 Overview of Metal Forming

 Material Behavior in Metal Forming

 Temperature in Metal Forming
 Strain Rate Sensitivity
 Friction and Lubrication in Metal Forming

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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 yield strength of metal
 The metal takes a shape determined by the
geometry of the die

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 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

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 Desirable material properties:
• Low yield strength and 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

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 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

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Figure 18.2 – Basic bulk deformation processes: (a) rolling

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Figure 18.2 – Basic bulk deformation processes: (b) forging

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Figure 18.2 – Basic bulk deformation processes: (c) extrusion

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Figure 18.2 – Basic bulk deformation processes: (d) drawing

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• 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

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Figure 18.3 ‑ Basic sheet metalworking operations: (a) bending

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Figure 18.3 ‑ Basic sheet metalworking operations: (b) drawing

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Figure 18.3 ‑ Basic sheet metalworking operations: (c) shearing

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• 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:

n
  K
where K = strength coefficient; and n = strain
hardening exponent
• Stress and strain in flow curve are true stress and
true strain

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• 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

Yf  K n
where Yf = flow stress, that is, the yield strength as a
function of strain

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Determined by integrating the flow curve
equation between zero and the final strain
value defining the range of interest
_
K n
Yf 
1 n
_
where Yf = average flow stress; and  =
maximum strain during deformation process

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 For any metal, K and n in the flow curve
depend on temperature
• Both strength and strain hardening are reduced at
higher temperatures
• In addition, ductility is increased at higher temperatures

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 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

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 Performed at room temperature or slightly
above
 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

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 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

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• Higher forces and power required
• Surfaces of starting workpiece must be free of
scale and dirt
• Ductility and strain hardening limit the amount
of forming that can be done
– In some operations, metal must be annealed to allow
further deformation
– In other cases, metal is simply not ductile enough to be
cold worked

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 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

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 Lower forces and power than in cold working
 More intricate work geometries possible
 Need for annealing may be reduced or
eliminated

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 Deformation at temperatures above
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

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Capability for substantial plastic deformation of
the metal ‑ far more than possible with cold
working or warm working
 Why?
• Strength coefficient is substantially less than at room
temperature
• Strain hardening exponent is zero (theoretically)
• Ductility is significantly increased

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• 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

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 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

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 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: Strain
rate sensitivity

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• Strain rate in forming is directly related to
speed of deformation v
• Deformation speed v = velocity of the ram or
other movement of the equipment
Strain rate is defined:
. v

h
.
where  = true strain rate; and h = instantaneous height
of workpiece being deformed

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 In most practical operations, valuation of strain
rate is complicated by
• Workpart geometry
• Variations in strain rate in different regions of the part
 Strain rate can reach 1000 s-1 or more for some
metal forming operations

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 Flow stress is a function of temperature
 At hot working temperatures, flow stress also
depends on strain rate
• As strain rate increases, resistance to deformation
increases
• This effect is known as strain‑rate sensitivity

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Figure 18.5 ‑ (a) Effect of strain rate on flow stress at an
elevated work temperature. (b) Same relationship
plotted on log‑log coordinates

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 Yf  C m

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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 Figure 18.6 ‑ Effect of
temperature on flow stress
for a typical metal. The
constant C in Eq. (18.4),
indicated by the intersection
of each plot with the vertical
dashed line at strain rate =
1.0, decreases, and m
(slope of each plot)
increases with increasing
temperature

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 Increasing temperature decreases C, increases
m
• At room temperature, effect of strain rate is almost
negligible
 Flow curve is a good representation of material behavior
• As temperature increases, strain rate becomes
increasingly important in determining flow stress

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 In most metal forming processes, friction is
undesirable:
• Metal flow is retarded
• Forces and power are increased
• Wears tooling faster
 Friction and tool wear are more severe in hot
working

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 Metalworking lubricants are applied to
tool‑work interface in many forming
operations to reduce harmful effects of friction
 Benefits:
• Reduced sticking, forces, power, tool wear
• Better surface finish
• Removes heat from the tooling

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 Type of forming process (rolling, forging,
sheet metal drawing, etc.)
 Hot working or cold working
 Work material
 Chemical reactivity with tool and work metals
 Ease of application
 Cost

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