FUNDAMENTALS OF METAL FORMING

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ME 350 Lecture 20 Chapter 18

• FUNDAMENTALS OF METAL FORMING
• Overview of Metal Forming
• Material Behavior in Metal Forming
• Temperature in Metal Forming
• Strain Rate Sensitivity

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Metal Forming

• Plastic deformation that changes the shape of a
  metal workpiece using a tool, called a die, by
  applying a stress that exceeds the metals
  yield strength
• Stresses are typically compressive
• Examples rolling, forging, extrusion
• However, some forming processes
• Stretch the metal (tensile stresses) wire
  drawing
• Others bend the metal (tensile and compressive)
• Still others apply shear

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Basic Types of Deformation Processes

• Bulk deformation starting material has low
  surface area to volume (e.g. billets bars)
• Rolling
• Forging
• Extrusion
• Wire and bar drawing
• Sheet metalworking starting material has high
  surface area to volume (e.g. sheet coils)
• Bending
• Deep drawing
• Cutting

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Rolling Forging
Extrusion Wire and Bar Drawing
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Sheet Metalworking

• Often called pressworking
• Parts are often called stampings
• Usual tooling punch and die

Bending Cutting
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Material Behavior in Metal Forming

• Plastic region of stress-strain curve is of
  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 (typical material values
  listed in Table 3.4)
• Flow curve based on true stress and true strain

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Flow Stress Average Flow Stress

• Flow stress, Yf, - the instantaneous (or maximum)
  stress required to continue deforming the
  material. (used in upset or impression forging)
• Average flow stress, Yf, - integrating area under
  the stress curve . (? maximum strain during
  deformation, used in open forging, rolling or
  drawing)

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Temperature in Metal Forming

• K and n in the flow curve depend on temperature
• At higher temperatures ductility is increased
• At higher temperatures both K and n are
  reduced
• Thus, the force and power required to perform
  deformation operations at elevated temperatures
  are lower
• Three temperature ranges in metal forming
• Cold working
• Warm working
• Hot working

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Cold Working

• Temperature performed at room temp
• Advantages
• Better accuracy, closer tolerances
• Due to strain hardening, strength and hardness
  are increased
• Directional properties in workpart due to grain
  flow
• No heating of work required
• Disadvantages
• Metal may not be ductile enough for large
  deformations
• Deformation forces and power are higher
• Surfaces must be clean - free of scale and dirt

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Warm Working

• Temperature between room and recrystallization
  temperature ( 0.3Tm in Kelvin)
• Advantages
• Lower forces and power than cold working
• More intricate work geometries possible
• Need for annealing may be reduced or eliminated

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Hot Working

• Temperature at or above the recrystallization
  temperature ( 0.5 Tm in Kelvin)
• Advantages
• Large deformations possible (fracture and
  cracking possibility eliminated or greatly
  reduced)
• Lower forces and power required
• Strength properties of product are generally
  isotropic
• Disadvantages
• Lower dimensional accuracy
• Higher total energy required (due to heating)
• Work surface oxidation (scale), poorer surface
  finish
• Shorter tool life

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Strain Rate Sensitivity

• Theoretically, a metal in hot working behaves
  like a perfectly plastic material, with strain
  hardening exponent n 0
• The metal should continue to flow at the same
  flow stress, once that stress is reached
• However, an additional phenomenon occurs during
  deformation, especially at elevated temperatures,
  where larger stress is needed as deformation
  velocity increases.

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What is Strain Rate?

• Strain rate in forming is directly related to
  speed of deformation, v (i.e. ram velocity)
• where h instantaneous workpiece height
• As strain rate increases, resistance to
  deformation increases

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Strain Rate Sensitivity

• where C strength constant (similar but not
  equal to strength coefficient in flow curve
  equation), and m strain-rate sensitivity
  exponent

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Effect of Temperature on Flow Stress
where C, is the intersection of each plot with
the vertical dashed line at strain rate 1.0,
and m is the slope of each plot.

• Observations
• Increasing temp. C decreases
• Increasing temp. m increases
• Effect of strain rate at room temperature is
  negligible

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Quotes

• The more a person knows, the more they forgive. -
  Confucius
• Living well and beautifully and justly are all
  one thing.- Socrates
• Adversity introduces a person to themselves.
• And in the end, it's not the years in your life
  that count. It's the life in your years.-
  Abraham Lincoln
• Beware the barrenness of a busy life. - Socrates