Structural Welding is a common topic of interest. Unfortunately, many structural engineers have had little or no exposure to welding. Without regard to your locale, structural welding is usually required to follow accepted standards, either by a building code, accepted practices or both.
Here are the four most common welding processes that are used in building and bridge construction, including some comments and limitations of each:
- Shielded Metal Arc Welding (SMAW)
This process is commonly known as “stick welding”. It is the most common field welding process used and is still used in a lot of shop welding as well. It is an easily controlled process and is not as “finicky” to the welding operator (welder) as some other processes. It consists of a welding machine, and positive and negative cable leads to complete an electrical circuit and thus, the arc that melts both the filler metal (welding electrode or rod) and the base metal. The electrode is coated with a meltable flux material that melts within the arc and covers the molten weld metal to protect it from impurities and anomalies caused by oxidation of the weld during the molten process and subsequent cooling. With an experienced welder, it can produce a strong, consistent quality weld that the structural engineer can rely on for its definable and predictable structural properties from which to achieve calculable designs.
This process can be used in any welding position. Its limitations are mostly based on the experience and capability of the welder and his/her capability to effect a proper, standard compliant weld.
- Gas Metal Arc Welding (GMAW)
This process is commonly known as “MIG” welding which stands for Metal Inert Gas, an older designation of the process.
This process consists of a welding machine, a wire feed device where the wire is the electrode and filler metal, a shielding gas and shielding gas delivery, all contained within a nozzle or “gun” that delivers both the wire and the shielding gas at the same point on the workpiece. This allows the shielding gas, which is an inert gas or combination of an inert gas and other gasses. The most common shielding gases are Argon (an inert gas) and carbon dioxide (not inert), with carbon dioxide being a minor percentage of the mixture.
The shielding gas serves the same purpose as the flux coating on an SMAW welding electrode.
One limitation of this process is control of the shielding gas. This is particularly evident when GMAW welding is attempted outdoors and the wind is blowing…even lightly. It blows the shielding gas away from the weld and causes numerous problems with the molten weld puddle including porosity, lack of fusion and other detrimental anomalies and defects. For this reason, it is better to use this process indoors in a welding shop rather than in the field.
- Flux Core Arc Welding (FCAW)
This process is almost identical to GMAW with the exception that instead of a solid wire electrode, the wire used in this process has a hollow center that is filled with flux. This makes the process somewhat a combination process between SMAW and GMAW with regard to protecting the molten weld metal. While shielding gas can also be used with FCAW, it is unnecessary as the flux core of the wire provides adequate protection to the weld.
This process can be used effectively in the field since wind mitigation is not necessary.
- Gas Tungsten Arc Welding (GTAW)
The GTAW process is somewhat different than the other processes in that it uses a tungsten electrode to cause an arc, but requires that the filler metal be fed manually into the arc for establishing a molten pool of filler and base metal. The technique is one that requires more practice and experience to do well. Further, the process also uses a shielding gas similar to GMAW as the filler metal is not coated with flux. In some respects it has similarities to oxy-acetylene welding and GMAW welding. It is the most common welding process for shop welding of aluminum and stainless steel, but because of the need for shielding gas, the process is generally not used in the field.