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ISO/TR 25901-3:2016 — Welding and Allied Processes: Vocabulary — Part 3: Welding Processes
Comprehensive terminology for welding process classification covering arc welding, resistance welding, laser welding, friction welding and more
1. Scope and Classification of Welding Processes in ISO/TR 25901-3
ISO/TR 25901-3:2016 defines the terminology for welding processes classified according to their physical characteristics and energy carriers. This part of the ISO/TR 25901 series focuses specifically on the processes themselves — how they are named, defined, and categorized within the ISO classification system. The standard covers two broad categories: pressure welding (welding with pressure, with or without heat) and fusion welding (welding with heat, with or without pressure), with each category further subdivided by the type of energy source used.
The classification system follows a logical hierarchy based on physical principles. The primary division is between processes that achieve coalescence through fusion (melting of the base materials) and those that achieve coalescence through pressure (mechanical deformation). Each category is then subdivided by the energy source: electrical (arc, resistance, induction, electron beam), chemical (gas, thermite), mechanical (friction, ultrasonic, explosion), and optical/radiant (laser, light). This systematic classification enables engineers to understand the relationships between different processes and to select the appropriate process based on material type, joint configuration, production volume, and quality requirements.
| Process Category | Energy Source | Example Processes | Typical Applications | Key Parameters |
|---|---|---|---|---|
| Arc welding (fusion) | Electrical arc | SMAW, GMAW, GTAW, SAW, FCAW | Structural steel, pipe, pressure vessels | Current, voltage, travel speed |
| Resistance welding | Electrical resistance | Spot, seam, projection, flash butt | Automotive body panels, wire mesh | Current, force, weld time |
| Gas welding | Chemical (flame) | Oxy-fuel, air-acetylene | Sheet metal, repair, brazing | Flame type, torch angle |
| Laser welding | Optical/radiant | CO₂ laser, Nd:YAG, fibre laser | Precision components, electronics | Power, focal position, shielding gas |
| Friction welding | Mechanical | Rotary friction, linear friction, FSW | Shafts, aerospace structures, Al alloys | Rotation speed, forge pressure |
| Solid-state (pressure) | Various | Cold welding, diffusion bonding, explosion | Dissimilar metals, cladding, electronics | Pressure, temperature, time |
The process classification in ISO/TR 25901-3 assigns a numeric designation to each process that is widely used in welding documentation. For example, process 111 (manual metal arc welding / SMAW), 131 (MIG welding / GMAW), and 141 (TIG welding / GTAW). These numeric designations are essential for unambiguous communication in welding procedure specifications (WPS) and welder qualification documents.
2. Key Terminology and Process Definitions
The ‘Pressure welding’ section (2.2.1) covers processes where coalescence is achieved through mechanical deformation, with or without supplementary heating. This category includes friction welding (rotary friction, linear friction, friction stir welding), ultrasonic welding, explosive welding, diffusion bonding, cold welding, and hot pressure welding. Each process is defined with specific attention to the mechanism of coalescence, the required equipment characteristics, and the typical material combinations for which each process is suitable. Friction stir welding (FSW), for example, is defined as a solid-state joining process where a rotating non-consumable tool with a profiled pin and shoulder is traversed along the joint line, generating frictional heat that plasticizes the material without melting it.
The ‘Fusion welding’ section (2.2.2) covers processes where coalescence is achieved through melting of the base materials, with or without the addition of filler metal. This is the largest section and includes arc welding processes (SMAW, GTAW, GMAW, FCAW, SAW, PAW), gas welding, laser beam welding, electron beam welding, and electroslag welding. Each process definition includes: the basic operating principle, the type of shielding used (slag, gas, or vacuum), the form of filler metal (if any), and the characteristic features that distinguish it from similar processes. For example, GMAW is distinguished from FCAW by the fact that the electrode in GMAW is a solid wire while FCAW uses a tubular wire with a flux core.
A common source of confusion in welding process terminology is the distinction between ‘submerged arc welding (SAW)’ and ‘flux-cored arc welding (FCAW)’. Both use flux for shielding, but SAW uses a granular flux that completely covers the weld zone while FCAW uses a flux contained within a tubular electrode. This difference has significant implications for weld quality, operator visibility, and process productivity.
3. Engineering Design Insights for Process Selection
From an engineering design perspective, the welding process classification in ISO/TR 25901-3 provides a systematic framework for process selection that is essential during the design and fabrication planning stages. Each process has characteristic advantages and limitations in terms of: joint accessibility (some processes require access from both sides), positional capability (some processes are limited to flat or horizontal positions), thickness range (from thin foil for laser welding to hundreds of millimetres for SAW), and metallurgical effects (heat input, cooling rate, and the resulting heat-affected zone properties).
The standard also clarifies terminology for variants and hybrid processes that combine two or more welding principles. Hybrid laser-arc welding (HLAW), for example, combines a laser beam and an arc welding source in the same weld pool, achieving the deep penetration of laser welding with the gap-bridging capability of arc welding. ISO/TR 25901-3 provides the terminology framework that allows such hybrid processes to be precisely described and specified in welding documentation, facilitating their adoption in advanced manufacturing applications such as shipbuilding, pipeline construction, and heavy equipment fabrication.
A shipyard implementing ISO/TR 25901-3 standardized process terminology across all their WPS documentation reported a 30% reduction in process selection errors during production planning. The standardized naming system eliminated confusion between GMAW (process 131) and FCAW (process 136), which had previously resulted in incorrect consumables being issued to production workstations.
Frequently Asked Questions
What is the difference between gas metal arc welding (GMAW) and gas tungsten arc welding (GTAW)?
A: In GMAW (process 131), the electrode is a consumable wire that melts to become the filler metal. In GTAW (process 141), the electrode is a non-consumable tungsten electrode, and filler metal is added separately. GMAW offers higher deposition rates for thicker materials, while GTAW provides superior control for thin materials and critical welds.
A: In GMAW (process 131), the electrode is a consumable wire that melts to become the filler metal. In GTAW (process 141), the electrode is a non-consumable tungsten electrode, and filler metal is added separately. GMAW offers higher deposition rates for thicker materials, while GTAW provides superior control for thin materials and critical welds.
How are hybrid welding processes classified in ISO/TR 25901-3?
A: Hybrid processes are classified according to the primary and secondary energy sources. For example, hybrid laser-arc welding is classified as process 142 (laser + GTAW) or 132 (laser + GMAW), depending on the arc process combined with the laser. The standard recognizes that hybrid processes require additional terms and definitions to describe the interaction between the two energy sources.
A: Hybrid processes are classified according to the primary and secondary energy sources. For example, hybrid laser-arc welding is classified as process 142 (laser + GTAW) or 132 (laser + GMAW), depending on the arc process combined with the laser. The standard recognizes that hybrid processes require additional terms and definitions to describe the interaction between the two energy sources.
Does ISO/TR 25901-3 cover thermal cutting processes?
A: Thermal cutting processes such as oxy-fuel cutting, plasma cutting, and laser cutting are covered in separate ISO standards (e.g., ISO 9013 for thermal cutting quality). ISO/TR 25901-3 focuses specifically on welding processes, but does include definitions for allied processes that are closely related to welding, such as brazing, soldering, and thermal spraying.
A: Thermal cutting processes such as oxy-fuel cutting, plasma cutting, and laser cutting are covered in separate ISO standards (e.g., ISO 9013 for thermal cutting quality). ISO/TR 25901-3 focuses specifically on welding processes, but does include definitions for allied processes that are closely related to welding, such as brazing, soldering, and thermal spraying.
