4. Conventional Fusion Welding Processes

Introduction and context

Fusion welding joins materials by melting the faying surfaces, with or without a separately supplied filler metal. The molten region solidifies to form a weld metal zone, while the adjacent parent material experiences a thermal cycle that produces a heat-affected zone (HAZ). Conventional fusion processes remain central to mechanical engineering because they can join carbon steels, stainless steels, aluminium alloys, nickel alloys, copper alloys and many engineering fabrications in forms ranging from thin sheet to heavy plate.

The choice of process is not determined by process name alone. It depends on joint geometry, material, thickness, production volume, required deposition rate, positional constraints, permissible distortion, quality standards, operator skill, access, shielding requirements and total cost. A process that is technically capable of producing a weld may still be unsuitable because its productivity, reliability or metallurgical control is inadequate.

This chapter compares shielded metal arc welding (SMAW), gas tungsten arc welding (GTAW), gas metal arc welding (GMAW), flux-cored arc welding (FCAW), submerged arc welding (SAW), plasma arc welding (PAW) and oxy-fuel welding. The comparison emphasizes equipment, operating variables, consumables, operating windows and industrial applications.

Learning objectives

After studying this chapter, the reader should be able to:

1. Explain the heat sources, shielding mechanisms and material-transfer characteristics of major fusion-welding processes.

2. Relate current, voltage, travel speed, polarity, shielding and consumable selection to weld formation.

3. Compare the productivity, quality, flexibility and limitations of conventional fusion processes.

4. Select a suitable process for a given material, thickness, joint configuration and production environment.

5. Identify common defects, risks and quality-control requirements.

6. Evaluate the role of mechanization, automation, sensing and data acquisition in modern welding practice.

The common process logic

Every fusion process contains four interacting elements: a heat source, a means of protecting the molten metal, a method of supplying filler metal when required, and a means of controlling solidification. Arc processes use an electric arc between an electrode and the workpiece. Oxy-fuel welding uses combustion heat from a fuel gas and oxygen. In all cases, the thermal cycle determines penetration, bead shape, dilution, residual stress, distortion and microstructure.

The principal variables are welding current, arc or flame voltage, travel speed, heat input, electrode or wire feed rate, polarity, shielding-gas flow and joint preparation. A useful nominal heat-input expression for arc welding is

`H = ηVI/v`

where `H` is heat input per unit length, `η` is process thermal efficiency, `V` is voltage, `I` is current and `v` is travel speed. The equation is a comparative tool rather than a complete thermal model. Efficiency varies with process and conditions, and actual weld temperature fields also depend on plate thickness, preheating, joint restraint and heat conduction.

Increasing current generally increases penetration and deposition rate, although excessive current can cause undercut, excessive reinforcement, spatter or burn-through. Increasing voltage lengthens the arc and usually produces a wider, flatter bead; excessive voltage may cause porosity and poor shielding. Increasing travel speed reduces heat input and bead size, but excessive speed can produce lack of fusion or an undersized weld. These tendencies must be interpreted together rather than applied as isolated rules.

Shielding prevents atmospheric oxygen, nitrogen and hydrogen from contaminating the molten pool. Shielding may be supplied by electrode coatings, granular flux, internally contained flux, externally supplied gas or a combination of these. Moisture control is particularly important because hydrogen can contribute to cold cracking in susceptible steels.

SMAW: flexibility through the coated electrode

Shielded metal arc welding, commonly called manual metal arc welding, uses a flux-coated consumable electrode. The arc melts both electrode and parent metal. The coating decomposes and melts to produce shielding gases and a slag layer. After cooling, the slag is removed mechanically.

The equipment is comparatively simple: a constant-current power source, electrode holder, work-return cable, welding leads and accessories for cleaning and electrode storage. Alternating current or direct current may be used, depending on electrode classification and application. The process is manually controlled, so arc length and travel speed depend strongly on operator skill.

Electrode coatings perform several functions: arc stabilization, shielding, slag formation, alloy addition, deoxidation and, in some classifications, hydrogen control. Cellulosic electrodes generate a forceful arc and deep penetration, making them useful for pipelines and field work. Rutile electrodes provide easy striking and a smooth appearance. Basic or low-hydrogen electrodes are selected where weld-metal toughness and resistance to hydrogen-assisted cracking are important; they require controlled storage and, in many cases, rebaking according to the applicable specification.

SMAW is effective over a broad range of thicknesses and in almost all welding positions. It is valuable for repair, construction, maintenance, shipbuilding, structural steelwork and pipeline work. Its limitations include low deposition rate compared with continuously fed processes, frequent electrode changes, slag removal, substantial dependence on operator technique and difficulty in mechanizing short or irregular welds.

Typical errors include excessive arc length, incorrect electrode angle, poor cleaning between passes, damp electrodes and unsuitable current. These can produce porosity, slag inclusions, undercut, lack of fusion and hydrogen cracking. Welding procedure specifications should define electrode type, diameter, current range, polarity, preheat, interpass temperature and post-weld treatment.

GTAW: control of the weld pool

Gas tungsten arc welding uses a non-consumable tungsten electrode and an inert shielding gas, normally argon, helium or a mixture. Filler metal may be added separately or the joint may be autogenously fused. Because the electrode does not normally become part of the weld, GTAW offers excellent control of arc energy and filler addition.

The equipment consists of a constant-current power source, torch, tungsten electrode, shielding-gas system, work-return circuit and often a high-frequency or lift-arc starting system. A foot control or remote current control is useful for varying current during welding. Alternating current is widely used for aluminium and magnesium because electrode-positive portions of the cycle assist oxide disruption, while direct-current electrode-negative operation is common for steels, nickel alloys, copper and titanium.

Tungsten preparation is important. Electrode diameter, tip geometry and current capacity influence arc stability and penetration. Gas flow must protect both the arc and the hot tungsten; excessive flow can create turbulence and entrain air. Trailing shields may be necessary for reactive alloys such as titanium. GTAW produces clean welds because it does not use flux, but cleanliness of the joint, filler and gloves is essential.

The process has a low deposition rate and is relatively slow. It is therefore preferred for root passes, thin materials, precision components, stainless-steel tubing, aerospace parts, pressure equipment and high-integrity joints rather than for large-volume filling. Common defects include tungsten inclusions, porosity, lack of fusion and oxidation caused by inadequate shielding. Touching the tungsten to the pool or using contaminated filler can introduce inclusions.

GMAW: continuous wire and controlled transfer

Gas metal arc welding uses a continuously fed consumable wire electrode and an externally supplied shielding gas. A constant-voltage power source is normally used. The wire-feed speed largely controls current and deposition rate, while voltage controls arc length and bead profile. This relationship enables semiautomatic, robotic and mechanized operation.

Shielding gases include argon, helium, carbon dioxide and mixtures. Argon-rich gases support smooth transfer and are common for aluminium and stainless steel. Carbon dioxide and argon–carbon-dioxide mixtures are widely used for carbon steel. Gas selection affects transfer mode, penetration, spatter, arc stability, bead shape and metallurgical reactions.

The principal transfer modes are short-circuiting, globular, spray and pulsed spray. Short-circuiting transfer operates at relatively low heat input and is useful for thin sheet and positional welding, but it can produce spatter and lack of fusion if poorly controlled. Globular transfer is generally less stable. Spray transfer provides high deposition and a smooth arc but requires suitable current, shielding and usually a favourable welding position. Pulsed GMAW alternates between peak and background current, allowing spray-like droplets at lower average heat input and improving positional capability and control of distortion.

GMAW offers high productivity, minimal post-weld cleaning and good compatibility with automation. It is extensively used in automotive manufacturing, pressure vessels, structural fabrication, railway equipment and general manufacturing. Its disadvantages include sensitivity to wind, shielding-gas disturbances, contact-tip wear, wire-feed problems and surface contamination. Outdoor welding often requires wind protection or a different process.

FCAW: flux inside the wire

Flux-cored arc welding uses a tubular wire containing flux ingredients. It may be self-shielded, in which the core supplies most shielding, or gas-shielded, in which external gas supplements the flux. The flux produces slag, shielding gases, deoxidizers and, when designed for the purpose, alloying additions.

FCAW combines the high productivity of continuous wire feeding with some advantages of SMAW. It can provide high deposition rates and is tolerant of mill scale and moderate surface contamination. Self-shielded FCAW is especially useful outdoors and for structural and pipeline applications because it does not require a separate gas cylinder. Gas-shielded FCAW is widely used for heavy fabrication and produces high-quality welds when shielding is stable.

FCAW requires attention to wire polarity, stick-out, voltage, current, travel angle, gas flow and slag removal. Excessive stick-out changes resistive heating and can reduce penetration. Incorrect polarity may destabilize the arc and alter penetration. Slag must be removed between passes; failure to do so can cause inclusions. Fume generation may be relatively high, requiring effective local exhaust ventilation and suitable respiratory protection.

SAW: high deposition beneath granular flux

Submerged arc welding uses a continuously fed bare wire or wires beneath a blanket of granular flux. The arc is invisible because it is submerged in the flux. The flux melts to form a protective slag and may contribute alloying or refining reactions. Equipment generally includes a wire-feeding head, power source, flux hopper, travel mechanism and flux recovery system.

SAW is well suited to long, straight or circumferential welds in the flat or horizontal position. High current and one or more wires can produce very high deposition rates and deep penetration. Applications include pressure-vessel shells, large-diameter pipes, ship panels, structural beams, storage tanks and heavy machinery.

The process provides stable shielding, high productivity and good weld appearance with low operator exposure to arc radiation. However, it is comparatively inflexible. Joint access must permit flux placement and recovery, and the work is usually positioned to keep molten flux and slag in place. Flux moisture and particle size affect weld quality. Slag removal, flux recycling and control of buried arc conditions are important.

Excessive heat input can enlarge the HAZ, reduce toughness in some steels and increase distortion. Improper flux-wire combinations may cause unsuitable weld chemistry or hydrogen pickup. SAW is therefore selected when production volume and weld length justify dedicated equipment.

Plasma arc welding: a constricted arc

Plasma arc welding constricts an arc through a water-cooled copper nozzle. The constriction increases energy density and produces a focused plasma jet. A separate plasma gas passes through the torch, while a shielding gas protects the outer weld region. The process may operate in transferred-arc mode, where the arc is established between electrode and workpiece, or non-transferred mode for specialized heating applications.

Plasma welding is commonly classified as microplasma, melt-in or keyhole welding. Microplasma is useful for very thin sheet. Keyhole operation can penetrate relatively thick material in a single pass when current, travel speed, stand-off, plasma flow and shielding are properly coordinated. The process can produce narrow welds and reduced distortion, but torch alignment and nozzle condition are critical.

Equipment is more complex than GTAW and includes a plasma torch, gas controls, cooling system and precision power source. Applications include stainless-steel tube, aerospace components, instrumentation, nuclear and chemical equipment, and mechanized production. Limitations include higher capital cost, sensitivity to parameter variation, nozzle wear and the need for accurate joint preparation.

Oxy-fuel welding: thermal flexibility without an arc

Oxy-fuel welding uses the heat of combustion of a fuel gas, commonly acetylene, in oxygen. A torch mixes the gases and produces a flame that may be neutral, carburizing or oxidizing. Filler rod is added separately when required. The neutral flame is widely used because it minimizes undesirable oxidation or carburization for many materials.

Equipment includes gas cylinders, regulators, hoses, flashback arrestors, check valves and a torch. Flame adjustment is central to process control. Oxy-fuel welding is portable and does not require electrical power, making it useful for maintenance, thin sheet, tubes, brazing-related operations and field repair. It also provides excellent control of preheating and is used for cutting and heating.

Compared with arc processes, it has lower energy density, slower welding speed, a larger HAZ and greater distortion on many joints. It is unsuitable for many high-productivity heavy-fabrication applications. Oxygen and fuel-gas handling also create fire, explosion and flashback hazards. Cylinders must be secured, hoses inspected and work areas controlled for combustible materials.

Comparing operating windows and selecting a process

An operating window is the range of parameter combinations that produces acceptable weld geometry, soundness and properties. It is bounded by defects and process instability. For example, a GMAW window may be defined by wire-feed speed, voltage, shielding gas, contact-tip-to-work distance and travel speed. A GTAW window additionally includes tungsten diameter, tip geometry, gas flow and current waveform. In SAW, wire diameter, flux type, current, voltage, travel speed and electrode extension are central variables.

A practical selection sequence is:

1. Define service requirements, material, thickness, joint type, position and required properties.

2. Establish whether the weld is a root, fill or finish operation and whether access is single-sided or double-sided.

3. Compare deposition rate, heat input, distortion risk, cleaning requirements and automation potential.

4. Select compatible consumables and shielding or flux systems.

5. Qualify a welding procedure through testing required by the governing code or specification.

6. Control production through welder qualification, inspection, traceability and parameter monitoring.

SMAW is often the most flexible field process; GTAW offers the highest manual precision; GMAW supports clean, automated production; FCAW combines productivity with strong field and heavy-fabrication capability; SAW is preferred for long, high-deposition welds; PAW provides a concentrated, mechanizable heat source; and oxy-fuel remains valuable for portability, thin materials, heating and repair.

Applied case: selecting a process for a stainless-steel pressure-vessel line

Background

An illustrative manufacturer produces stainless-steel pressure vessels with circumferential and longitudinal seams. The vessels require clean internal surfaces, controlled distortion and documented weld quality. Production volume is moderate, and several shell thicknesses are used.

Situation and analysis

GTAW provides excellent root quality and low risk of slag inclusion, but using it for the complete thickness would be slow. GMAW is productive and suitable for mechanization, but shielding stability and heat input must be controlled. SAW could provide high deposition on long seams, yet flux handling and access are less attractive for the smaller vessels. SMAW is available for repair but would introduce slag and lower productivity.

Approach

The manufacturer qualifies a two-stage procedure: mechanized GTAW with controlled gas purging for the root, followed by pulsed GMAW for filling and capping. Argon-based shielding is selected, interpass temperature is limited, and joint fit-up is controlled with fixtures. Weld current, voltage, wire-feed speed, travel speed and gas flow are recorded. Visual inspection is supplemented by radiographic or ultrasonic examination according to the applicable pressure-vessel code.

Outcome

The hybrid procedure combines root cleanliness with higher deposition during filling. Distortion is reduced through balanced welding sequences and controlled heat input. The process is more productive than all-position manual GTAW while maintaining the required quality.

Learning points

Process selection may be divided by weld stage rather than restricted to one process. Qualification must consider metallurgy, shielding, sequence and inspection—not merely nominal equipment settings. The least expensive consumable is not necessarily the least expensive production solution when rework and inspection are included.

Defects, safety and professional responsibility

Typical fusion-welding defects include porosity, lack of fusion, incomplete penetration, slag or tungsten inclusions, undercut, overlap, cracks and excessive distortion. Prevention begins with sound joint design, correct fit-up, clean surfaces, dry consumables, stable shielding and qualified procedures. Inspection should be matched to the likely failure mode: visual inspection is fundamental, while radiography, ultrasonics, magnetic-particle or liquid-penetrant methods may be added as required.

Welding safety includes control of electric shock, arc radiation, fumes, fire, compressed gases, noise and hot metal. Ventilation must be designed for the materials and consumables used. Welding stainless steel, coated materials or painted surfaces may generate hazardous fumes; coatings should be removed or assessed before heating. Gas cylinders require segregation, secure storage and compatible regulators. Employers must provide training, protective equipment and exposure controls, while engineers have a professional duty not to compromise procedure qualification or inspection records for production convenience.

Digital power sources increasingly provide waveform control, synergic parameter selection, arc monitoring and data logging. Robotic GMAW, FCAW and SAW cells use positioners, seam tracking and offline programming. Machine-learning methods are being investigated for detection of spatter, arc instability and weld defects from electrical signals, images and thermal data. These tools support quality assurance but do not eliminate the need for qualified procedures, competent personnel and independent inspection. Sensor data are only useful when calibrated, traceable and interpreted in relation to material and process conditions.

Current developments and best practice

Important developments include low-spatter pulsed transfer, tandem-wire systems, digitally controlled power sources, narrow-gap welding, adaptive seam tracking, collaborative robotic cells and integrated weld-data management. Energy efficiency and reduced fume generation are also becoming significant selection criteria. Hybrid processes combining arc welding with laser or other concentrated heat sources extend the operating window for some applications.

Best practice is to:

• treat the welding procedure as a controlled manufacturing system rather than a set of isolated machine settings;
• select consumables by required weld-metal chemistry, toughness, corrosion resistance and hydrogen control;
• maintain gas, flux and electrode storage conditions;
• verify calibration of power sources, wire feeders, flowmeters and temperature instruments;
• control fit-up, preheat, interpass temperature and welding sequence;
• record traceable parameters and nonconformities;
• use inspection results for process improvement rather than merely final acceptance; and
• select automation only after joint access, variability and maintenance capability have been assessed.

Chapter Summary

Conventional fusion welding processes differ principally in their heat source, shielding method, filler delivery, deposition rate, positional capability and degree of operator control. SMAW uses a coated consumable electrode and is highly portable. GTAW provides exceptional control and cleanliness but has low deposition rate. GMAW uses continuous wire and external gas, making it productive and highly automatable. FCAW adds flux within a tubular wire and is effective in heavy fabrication and field work. SAW achieves very high deposition beneath granular flux for long, accessible welds. PAW produces a constricted, concentrated arc suited to precision and mechanized work. Oxy-fuel welding is portable and useful for thin materials, heating and repair, although it generally produces a broader HAZ and lower productivity.

Sound process selection requires assessment of material, thickness, joint design, position, quality requirements, production volume, heat input, consumables, safety and total cost. Modern digital controls and automation improve consistency, but they supplement rather than replace welding science, procedure qualification, inspection and professional responsibility.

Key Terms

Heat-affected zone: Region of parent metal whose microstructure or properties are altered by welding heat without melting.

Heat input: Thermal energy introduced per unit length of weld, commonly estimated from voltage, current, travel speed and efficiency.

Shielding gas: Gas that protects the arc and molten weld pool from atmospheric contamination.

Flux: Material that forms shielding products, slag and refining reactions during welding.

Slag: Solidified nonmetallic layer formed from flux during or after welding.

SMAW: Arc welding using a flux-coated consumable electrode.

GTAW: Arc welding using a non-consumable tungsten electrode and inert shielding gas.

GMAW: Arc welding using a continuously fed consumable wire and external shielding gas.

FCAW: Arc welding using a tubular, flux-filled consumable wire.

SAW: Arc welding in which the arc is submerged beneath granular flux.

PAW: Arc welding using a constricted plasma arc through a nozzle.

Oxy-fuel welding: Fusion welding using heat generated by combustion of a fuel gas with oxygen.

Polarity: Arrangement of direct-current electrode and workpiece connections.

Deposition rate: Mass of weld metal deposited per unit time.

Transfer mode: Manner in which molten electrode metal moves across the arc.

Keyhole welding: Mode in which concentrated energy creates a penetrating cavity through the joint.

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

Hydrogen-assisted cracking: Cracking promoted by diffusible hydrogen, tensile stress and susceptible microstructure.

Procedure qualification: Demonstration that a defined welding procedure can produce acceptable welds.

Review Questions

Short Answer Questions

1. What functions are performed by the coating on an SMAW electrode?

2. Why is GTAW commonly selected for root passes?

3. Distinguish gas-shielded and self-shielded FCAW.

4. What is meant by heat input per unit length?

5. Why is SAW generally restricted to accessible welding positions?

Long Answer Questions

1. Compare SMAW, GTAW and GMAW in equipment, consumables, control, productivity and applications.

2. Explain the transfer modes of GMAW and their influence on weld quality and position capability.

3. Describe the operating principles, advantages and limitations of FCAW and SAW.

4. Discuss the factors governing selection between GTAW, PAW and oxy-fuel welding.

5. Explain how shielding, consumable storage, preheating and interpass control influence weld integrity.

Analytical / Application Questions

1. Select a suitable process combination for a stainless-steel pressure vessel requiring a clean root and productive fill passes. Justify the choice.

2. A field weld exhibits porosity and unstable arc behavior during GMAW. Analyze possible causes and propose corrective actions.

3. For a long, straight weld in thick carbon-steel plate, compare SAW, FCAW and SMAW using productivity, access, quality and cost as criteria.

22. Multiple Choice Questions with Answers

Question 1

Which process uses a non-consumable tungsten electrode and normally permits separate filler addition?

A. SMAW

B. GTAW

C. FCAW

D. SAW

Correct Answer: B. GTAW

Question 2

For a long, straight weld in thick plate where the work can be positioned suitably, which process normally offers the highest productivity?

A. GTAW

B. Oxy-fuel welding

C. SAW

D. SMAW

Correct Answer: C. SAW

Question 3

In GMAW, increasing wire-feed speed generally has the strongest direct effect on which variable?

A. Electrode diameter

B. Welding current and deposition rate

C. Flux moisture

D. Tungsten tip angle

Correct Answer: B. Welding current and deposition rate

Question 4

Which feature distinguishes self-shielded FCAW from gas-shielded FCAW?

A. It uses a non-consumable electrode

B. It requires granular external flux

C. The flux core supplies most of the shielding

D. It cannot be used outdoors

Correct Answer: C. The flux core supplies most of the shielding

Question 5

A major reason for using basic low-hydrogen SMAW electrodes is to reduce the risk of:

A. Hydrogen-assisted cracking

B. Arc constriction

C. Gas-cylinder flashback

D. Tungsten melting

Correct Answer: A. Hydrogen-assisted cracking

Question 6

Which GMAW transfer mode is particularly useful for lower average heat input while retaining controlled spray-like transfer?

A. Globular transfer

B. Pulsed transfer

C. Short-circuit transfer only

D. Submerged transfer

Correct Answer: B. Pulsed transfer

Question 7

What is the principal heat-source characteristic of PAW?

A. A broad oxy-fuel flame

B. A constricted, high-energy-density arc

C. A granular flux reaction without an arc

D. Resistance heating between plates

Correct Answer: B. A constricted, high-energy-density arc

Question 8

In the pressure-vessel case, why was GTAW used for the root and GMAW for filling?

A. GTAW has the highest deposition rate and GMAW needs no shielding

B. GTAW provides root control while GMAW improves filling productivity

C. GTAW is limited to carbon steel and GMAW to aluminium

D. GMAW cannot be mechanized

Correct Answer: B. GTAW provides root control while GMAW improves filling productivity

Question 9

Excessive voltage in a gas-shielded arc process may contribute to:

A. A shorter arc and narrower bead in all cases

B. Poor shielding and porosity

C. Elimination of all spatter

D. Removal of the HAZ

Correct Answer: B. Poor shielding and porosity

Question 10

Which process is most characterized by portability, no electrical arc requirement and a relatively broad heat-affected zone?

A. Oxy-fuel welding

B. SAW

C. GMAW

D. PAW

Correct Answer: A. Oxy-fuel welding