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Fundamentals of Metal Forming Chapter 18

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Title: Fundamentals of Metal Forming Chapter 18


1
Fundamentals of Metal FormingChapter 18
  • Manufacturing Processes, MET1311
  • Dr Simin Nasseri
  • Southern Polytechnic State University

2
FUNDAMENTALS OF METAL FORMING
  • Overview of Metal Forming
  • Material Behavior in Metal Forming
  • Temperature 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

5
Material Properties in Metal Forming
  • Desirable material properties
  • Low yield strength
  • High ductility

6
Basic Types of Deformation Processes
  • Bulk deformation
  • Rolling
  • Forging
  • Extrusion
  • Wire and bar drawing
  • Sheet metalworking
  • Bending
  • Deep drawing
  • Cutting
  • Miscellaneous processes

7
Bulk Deformation
8
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

9
Rolling
The slab is heated in a furnace and rolled
between powered rollers until the plate is made
with desirable thickness.
Figure 18.2 Basic bulk deformation processes
(a) rolling
10
Forging
Forging is the process of forming the metal by
impacting and/or squeezing a preheated part
between two halves of a die. A succession of dies
may be needed to achieve the final shape.
Figure 18.2 Basic bulk deformation processes
(b) forging
11
Extrusion
Billets are preheated and forced by a ram through
one or more dies to achieve the desired
cross-section. The product is long in relation to
its cross-sectional dimensions and has a cross
section other than that of rod and bar and pipe
and tube.
Aluminum part
Figure 18.2 Basic bulk deformation processes
(c) extrusion
12
Wire and Bar Drawing
Drawing is an operation in which the
cross-section of solid rod, wire or tubing is
reduced or changed in shape by pulling it through
a die.
Figure 18.2 Basic bulk deformation processes
(d) drawing
13
Sheet Metal Working
14
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

15
Sheet Metal Bending
Straining of a metal sheet or plate to take an
angle.
Figure 18.3 Basic sheet metalworking operations
(a) bending
16
Deep Drawing
Forming of a flat metal sheet into a hollow or
concave shape, by stretching the metal.
Figure 18.3 Basic sheet metalworking operations
(b) drawing
17
Shearing of Sheet Metal
Shearing operation which cuts the work by using a
punch and die.
Figure 18.3 Basic sheet metalworking operations
(c) shearing
18
Test yourself!
  • Name the metal forming process used for each
    object (bulk deformation or sheet metalworking).

sheet metalworking
19
Test yourself!
  • Name the metal forming process used for each
    object.

sheet metalworking
sheet metalworking
20
Test yourself!
  • Name the metal forming process used for each
    object.

Bulk deformation
21
Test yourself!
  • Name the metal forming process used for each
    object.

sheet metalworking
22
Test yourself!
  • Name the metal forming process used for each
    object.

Forging (Bulk deformation)
23
Test yourself!
Extrusion (bulk deformation)
24
Test yourself!
Gutter
25
Materials behavior
26
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

27
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, that is, the yield
strength as a function of strain
28
Average Flow Stress
  • Determined by integrating the flow curve equation
    between zero and the final strain value
  • where average flow stress and
  • ? maximum strain during deformation process

29
Temperature in Metal Forming
30
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

T K n
31
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

32
Temperature in Metal Forming
Cold working refers to plastic deformation that
occurs usually, but not necessarily, at room
temperature. Warm working as the name implies,
is carried out at intermediate temperatures. It
is a compromise between cold and hot working.
Hot working refers to plastic deformation
carried out above the recrystallization
temperature.
33
Temperature in Metal Forming
Increasing temperature
Melting temperature
Room temperature 0.3 Tm 0.5 Tm
Tm Above Tm
Casting
Hot working
Cold working
Warm Working
Recrystallization
34
Cold Working
  • Performed at room temperature or slightly above
  • Cold Forming is the primary manufacturing
    operation of the fastener industry.
  • 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

35
Advantages of Cold Forming
  • Better accuracy, closer tolerances
  • Better surface finish
  • Strain hardening increases strength and hardness
  • Grain flow during deformation can cause desirable
    directional properties in product
  • No heating of work required

Cold forming can make tiny, complex precision
parts in one machining step. Tolerances of 0.0005
in. are possible.
36
Disadvantages of Cold Forming
  • Higher forces and power required in the
    deformation operation
  • Surfaces of starting workpiece must be free of
    scale and dirt (Scalea usually black scaly
    coating of oxide forming on the surface of a
    metal (as iron) when it is heated for processing)
  • 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

Annealing a heat treatment that alters the
microstructure of a material causing changes in
properties such as strength and hardness
37
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

38
Advantages of Warm Working
  • Lower forces and power than in cold working
  • More complex work geometries possible
  • Need for annealing may be reduced or eliminated

39
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 0.5Tm
  • Metal continues to soften as temperature
    increases above 0.5Tm, enhancing advantage of hot
    working above this level

40
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

41
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 strengthening of part occurs from work
    hardening
  • Advantageous in cases when part is to be
    subsequently processed by cold forming

42
Disadvantages of Hot Working
  • Lower dimensional accuracy
  • Higher total energy required (due to the thermal
    energy to heat the workpiece)
  • Work surface oxidation (scale), poorer surface
    finish
  • Shorter tool life

43
Useful site
  • http//campus.umr.edu/mfge/ugumrmfge/HW2/Metal20F
    orming20Processes_files/frame.htm
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