Welding joins metals through different ways. From arc to oxyfuel, there are various techniques to fasten metal pieces together to create unique products. Contrary to popular belief, not all welding processes have to melt the metal. One such way of forming metal pieces without increasing the temperature is through solid-state welding, and there are different techniques under this branch too. Here are some of them:

1. Introduction to Solid-State Welding
Processes
Welding joins metals through different ways. From arc to oxyfuel, there
are various techniques to fasten metal pieces together to create unique
products.
Contrary to popular belief, not all welding processes have to melt the
metal. One such way of forming metal pieces without increasing the
temperature is through solid-state welding, and there are different
techniques under this branch too. Here are some of them:
Coextrusion
Coextrusion welding forces dissimilar
metals through the same die. Practical
application of this includes blending
corrosion-resistant and cheap alloys
together.
Cold pressure
Through cold pressure, copper, aluminium,
and similar soft alloys are formed under
their melting point. This is a great technique
in manufacturing electrical contacts.

2. Explosion
Explosion welding is one of the more recent
techniques. Developed only after World
War II, this uses chemical explosives to weld
metals together. Common applications
include cladding a steel plate with a layer of
corrosion-resistant material, bimetal
pipelines, and shipbuilding.
Diffusion
Diffusion welding heats metals to around
50-70% of the melting point to coalesce
them together. This technique does not
yield weld lines. The electronics, aerospace,
and nuclear industries use this process.
Electromagnetic pulse
Electromagnetic pulse welding joins two
pieces together through the use of strong
magnetic forces. Similar to explosion
welding, this technique accelerates tubes
and sheets to merge, and expels oxides
upon impact. Pressure vessels and cars have
parts made through this process.

3. Hot isostatic pressure
Hot isostatic pressing is a notable
manufacturing process that uses high
temperature and isostatic gas pressure
contained inside a high-pressure vessel. As
a result, the gases – which are inert, to
avoid chemical reaction – apply pressure on
all directions to form the metal and weld it.
While this technique reduces the porosity
of metals, it improves the workability and
mechanical properties of the material.
Roll
Roll welding uses two rotating wheels. The
work piece goes in between the wheels,
and they are joined together through force.
Ultrasonic
Ultrasonic welding uses pressure and high-
frequency vibrations to apply force to
workpieces and join them together. This is a
common technique in plastics, and metal
sheets, and in foils found in the automotive,
electronics, and solar industries.
The different techniques of solid-state welding show that metals do
not have to reach melting point to be formed according to a desired
shape. These are just some processes under solid-state welding, and
there are more that can yield interesting results.
Resources:
http://winward.co.uk/welding/
http://www.keytometals.com/Article51.htm
http://www.substech.com/dokuwiki/doku.php?id=solid_state_welding_ssw