Chapter 11IT 2081 Mechanical and Other Methods of Change of Form Chapter 11.
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Transcript of Chapter 11IT 2081 Mechanical and Other Methods of Change of Form Chapter 11.
Chapter 11 IT 208 2
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 3
Overview of Metal FormingCan 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 4
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 5
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 6
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 8
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 9
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 10
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 11
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 12
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 13
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 14
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 15
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 16
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 17
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 18
Drawing
The pulling of a bar through a Die to reduce the cross section.
• Used to make wire
• Seamless Tubing
Chapter 11 IT 208 19
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 20
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 23
TensileThe 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 24
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 25
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 26
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
o
of
L
LLEL
Chapter 11 IT 208 27
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
o
fo
A
AAAR
Chapter 11 IT 208 28
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 29
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
A
F
Chapter 11 IT 208 30
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 31
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
o
L
L L
L
L
dLo
ln
Chapter 11 IT 208 32
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.
Chapter 11 IT 208 33
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 34
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 metal’s tendency to work harden
n
Chapter 11 IT 208 35
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 36
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 37
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 38
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 39
Tensile
Manufacturing processes that deform materials through the application of tensile stresses include wire and bar drawing and stretch forming
Chapter 11 IT 208 40
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
Chapter 11 IT 208 42
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:
A
F
b
Where δ is the deflection of the element (in) and b = the orthogonal distance over which deflection occurs
Chapter 11 IT 208 43
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 44
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)
tR
T22
Chapter 11 IT 208 45
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)
L
R Where α = the angular deflection (radians)