SOLID STATE WELDING PROCESSES: Forge Welding, Cold Pressure Welding, Friction Welding, Explosive Welding, Diffusion Welding, Thermo-compression Welding


In these processes, the base materials to be
joined are heated to a temperature below or just up to the solidus temperature
and then continuous pressure is applied to form the welded joint. No filler
metal is used in solid-state welding processes. The various solid-state welding
processes are-

(1)  Forge Welding

(2)  Cold Pressure Welding

(3)  Friction Welding

(4)  Explosive Welding

(5)  Diffusion Welding

(6)  Thermo-compression Welding

of the above important welding processes are discussed as under

Forge Welding

In this welding process, the work-pieces to be
welded are heated to the plastic condition (above 1000°C), and then placed
together and forged while hot by applying force. Force
may be applied by hammering, rolling, drawing or squeezing to
achieve the forging action. Forge welding was originally the first process of
welding. In this process the two metal pieces to be joined are heated in a
forge or furnace to a plait condition and then they are united by pressure. The
ends to be joined are heated in a furnace to plastic condition and formed
   the required shape by upsetting.
Then they are brought together and hammered, so as to
get the finished joint similarly, a butt joint can be prepared by forge welding
as shown in Fig, Before joining the two pieces, their
ends are formed to the required shape according to the type of joint. The forge
welding is a manual process and is limited to light work because all forming
and welding are done with a hand sledge. It is a slow process and there is
considerable danger of an oxide scale forming on tile surfaces. The tendency to
can  be counteracted somewhat
by using a thick fuel bed and by covering the surfaces with a fluxing material,
which dissolves the oxides. Borax in combination with salt ammoniac is commonly
used as flux. The forge welding is recommended to such metals, which have a
large welding temperature range like low carbon steel and wrought iron. By the
increase of
carbon content, this
range decreases
rapidly. High carbon
steels alloy steels require
considerably more
care in controlling temperature and producing the welds. Large work may be
welded   in
hammer forges driven by steam.
Welded steel
pipe is made mechanically by running
the preheated
strips through rolls, which form the pipe to size and apply the necessary
pressure for the

Friction Welding

In this process, the
heat for welding is obtained from mechanically induced sliding motion between rubbing surfaces of
work-pieces as shown in Fig. In friction welding, one part is firmly
held while the other (usually cylindrical) is rotated under simultaneous
application of axial pressure. As
these parts are brought to rub against each other under pressure, they get heated due to friction. When the desired
forging temperature is attained, the rotation is stopped and the axial pressure
is increased to obtain forging action and hence welded joint. Most of the metals and their dissimilar combinations such as aluminium and titanium, copper
and steel, aluminium and steel etc. can be welded using
friction welding.

Friction Welding
Friction Welding

Explosive Welding

In explosive welding, strong metallurgical bonds can be produced between
metal combinations which cannot be welded by other methods or
processes. For example, tantalum can be explosively welded to steel although
the welding point of tantalum is higher than the vaporization temperature of
steel. Explosive welding process is shown in Fig. It is carried out by
bringing together properly paired metal surfaces with high relative velocity at
a high pressure and a proper orientation to each other so that a large amount
of plastic interaction occurs between the surfaces. The work piece, held fixed
is called the target plate and the other called flyer plate. While a variety
of procedures have been successfully employed, the main techniques of explosive welding can be
divided into contact techniques and impact techniques.
In critical space and nuclear application, explosive welding permits
fabrication of structures that cannot be made by any other means and, in some
commercial applications, explosive joining is the least costly method.
The main advantage of explosive welding
includes the simplicity of the process,
and the extremely large surface that can be welded. Incompatible
materials can also be bonded,
and thin foils can be bonded to heavier plates.

Explosive Welding

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