Download - III-2 Bulk Deformation
-
8/10/2019 III-2 Bulk Deformation
1/56
BULK DEFORMATION PROCESSES
IN METALWORKING
1. Rolling2. Other Deformation Processes Related to
Rolling
3. Forging
4. Other Deformation Processes Related to
Forging
5. Extrusion
6. Wire and Bar Drawing
Bulk Deformation
Metal forming operations which cause significant
shape change by deforming metal parts whose
initial form is bulk rather than sheet
Starting forms:
Cylindrical bars and billets,
Rectangular billets and slabs, and similar
shapes
These processes stress metal sufficiently to
cause plastic flow into desired shape
Performed as cold, warm, and hot working
operations
-
8/10/2019 III-2 Bulk Deformation
2/56
Importance of Bulk Deformation
In hot working, significant shape change canbe accomplished
In cold working, strength is increased during
shape change
Little or no waste - some operations are near
net shape or net shape processes
The parts require little or no subsequent
machining
Four Basic Bulk Deformation Processes
1. Rolling slab or plate is squeezed between
opposing rolls
2. Forging work is squeezed and shaped
between opposing dies
3. Extrusion work is squeezed through a die
opening, thereby taking the shape of the
opening4. Wire and bar drawing diameter of wire or bar
is reduced by pulling it through a die opening
-
8/10/2019 III-2 Bulk Deformation
3/56
Deformation process in which work thickness is reduced by
compressive forces exerted by two opposing rolls.
Rotating rolls perform two main functions:
Pull the work into the gap between them by friction between
workpart and rolls
Simultaneously squeeze the work to reduce its cross section
The rolling process (specifically, flat rolling).
ROLLING
Basic Rolling Process
Metal is passedbetween two rolls thatrotate in oppositedirections
Friction acts to propelthe material forward
Metal is squeezed andelongates tocompensate for thedecrease in cross-sectional area
Schematic representation of the hot-rolling process, showingthe deformation and recrystallization of the metal being rolled.
-
8/10/2019 III-2 Bulk Deformation
4/56
-
8/10/2019 III-2 Bulk Deformation
5/56
RECRYSTALLIZATION IN HOT ROLLING
One effect of a hot-working rolling operation is thegrain refinement brought about by recrystallization.
The coarse structure is broken up and elongated by
the rolling action. Because of the high temperature,
recrystallization starts immediately and small
grains begin to form. These grains grow rapidly until
recrystallization is complete.
Types of Rolling
Based on workpiece geometry:
Flat rolling - used to reduce thickness of arectangular cross section
Shape rolling - square cross section isformed into a shape such as an I-beam
-
8/10/2019 III-2 Bulk Deformation
6/56
Rolled Products
Hot-rolled products have little directionality in theirproperties
Hot-rolled products are therefore uniform and havedependable quality
Surfaces may be rough or may have a surface oxide
known as mill scale in hot rolling Dimensional tolerances vary with the kind of metal and
the size of the product in hot rolling.
Cold-rolled products exhibit superior surface finishand dimensional precision
ROLLING OF STEELFIRST, Steel has been cast into molds (INGOTS).
The ingot remains in molds until the solidification is about
complete and then removed. While still hot, the ingots are
placed in gas-fired furnaces called soaking pits, where they
remain until attaining working temperature of about 1200C.
The ingots are taken to the rolling mill where, because of the
large variety of finished shapes (plates, sheets, bar stock,
structural shapes, orfoils) to be made, they are first hot
rolled into intermediates shapes as blooms (blum, ktk),
billets (billet, ubuk), and slabs (yass ktk).
-
8/10/2019 III-2 Bulk Deformation
7/56
INTERMEDIATE SHAPES
-A bloom has a square cross section with a minimumsize of 150 by 150 mm. About 30 passes are required
to reduce a large ingot into a bloom.
-A billet is smaller than a bloom and may have any
square section from 40 mm up to the size of a bloom.
-Slabs may be rolled from either an ingot or a bloom.
They have a rectangular cross-sectional area with a
minimum width of 250 mm and a minimum thickness
of 40 mm. The width is always three or more times
the thickness, which may be as much as 380 mm.Plates, skelp, and thin strips are rolled from slabs.
ROLLING OF STEEL
-
8/10/2019 III-2 Bulk Deformation
8/56
Flowchart of Rolling Operations
Flow chart for the production of
various finished and semifinished
steel shapes. Note the abundance
of rolling operations.
Some of the steel products made in a rolling mill.
Rolled Products Made of Steel
-
8/10/2019 III-2 Bulk Deformation
9/56
Side view of flat rolling, indicating before and after thicknesses, workvelocities, angle of contact with rolls, and other features.
FLAT ROLLING
FLAT ROLLING
In rolling, the quantity of metal going into a roll and out of it is the
same, but the area and velocity are changed. Thus, stock
enters the rolls with a speed less than the peripheral roll speed.
The metal emerges from the rolls traveling at a higherspeed
than it enters.
Q1= Q2 = A1V1 = A2V2
Q1 = Quantity of metal going into roll ; Q2 = Quantity of metalleaving roll
A1 = area of an element in front of roll; A2 = Area of an element
after roll
V1 = Velocity in element before roll; V2 = Velocity in element after
roll
-
8/10/2019 III-2 Bulk Deformation
10/56 1
Flat Rolling Terminology
Reduction ratio (indirgeme/haddeleme oran) (r) =
ot
dr
where d = draft; to = starting thickness; and tf = final
thickness
fo ttd
SHAPE ROLLING
Work is deformed into a contoured cross section
rather than flat (rectangular)
Accomplished by passing work through rolls
that have the reverse of desired shape
Products include:
Construction shapes such as I-beams,
L-beams, and U-channels
Rails for railroad tracks
Round and square bars and rods
-
8/10/2019 III-2 Bulk Deformation
11/56
-
8/10/2019 III-2 Bulk Deformation
12/56 1
Rolling Mill Configurations
Various roll configurations
used in rolling operations.
Rolling Mill Configurations
Roll arrangements used in rolling mills. A, Two-high mill, continuous reversing. B,
Four-high mill with backing-up rolls for wide sheets. C, Three-high mill for back-and-
forth rolling. D, Cluster mill using four backing-up rolls.
-
8/10/2019 III-2 Bulk Deformation
13/56 1
Rolling Mill Configurations
Most primary rolling is done in either a two-high reversingmill or a three-high continuous rolling mill.
In the two-high reversing mill the piece passes through the rolls,
which are then stopped and reversed in direction, and the operation
is repeated. At frequent intervals the metal is turned 90 on its side
to keep the section uniform and to refine the metal throughout.
2-high rolling mill
(b) 3-high rolling mill (c) four-high rolling mill
Three-High Rolling Mill and Four-High Rolling Mill
-
8/10/2019 III-2 Bulk Deformation
14/56 1
WHY SMALLER ROLLS?
Smaller diameter rollsproduce less length of
contact for a given
reduction and require less
force to produce a given
change in shape
HOWEVER Smallercross
section provides a reduced
stiffness
Rolls may be prone to flexelastically because they
are only supported on the
ends
The effect of roll diameter on length of contact for a given reduction.
Multiple backing rolls allow even smaller roll diameters
Various configurations of rolling mills: (d) cluster mill
Cluster Mill
-
8/10/2019 III-2 Bulk Deformation
15/56 1
A series of rolling stands in sequence
CONTINOUS (Tandem) Rolling Mill
Continuous (Tandem) Rolling Mills
Billets, blooms, and
slabs are heated and
fed through an
integrated series of
nonreversing rolling
mills Synchronization of
rollers may pose issuesTypical roll-pass sequences used in producing structural shapes.
-
8/10/2019 III-2 Bulk Deformation
16/56 1
Thread Rolling
Bulk deformation process used to form threadson cylindrical parts by rolling them betweentwo dies
Important commercial process for massproducing bolts and screws
Performed by cold working in thread rollingmachines
Advantages over thread cutting (machining):
Higher production rates
Better material utilization
Stronger threads and better fatigueresistance due to work hardening
THREAD ROLLING
Thread-rolling processes: (a) flat dies and (b) two-roller dies.
-
8/10/2019 III-2 Bulk Deformation
17/56 1
Thread rolling with flat dies: (1) start of cycle, and (2) end of cycle.
Thread Rolling (FLAT DIES)
Ring Rolling
Deformation process in which a thick-walled ring of smaller
diameter is rolled into a thin-walled ring of larger diameter
As thick-walled ring is compressed, deformed metal
elongates, causing diameter of ring to be enlarged
Hot working process for large rings and cold working
process for smaller rings
Applications: ball and roller bearing races, steel tires for
railroad wheels, and rings for pipes, pressure vessels, and
rotating machinery
Advantages: material savings, ideal grain orientation,
strengthening through cold working
-
8/10/2019 III-2 Bulk Deformation
18/56 1
Ring rolling used to reduce the wall thickness and increase the diameter
of a ring: (1) start, and (2) completion of process.
Ring Rolling
FORGING (DVME)
Deformation process in which work is compressed between
two dies
Oldest of the metal forming operations, dating from about
5000 B C
Components: engine crankshafts, connecting rods, gears,
aircraft structural components, jet engine turbine parts
Forging is adaptable to carbon and alloy steels, wrought
iron, copper, and aluminum and magnesium alloys.
Also, basic metals industries use forging to establish basic
form of large parts that are subsequently machined to final
shape and size
-
8/10/2019 III-2 Bulk Deformation
19/56 1
Classification of Forging Operations
Cold vs. hot forging: Hot or warm forging most common, due to
the significant deformation and the need to
reduce strength and increase ductility of work
metal
Cold forging advantage: increased strength
that results from strain hardening
Impact vs. press forging:
Forge hammer- applies an impact load
Forge press - applies gradual pressure
HOT FORGING
The number of blows required varies according to
the size and shape of the part, the forging qualities
of the metal, and the tolerances required.
Forging temperature for steel is 1100-1250C.
Advantages of the hot forging operation include a finecrystalline structure of the metal, closing of any voids,
reduced machining time, and improved physical properties.
Disadvantages include scale inclusions and the highcost of dies, which prohibits short-run jobs.
-
8/10/2019 III-2 Bulk Deformation
20/56 2
Types of Forging Dies
Open-die forging - work is compressed betweentwo flat dies, allowing metal to flow laterally with
minimum constraint
Impression-die forging - die contains cavity or
impression that is imparted to workpart
Metal flow is constrained so that flash is created
Flashless forging - workpart is completely
constrained in die
No excess flash is created
(a) open-die forging.
OPEN-DIE Forging
-
8/10/2019 III-2 Bulk Deformation
21/56 2
Open-Die Forging
Compression of workpart between two flat dies Similar to compression test when workpart has
cylindrical cross section and is compressed along
its axis
Deformation operation reduces height and
increases diameter of work
Common names include upsettingor upset
forging
The accuracy of open-die forging is not high and
complicated shapes can not be obtained.
Open-Die Forging with No Friction
If no friction occurs between work and die surfaces,
then homogeneous deformation occurs, so that
radial flow is uniform throughout workpart height and
true strain is given by:
where ho= starting height; and h = height at some
point during compression
At h = final value hf, true strain is maximum value
h
holn
-
8/10/2019 III-2 Bulk Deformation
22/56 2
Homogeneous deformation of a cylindrical workpart under ideal
conditions in an open-die forging operation: (1) start of process withworkpiece at its original length and diameter, (2) partial compression,
and (3) final size.
Open-Die Forging with No Friction (HAPPENS
ONLY UNDER IDEAL CONDITIONS)
Open-Die Forging with Friction
Friction between work and die surfaces constrains lateral
flow of work, resulting in barreling effect
In hot open-die forging, effect is even more pronounced
due to heat transfer at and near die surfaces, which cools
the metal and increases its resistance to deformation
Actual deformation of a cylindrical workpart in open-die forging, showing pronouncedbarreling:
(1) start of process, (2) partial deformation, and (3) final shape.
-
8/10/2019 III-2 Bulk Deformation
23/56 2
(Top) Illustration of the
unrestrained flow of
material in open-die
forging. Note the barrel
shape that forms due to
friction between the die
and material.
(Middle) Open-die forging
of a multidiameter shaft.
(Bottom) Forging of aseamless ring by the
open-die method.
-
8/10/2019 III-2 Bulk Deformation
24/56 2
IMPRESSION-DIE Forging
Compression of workpart by dies with inverse of desired part shape
Closed-impression die forgings have better utilization of material than open (flat)dies, better physical properties, closer tolerances, higher production rates, and less
requirement of operator skill.
Flash is formed by metal that flows beyond die cavity into small gap between die
plates. Flash must be later trimmed, but it serves an important function during
compression:
As flash forms, friction resists continued metal flow into gap, constraining
material to fill die cavity
In hot forging, metal flow is further restricted by cooling against die plates
-
8/10/2019 III-2 Bulk Deformation
25/56 2
Sequence in impression-die forging: (1) just prior to initial contact
with raw workpiece, (2) partial compression, and (3) final die
closure, causing flash to form in gap between die plates.
Impression-Die Forging
Impression-Die Forging
The dies are shaped to control the flow of metal
The die halves are matched and separately attached to the movable ram
and the fixed anvil. The forging is produced by impact or pressure, which
compels the hot and pliable metal to conform to the shape of the dies.
Metal flows and completely fills the die
Schematic of the impression-die forging
process, showing partial die filling andthe beginning of flash formation in the
center sketch and the final shape with
flash in the right-hand sketch.
-
8/10/2019 III-2 Bulk Deformation
26/56 2
Impression-Die Forging Practice
Several forming steps often required, with separate diecavities for each step
Beginning steps redistribute metal for more uniform
deformation and desired metallurgical structure in
subsequent steps and final steps bring the part to final
geometry
First impression is called edging, fullering, or bending
Intermediate impressions are for blocking the metal to
approximately its final shape
Final shape is given in its final forging operation Impression-die forging is often performed manually by
skilled operator under adverse conditions
Impression drop-forging dies
and the product resulting from
each impression. The flash is
trimmed from the finished
connecting rod in a separate
trimming die. The sectional view
shows the grain flow resulting
from the forging process.
Impression-Die Forging Practice separate die cavities
-
8/10/2019 III-2 Bulk Deformation
27/56 2
Advantages and Limitations
Advantages of impression-die forging compared to
machining from solid stock:
Higher production rates
Less waste of metal
Greater strength
Favorable grain orientation in the metal
Limitations of impression-die forging:
Not capable of close tolerances
Machining often required to achieve accuracies and features needed
Upon completion all forgings are covered with scale (kabuk, tufal) and
must be cleaned. This can be done by pickling in acid (asitletemizleme), shot peening (bilyal dvme), or tumbling (deirmen),
depending on the size and composition of the forgings.
FLASHLESS FORGING
Compression of work in punch and die tooling whose
cavity does not allow for flash
Flashless forging can be performed if the metal is
deformed in a cavity that provides total confinement
Starting workpart volume must equal die cavity volume
within very close tolerance
Process control more demanding than impression-dieforging
Best suited to part geometries that are simple and
symmetrical
Often classified as aprecision forgingprocess
-
8/10/2019 III-2 Bulk Deformation
28/56 2
Three types of forging (c) flashless forging.
Flashless Forging
Flashless forging: (1) just before initial contact with workpiece, (2)
partial compression, and (3) final punch and die closure.
Flashless Forging
-
8/10/2019 III-2 Bulk Deformation
29/56 2
Forging Hammers (Drop Hammers)(ahmerdanda dvme, serbest dvme)
Apply impact load against workpart
Two types:
Gravity drop hammers - impact energy from falling weight
of a heavy ram
Power drop hammers - accelerate the ram by pressurized
air or steam
The two principal types of drop-forging hammers are the gravity drop (dm ahmerdan)
and the steam hammer (buharahmerdan). In the gravity-type hammer, the impact pressure
is developed by the force of the falling ram as it strikes upon the lower fixed die. It utilizes air
or steam to lift the ram. In the steam hammer the ram and hammer are lifted by steam, and
the force of the blow is controlled by throttling the steam. These hammers operate at over 300
blows/min. The capacity of steam hammers range from 2-200 kN (i.e. up to 20 t force).
Disadvantage: impact energy transmitted through anvil into
floor of building
Commonly used for impression-die forging
Drop forging hammer, fed by conveyor and heating units at the right of the
scene
Forging Hammers (Drop Hammers)
-
8/10/2019 III-2 Bulk Deformation
30/56 3
Diagram showing details of a drop hammer for impression-die forging.
Forging Hammers (Drop Hammers)Details
Forging Hammers (Drop Hammers)
(Left) Double-frame drop hammer. (Right) Schematic diagram
of a forging hammer.
-
8/10/2019 III-2 Bulk Deformation
31/56 3
Forging Presses (stroke restricted machines)(presle dvme)
Apply gradual pressure to accomplish compression operation
Press forging is used for large or thick products
Slow squeezing action penetrates completely through the metal.
Press forging employs a slow squeezing action in deforming the
plastic metal as contrasted to the rapid impact blows of a hammer.
Produces a more uniform deformation and flow
Longer time of contact between the die and workpiece
Dies may be heated (isothermal forging)
Presses are vertical type and either mechanically or hydraulically
operated.
Types:
Mechanical press - converts rotation of drive motor into linear motion of
ram. Faster
Hydraulic press - hydraulic piston actuates ram. More controllable
Screw press - screw mechanism drives ram
For small press forgings closed-impression dies are used,
and only one stroke of the ram is normally required to perform
the forging operation.
In the forging press a greater proportion of the total energy
input is transmitted to the metal than in a drop hammerpress.
Much of the impact of the drop hammer is absorbed by the
machine and foundation.
Most press forgings are symmetrical in shape with surfaces that
are smooth, and they provide a closer tolerance than is
obtained by a drop hammer. However, many parts of irregular
and complicated shapes can be forged more economically by
drop hammers. Forging presses are often used for sizing
operations on parts made by other processes.
Forging Presses
-
8/10/2019 III-2 Bulk Deformation
32/56 3
UPSET FORGING (yma, iirme)
Upset Forging is used to form heads on nails, bolts,and similar hardware products. It increases the
diameter of a material by compressing its length
More parts produced by upset forging than any
other forging operation
Performed cold, warm, or hot on machines called
headers or formers
Wire or bar stock is fed into machine, end is
headed, then piece is cut to length
For bolts and screws, thread rolling is then used toform threads
UPSET FORGING (Heading)
Upset forging entails gripping a bar of uniform section in dies and applying
pressure on the heated end, causing it to be upset or formed to shape.
The length of the stock to be upset cannot be more than two or three times
the diameter or else the material will bend rather than bulge out to fill the die
cavity.
-
8/10/2019 III-2 Bulk Deformation
33/56 3
An upset forging operation to form a head on a bolt or similar hardware
item The cycle consists of: (1) wire stock is fed to the stop, (2)gripping dies close on the stock and the stop is retracted, (3) punch
moves forward, (4) bottoms to form the head.
Upset Forging (Heading)
Examples of heading (upset forging) operations: (a) heading a nail
using open dies, (b) round head formed by punch, (c) and (d) two
common head styles for screws formed by die, (e) carriage bolt head
formed by punch and die.
Upset Forging (Heading)
-
8/10/2019 III-2 Bulk Deformation
34/56 3
Upset Forging Rules
RULE 1: The length of the unsupported material that can be gathered or upset in one blow without injurious bucklingshould be limited to three times the diameterof the bar.RULE 2: Lengths of stock greater than three times the diameter may be upset successfully provided that the diameterof the upset is not more than 1 times the diameterof the bar.RULE 3: In an upset requiring stock length greater than three times the diameter of the bar, and where the diameterof the cavity is not more than 1 times the diameter of the bar (the conditions of rule 2), the length of the unsupportedmetal beyond the face of the diemust not exceed the diameterof the bar.
Swaging
Accomplished by rotating dies that hammer a workpiece radially
inward to taper it as the piece is fed into the dies
Used to reduce diameter of tube or solid rod stock
Mandrel sometimes required to control shape and size of
internal diameter of tubular parts
Swaging process to reduce solid rod stock; the dies rotate as they hammer the work In
radial forging, the workpiece rotates while the dies remain in a fixed orientation as they
hammer the work.
-
8/10/2019 III-2 Bulk Deformation
35/56 3
Swaging
Also known as rotary
swaging and radial
forging
Uses external
hammering to reduce
the diameter or produce
tapers or points on
round bars of tubes Schematic of the roll-forging process showing the two shaped rollsand the stock being formed.
Swaging
Tube being
reduced in
a rotary
swaging
machine.
Basic components and motions of a rotary swaging machine.
(Note: The cover plate has been removed to reveal the interior
workings.)
A variety of swaged parts, some with internal details.
-
8/10/2019 III-2 Bulk Deformation
36/56 3
Roll Forging
Round or flat bar stock isreduced in thickness and
increased in length
Produces products such
as axles, tapered levers,
and leaf springs
Little or no flash is
produced
Rolls from a roll-forging machine and the various
stages in roll forging a part.
Roll-forging machine in operation
Trimming
Cutting operation to remove flash from workpart
in impression-die forging
Usually done while work is still hot, so a
separate trimming press is included at the
forging station
Trimming can also be done by alternative
methods, such as grinding or sawing
-
8/10/2019 III-2 Bulk Deformation
37/56 3
Trimming operation (shearing process) to remove the flash after
impression-die forging.
Trimming After Impression-Die Forging
EXTRUSION
Compression forming process in which work metal is forced to flow
through a die opening to produce a desired cross-sectional shape
Some metals, notably lead, tin, and aluminum, may be extruded cold,
whereas others require the application of heat to render them plastic
or semisolid before extrusion.
In general, extrusion is used to produce long parts of uniform cross
sections Process is similar to squeezing toothpaste out of a toothpaste tube
Rods, tubes, molding trim, structural shapes, brass cartridges (fiek),
and lead-covered cables are typical products of metal extrusion.
Two basic types:
Direct extrusion
Indirect extrusion
-
8/10/2019 III-2 Bulk Deformation
38/56 3
EXTRUSION
Almost unlimited lengths of a continuous cross section can
be produced, and because of low die costs production runs of
150 m may justify its use.
Typical
shapes
produced
by
extrusion
EXTRUSION TYPES
Direct extrusion Solid ram drives the entire billet to and through a stationary die
In direct extrusion, the ram and billet both move and friction betweenthe billet and the chamber opposes forward motion.
Must provide power to overcome friction
Indirect extrusion A hollow ram pushes the die back through a stationary, confined billet
For indirect extrusion, the billet is stationary. There is no billet-
chamber friction, since there is no relative motion.
-
8/10/2019 III-2 Bulk Deformation
39/56 3
Direct extrusion.
Direct Extrusion
A heated round billet is placed into the die chamber andram placed into position. The metal is extruded through
the die opening until only a small amount remains.
Direct Extrusion
Metal is compressed andforced to flow through ashaped die to form aproduct with a constantcross section
May be performed hot orcold
A ram advances from oneend of the die and causesthe metal to flow plasticallythrough the die
Commonly extruded metals:aluminum, magnesium,copper, and lead
Direct extrusion schematic showing the
various equipment components.
-
8/10/2019 III-2 Bulk Deformation
40/56 4
Comments on Direct Extrusion
Also called forward extrusion
As ram approaches die opening, a small portion of
billet remains that cannot be forced through die
opening
This extra portion, called the butt, must be
separated from extrudate by cutting it just beyond
the die exit Starting billet cross section usually round
Final shape of extrudate is determined by die
opening
-
8/10/2019 III-2 Bulk Deformation
41/56 4
(a) Direct extrusion to produce a hollow or semi-hollow cross
sections; (b) hollow and (c) semi-hollow cross sections.
Hollow and Semi-Hollow Shapes in Direct
Extrusion
Direct Extrusion of Hollow Shapes
Mandrels may be
used to produce
hollow shapes or
shapes with multiple
longitudinal cavities
Two methods of extruding hollow shapes using internal mandrels. In part (a) the
mandrel and ram have independent motions; in part (b) they move as a single unit.
-
8/10/2019 III-2 Bulk Deformation
42/56 4
Indirect extrusion to produce (a) a solid cross section and (b) a hollow cross
section.
INDIRECT EXTRUSION
Indirect extrusion is similar to direct extrusion except that the extruded part is forced
through the ram stem. Less force is required by this method because there is no
frictional force between the billet and the container wall. The weakening and
complexity of the ram when it is made hollow and the difficulty of providing good
support for the extruded part constitute limitations of this process.
Comments on Indirect Extrusion
Also called backward extrusion and reverse
extrusion
Limitations of indirect extrusion are imposed by
Lower rigidity of hollow ram
Difficulty in supporting extruded product as it
exits die
-
8/10/2019 III-2 Bulk Deformation
43/56 4
Advantages of Extrusion
Variety of shapes possible, especially in hot extrusion Limitation: part cross section must be uniform
throughout length
Grain structure and strength enhanced in cold and
warm extrusion
Close tolerances possible, especially in cold extrusion
In some operations, little or no waste of material
Hot vs. Cold Extrusion
Hot extrusion - prior heating of billet to above
its recrystallization temperature
Reduces strength and increases ductility of
the metal, permitting more size reductions
and more complex shapes
Cold extrusion - generally used to produce
discrete parts The term impact extrusion is used to
indicate high speed cold extrusion
-
8/10/2019 III-2 Bulk Deformation
44/56 4
Extrusion Ratio
Also called the reduction ratio, it is defined as
where rx = extrusion ratio;Ao = cross-sectional
area of the starting billet; andAf = final cross-
sectional area of the extruded section
Applies to both direct and indirect extrusion
f
ox
A
Ar
(a) Definition of die angle in direct extrusion; (b) effect of die angle on ram
force.
Extrusion Die Features
Low die angle - surface area is large, which increases friction at die-billet
interface. Higher friction results in larger ram force.
Large die angle - more turbulence in metal flow during reduction. Turbulence
increases ram force required
Optimum angle depends on work material, billet temperature, and lubrication.
-
8/10/2019 III-2 Bulk Deformation
45/56 4
Orifice Shape of Extrusion Die
Simplest cross section shape is circular dieorifice
Shape of die orifice affects ram pressure
As cross section becomes more complex,
higher pressure and greater force are required
Effect of cross-sectional shape on pressure
can be assessed by means the die shape
factor Kx
Extrusion Presses
Either horizontal or vertical
Horizontal more common
Extrusion presses - usually hydraulically
driven, which is especially suited to
semi-continuous direct extrusion of long
sections
Mechanical drives - often used for coldextrusion of individual parts
-
8/10/2019 III-2 Bulk Deformation
46/56 4
Forces in Extrusion
Lubrication is important
to reduce friction and
act as a heat barrier
Metal flow in extrusion
Flow can be complex
Surface cracks, interior
cracks and flow-related
cracks need to be
monitored Process control is
importantDiagram of the ram force versus ram position for both
direct and indirect extrusion of the same product. The
area under the curve corresponds to the amount of
work (force x distance) performed. The difference
between the two curves is attributed to billet-chamber
friction.
WIRE AND BAR DRAWING
Cross-section of a bar, rod, or wire is reduced by pulling it
through a die opening
Similar to extrusion except work is pulled through die in
drawing (it ispushedthrough in extrusion)
Although drawing applies tensile stress, compression also
plays a significant role since metal is squeezed as it passes
through die opening
-
8/10/2019 III-2 Bulk Deformation
47/56 4
Wire, Rod, and Tube Drawing
Schematic drawing of the rod-or bar-drawing process.
Cold-drawing smaller tubing from larger tubing. The die
sets the outer dimension while the stationary mandrel
sizes the inner diameter.
Tube drawing with a floating plug.
Schematic of wire
drawing with arotating draw block.
The rotating motor on
the draw block
provides a
continuous pull on
the incoming wire.
Area Reduction in Drawing
Change in size of work is usually given by area
reduction:
where r= area reduction in drawing;Ao
=
original area of work; andAr= final work
o
fo
A
AAr
-
8/10/2019 III-2 Bulk Deformation
48/56 4
Drawing Practice and Products
Drawing practice:
Usually performed as cold working
Most frequently used for round cross sections
Products:
Wire: electrical wire; wire stock for fences,coat hangers, and shopping carts
Rod stock for nails, screws, rivets, and springs
Bar stock: metal bars for machining, forging,and other processes
Wire Drawing vs. Bar Drawing
Difference between bar drawing and wire
drawing is stock size
Bar drawing - large diameter bar and rod
stock
Wire drawing - small diameter stock - wire
sizes down to 0.03 mm (0.001 in.) are
possible Although the mechanics are the same, the
methods, equipment, and even terminology are
different
-
8/10/2019 III-2 Bulk Deformation
49/56 4
Bar Drawing
Accomplished as a single-draftoperation - the stock ispulled through one die opening
Beginning stock has large diameter and is a straight
cylinder
Requires a batch type operation
Hydraulically
operated drawbench for drawing
metal bars.
Bar Drawing Bench
Wire Drawing
Wire is made by cold-drawing hot-rolled wire or rod through one or
more dies to decrease its size and increase the physical properties.
Continuous drawing machines consisting of multiple draw dies
(typically 4 to 12 successive dies) separated by accumulating
drums. The end is grasped by tongs (maa, ene) on a drawbench
and pulled through to such length as may be wound around a
drawing block or reel. The number of dies in the series will depend
on the kind and final diameter of metal or alloy being processed.
Each drum (capstan) provides proper force to draw wire stock
through upstream die
Each die provides a small reduction, so desired total reduction is
achieved by the series
Annealing sometimes required between dies to relieve work
hardening
-
8/10/2019 III-2 Bulk Deformation
50/56 5
WIRE DRAWING
Wire Drawing machine and a drawing reel
Schematic of a multistation synchronized wire-drawing machine. To prevent accumulation or
breakage, it is necessary to ensure that the same volume of material passes through each station in a
given time. The loops around the sheaves between the stations use wire tensions and feedback
electronics to provide the necessary speed control.
Cross section through a typical carbide
wire-drawing die showing the
characteristic regions of the contour. The
dies are usually made from tool steels or
tungsten carbide, although diamond dies
can be used for drawing small diameters.
WIRE DRAWING
-
8/10/2019 III-2 Bulk Deformation
51/56 5
WIRE DRAWING
Wire drawing is often determined by the followingrelationships:
Reduction in area (r) [%] = ((Ao - Af)/Ao)x100
where Ao and Afare the original and final area of the
wire.
Features of a Draw Die
Entry region - funnels lubricant into the die to prevent scoring of work
and die
Approach - cone-shaped region where drawing occurs
Bearing surface - determines final stock size
Back relief - exit zone - provided with a back relief angle (half-angle) of
about 30
Draw die for
drawing of
round rod or
wire
-
8/10/2019 III-2 Bulk Deformation
52/56 5
Preparation of Work for Drawing
Annealing to increase ductility of stock Cleaning - to prevent damage to work surface and
draw die
Pointing to reduce diameter of starting end to
allow insertion through draw die
IMPACT EXTRUSION
Impact Extrusion (svama ekstrzyon): In impact extrusion a
punch is directed to a slug (or blank) (taslak) with such a force
that the metal from the slug is pushed up and around it.
Most impact extrusion operations, such as the manufacture of
collapsible tubes (shaving cream, toothpaste, paint pigment
tubes) are cold-working ones.
Production
of
toothpaste
tube by
impact
extrusion
-
8/10/2019 III-2 Bulk Deformation
53/56 5
Impact Extrusion
A metal slug ispositioned in a diecavity where it is struckby a single blow
Metal may flow forward,backward or somecombination
The punch controls theinside shape while thedie controls the exteriorshape Backward and forward extrusion
with open and closed dies.
Impact Extrusion
Steps in the forming of a bolt by cold extrusion, cold heading, and thread rolling.
(a) Reverse
(b) Forward
(c) Combined
forms of cold
extrusion.
Forming a Bolt with Cold-Forming Sequence
-
8/10/2019 III-2 Bulk Deformation
54/56 5
Cold-forming sequence involving cutoff, squaring, two
extrusions, an upset, and a trimming operation.
Typical parts made by upsetting and related operations.
Cold-Forming Sequence Examples
Piercing
Thick-walled seamless tubing can be made by rotarypiercing
Heated billet is fed into the gap between two large,convex-tapered rolls
Forces the billet to deform into a rotating ellipse
Principle of the
Mannesmann process
of producing seamless
tubing.
-
8/10/2019 III-2 Bulk Deformation
55/56
-
8/10/2019 III-2 Bulk Deformation
56/56
Other Squeezing Operations
Hubbing a die block in a hydraulic press. Inset shows
close-up of the hardened hub and the impression in the
die block. The die block is contained in a reinforcing ring.The upper surface of the die block is then machined flat
to remove the bulged metal.
The
coining
process.
Summary
There are a variety of bulk deformation
processes
The main processes are rolling, forging,
extrusion, and drawing
Each has limits and advantages as to its
capabilities
The correct process depends on the desiredshape, surface finish, quantity, etc.