Transcript Chapter 11 Mechancial and Other Methods of Change of Form

Mechanical and Other
Methods of Change of
Form
Chapter 11
Chapter 11
IT 208
1
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
Chapter 11
IT 208
2
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
Chapter 11
IT 208
3
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
Chapter 11
IT 208
4
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.
Chapter 11
IT 208
5
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.
Chapter 11
IT 208
6
Chapter 11
IT 208
7
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
Chapter 11
IT 208
8
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.
Chapter 11
IT 208
9
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.
Chapter 11
IT 208
10
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.
Chapter 11
IT 208
11
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.
Chapter 11
IT 208
12
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
Chapter 11
IT 208
13
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
Chapter 11
IT 208
14
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.
Chapter 11
IT 208
15
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.
Chapter 11
IT 208
16
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
Chapter 11
IT 208
17
Drawing
The pulling of a bar through a Die to reduce the
cross section.
• Used to make wire
• Seamless Tubing
Chapter 11
IT 208
18
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.
Chapter 11
IT 208
19
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
Chapter 11
IT 208
20
Spin forming
Chapter 11
IT 208
21
Material Properties



Tensile
Compression
Shear
Chapter 11
IT 208
22
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 Hooke’s Law:
σe = E е
where E = modulus of elasticity (psi) which is the inherent
stiffness of a material; e = engineering strain
Chapter 11
IT 208
23
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, Hooke’s 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.

Chapter 11
IT 208
24
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.
Chapter 11
IT 208
25
Tensile Stress – Strain Curve


This measure can be taken as either elongation or
area reduction
Elongation often expressed as a percent.
EL 

L f  Lo
Lo
where Lf = specimen length after fracture and Lo =
original specimen length
Chapter 11
IT 208
26
Tensile Stress – Strain Curve

Area reduction often expressed as a percent
AR 

Ao  A f
Ao
where Ao = original area and Af = area of the
cross-section at the point of fracture
Chapter 11
IT 208
27
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.
Chapter 11
IT 208
28
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
F
 
A
Where F = force (lb) and A = actual (instantaneous)
area resisting the load
Chapter 11
IT 208
29
True Stress-Strain
Similarly, true strain provides a more realistic
assessment of the instantaneous elongation per unit
length of the material.
Chapter 11
IT 208
30
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
 
L
Lo
dL
L
 ln
L
Lo
Where L = instantaneous length at any moment during
elongation
Chapter 11
IT 208
31
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 stressstrain 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.
Chapter 11
IT 208
32
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.
Chapter 11
IT 208
33
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
  
n
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 metal’s tendency to
work harden
Chapter 11
IT 208
34
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
Chapter 11
IT 208
35
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.
Chapter 11
IT 208
36
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.
Chapter 11
IT 208
37
Types of Stress-Strain relationships

Elastic and strain hardening
•
•
•
•
This material obeys Hooke’s 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.
Chapter 11
IT 208
38
Tensile
Manufacturing processes that deform materials through
the application of tensile stresses include wire and
bar drawing and stretch forming
Chapter 11
IT 208
39
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.
Chapter 11
IT 208
40
Compression Properties
Although differences exist between the engineering
stress-strain curve in tension and compression,
when the respective data are plotted as true stressstrain, the relationships are nearly identical

Important compression processes in industry include
rolling, forging, and extrusion
Chapter 11
IT 208
41
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:
F

A

Shear strain (in/in) is defined by:
 
Chapter 11

b
Where δ is the deflection of the element (in)
and b = the orthogonal distance over which
deflection occurs
IT 208
42
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.
Chapter 11
IT 208
43
Shearing Properties

The shear stress can be determined in the test by
the equation
T
 
2R 2 t

Where T = applied torque (lb-in); R = radius of the
tube measured from the neutral axis of the wall (in);
t = wall thickness (in)
Chapter 11
IT 208
44
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.
R
 
L

Where α = the angular
deflection (radians)
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)
Chapter 11
IT 208
45