Types of welding
Contenido
Se distinguen los siguientes procesos de soldadura basados en el principio del arco eléctrico:.
Shielded Metal Arc Welding (SMAW)
Welding is distinguished by being SMAW (from Shielded Metal Arc Welding), or MMA (from Manual Metal Arc welding).
The most important characteristic of welding with coated electrodes is that the electric arc is produced between the piece and a coated metal electrode. The coating protects the interior of the electrode until fusion. With the heat of the arc, the end of the electrode melts and the coating is burned, so that the appropriate atmosphere is obtained for the transfer of molten metal from the core of the electrode to the molten pool in the base material.
These drops of molten metal fall covered with molten slag from the melting of the arc coating. The slag floats on the surface and forms a protective layer of the molten metal above the weld bead.
As the electrodes themselves provide the flow of molten metal, it will be necessary to replace them when they wear out. Electrodes are made up of two pieces: the core and the coating.
The core or rod is a wire (original diameter 5.5 mm). After obtaining the material, the manufacturer mechanically pickles it (in order to eliminate rust and increase purity) and then draws it to reduce its diameter.
The coating is produced by combining a wide variety of elements (various minerals, cellulose, marble, alloys, etc.) conveniently selected and tested by the manufacturers, who keep the process, quantities and dosages strictly secret.
The composition and classification of each type of electrode is regulated by AWS (American Welding Society), a world reference body in the field of welding.
This type of welding can be carried out under both direct and alternating current. In direct current the arc is more stable and easier to ignite, and splashes are infrequent; However, the method is not very effective with welding thick pieces. Alternating current makes it possible to use larger diameter electrodes, so performance on a larger scale also increases. In any case, current intensities range between 10 and 500 amperes.
The main factor that makes this welding process such a useful method is its simplicity and, therefore, its low price. Despite the wide variety of welding processes available, stick welding has not been displaced from the market. Simplicity makes it a practical procedure; All a welder needs to work is a power source, cables, an electrode holder and electrodes. The welder does not have to be next to the source and there is no need to use compressed gases as protection. The procedure is excellent for repair, manufacturing and construction work. Additionally, SMAW welding is very versatile. Its field of applications is enormous: almost all small and medium workshop welding jobs are carried out with a coated electrode; metal of almost any thickness can be welded and joints of any type can be made.
However, the stick welding procedure does not lend itself to automation or semi-automation; Its application is essentially manual. The length of the electrodes is relatively short: from 230 to 700 mm. Therefore, it is a process primarily for small-scale welding. The welder has to stop work at regular intervals to change the electrode and must clean the starting point before starting to use a new electrode. However, even with all this downtime and preparation time, an efficient welder can be very productive.
Advantages:.
Disadvantages:.
Characteristics of the electrodes.
Electrodes are classified by a combined system of numbers that identify them, and allow the most appropriate type of electrode to be selected for a given job.
E XX XX.
The combination of numbers allows you to identify:
to. The type of current to be used (direct current “DC”/alternating current “AC”).
b. The welding position that can be performed (Overhead, Vertical, Horizontal).
c. Tensile strength of welding.
The prefix “E” means “electrode for electric arc welding”.
The first two digits, of a total of four, indicate the tensile strength, in thousands of pounds per square inch.
The third digit indicates.
- All positions.
- Joints at an internal angle, in a horizontal or flat position.
- Flat position only.
The last two digits together indicate the current class to be used and the coating class.
10 - C C (+) cellulosic coating.
11 - C C (+) cellulosic coating.
12 - DC or AC (-) rutile coating.
13 - C A or C C (±), coating with rutile and iron powder (approximately 30%).
16 - C C (+) low level, hydrogen.
18 - DC or AC (±) low hydrogen and iron powder coating.
20 - DC or AC (±) low hydrogen coating with iron powder (approximately 25%).
24 - AC or DC (±) with rutile and iron powder (approximately 50% of the latter element).
Example.
E – 6013.
Electrode, with a resistance of 60,000 Lb per square inch, for all positions, for DC or AC and has a rutile coating with Fe powder.
Shielded non-consumable electrode welding (TIG)
Non-consumable electrode welding, also called TIG welding (acronym for Tungsten Inert Gas), is characterized by the use of a permanent electrode that is normally, as the name indicates, made of tungsten (tungsten). In this type of welding, a gas jet is used as a means of protection to prevent contamination of the joint. Both this and the following welding process have in common the protection of the electrode by means of said gas. The production of this type of electrodes is very expensive. Currently there are materials that replace it. In addition to reducing costs, they have thermal characteristics that improve the process.[2].
This welding method was patented in 1920 but did not come into widespread use until 1940, given its cost and technical complexity.
Unlike consumable electrode welding, in this case the metal that will form the weld bead must be added externally, unless the pieces to be welded are specifically thin and it is not necessary. The filler metal must be of the same or similar composition as the base metal; Even, in some cases, a strip obtained from the sheets themselves to be welded can be used satisfactorily as a filler material.
The injection of the gas into the welding area is achieved through a channel that reaches directly to the tip of the electrode, surrounding it. Due to the high temperature resistance of tungsten, which melts at 3410 °C, accompanied by gas protection, the tip of the electrode hardly wears out after prolonged use. It is advisable, however, to review the pointed finish, since an inadequate geometry would greatly impair the quality of the weld. Regarding gas, the most used are argon, helium and mixtures of both. Helium, an inert noble gas (hence the name inert gas welding), is most used in the United States, since it is obtained economically there from natural gas fields. This gas leaves a flatter and shallower weld bead than argon. The latter, more used in Europe due to its low price compared to helium, leaves a more triangular bead that infiltrates the weld. A mixture of both gases will provide a weld bead with intermediate characteristics.
TIG welding works with direct and alternating currents. In direct current and direct polarity, current intensities are of the order of 50 to 500 amperes. With this polarization, greater penetration and an increase in the duration of the electrode are achieved. With reverse polarization, the melt pool is larger but there is less penetration; The intensities range between 5 and 60 A. Alternating current combines the advantages of the previous two, but on the other hand it gives an arc that is not very stable and difficult to start.
The great advantage of this welding method is, basically, obtaining more resistant, more ductile and less sensitive to corrosion beads than in other procedures, since the protective gas prevents contact between the atmosphere and the weld pool. Furthermore, this gas significantly simplifies the welding of non-ferrous metals, as it does not require the use of deoxidizers, with the deformations or inclusions of slag that may entail. Another advantage of gas shielded arc welding is that it allows you to obtain clean and uniform welds due to the lack of fumes and projections; The mobility of the gas surrounding the transparent arc allows the welder to clearly see what he is doing at all times, which has a favorable impact on the quality of the weld. The cord obtained therefore has a good surface finish, which can be improved with simple finishing operations, which has a favorable impact on production costs. Furthermore, the deformation that occurs in the vicinity of the weld bead is lower.
Shielded consumable electrode welding (MIG/MAG)
This method is similar to the previous one, with the exception that in the two types of welding using a protected consumable electrode, MIG (Metal Inert Gas) and MAG (Metal Active Gas), this electrode is the food for the welding bead. The electric arc is protected, as in the previous case, by a continuous flow of gas that guarantees a clean and good joint.[3].
In MIG welding, as the name suggests, the gas is inert; It does not participate in any way in the welding reaction. Its function is to protect the critical area of the weld from oxidation and external impurities. The same gases are usually used as in the case of a non-consumable electrode: argon, less frequently helium, and a mixture of both.
In MAG welding, on the other hand, the gas used actively participates in the welding. Its zone of influence can be oxidizing or reducing, whether gases such as carbon dioxide or argon mixed with oxygen are used. The problem with using CO in welding is that the resulting joint, due to the oxygen released, is very porous. Furthermore, it can only be used to weld steel, so its use is restricted to those occasions in which it is necessary to weld large quantities of material and in which the resulting porosity is not a problem to be taken into account.
The use of MIG and MAG welding methods is increasingly common in the industrial sector. Currently, it is one of the most used methods in Western Europe, the United States and Japan in factory welding. This is due, among other things, to its high productivity and ease of automation, which has earned it a place in the automotive industry. Flexibility is the most outstanding characteristic of the MIG / MAG method, since it allows welding low-alloy steels, stainless steels, aluminum and copper, in thicknesses from 0.5 mm and in all positions. Gas protection guarantees a continuous and uniform weld bead, as well as free of impurities and slag. In addition, MIG / MAG welding is a clean method and compatible with all environmental protection measures.
On the contrary, its biggest problem is the need to supply both gas and electrode, which multiplies the possibilities of failure of the device, in addition to the logical increase in the cost of the process.
MIG/MAG welding is intrinsically more productive than MMA welding, where productivity is lost every time there is a stop to replace the consumed electrode. Material losses also occur with MMA welding, when the last part of the electrode is discarded. For every kilogram of coated electrode purchased, around 65% is part of the material deposited (the rest is discarded). The use of solid yarns and flux cored yarns has increased this efficiency to 80-95%. MIG/MAG welding is a versatile process, being able to deposit metal at high speed and in all positions. The procedure is widely used in thin and medium thicknesses, in steel manufacturing and aluminum alloy structures, especially where a large percentage of manual work is required. The introduction of tubular wires is increasingly finding its application in the strong thicknesses that occur in heavy steel structures.
Submerged arc welding (SAW)
Submerged arc welding (SAW Submerged Arc Welding) is an arc welding process. Originally developed by the Linde - Union Carbide Company"). It requires a continuous consumable electrode feed, either solid or tubular (flux). The molten zone and arc zone are protected from atmospheric contamination by being "submerged" under a blanket of granular flux composed of calcium oxide, silicon dioxide, manganese oxide, calcium fluoride and other compounds. In the liquid state, the flux becomes conductive, providing a current path between the electrode and the piece. This thick layer of flux completely covers the molten metal, thus preventing splashes and sparks, as well as reducing the intense ultraviolet radiation and fume emission, which are very common in manual shielded metal arc welding (SMAW).[4].
The SAW can be operated in both automatic and mechanized modes, although there is also the semi-automatic pistol SAW (portable) with pressure or gravity feed flow emission.
The process is typically limited to flat or horizontal welding positions (although horizontal position welds are made with a special structure to deposit flux). Deposition rates approach 45 kg/h compared to approximately 5 kg/h (maximum) for manual shielded metal arc welding (SMAW). Although the range of intensities normally used goes from 300 to 2000 A,[5] currents of up to 5000 A are also used (multiple arcs).
Whether single or multiple (2 to 5) there are variations of the electrode wire in the process. SAW uses a flat ribbon electrode coating (e.g. 60 mm wide x 0.5 mm thick). DC or AC power can be used, although combinations of the two are very common in multi-electrode systems. The most used power supplies are constant voltage ones, although current systems have a combination of constant voltages with a voltage detector in the power cable.
The filler material for the SAW is generally a standard wire, as well as other special shapes. This wire is normally between 1.6mm and 6mm thick. In certain circumstances, a twisted wire can be used to give the bow a swinging motion. This helps melt the solder tip to the base metal.[6].
Disadvantages: