Welding – explanation from a technical point of view

Table of Contents

The beginning of the subject of welding dates back to about 3000 years before Christ with the Sumerians in southern Mesopotamia. Even then, people were already welding similar base materials, here – gold with gold. Later, the Egyptians used welding techniques to build pipelines from copper materials. But ancient welding methods no longer have much in common with modern applications of the 21st century.

Modern welding methods

Welding applications have become economically interesting since Oscar Kjellberg had the idea to coat a stick electrode in 1907. The filler material used in this way served 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.

In the meantime, there are various welding processes, all of which have their advantages and disadvantages, including special areas of application. DIN EN ISO 4063 provides a clear list of the welding processes, the most important of which are listed below:

  • 111 Manual arc welding
  • 121 Submerged arc welding with solid wire electrode
  • 131 Metal-Intertgas welding with solid wire electrode
  • 135 Metal active gas welding with solid wire electrode
  • 136 Metal active gas welding with flux-filled wire electrode (flux-cored wire welding)
  • 141 Tungsten inert gas welding with solid wire or solid wire filler (TIG )

Separation from soldering/welding

Another very similar joining method is soldering. In contrast to welding, soldering leaves the base material in a solid-state.

Shielding gases

Because metals tend to react with the environment at high temperatures (e.g. burn-off of alloy components), protective atmospheres are created during welding. The main aim is to protect the weld pool and the arc from oxygen, nitrogen and other components contained in the air. The shielding gas is supplied on the one hand through outlet nozzles directly at the welding torch or other measures, such as forming. Forming gases are preferably used for closed profile cross-sections such as tubes, but also by special devices when welding sheet metal. By closing the tube with tube plugs, trapped gas is kept in the profile. The gas is then fed into the hollow cross-section through gas passages in the plug. In this way, the back of the weld pool is effectively protected against unwanted gases. In welding processes such as electric hand welding, a small amount of shielding gas is formed by burning off the coating. In submerged arc welding, a certain amount of shielding gas is also formed by the welding flux. A distinction is made between active, inert and mixed gases. A selection of shielding gases is provided in the ISO 14175 standard.

Rule of thumb for MIG and MAG welding: wire diameter * 10 = volume flow shielding gas

If burn-off of alloying elements during welding is not a problem (e.g. with unalloyed structural steels), welding is usually carried out with active gases. The active best example is pure carbon dioxide (CO2, carbonic acid). But also other gases such as nitrogen (N2) are used in welding technology. By the way, carbon dioxide is not extracted from our air, but from economic combustion processes such as lime burning or the combustion of fossil fuels.

The gases used are referred to as active. However, the activity is only deficient. The purpose is to protect the melt pool from the ambient air.

Inert gases

If a reaction of the weld pool or the arc with the environment is to be almost completely prevented, inert gases are used. Commonly 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: Inters Gases). This fully occupied shell prevents chemical bonds with other atoms or molecules from being formed undesirably. These would need free electron pairs.

In most cases, argon is used for inert gas welding. If a base material has high thermal conductivity (copper, aluminium), 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

Mixed gases are used in around 80% of applications. Here a synergy of properties of different gases is to be used. Typical mixtures have a high proportion of CO2. Argon, CO2, O2, He, N2 are often added.

Bezeichnung ISO 14175ZusammensetzungFunction
I1100% ArInert
I2100% HeInert
I30,5 – 95 % Ar, rest HeInert
M2115-25% CO2, Rest Arweakly oxidising
C1100% CO2oxidizing
N1100% N2slow-reacting
O1100% O2stark oxidizing
ZNon-captured gases
Selection Gases ISO 14175

Filler materials for welding

In many processes, a filler material is used during the welding process. This is fed either manually, as in TIG or gas welding, or by a conveyor system built into the machine. A filler material of the same type must always be used during welding. This means that steel can only be welded with a filler material of steel.

Manual Metal Arc Welding

Welding using an arc and stick electrode is called manual arc welding (process number ISO 4063: 111). The stick electrodes used are usually covered with a coating. This makes it possible to weld without protective gas measures. Thus it is also possible to weld outdoors or even underwater. Most ferrous materials, nickel materials and other non-ferrous metals can be welded. The welding of aluminium materials is hardly used any more and is no longer considered in standards. 

To protect the weld metal from atmospheric oxygen, coatings are used. These form a flue gas during the burning process, which surrounds the weld. Furthermore, slag formers are present which 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 a hammer. Typical coating types are:

  • Basic coatings (of fluorspar and calcite)
  • Acid cladding (made of magnetite)
  • Cellulose wrappings (made of cellulose, also colloquially called “paper electrode”)
  • Rutile coating (made of rutile TiO2)

Advantages and disadvantages of Metal Manual Arc Welding

Advantages:

  • As no gas is required, the process can be applied regardless of location. Welding technology often takes place on construction sites.
  • Low-cost devices. In contrast to large MIG/MAG systems, systems for manual electric welding are simpler and therefore often cheaper.
  • Due to the flat sloping characteristic curve of themachines, they are also often suitable for TIG. Sometimes even necessary devices are already available in the welding power source (a device for gas supply, etc.).
  • Welding with alternating and direct current is possible (depending on the stick electrode, pure 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. Therefore quick adaptation to the welding task. Diameter standardised in EN 759.
  • High availability of stick electrodes. Ex, the appropriate electrodes are available on the market for many applications.
  • Can be used in all positions.

Disadvantages 

  • Low melting capacity. Therefore slow and time-consuming process.
  • Partially toxic and even carcinogenic substances in the welding smoke. PPE required!
  • High heat input.
  • High demands on the welder’s manual dexterity.
  • Possible problems with hydrogen. Redrying of the electrodes often necessary.

Gas Metal Arc Welding

The most widespread process in craft enterprises is metal active gas or metal inert gas welding. MIG here refers to the process of arc welding with a melting wire electrode using an inert gas. MAG is welded with an active gas. GMAW (metal inert gas welding) is the generic term for both processes. According to ISO 4063, the process number for GMAW welding is 131 and for MAG welding 135. Usually, steels, aluminium and nickel materials and their alloys are welded with this process.

Filler material

Wire electrodes for gas shielded arc welding are usually wound on 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 usually used for flux-cords. Solid wire electrodes are usually plus-pole, cored wire electrodes are minus-pole.

The classification of different filler materials is explained using the following example:

ISO 14341-A-G 46 5 M21 3Si1

  • ISO 14341-A: Filler metal standard
  • G: Wire electrode
  • 46: Elongation at break
  • 5: Notched bar impact work
  • M21: shielding gas
  • 3Si1: composition of the filler material

Arc types in GMAW

The arc converts the material transition from the welding torch to the component. The most important lever here is the pinch force, an electromagnetic force that acts on every conductor through which current flows. The pinch force increases with increasing current intensity and is only sufficient for a very coarse-droplet material transition at low amperage settings on the power source. With increasing current intensity, the electromagnetic forces cause the arc to constrict, thus ensuring a fine drop transition, up to a spray arc. However, the prerequisites for a spray arc are not only the set current intensity but also the gas. A gas with low thermal conductivity is therefore required here.

Long Arc

A long arc occurs during the process in gases rich in CO2 with at least 25% CO2. The arc is kept long and only “rarely”, but violent short circuits occur. The high short-circuit currents ensure high spatter quantities.

Short Arc

The short arc burns under constant short circuits. The short-circuit current is lower than that of the long-arc. The material transfer takes place during the short circuit, and the arc is also extinguished again and again during welding. The arc is re-ignited by increasing currents when immersing into the weld pool.

Spray Arc

At higher current intensities, the very advantageous spray arc occurs. This burns almost without short circuit and offers the following advantages:

  • Good directional stability
  • High penetration depth
  • Less energy loss
  • Lower combustion losses
  • The lower tendency to notches
  • Less tendency to splash and pores

Special position impulse arc

An internal circuit can produce a pulsed arc in the power source. The main advantages are the lower heat effect because the arc does not burn constantly. However, there is also a better penetration, as higher peak currents can be set.

Advantages and disadvantages of GMAW

Advantages:

  • Many areas of application. There are applications for almost all common materials.
  • Good seam quality
  • Quick to learn manual skills
  • All positions possible
  • Small as well as high thicknesses weldable
  • High availability of filler materials

Disadvantages:

  • Can only be used in a protected environment. Protective gas can be blown away in windy conditions
  • Higher acquisition costs
  • In some cases, seam defects are hardly avoidable

TIG Welding

Tungsten inert gas welding (TIG) is a welding process, according to EN 14640. Terms such as TIG (tungsten inter gas welding) or GTA (gas tungsten welding) are commonly used in other language areas. Process numbers, according to ISO 4063:

Das Wolfram-Inertgasschweißen (WIG) ist ein Schweißverfahren, das nach EN 14640 als Wolfram-Inertgasschweißen klassifiziert ist. Begriffe wie WIG (Wolfram-Inertgasschweißen) oder GTA (Wolfram-Inertgasschweißen) sind in anderen Sprachräumen gebräuchlich. Verfahrensnummern nach ISO 4063:

  • 141: TIG with solid wire electrode
  • 142: TIG without filler metal
  • 143: TIG with cored wire

In the TIG process, the arc burns between the workpiece and a non-burning tungsten electrode. Tungsten is chosen here as the electrode material because a very high melting point is guaranteed. This prevents the electrode from melting during welding. Used filler material is fed in manually or mechanically. The wire is fed cold or hot using resistance heating by an additional device. The process is used for joint welding and build-up welding. Preferably inert gases such as argon or helium are used. In some cases, small amounts of hydrogen are added.

The special feature of this process is the possibility of producing precise and high-quality seams. The main disadvantage is the low melting rate. An application has to be weighed up carefully.

As in manual electric welding, power sources with a falling characteristic are used. This has the advantage that a constant current can be maintained at variable arc lengths. In contrast to GMAW, where the arc length remains constant due to the automatically guided wire, in TIG every movement means a change in the arc length.

High-frequency 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 by tungsten can be avoided. Furthermore, the tungsten electrode also remains free of contamination by the base material and requires less reworking.

Tungsten Electrodes

A tungsten electrode has a relatively high melting point. Approx. 3400°C and thus wears out very little at moderate current intensity. Also, the arc is only ignited in the inert gas, so that oxidations on the electrode hardly take place. TIG electrodes are standardised in ISO 6848, and the properties can be influenced by adding various oxide additives:

Material of the electrodeZeichenIdentification colour
Pure TungstenWPgreen
Tungsten with thorium oxideWT10
WT20
WT30
yellow
red
purple
Tungsten with zirconium oxideWZr3
WZr8
brown
white
Tungsten with lanthanum oxideWLa10
WLa20
WLa30
bleck
gold
blue
Tungsten with cerium oxideWCe20grey
Composition Tungsten electrodes according to ISO 6848

Attention: Tungsten electrodes with thorium oxide are radioactive and should no longer be used!

Welding quality assurance

ISO 9000 ff. makes the basis for many quality management systems. However, this standard requires separate approaches for “special processes”. EN ISO 3834 can provide a remedy here. This offers a QM system for welding technology. Thus, “quality levels” are used as a basis for various requirements. Thus many design standards (e.g. EN 1090) refer to adapted levels of ISO 3834.

Training to become a welder

Contrary to popular opinion, the profession of a welder is not an independent vocational training programme. Other metal professions often learn the manual skills. Welders are qualified with an examination according to EN ISO 6906, which is often followed by training. However, this is not mandatory. There is also no legal requirement for welders to pass a examination in the unregulated area. Every technically qualified company and every technically qualified person may carry out examinations according to EN ISO 9606. However, it is strongly recommended to take examinations at a testing organisation. This way, in case of damage, care can be proven. If welders are tested within a company, this should be carried out by a supervisor, according to EN 14731. Welding specialists, technicians and welding engineers have the necessary background knowledge to be able to conduct such an examination. However, the obligation to provide evidence of the correct qualification remains with the company.

Dangers during welding

Welding applications are also associated with considerable risks. On the one hand, high current intensities and voltages create the danger of an electric shock. On the other hand, high radiation exposure to UV light and the development of smoke must be dealt with. The training standard for welders, EN ISO 6906, includes a consideration of these dangers in any case. Thus, welding work should only be carried out by qualified personnel. Employers are also obliged to create an appropriate working environment. Suitable high-quality personal protective equipment (PPE) and systems for disposing of fumes are mandatory. If necessary, the use of a breathing mask (especially when welding chromium-containing materials (!!!)) is useful.

Disclaimer: This article does not constitute advice, but only the personal opinion of the author.

More To Explore

Quality Assurance

Visual test (VT) – with free template

Steel construction products that receive a declaration of conformity in the sense of EN 1090 (independent of the EXC) must, among other things, be subjected

contact us

Do you have questions, suggestions or need information for your listening?

Please contact us.

send us an email:

inbox@certreg.eu