Welding processes and types.
A. Welding is a process in which two pieces metal are joined together
permanently to form one piece.
There are two types of welding: including melting and without melting. The processes
without melting can be divided into two categories: including softening and without
softening.
B. The processes with melting consist of heating metals (or just one metal) to
their melting points. In most of these processes, additional metal (also called filler
metal) is added during the heating process to help bond the two pieces together. No
matter what is happening to the joined pieces, this filler is always melted.
According to this, there are two kinds of welding with melting: one, in which all metals
are melted on the welding spot (also named weld bath or pool), and another, in which
only filler is melted (this process is called soldering. In soldering, they usually employ
fillers made of tin or copper, due to their low melting points relatively to those of the
joined pieces. Soldering is used in electronics).
The processes including melting of all metals can be performed with or without
application of pressure and with or without fillers. The list below contains most
common types of the processes, according to source of heat which is obviously
needed to melt metals.
Type of heat source Names of processes Purpose
Electric arc (in air) MIG, TIG, Flux Cord Creating long seams
Electric current Spot Welding Creating points of
connection
Gas burning Gas Welding Creating long seams
Chemical reaction Thermite Welding Creating casts
C. Electric arc is a strong electric current formed by a spark in the air when
two electrodes are held close but not touching. Due to high electric resistance of air,
a large quantity of heat is emitted. This heat is enough to melt the metals in the
welding area. But: molten metals tend to react with oxygen, so the weld bath must be
shielded of it by a non-flammable gas (as argon or CO2).
MIG welding: the electrode is a thin wire which is fed from a spool attached on a gun
through a flexible tube and comes out of the nozzle on the welding gun or torch. The
wire is fed continuously when the trigger is pulled on the welding gun. Addition of a
non-flammable gas is needed. This is the reason for using a variant of MIG welding
called Stick Welding (meant, Shielded Metal Arc Welding, also identified as hand-
operated metal arc welding, flux shielded arc welding, or stick welding). In this type of
welding process in which the arc is struck between the metal rod or electrode (flux
coated) and the workpiece, the surface of both the rod and the workpiece melting to
create a weld bath. The simultaneous melting of the flux coating on the rod will
produce gas and slag, which shields the weld joint from the environment.
TIG welding: stands for Tungsten Inert Gas arc welding (by American welding society
it is also identified as GTAW). Weld electrode gets hot, but it doesn’t melt, it is a non-
consumable electrode. The term "non-consumable" does not mean that it lasts
forever, but that it does not melt and become part of the weld. This welding type uses
an outer source of shielding gas.
Flux Cored Arc Welding: this type of welding is almost like MIG welding. In fact, MIG
welders can often perform flux-cored arc welding. In this welding, the wire has a core
of flux that forms a gas shield around the weld. This reduces the demand for external
gas supply.
Spot welding: it is known that electric current choses a path with minimal resistance,
so when two electrodes are connected close to the welding point, due to low
resistance of metal, a very strong current passes through the point of contact, melting
the would be connected pieces on this point only, as the method is named.
Gas welding: it is performed by melting the sides or surfaces to be connected by gas
flame and providing the molten metal to flow together, thus creating a solid
continuous joint upon cooling.
Oxygen-acetylene (or propane) mixtures are used to a very greater extent. The
temperature of the oxy-acetylene flame in its hottest area is about 3200°C, while the
temperature reached in the oxy-propane flame is about 1900°C.
Thermite welding: a mix of iron oxide and aluminium in a box is ignited by spark.
Through the chemical reaction, iron frees up, aluminium oxidizes, temperature rises
and iron melt. Melted iron flows out of the box to the cast. This is a popular method of
connecting rails.
D. Welding without melting but including softening of would be connected
areas employs friction as the source of heat. This type is used for creating
"seamless" (almost) tubes and rods and for connecting aluminium profiles.
E. A standing out method of connecting large plates made of different metals
is called Explosion Welding. The method does not use heat, so the plates are not
melted or even softened. A stack of would be connected plates is formed, then
covered whit special explosives. They are ignited on one corner and burn evenly,
creating large pressure on the plates, thus forcing them together.
