What is the technical application of welding
Welding - explanation from a technical point of view
The topic of welding began around 3000 BC with the Sumerians in southern Mesopotamia. Even then, people welded the same basic materials, here - gold with gold. The Egyptians later used welding processes to build cables from copper materials. But ancient welding processes no longer have much to do with modern welding applications of the 21st century.
Modern welding processes
Welding applications have been economically interesting since Oscar Kjellberg in 1907 had the idea of providing a stick electrode with a coating. The elements introduced in this way serve to improve the properties of the arc and those of the weld seam, as well as to protect the weld pool from atmospheric oxygen and thus from unwanted oxidation.
There are now various welding processes, all of which have special fields of application as well as advantages and disadvantages. DIN EN ISO 4063 provides a list of the welding processes. The most important ones are as follows:
- 111 manual arc welding (MMA welding)
- 121 Submerged arc welding with solid wire electrode (SAW welding)
- 131 Metal inert gas welding with solid wire electrode (MIG welding)
- 135 Metal active gas welding with solid wire electrode (MAG welding)
- 136 Metal active gas welding with welding powder-filled wire electrode (cored wire welding)
- 141 Tungsten inert gas welding with solid wire or solid wire filler (TIG welding)
Differentiation from soldering / welding
Another very similar joining process is soldering. In contrast to welding, the base material remains in a solid state when soldering.
Since metals tend to react with the environment at high temperatures (including burn-off of alloy components), protective atmospheres are built up during welding. The main aim is to protect the weld pool and the arc from the oxygen, nitrogen and other gases contained in the air. The protective gas is supplied on the one hand through discharge nozzles directly on the welding torch or other measures such as forming. The use of forming gases is preferably used for closed profile cross-sections such as pipes, but also with special devices when welding sheet metal. By closing the pipe with pipe plugs, trapped gas is kept in the profile. The gas is then passed into the hollow cross section through gas feedthroughs in the plug. In this way, the back of the weld pool is effectively protected against undesired gases. In welding processes such as MMA welding, a small amount of shielding gas is formed when the cover burns off. In submerged arc welding, too, a certain amount of shielding gas is created by the welding powder. A distinction is made between active, inert and mixed gases. The ISO 14175 standard sheet offers a selection of shielding gases.Note rule for MIG and MAG welding: Wire diameter * 10 = volume flow of shielding gas
If burn-off of alloy components during welding is not a problem (for example with unalloyed structural steels), active gases are usually used for welding. The best example here is pure carbon dioxide (CO2, carbonic acid). But other gases such as nitrogen (N2) are also used in welding technology. Incidentally, carbon dioxide is not obtained from our air, but from economical combustion processes such as burning lime or burning fossil fuels.
The gases used are described as active. However, the activity is very low. The purpose is to protect the weld pool from the ambient air.
If a reaction of the weld pool or the arc with the environment is to be almost completely prevented, inert gases are used. Frequently used products are argon (Ar) and helium (He). These are noble gases of the eighth main group of the periodic table. Noble gases are inert because atoms have a completely occupied (or empty) electron shell (see also Wikipedia: Noble gas configuration). This fully occupied shell prevents chemical compounds from being undesirably established with other atoms or molecules. These would need free electron pairs.
In most cases, argon is used for inert gas welding. If a base material has a high thermal conductivity (copper, aluminum), a mixed gas containing helium is used. Pure helium, on the other hand, is only used in special applications and is also very expensive.
Mixed gases are used in around 80% of welding applications. Here a synergy of properties of different gases is to be used. Typical mixtures have high proportions of CO2. Argon, CO2, O2, He, N2 are often added.
Selection of gases according to ISO 14175
Welding consumables used in welding
In many welding experiences, a filler metal is used in the welding process. This is either fed in manually, as in TIG welding or gas welding, or by a conveyor built into the welding device. It is important here that a filler material of the same type is always used when welding. Steel can only be welded with a steel filler metal.
Manual arc welding
Welding with an electric arc and stick electrode is called manual arc welding (process number ISO 4063: 111). Common abbreviations are MMA welding, MMA or MMAW (Manual Metal Arc Welding). The stick electrodes used are usually provided with a cover. This makes it possible to weld without protective gas measures. It is the same for welding outdoors or even under water. Most ferrous materials, nickel materials and other non-ferrous metals can be welded. The welding of aluminum materials is hardly used any more and is no longer considered normatively.
Coverings are used to protect the weld metal from atmospheric oxygen. When burned, these form a flue gas that surrounds the weld. Furthermore, there are slag formers that form a solid, glass-like and gas-tight layer over the weld seam. This can be removed after the welding curtain by lightly tapping with the welding hammer. Typical wrapping types are:
- Basic coatings (made from fluoro and calcite)
- Acid coatings (made of magnetite)
- Cellulose coverings (made of cellulose, also known colloquially as "paper electrode")
- Rutile coating (made of rutile TiO2)
Advantages and disadvantages of MMA welding
- Since no welding gas is required, the process can be used anywhere. Welding technology often takes place on construction sites
- Inexpensive devices. In contrast to large MIG / MAG systems, systems for MMA welding are more simple and therefore often cheaper.
- Due to the gently sloping characteristic curve of the welding machines, they are also often suitable for TIG welding. In some cases, the necessary devices are already present in the welding power source (device for gas supply, etc.).
- Welding with alternating and direct current possible (depending on the rod electrode, purely basic electrodes are normally welded with direct current at the positive pole. Others with alternating current or with direct current at the negative pole. Please note the manufacturer's information here)
- Quickly changeable electrode diameter. This enables quick adaptation to the welding task. Diameter standardized in DIN EN 759.
- High availability of stick electrodes. Ex suitable electrodes are offered in the trade for many purposes.
- Can be used in all welding positions.
- Low deposition rate. This is a slow and time-consuming process.
- Partly toxic and even carcinogenic substances in welding fumes. PPE required!
- High heat input.
- High demands on the skill of the welder.
- Possible problems with hydrogen. It is often necessary to re-dry the electrodes.
MIG / MAG (MSG) welding
The most common method in craft businesses is metal active gas or metal inert gas welding. MIG refers to the process of arc welding with a melting wire electrode with an inert gas. MAG is welded with an active gas. MSG (gas-shielded metal arc welding) denotes the generic term for both processes. According to ISO 4063, the process number for MIG welding is 131 and for MAG welding 135. This welding process is usually used to weld steels, aluminum and nickel materials and their alloys.
Wire electrodes for gas-shielded welding are usually wound onto bobbins. Common diameters are 0.6, 0.8, 1.0, 1.2 and 1.6mm. Wire diameters of 0.9mm are also often used in the automotive industry. Wires with a powder filling are sold from 1.6 to 3.2mm and are commonly used for build-up welds. Solid wire electrodes are usually plus-polarized, flux-cored wire electrodes are minus-polarized.
The classification of different welding consumables is explained using the following example:
ISO 14341-A-G 46 5 M21 3Si1
- ISO 14341-A: Standard of the welding consumable
- G: wire electrode
- 46: Elongation at break
- 5: Impact work
- M21: shielding gas
- 3Si1: Composition of the additive
Types of arc in MIG welding
The material transfer from the welding torch to the component is implemented by the arc. The most important lever here is the pinch force, an electromagnetic force that acts on every conductor through which current flows. The pinching force increases with increasing amperage and, with low ampere settings on the welding power source, is only sufficient for a very large drop in material transfer. With increasing current strength, the electromagnetic forces cause the arc to constrict and thus ensure a fine droplet transition, up to a spray arc. However, in addition to the current strength set, the welding gas is also required for a spray arc. A gas with low thermal conductivity is required here.
A long arc occurs when welding with CO2-rich gases with at least 25% CO2. The arc is held for a long time and there are only "seldom" but violent short circuits. The high short-circuit currents cause high amounts of stritzer.
The short arc burns with constant short circuits. The short-circuit current is lower in contrast to the long arc. The material transfer takes place during the short circuit, and the arc is extinguished again and again during welding. The arc is reignited by increasing currents when it is immersed in the weld pool.
The very advantageous spray arc occurs at higher currents. This burns almost without short circuits and offers the following advantages:
- Good directional stability
- High penetration depth
- less energy loss
- lower combustion losses
- Less tendency towards undercuts
- less tendency to spatter and pores
Special position pulsed arc
An internal circuit in the welding power source can produce a pulsed arc. The main advantages are the lower exposure to heat because the arc does not burn all the time. But also a better penetration, since higher peak currents can be set.
Advantages and disadvantages of MIG / MAG welding
- Many areas of application. Applications exist for almost all common materials
- Good seam quality
- Quickly learnable manual skills
- All positions possible
- Small as well as high thicknesses can be welded
- High availability of welding consumables
- Can only be used in a protected environment. Shielding gas can be blown away in windy conditions
- Higher acquisition costs
- In some cases, seam defects can hardly be avoided
Tungsten inert gas welding (TIG) is a welding process that is assigned to tungsten inert gas welding in accordance with EN 14640. Terms such as TIG (tungsten inter gas welding) or GTA (gas tugsten welding) are used in other language areas. Process numbers according to ISO 4063 are:
- 141: TIG welding with solid wire electrode
- 142: TIG welding without filler material
- 143: TIG welding with filler wire
In the TIG process, the arc burns between the workpiece and a non-burning tungsten electrode. Tungsten is chosen as the electrode material here because a very high melting point is guaranteed. This prevents the electrode from melting off during welding. The filler metal is added manually or by machine. The wire is fed in cold or warm by means of resistance heating by an additional device. Joint welds and build-up welds are realized with the welding process. Inert gases such as argon or helium are preferably used. Sometimes there is an admixture of small proportions of hydrogen.
The specialty of this process is the possibility to produce precise and high quality seams. The main disadvantage is the low deposition rate. An application must be carefully considered.
As with MMA welding, welding power sources with a falling characteristic are used. This has the advantage that a constant current can be maintained with varying arc lengths. In contrast to MIG / MAG welding, where the arc length remains constant thanks to the automatically guided wire, with TIG welding every movement means a change in the arc length.
HF ignition devices are preferably used. This has the advantage that the arc can be ignited without contact. In this way, contamination of the weld pool with tungsten can be avoided. In addition, the tungsten electrode remains free of contamination from the base material and does not need to be reworked.
A tungsten electrode has a relatively high melting point. Approx. 3400 ° C and thus only wears out little with moderate amperage. The arc is only ignited in the protective gas, so that there is hardly any oxidation on the electrode. TIG electrodes are standardized in ISO 6848 and the properties can be influenced by adding various oxide additives:
Attention: Tungsten electrodes with thorium oxide are radioactive and should no longer be used!
Quality assurance in welding
DIN EN ISO 9000 ff. Forms the basis for many quality management systems. However, this standard requires separate approaches for “special processes”. DIN EN ISO 3834 can provide a remedy here. This offers a QM system for welding technology. So here “quality levels” are used as a basis for various requirements for welds. Many implementation standards (e.g. DIN EN 1090) refer to adapted levels of DIN EN ISO 3834.
Training as a welder
Contrary to popular belief, the welder's profession is not an independent professional training. Often other metal trades learn the skill of welding. Welders are qualified with an examination in accordance with DIN EN ISO 6906. Often the examination requires training. However, this is not mandatory. There is also no legal requirement that welders take a welder's examination in the unregulated area. Every professionally qualified company and every professionally qualified person may carry out tests in accordance with DIN EN ISO 6906. However, it is strongly recommended to take exams with a testing organization. In this way, care can be proven in the event of damage. If welders are tested within a company, this should be carried out by a welding supervisor in accordance with DIN EN 14731. Welding specialists, welding technicians and welding engineers have the necessary background knowledge to be able to take such an examination. Here, however, the obligation to provide evidence of correct qualification remains with the company.
Welding applications are also associated with considerable dangers. On the one hand, there is a risk of electric shock due to high currents and voltages. On the other hand, high levels of radiation from UV light and smoke development must be dealt with. The training standard for welders, DIN EN ISO 6906, always takes these hazards into account. Welding work should only be carried out by qualified personnel. Employers are also obliged to create a suitable working environment. Suitable high-quality personal protective equipment (PPE) and systems to dispose of the welding fumes are mandatory. It may also be useful to use a breathing mask (especially when welding chromium-containing materials (!!!)).
Disclaimer: This article does not constitute advice, just the personal opinion of the author.
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