Chapter 2
Bulk Deformation
Forming - Forging
1
Forging Process
Application of compressive force applied
through various mechanisms
 The forming of workpieces through a
succession of tools and dies
 One of the oldest metalworking operations
 Initially just a hammer on an anvil (jewelry,
horse shoes, sword making)
 Used to improved properties as well as form
a shape
 Produces discrete parts
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Forging Process History
 Molds
of stone helped initial forming
efforts
 Now forces are
– Mechanical (hammer presses)
– Hydraulic
 Dies are tool steel
 Near net shape forming
3
Forging Practice
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Prepare raw material including cleaning
Heat workpiece (for hot forging)
Descale if necessary
Preheat and lubricate dies (hot forging)
Forge in appropriate dies and in correct sequence
Remove excess material (flashing)
Clean
Check dimensions
Straighten if necessary
Machine to final dimensions
Heat treat if necessary
Inspect
4
Forging Process Capabilities
 Tolerances
of 0.5% to 1% can be
achieved
 Material properties can be tailored by
appropriate die design
– Directed material flow
5
Forging Processes
Advantages
– Metal flow and grain structure can be
controlled
– Results in good strength and toughness
– Near net shape
– Parts of reasonable complexity can be created
• Landing gear
• Connecting rods
• Complex shafts
 Disadvantages
– Dies are expensive, particularly for hot forging
– Highly skilled labor required
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Forging Process Categories
7
Open Die Forging and Cogging
Simplest and cheapest
 Also called upsetting or flat-die forging
 Advantages
– Cheap
– Can form a wide variety of simple shapes with
the same dies
• Squares, cylindrical
– Useful for preparing material for other forms
of forging or machining
– Can handle large items (35 tons)
 Disadvantages
– Barreling of shape due to high friction
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Open Die Forging and Cogging
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Open Die Forging Force
F = Yf p r2 (1 + 2mr/3h)
where Yf is the flow stress of the material
m is the coefficient of friction
r is the radius
h is the height of the workpiece
Examples
– Stainless steel workpiece, 150 mm diameter,
100 mm high reduced with flat dies to 50% of
original height. Coefficient of friction is 0.2
– Force is 5000 tons
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Impression and Closed Die
Forging
Use dies with the approximate end shape
 Usually requires more than one die to
complete process
 Fullering and Edging dies prepare material to
take up die shape
– Fullering moves material away from
center
– Edging moves material away from edges
 Flashing produced from excess material
 Often used to ensure good die filling
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Stages in Impression Die
Forging
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Load in impression-die
forging
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Stages in the forging of a conrod
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Terminology of Impression
Forging
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Impression and Closed Die
Forging
 Advantages
– Produces near net shape
– Material properties tailored to
application
 Disadvantages
– High die costs
– Highly skilled labor required
16
Precision Forging
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A further development of closed die forging
Close calculation of material required to fill die
minimizes scrap and flashing
Dies have more detail minimizing subsequent
shaping operations
Advantages
– Little subsequent shaping
– Good to excellent properties
Disadvantages
– Expensive
– Difficult to control
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Closed Die Forging Force
F = k Yf A
where Yf is the flow stress
A is the area and
k is a factor given below
Shapes
k
Simple, no flashing
3-5
simple, with flashing
5-8
Complex, with flashing
8-12
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Related Processes
Coining
– Similar to precision forging but much older
– Die cavity completely closed
– Very high pressures involved
– Used in coin making
 Heading
– Used mostly for bolts
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Related Processes
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Piercing
– Exactly as it sounds
– Makes holes
– Used in conjunction with
closed die forging
Hubbing
– Like piercing but for making cavities, not complete penetrations
larger areas
Roll Forging
– Uses rolls to shape parts
– Similar to shape rolling but makes discrete parts
– (cross-rolling) operation. Tapered leaf springs and knives can
be made by this process with specially designed rolls.
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Skew rolling
Production of steel balls for
bearings by the skew rolling
process.
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Orbital Forging
– Forms the part incrementally
– Small forging forces because the die contact is
– concentrated on a small part of the workpiece at anyone time
– Applicable to mostly cylindrical shapes
 Incremental forging
– Blank formed in several small steps like orbital
– non-rotational parts can be made
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Isothermal forging
– Dies at same temperature of workpiece
– No workpiece cooling
– Low flow stresses
– Better material flow
– More close tolerances and finer details can be
achieved
Swaging
– Cylindrical parts subjected to radial impact forces by
reciprocating dies
– Used to reduce tube diameter and introduce rifling
into gun barrels
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Die Design
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Requires knowledge of
– Material strength
– Sensitivity of these to deformation rate and
temperature
– Friction and its control
– Shape and complexity of workpiece
– How the metal will flow to fill the die cavity
– Great skill and expertise
– Multiple dies to move the material in the right
direction
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Forgeability
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Defined as the capability of a material to undergo
deformation without cracking
Common test is the upset test
– Upset cylindrical specimen to fixed, large deformation
– Examine barrel surfaces for cracks
Another is the hot torsion test
– Twist long cylindrical specimen around its axis
– No of twists to failure is forgeability
– Also used for rolling and extrusion deformation
capabilities
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Hot forging Temperatures
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Product Quality Issues
 Surface
cracks (forgeability limitation)
 Buckling
 Laps
 Internal cracks
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Defects
Laps formed by buckling of the web during forging.
Internal defects produced in a forging because of an oversized billet. The
die cavities are filled prematurely, and the material at the center of the
part flows past the filled regions as deformation continues.
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Defects
Effect of fillet radius on defect formation in forging. Small fillets (right side
of drawings) cause the defects.
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Forging Machines
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Mechanical Presses
– Hydraulic
– Mechanical
– Screw
– Hammers
– Gravity Drop
– Power Drop
– Counterblow
– High Energy Rate
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Hydraulic Presses
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Constant speed
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Load limited
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Compared to mechanical
– Typically slower
– Higher initial cost
– Less maintenance
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Large amount of energy can be
transmitted to the workpiece
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suited for extrusion-type forging
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Used for both open-die and closeddie forging
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The largest H-press in the world is
75000 tons, The largest of our
country is 25000tons
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Mechanical Presses
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Crank or eccentric types
Stroke limited
Energy dependent on that stored
in flywheel
Very large forces can be generated at bottom
dead center
Hence must be careful in die design and
placement to avoid die fracture
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Screw Presses
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Derive energy from flywheel
like mechanical presses
Flywheel drives a screw, not a ram
Energy limited
Process stops when flywheel energy exhausted
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Suitable for producing small quantities, for parts requiring
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precision (such as turbine blades), and for control of ram
speed
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The largest screw press has a capacity of 16000 tons
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Hammers
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Ram is raised by some mechanism and let fall
onto workpiece
Derives energy from potential energy of the
hammer
They are energy limited
High speeds
Minimal cooling
Different types
– Gravity drop
– Power drop
– Counterblow
– High energy rate machines
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Equipment Selection
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The selection of forging equipment depend on:
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The size and complexity of the forging
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The strength of the material and its sensitivity to strain rate
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The degree of deformation
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Guideline
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Presses are generally preferred for aluminium,
magnesium, beryllium, bronze and brass
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Hammer are preferred for copper, steel, titanium and
refractory alloys
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Characteristics of Forging
Processes
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Forging Economics
Setup and tooling costs are high initially
 Good for large production quantities
 Material costs as a fraction of total costs vary
with material
– High percentage for stainless steels (7085%)
– Low percentage for carbon steel (2545%)
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