Arc Physics, Power Sources and Consumables

Introduction

Arc welding is a controlled electrical, thermal, metallurgical and chemical process. An electric arc provides concentrated heat, melts the parent material and, when applicable, melts a filler metal to form a joint. The quality of the weld depends not only on operator skill but also on the interaction among arc physics, power-source behavior, polarity, shielding, electrode composition, filler-metal deposition and flux chemistry.

For a welding technologist, these relationships are practical design variables. A change from direct current electrode negative to electrode positive changes heat distribution and penetration. A change in shielding gas can alter arc voltage, droplet transfer, spatter and weld-metal chemistry. A power source with an inappropriate volt–ampere characteristic can make arc length difficult to control or cause unstable metal transfer. Consumable selection must therefore be matched to the base metal, joint design, welding position, service environment and applicable classification standard.

This chapter develops these principles for common fusion-welding processes, especially shielded metal arc welding (SMAW), gas metal arc welding (GMAW), flux-cored arc welding (FCAW), gas tungsten arc welding (GTAW) and submerged arc welding (SAW).

Learning Objectives

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

1. Explain how an arc is initiated and maintained.
2. Relate arc length, voltage, current and heat input to weld behavior.
3. Distinguish electrode polarity and explain its influence on heating and penetration.
4. Describe globular, short-circuiting, spray and pulsed-spray metal transfer.
5. Compare constant-current and constant-voltage power sources.
6. Select shielding gases and recognize their effects on arc and weld-metal properties.
7. Distinguish electrodes, filler metals and fluxes by function and composition.
8. Interpret the principal information contained in consumable classifications.
9. Diagnose common instability, porosity, spatter and deposition problems.
10. Apply these principles to a practical welding-process selection problem.

1. The Welding Arc as a Plasma Conductor

An arc is a sustained electrical discharge through an ionized gas. The gas between the electrode and workpiece is initially a poor conductor, but sufficient electric-field strength and heating cause electrons to be released and gas molecules to become ionized. The resulting mixture of electrons, positive ions and neutral particles is called plasma.

The arc has three principal regions:

• Cathode region: the negative electrode surface, where electrons are emitted.
• Arc column: the hot, ionized gas through which current flows.
• Anode region: the positive surface, where electrons are received.

The arc voltage is the electrical potential difference across the arc. A useful approximation for arc power is:

where P_arc is arc power in watts, V_arc is arc voltage in volts and I is welding current in amperes. This relationship assumes that the measured voltage is the effective voltage across the arc and that electrical power losses in cables and connections are excluded.

The energy supplied during a welding time interval is:

where E is electrical energy in joules and t is time in seconds. In production welding, heat input is commonly expressed per unit length:

where H_net is net heat input per unit length, η is the process thermal efficiency, dimensionless, and v is travel speed in millimetres per second. If voltage is in volts, current in amperes and travel speed in millimetres per second, the result is joules per millimetre. For practical welding calculations using volts, amperes and travel speed in millimetres per minute:

The efficiency factor accounts for heat that does not enter the workpiece. It varies with process, joint geometry, transfer mode and operating conditions; it should not be treated as a universal constant. Heat input affects fusion, cooling rate, distortion, hardness and heat-affected-zone behavior, but it does not alone determine weld quality.

Arc initiation

Before ignition, the electrode and workpiece are separated by a gas gap. In SMAW, the operator briefly touches the electrode to the workpiece and withdraws it. Contact creates a short circuit and locally heats the surfaces. As the electrode is withdrawn, the hot material and vaporized metal help ionize the gap, allowing current to continue through the arc.

In GTAW, high-frequency or lift-arc starting may be used. High-frequency starting establishes ionization without physically touching the tungsten to the workpiece. Lift-arc starting briefly contacts the tungsten at low current and then lifts it, reducing the risk of tungsten inclusion. GMAW normally uses a wire-feed system: the wire approaches the workpiece, short-circuits or establishes an arc, and the power source responds to the changing arc condition.

A stable arc requires a balance among voltage, current, electrode extension, arc length, shielding and power-source response. Contamination, excessive arc length, inadequate shielding or an incorrect parameter combination can cause erratic ignition and repeated arc extinction.

Arc stability

Arc stability means that the arc can be maintained with predictable current, voltage, shape, sound and metal transfer. Stability is influenced by:

• ionization potential and thermal conductivity of the shielding gas;
• electrode coating or flux composition;
• polarity and current type;
• arc length and electrode extension;
• power-source dynamic response;
• surface contamination, moisture and oxide films; and
• travel speed and operator manipulation.

The visible arc is not necessarily a direct measure of weld quality. A quiet arc may still produce inadequate fusion, while a louder arc may be appropriate for a particular transfer mode. Sound, bead appearance and instrument readings should be interpreted together.

2. Current, Voltage, Arc Length and Polarity

Current primarily influences melting rate, penetration and deposition rate. Voltage is strongly related to arc length in many processes. Increasing arc length generally increases arc voltage and broadens the arc, while reducing arc length generally produces a narrower, more concentrated arc. The exact response depends on process and power-source design.

In SMAW, an excessively long arc can cause spatter, porosity, undercut and atmospheric contamination. An excessively short arc may cause electrode sticking, unstable transfer and slag entrapment. In GMAW, arc length is closely associated with the distance between the wire tip and molten pool; this distance is often called electrical stick-out or electrode extension when measured from contact tip to arc.

Polarity

Direct current has two polarity arrangements:

• Direct current electrode negative (DCEN): the electrode is negative and the workpiece is positive.
• Direct current electrode positive (DCEP): the electrode is positive and the workpiece is negative.

Alternating current (AC) reverses polarity periodically. Actual heat distribution is affected by electron flow, ion bombardment, electrode material, arc attachments and process conditions. The simplified rule that one fixed percentage of heat always enters each side is only an approximation and should not be used universally.

DCEP is widely used in GMAW because it generally provides stable droplet transfer and good electrode melting behavior. DCEN can increase electrode melting rate in some wire processes and is used with selected consumables and applications. GTAW commonly uses DCEN for steels, stainless steels and nickel alloys because it concentrates more useful heating at the workpiece while limiting tungsten heating. AC is important for aluminium and magnesium GTAW because the electrode-positive portion assists oxide removal, while the electrode-negative portion supplies penetration.

Polarity must always comply with the consumable manufacturer’s instructions and the applicable welding procedure specification. Incorrect polarity can produce poor penetration, excessive spatter, unstable transfer, rapid electrode overheating or defective weld-metal chemistry.

3. Metal Transfer Across the Arc

Metal transfer describes how molten electrode material travels from the electrode to the weld pool. Transfer mode is determined by current, voltage, wire diameter, shielding gas, electrode extension, inductance and material properties.

Short-circuiting transfer

In short-circuiting GMAW, the electrode repeatedly contacts the weld pool. Current rises rapidly during the short circuit, the molten bridge pinches off, and the arc re-establishes. This mode uses relatively low heat input and is useful for thin material, root passes and positional welding. Its limitations include the possibility of excessive spatter, lack of fusion and arc instability if inductance, voltage or wire-feed speed is poorly adjusted.

Globular transfer

Globular transfer produces droplets larger than the electrode diameter. Gravity and electromagnetic forces move the droplets irregularly toward the pool. It commonly occurs in an intermediate current range, particularly with carbon-dioxide-rich shielding. Globular transfer may provide high deposition but generally produces more spatter and a less controlled arc than spray transfer.

Spray transfer

Spray transfer produces a stream of small droplets that travel axially across the arc. It requires sufficient current and a shielding-gas composition capable of supporting the mode, often an argon-rich mixture. Spray transfer provides high deposition, good fusion and low spatter, but its relatively high heat input and fluid weld pool can restrict it mainly to flat and horizontal positions unless specialized techniques are used.

Pulsed-spray transfer

Pulsed GMAW alternates between a peak current and a lower background current. The peak current detaches one or more droplets, while the background current maintains the arc with reduced average heat input. Properly controlled pulsing can provide spray-like transfer in positions and on thicknesses where continuous spray would be unsuitable. Pulse parameters are waveform-specific; a generic pulse equation should not be assumed without the power-source manufacturer’s data.

Transfer in other processes

In FCAW and SAW, transfer is influenced by the flux system as well as the shielding environment. In GTAW, the nonconsumable tungsten does not normally transfer to the joint; filler metal, if used, is added separately. The electrode serves as an arc source rather than the deposited material.

Figure: Conceptual comparison of common GMAW transfer modes. The boundaries between modes depend on material, wire diameter, shielding gas and power-source settings.

4. Power Sources and Their Characteristics

A welding power source converts electrical supply into a controlled welding output. Important characteristics include output current range, voltage range, duty cycle, polarity capability, dynamic response, arc-starting behavior and control of inductance or waveform.

Constant-current sources

A constant-current (CC) source is designed to maintain approximately constant current despite moderate changes in arc length. A change in arc length causes a relatively larger change in voltage than in current. CC output is appropriate for processes in which the operator controls arc length, especially SMAW and GTAW.

In GTAW, a foot pedal or hand control may vary current during starting, welding and crater filling. A CC source helps prevent large current changes when the tungsten-to-work distance changes slightly.

Constant-voltage sources

A constant-voltage (CV) source attempts to maintain a relatively constant voltage. In GMAW and many FCAW applications, the wire feeder controls wire-feed speed while the power source responds rapidly to changes in arc length. If the arc becomes shorter, current rises and melts wire more rapidly; if it becomes longer, current tends to fall and the wire advances toward the pool. This self-regulating interaction is called burn-off control.

The relationship is not perfectly instantaneous. Cable inductance, control-loop delay, wire-feed inertia and short-circuit dynamics influence stability. Consequently, voltage and wire-feed speed must be selected as a coordinated parameter set.

Drooping and flat characteristics

A power-source volt–ampere characteristic describes how output voltage changes with current. A drooping characteristic is associated with CC operation; a comparatively flat characteristic is associated with CV operation. Modern inverter sources can synthesize several characteristics and waveforms, but the process still requires a suitable control strategy.

Duty cycle and ratings

Duty cycle is the percentage of a specified time interval during which a source can deliver its rated output without exceeding its thermal design limit. For example, a rating of 300 A at 60% duty cycle means 6 minutes of operation at 300 A in a 10-minute test period, followed by 4 minutes of cooling under the stated standard and ambient conditions. It does not mean that the machine must be switched off after exactly six minutes in every real application. Actual allowable operation depends on current, ambient temperature, ventilation and maintenance.

5. Shielding Gases and Flux Shielding

Atmospheric oxygen, nitrogen, hydrogen and moisture can contaminate molten weld metal. Oxygen may form oxides; nitrogen can contribute to porosity and loss of toughness in some systems; hydrogen can cause cracking in susceptible steels. Shielding prevents or reduces contact between the molten pool and the atmosphere.

Common shielding gases

Argon has relatively low thermal conductivity and supports easy arc starting, a stable arc and good cleaning behavior in AC GTAW. It is widely used for aluminium, stainless steel and nonferrous GMAW. Argon-rich mixtures support spray transfer at lower current than carbon-dioxide-rich mixtures.

Helium has higher thermal conductivity than argon and can produce a hotter, broader arc. It may increase welding speed or penetration in suitable applications, but it is more expensive and generally requires greater flow rates.

Carbon dioxide is chemically active. It can provide deep penetration and is economical, but it tends to increase oxidation and spatter compared with argon-rich mixtures. It is used in GMAW and some FCAW applications.

Oxygen, normally used only in small controlled additions to argon, can improve arc stability and wetting for certain ferrous-metal applications. Excessive oxygen increases oxidation and may change weld-metal chemistry.

Gas mixtures are selected according to base metal, transfer mode, position, desired bead shape, productivity and metallurgical requirements. Shielding-gas flow must be high enough to exclude air but not so high that turbulence entrains air. Drafts, leaks, damaged hoses, poor nozzle position and excessive stick-out can defeat an otherwise correct gas selection.

Fluxes

Flux is a granular, powdered or coating material that performs chemical and physical functions during welding. It may:

• generate shielding gases;
• form a protective slag layer;
• absorb or react with oxides and impurities;
• stabilize the arc through ionizable compounds;
• control bead shape and slag detachability; and
• add alloying elements or scavengers to the weld metal.

SMAW electrodes contain a coating around a metal core. FCAW wires contain a flux-filled tubular core, while SAW uses a separate granular flux. Fluxes may be basic, rutile, acidic or cellulosic in broad classification systems. These labels indicate characteristic chemistry and behavior, not a complete prediction of weld performance.

Basic or low-hydrogen flux systems are designed to limit diffusible hydrogen and often provide good toughness, but they require careful storage and rebaking according to manufacturer instructions. Rutile systems generally provide smooth operation, attractive beads and easy slag removal. Cellulosic electrodes generate substantial shielding gas and are valued for deep penetration, especially in pipeline work, but they produce higher hydrogen levels and require controlled application.

6. Electrodes and Filler Metals

An electrode is the electrical conductor that carries current to the arc. It may be consumable, as in SMAW and GMAW, or nonconsumable, as in GTAW. A filler metal is metal added to the joint. A consumable electrode can therefore be both electrode and filler metal, whereas a GTAW tungsten electrode is an electrode but not normally a filler metal.

Consumable selection should consider:

• chemical compatibility with the base metal;
• required tensile strength and yield strength;
• toughness at service temperature;
• corrosion, wear or heat resistance;
• welding position and process;
• hydrogen-control requirements;
• deposition rate and productivity;
• postweld heat treatment; and
• applicable code or standard.

Electrode types

SMAW electrodes consist of a core wire and coating. The core supplies metal, while the coating provides shielding, slag, arc stabilization and, in some products, alloying or hydrogen control.

GMAW electrodes are continuously fed solid wires or metal-cored wires. Solid wires rely on external shielding gas. Metal-cored wires contain metallic powders and arc conditioners; they may deliver high deposition rates and good productivity.

FCAW electrodes are tubular wires containing flux. Self-shielded FCAW generates shielding from the flux and is useful outdoors, where wind makes external gas shielding difficult. Gas-shielded FCAW combines tubular wire with an external gas and can provide high deposition and good mechanical properties.

GTAW electrodes are commonly tungsten or tungsten alloys. The electrode tip geometry affects arc concentration and penetration. Contamination of tungsten by contact with the pool or filler metal can destabilize the arc and introduce inclusions; the contaminated tip should be removed and properly reground.

Classification systems

Consumable classifications are standardized by organizations such as the American Welding Society, ISO and national standards bodies. The exact designation depends on the standard and product family. A classification may communicate tensile-strength level, yield-strength level, elongation, impact-test temperature, coating type, welding positions, current or polarity, shielding condition and diffusible-hydrogen designation.

A designation must never be interpreted from memory alone. The applicable standard, manufacturer data sheet and welding procedure specification should be consulted because similar-looking symbols can have different meanings in different standards.

For low-alloy steel electrodes, a hydrogen suffix may identify a maximum diffusible-hydrogen level under specified test conditions. This designation supports hydrogen-control planning but does not guarantee immunity from hydrogen cracking. Joint restraint, preheat, interpass temperature, cleanliness, moisture exposure and postweld conditions remain important.

7. Practical Parameter Relationships

Welding parameters are interdependent. In GMAW, wire-feed speed is closely related to current for a given wire type, diameter, electrode extension and polarity. Increasing wire-feed speed usually increases current and deposition rate. Increasing voltage tends to lengthen the arc and flatten or widen the bead, but excessive voltage can cause undercut, porosity or an unstable arc.

Travel speed affects heat input per unit length. For constant voltage and current, increasing travel speed reduces heat input per unit length; decreasing travel speed increases it. A simple worked example illustrates the calculation.

Worked Example: Net heat input

A GMAW operation uses 24 V, 180 A and a travel speed of 300 mm/min. Assume a process efficiency of 0.80. Determine net heat input in kJ/mm.

Use:

Substitution gives:

The electrical heat input before efficiency correction is:

Therefore:

The net heat input is approximately 0.69 kJ/mm. This result applies only to the stated parameters and assumed efficiency. It does not by itself predict penetration or cooling time.

8. Defects Related to Arc and Consumable Control

Porosity

Porosity may result from inadequate shielding, excessive gas flow, leaks, moisture, contaminated surfaces, excessive arc length or damp electrodes and fluxes. Corrective action includes leak testing, protection from drafts, cleaning, correct storage and verification of gas-flow conditions.

Lack of fusion and penetration

These defects may result from insufficient current, excessive travel speed, incorrect torch angle, poor joint preparation, an excessively cold transfer mode or slag remaining between passes. Increasing heat input is not always the best solution; joint access, manipulation and cleaning must also be examined.

Excessive spatter

Spatter may be associated with incorrect voltage–current matching, unsuitable shielding gas, long arc length, magnetic arc blow, contaminated wire or unstable short-circuit transfer. Parameter adjustment should follow a controlled sequence rather than random changes.

Hydrogen cracking

Hydrogen cracking requires a susceptible microstructure, sufficient diffusible hydrogen and tensile restraint. Prevention may require low-hydrogen consumables, dry storage, preheating, controlled interpass temperature, clean joint surfaces and appropriate postweld hydrogen diffusion time. The absence of visible cracking immediately after welding does not prove that the weld is safe.

Slag inclusions

Slag inclusions arise when slag becomes trapped in the weld metal, often because of inadequate interpass cleaning, unfavorable bead placement, insufficient fusion or incorrect electrode angle. Slag systems are useful only when their removal and flow characteristics are compatible with the joint and welding position.

9. Applied Case: Selecting a Process for an Outdoor Structural Repair

The following is an illustrative case, not a record of a specific industrial project.

Background

A fabrication team must repair a 12 mm carbon-steel bracket in an outdoor location. The joint is restrained, access is limited, and wind conditions vary. The repair must achieve adequate toughness and must be inspected by visual and ultrasonic methods.

Problem

The team initially proposes gas-shielded GMAW because it offers high productivity. During trial welding, wind disturbs the shielding gas, producing porosity. A second trial uses a high-deposition FCAW wire, but the operator leaves slag between passes and ultrasonic indications occur. The team must select a more reliable process and consumable system.

Analysis

The principal constraints are outdoor wind, restraint, hydrogen-cracking risk, limited access and inspection requirements. External gas shielding is vulnerable unless wind screens and adequate flow control are provided. A self-shielded, low-hydrogen-controlled FCAW system or low-hydrogen SMAW electrode may be more robust. However, the selected consumable must be approved for the base metal, position, strength level and impact requirements.

The repair engineer reviews the material certificate, joint design, code requirements and manufacturer classification. A qualified procedure specifies preheat, maximum interpass temperature, electrode or wire storage, cleaning between passes and inspection hold points. Trial welds confirm bead shape, slag removal and mechanical performance.

Approach and solution

The team installs a wind screen, removes paint and rust, checks joint restraint and establishes the specified preheat. For the most exposed passes, a suitable self-shielded FCAW consumable is selected. Where access permits and wind can be controlled, low-hydrogen SMAW electrodes are used. Electrodes are stored in heated containers and exposed electrodes are controlled according to the manufacturer’s instructions. Each pass is visually examined and thoroughly cleaned before the next pass.

Outcome

The revised procedure eliminates the trial porosity and reduces slag-related indications. The repair passes inspection because process selection, consumable control, environmental protection and interpass discipline were treated as one system rather than as separate choices.

Learning points

1. Productivity alone is not a sufficient basis for process selection.
2. Shielding must be evaluated against the actual environment.
3. Consumable classification and storage are part of quality control.
4. Hydrogen control requires attention to material, process, moisture and restraint.
5. A welding procedure must convert theory into controlled, verifiable practice.

10. Best Practices and Professional Responsibility

A competent welding technologist should use the following sequence:

1. Identify the base-metal grade, thickness, joint type and service requirements.
2. Select a qualified process and consumable classification.
3. Confirm polarity, current range, shielding gas or flux requirements and position capability.
4. Verify power-source capacity, duty cycle and cable condition.
5. Control electrode, wire and flux storage according to manufacturer and code requirements.
6. Establish parameters through a qualified welding procedure rather than informal trial and error.
7. Monitor preheat, interpass temperature, travel speed, arc length and shielding conditions.
8. Inspect the joint, record nonconformities and correct root causes.

Safety and regulatory responsibility are inseparable from technical performance. Arc radiation, fumes, electric shock, fire, compressed gases, noise and hot metal require risk assessment and appropriate controls. Fume exposure should be reduced through substitution where feasible, local exhaust ventilation and suitable respiratory protection. Welding procedures must comply with applicable occupational-safety rules, construction codes and environmental requirements. Digital power sources may record parameters and support automated quality monitoring, but data logging does not replace qualified personnel, inspection or sound engineering judgment.

Contemporary Developments

Modern inverter power sources provide rapid control of current and voltage, programmable waveforms, synergic parameter selection and pulsed transfer. Advanced systems can compensate for variations in wire-feed speed and arc length, improving repeatability. Robotic welding combines controlled torch motion with parameter monitoring, while machine-learning methods are being investigated for defect prediction and adaptive control.

These technologies have limitations. Algorithms depend on representative data, sensors may drift, and a visually acceptable signal may not guarantee required mechanical properties. Cybersecurity, software validation, traceability and human oversight become important when welding data are used for production acceptance. Technology should therefore be integrated into a qualified quality system rather than treated as an independent guarantee of weld integrity.

Chapter Summary

An arc is an ionized plasma discharge whose behavior depends on electrical, thermal, chemical and mechanical variables. Arc power is approximately the product of arc voltage and current, while net heat input per unit length additionally depends on travel speed and process efficiency. Arc initiation requires ionization of the gap; stable operation requires coordinated control of arc length, polarity, shielding, consumable and power-source response.

CC power sources are suited to processes in which arc length is manually controlled, such as SMAW and GTAW. CV sources support the self-regulating wire-feed behavior used in GMAW and many FCAW applications. Polarity changes heat distribution, arc behavior and metal-transfer characteristics, while AC is particularly valuable for oxide control in aluminium and magnesium GTAW.

Short-circuiting, globular, spray and pulsed-spray transfer provide different combinations of heat input, deposition rate, spatter and positional capability. Shielding gases prevent atmospheric contamination, whereas fluxes provide shielding, slag formation, arc stabilization and chemical conditioning. Electrodes and filler metals must be selected using base-metal compatibility, mechanical properties, hydrogen control, position, productivity and applicable classification standards.

Reliable welding results from an integrated system: qualified procedures, appropriate power sources, controlled consumables, suitable shielding, parameter discipline, environmental protection and inspection. New inverter, robotic and data-driven technologies improve control but do not replace engineering judgment or regulatory compliance.

Key Terms

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

Arc length: Distance across the active arc between the electrode and workpiece or weld pool.

Arc stability: Ability of an arc to remain established with predictable electrical and transfer behavior.

Cathode: Negative arc region from which electrons are emitted.

Anode: Positive arc region that receives electrons.

Plasma: Ionized gas containing electrons, ions and neutral particles.

Polarity: Electrical arrangement defining the positive and negative sides of a direct-current welding circuit.

DCEN: Direct current electrode negative arrangement.

DCEP: Direct current electrode positive arrangement.

Constant-current source: Power source designed to maintain nearly constant current as arc voltage changes.

Constant-voltage source: Power source designed to maintain nearly constant voltage while current responds to arc-length changes.

Short-circuiting transfer: Metal transfer in which the electrode repeatedly contacts the weld pool.

Globular transfer: Transfer of relatively large, irregular droplets across the arc.

Spray transfer: Axial transfer of fine droplets across a stable arc.

Shielding gas: Gas supplied to protect the arc and molten weld metal from atmospheric contamination.

Flux: Material that provides shielding, slag, chemical cleaning, arc stabilization or alloying effects.

Electrode: Conductor through which welding current enters or leaves the arc.

Filler metal: Metal intentionally added to the joint.

Diffusible hydrogen: Hydrogen capable of moving through weld metal and contributing to delayed cracking under suitable conditions.

Duty cycle: Percentage of a defined test period during which a power source can operate at its rated output.

Review Questions

Short Answer Questions

1. What physical process allows a gas gap to conduct current after arc initiation?
2. Distinguish DCEN, DCEP and AC polarity arrangements.
3. Why is shielding necessary during fusion welding?
4. What is the principal difference between a CC and a CV power source?
5. Name two functions performed by a welding flux.

Long Answer Questions

1. Explain arc initiation and stability in SMAW, GMAW and GTAW, identifying the principal sources of instability.
2. Compare short-circuiting, globular, spray and pulsed-spray transfer in terms of heat input, spatter, deposition and position capability.
3. Explain how polarity affects GTAW of steels and aluminium alloys.
4. Discuss the factors that should be considered when selecting an electrode or filler metal classification.
5. Describe the causes and prevention of porosity, slag inclusions and hydrogen cracking.

Analytical / Application Questions

1. A weld uses 26 V, 220 A, 250 mm/min travel speed and an assumed efficiency of 0.75. Calculate net heat input in kJ/mm and explain two limitations of interpreting weld quality from this value alone.
2. A GMAW operation exhibits high spatter and irregular transfer after the operator increases wire-feed speed without changing voltage. Diagnose the likely interaction and propose a controlled troubleshooting sequence.
3. For an outdoor repair of restrained carbon steel, compare gas-shielded GMAW, self-shielded FCAW and low-hydrogen SMAW. Recommend a process strategy and justify it using shielding, hydrogen control, productivity and inspection considerations.

Multiple Choice Questions with Answers

Question 1

Which region of an arc is the hot ionized gas through which current flows?

A. Cathode region

B. Arc column

C. Anode region

D. Flux layer

Correct Answer: B. Arc column

Question 2

A constant-current power source is most characteristically associated with which process group?

A. SMAW and GTAW

B. GMAW and most CV FCAW

C. Resistance spot welding only

D. Submerged arc welding only

Correct Answer: A. SMAW and GTAW

Question 3

Which GMAW transfer mode uses repeated contact between the electrode and weld pool?

A. Spray transfer

B. Globular transfer

C. Short-circuiting transfer

D. Pulsed-spray transfer

Correct Answer: C. Short-circuiting transfer

Question 4

Which shielding gas generally supports easy arc starting and is widely used for GTAW of aluminium?

A. Argon

B. Carbon dioxide

C. Oxygen

D. Nitrogen

Correct Answer: A. Argon

Question 5

In GTAW of steels and nickel alloys, which polarity is commonly used?

A. DCEP

B. DCEN

C. AC with no polarity reversal

D. Reversed AC only

Correct Answer: B. DCEN

Question 6

Using H_net = η(60VI)/(1000v), what is the net heat input for η = 0.80, V = 24 V, I = 180 A and v = 300 mm/min?

A. 0.35 kJ/mm

B. 0.69 kJ/mm

C. 0.86 kJ/mm

D. 1.15 kJ/mm

Correct Answer: B. 0.69 kJ/mm

Question 7

What is a principal function of flux in arc welding?

A. To eliminate all heat from the arc

B. To provide shielding and form protective slag

C. To prevent the electrode from carrying current

D. To replace the welding power source

Correct Answer: B. To provide shielding and form protective slag

Question 8

What is the usual effect of increasing travel speed while voltage and current remain constant?

A. Heat input per unit length decreases

B. Heat input per unit length increases

C. Polarity changes automatically

D. Shielding gas becomes inert

Correct Answer: A. Heat input per unit length decreases

Question 9

Which condition is most directly associated with hydrogen cracking risk?

A. Susceptible microstructure, diffusible hydrogen and tensile restraint

B. High argon purity alone

C. Low travel speed alone

D. A clean, unrestrained joint with dry consumables

Correct Answer: A. Susceptible microstructure, diffusible hydrogen and tensile restraint

Question 10

Why is pulsed-spray transfer useful in some applications?

A. It eliminates the need for shielding gas in every process

B. It can provide spray-like droplet transfer with reduced average heat input

C. It prevents the electrode from melting

D. It is restricted to nonmetallic materials

Correct Answer: B. It can provide spray-like droplet transfer with reduced average heat input