1WARM AND COLD FORGING
PRECISION FORGINGCOLD HEADING
COLD EXTRUSION
2Precision forging
Main characteristics: Closed die forging Temperature is below the hot range, it
is warm forging (T~0,5*Tm oK) Close tolerances, acceptable surface
finish near net shaping process Good material yield Good mechanical properties
3Economic considerations
Precision forging is more costly thanconventional forging, but!
Savings in material and machiningcosts are significant
Forming complex shapes is possible Precision forging represents a higher
value product than a conventionalforging (higher added value)
4Temperature of forging
Low alloy steels:
950 1150 oCHot forging
540 815 oCWarm forging
< 250 oCCold forging
Controlled cooling may be necessary afterforging to avoid distorsion and to control themicrostructure of the workoiece
5Tooling (1)
Dimensions Allowances (thermal contraction,
machining) Draft angles, no sharp corners good
material flow
Workability High deformation levels needs good
formability Try to avoid cracking!
6Tooling (2)
Precision: Tolerance bands of tool: 1030% of the
workpiece High-precision machining (EDM) Rigid alignment Preform considerations (volume, weighting,
appropriate shape)
Conditions: good lubrication, removecontaminants, good control of billet and tooltemperature
7Forging equipment
Billet separation: shearing or sawing Heating:
Furnace oxide formation Induction heating Resistance heating
Presses: Hammers Crank presses Hydraulic presses
8Comparison of forgings
9Cold forging
Processes: Upsetting Cold forging of components
Effect of cold working on material properties the ductility of the material drops, strengths and
hardness increases (because of higher dislocation density - strain hardening)
the microstructure changes, crystals (grains) become elongated in the direction of major deformation
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Cold heading
To upset the metal in a portionof wire or rod blank
The cross-sectional area ofthe initial material is increasedas the height of the workpieceis decreased
variants: Free (head formed between flat
punch and die) Closed (head formed in punch
and die)
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Limits of deformation
Material dependent:
7.2...2 dD
3...2 kD
3,2dL
Material independent:
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Calculation of force
Where:
kf flow stress
A cross-sectional area
R radius of head
h height of head
coefficient of friction
)Rh321(AfkF+=
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Two-stroke upsetting
If L/d > 2,3 then twooperations areneeded: Preforming Finish heading
Preforming head: conic-cylindrical
Finish: closedheading punch
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Example: screw
head heading reduction shearingshearing
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Example: screws and bolts
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Example: heading
Closed-die cold heading
Die can be opened for feeding and removing theworkpiece
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Head shearing
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Cold extrusion of parts
Principle: a punch applies pressure tothe preform or billet, causing the workmetal to flow in the required direction
Process variants: Direct indirect or forward backward Extrusion of rod, can and hollow part Single or combined operations
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Forward extrusion of rod
billetpunch
dieworkpiece
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Backward extrusion of can
billet
punch
workpiece
diecounter punch
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Combined : forward andbackward extrusion of can
punch
die
counter punchworkpiece
billet
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Combined: forward extrusion ofrod, backward extrusion of can
die
workpiece
punch
counter punch
billet
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Cold extrusions
Forward extrusion of rod
Back extrusion of can
Forward extrusion of can
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Extrusion pressure and force
Extrusion pressure:
where:kfm mean flow stress logarithmic strain: ln(Ao/A1) coefficient of extrusion (0,40,7)
fmkp =
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Extrusion pressure and force
Punch load (force)F = p A
Work (energy)W = c F h(c=0,60,8)
where: A cross sectional
area h punch travel
Load
Punchtravel
h
Fmax
W
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Materials and lubrication
Good formability: mild steel, copper, aluminium
Medium formability:low alloy steels, Zn
Preparation for cold forging: Surface treatment Lubrication Forging
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Equipment
Machines: Crank presses Knuckle-joint presses Hydraulic presses Special purpose cold forging and heading
machines Requirements:
Sufficient flywheel energy and loadcapacity
Rigid frame
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Presses
crank press knuckle-joint press
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Automated press
Coining andsizing
Coldheading
Preformingof head
Feeding, shearing
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Cold Forging Machines
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Cold forged parts
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Example: multiple-step forging
1 2 3
Forward extrusionof rod, back
extrusion of canand rod Forward
extrusionof hollow
part radialextrusion
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Example: multiple-step forging
Radialextrusion
PiercingBackextrusion
of can
Billet
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Example: production of nut
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Comparison of material yield andenergy consumation
66824050Cutting
46497580Hot forging
4185Warm forging
4185Cold forging
Energy, 106 J/kg
Materialyield, %
Process