Fundamentals of Welding and Welding Processes PDF

**Fundamentals of welding and welding processes** The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts -- a joint that cannot easily be separated. The term assembly usually refers to mechanical methods of fastening...

**Fundamentals of welding and welding processes** The term joining is generally used for welding, brazing, soldering, and adhesive bonding, which form a permanent joint between the parts -- a joint that cannot easily be separated. The term assembly usually refers to mechanical methods of fastening parts together, some of which allow for disassembly, while others do not. Welding is a materials-joining process in which two or more parts are coalesced at their contacting surfaces by a suitable application of heat and/ or pressure. Many welding processes are accomplished by heat alone, with no pressure applied; others by a combination of heat and pressure; and still others by pressure alone, with no external heat supplied. In some welding processes, a filler material is added to facilitate coalescence. The assemblage of parts that are joined by welding is called a weldment. Welding is most commonly associated with metal parts, but the process is also used for joining plastics. - Welding provides a permanent joint. The welded parts become a single entity. - The welded joint can be stronger than the parent materials if a filler metal is used that has strength properties superior to those of the parents, and if proper welding techniques are used. - Welding is usually the most economical way to join components in terms of material usage and fabrication costs. Alternative mechanical methods of assembly require more complex shape alterations (e.g., drilling of holes) and the addition of fasteners (e.g., rivets or bolts). The resulting mechanical assembly is usually heavier than a corresponding weldment. - Welding is not restricted to the factory environment. It can be accomplished "in the field." Although welding has the advantages indicated above, it also has certain limitations and drawbacks (or potential drawbacks): - Many welding operations are performed manually and are expensive in terms of labor cost. Most welding operations are considered "skilled trades," and the labor to perform these operations many be scarce. - Most welding processes are inherently dangerous because they involve the use of high energy - Because welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly. If the product must occasionally be disassembled (e.g., for repair or maintenance), then welding should not be used as the assembly method. - The welded joint can suffer from certain quality defects that are difficult to detect. The defects can reduce the strength of the joint. Welding processes divide into two major categories: (1) fusion welding, in which coalescence is accomplished by melting the two part surfaces to be joined, in some cases adding filler metal to the joint; and (2) solid-state welding, in which heat and/ or pressure are used to achieve coalescence, but no melting of the base metals occurs and no filler metal is added. Fusion welding is by far the more important category. It includes (1) arc welding, (2) resistance welding, (3) oxyfuel gas welding, and (4) other fusion-welding processes -- those that cannot be classified as any of the first three types. **Overview of welding technology** Welding involves localized coalescence or joining together of two metallic parts at their faying surfaces. The faying surfaces are the part surfaces in contact or close proximity that to be joined. Welding is usually performed on parts made of the same metal, but some welding operations can be used to join dissimilar metals. **Types of welding processes** Some 50 different types of welding operations have been catalogued by the American welding society. They use various types or combinations of energy to provide the required power. The welding processes can be divided into major groups: (1) fusion welding and (2) solid-state welding. **Fusion welding** These processes use heat to melt the base metals. In many fusion welding operations, a filler metal is added to the molten pool to facilitate the process and provide bulk and strength to the welded joint. A fusion-welding operation in which no filler metal is added is referred to as an autogenous weld. The fusion category includes the most widely used welding processes, which can be organized into the following general groups. - Arc welding (AW). Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc, as shown in figure 29.1. Some arc-welding operations also apply pressure during the process and most utilize a filler metal. Diagram of a welding process Description automatically generated - Resistance welding (RW). Resistance welding achieves coalescence using heat from electrical resistance to the flow of current passing between the faying surfaces of two parts held together under pressure. - Oxyfuel gas welding (OFW). These joining processes use an oxyfuel gas, such as a mixture of oxygen and acetylene, to produce a hot flame for melting the base metal and filler metal, if one is used. - Other fusion-welding processes. Other welding processes that produce fusion of the metals joined include electron-beam welding and laser-beam welding. Certain arc and oxyfuel processes are also used to cut metals **Solid-state Welding** Solid-state welding refers to joining processes in which coalescence results from the application of pressure alone or a combination of heat and pressure. If heat is used, the temperature in the process is below the melting point of the metals being welded. No filler metal is utilized. Representative welding processes in this group include: - Diffusion welding (DFW). Two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state diffusion. - Friction welding (FRW). Coalescence is achieved by the heat of friction between two surfaces. - Ultrasonic welding (USW). Moderate pressure is applied between the two parts and an oscillating motion at ultrasonic frequencies is used in a direction parallel to the contacting surfaces. The combination of normal and vibratory forces results in shear stresses that remove surface films and achieve atomic bonding of the surfaces. **Welding as a commercial operation** The principal applications of welding are (1) construction, such as buildings and bridges; (2) piping, pressure vessels, boilers, and storage tanks; (3) shipbuilding; (4) aircraft and aerospace; and (5) automotive and railroad. Welding is performed in a variety of locations and in a variety of industries. Owing to its versatility as an assembly technique for commercial products, many welding operations are performed in factories. However, several of the traditional processes, such as arc welding and oxyfuel gas welding, use equipment that can be readily moved, so these operations are not limited to the factory. They can be performed at construction sites, in shipyards, at customers' plants, and in automotive repair shops. Most welding operations are labor-intensive. For example, arc welding is usually performed by a skilled worker, called a welder, who manually controls the path or placement of the weld to join individual parts into a larger unit. In factory operations in which arc welding is manually performed, the welder often works with a second worker, called a fitter. The fitter's job is to arrange the individual components for the welder prior to making the weld. Welding fixtures and positioners are used for this purpose. A welding fixture is a device for clamping and holding the components in fixed position for welding. It is custom-fabricated for the particular geometry of the weldment and therefore must be economically justified on the basis of the quantities of assemblies to be produced. A welding positioner is a device that holds parts and also moves the assemblage to the desired position for welding. This differs from a welding fixture and only holds the parts in a single fixed position. The desired position is usually one which the weld path is flat and horizontal.