The goal of the joining processes is to cause diverse pieces of material to become a unified whole. In the case of two pieces of metal, when the atoms at the edge of one piece come close enough to the atoms at the edge of another piece for interatomic attraction to develop, the two pieces become one.
There are two main categories for welding:
- Fusion welding. In fusion welding, two edges or surfaces to be joined are heated to the melting point, and, where necessary, the molten filler metal is added to fill the joint gap. The high-temperature phase transitions inherent to these processes create a heat-affected zone (HAZ) in the material. Fusion welds are created by the coalescence of molten base metals mixed with molten filler metals. Heat for melting is either developed at the intended weld joint or applied to the intended joint from an external source. An example of a means of developing heat at the weld joint is the passing of current through the electrical contact resistance between the contacting surfaces of the materials to be welded. Most fusion welding processes apply heat from an external source to the weld joint to produce the weld bond. Heat is transported from the heat source to the joint by conduction, convection, and radiation. Sources of externally developed heat include electron beams, laser beams, exothermic chemical reactions (used in oxyfuel gas welding and thermite welding), and electric arcs. Electric arcs, the most widely used heat source, are the basis for the various arc welding processes. Fusion welding manufactures many everyday items, including airplanes, cars, and structures.
- Solid-state welding. Two clean, solid metal surfaces are brought into sufficiently close contact for a metallic bond to be formed for solid phase welding. Solid phase welding can be accomplished at temperatures as low as room temperature. The bonding process is based either on deformation or diffusion and limited deformation, so atomic movement (diffusion) creates new bonds between atoms of two surfaces. Forge welding is a solid-state welding technique known for centuries. Many metals can be forged welded, the most common being both high and low-carbon steels. One of the most popular, ultrasonic welding, is used to connect thin sheets or wires made of metal or thermoplastic by vibrating them at high frequency and under high pressure. Another common process, explosion welding, involves joining materials by pushing them together under extremely high pressure. The energy from the impact plasticizes the materials, forming a weld, even though only a limited amount of heat is generated.
This section will focus on fusion welding, which is more common than solid-state welding. Fusion welding manufactures many everyday items, including airplanes, cars, and structures. Using a heat source with sufficient power makes it possible to fuse through a complete section of a very thick plate. The weld pool produced is difficult to control, and the heat-affected zone (HAZ) of such welds has a relatively coarse grain, adversely affecting the mechanical properties of the steel. The heat-affected zone (HAZ) is a ring surrounding the weld in which the temperature of the welding process, combined with the stresses of uneven heating and cooling, alters the heat-treatment properties of the alloy. The effects of welding on the material surrounding the weld can be detrimental—depending on the materials used and the heat input of the welding process used. The HAZ can be of varying size and strength. In the weld pool, heat is transported using convection and conduction.
An understanding of heat transfer is important in producing welds since the properties of a weldment are controlled by its geometry and by the composition and structure of the materials being welded.