Welding – An Overview

Welding is the process of fusing two or more metal or thermoplastic parts together to form a strong and permanent joint. The welding process involves heating the parts to be joined to a specified temperature, which can be achieved by various means such as an electric arc, gas flame or laser beam. The pieces are then brought close together and pressure is applied to fuse them together. A filler material may also be used to increase the strength of the weld and to fill any gaps between the parts.

There are many different types of welding, each with its own unique characteristics and advantages. Some of the more common types are

• Arc welding: This type of welding uses an electric arc to generate heat and can be further divided into several sub-categories such as Shielded Metal Arc Welding (SMAW) and Gas Tungsten Arc Welding (GTAW).
• Gas welding: This type of welding uses a flame produced by burning a gas such as acetylene, propane or natural gas. It is typically used to weld thin materials or for minor repairs.
• TIG (Tungsten Inert Gas) welding: This type of welding uses a tungsten electrode to create an arc and a separate filler wire to add material. It is mainly used for welding non-ferrous metals such as aluminium, magnesium and copper alloys.
• MIG (Metal Inert Gas) welding: This type of welding uses a wire electrode continuously fed through a gun and an inert gas, such as argon or helium, to shield the weld area. It is mainly used to weld ferrous metals such as steel and stainless steel.

Welding is a skilled trade that requires proper training and certification. Welders must be able to select the appropriate welding technique and materials for a given job, and be able to perform the welding process safely and accurately. Quality control is also an important aspect of welding, as the finished welds must be properly inspected to ensure that they meet the required specifications and standards.

Welding can also be hazardous if proper safety precautions are not taken, as it produces intense heat, ultraviolet and infrared radiation, and potentially toxic fumes. Therefore, welders must wear protective equipment such as gloves, goggles and masks, and work in well-ventilated areas to minimise exposure to hazardous materials.

History of welding

The subject of welding began about 3000 years before Christ with the Sumerians in southern Mesopotamia. Even then, people welded similar basic materials, in this case gold with gold. Later, the Egyptians used welding techniques to build pipes 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 become economically interesting since Oscar Kjellberg had the idea in 1907 to provide a rod electrode with a coating. The elements introduced with this 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.

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

• 111 Manual arc welding (manual electric arc welding)
• 121 Submerged arc welding with solid wire electrode (submerged arc welding)
• [131 Metal inert gas welding with solid wire electrode (MIG welding)
• 135 Metal active gas welding with solid wire electrode (MAG welding)](/welding/gmaw-mig-mag)
• 136 Metal-inert-gas welding with flux-cored wire electrode (flux-cored welding)
• 141 Tungsten inert gas welding with solid wire or solid wire addition (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 during brazing.

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 molten pool and the arc from oxygen, nitrogen and other gases present in the air. The shielding gas can be supplied either by discharge nozzles directly on the torch or by other means, such as forming. Shielding gases are preferably used for closed profile sections such as pipes, but also by means of special devices when welding sheet metal. By closing the pipes with pipe plugs, the trapped gas is held in the profile. The gas is then directed into the hollow section through gas passages in the plug. This effectively protects the back of the weld pool from unwanted gases. In welding processes such as manual arc welding, a small amount of shielding gas is produced by burning off the cladding. In submerged arc welding, a certain amount of shielding gas is also produced by the welding powder. A distinction is made between active, inert and mixed gases. A selection of shielding gases is given in ISO 14175.

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

Active gases

Where the burning off of alloy components during welding is not a problem (e.g. unalloyed structural steels), welding is usually carried out using active gases. The best example is pure carbon dioxide (CO2, carbonic acid). However, other gases such as nitrogen (N2) are also used in welding. Incidentally, carbon dioxide is not extracted from the air, but from economic combustion processes such as lime burning or the burning of fossil fuels.

The gases used are called active. But the activity is very low. The purpose is to protect the molten bath from the ambient air.

Inert gases

If the reaction of the weld pool or arc with the environment is to be almost completely prevented, inert gases are used. Argon (Ar) and helium (He) are commonly used. These are noble gases in the eighth main group of the periodic table. Noble gases are inert because the atoms have a fully occupied (or empty) electron shell (see also Wikipedia: Noble gas configuration). This fully occupied shell prevents the atoms from forming unwanted chemical bonds with other atoms or molecules. These would require free electron pairs.

In most cases, argon is used for intergas welding. If a base material has a high thermal conductivity (copper, aluminium), a mixed gas containing helium is used. Pure helium is only used in special applications and is very expensive.

Mixed gases

Mixed gases are used in about 80% of welding applications. The aim is to exploit the synergy between the properties of different gases. Typical mixtures have a high proportion of CO2. Argon, CO2, O2, He, N2 are often added.

Designation ISO 14175	Composition	Function
I1	100% Ar	Inert
I2	100% He	Inert
I3	0,5 – 95 % Ar, rest He	Inert
M21	15-25% CO2, rest Ar	weakly oxidising
C1	100% Ar	oxidising
N1	100% Ar	slow-reacting
O1	100% O2	strongly oxidising
Z	Gases not covered

Filler materials for welding

Many welding processes use a filler metal during the welding process. This is either fed manually, as in TIG or gas welding, or by a conveyor built into the welding machine. It is important that the same type of filler metal is used throughout the welding process. For example, steel can only be welded with a steel filler metal.

Manual arc welding

Welding by means of arc and stick electrode is called manual arc welding (process number ISO 4063: 111). Common abbreviations are E-hand welding, MMA or MMAW (Manual Metal Arc Welding). The applied stick electrodes are usually provided with a coating. This makes it possible to weld without protective gas measures. Thus it is also possible to weld in the open or even under water. 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 the standards.

To protect the weld metal from atmospheric oxygen, claddings are used. During burning, these form a flue gas that 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 welding hammer. Typical cladding types are:

• Basic claddings (made of fluorspar and calcite).
• Acidic claddings (made of magnetite)
• Cellulose sheaths (made of cellulose, also colloquially called “paper electrode”)
• Rutile coating (made of rutile TiO2)

Advantages and disadvantages of electric manual welding

Advantages:

• As no welding gas is required, the process can be used anywhere. Welding technology is therefore often used on construction sites.
• Cost-effective equipment. Compared to large MIG/MAG systems, manual electric welding equipment is simpler and therefore often less expensive.
• They are often suitable for TIG welding due to the flat, sloping characteristic of the welding units. In some cases, the necessary equipment is already included in the welding power source (gas supply unit, etc.).
• Welding with alternating and direct current possible (depending on the electrode, pure basic electrodes are usually welded with direct current on the positive pole. Others are welded with alternating current or direct current on the negative pole. Please refer to the manufacturer’s instructions)
• Quickly changeable electrode diameter. This allows quick adaptation to the welding task. Diameter is standardised in DIN EN 759.
• High availability of stick electrodes. Ex electrodes are available for many applications.
• Can be used in all welding positions.

Disadvantages:

• Low deposition rate. This makes the process slow and time consuming.
• Some toxic and even carcinogenic substances in the welding fumes. PPE required!
• High heat input.
• High manual dexterity requirements for the welder.
• Possible problems with hydrogen. Often requires re-drying of electrodes.

Gas metal arc welding

The most widely used process in the workshop is Metal Active Gas or Metal Inert Gas welding. MIG refers to the process of arc welding with a consumable wire electrode using an inert gas. MAG is welding with an active gas. MSG (Metal Inert Gas Welding) is the generic term for both processes …. According to ISO 4063, the process number for MIG welding is 131 and for MAG welding is 135. Steels, aluminium and nickel materials and their alloys are usually welded with this welding process.

Filler material

Wire electrodes for gas-shielded welding are usually wound on spools. 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 welding. Solid wire electrodes are usually plus-poled, flux cored electrodes are minus-poled.

The classification of different welding consumables is explained in the following example:

ISO 14341-A-G 46 5 M21 3Si1

• ISO 14341-A: Standard of the filler metal
• G: Wire electrode
• 46: Elongation at fracture
• 5: Notched bar impact energy
• M21: shielding gas
• 3Si1: Filler metal composition

Arc types in GMAW welding

The material transfer from the torch to the component is performed by the arc. The most important lever is the pinch force, an electromagnetic force that acts on any conductor through which current flows. The pinch force increases with increasing amperage and is only sufficient for very coarse material transfer at low amperage settings on the welding power source. As the amperage increases, the electromagnetic forces cause the arc to constrict, resulting in a fine droplet transition up to and including a spray arc. However, a spray arc is not only dependent on the current setting, but also on the welding gas. A gas with low thermal conductivity is required.

Long arc

A long arc occurs when welding with CO2-rich gases containing at least 25% CO2. The arc is maintained for a long time and short-circuits occur only “rarely” but violently. The high short-circuit currents result in high deposition rates.

Short arc

The short arc burns under continuous short circuits. In contrast to the long arc, the short-circuit current is lower. Material transfer takes place during the short circuit and the arc is also repeatedly extinguished during welding. The arc is re-ignited by increasing currents as it is immersed into the weld pool.

Spray arc

At higher currents the very advantageous spray arc occurs. It burns almost without short circuits and has the following advantages

• Good directional stability
• High penetration depth
• Less energy loss
• Lower burn-up losses
• Less tendency to notch
• Less tendency to spatter and porosity

Special position pulse arc

An internal circuit in the welding power source can be used to produce a pulsed arc. The main advantages are less heat exposure as the arc does not burn continuously. But also better penetration, as higher peak currents can be set.

Advantages and disadvantages of MIG/MAG welding

Advantages

• Many applications. There are applications for almost all common materials.
• Good seam quality
• Quick to learn manual skills
• All positions possible
• Both small and large thicknesses can be welded
• High availability of filler metals

Disadvantages

• Can only be used in a protected environment Shielding gas can be blown away in windy conditions
• Higher initial cost
• Seam defects can be difficult to avoid in some cases

TIG Welding

Gas Tungsten Arc Welding (GTAW) is a welding process classified as tungsten inert gas welding according to EN 14640. Terms such as TIG (Tungsten Inter Gas Welding) or GTA (Gas Tungsten Welding) are common in other language areas. Process numbers according to ISO 4063 are

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

In the TIG process, the arc burns between the workpiece and a tungsten electrode, which does not burn off. Tungsten is chosen as the electrode material because it has a very high melting point. This prevents the electrode from melting during welding. The filler metal is fed by hand or machine. The wire is fed cold or hot by means of resistance heating by an additional device. The welding process is used for joint welding and build-up welding. Inert gases such as argon or helium are preferably used. In some cases a small amount of hydrogen is added.

The special feature of this process is the ability to produce precise and high quality seams. The main disadvantage is the low melting capacity. Each application must be carefully considered.

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

HF torches are preferably used. This has the advantage that the arc can be struck without contact. This avoids contamination of the weld pool with tungsten. In addition, the tungsten electrode remains free of contamination from the base material and requires less reworking.

Tungsten Electrodes

A tungsten electrode has a relatively high melting point. Approximately 3400°C and therefore wears very little at moderate amperage. In addition, the arc is only ignited in the shielding gas, so there is little oxidation on the electrode. TIG electrodes are standardised in ISO 6848 and the properties can be influenced by the addition of various oxide additives:

Electrode material	Sign	Identification colour
Pure tungsten	WP	green
Tungsten with thorium oxide	WT10, WT20, WT30	yellow, red, violet
Tungsten with zirconium oxide	WZr3, WZr8	brown, white
Tungsten with lanthanum oxide	WLa10, WLa20, WLa30	black, gold, blue
Tungsten with cerium oxide	WCe20	grey

Composition of tungsten electrodes according to ISO 6848 Attention: Tungsten electrodes with thorium oxide are radioactive and should no longer be used!

Quality assurance during welding

DIN EN ISO 9000 ff. is the basis for many quality management systems. However, this standard requires separate approaches for “special processes”. This is where DIN EN ISO 3834 comes in. It provides a QM system for welding. It is based on “quality levels” for different welding requirements. Many execution standards (e.g. DIN EN 1090) refer to adapted levels of DIN EN ISO 3834.

Welder training

Contrary to popular belief, welding is not a stand-alone trade. The manual skills of welding are often learned in other metal trades. Welders are qualified by an examination according to DIN EN ISO 6906. The examination is often preceded by an apprenticeship. However, this is not compulsory. There is also no legal requirement for welders in the non-regulated sector to pass a welding examination. Examinations to DIN EN ISO 6906 can be carried out by any suitably qualified company or person. However, it is strongly recommended that the examinations are carried out by a testing organisation. In this way, due diligence can be demonstrated in the event of a claim. If welders are tested within a company, this should be done by a welding supervisor in accordance with DIN EN 14731. Welding experts, welding technicians and welding engineers have the necessary background knowledge to carry out such an inspection. However, it is the company’s responsibility to provide evidence of the correct qualification.

Dangers during welding

Welding applications are also associated with considerable dangers. On the one hand, there is the danger of electric shock due to high currents and voltages. On the other hand, high radiation exposure due to UV light and smoke development must be dealt with. The training standard for welders, DIN EN ISO 6906, always takes these dangers into account. 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 facilities for disposing of welding fumes are mandatory. It may also be advisable to use a respiratory mask (especially for welding on materials containing chrome (!!!)).

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