Arc Welding Processes

Introduction

Arc welding joins metals by producing an electric arc between an electrode and the workpiece, or between two electrodes. The arc converts electrical energy into intense heat, melting the joint faces and, in many processes, a consumable filler metal. On cooling, the molten weld pool solidifies to form a metallurgical bond. Arc welding is central to machine technology because it can join carbon steels, stainless steels, aluminium alloys, nickel alloys and many engineered components in workshops, fabrication plants, shipyards, pipelines and automated production lines.

The principal processes considered in this chapter are shielded metal arc welding (SMAW), gas metal arc welding (GMAW, commonly MIG/MAG), gas tungsten arc welding (GTAW, commonly TIG), flux-cored arc welding (FCAW), submerged arc welding (SAW), plasma arc welding (PAW) and electroslag welding (ESW). They differ in electrode type, shielding method, heat transfer, deposition rate, positional capability, equipment, operator skill and suitability for particular materials and geometries.

Successful process selection is not based on production rate alone. A competent welding technologist must balance joint design, material composition, thickness, welding position, required mechanical properties, access, distortion, quality requirements, environmental conditions, available equipment and total cost. The welding procedure must then be qualified and controlled in accordance with the applicable code or standard, such as ISO 15614, ISO 2553, ISO 9606, AWS D1.1, ASME Section IX or another contractually specified requirement.

Learning Objectives

By the end of this chapter, the reader should be able to:

1. Explain the electrical, thermal and metallurgical principles of arc welding.
2. Describe the equipment, operating variables, advantages and limitations of major arc-welding processes.
3. Select a suitable process for different materials, thicknesses, geometries and production conditions.
4. Establish a logical setup sequence and identify essential safety controls.
5. Interpret weld discontinuities and evaluate weld quality using appropriate inspection methods.
6. Compare productivity, heat input, positional capability and automation potential.
7. Apply process-selection reasoning to an industrial case.

1. Physical Basis of Arc Welding

1.1 The welding arc

An arc is a sustained electrical discharge through an ionized gas between the electrode and workpiece. Electrons and positive ions move through the plasma column, producing heat, visible light and electromagnetic effects. Heat is generated at the cathode, anode and in the arc column. Depending on polarity, the relative distribution of heat between the electrode and workpiece changes; this influences penetration, melting rate and electrode behavior.

A simplified electrical power relationship is:

where P is arc power in watts (W), V is arc voltage in volts (V), and I is welding current in amperes (A). The relationship assumes that voltage and current represent the effective arc values and does not by itself account for power-source losses.

Only part of the electrical power becomes useful heat in the workpiece. A practical heat-input expression per unit weld length is:

where H is nominal heat input per unit length, η is the process thermal efficiency (dimensionless), V is voltage, I is current, and v is travel speed in length per unit time. If voltage is in volts, current in amperes and travel speed in millimetres per second, H is obtained in joules per millimetre. Welding procedures frequently report heat input in kJ/mm. The formula is an engineering estimate: actual thermal efficiency varies with process, joint geometry, polarity, plate thickness and operating conditions.

For a constant-current SMAW or GTAW power source, a change in arc length tends to change voltage more strongly than current. For a constant-voltage GMAW or FCAW system, wire feed speed is closely related to current, while the power source adjusts current to maintain approximately constant arc voltage. These are control tendencies rather than absolute laws; modern inverter systems may combine multiple control modes.

1.2 Polarity and current type

Direct current electrode positive (DCEP) and direct current electrode negative (DCEN) produce different electrode and workpiece heating patterns. Alternating current (AC) periodically reverses polarity and can reduce arc blow, while also supporting special applications such as aluminium GTAW with oxide-cleaning action. The correct polarity depends on the electrode classification, shielding gas, material and manufacturer instructions.

Arc length affects voltage, bead width, penetration and spatter. Excessive arc length generally increases voltage, atmospheric exposure and spatter, while an excessively short arc can cause sticking, unstable transfer or inadequate fusion. Current, voltage, travel speed, electrode or wire diameter, torch angle, work angle and shielding flow must be treated as an interacting system.

1.3 Weld-zone metallurgy

The weld consists broadly of deposited weld metal, the fusion zone and the heat-affected zone (HAZ). The fusion zone is melted base metal and filler metal that solidifies. The HAZ does not melt but experiences a thermal cycle that can alter grain size, hardness, toughness and residual stress. In carbon and low-alloy steels, rapid cooling may produce hard, brittle microstructures; preheating and controlled interpass temperature can reduce this risk.

Hydrogen-assisted cold cracking requires three conditions: susceptible microstructure, diffusible hydrogen and tensile restraint. Control measures include low-hydrogen consumables, dry storage, clean joint surfaces, suitable preheat, controlled heat input and reduction of restraint where practicable. Stainless steels require control of contamination and heat input; excessive heat can promote distortion or undesirable phase changes. Aluminium alloys require effective oxide removal and careful control of porosity and hot cracking.

2. Principal Arc-Welding Processes

2.1 Shielded Metal Arc Welding (SMAW)

SMAW uses a flux-coated consumable electrode held in an electrode holder. The coating decomposes and melts to provide shielding gas and a slag layer. The operator establishes and maintains the arc manually, then removes slag between passes.

SMAW equipment is comparatively portable: a power source, welding leads, holder, work return, electrodes and personal protective equipment are required. It is effective outdoors because the flux-generated shield is less vulnerable to moderate air movement than an external gas shield. Common electrode classifications include cellulosic, rutile and low-hydrogen types, each providing different arc characteristics and hydrogen levels.

Advantages include low capital cost, wide material capability, portability and suitability for repair and site work. Limitations include lower deposition rate, frequent electrode changes, slag removal, variable operator dependence and difficulty in mechanization. Short electrodes and stub losses also reduce deposition efficiency. SMAW is commonly used for structural steel, maintenance, pressure equipment and pipeline work, subject to applicable procedure qualifications.

2.2 Gas Metal Arc Welding: MIG and MAG

GMAW uses a continuously fed solid wire electrode and an externally supplied shielding gas. MIG technically refers to inert-gas shielding, usually argon or helium, while MAG uses active gases such as carbon dioxide or argon–carbon dioxide mixtures. Industrial usage often calls the entire family MIG welding.

A wire feeder, power source, gun, contact tip, liner, shielding-gas system and work return form the principal equipment. The process can operate in short-circuiting, globular, spray or pulsed-spray transfer. Short-circuiting transfer is useful for thin materials and positional welding but may produce spatter and lack of fusion if poorly controlled. Spray transfer gives high deposition and a stable arc but generally requires higher current and is more restricted in position. Pulsed transfer alternates peak and background current, allowing spray-like metal transfer at lower average heat input.

GMAW offers high productivity, continuous wire feed, low slag production and excellent automation potential. Its limitations include sensitivity to wind, contamination, poor gas coverage, liner or contact-tip wear and incorrect parameter selection. Aluminium GMAW commonly uses argon or argon-rich mixtures and requires attention to wire feeding, cleanliness and oxide control.

2.3 Gas Tungsten Arc Welding (GTAW)

GTAW uses a non-consumable tungsten electrode and an inert shielding gas, normally argon, helium or a mixture. Filler metal may be added separately or omitted for autogenous welding. The process provides exceptional control of the arc and weld pool, making it suitable for thin sections, root passes, stainless steel, aluminium, nickel alloys and high-integrity piping.

The operator controls torch movement, filler addition and often current through a foot or hand control. Equipment commonly includes a high-frequency or lift-arc starting system, gas regulator, torch, tungsten electrode, collet and cooling system. Tungsten must be correctly prepared and must not contact the weld pool. Contamination can cause arc instability and tungsten inclusions.

GTAW produces clean, precise welds with no flux slag, but deposition rate is low and operator skill requirements are high. Shielding must continue during solidification and cooling to prevent oxidation. Alternating current is widely used for aluminium and magnesium because it combines electrode cleaning and penetration effects; direct current electrode negative is commonly used for steels and many nickel alloys.

2.4 Flux-Cored Arc Welding (FCAW)

FCAW uses a continuously fed tubular wire containing fluxing ingredients. In self-shielded FCAW, the flux generates the protective atmosphere; in gas-shielded FCAW, an external gas supplements the flux. The process may be semi-automatic or mechanized.

FCAW generally provides higher deposition rates and better tolerance of mill scale than GMAW. Self-shielded FCAW is useful outdoors and for structural fabrication because it is less dependent on a separate gas shield. Gas-shielded FCAW can produce high-quality welds at high productivity. Slag must be removed, and fumes, spatter and wire storage require control. Incorrect travel angle or inadequate cleaning can trap slag between passes.

2.5 Submerged Arc Welding (SAW)

SAW uses a continuously fed wire or strip electrode beneath a blanket of granular flux. The arc is not visible during normal operation because it is submerged under the flux. The flux provides shielding, forms slag and may influence weld chemistry.

SAW is highly productive and can use high currents and multiple wires. It is particularly suitable for long, straight or circumferential welds in thick plate, pressure vessels, beams, rail components and large fabricated structures. It normally requires a relatively flat or horizontal welding position and good joint fit-up. Equipment may include a tractor, gantry or rotating positioner.

The process provides deep penetration, high deposition rate and consistent weld quality when controlled. Limitations include high equipment cost, restricted access and position, flux handling and the need to remove or recycle unused flux. Moisture control is important because damp flux can contribute to hydrogen-related defects.

2.6 Plasma Arc Welding (PAW)

PAW constricts the arc through a fine copper nozzle, producing a high-energy plasma jet. A separate shielding gas surrounds the plasma. Operating modes include microplasma for very thin material, melt-in plasma and keyhole plasma for deeper penetration.

Compared with GTAW, PAW can provide a more concentrated arc, higher energy density and greater penetration. It is used for precision fabrication, stainless steel, nickel alloys, aerospace components, tube and sheet production, and some automated applications. The equipment is more complex and costly, nozzle condition is important, and the process can generate intense noise, ultraviolet radiation and fumes. Accurate torch height and gas control are essential.

2.7 Electroslag Welding (ESW)

ESW joins very thick sections in a single vertical progression. After an initial arc establishes a molten pool, granular flux melts and becomes electrically conductive. Subsequent heating is primarily produced by resistance to current flow through the molten slag rather than by a continuously exposed arc. Water-cooled shoes contain the molten metal and slag as the weld rises.

ESW offers very high deposition rates and can weld great thicknesses with minimal edge preparation. It is used for heavy structural sections, large castings and pressure-related fabrications. However, the high heat input produces a broad HAZ and coarse-grained structure, which may reduce toughness. ESW therefore demands careful procedure qualification, control of consumables and, where required, post-weld heat treatment. It is unsuitable for thin sections and many small or complex geometries.

3. Comparison and Process Selection

The following comparison supports an initial selection; final selection requires a qualified welding procedure and applicable code review.

A practical selection sequence is:

1. Identify base material, thickness, coating, cleanliness and metallurgical risks.
2. Define joint type, access, welding position, root condition and required penetration.
3. Determine quality class, inspection requirements, mechanical-property requirements and code obligations.
4. Compare deposition rate, duty cycle, labour, consumables, gas, flux, equipment and likely rework.
5. Consider environment: wind, temperature, humidity, confined space and site access.
6. Select a process and consumable combination, then qualify and document the procedure.
7. Verify production controls through inspection, monitoring and welder qualification.

For example, GTAW is often selected for a thin stainless-steel tube root where cleanliness and control dominate. FCAW or GMAW may be selected for subsequent structural fill passes. SAW may be preferred for a long weld on a thick plate assembly, whereas SMAW may be the only practical option for a remote repair.

4. Setup and Control of a Welding Operation

A sound setup begins with approved drawings, a welding procedure specification (WPS), material certificates and a joint-preparation inspection. The operator should verify material identification, bevel angle, root face, root gap, alignment, tack weld quality and cleanliness. Oil, paint, moisture, rust and scale can cause porosity, hydrogen pickup or lack of fusion.

The power source must match the process, current range, polarity and duty cycle. Leads should be correctly sized and connections tight. The work return should be placed to minimize current paths through bearings, instrumentation or other sensitive components. Gas systems require the correct cylinder, regulator, hose, flowmeter and leak check. Gas flow should be sufficient for stable shielding but not so high that turbulence draws air into the arc.

Consumables must be identified by classification, diameter and batch where required. Low-hydrogen electrodes and fluxes should be stored, dried and reconditioned in accordance with manufacturer and code requirements. Wire should be protected from rust and contamination. Tungsten preparation should be consistent with the selected current and polarity.

During welding, record or monitor current, voltage, travel speed, preheat, interpass temperature, shielding-gas flow and consumable details when required. A useful travel-speed relation is:

where v is travel speed, L is distance travelled and t is elapsed time. Units must be consistent. If a welder travels 180 mm in 30 s, the speed is 6 mm/s, or 360 mm/min. This simple measurement can reveal why a weld is too narrow, too wide or has excessive heat input.

Distortion results from non-uniform heating and cooling. Control methods include balanced welding sequences, intermittent welds where permitted by design, strongbacks and fixtures, presetting, reduced heat input, smaller welds and alternating sides. Restraint reduces movement but can increase residual stress and cracking; it is not a universal solution.

5. Weld Discontinuities and Evaluation

A discontinuity is an interruption in the expected structure; it becomes a defect only when it exceeds acceptance criteria. Common discontinuities include porosity, slag inclusion, lack of fusion, incomplete penetration, undercut, overlap, cracks, crater defects, excessive reinforcement, burn-through and distortion.

Typical causes and corrective actions include:

• Porosity: inadequate shielding, damp consumables, contamination or excessive arc length. Check gas coverage, wind protection, cleanliness and storage.
• Lack of fusion: low heat input, excessive travel speed, incorrect angle or a cold sidewall. Improve joint access, parameters and manipulation.
• Slag inclusion: inadequate interpass cleaning, poor angle or trapped slag geometry. Remove slag completely and correct bead placement.
• Undercut: excessive current or voltage, high travel speed or incorrect angle. Adjust parameters and technique.
• Cracking: hydrogen, high restraint, unsuitable chemistry, excessive hardness or poor crater filling. Apply qualified preheat, consumable and sequence controls.

Visual testing is the first and most widely applied method. Dimensional gauges, borescopes and lighting support visual inspection. Dye penetrant testing detects surface-breaking defects in nonporous materials. Magnetic-particle testing detects surface and near-surface discontinuities in ferromagnetic materials. Ultrasonic testing detects internal reflectors and is useful for thick sections. Radiographic testing provides a record of volumetric discontinuities but involves radiation controls. No method detects every defect; selection depends on material, geometry, orientation, thickness, acceptance standard and safety requirements.

6. Worked Example: Estimating Heat Input

A GMAW operation uses 24 V, 220 A and a travel speed of 300 mm/min. Assume a thermal efficiency of 0.80. Estimate nominal heat input in kJ/mm.

First convert travel speed:

300 mm/min = 5 mm/s.

Apply the heat-input relation:

The result is a nominal estimate, not a direct measurement of the thermal cycle. If travel speed is reduced while voltage and current remain unchanged, heat input per unit length increases. The resulting larger HAZ or slower cooling may improve fusion but increase distortion and may adversely affect properties in heat-sensitive materials. A qualified procedure must therefore consider more than this single calculated value.

7. Applied Case: Fabrication of a Stainless-Steel Process Skid

Background

The following is an illustrative industrial case. A fabrication company must produce a stainless-steel process skid containing 2 mm sheet panels, 8 mm structural brackets and thin-wall process tubing. Welds must be clean, dimensionally controlled and suitable for visual and leak inspection.

Problem

A single process would be inefficient. SMAW is unsuitable for the thin sheet, while high-productivity processes could introduce excessive heat or contamination. The plant also has limited tolerance for spatter and post-weld cleaning.

Analysis

The sheet and tubing require low heat input, accurate arc control and a clean weld surface. GTAW is appropriate for visible tube roots and thin sheet joints. The 8 mm brackets require greater deposition rate, but stainless-steel GMAW with an argon-rich gas and pulsed transfer can reduce spatter and control heat. Joint surfaces must be cleaned with dedicated stainless-steel tools to avoid iron contamination.

The design team specifies accessible joints, controlled root gaps and temporary fixtures that do not create crevices. The WPS defines filler classification, tungsten preparation, shielding-gas flow, current range, travel speed, preheat policy and maximum interpass temperature. Welders are qualified for the process and position.

Approach and outcome

GTAW is used for tube roots and thin panels, with back purging where the root surface must remain oxidation-free. Pulsed GMAW is used for brackets and longer fillet welds. Weld sequence is arranged symmetrically to reduce distortion. Visual inspection is performed on all welds; leak testing and selected penetrant testing are applied according to the quality plan. The result is a clean assembly with reduced rework and acceptable dimensional accuracy.

Learning points

The case demonstrates that process selection can be hybrid rather than exclusive. Geometry, thickness and quality requirements justify different processes within one product. Cleanliness, shielding and heat control are as important as nominal deposition rate. Procedure qualification and inspection planning must be integrated before production begins.

8. Safety, Standards and Professional Responsibility

Arc welding creates electrical, optical, thermal, fume, fire, noise and compressed-gas hazards. Controls include electrical isolation and sound equipment grounding, welding helmets with appropriate shade, flame-resistant clothing, gloves, ventilation or local exhaust, fire watches, safe cylinder storage, confined-space procedures and protection from ultraviolet radiation. Welding fumes may contain hazardous metals and fluorides; ventilation must be designed for the contaminant and task rather than relying only on a general fan.

Safety responsibilities include hazard assessment, training, equipment inspection, exposure control and compliance with local legislation. Welding procedures should identify essential variables and acceptance criteria. Traceability is especially important in pressure equipment, lifting structures, transport equipment and safety-critical products. Records should link material, consumable, welder, WPS, inspection results and repair history.

Digital power sources increasingly provide waveform control, parameter logging and remote monitoring. Robotic GMAW, FCAW and SAW systems use positioners, seam tracking, vision and adaptive control. Data analytics can identify drift in current, voltage, wire feed speed or gas flow. Artificial intelligence may assist defect classification and predictive maintenance, but automated recommendations require validated data, calibrated sensors and competent human oversight. AI does not replace code compliance, welding procedure qualification or responsible inspection decisions.

9. Limitations and Best Practices

No arc process is universally superior. High deposition can increase heat input and distortion; low heat input can reduce fusion or productivity. Gas-shielded processes are vulnerable to drafts, while flux processes create slag and fumes. Automated processes improve repeatability but require accurate fit-up and robust sensing. Manual processes accommodate variation but depend heavily on welder skill.

Best practice is to:

• select the process from the complete production requirement, not one parameter;
• use qualified WPSs and qualified welders;
• control consumable identity, dryness and storage;
• verify joint preparation and cleanliness before welding;
• monitor heat input, preheat and interpass temperature when specified;
• provide effective shielding and wind protection;
• inspect progressively rather than waiting until final assembly;
• document nonconformities and repair them using approved procedures;
• maintain equipment, calibration and traceability;
• review productivity together with safety, quality, energy and life-cycle cost.

Chapter Summary

Arc welding processes use an electric arc to generate heat for fusion and filler deposition. SMAW is portable and versatile; GMAW provides clean, continuous-wire productivity; GTAW offers precise control and high integrity; FCAW combines high deposition with flux protection; SAW is highly productive for long, heavy welds; PAW provides a concentrated, controllable plasma arc; and ESW joins very thick sections through conductive molten slag.

Process selection depends on material, thickness, joint geometry, position, access, quality, environment, productivity and economic conditions. Electrical power, heat input, polarity, arc length and travel speed influence penetration, HAZ behavior and distortion. Safe and reliable production requires correct setup, consumable control, qualified procedures, competent operators and appropriate inspection. Contemporary automation and data systems can improve consistency, but they must operate within validated engineering and regulatory controls.

Key Terms

Arc: Sustained electrical discharge through ionized gas between electrode and workpiece.

SMAW: Manual process using a flux-coated consumable electrode.

GMAW: Continuous solid-wire arc welding using externally supplied shielding gas.

MIG: GMAW using primarily inert shielding gas.

MAG: GMAW using active shielding gas.

GTAW: Welding with a non-consumable tungsten electrode and shielding gas.

FCAW: Welding with a tubular flux-cored consumable wire.

SAW: Welding beneath a blanket of granular flux.

PAW: Welding using a constricted plasma arc.

ESW: Vertical thick-section welding in which conductive molten slag supplies heat.

Heat input: Nominal energy transferred to the weld per unit length.

HAZ: Base-metal region altered by welding heat without melting.

Polarity: Directional relationship of electrode and workpiece in direct-current welding.

Deposition rate: Rate at which filler metal is deposited into the joint.

Preheat: Heating the base material before and during welding.

Interpass temperature: Temperature of the weld area immediately before the next pass.

Porosity: Gas cavities retained in solidified weld metal.

Lack of fusion: Failure of weld metal to fuse with a joint face or previous pass.

WPS: Welding procedure specification defining controlled welding variables.

HAZ toughness: Ability of the heat-affected zone to resist fracture under specified conditions.

Review Questions

5 Short Answer Questions

1. What is the function of flux in SMAW and FCAW?
2. Distinguish MIG from MAG welding.
3. Why is GTAW commonly selected for thin stainless-steel tubing?
4. Define heat input per unit length and identify its main variables.
5. Name two non-destructive testing methods and state the type of discontinuity each can reveal.

5 Long Answer Questions

1. Explain the operating principle, equipment, advantages and limitations of SMAW, GMAW and GTAW.
2. Compare FCAW and SAW for heavy structural fabrication, including productivity, shielding, position and inspection considerations.
3. Discuss how polarity, arc length, current, voltage and travel speed influence weld quality.
4. Explain the metallurgical significance of the fusion zone and HAZ, including hydrogen-assisted cracking.
5. Describe a complete setup and control sequence for a qualified arc-welding operation.

3 Analytical / Application Questions

1. A manufacturer must weld a long, straight, 30 mm thick carbon-steel seam in a controlled shop environment. Select a process and justify the choice against at least two alternatives.
2. A field repair on a carbon-steel structure must be completed outdoors with limited access and no shielding-gas supply. Recommend a process, consumable strategy and safety controls.
3. A GMAW operator reports porosity and occasional lack of fusion in a stainless-steel assembly. Develop a systematic diagnostic plan covering material preparation, gas shielding, parameters, technique and inspection.

Multiple Choice Questions with Answers

Question 1

Which process uses a non-consumable tungsten electrode?

A. SMAW

B. GTAW

C. FCAW

D. SAW

Correct Answer: B. GTAW

Question 2

What is the principal shielding method in SAW?

A. A blanket of granular flux

B. A helium jet only

C. A ceramic-coated tungsten electrode

D. Water-cooled copper shoes only

Correct Answer: A. A blanket of granular flux

Question 3

Which process is generally most suitable for a portable outdoor repair when a separate shielding-gas supply is impractical?

A. SMAW

B. GTAW

C. PAW

D. Gas-shielded GMAW

Correct Answer: A. SMAW

Question 4

In the relation P = VI, what does I represent?

A. Travel distance

B. Thermal efficiency

C. Welding current

D. Heat input per unit length

Correct Answer: C. Welding current

Question 5

A weld travels 180 mm in 30 s. What is the travel speed?

A. 3 mm/s

B. 6 mm/s

C. 30 mm/s

D. 180 mm/s

Correct Answer: B. 6 mm/s

Question 6

Which discontinuity is most directly associated with incomplete bonding between weld metal and a sidewall or previous pass?

A. Porosity

B. Lack of fusion

C. Undercut

D. Crater pipe

Correct Answer: B. Lack of fusion

Question 7

Which process is particularly suited to very thick sections welded in a vertical progression using conductive molten slag?

A. ESW

B. GTAW

C. SMAW

D. MIG

Correct Answer: A. ESW

Question 8

What is a major limitation of gas-shielded arc welding in a windy outdoor environment?

A. The electrode cannot melt

B. Wind can disturb shielding and cause porosity

C. The workpiece becomes nonconductive

D. Slag becomes electrically insulating

Correct Answer: B. Wind can disturb shielding and cause porosity

Question 9

Which condition most directly helps reduce hydrogen-assisted cold cracking?

A. Using damp low-hydrogen electrodes

B. Increasing joint contamination

C. Using controlled preheat and dry consumables

D. Maximizing restraint without procedure review

Correct Answer: C. Using controlled preheat and dry consumables

Question 10

For a precision weld on thin stainless-steel tubing where cleanliness and control dominate over deposition rate, which process is generally the best initial choice?

A. GTAW

B. ESW

C. SAW

D. High-deposition FCAW

Correct Answer: A. GTAW