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Bulk Deformation Processes
in Metalworking
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Rolling
Rolling - deformation process with thickness
reduced by compressive forces exerted by
two opposing rolls.
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Rolling Analysis
Conservation of material
to wo Lo  t f w f L f
Continuity of volume flow
rate
to wo vo  t f w f v f
Forward slip
s
v f  vr
vr
Rolling force, F
L
F  w pdL  Y f wL
0
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Rolling Analysis
Where the deformation strain
  ln
to
tf
and the average flow stress
K n
Yf 
1 n
The torque required for the deformation process
T  0.5FL
The power required by the process is
P  2FL
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Rolling Mechanics
The rolling process is governed by the
frictional force between the rollers and the
workpiece. The frictional force at the
entrance side is higher than that at the exit
side. This allows the roller to pull the
workpiece towards the exit end.
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Friction
vo<vr<vf
Maximum draft, which is the
thickness reduction, is given
as 2R.
Coefficient of friction depends
on lubrication, typically:
cold working 0.1
warm working 0.2
hot working 0.4
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Material and Process Parameters
Material Parameters
– ductility
– coefficient of friction
– strength, modulus and Poisson’s ratio
Process Parameters
–
–
–
–
roller speed
power
draft
lubrication
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Shape Rolling
In addition to the material and process
parameters, the rollers will acts as a set of
dies and have to be pre-formed to take the
negative shape of the cross-section.
There may be more than one set of rollers
required to reduce the workpiece to the
appropriate shape.
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Rolling Mill Configurations
a) two high b) three high c) four high
d) cluster mill e) tandem rolling mill
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Ring Rolling
• To make a larger and thinner ring from the
original ring
• Usually a hot rolling process for large rings
and cold rolling for small rings
• Typical applications: bearing races, steel
tires, rings for pressure vessel.
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Thread Rolling
• Production of external thread
• Cold rolling
• High and competitive production rate (up to
8 parts per second)
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Gear Rolling
• Similar to the screw thread.
• Typically for helical gears
• Shares the same advantages:
–
–
–
–
better material usage
smoother surface
stronger thread due to work hardening
better fatigue resistance due to compression
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Roll Piercing
• Hot working process
• Production of Seamless thick-wall tubes
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Forging
Open-die forging
Flashless forging
Impression-die forging
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Mechanics of Forging
Under ideal condition:
h
  ln o
h
F  Yf A
Where F = forging force
Yf = flow stress
A = cross-section of part
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Forging
In open-die forging, barreling occurs.
But with hot forging, the issue is complicated by the thermal
distribution within the workpiece and the associated flow of
metal.
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Shape factor
Load-stroke curve
The actual forging force is
greater than the ideal case.
F  K f Yf A
The shape factor is to cover
the effect of barreling and
the friction effect.
0.4D
K f  1
h
Open-die forging is not a
net-shape process and will
require subsequent
machining to dimension.
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Open-die Forging
Fullering
Edging
Cogging
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Open-die Forging
• Fullering
Reducing workpiece cross section to prepare for
subsequent shaping action. Dies with convex
surface cavity are used.
• Edging
Similar to Fullering, but the dies have concave
surface cavitiy.
• Cogging
Open dies with flat or slightly contoured surfaces
to reduce cross-section and to increase length.
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Impression-die Forging
Dies containing the inverse of the shape of the
part. Flash is allowed on the parting surface.
The flash serves as a constraint for metal
flow in the die and help to fill the intricate
details of the cavity.
Higher forging forces are required in this
process than open-die forging. The shape
factor generally will have a higher value.
F  K f Yf A
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Impression-die Forging
• The forces are largest at the end of the
process when the projected area of the blank
and the friction is largest.
• Again, progressive dies are needed to
transform the starting blank into a final
desired geometry.
• Machining is needed to produce the fine
tolerance needed.
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Impression-die Forging
Pros:
• high production rate
• conservation of metal
• greater strength
• favorable grain orientation
Forging
Machining
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Shape Factor
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Flashless Forging
Conventional forging part
Precision forging part
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Flashless Forging
The volume control is important and the outcome is
precision re-production of inverse of cavity geometry.
Typically for aluminum and magnesium alloy.
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Corning
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Drop Hammer and Dies
Webs - Thin section parallel to parting line.
Ribs - thin section perpendicular to parting line
Gutter - area for containing flash
Dies are normally made from tool steel with high
impact strength and high wear resistance.
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Upsetting and Heading
Upsetting and Heading
The leading section of the stock is forged to form a
head section using closed-die forging.
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Upsetting and Heading
Upsetting is used to form heads of screw and bolt
with different geometric forms.
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Swaging
Swaging used to
reduce the crosssections of forged
rods or tubes using
a set of rotating
dies. A mandrel is
sometimes used to
control the internal
form of the tube.
Radial forging
rotates the stock
rather than the die.
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Roll Forging
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Orbital Forging
Small contact area reduce
the forging force
required substantially.
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Hobbing
To press the die against the softer blank to form the final shape.
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Trimming
Trimming is a shearing
process to remove
the flash from the
workpiece.
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Design Considerations
• Material
• Die design
• Machine
– Machine processing range
– Machine process setting
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Design Considerations
Material
– Ductility
– Strength
– Plastic deformation law (constitutive
relationship)
– Coefficient (Die/workpiece)
– Variation of properties at processing
temperature range
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Design Considerations
Die Design
– Number of die stations (progressive die)
– Geometric complexity of the part
– Die geometric details
• Draft angle, fillet, radii
• Webs and ribs
• Flash
– Parting surface and parting direction
– Die material
– Die life
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Design Considerations
Machine processing range
–
–
–
–
Maximum forging force
Maximum power
Maximum speed
Maximum die size
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Design Considerations
Machine process setting
–
–
–
–
No. of stations
Velocity profile
Temperature / time profile
Force