Unit 5 : Advanced Welding Processes

By: Mr. A V Kakade

Electron Beam Welding

Introduction

• EBW is a fusion welding process where a fast-moving electron beam hits the metal surface, transforming kinetic energy into thermal energy to melt and fuse the material.

• A major feature of this process is the greater depth of penetration that can be achieved due to the use of highly accelerated electrons.

Working

• The welding is performed at high voltage and under a vacuum environment.

• A Tungsten filament is heated to approximately 2500°C to generate a beam of electrons.

• This electron beam is accelerated towards the anode by a high electric field (up to 100 kW for deep section welding).

• Electromagnetic lenses are used to focus the beam of electrons onto the target workpiece.

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• This process enables very deep penetration, often creating a keyhole at fast travel speeds, which provides low overall heat input.

• The hole is immediately filled by molten metal from a reservoir, which then forms the weld upon solidification.

• Three-axis tables are used inside the vacuum chamber to hold and position the workpiece

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Advantages

1. Materials with high hardness values can be joined.
2. The process is done in a vacuum, so there are no impurities left, resulting in high-quality weld properties.
3. The absence of filler materials brings the operating cost down.
4. It creates a small heat-affected zone, which minimises distortion and material shrinkage, leading to fewer welding defects.

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Disadvantages:

1.     A skilled operator is required.
2.      The workpiece size is limited by the vacuum chamber's size.
3.     The initial investment is high.

Applications:

1.      Used in the aerospace and automobile industries.
2.      Materials like Titanium can be welded.

Process Parameters

1. Electron beam power: The total power of the beam is constant over a large working distance, but power density decreases with distance. A larger distance gives a broader, less penetrating weld.
2. Accelerating voltage: The depth of penetration depends on the accelerating voltage and can be increased by increasing the voltage.
3. Beam current: Penetration is directly proportional to beam current. Increasing or decreasing the current will increase or decrease the penetration depth.
4. Electron travel speed: As travel speed is increased, penetration is reduced because the heat input is lower.
5. Electron beam spot size: The beam spot size can be concentrated to 0.5–3 mm. A smaller spot size produces a narrow weld geometry.

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Laser Beam Welding �

• LASER stands for Light Amplification by Stimulated Emission of Radiation.
• When atoms get energy, their electrons are at an excited state. When they come down to a lower energy level, they emit photons. When these photons are amplified and directed, it is called a LASER.

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LBW Working

1. Laser beams can be concentrated on a tiny spot, which is used to focus the beam on the metal interfaces to be welded.
2. The targeted laser beam melts a portion of the metal, and the molten metal gets diffused into one another.
3. The joint is then allowed to cool, after which a strong welding joint is established.
4. This technique is widely used in the automobile industry.
5.  Dissimilar metals can be welded.

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Materials: Can weld Titanium, Molybdenum, Nickel, Aluminum, and Stainless Steel.

LBW - System Components

1. Power Source: Required to put the flash lamps on.

2. Flash Lamps: Used to emit light.

3. Ruby Crystal: The atoms of the ruby crystal absorb energy and the electrons of those atoms get excited. When they come down to ground level, they emit a photon of light.

4. Lens: Emitted photons are concentrated with the help of a lens on a target.

5. CAM (Computer-Aided Manufacturing): Takes care of controlling the operations when welding is in process. It can speed up or slow down the speed also.

6. CAD (Computer-Aided Design): Used to design the welding joint.

Advantages:

1.      Intricate areas can easily be welded.
2.      Good quality welds can be achieved.
3.      It is a 'contact less' process, so no tooling is worn out.
4.      Less time is required for welding thick sections.
5.      Dissimilar metals can be welded.

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Disadvantages:

1.      The equipment cost is high.
2.      High skills are required.
3.      Cracks may get initiated due to the high cooling rate.

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Materials: Can weld Titanium, Molybdenum, Nickel, Aluminum, and Stainless Steel.

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Electroslag Welding

Introduction

• ESW is a fusion welding process invented for welding thick mild and low-alloy steels in a vertical or near-vertical position.
• It is an efficient process to weld plates with thicknesses from 50 mm to over 900 mm in a single pass.
• The process generates heat to melt the electrode and fuse plates using the resistive heating of a molten slag pool.

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Working

1. Before welding, a gap of about 20-50 mm is maintained between the two plates.
2. At the bottom, a starting plate is attached, and the gap is filled with welding flux.
3. Water-cooled copper shoes are used to confine the molten pool and prevent it from flowing out.
4. An electric arc is initiated to melt the flux and form a molten slag pool. The arc occurs only at the beginning and is then extinguished.
5. Electric current is then passed through the low-electric-conductive molten pool to sustain the liquid state.
6. At sufficient temperature, both the electrode (feed wire) and the edges of the plates melt.
7. The solidification process starts from the bottom and moves upwards. As it moves up, the copper shoes also move upwards.

Weld Properties and Parameters

1. The process is sometimes called a joint to be 'cast' within the edges of two plates.
2. The weld has a large grain size with a columnar structure, which generally leads to inferior mechanical properties.
3. Post-weld normalizing heat treatment is performed to mitigate these poor properties.

Process Parameters:

•     The heat produced is governed by the depth and width of the slag, electrode dimensions, and electric parameters.
•     Voltage: 40 to 50 volts (influences penetration and bead width).
•     Current: Up to 600A (responsible for electrode melt rate).
•     Travel speed: 13-36 mm/min.

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Advantages:

1.     The high deposition rate leads to high productivity.
2.     It is economic for welding thick plates.
3.     Can weld a wide range of plate thicknesses.
4.     Suitable for long butt joints in heavy metal plates and has little transverse shrinkage.

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Disadvantage:

1.     Weld metal and HAZ generally have inferior mechanical properties, requiring post-weld heat treatment.

Applications:

1.     Used to weld heavy plates of thickness over 50-75 mm.
2.     Used for large steel structures like bridges, machinery frames, and offshore structures

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Electrogas Welding (EGW)

Introduction

• EGW is a variant of Electroslag welding.
• It uses a shielding gas (as in MIG/MAG welding) instead of a pool of slag to protect the weld.
• In EGW, arc heat is generated to melt the electrode (feed wire) rather than using resistance heat.
• A suitable gap is maintained, and a water-cooled copper shoe is attached to form a cavity.
• A continuous electrode is fed into the zone while an inert gas is applied to prevent oxidation.
• An electric arc is initiated, which melts the electrode and edges of the plates to form a melt pool.
• Water-cooled copper shoes confine the molten pool as solidification starts from the bottom and rises in a single pass.

Shielding Gas in EGW

• A key feature of EGW is its use of shielding gas.

• When a bare electrode wire is used, an external shielding gas like CO₂ is supplied from an external source to protect the arc and molten pool.

• If a flux-cored electrode wire is used, the flux itself can provide the necessary shielding, eliminating the need for an external inert gas supply

Applications:

1.     Welding of low and medium carbon steels.
2.      Used to weld metal plates of thickness under 50-75 mm.
3.     Used in the erection of storage vessels and less critical structures

Atomic Hydrogen Welding

Introduction

• This is an electric arc welding process invented in the early 19th century by Irving Langmuir.
• It has been largely replaced by modern MIG and TIG welding processes.
• An arc is generated between two non-consumable tungsten electrodes in a shielding atmosphere of hydrogen gas.
• A high temperature of 3400 to 4000°C is obtained by using the electric arc and hydrogen gas.

Working

• An alternate arc is maintained between the two electrodes.
• When hydrogen is fed through the arc, the hydrogen molecules (H₂) break down into hydrogen ions (atomic hydrogen).
• This reaction is endothermic, meaning it absorbs heat (422 kJ).
•      H₂ + Heat (422 kJ) → H + H
• When this stream of hydrogen ions hits the cooler workpiece surface, the ions recombine to form hydrogen molecules.
• This recombination is exothermic, releasing a high amount of heat (422 kJ) at the point of contact, creating a high-temperature flame of around 3400 to 4000°C.
•      H + H → H₂ + Heat (422 kJ)
• This heat is sufficient to melt the surfaces, and a filler rod can be used.
• The hydrogen also acts as a shield for the weld zone, avoiding oxidation and resulting in a clean and homogeneous weld.

Advantages:

    1. The hydrogen shields the weld zone, so no flux or other inert gas is needed.

    2. It offers a faster rate of welding with little distortion of the workpiece.

    3. Welding of thin material is possible.

Disadvantages:

    1. High operating cost.

    2. Requires high safety standards due to the flammable nature of hydrogen.

Applications:

    ◦ Used to weld tool steel and hardened steel containing tungsten, nickel, and molybdenum.

    ◦ Used to weld non-ferrous metals and thin sheets or smaller diameter rods

Underwater Welding

• Also called hyperbaric welding, it is used to weld marine structures submerged in water, often at elevated pressures.
• It is generally used for offshore structures, pipelines, ships, submarines, and nuclear reactors.
• The operator must be specially trained and qualified for the task.
• There are two main types: Dry Welding and Wet Welding

Dry Welding

Working

• Welding is performed at ambient pressure inside a chamber (a hyperbaric chamber) from which water has been displaced with a gas like air.
• The welder may be entirely or partially inside the chamber.
• The weld mechanical properties are equal to similar welds performed above water.
• The pressure range is 0 to 25 bar, and the temperature is normally 25-35°C.
• The preferred depth should not be more than 300 m.
• Processes like TIG, SMAW, MIG, and Flux-cored Arc Welding can be used.

Advantages:

    1. Weld quality is as good as the quality obtained in normal conditions.

    2. Pre- and post-heat treatment is possible.

    3. Offers better welder safety as compared to wet welding.

Disadvantages:

    1. A specially designed chamber is required for each application.

    2. Large equipment setup is required.

    3. High cost of operation.

Wet Welding

• The diver (welder) and the electrode are directly exposed to the water and the surrounding environment.
• There is no physical barrier between the water and the welding arc.
• A special waterproof stick electrode is used.
• Shielded Metal Arc Welding (SMAW) and Flux-cored Arc Welding are commonly used.

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Advantages:

    1. Does not have a complicated setup and can be deployed immediately.

    2. Minimum set of equipment is required, making it economical.

    3. Less costly compared to dry welding.

    4. The cooling rate is higher, so the welding rate is also high.

Disadvantages:

    1. The welder is exposed to more electrical hazards.

    2. The weld contains high porosity due to water vapor and entrapped carbon monoxide.

    3. Hydrogen embrittlement occurs due to molecular hydrogen generation, which causes cracks and porosity.

    4. Poor visibility for the underwater welder causes operation issues.

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Challenges In Underwater Welding

• Electric shock prevention: This is the biggest challenge as water offers little resistance to electricity. Special waterproof equipment must be used.

• Explosive potential: Oxygen and hydrogen are generated under pressure and can cause an explosion. Proper safety measures must be taken.

• Drowning: The welder must have good diving skills and be fully trained.

• Health hazards: Decompression sickness can occur when divers inhale harmful gases. Long-term high-pressure work can lead to ear, lung, and nose damage.

Plasma Arc Welding

• Plasma arc welding was introduced in welding application in 1964 to provide an advanced level of control and provide high weld quality.
• When inert gas is heated at an extremely high temperature, it forms plasma consisting of equal numbers of electrons and ions and carries electric current.
• The plasma carries a high temperature in the range of 16600 to 20000°C.
• Plasma arc welding (PAW) is a variant of gas tungsten arc welding (GTAW) in which heat generated with plasma is produced between a non-consumable electrode and the workpiece.
• However, the construction of Plasma arc welding is different from GTAW.

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Working

• Plasma arc welding (PAW) consists of a welding torch.
• In the torch, two nozzles are mounted as shown in fig.
• An inner copper nozzle has a small orifice and contains a non-consumable tungsten electrode.
• Inert gas is supplied through this inner nozzle for plasma generation.
• The outer nozzle is used to supply shielding gas.
• A low current pilot arc is created between the electrode and nozzle orifice or workpiece.
• Inert gas is passed through the nozzle, where the pilot arc heats the gas to form a plasma.
• Because of the nozzle shape, the plasma arc is concentrated and jetted at high velocity with a small diameter.

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Therefore the plasma jet has high energy density, higher than other arc welding processes.

This high-temperature plasma arc is used to melt the filler rod and workpiece material to form a weld joint.

The shielding gas surrounds the plasma arc and protects the weld from oxidation.

Argon, helium, or a mixture of argon-hydrogen is used as a shielding gas.

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Types of Torches

There are two types of torches used in plasma arc welding.

1. Transferred arc: The workpiece is part of the electrical circuit i.e., the arc is transferred from the electrode to the workpiece.

2. Non-transferred arc: the arc occurs between the electrode and the nozzle.

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Advantages of Plasma Arc Welding

1. The plasma jet is concentrated; hence it has better arc control during the welding process and has a less heat-affected zone.
2. Because of the less heat-affected zone, distortion of the weld and weld part is less.
3. The plasma arc temperature is around 20000°C, hence it can be used to melt and weld any metal.
4. The concentrated plasma jet has high velocity and penetration depth, hence it has a high speed compared to any welding process.

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Disadvantages of Plasma Arc Welding

1. High equipment cost.
2. A skilled operator is required.

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Application of Plasma Arc Welding

1. Used to weld hard metal such as tungsten, bronze, cast iron, lead etc.

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Cold Metal Transfer

Introduction and Need

• Cold Metal Transfer is one of the variants of gas metal arc welding (GMAW), specifically MIG, developed in 2004.
• It is well suited for welding aluminum and dissimilar metal, also ideal for a thin sheet of metals.
• The word ‘Cold’ implies generating a lower amount of heat than other gas metal arc welding processes.
• During the arc welding process, variation in the heat input or temperature generates residual stresses in the weld.
• Upon releasing residual stresses, distortion in the weld joint and weld metal occurs, which poses a dimensional accuracy problem.
• When high heat is applied, the filler rod melts before reaching the workpiece and causes a splatter of metal droplets onto the workpiece surface.
• Excess heat of droplets also creates holes on the workpiece if the workpiece is too thin.
• For thermally conductive metal such as aluminum, it is critical to control the heat in the welding.

Principle and Working

• CMT is ideal for welding aluminum where the process provides less heat sufficient for the weld and eliminates the platter of metal droplets.
• CMT uses a digital control process where the feed wire moves forward and is pulled back as soon as a short circuit occurs, introducing heat for a very brief period.
• The CMT process works on short circuit mode, which uses low voltage and low current to generate low heat compared to the MIG welding process.
• CMT uses advanced digital control over the rate of feed wire as compared to the conventional MIG welding process.

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Phases of Working

Phase 1: Peak current phase: An initial high pulse of current is supplied to form an arc between the feed wire (electrode) and the workpiece, melting the tip of the wire to form a droplet.

Phase 2: The background current phase: Low current is supplied until a short circuit happens.

Phase 3: The short-circuiting phase: The wire touches the workpiece, arc voltage changes to zero, and the arc is extinguished. The feed wire is retracted, leading to break-away droplets from the tip of the feed wire which transfer to the weld pool.

Phase 4: Arc is again reignited, and the cycle is repeated.

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Advantages of CMT

1. Low thermal distortion at the weld joint.
2. The process can be used for cladding and producing a smooth coating.
3. The good deposition rate of 5 kg/h with a single wire.
4. The process can be easily automated.
5. A more efficient process for welding of thin sheets.
6. Precise welding for materials like aluminum and steel with little splatter.
7. Efficient arc management.
8. Aesthetically cleaner weld.

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Disadvantage of CMT Suitable only for welding of thin sheets.

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Robotic Welding

Introduction

• Welding is an essential part of manufacturing, and robotic welding is considered a modern part of the manufacturing system.
• Robotic welding is preferred to increase accuracy, enhance safety and reduce the time required for welding.
• According to the Robotics Institute of America, a robot is a "reprogrammable, multifunctional manipulator designed to move materials, parts, tools, or specialized devices, through variable programmed motions for the performance of a variety of tasks".

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In the case of welding robots, the ‘tool or specialized device’ is the welding heads, wire feed system, and tracking device, whereas the ‘task’ is welding.

The robotic welding system consists of two subsystems:

1. Robot configuration and

2. Welding package.

Fig : Articulated arm

Fig: SCARA

Fig: Cartesian Robot

Robot Configuration

1. The most common configuration used in robotic welding is a vertical articulated arm robot which provides six or more axis movement.
2. The vertical articulated arm robot provides more flexibility and can reach out to difficult access areas for welding.
3. Another configuration of a robot is a Cartesian robot and a SCARA robot configuration.
4. Cartesian robots are used where a large operating area is available and requires high precision, but cannot be used for jobs with complex access areas.
5. SCARA (Selective Compliance Assembly Robot Arm) provides more flexibility than Cartesian robots and has limited position capabilities.

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Robotic System - Actuators and Control

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Actuators

1. Pneumatic, hydraulic, or electrical actuators may drive the arm.
2. An electrical actuator like a stepper motor or DC motor is used when the arm carries a low load and requires high positional accuracy.
3. Pneumatic and hydraulic actuators are used when high precision in positioning is not required. A hydraulic system is used when the application requires high load-carrying capacity.

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Control system

1. The robot controller uses feedback from a variety of sensors to monitor the robotic system.
2. The sensors are applied to observe and measure process parameters, acting as input sources to the control system.
3. The controller adapts the output of the robotized welding process in accordance with the defined welding procedure specification.
4. The essential function of the robot controller is to execute the program.