Welding is the process of permanently joining two or more parts – usually metals or thermoplastics – by melting them and adding a filler material, causing them to fuse together into a strong joint. Once cooled, the two parts will be permanently bonded.
Different heating methods are used for welding, among them, electric arc, gas flame, laser, electron beam, friction and ultrasound. The most popular methods of modern welding include shielded metal arc welding, gas metal arc welding, submerged arc welding, flux-cored arc welding and electro-slag welding. Robot welding is also commonplace in industrial settings.
Spot welding and seam welding are two popular methods used for joining sheet metal parts. Low carbon steel is most suitable in spot welding, primarily used for materials that are normally 3mm thick. Higher carbon content or alloy steels tend to form hard welds that are brittle with a tendency to crack. This can be reduced to tempering. Aluminum can be welded using high-powered, very clean oxide-free surfaces. Dissimilar materials cannot be spot welded due to different melt properties and thermal conductivity. Plated steel welding takes on the characteristics of the coating. Nickel and chrome plated steel are relatively easy to spot weld. Aluminum, tin and zinc need special preparation inherent to the coating metals.
Commonly an industrial fabrication or repair process, welding can also be undertaken as a sculptural process in producing art pieces.
Parts of a weld and weld quality
After welding, a number of distinct regions can be identified in the weld area:
- Fusion zone – This is the weld itself. It is where the filler metal was applied during the welding process. The properties of the fusion zone depend primarily on the filler metal used, and its compatibility with the base materials.
- Heat-affected zone – This is the area that had its microstructure and properties altered by the weld. These properties depend on the base material’s behavior when subjected to heat. The metal in this area is often weaker than both the base material and the fusion zone, and is also where residual stresses are found.
Many factors influence the strength of welds and the material around them: the welding method, the amount and concentration of energy input, the weldability of the base material, filler material, and flux material, the design of the joint, and the interactions between all these factors. To test the quality of a weld, destructive or nondestructive testing methods are commonly used to verify that welds are free of defects, have acceptable levels of residual stresses and distortion, and have acceptable heat-affected zone (HAZ) properties. Methods such as visual inspection, radiography, ultrasonic testing, dye penetrant inspection, magnetic-particle inspection or industrial CT scanning can help with detection and analysis of defects.
Welding defects include cracks, distortion, gas inclusions (porosity), non-metallic inclusions, lack of fusion, incomplete penetration, lamellar tearing, and undercutting. Welding codes and specifications exist to guide welders in proper welding technique and in how to judge the quality of welds.
Welding is a dangerous occupation. Precautions should be taken during welding to prevent accidents. Special protective gear such as heavy leather gloves, long-sleeved jackets, welding masks and goggles are required to avoid burns, electric shock, damage to the eyes, inhalation of poisonous gases and fumes, and exposure to radiation.
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