Welding Process

Introduction to Welding Classification Systems

The American Welding Society has developed comprehensive definitions for welding processes, making each definition complete enough to stand alone without reference to others. They define a process as "a distinctive progressive action or series of actions involved in the course of producing a basic type of result."

The official listing of processes and their grouping is shown in Figure 1, the AWS Master Chart of Welding and Allied Processes. The welding society formulated process definitions from an operational perspective rather than a metallurgical one, prescribing the significant elements of operation instead of metallurgical characteristics.

Figure 1: AWS master chart of welding and allied processes

According to AWS, a welding process is defined as "a materials joining process which produces coalescence of materials by heating them to suitable temperatures with or without the application of pressure or by the application of pressure alone and with or without the use of filler material."

Classification Principles of Welding Processes

AWS has grouped welding processes primarily according to the "mode of energy transfer," with a secondary consideration being the "influence of capillary attraction in effecting distribution of filler metal" in the joint. Capillary attraction distinguishes the welding processes grouped under "Brazing" and "Soldering" from "Arc Welding," "Gas Welding," "Resistance Welding," "Solid State Welding," and "Other Processes."

The welding processes, in their official groupings, are shown in Table 1, which also displays the letter designation for each process. These letter designations can be used for identification on drawings, technical documentation, and tables. Allied and related processes include adhesive bonding, thermal spraying, and thermal cutting.

Table 1. Welding processes and letter designation

Arc Welding Processes and Applications

The arc welding group includes eight specific processes, each distinct yet sharing similar characteristics.

Carbon arc welding (CAW), the oldest of all arc welding processes, is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a carbon electrode and the work-piece. No shielding is used. Pressure and filler metal may or may not be used." While its applications are limited today, variations like twin carbon arc welding remain popular.

Shielded metal arc welding (SMAW) evolved from the metal arc process and is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a covered metal electrode and the work-piece. Shielding is obtained from decomposition of the electrode covering. Pressure is not used and filler metal is obtained from the electrode."

Submerged arc welding (SAW) made automatic welding popular and is defined as "an arc welding process which produces coalescence of metals by heating them with an arc or arcs between a bare metal electrode or electrodes and the work piece. Pressure is not used and filler metal is obtained from the electrode and sometimes from a supplementary welding rod." This process is typically limited to flat or horizontal positions.

Gas tungsten arc welding (GTAW) was developed to address the challenge of welding non-ferrous metals like magnesium and aluminum. It is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a tungsten (non-consumable) electrode and the work piece. Shielding is obtained from a gas or gas mixture."

Plasma arc welding (PAW) uses "a constricted arc between an electrode and the work piece (transferred arc) or the electrode and the constricting nozzle (non-transferred arc)." This process is particularly useful for joining thinner materials.

Gas metal arc welding (GMAW), developed in the late 1940s for welding aluminum, has become extremely popular. It is defined as "an arc welding process which produces coalescence of metals by heating them with an arc between a continuous filler metal (consumable) electrode and the work piece. Shielding is obtained entirely from an externally supplied gas or gas mixture."

Flux-cored arc welding (FCAW), a variation of GMAW, uses a tubular electrode containing flux for shielding. Additional shielding may be obtained from an externally supplied gas or gas mixture.

Stud arc welding (SW) "produces coalescence of metals by heating them with an arc between a metal stud or similar part and the work piece." When the surfaces reach proper temperature, they are joined under pressure, with partial shielding potentially provided by a ceramic ferrule surrounding the stud.

Brazing and Soldering Techniques

Brazing is defined as "a group of welding processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal, having a liquidus above 450°C and below the solidus of the base materials." The filler metal is distributed between closely fitted surfaces by capillary attraction. In brazing, the base metal theoretically remains unmelted. Seven popular different processes exist within the brazing group, differentiated primarily by their heat sources.

Soldering follows similar principles but uses lower temperatures. It is defined as "a group of joining processes which produces coalescence of materials by heating them to a suitable temperature and by using a filler metal having a liquidus not exceeding 450°C (840°F) and below the solidus of the base materials." As with brazing, the filler metal is distributed by capillary attraction between closely fitted surfaces.

Oxyfuel, Resistance, and Solid State Welding

Oxyfuel gas welding (OFW) is "a group of welding processes which produces coalescence by heating materials with an oxyfuel gas flame or flames with or without the application of pressure and with or without the use of filler metal." This group contains four distinct processes, with classifications based primarily on the fuel gas used. The heat comes from the chemical reaction of burning gases.

Resistance welding (RW) produces coalescence "with the heat obtained from resistance of the work to electric current in a circuit of which the work is a part, and by the application of pressure." The processes within this group differ mainly in weld design and the machinery required. These processes are typically automated, with welding machines incorporating both electrical and mechanical functions.

Solid state welding (SSW) is "a group of welding processes which produces coalescence at temperatures essentially below the melting point of the base materials being joined without the addition of a brazing filler metal." This group includes some of the oldest welding methods, such as forge welding, as well as cold welding, diffusion welding, explosion welding, friction welding, hot pressure welding, and ultrasonic welding—each using different energy forms to create welds.

Advanced and Specialized Welding Techniques

The "Other Welding Processes" group includes techniques that don't fit neatly into other categories. These include electron beam welding, laser beam welding, thermit welding, and various miscellaneous processes, along with electroslag welding. These specialized techniques often employ advanced technologies or unique energy sources to achieve specific joining results for particular applications or materials.

Conclusion

The diverse range of welding processes represents the evolution of materials joining technology from ancient forge welding to today's high-tech laser and electron beam methods. Each process offers specific advantages for particular applications, materials, and production requirements. Understanding the classification, principles, and characteristics of these processes is essential for proper process selection and implementation in manufacturing and fabrication settings.