a) Process code: SMAW, shielded metal arc welding; SAW, submerged arc welding; GMAW, gas−metal arc welding; FCAW, flux cored arc welding; GTAW, gas–tungsten arc welding; PAW, plasma arc welding; ESW, electroslag welding; OFW, oxyfuel gas welding; EBW, electron beam welding; LBW, laser beam welding.
b) Thickness: S, sheet, up to 3 mm (1/8 in.); I, intermediate, 3−6 mm (1/8−¼ in.); M, medium, 6−19 mm (¼−¾ in.); T, thick, 19 mm (¾ in. and up); X, recommended.
1.1.1.3 Types of Joints and Welding Positions
Figure 1.5 shows the basic weld joint designs in fusion welding: the butt, lap, T‐, edge, and corner joints. Figure 1.6 shows the transverse cross sections of some typical weld joint variations. The surface of the weld is called the face, the two junctions between the face and the workpiece surface are called the toes, and the portion of the weld beyond the workpiece surface is called the reinforcement. Figure 1.7 shows four basic welding positions.
Figure 1.5 Five basic types of weld joint designs.
Figure 1.6 Typical weld joint variations.
Figure 1.7 Four welding positions.
1.1.2 Solid‐State Welding Processes
In addition to fusion welding processes, various solid‐state welding processes have also been developed. Some of them are shown as follows:
1 (a) Friction Stir Welding:Friction stir welding (FSW)Friction stir spot welding (FSSW)
2 (b) Friction Welding:Linear friction welding (LFW)Rotary friction welding (RFW)
3 (c) Fast‐Collision Welding:Explosion welding (EXW)Magnetic pulse welding (MPW)
4 (d) Diffusion Welding
1.2 Gas Welding
1.2.1 The Process
Gas welding is a welding process that melts and joins metals by heating them with a flame caused by the reaction between a fuel gas and oxygen. Oxyacetylene welding (OAW), shown in Figure 1.8, is the most commonly used gas welding process because of its high flame temperature. A flux may be used to deoxidize and cleanse the weld metal. The flux melts, solidifies, and forms a slag skin on the resultant weld metal. Figure 1.9 shows three different types of flames in OAW: neutral, reducing, and oxidizing [4], which are described next.
Figure 1.8 Oxyacetylene welding: (a) overall process; (b) welding area enlarged.
Figure 1.9 Three types of flames in oxyacetylene welding [4].
Source: Welding Journal, 1969, © American Welding Society.
1.2.2 Three Types of Flames
1.2.2.1 Neutral Flame
This refers to the case where oxygen (O2) and acetylene (C2H2) are mixed in equal amounts and burned at the tip of the welding torch. A short inner cone and a longer outer envelope characterize a neutral flame (Figure 1.9a). The inner cone is the area where the primary combustion takes place through the chemical reaction between O2 and C2H2, as shown in Figure 1.10. The heat of this reaction accounts for about two‐thirds of the total heat generated. The products of the primary combustion, CO and H2, react with O2 from the surrounding air to form CO2 and H2O. This is the secondary combustion, which accounts for about one‐third of the total heat generated. The area where this secondary combustion takes place is called the outer envelope. It is also called the protection envelope since CO and H2 here consume the O2 entering from the surrounding air, thereby protecting the weld metal from oxidation. For most metals, a neutral flame is used.
Figure 1.10 Chemical reactions and temperature distribution in a neutral oxyacetylene flame.
1.2.2.2 Reducing Flame
When excess acetylene is used, the resulting flame is called a reducing flame. The combustion of acetylene is incomplete. As a result, a greenish acetylene feather between the inert cone and the outer envelope characterizes a reducing flame (Figure 1.9b). This flame is reducing in nature and is good for welding aluminum alloys because aluminum oxidizes easily. It is also good for welding high‐carbon steels (also called carburizing flame in this case) because excess oxygen can oxidize carbon and form CO gas porosity in the weld metal.
1.2.2.3 Oxidizing Flame
When excess oxygen is used, the flame becomes oxidizing because of the presence of unconsumed oxygen. A short white inner cone characterizes an oxidizing flame (Figure 1.9c). This flame is preferred when welding brass because copper oxide covers the weld pool and thus prevents zinc from evaporating from the weld pool.
1.2.3 Advantages and Disadvantages
The main advantage of the OAW process is that the equipment is simple, portable, and inexpensive. Therefore, it is convenient for maintenance and repair applications. However, due to its limited power density, the welding speed is very low and the total heat input per unit length of the weld is rather high, resulting in large heat‐affected zones (HAZs) and severe distortion. The OAW process is not recommended for welding reactive metals such as titanium and zirconium because of reactions with CO and H2.
1.3 Arc Welding