Metal Forming
Topic includes—
Hot &Cold working
Forging
Extrusion
Rolling
Drawing
1) COLD WORKING
Plastic deformation which is carried out in a temperature region and over
a time interval such that the strain hardening is not relieved is called
cold working.
Some Cold Working Processes:
Cold rolling
Cold forging
Cold extrusion
Bending
Drawing
Shearing
Reason for Cold Working:
Provides better surface finish and dimensional precision.
The advantages of cold working are
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A better surface finish may be achieved
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Dimensional accuracy can be excellent because the work
is not hot so it doesn't shrink on cooling; also the low
temperatures mean the tools such as dies and rollers
can last a long time without wearing out.
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Usually there is no problem with oxidative effects such
as scale formation. In fact, cold rolling (for example) can
make such scale come off the surface of a previously
hot-worked object.
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Controlled amounts of cold work may be introduced.
Effect of cold working on tensile strength, hardness, ductility and
grain size. (The curve below ductility represents the change in
grain size)
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As with hot working, the grain structure of the material
is made to follow the deformation direction, which can
be good for the strength of the final product.
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Strength and hardness are increased, although at the
expense of ductility.
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OH & S problems related to working near hot metal are
eliminated.
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There is a limit to how much cold work can be done
on a given piece of metal. See the discussion above
about accumulation of damage in the form of piled up
dislocations.
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Higher forces are required to produce a given
deformation, which means we need heavily built,
strong forming machines .
HOT WORKING
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Hot working refers to the process where metals are defromed above
their recrystallizatıon tempereture and strain hardening does not
occur.Hot working performed at elevated tempreature.However is hot
working at room temperature because of its low melting temperature.
Hot ingots
Some Hot Working Processes:
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Rolling
Forging
Extrusion
Hot drawing
Pipe welding
Piercing
Reason for Hot Working:
At elevated temperatures, metals weaken and become more ductile.
The most important continuous hot working processes
The advantages of hot working are
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Lower working forces to produce a given shape, which
means the machines involved don't have to be as strong,
which means they can be built more cheaply;
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The possibility of producing a very dramatic shape change in
a single working step, without causing large amounts of
internal stress, cracks or cold working;
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Sometimes hot working can be combined with a casting
process so that metal is cast and then immediately hot
worked. This saves money because we don't have to pay for
the energy to reheat the metal.
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Hot working tends to break up large crystals in the metal and
can produce a favourable alignment of elongated crystals
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Hot working can remove some kinds of defects that occur in
cast metals. It can close gas pockets (bubbles) or voids in a
cast billet; and it may also break up non-metallic slag which
can sometimes get caught in the melt (inclusions).
The main problems,
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If the recrystallisation temperature of the worked metal is high
e.g. if we are talking about steel, specialised methods are
needed to protect the machines that work the metal. The
working processes are also dangerous to human operators
and very unpleasant to work near.
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The surface finish of hot worked steel tends to be pretty crude
because ;
the dies or rollers wear quite rapidly
there is a lot of dimensional change as the worked object
cools
there is the constant annoying problem of scale
formation on the surface of the hot steel.
Forging
• Forging is manufacturing process where
metal is pressed, pounded or squeezed
under great pressure into high strength parts
known as forgings. The process is normally
(but not always) performed hot by preheating
the metal to a desired temperature before it
is worked. It is important to note that the
forging process is entirely different from the
casting (or foundry) process, as metal used
to make forged parts is never melted and
poured (as in the casting process).
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Types of Forging
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Hand forging :- Hand forging is emplayed only to
shape a small number of light forgings chiefly in
repair shops.
Hammer forgings :- Usually used for small item
forging.
Press forging :- Usually used for heavy item
forging.
Machine forging :- For medium sized and large
articles requiring very heavy blows.
Drop forging :- For mass production of identical
parts.
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Most open die forgings are produced on flat dies.
Round swaging dies and V dies also are used in
pairs or with a flat die. Operations performed on
open die presses include:
• Drawing out or reducing the cross-section of an
ingot or billet to lengthen it.
• Upsetting or reducing the length of an ingot or billet
to a larger diameter.
• Upsetting, drawing out, and piercing--processes
sometimes combined with forging over a mandrel
for forging rough-contoured rings.
Forged Products
• In automotive applications, forged components
are commonly found at points of shock and
stress. Forged automobile components include
connecting rods, crankshafts, wheel spindles,
axle beams, pistons, gears, and steering arms.
• Forgings are also used in helicopters, pistonengine planes, commercial jets, and supersonic
military aircraft. Many aircraft are "designed
around" forgings and contain more than 450
structural forgings, including hundreds of forged
engine parts.
• "Forged" is the mark of quality in hand tools and
hardware. Pliers, hammers, sledgers, wrenches,
garden implements, and surgical tools are
almost always produced through forging.
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Rolling
Aluminium is first passed through a hot rolling mill and then transferred to a cold rolling
mill.
• Hot rolling mils:
Prior to rolling the aluminium is in the form of an ingot which can be up to 600mm
thick. This ingot is then heated to around 500°C and passed several times
through the hot rolling mill. This gradually reduces the thickness of the metal to
around 6mm.
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This thinner aluminium is then coiled and transported to the cold rolling mill for
further processing.
• Cold rolling mils:
There are various types of cold rolling mill, and they produce various types of rolled
product, with thicknesses as low as 0.05mm. In general the type of product
depends on the alloy used, the rolling deformation and thermal treatment used
in the process as well as careful adjustments to the mechanics and chemistry
of the process. Rolling mills are controlled by very precise mechanisms and
measuring systems.
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Rolling??
Rolling is a fabricating process in which the metal, plastic,
paper, glass, etc. is passed through a pair (or pairs) of rolls.
There are two types of rolling process, flat and profile rolling.
In flat rolling the final shape of the product is either classed
as sheet (typically thickness less than 3 mm, also called
"strip") or plate (typically thickness more than 3 mm).
In profile rolling, the final product may be a round rod or other
shaped bar such as a structural section (beam, channel, joist
etc). Rolling is also classified according to the temperature of
the metal rolled. If the temperature of the metal is above its
recrystallization temperature then the process is termed as
hot rolling, If the temperature of metal is below its
recrystallization temperature the process is termed as cold
rolling.
A material is pushed or drawn through a die of the desired crosssection .Any solid or hollow cross-section may be produced by
extrusion, which can create essentially semi-finished parts. The
metal can forcing through a die in the same direction or opposite
direction.
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The cross-sections that can be produced vary from solid round,
rectangular, to L shapes, T shapes.
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Extrusion may be continuous (theoretically producing indefinitely
long material) or semi-continuous (producing many pieces).
Extrusions can be done with the material hot or cold.
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Commonly extruded materials include metals, polymers,
ceramics, and foodstuffs.
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Direct extrusion: A metal
billet is located into a
container, and a ram
compresses the material,
forcing it to flow through
one or more openings in a
die at the opposite end of
the container.
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Indirect extrusion: The die
is mounted to the ram
rather than at the opposite
end of the container. One
advantage of the indirect
extrusion process is that
there is no friction, during
the process, between the
billet and the container
liner
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Hot Extrusion
Hot extrusion is done at an elevated temperature to keep the material
from work hardening and to make it easier to push the material through
the die. Most hot extrusions are done on horizontal hydraulic presses
that range from 250 to 12,000 tons.
The biggest disadvantage of this process is its cost for machinery and
its upkeep.
Typical parts produced by extrusions are trim parts used in automotive
and construction applications, window frame members, railings,
aircraft structural parts.
Cold Extrusion
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Cold extrusion is performed at temperatures significantly below
the melting temperature of the alloy being deformed, and
generally at room temperature.
The process can be used for most materials, provided that
sufficiently robust machinery can be designed.
Products of cold extrusion include aluminium cans, collapsible
tubes and gear blanks.