Download - Chapter 18-Fundamentals Metal Forming
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Chapter 18:
Fundamentals of Metal Forming
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FUNDAMENTALS OF METAL FORMING
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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Metal Forming
Metal Forming includes a 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 ashape determined by the geometry of the die
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 shearstresses
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Material Properties Required in Metal
Forming
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
Effect of temperature gives rise to distinctions
between cold, warm and hot working Other factors:
Strain rate and friction
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Bulk Deformation Processes
Metal Forming processes can be classified as (1)
bulk deformation processes and (2) sheet
metalworking processes Bulk deformation processes are 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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BASIC BULK DEFORMATION PROCESSES
(a) Rolling (b) Forging
(c) Extrusion (d) Drawing
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Sheet Metalworking
Sheet metalworking processes are 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 pressworkingbecause presses perform
these operations
Parts are called stampings
Usual tooling: punch and die
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BASIC SHEET METALWORKING OPERATIONS
(a) Bending (b) Deep Drawing
(c) Shearing
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Material Behavior in Metal Forming
Typical stress-strain curve for most metals is dividedinto an elastic region and a plastic region
Plastic region of stress-strain curve is primary interestbecause material is plastically or permanentlydeformed
In plastic region, metal's behavior is expressed by theflow curve:
nKIW !
where K= strength coefficient; and n = strain
hardening exponent
Stress and strain in flow curve are true stress and
true strain
Flow curve is a valid relationship for cold working
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Flow Stress
For most metals at room temperature, strengthincreases when deformed due to strain hardening
Flow stress = instantaneous value of stress requiredto continue deforming the material-to keep metalflowing
where Yf = flow stress, that is, the yield strength as a
function of strain The flow stress of the material is the strength
property that determines forces and power requiredto accomplish a particular forming operation
nf KY I!
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Average Flow Stress
In some processes, like extrusion, average flowstress ( the average value of stress overthe stress-straincurve fromthe beginningof strain tothe finalmaximum
value thatoccurs during deformation) is more useful
than the instantaneous flow stress Determined by integrating the flow curve equation
between zero and the final strain value defining the
range of interest
where = average flow stress; and I = maximum
strain during deformation process
n
KY
n
f
!
1
I_
_
fY
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Material Behavior in Metal Forming
n
f KY I!
n
KY
n
f
!
1
I
Yf Flow Stress
I Maximum strain
for forming process
K Strength coefficient
Average flow stressfY
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Temperature in Metal Forming
For any metal, Kand n in the flow curve depend on
temperature
Both strength and strain hardening are reduced at
higher temperatures
In addition, ductility is increased at highertemperatures
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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Cold Working
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 nearnetshape ornetshape
processes
Due to strain hardening the strength of the metalincreases and ductility decreases during cold working
Advantages Vs. Disadvantages
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Advantages of Cold Forming vs.
Hot Working
1. Better accuracy closer tolerances
2. Better surface finish
3. Strain hardening increases strength and hardness4. Grain flow during deformation can cause desirable
directional properties in product
5. No heating of work required lower fuel cost and
higher production rates
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Disadvantages of Cold Forming
1. Higher forces and power required
2. Surfaces of starting workpiece must be free of scale
and dirt3. 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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Warm Working
Performed at temperatures above room temperature
but below recrystallization temperature
Dividing line between cold working and warm workingoften expressed in terms of melting point:
0.3Tm, where Tm = melting point (absolute
temperature) for metal
Lower strength and strain hardening while higherductility
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Advantages of Warm Working
1. Lower forces and power than in cold working
2. More intricate work geometries possible
3. Need for annealing may be reduced or eliminated
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Hot Working
Deformation at temperatures above recrystallizationtemperature
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
0.75 Tmis generally considered as upper limit asheat generated due to deformation can lead to
melting above that temperature
Recrystallization may take place during or after the
deformation
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Why Hot Working?
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
Advantages Vs. Disadvantages
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Advantages of Hot Working vs. Cold Working
1. Workpart shape can be significantly altered
2. Lower forces and power required
3. Metals that usually fracture in cold working can behot formed
4. Strength properties of product are generally isotropic
(no grain orientation)
5. 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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Disadvantages of Hot Working
1. Lower dimensional accuracy
2. Higher total energy required (due to the thermal
energy to heat the workpiece)3. Work surface oxidation (scale), poorer surface finish
4. 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: Strain rate sensitivity
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What is Strain Rate?
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:
where =true strain rate; and h = instantaneous height
of workpiece being deformed
h
v!
.
I.
I
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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Effect of Strain Rate on Flow Stress
Flow stress is a strong function of temperature However, 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
As strain rate is increased, resistance to deformation
increases. This usually plots as a straight line on a
log-log graph:
Strain Rate Sensitivity Equation: mf CY I!
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 18.5 - (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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Figure 18.6 - Effect of
temperature on flow stress
for a typical metal. Theconstant Cin 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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Friction in Metal Forming
Friction in metal forming arises because of the closecontact between the tool and work surfaces and the high
pressures that drive the surfaces together in these
operations
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 (see
table)
Friction in metal forming is more severe than machine
parts (gears, cams, bearings etc) where there is low
contact pressures, low to moderate temperatures, and
ample lubrication to avoid metal to metal contact
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Lubrication in Metal Forming
Metalworking lubricants are applied to tool-workinterface in many forming operations to reduce the
harmful effects of friction
Benefits:
Reduced sticking, forces, power, tool wear Better surface finish
Removes heat from the tooling
Lubricants used for cold working operations include:
mineral oils, fats and fatty oils, water basedemulsions, soaps, and other coatings
Hot working is sometimes performed dry; the used
lubricants include minerals oils, graphite and glass
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Considerations in Choosing an
Appropriate Metalworking Lubricant
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
Toxicity, flammability and cost