Mechanical and Other Methods of Change of Form

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Mechanical and Other Methods of Change of Form

• Chapter 11

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• 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

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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

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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

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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.

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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.

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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

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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.

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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.

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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.

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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.

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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

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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

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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.

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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.

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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

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Drawing

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

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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.

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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

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Spin forming
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Material Properties

• Tensile
• Compression
• Shear

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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

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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.

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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.

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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

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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

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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.

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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

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True Stress-Strain

• Similarly, true strain provides a more realistic
  assessment of the instantaneous elongation per
  unit length of the material.

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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

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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.

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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.

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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

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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

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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.

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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.

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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.

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Tensile

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

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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.

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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

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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
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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.

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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)

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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)