20-dss.ppt

Principles of
Major
Manufacturing
Processes
1

FUNDAMENTALS OF METAL FORMING
1. Material Behavior in Metal Forming
2. Overview of Metal Forming
3. Temperature in Metal Forming
4. Strain Rate Sensitivity
5. Friction and Lubrication in Metal Forming
2

Metal Forming
Large group of manufacturing processes in which
plastic deformation is used to change the shape of
metal workpieces
 The tool, usually called a die, applies stresses that
exceed the yield strength of the metal
 The metal takes a shape determined by the geometry
of the die
3

Stresses in Metal Forming
 Stresses to plastically deform the metal are usually
compressive
 Examples: rolling, forging, extrusion
 However, some forming processes
 Stretch the metal (tensile stresses)
 Others bend the metal (tensile and compressive)
 Still others apply shear stresses (shear spinning)
4

Material Properties in Metal Forming
 Desirable material properties:
 Low yield strength
 High ductility
 These properties are affected by temperature:
 Ductility increases and yield strength decreases
when work temperature is raised
 Other factors:
 Strain rate and friction
5

Basic Types of Deformation Processes
1. Bulk deformation
 Rolling
 Forging
 Extrusion
 Wire and bar drawing
2. Sheet metalworking
 Bending
 Deep drawing
 Cutting
6
(stock has high V/A)
(stock has low V/A)

Bulk Deformation Processes
 Characterized by significant deformations and
massive shape changes
 "Bulk" refers to workparts with relatively low
surface area-to-volume ratios
 Starting work shapes include cylindrical billets and
rectangular bars
7

Basic bulk deformation processes: rolling
Rolling
8

Basic bulk deformation processes: forging
Forging
9

Basic bulk deformation processes: (c) extrusion
Extrusion
10

Basic bulk deformation processes: (d) drawing
Wire and Bar Drawing
11

Sheet Metalworking
 Forming and related operations performed on metal
sheets, strips, and coils
 High surface area-to-volume ratio of starting metal,
which distinguishes these from bulk deformation
 Often called pressworking because presses perform
these operations
 Parts are called stampings
 Usual tooling: punch and die
12

Basic sheet metalworking operations: bending
Sheet Metal Bending
13

Basic sheet metalworking operations: drawing
Deep Drawing
14

Basic sheet metalworking operations: shearing
Shearing of Sheet Metal
15

Material Behavior in Metal Forming
 Plastic region of stress-strain curve is primary
interest because material is plastically deformed
 In plastic region, metal's behavior is expressed by
the flow curve:
16
where K = strength coefficient;
and n = strain hardening exponent
 Flow curve based on true stress
and true strain
n
f
Y K


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
17
where Yf = flow stress, i.e., the yield strength as
a function of strain
n
f
Y K


Average Flow Stress
 Determined by integrating the flow curve equation
between zero and the final strain value defining the
range of interest
where = average flow stress; and  = maximum
strain during deformation process. n = strain
hardening exponent
18
_
1
n
f
K
Y
n



_
f
Y

Temperature in Metal Forming
 For any metal, K and n in the flow curve depend on
temperature
 Both strength (K) and strain hardening (n) are
reduced at higher temperatures
 In addition, ductility is increased at higher
temperatures
19

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
 Warm working
 Hot working
20

1. Cold Working
 Performed at room temperature or slightly above
 Many cold forming processes are important mass
production operations
 Minimum or no machining usually required
21

Advantages of Cold Forming
 Better accuracy, closer tolerances
 Better surface finish
 Strain hardening increases strength and hardness
 No heating of work required
22

Disadvantages of Cold Forming
 Higher forces and power required in the
deformation operation
 Ductility and strain hardening limit the amount of
forming that can be done
 In some cases, metal must be annealed to allow
further deformation
 In other cases, metal is simply not ductile
enough to be cold worked
23

24
Impact of Cold Work
Adapted from Fig. 8.20,
Callister & Rethwisch 4e.
• Yield strength (sy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
As cold work is increased
low carbon steel

• What are the values of yield strength, tensile strength &
ductility after cold working Cu?
100
x
4
4
4
%CW
2
2
2
o
d
o
D
D
D
p
p
-
p
=
Mechanical Property Alterations
Due to Cold Working
Do = 15.2 mm
Cold
Work
Dd = 12.2 mm
Copper
%
6
.
35
100
x
mm)
2
.
15
(
mm)
2
.
12
(
mm)
2
.
15
(
CW
%
2
2
2
=
-
=
100
x
2
2
2
o
d
o
D
D
D -
=
25

Mechanical Property Alterations
Due to Cold Working
% Cold Work
100
300
500
700
Cu
20
0 40 60
sy = 300 MPa
300 MPa
% Cold Work
200
Cu
0
400
600
800
20 40 60
% Cold Work
20
40
60
20 40 60
0
0
Cu
340 MPa
TS = 340 MPa
7%
%EL = 7%
• What are the values of yield strength, tensile strength &
ductility for Cu for %CW = 35.6%?
yield
strength
(MPa)
tensile
strength
(MPa)
ductility
(%EL)
26
Adapted from Fig. 8.19, Callister & Rethwisch 4e. (Fig. 8.19 is adapted from Metals Handbook: Properties
and Selection: Iron and Steels, Vol. 1, 9th ed., B. Bardes (Ed.), American Society for Metals, 1978, p. 226;
and Metals Handbook: Properties and Selection: Nonferrous Alloys and Pure Metals, Vol. 2, 9th ed., H.
Baker (Managing Ed.), American Society for Metals, 1979, p. 276 and 327.)

27
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are nullified!
Adapted from Fig. 8.22, Callister & Rethwisch
4e. (Fig. 8.22 is adapted from G. Sachs and
K.R. van Horn, Practical Metallurgy, Applied
Metallurgy, and the Industrial Processing of
Ferrous and Nonferrous Metals and Alloys,
American Society for Metals, 1940, p. 139.)
Effect of Heat Treating After Cold Working
tensile
strength
(MPa)
ductility
(%EL)
tensile strength
ductility
600
300
400
500
60
50
40
30
20
annealing temperature (ºC)
200
100 300 400 500 600 700 • Three Annealing stages:
1. Recovery
2. Recrystallization
3. Grain Growth

28
Three Stages During Heat Treatment:
1. Recovery
•During recovery, some of the stored internal strain energy
is relieved. In addition, physical properties such as
electrical and thermal conductivities are recovered to their
precold-worked states.

29
Adapted from
Fig. 8.21 (a),(b),
Callister &
Rethwisch 4e.
(Fig. 8.21 (a),(b)
are courtesy of
J.E. Burke,
General Electric
Company.)
33% cold
worked
brass
New crystals
nucleate after
3 sec. at 580C.
0.6 mm 0.6 mm
2. Recrystallization
• New grains are formed that:
-- have low dislocation densities
-- are small in size
-- consume and replace parent cold-worked grains.

30
• All cold-worked grains are eventually consumed/replaced.
Adapted from
Fig. 8.21 (c),(d),
Callister &
Rethwisch 4e.
(Fig. 8.21 (c),(d)
are courtesy of
J.E. Burke,
General Electric
Company.)
After 4
seconds
After 8
seconds
0.6 mm
0.6 mm
As Recrystallization Continues…

31
• Can be induced by rolling a polycrystalline metal
- before rolling
235 mm
- after rolling
- anisotropic
since rolling affects grain
orientation and shape.
rolling direction
(Fig. 8.11 is from W.G. Moffatt,
G.W. Pearsall, and J. Wulff,
The Structure and Properties
of Materials, Vol. I, Structure,
p. 140, John Wiley and Sons,
New York, 1964.)
Anisotropy in sy
- isotropic
since grains are
equiaxed &
randomly oriented.

32
Adapted from
Fig. 8.21 (d),(e),
Callister &
Rethwisch 4e.
(Fig. 8.21 (d),(e)
are courtesy of
J.E. Burke,
General Electric
Company.)
3. Grain Growth
• At longer times, average grain size increases.
After 8 s,
580ºC
After 15 min,
580ºC
0.6 mm 0.6 mm
• Empirical Relation:
Kt
d
d n
o
n
=
-
elapsed time
coefficient dependent
on material and T.
grain diam.
at time t.
exponent typ. ~ 2
-- Small grains shrink (and ultimately disappear)
-- Large grains continue to grow

33
TR
TR = recrystallization
temperature
º

34
Recrystallization Temperature
TR = recrystallization temperature = temperature
at which recrystallization just reaches
completion in 1 h.
0.3Tm < TR < 0.6Tm
For a specific metal/alloy, TR depends on:
• %CW -- TR decreases with increasing %CW
• Purity of metal -- TR decreases with
increasing purity

2. 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, where Tm = melting point (absolute
temperature) for metal
35

Advantages of Warm Working
 Lower forces and power than in cold working
 More intricate work geometries possible
 Need for annealing may be reduced or eliminated
 Low spring back
Disadvantage:
1. Scaling of part surface
36

3. Hot Working
 Deformation at temperatures above the
recrystallization temperature
 Recrystallization temperature = about one-half of
melting point on absolute scale
 In practice, hot working usually performed
somewhat above 0.6Tm
 Metal continues to soften as temperature
increases above 0.6Tm, enhancing advantage of
hot working above this level
37

Why Hot Working?
Capability for substantial plastic deformation of the
metal - far more than possible with cold working or
warm working
 Why?
 Strength coefficient (K) is substantially less than
at room temperature
 Strain hardening exponent (n) is zero
(theoretically)
 Ductility is significantly increased
38

Advantages of Hot Working
 Workpart shape can be significantly altered
 Lower forces and power required
 Metals that usually fracture in cold working can be
hot formed
 Strength properties of product are generally
isotropic
 No work hardening occurs during forming
39

Disadvantages of Hot Working
 Lower dimensional accuracy in case of bulk
forming
 Higher total energy required (due to the thermal
energy to heat the workpiece)
 Work surface oxidation (scale), poorer surface
finish
 Shorter tool life
40

20-dss.ppt

More Related Content

Similar to 20-dss.ppt (20)

Recently uploaded (20)

20-dss.ppt