thin sheets, quickly heating up and easily forming a welded bath. Slow motion electrode is applied on thick massive joints.
The shape of the movement of the electrode (straight, zigzag, loops) is determined by the width of the weld and the depth of penetration. The electrode can move straight (straight) with a small welding width. He can move loops, zigzag, if you need to boil sufficient width and depth of the connection. Variants of the movement of the electrode are shown in Figure 1.
Figure 1. Modes of movement of the electrode.
The protuberance of the seam after solidification of the weld pool is determined by the position of the electrode during welding. If the electrode is located almost vertically, the seam will be smooth, and penetration will be deep. A more inclined arrangement of the electrode forms a convex surface of the welded joint and a reduction in the depth of penetration. Tilting the electrode too much places the arc in the direction of the seam, making the welding process difficult to control.
For a high-quality compound, the molten bath should have thin edges, be sufficiently liquid and obediently move behind the electrode.
A bath in a light filter (through dark glass) looks like an orange surface with ripples. The appearance of the orange-colored bath (a drop of liquid melt) can be regarded as an indicator for the further movement of the electrode. That is, if an orange color appears, then move the electrode further a few millimeters.
Diagram of the device and the main indicators of the weld pool.
At the end of penetration it is necessary to increase the size of the weld pool. To do this, the electrode must be held above this point for a few seconds longer.
If a penetration of material occurs, it is necessary to reduce the amount of current and take another electrode (of smaller diameter). Burned holes are allowed to cool, knock down slag from them, and then brew.
After welding, you need to knock with a hammer on the weld. This will remove scale from it and visually check the welded joint for any discontinuities or poor penetration.Back to table of contents
Technology of contact, seam and gas welding of metals
The technology of metal welding by contacts has some peculiarities. The current is connected to the parts to be welded, after which they are brought together to contact. Contact points appear along the butt joint surface, in which for several seconds the metal is heated up before its melting. After that, the current is turned off and the butt surfaces are pressed against each other, ensuring close contact with the melting points.
Seam welding technology.
When seam welding works welding machine. This type of welding allows to obtain an even solid seam on long sheet surfaces. In the apparatus for seam welding electrodes are rotating rollers. Connected metal sheets are passed between them.
Gas welding uses heat to oxidize combustible gas with a high calorific value, such as acetylene, propane, or butane. Gas and oxygen is mixed inside the burner, from which the flame comes out.
Electroslag welding is a type of welding in a protective environment. In this technological operation, the slag is a protective material that protects the molten metal from contact with air. This type of welding is carried out automatically.Back to table of contents
Equipment: selection of welding machine and means of protection
To protect your eyes from burns during welding, you must use a mask with a light filter.
To perform welding, a large amount of electrical current is required, which is fed to the electrode. A modern device that provides a constant flow of current to the welding site is called an inverter. Older models of welding machines had bulky size and considerable weight, new inverters are easily transferred, do not cause subsidence of the network (this state is reflected in the loss of voltage and blinking of light bulbs throughout the apartment building or throughout the street of the private sector). Many modern inverters have short-circuit protection. When the electrode sticks, the inverter device automatically turns off.
Protective inventory: a mask with a light filter (dark glass). Light filter protects the eyes from burns. Without it, you can get corneal burns of varying degrees: from the lungs, when there is a feeling of presence of sand in the eyes, to severe, when it is impossible to restore vision.
The quality of protection of the light filter is determined by the number. The thicker the electrode and the greater the welding current, the more powerful a light filter is needed to protect eyesight.
Mastering the subtleties of working with a welding machine, maintaining the correct distance of the arc, the inclination of the electrode forms the skills of the welder. Professionalism is determined by the ability to manage the process, to obtain high-quality connection surfaces.
Modern welding inverters provide an opportunity to master the art of the welder independently and perform welding work with his own hands.