ENM208
BULK DEFORMATION •
PROCESS 1
ANADOLU UNIVERSITY •
Industrial Eng. Dep.
•
Manufacturing Processes
Bulk Deformation Processes in Metal Forming
Metal forming operations which cause significant shape
change by deformation in metal parts whose initial
form is bulk rather than sheet
"Bulk" refers to workparts with relatively low surface
area-to-volume ratios
• Starting forms: cylindrical bars and billets, rectangular
billets and slabs, and similar shapes
• 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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Manufacturing Processes
Bulk Deformation Processes in Metal Forming
Bulk deformation processes are performed as Cold,
warm and hot working operations
Cold and Warm working is appropriate when
• Shape change is less sever;
• There is a need to improve mechanical properties;
• Achieve good surface finish on the part.
Hot working is generally required when massive
deformation of large workparts is involved
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Manufacturing Processes
Importance of Bulk Deformation Processes
When performed as hot working, they can achieve
significant change in shape of the workpart.
When performed as cold working operations, they can
be used not only to shape the product, but also to
increase its strength.
These processes produce little or no waste
Some bulk deformation operations are near net shape
or net shape processes.
They achieve final product geometry with little or no
subsequent machining
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Manufacturing Processes
Four Basic Bulk Deformation Processes
Rolling – slab or plate is squeezed between opposing rolls
Forging – work is squeezed and shaped between opposing
dies
Extrusion – work is squeezed through a die opening,
thereby taking the shape of the opening
Wire and bar drawing – diameter of wire or bar is
reduced by pulling it through a die opening
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Manufacturing Processes
Products of Bulk Deformation Processes
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Manufacturing Processes
Rolling
Deformation process in which work thickness is reduced
by compressive forces exerted by two opposing rolls
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Manufacturing Processes
The 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
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Manufacturing Processes
Types of Rolling
• By geometry of work:
– Flat rolling - used to reduce thickness of a
rectangular cross-section
– Shape rolling - a square cross-section is formed into
a shape such as an I-beam
• By temperature of work:
– Hot Rolling – most common due to the large amount
of deformation required
– Cold rolling – produces finished sheet and plate stock
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Manufacturing Processes
Rolled Steel Products
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Manufacturing Processes
Flat Rolling
It involves the rolling of slabs, strips, sheets, and plates,
workparts of rectangular cross section in which the width
is greater than the thickness.
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Manufacturing Processes
Flat Rolling Analysis
Draft: The work is squeezed between two rolls so that its
thickness is reduced by an amount called the draft.
where
d t o t f
d = draft; to = starting thickness; and tf = final thickness
Reduction: The draft is sometimes expressed as a fraction of
the starting stock thickness, called the reduction
Reduction =
d
r 
to
When a series of rolling operations is used, reduction is taken as the
sum of the drafts divided by the original thickness
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Manufacturing Processes
Flat Rolling Analysis
Spreading: In addition to thickness reduction, rolling
usually increases work width. This is called spreading
Conservation of material is preserved, so the volume of metal
exiting the rolls equals the volume entering:
t0 w0 L0  t f w f L f
Where
w0 and wf are the before and after work widths (mm)
L0 and Lf are the before and after work lengths (mm)
Similarly, before and after volume rates of materials flow must be
the same, so the before and after velocities can be related:
t0 w0 v0  t f w f v f
Where v0 and vf are the entering and exiting velocities of the work
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Manufacturing Processes
Flat Rolling Analysis
Maximum Draft: There is a limit to the maximum possible
draft that can be accomplished in flat rolling with a given
coefficient of friction
d max   R
2
where
dmax = maximum draft (mm).
μ = coefficient of friction
R= roll radius (mm)
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Manufacturing Processes
Flat Rolling Analysis
Rolling Force: The rolling force can be calculated based
on the average flow stress experienced by the work
material in the roll gap
F  Y wL
Where
Y
= average flow stress (MPa)
wL = the roll-work contact area (mm2)
- Contact length can be approximated by
L  R (t0  t f )
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Manufacturing Processes
Flat Rolling Analysis
The torque in rolling can be estimated by assuming that
the roll force is centered on the work as it passes the
rolls and that it acts with a moment arm of one-half the
contact length L.
T  0.5FL
The power required to drive each roll is the product of
torque and angular velocity
P  2NFL
Where
P = power (J/s)
F = rolling force (N)
N = rotational Speed (rev/min)
L = contact length (m)
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Manufacturing Processes
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
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Manufacturing Processes
Rolling Mill Configuration
Two-high – two opposing large diameter rolls
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Manufacturing Processes
Rolling Mill Configuration
Three-high – work passes through both directions
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Manufacturing Processes
Rolling Mill Configuration
Four-high – backing rolls support smaller work rolls
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Manufacturing Processes
Rolling Mill Configuration
Cluster mill – multiple backing rolls on smaller rolls
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Manufacturing Processes
Rolling Mill Configuration
Tandem rolling mill – sequence of two-high mills
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Manufacturing Processes
Thread Rolling
• 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
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Manufacturing Processes
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, strengthening through cold working
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Manufacturing Processes
Forging
Forging is a deformation process in which work is
compressed between two dies
Oldest of the metal forming operations, dating from about 5000 BC
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 are
subsequently machined to final shape and size
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Manufacturing Processes
Classification of Forging Processes
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 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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Manufacturing Processes
Classification of Forging Processes
Another difference among forging operations is the
degree to which the flow of the work metal is
constrained by the dies.
1- Open-die forging operation.
2- Impression-die forging operation.
3- Flashless forging operation.
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Manufacturing Processes
Open-die Forging Operation
The work is compressed between two flat dies, thus
allowing the metal to flow without constraint in a lateral
direction relative to the die surfaces.
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Manufacturing Processes
Impression-die Forging Operation
The die surfaces contain a cavity or impression that is
imparted to workpart, thus constraining metal flow flash is created
- Flash is excess metal that must be trimmed off later.
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Manufacturing Processes
Flashless Forging Operation
The work is completely constrained within the die and
no excess flash is produced.
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Manufacturing Processes
Open-die Forging Operation
• Compression
of
workpart
with
cross-section between two flat dies
cylindrical
• Similar to compression test
• Deformation operation reduces
increases diameter of work
height
and
• Common names include upsetting or upset forging
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Manufacturing Processes
Open-die Forging Operation 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
(1) start of process with workpiece at its original length and diameter,
(2) partial compression, and (3) final size
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Manufacturing Processes
Open-die Forging Operation with Friction
Friction between work and die surfaces constrains
lateral flow of work, resulting in barreling effect
(1) start of process, (2) partial deformation, and (3) final shape
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Manufacturing Processes
Open-die Forging Analysis
Under ideal conditions, the true strain experienced by
the work during the process is determined by
ho
  ln
h
where
ho= starting height; and
h = height at some point during compression
• At h = final value hf, true strain is maximum value
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Manufacturing Processes
Open-die Forging Analysis
The forging force required to continue compression at
any height h during the process is obtained by
F  Yf A
Where
F = Upsetting (Forging) force (N).
A = Cross sectional area of the part (mm2)
Yf = Flow stress corresponding to the true strain (MPa)
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Manufacturing Processes
Open-die Forging Analysis
In actual forging process, the force is obtained by
F  K f Yf A
Where Kf = The Forging Shape Factor defined as
Where
0.4D
K f  1
h
μ = Coefficient of friction.
h = Work height (mm).
D = Workpart diameter or other dimension representing contact length
with die surface (mm)
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Manufacturing Processes
Impression-die Forging Operation
• 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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Manufacturing Processes
Impression-die Forging Operation
Several forming steps often required, with separate die cavities for each step
Beginning steps redistribute metal for more uniform deformation
Final steps bring the part to its final geometry
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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Manufacturing Processes
Impression-die Forging Operation Advantages and Limitations
• Advantages compared to machining from solid stock:
– Higher production rates
– Conservation of metal (less waste)
– Greater strength
• 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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Manufacturing Processes
Flashless Forging Operation
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.
Best suited to part geometries that are simple and
symmetrical
Often classified as a
precision forging process
(1) just before initial contact
with workpiece,
(2) partial compression, and
(3) final punch and die closure
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Manufacturing Processes
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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Manufacturing Processes
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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Manufacturing Processes
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
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 to
form threads
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Manufacturing Processes
Upsetting and Heading
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
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Manufacturing Processes
Upsetting and 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
Spring 2005