Fundamentals of welding

Welding is a materials joining process in which two (or more) parts are coalesced at their contacting surfaces by the suitable application of heat and/or pressure. The assemblage of parts that are joined by welding is called a weldment.

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. Welding is most commonly associated with metal parts, but the process is also used for joining plastics.

Welding is a relatively new process, its commercial and technological importance derives from the following:

• 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 material is used that has strength properties superior to those of the parents, and 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 shapes alterations (for example, drilling of holes) and addition of fasteners (for example, 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, it also has certain limitations and drawbacks (or potential drawbacks):

• Most welding operations are performed manually and are expensive in terms of labour cost. Many welding operations are considered skilled trades, and the labour to perform these operations may be scarce.
• Most welding processes, involving the use of high energy, are inherently dangerous.
• Since welding accomplishes a permanent bond between the components, it does not allow for convenient disassembly. If there is a need for occasional disassembly of the product (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 involves localised 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 are 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

Over 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. Welding processes can be divided into two major groups: (1) fusion welding and (2) solid-state welding.

Fusion Welding

Fusion welding 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 comprises the most widely used welding processes and includes the following general groups (initials in parentheses are designations of the American Welding Society):

Arc welding (AW). Arc welding refers to a group of welding processes in which heating of the metals is accomplished by an electric arc. Some arc welding operations also apply pressure during the process, and most utilise a filler metal.

Resistance welding (RW). Resistance welding achieves coalescence using heat from electrical resistance to the flow of a 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. In addition to the preceding types, there are other welding processes that produce fusion of the metals joined. Examples include electron beam welding and laser beam welding.

Certain arc and oxyfuel processes are also used for cutting metals.

Solid-state Welding

Solid-state welding refers to joining processes in which coalescence results from 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 utilised in solid-state processes. Some representative welding processes in this group include the following:

Diffusion welding (DFW). In diffusion welding, two surfaces are held together under pressure at an elevated temperature and the parts coalesce by solid-state fusion.

Friction welding (FRW). In this process, coalescence is achieved by the heat between two surfaces.

Ultrasonic welding (USW). Ultrasonic welding is performed by applying a moderate pressure between the two parts and using an oscillating motion at ultrasonic frequencies 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.

 (GMAW or MIG)