Download - Ch 4 Metal Forming
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Chapter 4: Metal Forming
Learning objectives
Understand the basic metal forming processes, including
forging and sheet metal stamping
Understand how the metal forming process changes the
shape and the material properties of the metal
Recognize different metal forming machines (presses),
including mechanical press and hydraulic press
Recognize the dies for sheet metal forming, their design
and constructions
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The aspects of metal forming:
The material (metal)
The machine
The tool (dies)
The operation (process)
Process
Material
Press
Die
Product
Heat and Beat
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A list of different metal forming processes:
Metal Forming
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Types of metal forming processes
Rolling (Chapter 13)
Bulk deforming (Chapter 14)
Extrusion and Drawing (Chapter 15)
Sheet metal forming (Chapter 16)
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Rolling
Different types of rolling process
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Rolling
Rolling machines
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Rolling
Rolling process
The forces act on the workpiece
The force and torque act on the roll
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Rolling
The rolling force
The total power (for two rolls)
avgLwYF L = roll-strip contact length
w = width of the strip
Yavg = average true stress of the strip
000,60
2kW)(in Power
FLN N = rolling speed (RPM)
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Rolling
An example: An annealed copper strip 228 mm wide and
25 mm thick is rolled to a thickness of 20 mm in one pass.
The roll radius is 300 mm, and the rolls rotate at 100 rpm.
Calculate the roll force and the power required in this
operation
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Rolling
Solution
Roll-strip contact length is calculated through geometry,
Absolute true strain of the strip is
Average true stress is
The roll force is
Total power is
223.020
25ln
mm 7.3820253000 fo hhRL
MN 4.171801000250
10007.38 avgLwYF
MPa 1802/28080
W705000,66
100
1000
7.381074.12
000,66
2 6 FLN
Power
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Rolling
Dimensional Tolerances
Thickness tolerances for cold-rolled sheets range from
0.1~0.35 mm
Flatness tolerances are within 15 mm/m for cold rolling
and 55 mm/m for hot rolling
Surface Roughness
Cold rolling can produce a very fine surface finish
Cold-rolled sheets products may not require additional
finishing operations
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Rolling
Straight and long structural shapes can be formed by shape
rolling
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Rolling
Roll forging: Cross section of a round bar is shaped by
passing it through a pair of rolls with profiled grooves
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Rolling
Skew rolling: Similar to roll forging and used for making
ball bearings
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Rolling
Ring rolling:
A thick ring is expanded into a large-diameter thinner one
Thickness is reduced by bringing the rolls closer together as they
rotate
Short production times, material savings and close dimensional
tolerances
Video: https://www.youtube.com/watch?v=wSbywBfXlHg
https://www.youtube.com/watch?v=wSbywBfXlHg
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Rolling
Thread rolling:
Thread rolling is a cold-forming process by which straight or
tapered threads are formed on round rods or wire
Threads are formed with rotary dies at high production rates
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Rolling
Rotary tube piercing:
Also known as the Mannesmann process
It is a hot-working operation for making long, thick-walled
seamless pipe and tubing
The round bar is subjected to radial compressive forces while
tensile stresses develop at the center of the bar
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Rolling
Tube rolling:
Diameter and thickness of pipes and tubing can be reduced by
tube rolling, which utilizes shaped rolls
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Rolling
Computer simulation of rolling:
https://www.youtube.com/watch?v=k6iODHla6qY
Concluding remarks
Rolling is highly efficient
Rolling is reasonably accurate
Rolling can generate good surface
Rolling results in residual stress
https://www.youtube.com/watch?v=k6iODHla6qY
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Forging
workpiece is shaped by compressive forces applied
through dies and tools
produce discrete parts
Forged parts have good strength and toughness, and are
reliable for highly stressed and critical applications
Types of forging
Open die forging and closed die forging
Hot forging and cold forging
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Characteristics of forging
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Forging
The machine
The process: https://www.youtube.com/watch?v=XTU0Z-
FkhtU
https://www.youtube.com/watch?v=XTU0Z-FkhtU
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Open-die forging
Workpiece is deformed uniformly under frictionless
conditions
Barreling occurs because of the friction force. It can be
reduced by adding lubrications
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Open-die forging
Different types of open-die forging
In open die forging, the stress varies continuously
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Open-die forging (continue)
The directional flow of the material in forging
Folding
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Open-die forging
The forging force, F, in an open-die forging operation on
a solid cylindrical workpiece can be estimated from
h
rrYF f
3
212
Yf = flow stress of the material
= coefficient of friction between the workpiece and die
r = the instantaneous radius
h = height of the workpiece
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Open-die forging
An example: A solid cylindrical slug made of 304
stainless steel is 150 mm in diameter and 100 mm high.
It is reduced in height by 50% at room temperature by
open-die forging with flat dies. Assuming that the
coefficient of friction is 0.2, calculate the forging force
at the end of the stroke.
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Open-die forging
Solution
The final radius is
Absolute value of the true strain is
From Table 2.3, 304 stainless steel has K = 1275 MPa and n =
0.45. Thus for a true strain of 0.69, the flow stress is 1100 MPa.
The forging force is
mm 106100752
10022 rr
69.050
100ln
MN 4505.03
106.02.021106.0101000
26 F
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Analysis of open die forging
This is the simplest case in metal forming. Consider the
deformation of a square workpiece:
h0 h1d0 d1
0
101
h
hhe
1
01 ln
h
h
F, v
0
1h
ve
1
1h
v
h0d0
F, v
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The reaction force
The required work for deformation
h0
h1
d0
d1
1YAF
1
001
h
hAA
F, v
10
0
Volume
Volume
VolumeWork
1
1
Y
dK
d
n
1
1
1
0
1
n
KdK
Yn
n
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Note:
We are primary interested in the plastic deformation and hence,
always use true strain and stress
How to compute the average yield stress and average work
= F/A
5
0
0
555, lnln
A
AY
l
lYYWs
A
A
l
l 0
0
lnln
Average yield stress
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In order to study the stress distribution, in general, we
need to understand the 3D strain and stress
In general, strain and stress are three dimensional
y
tyzx
txyz
tzx
z
zyy
zxyxx
ij
t
tt
z
zyy
zxyxx
ij
x
v
y
uyx
Shear stressShear strain
h0d0
F, v
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In open die forging, it can be simplified as a 2D problem. In the
horizontal direction:
Using the distortion-energy criterion:
Therefore,
02 hdxhd xzxx
xz dd
dxh
d
x
x
2
x
dx
x x + dx
zz
z
za
x(a) = 0
z(a) = Y
Square
specimenh
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Solving the differential equation results in
Similarly, we can find the solution for y The stress distribution is shown below
hxaz eY
'
1' 2 hxax eY
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Open die forging (continue)
The facture
Under excessive force, the barreling will become fracturing
Improved lubrication can reduce fracture
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Polycrystalline deformation
Crystal elongation
Cold forming and hot forming
Cold forming is used for near shape manufacturing
The micro-process of cold forming
Reorientation (deformation ratio = 3)
Single crystal slip
Polycrystalline deformation
Crystal elongation (deformation ratio = 8 ~ 10)
Crystallographic fibrous structure
Slip line
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Cold forming and hot forming
Hot forging is used to form large bulk steels
The temperature effect is significant
Finding the right temperature range using phase diagram
Overheat, steel may
burn or oxidize
Under-heat, much
large force is required
Right temperatureoC
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The forming process affects the grain size
Finer grain have higher yield strength and toughness and lower
internal strains and stresses
The process of forging and grain size
Grain size increase
because of heating
Grain size after one
forging
Grain size after many
forgings
Heating
punching
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Closed-die forging
Application example
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Closed-die forging (continue)
Application example
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Closed die forging
making an aircraft landing gear
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Closed-die forging
The workpiece resemble the shape of the die
Anatomy of a die set: gutter and flash, parting line
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Closed-die forging
Forging Force
The forging force, F, required to carry out an impression-die
forging operation is
AkYF f
k = multiplying factor obtained
Yf = flow stress of the material at the forging temperature
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Extrusion and drawing
fA
AkAF 00 ln