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Chapter 19:
Bulk Deformation Processes
Rizwan M. Gul
NWFP UET
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BULK DEFORMATION PROCESSES
IN METALWORKING Rolling
Other Deformation Processes Related to Rolling
Forging Other Deformation Processes Related to Forging
Extrusion
Wire and Bar Drawing
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Bulk Deformation
Metal forming operations which cause significant
shape change by deformation in metal parts whose
initial form is bulk rather than sheet
Starting forms: cylindrical bars and billets, rectangular
billets and slabs, and similar shapes
Bulk deformation process also sometimes improve
the mechanical properties of materials
These processes work by stressing metal sufficiently
to cause plastic flow into desired shape
Performed as cold, warm, and hot working operations
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Importance of Bulk Deformation
When performed as hot working, significant shape
change can be accomplished
When performed as cold working , strength can be
increased during shape change
Little or no waste - some bulk deformation operations
are near net shapeor net shapeprocesses
The parts require little or no subsequent
machining
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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 opening
4. Wire and bar drawing diameter of wire or bar is
reduced by pulling it through a die opening
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BASIC BULK DEFORMATION PROCESSES
(a) Rolling (b) Forging
(c) Extrusion (d) Drawing
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Rolling
Rollingis a deformation process in which work thickness is
reduced by compressive forces exerted by two opposing rolls
The 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 cross section
Figure 19.1 - The rolling process (specifically, flat rolling)
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Types of Rolling
By geometry of work:
Flat rolling- used to reduce thickness of a rectangularcross-section
Shape rolling- a square cross-section is formed into ashape such as an I-beam
By temperature of work:
Hot Rollingmost common due to the large amount ofdeformation required in shaping
Cold rollingproduces finished sheet and plate stock
Rolling is a very capital intensive process, as massive
pieces of equipment, called rolling mills, are required toperform it:
The high investment cost requires the mills to be usedfor large production of standard items such as sheetsand plates
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Steel Rolling Practice
The work starts out as a cast steel ingot that has just solidified
by casting While it is still hot, the ingot is placed in a furnace where it
remains for many hours until it has reached a uniform
temperature throughout
For steel the temperature is around 1200 C
The heating operation is called soaking and furnace is
called soaking pits
The ingot is then rolled into one of three intermediate products
called: Bloom, Slab, or Billet
Bloom: Square cross-section of 150 mm or higher Slab: Rectangular cross-section of width 20 mm o more and
thickness 40 mm or more
Billet: Square cross-section of 40 mm or higher
These intermediate shapes are subsequently rolled into
product shapes
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Flat- and Shape-Rolling Processes
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Figure 19.2 - Some of the steel products made in a rolling mill
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Flat Rolling and Its Analysis
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Flat Rolling Terminology
Draft= amount of thickness reduction
fo ttd
where d = draft; to = starting thickness; and tf = final
thickness
Reduction = draft expressed as a fraction of starting
stock thickness:
ot
d
r
where r = reduction
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Flat-Rolling
Figure 13.2 (a) Schematic illustration of the flat-rolling process. (b) Friction forces acting on stripsurfaces. (c) The roll force, F, and the torque acting on the rolls. The width wof the strip usually increasesduring rolling, as is shown in Fig. 13.5.
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Ways to Reduce Force and/or Power in Rolling
Force and/or power to roll a strip of a given width and workmaterial can be reduced by any of the following:
1. Using hot rolling rather than cold rolling to reduce
strength and strain hardening (Kand n) of the work
material [ ]2. Reducing the draft in each pass [ ]
3. Using a smaller roll radius Rto reduce force
[ ]
4. Using a lower rolling speed Nto reduce power
nf KY
fo ttRL (
fo ttRL (
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Example 9.1
A 300 mm wide strip 25 mm thick is fed through a
rolling mill with two powered rolls each of radius =
250 mm. The work thickness is to be reduced to 22
mm in one pass at a roll speed of 50 rev/min. Thework material has a flow curve defined by K= 275
MPa and n= 0.15, and the coefficient of friction
between the rolls and the work is assumed to be
0.12. Determine if the friction is sufficient to permitthe rolling operation to be accomplished. If so,
calculate the roll force, torque, and the horsepower.
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Shape Rolling
In shape rolling, the work is deformed into acontoured cross-section rather than flat (rectangular)
Accomplished by passing work through rolls thathave the reverse of desired shape
Gradual transformation through several rolls toachieve final cross-section is achieved by designing aspecific roll-pass design
Products include:
Construction shapes such as I-beams, L-beams,
and U-channels Rails for railroad tracks
Round and square bars and rods
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Shape Rolling
Figure 13.13 Stages in theshape rolling of an H-section
part. Various other structuralsections, such as channels andI-beams, are also rolled by thiskind of process.
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Figure 19.5 - Arolling mill for hot
flat rolling; the
steel plate is
seen as the
glowing strip
extending
diagonally from
the lower left
corner
(photo courtesy
of Bethlehem
Steel Company)
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Rolling Mill
Figure 13.10 A generalview of a rolling mill.Source: Inland Steel.
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Rolling Mills
Equipment is massive and expensive
Various rolling-mill configurations are available to deal
with the variety of applications and technical problems inthe rolling process
Rolling mill configurations:
Two-high:
two opposing large diameter rolls can be either reversing or nonreversing
Three-high: work passes through both directions
Four-high:
backing rolls support smaller work rolls, smaller diameter rolls means lower forces, torque
and power
Cluster mill multiple backing rolls on smaller rolls
Tandem rolling mill
sequence of two-high mills
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(a) 2-high rolling mill
Various Configurations of Rolling mills
(b) 3-high rolling mill
(c) four-high rolling mill
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Cluster MillMultiple backing rolls allow even smaller roll diameters
Figure 19 6 - Various configurations of rolling mills: (d) cluster mill
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Backing Roll Arrangements
Figure 13.11 Schematic illustration of various roll arrangements: (a) two-high; (b) three- high; (c)four-high; (d) cluster (Sendzimir) mill.
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Tandem Rolling Mill A series of rolling stands (two-high mills) in sequence
Helps achieve higher throughput rates in standard
products
As thickness is deccreased in each rolling step, work
velocity increases, and the problem of synchronizing
the roll speeds at each stand is a significant one
Figure 19.6 - Various configurations of rolling mills: (e) tandem rolling mill
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Tandem Rolling
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OTHER DEFORMATION PROCESSES
RELATED TO ROLLING
Thread Rolling
Gear Rolling
Ring Rolling
Roll Piercing or Mannesmann Mill
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Bulk deformation process used to form threads on
cylindrical parts by rolling them between two dies Most important commercial process for mass
producing bolts and screws
Performed by cold working in thread rolling machines
Advantages over thread cutting (machining): Higher production rates
Better material utilization
Stronger threads due to work hardening
Better fatigue resistance due to compressivestresses introduced by rolling
Gear rolling is a cold working process similar tothread rolling
Thread Rolling and Gear Rolling
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Figure 19.7 - Thread rolling with flat dies:
(1) start of cycle, and (2) end of cycle
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Ring Rolling
Deformation process in which a thick-walled ring ofsmaller diameter is rolled into a thin-walled ring oflarger 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 tiresfor railroad wheels, and rings for pipes, pressurevessels, and rotating machinery
Advantages: material savings, ideal grain orientation,strengthening through cold working
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Figure 19.8 - Ring rolling used to reduce the wall thickness andincrease the diameter of a ring:
(1) start, and (2) completion of process
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Roll Piercing or Mannesmann Process
A specialized hot working process for making seamlessthick-walled tubes
Process is based on principle:
When a solid cylindrical part is compressed on its
circumference, high tensile stresses are developed at
its center
If compression is high enough, an internal crack is
formed
Compressive stresses on a solid cylindrical billet are
applied by two rolls, whose axes are oriented at slightangles from the axis of the billet, so that their rotation
tends to pull the billet though the rolls
A mandrel is used to control the size and finish of the hole
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Roll Piercing or
Mannesmann Process or
Rotary Tube Piercing
Figure 13.17 Cavity formation in a solid round bar and its utilization in the rotary tube piercing
process for making seamless pipe and tubing. (The Mannesmann mill was developed in the 1880s.)
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Forging
Deformation process in which work is compressed
between two dies, using either impact or gradual
pressure to form the part
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
In addition, basic metals industries use forging to
establish basic form of large components that aresubsequently machined to final shape and size
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Classification of Forging Operations
Cold vs. hot forging:
Hotor warmforgingmost common, due to the
significant deformation and the need to reduce
strength and increase ductility of work metal
Cold forging- advantage is increased strength that
results from strain hardening
Impact vs. press forging:
Forge hammer- applies an impact load
Forge press- applies gradual pressure
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Types of Forging Dies
The distinction is based on the degree to which the flow
of work metal s constrained by the dies
Open-die forging- work is compressed between two
flat (or almost flat) dies, allowing metal to flow
laterally without constraint
Impression-die forging- die surfaces contain a cavity
or impression that is imparted to workpart, thus
constraining metal flow - flashis created (excess
material that is trimmed off)
Flashless forging- workpart is completely
constrained in die and no excess flash is produced
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Figure 19.10 - Three types of forging: (a) open-die forging
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Figure 19.10 - Three types of forging (b) impression-die forging
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Figure 19.10 - Three types of forging (c) flashless forging
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Open-Die Forging
Involves compression of workpart with cylindrical
cross-section between two flat dies
Similar to compression test
Deformation operation reduces height and increases
diameter of work
Common names include upsettingor upset forging
Open die hot forging is an important process to
produce shafts, disks and rings: Used for shaping a
large square ingot into a round cross-section creating
favorable grain flow and metallurgical structure
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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
Force required to continue the compression at any
given height h: F =YfA
h
holn
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Figure 19.11 - Homogeneous deformation of a cylindrical workpartunder ideal conditions in an open-die forging operation:
(1) start of process with workpiece at its original length and
diameter, (2) partial compression, and (3) final size
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Open-Die Forging with Friction
Friction between work and die surfaces constrainslateral 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 itsresistance to deformation
The above factors causes the actual upsetting force
to be greater than with no friction case:F =KfYfA
whereKfis the forging shape factor given byequation:
h
DKf
4.01
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Figure 19.12 - Actual deformation of a cylindrical workpart in
open-die forging, showing pronounced barreling:
(1) start of process, (2) partial deformation, and (3) final shape
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Example 19.2
A cylindrical workpiece is subjected to a cold upset
forging operation. The starting piece is 75 mm in
height and 50 mm in diameter. It is reduced in the
operation to a height of 36 mm. The work materialhas a flow curve defined by K=350 MPa and n=0.17.
Assume a coefficient of friction of 0.1. Determine the
force as the process begins, at intermediate heights
of 62 mm, 49 mm, and at the final height of 36 mm.
Yf=Kn
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Impression Die or Closed Die Forging
Compression of workpart by dies with inverse of
desired part shape
Flash is formed by metal that flows beyond die cavity
into small gap between die plates
Flash must be later trimmed from part, 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
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Figure 19.15 - 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
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Impression-Die Forging Practice
Several forming steps often required, with separate
die cavities for each step
Beginning steps redistribute metal for more
uniform deformation and desired metallurgicalstructure in subsequent steps
Final steps bring the part to its final geometry
Impression-die forging is often performed
manually by skilled operator under adverseconditions
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Forging a Connecting Rod
Figure 14.7 (a) Stages in
forging a connecting rod for an
internal combustion engine.
Note the amount of flash
required to ensure proper
filling of the die cavities. (b)
Fullering, and (c) edgingoperations to distribute the
material when preshaping the
blank for forging.
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Impression-Die Forging
Advantages and Limitations
Advantages compared to machining from solid stock:
Higher production rates
Conservation of metal (less waste)
Greater strength
Favorable grain orientation in the metal
Limitations:
Not capable of close tolerances
Machining often required to achieve accuracies
and features needed, such as holes, threads, and
mating surfaces that fit with other components
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Flashless Forging
Compression of work in punch and die tooling whose
cavity does allow for flash
Starting workpart volume must equal die cavity
volume within very close tolerance Process control more demanding than impression-die
forging
Best suited to part geometries that are simple and
symmetrical Often classified as aprecision forgingprocess
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Figure 19.18 - Flashless forging:
(1) just before initial contact with workpiece,(2) partial compression, and
(3) final punch and die closure
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Forging Hammers (Drop Hammers)
Apply an 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
Disadvantage: impact energy transmitted through
anvil into floor of building
Most commonly used for impression-die forging
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Figure 19.20 - Drop forging hammer, fed by conveyor and heatingunits at the right of the scene
(photo courtesy of Chambersburg Engineering Company)
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Figure 19.21 - Diagram showing details of a drop hammer for
impression-die forging
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Forging Presses
Apply gradual pressure to accomplish compression
operation - types:
Mechanical presses- converts rotation of drive
motor into linear motion of ram
Hydraulic presses- hydraulic piston actuates ram
Screw presses- screw mechanism drives ram
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Impression-Forging Dies
Figure 14.18 Standard terminology for
various features of a typical impression-
forging die.
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Upsetting and Heading
Forging process used to form heads on nails, bolts, and
similar hardware products
More parts produced by upsetting than any other
forging operation
Performed cold, warm, or hot on machines called
headersor 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 to
form threads
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Figure 19.23 - An upset forging operation to form a head on a boltor 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
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Figure 19.24 - 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
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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
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Figure 19.25 - 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 theyhammer the work
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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
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Figure 19.30 - Trimming operation (shearing process)to remove
the flash after impression-die forging
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Extrusion
Compression forming process in which the work metalis forced to flow through a die opening to produce adesired cross-sectional shape
Process is similar to squeezing toothpaste out of atoothpaste tube
In general, extrusion is used to produce long parts ofuniform cross-sections
Two basic types of extrusion:
Direct extrusion
Indirect extrusion
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Figure 19.31 - Direct extrusion
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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 extruded product by cutting it just beyond the
die exit
Starting billet cross section usually round, but finalshape is determined by die opening
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Figure 19.32 - (a) Direct extrusion to produce a hollow or semi-hollow
cross-section; (b) hollow and (c) semi-hollow cross- sections
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Figure 19.33 - Indirect extrusion to produce
(a) a solid cross-section and (b) a hollow cross-section
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Comments on Indirect Extrusion
Also called backward extrusionand reverse extrusion
Limitations of indirect extrusion are imposed by the
lower rigidity of hollow ram and difficulty in supporting
extruded product as it exits die
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General 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
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Hot vs. Cold Extrusion
Hot extrusion- prior heating of billet to above its
recrystallization temperature
This reduces strength and increases ductility of
the metal, permitting more size reductions andmore complex shapes
Cold extrusion- generally used to produce discrete
parts
The term impact extrusionis used to indicate highspeed cold extrusion
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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
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Figure 19.36 -(a) Definition of die angle in direct extrusion;
(b) effect of die angle on ram force
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Comments on Die Angle
Low die angle - surface area is large, leading to
increased friction at die-billet interface
Higher friction results in larger ram force
Large die angle - more turbulence in metal flowduring reduction
Turbulence increases ram force required
Optimum angle depends on work material, billet
temperature, and lubrication
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Comments on Orifice Shape
of Extrusion Die
Simplest cross section shape = circular die orifice
Shape of die orifice affects ram pressure
As cross-section becomes more complex, higher
pressure and greater force are required
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Figure 19.37 - A complex extruded cross-section for a heat sink
(photo courtesy of Aluminum Company of America)
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Extrusion Presses
Either horizontal or vertical
Horizontal more common
Extrusion presses - usually hydraulically driven,
which is especially suited to semi-continuous directextrusion of long sections
Mechanical drives - often used for cold extrusion of
individual parts
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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 ispulledthrough 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
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Figure 19.41 - Drawing of bar, rod, or wire
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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
AA
r
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Wire Drawing vs. Bar Drawing
Difference between bar drawing and wire drawing is
stock size
Bar drawing- large diameter bar and rod stock
Wiredrawing- small diameter stock - wire sizesdown to 0.03 mm (0.001 in.) are possible
Although the mechanics are the same, the methods,
equipment, and even terminology are different
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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 stockfor nails, screws, rivets, and springs
Bar stock: metal bars for machining, forging, and
other processes
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Bar Drawing
Accomplished as a single-draftoperation - the stock
is pulled through one die opening
Beginning stock has large diameter and is a straight
cylinder This necessitates a batch type operation
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Figure 19.42 - Hydraulically operated draw bench
for drawing metal bars
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Wire Drawing
Continuous drawing machines consisting of multiple
draw dies (typically 4 to 12) separated by
accumulating drums
Each drum (capstan) provides proper force todraw 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
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Figure 19.43 - Continuous drawing of wire
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Features of a Draw Die
Entryregion - 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
Die materials: tool steels or cemented carbides
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Figure 19.44 - Draw die for drawing of round rod or wire
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Preparation of the Work for
Wire or Bar Drawing
Annealingto increase ductility of stock
Cleaning- to prevent damage to work surface and
draw die
Pointingto reduce diameter of starting end to allowinsertion through draw die