Chapter 19-Bulk Deformation Processes.ppt

5/27/2018 Chapter 19-Bulk Deformation Processes.ppt

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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

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Chapter 19-Bulk Deformation Processes.ppt

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