How does the Welding Process Work?

Every type of welding is carried out through specific  procedures and machinery. However, a general procedure that unites various welding processes can be described.

To perform a weld between two parts, it is  necessary to prepare beforehand the two edges of the joint through a process known as beveling. Afterwards, the joint is heated to various temperatures depending on the process used.

Welding can  be done through fusion (solid/liquid phase transition) or in the solid state. In the first case, it is necessary to reach the melting temperature of the base materials used; in the second case, temperatures lower than the melting point are required, along with the application of pressure between the edges to be joined.

The heat required for the process can be reached through several systems:

• A flame produced by the combustion of a gas with air or oxygen.
• An electric arc formed between two electrodes (one of which may be the piece itself).
• Electric resistance generated by the Joule effect when current passes through the pieces to be welded.
• A high-power laser or other energy delivery systems not based on flame.
• Friction, where the components heat up due to friction.

To achieve a durable, technically satisfactory, and defect-free weld, the fusion zone must be protected from oxidation, and the molten metal must be purified from slag. To prevent oxidation, welding must be carried out in an atmosphere  as oxygen-free as possible (inert); Therefore, substances like gases, borax, silicates, and carbonates are added near the fusion bath to create a “protective cloud” around it and to allow the expulsion of slag. In oxy-acetylene welding, a reducing atmosphere is created, while arc welding must take place in the atmosphere produced by the combustion of the electrode coating or under a gas flux.

Filler metal can come in the form of rods or continuous wire, which are brought to the fusion zone (flame welding and TIG welding) or constitute the electrode itself, which melts due to the electric arc it generates.

What are the Welding Processes?

Shielded Metal Arc Welding (SMAW)

Shielded metal arc welding (SMAW) is a manual process in which the heat source is the electric arc, formed between a coated electrode (held by the electrode holder) and the workpiece (base material). The heat generated from the arc causes rapid fusion of both the base material and the electrode (filler material).

Also known as SMAW (according to the American classification) or 111 (according to the European classification), it is typically used in the filling phase (filling) and sometimes, depending on the size of the piece to be joined in terms of diameters and thicknesses, in the finishing phase (cap) of the welded joint.

In some situations, this process is also used for root passes (root pass and/or hot pass), leveraging the high penetration capacity of the cellulose-coated electrode.

Tungsten Inert Gas Welding (TIG)

Tungsten Inert Gas (TIG) welding is a process in which the heat needed for welding is generated by an electrode made of tungsten or tungsten alloy, which does not melt.

The weld zone, molten metal, and the non-consumable electrode are protected from atmospheric influence by an inert gas supplied through the torch holder.

TIG welding can occur with the addition of filler material (filler rod) or by melting the base material through the heat produced by the electric arc.

Also known as Gas Tungsten Arc Welding (GTAW) or 141 (according to the European classification), TIG welding is generally used in the early passes of a welded joint (root & hot) due to its characteristic of providing high penetration.

Submerged Arc Welding (SAW)

Submerged Arc Welding (SAW) is an arc welding process that uses a continuous wire electrode under a layer of flux. The general morphology of the welding zone (i.e., the arc being submerged under flux) generates a large amount of heat, which, being shielded by the flux (a poor conductor of heat), remains localized in the welding bath.

Therefore, submerged arc welding allows for high welding speeds and deposition rates, making it ideal for thick filling applications or welds with large diameters and/or lengths, as well as for finish welding (cap).

Submerged arc welding is a process that can be fully automated and is capable of making both longitudinal welds in a flat position and circumferential welds on positioners.

Gas Metal Arc Welding (GMAW)

Gas Metal Arc Welding (GMAW) with or without gas protection (Flux-Cored or Self-Shielded Wire)
MIG (Metal Inert Gas) or MAG (Metal Active Gas) welding ( with the only difference between the two being the gas used to protect the welding bath) is a process developed after World War II and has steadily grown in terms of welded product output per year. One of the main reason for its development was the reduction of costs of electronic products, leading to the creation of semi-automatic welding machines at affordable prices for small and medium-sized companies.

Flux-Cored Arc Welding (FCAW) is an automatic or semi-automatic arc welding process. FCAW uses a continuously fed tubular electrode containing flux and a constant voltage or, less commonly, a constant current welding power supply. Sometimes, an external shielding gas is used, but often the flux itself provides the necessary protection by creating both a gas shield and slag to protect the weld.

Common Welding Defects

Various defects can occur during the welding process, affecting the quality and structural integrity of the weld. Here are some of the most common welding defects:

1. Porosity: The presence of small bubbles or pores in the weld zone due to the entry of gas during the fusion process. Causes include inadequate gas protection, contamination of filler material, or improper gas flow settings. Prevention can be done by involing the use of  appropriate shielding gases, thoroughly cleaning surfaces, and controlling gas flow.
2. Slag Inclusions or Foreign Material Inclusions: The presence of foreign material or slag in the weld zone. Causes include poor surface preparation and inadequate removal of slag between welding passes.
3. Cracks: Fissures or fractures in the welded zone. Causes include rapid cooling, residual stresses, and improper welding parameters.
4. Distortion: Unwanted deformation or shape changes in the welded materials. Caused by uneven cooling, residual stresses, or welding in difficult positions. This can be prevented with proper temperature control during cooling and managing residual stresses.
5. Lack of Penetration: The weld does not fully penetrate the base material. Causes include incorrect welding parameters and excessive travel speed.
6. Cold Welding: The temperature during welding is insufficient to fully melt the materials. Causes include inadequate power or excessive travel speed. This defect can be prevented by increasing the power and adjusting the travel speed.
7. Overlapping: Excessive overlap of weld passes, leading to protrusions. This is the result of  excessive overlap during the welding passes.