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Mechanical and Other Methods of Change of Form

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Title: Mechanical and Other Methods of Change of Form


1
Mechanical and Other Methods of Change of Form
  • Chapter 11

2
  • Competencies
  • Define Forging
  • Describe the fundamental characteristics of
    extrusion
  • Describe the process of Coining and Heading
  • Describe the reasons for using lubrication in
    forging
  • Describe the fundamental characteristics of
    rolling
  • List the common material change of form
    mechanical methods

3
Overview of Metal Forming
  • Can be classified as
  • Bulk deformation processes generally
    characterized by significant deformations and
    massive shape changes and the surface area-to-
    volume of to work is relatively small.
  • Forging
  • Extrusion
  • Rolling
  • Wire and bar drawing
  • Sheet metalworking process
  • Bending operations
  • Deep or cup drawing
  • Shearing processes
  • Miscellaneous

4
Forging
  • Forging - plastic deformation by compressive
    forces
  • Hand Forging exactly what the blacksmiths did.
  • Drop Forging a drop forge raises a massive
    weight and lets it fall.
  • The two basic types of forging machines are
    presses and hammers.
  • Presses exert enormous forces, which are applied
    slowly enough that the metal has time to flow.
  • The hammer machines are designed to raise a
    massive weight and let it drop.
  • Power hammers add to gravity with pneumatic or
    hydraulic assistance.
  • Counterblow hammers use two opposed hammers

5
Forging
  • Open Forging - Presses the billet between two
    flat plates to reduce its thickness.
  • Cogging is a forging process that reduces the
    thickness of a single BILLET by small increments.
  • Closed forging - The billet is forced into the
    cavities of one or more dies.
  • Flashing is the excess material squeezed out from
    a BILLET in a CLOSED FORGING or stamping process.

6
Forging
  • Coining - the process used to form faces on coin
    blanks. It is a very intricate process.
  • Heading - is the process of upsetting metal to
    form heads on nails or screws.
  • Swaging is the forging process by which a hollow
    cylindrical part is forced tightly around a rod
    or wire to permanently attach the two parts. It
    is also known as RADIAL FORGING.

7
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8
Forging
  • Lubricants for Forging
  • improve the flow of the material into the dies
  • to reduce die wear
  • to control the cooling rate
  • to serve as a parting agent

9
Forging
  • Pressures Involved in Forging
  • The force needed to forge a part depends on
  • the compressive strength of the metal
  • the area including flashings of the metal being
    forged
  • the temperature at which the forging is being
    done
  • the amount of deformation each compressive stroke
    of the ram or hammer performs.

10
Extrusion
  • Extrusion is the process of forcing a material
    through a DIE to produce a very long WORKPIECE of
    constant shape and cross section. Extrusion can
    be done cold (at room temperature) or hot so
    that the material is softened slightly.

11
Extrusion
  • Direct or forward - The product moves though a
    die
  • Indirect (reverse or backward) - product
    stationary, die moves
  • Hydrostatic Extrusion In hydrostatic extrusion
    a fluid is placed between the ram and the metal
    being extruded. This produces two advantages
  • (1) The fluid presses radially inward on the
    billet, which helps guide it into the opening in
    the die
  • (2) the fluid lubricates the walls of the
    cylinder, which reduces the friction forces in
    the extrusion process.
  • Hollow Extrusion Hollow pieces such as pipes
    and tubing can be made by extrusion if some
    obstacle is part of the die design.

12
Rolling
  • A compressive deformation process in which the
    thickness of a slab or plate is reduced by two
    opposing cylindrical tools called rolls.
  • The rolls rotate so as to draw the work into the
    gap between them and squeeze it. Rollers are
    pressed together with enough force so that
    whatever passes between them must take the shape
    of the space between the rollers.

13
Rolling
  • Bend rods or sheets into curved surfaces
  • Change the grain structure of cast bars or sheets
  • Form billets into structural shapes such as
    flanges, channels, or railroad rails
  • Produce tapers or threads on rods
  • Straighten bent sheets, rods, or tubing

14
Bending by Rolling
  • Crimped by rolling.
  • Tube forming by rolling
  • Threaded parts by rolling - faster than machining
    the threads and leaves a harder grain structure.
  • Forming ball bearings
  • Straightening flat stock

15
Rolling Shapes
  • Plate is defined as stock that is thicker than
    0.25 inch (6 millimeters)
  • Sheet runs from 0.25 inch down to about 0.0003
    inch (0.008 millimeter)
  • Foil is considered to be less than 0.0003 inch
    thick.
  • Large flange beams (I-beams), channels, and even
    wire are made by rolling.

16
Hot Versus Cold Rolling
  • Hot rolling Billets heated to the red hot range
    rapidly form an oxide coating or scale.
  • Cold rolling - Softer materials such as aluminum
    and copper are cold rolled.
  • rolling material at room temperature provides
    better surface finish and closer tolerances
  • characterized by fine grain size. The finer the
    grain, the harder and less malleable the metal
    becomes.

17
Factors Affecting Rolling
  • The material being rolled
  • The material of the rollers
  • The shape being rolled
  • The size of the stock being rolled
  • The size of the rollers
  • Power requirements

18
Drawing
  • The pulling of a bar through a Die to reduce the
    cross section.
  • Used to make wire
  • Seamless Tubing

19
Sheet metalworking Processes
  • Bending
  • Brake general use device for bending sheet
    metal.
  • Punch and Dies shaping material by punching it
    into a die. Punch is the moving form, Die is the
    stationary form.
  • Press brake - an extension of the punch-and-die
    set extended along one dimension to make complex
    bends in a long piece of sheet stock.

20
Sheet Metalworking Processes
  • Drawing - in sheet metal working, drawing refers
    to the forming of a flat metal sheet into a
    hollow or concave shape, such as a cup, by
    stretching the metal.
  • Spin forming - A forming process in which a sheet
    of metal is held to a mandrel, rotated, and
    forced onto the mandrel to shape the sheet.
  • Miscellaneous stretch forming, roll bending,
    spinning, and bending of tube stock

21
Spin forming
22
Material Properties
  • Tensile
  • Compression
  • Shear

23
Tensile
  • The stress-strain relationship has two regions,
    indicating two distinct forms of behavior
    elastic and plastic.
  • In the elastic region, the relationship between
    stress and strain is linear, and the material
    exhibits elastic behavior by returning to its
    original length when the load is released. This
    relationship is defined by Hookes Law
  • se E ?
  • where E modulus of elasticity (psi) which is
    the inherent stiffness of a material e
    engineering strain

24
Tensile Stress Strain Curve
  • As stress increases, some point in the linear
    relationship is finally reached at which the
    material begins to yield (yield point Y) Often
    referred to as the yield strength, yield stress
    and elastic limit.
  • Beyond this point, Hookes Law does not apply.
    As the elongation increases at a much faster
    rate, this causes the slope of the curve to
    change dramatically.
  • Finally, the applied load F reaches maximum
    value, and the engineering stress calculated at
    this point is called the tensile strength or
    ultimate tensile strength of the material.

25
Tensile Stress Strain Curve
  • The amount of strain that the material can endure
    before failure is also a mechanical property of
    interest in many manufacturing processes. The
    common measure of this property if ductility, the
    ability of a material to plastically strain
    without fracture.

26
Tensile Stress Strain Curve
  • This measure can be taken as either elongation or
    area reduction
  • Elongation often expressed as a percent.
  • where Lf specimen length after fracture and Lo
    original specimen length

27
Tensile Stress Strain Curve
  • Area reduction often expressed as a percent
  • where Ao original area and Af area of the
    cross-section at the point of fracture

28
True Stress-Strain
  • There is a small problem with using the original
    area of the material the calculate engineering
    stress, rather than the actual (instantaneous)
    area that becomes increasing smaller as the test
    proceeds.

29
True Stress-Strain
  • If the actual area were used, the calculated
    stress value would be higher. The stress value
    obtained by dividing the instantaneous value of
    area into the applied load is defined as the true
    stress
  • Where F force (lb) and A actual
    (instantaneous) area resisting the load

30
True Stress-Strain
  • Similarly, true strain provides a more realistic
    assessment of the instantaneous elongation per
    unit length of the material.

31
True Stress-Strain
  • The value of true stain in a tensile test can be
    estimated by dividing the total elongation into
    small increments, calculating the engineering
    strain for each increment on the basis of its
    starting length, and then adding up the strain
    values, in the limit, true strain is defined as
  • Where L instantaneous length at any moment
    during elongation

32
True Stress-Strain
  • At this point if the engineering stress-strain
    curve is replotted using the true stress-strain,
    then we would see very little difference in the
    elastic region.
  • The difference occurs at the point in which the
    stress-strain exceeds the yield point and enters
    the plastic region.
  • The true stress-strain values are high due to a
    smaller cross sectional area being used, which is
    continuously reduced during elongation.
  • As in the engineering stress-strain curve,
    necking occurs and therefore a downturn leading
    to fracture.

33
True Stress-Strain
  • Unlike engineering stress-strain, true stress
    values indicate that the material is actually
    becoming stronger as strain increases.
  • This property is called strain hardening. Stain
    hardening (work hardening) is an important factor
    in certain manufacturing processes, particularly
    metal forming.

34
True Stress-Strain
  • By replotting the plastic region of the true
    stress curve on a Log/Log scale, the result is a
    linear relationship expressed as
  • Known as the flow curve which captures a good
    approximation of the behavior of metals in the
    plastic region, including their capacity for
    strain hardening
  • Where K strength coefficient (psi) it equals
    the value of true stress at a true strain value
    equal to one.
  • n strain hardening exponent, and is the slope
    of the line. Its value is directly related to a
    metals tendency to work harden

35
True Stress-Strain
  • Empirical evident reveals that necking begins for
    a particular metal when the true strain reaches a
    value equal to the strain hardening exponent.
  • Therefore, a higher n value means that the metal
    can be strained further before the onset of
    necking

36
Types of Stress-Strain relationships
  • Perfectly elastic
  • the behavior of this material is defined
    completely by its stiffness, indicated by the
    modulus of elasticity E. It fractures rather
    than yielding to plastic flow.
  • Brittle material such as ceramics, many cast
    irons, and thermosetting polymers possess
    stress-strain curves that fall into this
    category.
  • These material are not good candidates for
    forming operations.

37
Types of Stress-Strain relationships
  • Elastic and perfectly plastic
  • This material has a stiffness defined by E. Once
    the yield strength Y is reached, the material
    deforms plastically at the same stress level.
  • The flow curve is given by K Y and n 0.
    Metals behave in this fashion when they have been
    heated to sufficiently high temperatures that
    they recrystallize rather than strain harden
    during deformation.
  • Lead exhibits this behavior at room temperature
    because room temperature is above the
    recrystallization point for lead.

38
Types of Stress-Strain relationships
  • Elastic and strain hardening
  • This material obeys Hookes Law in the elastic
    region.
  • It begins to flow at its yield strength Y.
    Continued deformation requires an
    every-increasing stress, given by a flow curve
    whose strength coefficient K is greater that Y
    and whose strain hardening exponent n is greater
    than zero.
  • The flow curve is generally represented as a
    linear function on a natural logarithmic plot.
  • Most ductile metals behave this way when cold
    worked.

39
Tensile
  • Manufacturing processes that deform materials
    through the application of tensile stresses
    include wire and bar drawing and stretch forming

40
Compression Properties
  • Applies a load that squeezes a cylindrical
    specimen between two platens. The specimen
    height is reduced and its cross-sectional area is
    increased.
  • Engineering stress and strain are calculated much
    like that in tensile engineering stress and
    strain.
  • The engineering stress strain curve is different
    in plastic portion of the curve. Since
    compression causes the cross section to increase,
    the load increases more rapidly than previously.
    The result is a higher calculated engineering
    stress.

41
Compression Properties
  • Although differences exist between the
    engineering stress-strain curve in tension and
    compression, when the respective data are plotted
    as true stress-strain, the relationships are
    nearly identical
  • Important compression processes in industry
    include rolling, forging, and extrusion

42
Shearing Properties
  • Shear involves application of stresses in
    opposite directions on either side of a thin
    element to deflect it.
  • Shear stress (psi) is defined by
  • Shear strain (in/in) is defined by

Where d is the deflection of the element (in) and
b the orthogonal distance over which deflection
occurs
43
Shearing Properties
  • Shear stress and strain are commonly tested in a
    torsion test, in which a thin-walled tubular
    specimen is subjected to a torque.
  • As torque is increased, the tube deflects by
    twisting, which is a shear strain for this
    geometry.

44
Shearing Properties
  • The shear stress can be determined in the test by
    the equation
  • Where T applied torque (lb-in) R radius of
    the tube measured from the neutral axis of the
    wall (in) t wall thickness (in)

45
Shearing Properties
  • Shear strain can be determined by measuring the
    amount of angular deflection of the tube,
    converting this into a distance, and dividing by
    the gauge length (L). Reducing this to a simple
    expression.
  • The shear stress at fracture can be calculated,
    and this is used as the shear strength S of the
    material. Shear strength can be estimated from
    tensile strength data by approximation S 0.7(TS)

Where a the angular deflection (radians)
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