WELDING

1 Fusion Welding Processes
Fusion welding processes will be described in this chapter, including gas
welding, arc welding, and high-energy beam welding. The advantages and disadvantages
of each process will be discussed.
1.1 OVERVIEW
1.1.1 Fusion Welding Processes
Fusion welding is a joining process that uses fusion of the base metal to make
the weld. The three major types of fusion welding processes are as follows:
1. Gas welding:
Oxyacetylene welding (OAW)
2. Arc welding:
Shielded metal arc welding (SMAW)
Gas–tungsten arc welding (GTAW)
Plasma arc welding (PAW)
Gas–metal arc welding (GMAW)
Flux-cored arc welding (FCAW)
Submerged arc welding (SAW)
Electroslag welding (ESW)
3. High-energy beam welding:
Electron beam welding (EBW)
Laser beam welding (LBW)
Since there is no arc involved in the electroslag welding process, it is not
exactly an arc welding process. For convenience of discussion, it is grouped
with arc welding processes.
1.1.2 Power Density of Heat Source
Consider directing a 1.5-kW hair drier very closely to a 304 stainless steel sheet
1.6mm (1/16 in.) thick. Obviously, the power spreads out over an area of roughly
3
50mm (2in.) diameter, and the sheet just heats up gradually but will not melt.
With GTAW at 1.5kW, however, the arc concentrates on a small area of about
6mm (1/4 in.) diameter and can easily produce a weld pool.This example clearly
demonstrates the importance of the power density of the heat source in
welding.
The heat sources for the gas, arc, and high-energy beam welding processes
are a gas flame, an electric arc, and a high-energy beam, respectively. The
power density increases from a gas flame to an electric arc and a high-energy
beam. As shown in Figure 1.1, as the power density of the heat source
increases, the heat input to the workpiece that is required for welding
decreases.The portion of the workpiece material exposed to a gas flame heats
up so slowly that, before any melting occurs, a large amount of heat is already
conducted away into the bulk of the workpiece. Excessive heating can cause
damage to the workpiece, including weakening and distortion. On the contrary,
the same material exposed to a sharply focused electron or laser beam
can melt or even vaporize to form a deep keyhole instantaneously, and before
much heat is conducted away into the bulk of the workpiece, welding is completed
(1).
Therefore, the advantages of increasing the power density of the heat
source are deeper weld penetration, higher welding speeds, and better weld
quality with less damage to the workpiece, as indicated in Figure 1.1. Figure
1.2 shows that the weld strength (of aluminum alloys) increases as the heat
input per unit length of the weld per unit thickness of the workpiece decreases
(2). Figure 1.3a shows that angular distortion is much smaller in EBW than in
4 FUSION WELDING PROCESSES
Increasing
damage to
workpiece
Increasing
penetration,
welding speed,
weld quality,
equipment cost
Power density of heat source
high energy
beam welding
arc
welding
gas
welding
Heat input to workpiece
Figure 1.1 Variation of heat input to the workpiece with power density of the heat
source.
1.3 SHIELDED METAL ARC WELDING
1.3.1 The Process
Shielded metal arc welding (SMAW) is a process that melts and joins metals
by heating them with an arc established between a sticklike covered electrode
and the metals, as shown in Figure 1.10. It is often called stick welding.
The electrode holder is connected through a welding cable to one terminal
of the power source and the workpiece is connected through a second cable
to the other terminal of the power source (Figure 1.10a).
The core of the covered electrode, the core wire, conducts the electric
current to the arc and provides filler metal for the joint. For electrical contact,
the top 1.5 cm of the core wire is bare and held by the electrode holder. The
electrode holder is essentially a metal clamp with an electrically insulated
outside shell for the welder to hold safely.
The heat of the arc causes both the core wire and the flux covering at the
electrode tip to melt off as droplets (Figure 1.10b). The molten metal collects
in the weld pool and solidifies into the weld metal.The lighter molten flux, on
the other hand, floats on the pool surface and solidifies into a slag layer at the
top of the weld metal.
1.3.2 Functions of Electrode Covering
The covering of the electrode contains various chemicals and even metal
powder in order to perform one or more of the functions described below.
A. Protection It provides a gaseous shield to protect the molten metal from
air. For a cellulose-type electrode, the covering contains cellulose, (C6H10O5)x.
A large volume of gas mixture of H2, CO, H2O, and CO2 is produced when
cellulose in the electrode covering is heated and decomposes. For a limestone-
(CaCO3) type electrode, on the other hand,CO2 gas and CaO slag form when
the limestone decomposes. The limestone-type electrode is a low-hydrogentype
electrode because it produces a gaseous shield low in hydrogen. It is often
used for welding metals that are susceptible to hydrogen cracking, such as
high-strength steels.
B. Deoxidation It provides deoxidizers and fluxing agents to deoxidize and
cleanse the weld metal. The solid slag formed also protects the already solidified
but still hot weld metal from oxidation.

4 FUSION WELDING PROCESSES