METAL
FORMING
1
2
FUNDAMENTALS OF METAL FORMING
1.
2.
3.
4.
5.
Material Behavior in Metal Forming
Overview of Metal Forming
Temperature in Metal Forming
Strain Rate Sensitivity
Friction and Lubrication in Metal Forming
3
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
4
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)
5
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
6
Basic Types of Deformation Processes
1.
2.
(stock has high V/A)
Bulk deformation
▪ Rolling
▪ Forging
▪ Extrusion
▪ Wire and bar drawing
(stock has low V/A)
Sheet metalworking
▪ Bending
▪ Deep drawing
▪ Cutting
7
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
8
Rolling
Basic bulk deformation processes: rolling
9
Forging
Basic bulk deformation processes: forging
10
Extrusion
Basic bulk deformation processes: (c) extrusion
11
Wire and Bar Drawing
Basic bulk deformation processes: (d) drawing
12
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
13
Sheet Metal Bending
Basic sheet metalworking operations: bending
14
Deep Drawing
Basic sheet metalworking operations: drawing
15
Shearing of Sheet Metal
Basic sheet metalworking operations: shearing
16
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:
where K = strength coefficient;
and n = strain hardening exponent
▪ Flow curve based on true stress
and true strain
17
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, i.e., the yield strength as
a function of strain
18
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 ε = maximum
strain during deformation process. n = strain
hardening exponent
19
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
20
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
21
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
22
Advantages of Cold Forming
▪
▪
▪
▪
Better accuracy, closer tolerances
Better surface finish
Strain hardening increases strength and hardness
No heating of work required
23
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
Impact
of
Cold
Work
As cold work is increased
• Yield strength (σy) increases.
• Tensile strength (TS) increases.
• Ductility (%EL or %AR) decreases.
Adapted from Fig. 8.20,
Callister & Rethwisch 4e.
low carbon steel
24
Mechanical Property Alterations
Due to Cold Working
• What are the values of yield strength, tensile strength &
ductility after cold working Cu?
Copper
Cold
Work
Do = 15.2 mm
Dd = 12.2 mm
25
Mechanical Property Alterations
Due to Cold Working
500
300
300
MPa
100
0
2
4
% 0Cold 0
Work
C
u
6
0
σy = 300 MPa
800
600
400 340
MPa
200
0
2
4
% Cold
0
0
Work
C
u6
0
TS = 340 MPa
ductility (%EL)
700
tensile strength (MPa)
yield strength (MPa)
• What are the values of yield strength, tensile strength &
ductility for Cu for %CW = 35.6%?
6
0
4
0
2
0
7
%00
C
u
2
4
%0 Cold0
Work
6
0
%EL = 7%
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.)
26
Effect of Heat Treating After Cold Working
• 1 hour treatment at Tanneal...
decreases TS and increases %EL.
• Effects of cold work are nullified!
100 200 300 400 500 600 700
600
6
0
tensile strength
5
0
500
4
0
400
3
0
ductility
2
0
300
Re
co
ry
cry
• Three Annealing stages:
1. Recovery
2. Recrystallization
3. Grain Growth
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.)
Gr
Re
ve
ductility (%EL)
tensile strength (MPa)
annealing temperature (ºC)
sta
lliz
ain
ati
o
n
Gr
ow
th
27
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.
28
Three Stages During Heat Treatment:
2. Recrystallization
• New grains are formed that:
-- have low dislocation densities
-- are small in size
-- consume and replace parent cold-worked grains.
0.6 mm
0.6 mm
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.
29
As Recrystallization Continues…
• All cold-worked grains are eventually consumed/replaced.
0.6 mm
0.6 mm
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
30
Anisotropy in σy
• Can be induced by rolling a polycrystalline metal
- before rolling
- after rolling
Adapted from Fig. 8.11,
Callister & Rethwisch 4e.
(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.)
rolling direction
235 μm
- isotropic
since grains are
equiaxed &
randomly oriented.
- anisotropic
since rolling affects grain
orientation and shape.
31
Three Stages During Heat Treatment:
3. Grain Growth
• At longer times, average grain size increases.
-- Small grains shrink (and ultimately disappear)
-- Large grains continue to grow
0.6 mm
After 8 s,
580ºC
0.6 mm
After 15 min,
580ºC
• Empirical Relation:
exponent typ. ~ 2
grain diam.
at time t.
Adapted from
Fig. 8.21 (d),(e),
Callister &
Rethwisch 4e.
(Fig. 8.21 (d),(e)
are courtesy of
J.E. Burke,
General Electric
Company.)
coefficient dependent
on material and T.
elapsed time
32
TR = recrystallization
temperature
TR
Adapted from Fig. 8.22,
Callister & Rethwisch 4e.
º
33
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
34
35
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
36
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
37
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
38
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
39
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
40
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