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WEEK-9
O C T 7 T H 2 0 1 4
2 0 1 4 - 2 0 1 5 S E M E S T E R - I
TA201 Manufacturing Processes
F U N D A M E N T A L S O F M E T A L F O R M I N G
Classification Material behavior Strain-rate sensitivity Temperature
B U L K D E F O R M A T I O N P R O C E S S E S
Rolling Forging Extrusion
S H E E T M E T A L W O R K I N G
Cutting Bending Drawing
Dr. Shashank Shekhar TA 201
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Metal Forming [Chap-18, 19 & 20:Groover]
2014-15 Semester-I
Metal Forming Processes
Forging
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TA 201 Dr. Shashank Shekhar
2014-15 Semester-I
Classification of Metal Working Processes
Plastic deformation based processes
[Metal forming]
Metal removal processes
[Metal Machining]
Direct compression
Indirect compression
Bending
Tension type
Shearing
Cold working
Warm working
Hot working
On
th
e b
as
is o
f fo
rce
Dir
ec
tio
n
On
the b
asis
of w
ork
ing
tem
pera
ture
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Dr. Shashank Shekhar
2014-15 Semester-I
5 Classification of deformation based Metal Forming
Processes
(Thickness: 0.4mm to 6mm)
(Thickness> 6mm)
(on
the b
asis
of t
hickne
ss o
f th
e
wor
kpiece
to
be f
ormed
)
TA 201 Dr. Shashank Shekhar
2014-15 Semester-I
Metal Forming Large group of manufacturing processes in which
plastic deformation is used to change the shape of metal work pieces
The tool, usually called a die, applies stresses that exceed yield strength of metal
The metal takes a shape determined by the geometry of the die
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Dr. Shashank Shekhar
2014-15 Semester-I
Bulk Deformation Processes
Characterized by significant deformations and massive shape changes
"Bulk" refers to work parts with relatively low surface area to volume ratios
Starting work shapes include cylindrical billets and rectangular bars
Stresses to plastically deform the metal are usually compressive - Examples: rolling, forging, extrusion
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Dr. Shashank Shekhar
2014-15 Semester-I
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Strain calculation In metalworking, compressive stress and strain are predominant. If a block of initial height ho is compressed to h1, the axial compressive strain will be:
True strain
Engineering strain
Note: the calculated strain is negative for compressive strains. However the convention is reversed in metalworking problems so that compressive stresses and strains are defined as positive.
TA 201 Dr. Shashank Shekhar
h1
2014-15 Semester-I
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And the fractional reduction (reduction of area) in
metal working deformation is given by
Again from constancy in volume before and after deformation
TA 201 Dr. Shashank Shekhar
Strain calculation (rolling)
A0 A1
Compressive stresses
L0 L1
W W
2014-15 Semester-I
Sheet Metalworking
Characterized by localized deformation and configuration changes
Includes processes like cutting, bending and drawing
Stresses required are combination of tensile, compressive and shear
- Stretch the metal (tensile stresses)
- Bend the metal (tensile and compressive)
- Still others apply shear stresses
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Dr. Shashank Shekhar
2014-15 Semester-I
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Direct Compression
Indirect Compression
Tension type Bending Shearing
Bulk deformation
Sheet Metalworking TA 201 Dr. Shashank Shekhar
2014-15 Semester-I
Not always Uniaxial!
TA 201 Dr. Shashank Shekhar
Process State of Stress in Main Part During
Forming
Rolling Bi-axial compression
Forging Tri-axial compression
Extrusion Tri-axial compression
swaging Bi-axial compression
Deep drawing
In flange of blank, bi-axial tension and
compression. In wall of cup, simple uni-
axial tension.
Wire and tube drawing
Bi-axial compression, tension.
Straight bending At bend, bi-axial
compression and bi-axial tension
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2014-15 Semester-I
Plasticity
TA 201 Dr. Shashank Shekhar
Engineering or true?
• Elastic region • Plastic region • Yielding • Necking • Elastic Strain • Plastic strain
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2014-15 Semester-I
True Stress (σt) & Strain (ε)
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Dr. Shashank Shekhar
a
2014-15 Semester-I
Flow Curve
TA 201 Dr. Shashank Shekhar
A true stress-strain curve is frequently called a flow curve, because it gives the stress required to cause the metal to flow plastically to given strain
Loading-Unloading causes ‘Hardening’, which is obvious with increasing flow (yield) stress
Plastic stress-strain is a complex behavior (unlike elastic) and many mathematical equations have been used to fit this behavior
Most common is ‘Power Law’ =Kn
Flow curve is strongly dependent
on strain rate and temperature
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Flow Stress
For most metals at room temperature, strength increases when deformed due to strain hardening
Flow stress = instantaneous value of stress required to continue deforming the material
where Yf = flow stress (true
stress), that is, the yield
strength as a function of
strain
n
f KY
Dr. Shashank Shekhar
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Average Flow Stress
Determined by integrating the flow curve equation between zero and the final strain value defining the range of interest
Where = maximum strain during deformation process
n
KY
n
f
1
_
True strain
0
1)( dYmeanY ff
Given that 2 = and 1 (initial) = 0
Dr. Shashank Shekhar
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nKn
1
1
2014-15 Semester-I
Plasticity
TA 201 Dr. Shashank Shekhar
In elastic regime, Hooke’s law relates stress and strain, where any stress results in strain
In plastic regime, a minimum stress (yield stress) must be reached before deformation can be attained
The relation between plastic stress-strain is mere approximate, compared to elastic-stress-strain relations which are more “exact”
Plastic stress-strain is a complex behavior (unlike elastic) and many mathematical equations have been used to fit this behavior. Most common is ‘Power Law’ =Kn
Flow stress, the stress required to keep the material deforming, is usually given by and average flow stress is given by
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n
f KY
n
KY
n
f
1
_
2014-15 Semester-I TA 201 Dr. Shashank Shekhar
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Flow curve is strongly dependent on
• strain rate
• temperature
2014-15 Semester-I
Effect of Strain Rate on Flow Stress
A flow stress simply gives the stress required to impose certain strain and should be valid at whatever rate you reach that strain. However this is not correct
Theoretically, a metal in hot working, behaves like a perfectly plastic material, with strain hardening exponent n = 0 and the metal should continue to flow at the same flow stress, once that stress is reached. However this is also not the case
Flow stress also depends on strain rate (particularly at hot working temperatures) - As strain rate increases, resistance to deformation increases - This effect is known as strain rate sensitivity
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Dr. Shashank Shekhar
2014-15 Semester-I
What is Strain Rate?
Dr. Shashank Shekhar
Rate of straining is directly related to speed of deformation v
Deformation speed v = velocity of the ram or other movement of the equipment
Strain rate is defined:
Strain-rate is complicated by geometry of workpart
Values can range from 10-3 to 100 s-1
h
v
.
where = true strain rate; and h = instantaneous height of workpiece being
deformed
.
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2014-15 Semester-I
(a) Effect of strain rate on flow stress at an elevated work temperature. (b) Same relationship plotted on log-log coordinates
Log( )
Log(
)
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Strain Rate Sensitivity Equation
where C = strength constant (similar but not equal to strength coefficient in flow curve equation), and m = strain-rate sensitivity exponent
m
f CY
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TA 201
2014-15 Semester-I
Effect of temperature on flow stress for a typical metal. The constant C in the above Eq. indicated by the intersection of each plot with the vertical dashed line at strain rate = 1.0, decreases, and m (slope of each plot) increases with increasing temperature
m
f CY
TA 201 Dr. Shashank Shekhar
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2014-15 Semester-I
Observations about Strain Rate Sensitivity
Increasing temperature decreases C, increases m - At room temperature, effect of strain rate is almost negligible
Flow curve is a good representation of material behavior
- As temperature increases, strain rate becomes increasingly important in determining flow stress
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Dr. Shashank Shekhar
2014-15 Semester-I
Temperature in Metal Forming
For any metal, K and n in the flow curve depend on temperature Both strength and strain hardening are reduced at higher
temperatures
In addition, ductility is increased at higher temperatures
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Dr. Shashank Shekhar
2014-15 Semester-I
Temperature in Metal Forming
Any deformation operation can be accomplished with lower forces and power at elevated temperature
Three temperature ranges in metal forming: - Cold working (<0.3Tm)
- Warm working (0.3Tm to 0.5Tm)
- Hot working (>0.5Tm)
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Dr. Shashank Shekhar
2014-15 Semester-I
Influence of Annealing Temperature on the Tensile Strength and Ductility
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Dr. Shashank Shekhar
2014-15 Semester-I
Cold Working
Performed at room temperature or slightly below (≤0.3Tm )
Many cold forming processes are important mass production operations
Minimum or no machining usually required - These operations are near net shape or net shape processes
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Dr. Shashank Shekhar
2014-15 Semester-I
Hot Working
Deformation at temperatures above recrystallization temperature
Recrystallization temperature = about one-half of melting point on absolute scale 1. In practice, hot working usually performed somewhat above
0.5Tm
2. Metal continues to soften as temperature increases above 0.5Tm, enhancing advantage of hot working above this level
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Dr. Shashank Shekhar
2014-15 Semester-I
Why Hot Working?
Capability for substantial plastic deformation of the metal - far more than possible with cold working or warm working
Why? - Strength coefficient is substantially less than at room
temperature
- Strain hardening exponent is zero (theoretically)
- Ductility is significantly increased
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Dr. Shashank Shekhar
2014-15 Semester-I
Warm Working
Performed at temperatures above room temperature but below recrystallization temperature
Dividing line between cold working and warm working often expressed in terms of melting point: 0.3Tm – 0.5 Tm, where Tm = melting point (absolute
temperature) for metal
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Dr. Shashank Shekhar