Welding is the process of joining two materials with identical or slightly different chemical structures in such a way that they cannot be separated under the influence of heat, pressure, or both. During welding, the parts to be joined mix under the effect of applied heat and pressure. Once these effects are removed, the parts solidify and are fused together depending on the material. The welding process has evolved from the ancient method of forging heated iron strips together for sword-making to a sophisticated technique that now involves metallurgy, physics, chemistry, electronics, and quantum mechanics. Modern welding methods are categorized into two main groups based on whether heat or pressure plays the primary role in the process: **pressure welding** and **fusion welding**. A. Pressure Welding In pressure welding, mechanical force plays the primary role in joining the materials. Here are the main types: 1. **Forge Welding**: The parts are heated to welding temperature, overlapped, and hammered together to achieve a bond. 2. **Gas Pressure Welding**: Combustible gases like acetylene, propane, or hydrogen are burned with oxygen to reach the desired welding temperature. Adjustments are made according to the shape, material type, and thickness. Temperatures up to 3000°C can be achieved. 3. **Electric Resistance Welding**: This method uses the heat generated by electric current passing through a conductor. The heating depends on the material's cross-section, specific resistance, and current intensity. **Types of Electric Resistance Welding**: - **Butt Welding**: Parts are placed between two clamps connected to a transformer. Upon applying current and pressure, the parts heat up and join together. - **Spot Welding**: Used for joining thin metal sheets in automobile, aircraft, and lightweight structures. Electrodes press the sheets while applying heat and pressure. - **Seam Welding**: Ensures leak-proof joints in constructions like tanks and reservoirs by creating continuous welds with rotating electrodes. - **Tube Resistance Welding**: Used in pipe manufacturing, where edges are heated by electric current and pressed together for fusion. - **Induction Welding**: In modern pipe manufacturing, induction currents heat the tube edges for welding under pressure. - **Thermite Welding**: Commonly used in rail joining. Iron oxide and aluminum powder react to produce intense heat, melting the edges, which are then pressed together. - **Friction Welding**: One part is stationary while the other rotates under pressure, generating heat through friction for welding. - **Ultrasonic Welding**: High-frequency vibrations generate localized heat, fusing the parts under pressure. - **Explosion Welding**: Overlapping metal sheets are welded by the intense pressure and heat generated by an explosion. - **Cold Pressure Welding**: Commonly used for plastics, it involves high pressure without heat, resulting in recrystallization and bonding. B. Fusion Welding Fusion welding does not apply pressure to the parts. Instead, the joint surfaces are melted and fused together. A filler material of the same type is often added to fill gaps. 1. **Gas Welding**: Also called oxy-acetylene welding, where acetylene burns with oxygen to produce a flame at temperatures up to 3200°C. The flame melts the parts and filler material to create the joint. 2. **Electric Arc Welding**: The heat of an electric arc melts the joint area. The arc can form between an electrode and the material or between two electrodes. A filler rod is typically melted to complete the weld. **Types of Arc Welding**: - **Open Arc Welding**: - **Carbon Electrode Arc Welding**: The arc forms between a carbon electrode and the part. While CO and CO2 gases provide some protection, the weld quality is relatively low due to atmospheric exposure. - **Metal Electrode Arc Welding**: The arc forms between a metallic electrode and the material. The electrode melts to fill the joint and is often coated with flux to shield against atmospheric effects. - **Closed Arc Welding**: Includes submerged arc welding and gas-shielded arc welding, where the weld area is shielded from atmospheric contact. Welding techniques have advanced significantly and are now indispensable in modern industry, ensuring strong, durable joints for various applications.