MT UNIT-2
NOTES
Submerged Arc Welding (SAW)
Principle:
Submerged Arc Welding (SAW) is an arc welding process that utilizes heat generated by an electric
arc formed between a continuously fed consumable electrode and the workpiece. The distinguishing
feature of SAW is that the welding arc and molten weld pool are completely submerged under a
protective layer of fusible granular flux. This prevents spatter, minimizes fumes, and protects the weld
from atmospheric contamination.
During the welding process:
1. The arc melts the electrode wire and the base metal.
2. The granular flux is also melted, forming a molten slag that covers the weld pool.
3. The molten flux reacts with impurities in the weld metal, forming slag, which then solidifies
and floats on top of the weld.
4. After welding, the slag is removed, revealing the final weld.
SAW System and Applications:
SAW is a high-current welding process, typically used for:
• Joining thick steel plates in shipbuilding, structural steel construction, and heavy machinery
manufacturing.
• Pipeline welding (both longitudinal and spiral).
• Pressure vessel fabrication where deep penetration and high weld quality are required.
Advantages of SAW:
• High deposition rates and deep penetration.
• High-speed welding with minimal spatter.
• High-quality and consistent welds due to controlled arc characteristics.
Limitations:
• Not suitable for thin materials due to high heat input.
• Limited to horizontal and flat positions.
• Requires a flux handling and recycling system.
Essential Components of SAW Equipment:
1. Power Source:
o Can be AC or DC.
o DC polarity options:
▪ Direct Current Electrode Positive (DCEP): Provides stable arc, smaller weld
puddle, and deep penetration.
▪ Direct Current Electrode Negative (DCEN): Increases deposition rate but
reduces penetration.
o AC power sources balance arc stability and penetration.
2. Electrode (Welding Wire):
o Solid or tubular electrodes ranging from 1-5 mm in diameter.
o Continuous feed through a wire feeder.
o Copper-coated steel electrodes are used for better electrical conductivity and
corrosion resistance.
3. Flux System:
o The granular flux forms a protective slag over the weld.
o Flux prevents oxidation and stabilizes the arc.
o Types of flux include:
▪ Fused Flux (melted and solidified).
▪ Agglomerated Flux (powder-based).
▪ Bonded Flux (mix of materials bound by a binder).
▪ Mechanical Flux (for specific applications).
o Flux Recycling: Used flux can be collected and reused after removing impurities.
4. Wire Feeder & Control System:
o Ensures a constant supply of electrode wire.
o Regulates arc length and welding speed.
5. Torch and Flux Hopper:
o The torch directs the arc, while the flux hopper deposits flux over the welding area.
6. Travel Mechanism:
o Can be manual (semi-automatic) or automatic.
o In automated welding, either the torch moves over a stationary workpiece, or the
workpiece moves under a stationary torch.
Welding Procedure:
1. Flux Deposition:
o Flux is first deposited over the joint.
o When cold, flux is non-conductive, so the arc is initiated using a high-frequency unit
or by touching the electrode to the workpiece.
2. Arc Formation and Welding:
o Once the flux melts, it becomes highly conductive, allowing current to flow between
the electrode and workpiece.
o The welding electrode is continuously fed at a predetermined speed.
3. Slag Formation and Removal:
o The upper portion of flux remains granular and can be reused.
o The lower melted flux turns into slag, which must be removed.
4. Penetration and Backing Plate:
o Deep penetration may require a backing plate (steel or copper) to support the molten
metal.
Gas Metal Arc Welding (GMAW)
Principle of GMAW:
Gas Metal Arc Welding (GMAW), also known as Metal Inert Gas (MIG) or Metal Active Gas (MAG)
welding, is a semi-automatic or automatic arc welding process. It involves:
• A continuous, consumable wire electrode that feeds through a welding gun.
• A shielding gas supplied externally to protect the weld pool from atmospheric contamination.
• An electric arc generated between the consumable wire electrode and the workpiece, which
melts the metal, forming a strong welded joint.
The process allows high-speed welding with good control over the weld bead.
Power Supply:
• Direct Current Electrode Positive (DCEP) is most commonly used as it:
o Increases the metal deposition rate.
o Provides a stable arc and smooth metal transfer.
o Reduces spatter and results in a high-quality weld bead.
• Alternating Current (AC) is generally not used in GMAW due to:
o The arc extinguishing at each current cycle, making reignition difficult.
o Instability caused by polarity reversal.
• Direct Current Electrode Negative (DCEN) is not recommended as it causes an unstable
arc and excessive spatter.
• Constant Voltage Power Supply: This is preferred in GMAW since any change in arc length
affects voltage, automatically adjusting heat input and maintaining stable welding.
Metal Transfer Modes in GMAW:
GMAW employs different modes of transferring metal from the electrode to the weld pool:
1. Short-Circuit Transfer:
o The electrode touches the molten pool, creating a short circuit.
o The arc reignites as the electrode pulls away, depositing small droplets.
o Suitable for thin materials and out-of-position welding.
2. Globular Transfer:
o Larger metal droplets are transferred across the arc gap.
o It causes spatter and is mainly used with CO₂ shielding gas.
o Not preferred for precision work.
3. Spray Transfer:
o Fine, small metal droplets are transferred in a steady stream.
o Requires high current and an inert gas shield (argon-based).
o Produces smooth, high-quality welds with deep penetration.
4. Pulsed-Spray Transfer:
o Uses pulsed current to control droplet formation and transfer.
o Allows spray transfer at lower average currents, reducing heat input.
o Ideal for thin materials and out-of-position welding.
Factors influencing metal transfer include:
• Welding current and voltage.
• Electrode composition and diameter.
• Shielding gas type.
• Wire feed speed.
GMAW Equipment Components:
1. Welding Gun:
o Includes a control switch, contact tip, gas nozzle, power cable, and gas hose.
o The control switch triggers wire feed, power supply, and gas flow.
2. Wire Feeder:
o Feeds the electrode wire at a constant or variable rate.
o Some advanced systems adjust wire feed based on arc voltage.
3. Shielding Gas Supply:
o Protects the weld pool from atmospheric contamination.
o Common gases used:
▪ Argon: Provides deep penetration and arc stability.
▪ Helium: High thermal conductivity, used for thick materials.
▪ Carbon Dioxide (CO₂): Inexpensive but causes higher spatter.
▪ Gas Mixtures: Argon-CO₂, Argon-O₂, or Argon-Helium for different
applications.
4. Power Source:
o Maintains a constant voltage.
o Controls arc stability and metal transfer.
Welding Procedure in GMAW:
1. Workpiece Preparation:
o Clean the surface to remove rust, oil, or contaminants.
o Proper clamping and fixture setup for stability.
2. Electrode Selection:
o Choose the appropriate wire electrode based on material type and thickness.
o Typical diameters: 0.6 mm – 1.6 mm.
3. Shielding Gas Flow:
o Gas flow rate adjusted for adequate protection.
o Higher flow rates required in outdoor environments.
4. Arc Initiation:
o The welding gun is positioned at a suitable distance from the workpiece.
o A trigger starts the arc, wire feed, and gas flow.
5. Stick-Out Distance & Torch Angling:
o Maintaining an appropriate stick-out distance prevents overheating.
o Torch should be angled 45° for fillet welds and 90° for flat welding.
o Travel angle affects penetration and bead shape.
6. Weld Bead Formation:
o Consistent travel speed ensures uniform bead width.
o Trial welds help optimize voltage, current, and wire feed rate.
Gas Tungsten Arc Welding (GTAW/TIG)
Principle of GTAW:
Gas Tungsten Arc Welding (GTAW), commonly known as Tungsten Inert Gas (TIG) welding, is an
arc welding process that utilizes a non-consumable tungsten electrode to generate an arc between
the electrode and the workpiece. The arc melts the base metal, and a separate filler metal can be
manually added if needed. The entire process is protected by a shielding gas (such as argon or
helium), which prevents oxidation and contamination of the weld.
Key features:
• Uses a non-consumable tungsten electrode.
• Shielding gas prevents atmospheric contamination.
• Can be autogenous (without filler metal) or use a filler rod.
• A constant-current power supply generates the arc, which ionizes the gas and creates
plasma.
GTAW is widely used for welding thin sections of stainless steel and non-ferrous metals like
aluminum, magnesium, and copper alloys due to its high-quality welds.
Advantages of GTAW:
• Produces high-quality, precise welds with good aesthetic appearance.
• Suitable for thin metals.
• No spatter or slag formation.
• Provides better control over the weld pool than other arc welding processes.
Limitations:
• Slower welding speed compared to processes like GMAW and SAW.
• More complex and requires higher skill levels.
• Low deposition rate.
• Not suitable for very thick sections without multiple passes.
GTAW Equipment Components:
1. Welding Torch:
o Contains the tungsten electrode, gas nozzle, and control switch.
o Torches can be manual or automatic.
o Equipped with air- or water-cooling systems:
▪ Air-cooled torches for low-current welding (up to 200 A).
▪ Water-cooled torches for high-current welding (up to 600 A).
2. Power Supply:
o Uses a constant-current power source to maintain a stable arc.
o Can operate in DCEN (Direct Current Electrode Negative), DCEP (Direct Current
Electrode Positive), or AC (Alternating Current) modes.
o DCEN (Straight Polarity): Generates more heat near the workpiece, reducing
electrode overheating and increasing penetration.
o DCEP (Reverse Polarity): Used for oxide removal on aluminium, but generates
excessive heat at the electrode.
o AC (Alternating Current): Ideal for aluminium and magnesium as it removes oxides
while maintaining penetration.
3. Shielding Gas Supply:
o Protects the weld from atmospheric contamination.
o Common gases:
▪ Argon: Provides arc stability and is best for thin metals.
▪ Helium: Higher heat input, good for thicker materials and deep penetration.
▪ Argon-Helium Mix: Combines control and penetration benefits.
▪ Nitrogen (rarely used): Suitable for welding copper.
4. Electrodes (Tungsten Alloy Electrodes):
o Tungsten has a high melting point (3,410°C), making it ideal for non-consumable
applications.
o Electrode types:
▪ Pure Tungsten: Used for aluminum and magnesium (AC welding).
▪ Thoriated Tungsten: Best for stainless steel and carbon steel; provides
excellent arc stability.
▪ Zirconiated Tungsten: Used for high AC currents on non-ferrous metals.
5. Filler Rods (if used):
o Filler metal is selected based on the material being welded.
o Aluminum welding uses ER4045 or ER5356.
o Stainless steel welding uses ER308 or ER316.
Welding Procedure in GTAW:
1. Workpiece Preparation:
o Clean the metal surface to remove oxides, oil, and contaminants.
o Proper clamping and fixturing to ensure workpiece stability.
2. Electrode Setup:
o Select the correct tungsten electrode type and diameter.
o Shape the electrode tip for desired arc characteristics.
o For DC welding, electrodes are sharpened to a point.
o For AC welding, a balled-end electrode is preferred.
3. Shielding Gas Flow Adjustment:
o Argon flow rate: 10–20 CFH (Cubic Feet per Hour).
o Helium flow rate: Higher than argon due to its lightness.
4. Arc Initiation:
o A high-frequency generator provides an electric spark to establish the arc without
touching the workpiece.
o The arc is struck at 1.5–3 mm distance.
5. Welding Motion:
o The welder maintains a short arc length while controlling the molten pool.
o Torch angle is typically 10–15° backward.
o The electrode should never touch the workpiece.
6. Filler Metal Addition (if required):
o Manually fed into the molten pool while maintaining proper torch movement.
o The filler rod should not directly touch the tungsten electrode.
7. Post-Weld Cleaning:
o Oxide layer removal (especially for aluminium).
o Wire brushing or chemical cleaning for better weld appearance.
Plasma Arc Welding (PAW)
Principle of PAW:
Plasma Arc Welding (PAW) is an advanced arc welding process that is similar to Gas Tungsten Arc
Welding (GTAW/TIG) but utilizes a constricted plasma arc for higher energy density and precision.
In PAW:
1. A tungsten electrode generates an electric arc inside a narrow nozzle, which constricts and
ionizes a plasma gas (typically argon).
2. The plasma reaches extremely high temperatures (~30,000°C) and is directed onto the
workpiece, forming a weld pool.
3. The process can operate in two arc modes:
o Transferred Arc Mode: The arc transfers from the tungsten electrode to the
workpiece, providing deep penetration.
o Non-Transferred Arc Mode: The arc remains within the torch and is used for
applications like cutting or heating.
The result is a highly focused, stable, and controlled heat source, enabling precise, deep-
penetration welding.
Plasma as a State of Matter
PAW utilizes plasma, which is an ionized state of matter where atoms lose electrons, creating charged
particles (ions and free electrons). This ionization process:
• Increases thermal conductivity.
• Produces a narrow, high-energy arc.
• Allows high-speed welding with minimal heat distortion.
The key advantage of plasma is that it concentrates the energy into a smaller area than conventional
arc welding, making it ideal for thin materials and high-precision applications.
Equipment for PAW
PAW requires specialized equipment similar to GTAW but with added components:
1. Power Supply: Provides DC current, typically with pulsed modes for precision.
2. Plasma Torch:
o Contains the tungsten electrode and constriction nozzle.
o The arc is constricted to form a narrow, focused plasma jet.
3. Gas Supply System:
o Plasma gas (argon, hydrogen, or nitrogen): Ionized to create plasma.
o Shielding gas (argon or helium): Protects the weld pool.
o Secondary gas (optional): Enhances penetration or stabilizes the arc.
4. Cooling System:
o Uses water cooling for high-current operations to prevent overheating.
5. Control System:
o Regulates arc stability, travel speed, and penetration depth.
Modes of Operation
PAW can be performed in two different arc modes:
1. Transferred Arc Mode:
o The arc transfers from the tungsten electrode to the workpiece.
o Produces deep penetration.
o Used for welding and cutting applications.
2. Non-Transferred Arc Mode:
o The arc remains within the nozzle.
o Used for precision heating applications or plasma spraying.
Advantages of PAW
• Higher Arc Stability: More concentrated and stable than GTAW.
• Better Penetration Control: Due to the high energy density.
• High Travel Speeds: Faster than traditional TIG welding.
• Minimal Heat Distortion: Ideal for thin sheets and delicate materials.
• Excellent Weld Quality: Produces precise, defect-free welds.
Limitations:
• Higher Equipment Cost: More complex setup than GTAW.
• Requires Skilled Operators: More challenging to control.
• Not Suitable for All Applications: Overkill for simple welding tasks.
Fusion-Weld Zone in PAW
Like other arc welding methods, PAW creates a fusion weld zone, consisting of:
1. Fusion Zone: The area where metal melts and solidifies (weld metal).
2. Heat-Affected Zone (HAZ): The region where the microstructure changes due to heat
exposure.
3. Base Metal: The unaffected part of the workpiece.
The higher energy concentration in PAW means that:
• The HAZ is smaller, reducing the risk of warping.
• Penetration is deeper, making it suitable for thin sheets or critical aerospace components.
