Chapter 6 - Welding as a Joining Process

Abstract

Welding is an extraordinarily effective and versatile process for joining similar types of materials, including metals and their alloys, thermoplastic polymers, glasses, some ceramics, and even composites with these materials as their matrix phase. It uses the combination of heat and pressure to bring atoms, ions, or molecules to their equilibrium spacing to allow atomic-level bond formation like those in the parent materials to form joints and produce joint strength. The welding of real materials is complicated by the existence of adsorbed layers of atmospheric gases, moisture, oxides or other tarnish compounds, and other contaminants on surfaces. The challenge in practical welding is to overcome surface contaminants by cleaning and surface asperities by causing melting or solid-state plastic deformation. In fact, welding processes are broadly classified as fusion or non-fusion types, depending on whether melting or plastic deformation is the principal means for bringing atoms, ions, or molecules together to effect a weld. Other classification schemes consider whether or not the process requires pressure, while still others consider the source of energy to cause the required bonding. The most common gas welding process, electric arc welding processes, high-energy-density beam welding processes, and resistance welding processes are described in this chapter, along with non-fusion pressure, friction, and diffusion processes. The design of a weld joint or the joint type was described in the chapter as being dependent on the geometric requirements and restraints of the structure being welded, as well as on the expected loading. Specific preparations of joint types depend on the process, loading, and cost constraints.