1. Introduction
Welding is a permanent joining process in which two or more metal parts are joined together by the application of heat, pressure, or both, with or without the use of a filler material. During welding, the base metals are heated to a molten or plastic state, allowing them to fuse and form a strong joint upon cooling.
Welding is widely used in manufacturing, construction, automotive, shipbuilding, pipelines, and aerospace industries due to its strength, reliability, and economy.
Table of Contents
1.1 Advantage and Disadvantage of welding
| Advantage | Disadvantage |
|---|---|
| 1. Permanent and strong joint 2. Weight saving compared to riveted joints 3. Leak-proof joints possible 4. High structural efficiency 5. Suitable for complex shapes | 1. Residual stresses and distortion 2. Requires skilled labor 3. Inspection is difficult 4. Heat-affected zone may weaken material |
2. Principle of Welding
Two materials are joined by creating atomic bonding at their interface, either by melting them or by forcing them together in the solid state. The basic principle of welding is:
- Heat is applied to the joint area
- Metal edges melt or soften
- Filler metal (if used) melts and mixes with base metal
- On cooling, a metallurgical bond is formed
3. Classification of Welding Processes
A. Fusion Welding
Fusion welding is a joining process that uses heat to melt the base and weld metal and fuse into each other. Fusion welding is 3 types.
- Gas Welding
- Arc Welding
- Thermit Welding
B. Pressure Welding
Pressure welding is a welding process by which two metals joined by applying pressure with or without heat. In this process Heat may be absent or only moderate to bond the surface of metals without melting. This process is also called as solid state welding.
- Forge Welding
- Resistance Welding
- Friction Welding
- Diffusion Welding
A.1. Gas Welding
- Heat produced by combustion of fuel gas (usually acetylene) and oxygen
- Common type: Oxy-acetylene welding
Flame types:
- Neutral flame
- Carburizing flame
- Oxidizing flame
Applications:
- Thin sheets
- Repair works
- Pipes and tubes
A.2. Arc Welding
Heat is generated by an electric arc between electrode and workpiece.
Types of Arc Welding
- Shielded Metal Arc Welding (SMAW)
- Gas Metal Arc Welding (MIG/GMAW)
- Gas Tungsten Arc Welding (TIG/GTAW)
- Submerged Arc Welding (SAW)
A.2.1 Shielded Metal Arc Welding (SMAW)
Shielded Metal Arc Welding (SMAW), also known as stick welding, is a manual arc welding process in which heat is generated by an electric arc between a flux-coated electrode and the workpiece.
The flux coating produces a protective gas and slag that protect the molten metal from atmospheric contamination.
How it Works?
- An electric current passes through the electrode.
- An arc is formed between the electrode and base metal.
- The heat melts both the electrode and base metal.
- The molten electrode fills the joint.
- Flux coating forms a protective shield and slag.
A.2.2 Gas Metal Arc Welding (MIG/GMAW)
A.3. Thermit Welding
Thermit welding is a fusion welding process in which heat is produced by a chemical reaction between aluminium powder and iron oxide. The reaction generates extremely high temperature (around 2500–3000°C) that melts the metal and joins the parts.
The welding is based on the thermite reaction, which is an exothermic reaction:
Iron oxide + Aluminium → Molten iron + Aluminium oxide + Heat
Chemical equation:
The molten iron formed is poured into the gap between the workpieces to form a strong joint.
Applications:
- Railway track welding
- Heavy machine parts
B.1 Forge Welding
Forge welding is the classic blacksmith trick of joining two pieces of metal by heating them until they’re plastic (almost molten on the surface) and then hammering them together, so they fuse.
How it works:
- Prep the joint – Clean metal is everything. Grind or file off rust, scale, oil.
- Heat – Bring both pieces to welding heat in the forge.
- Mild steel: bright yellow to near-white.
- Flux – Sprinkle borax (or welding flux) to stop oxidation and help the surfaces flow.
- Set the weld – Light, controlled hammer blows at first to “stick” the joint.
- Consolidate – Reheat and hammer more firmly to fully fuse the metal.
B.2. Resistance Welding
- Heat produced by electrical resistance at the joint
- Pressure is applied during welding
Types:
- Spot welding
- Seam welding
- Projection welding
Applications:
- Automobile bodies
- Sheet metal fabrication
B.3 Friction Welding
Friction welding is a solid-state welding process in which heat is generated by mechanical friction between two workpieces in relative motion, followed by the application of axial pressure to form a strong joint. The materials are joined without melting, which helps preserve their original properties.
B.4 Diffusion Welding
Diffusion welding is a solid-state welding process in which two materials are joined by atomic diffusion at their interface under high temperature and pressure, without melting the base metals. The process is carried out in a vacuum or controlled atmosphere to prevent oxidation.
3. Welding Electrodes
A welding electrode is a metal rod or wire that conducts electric current to create an arc and may also supply filler metal to the weld joint. In arc welding, electrodes can be consumable (melt and become part of the weld) or non-consumable (do not melt).
Types:
- Consumable electrodes
- Non-consumable electrodes (tungsten)
Functions of electrode coating:
- Stabilizes arc
- Produces shielding gas
- Forms slag to protect weld pool
4. Welding Joints
Common types of welded joints:
- Butt joint
- Lap joint
- Tee joint
- Corner joint
- Edge joint
5. Welding Positions
Welding position refers to the orientation of the weld joint with respect to gravity during welding. Proper selection of welding position affects weld quality, strength, and ease of operation.
Main welding positions:
- Flat position (1G / 1F):
Easiest position with good control and penetration. - Horizontal position (2G / 2F):
Weld axis is horizontal; molten metal tends to sag. - Vertical position (3G / 3F):
Welding is done vertically (upward or downward); requires skilled control. - Overhead position (4G / 4F):
Welding is performed from below the joint; most difficult due to gravity.
13. Welding Defects
Welding defects are imperfections or flaws that occur in welded joints. These defects reduce the strength, durability, and safety of the welded structure. They can occur due to improper welding techniques, poor material, or incorrect parameters. Common welding defects include:
Crack: Cracks are the most dangerous welding defects. They may appear during or after welding.
| Cause: Improper weld design, High stress, Rapid cooling, Improper filler metal. |
| Remedy: Good weld design, Preheating, Controlled cooling, Correct filler material. |
Porosity: Porosity is the presence of gas pores or bubbles trapped inside the weld metal.
| Cause: Moisture in electrodes or base metal, Dirty/Contaminated surface, Poor shielding gas |
| Remedy: Clean the surface, Use dry electrodes, Proper shielding gas flow |
Lack of Fusion: This occurs when weld metal does not properly fuse with the base metal.
| Cause: Low heat input, Improper electrode angle, Fast welding speed |
| Remedy: Increase current, Correct welding technique |
Undercut: Undercut is a groove formed along the weld toe, reducing strength.
| Cause: High current, Fast welding speed, Wrong electrode angle. |
| Remedy: Reduce current, Correct travel speed |
Slag Inclusion:
14. Inspection and Testing of Welds
- Visual inspection
- Dye penetrant test
- Radiographic test
- Ultrasonic testing
- Magnetic particle testing
15. Applications of Welding
- Bridges and buildings
- Automobiles and railways
- Pressure vessels and boilers
- Shipbuilding
- Pipelines and storage tanks