chapter 11it 2081 mechanical and other methods of change of form chapter 11

Chapter 11 IT 208 1

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 7

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.


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.


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.


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


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


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.


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.


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


Drawing

The pulling of a bar through a Die to reduce the cross section.

• Used to make wire

• Seamless Tubing


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.


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


Spin forming


Material Properties

Tensile Compression Shear


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


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.


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.


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


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


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.


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


True Stress-Strain

Similarly, true strain provides a more realistic assessment of the instantaneous elongation per unit length of the material.


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


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.


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.


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


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


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.



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.



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.


Tensile

Manufacturing processes that deform materials through the application of tensile stresses include wire and bar drawing and stretch forming


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.


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


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


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.


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


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)

chapter 11it 2081 mechanical and other methods of change of form chapter 11

Documents

forging process

forging extrusion

forging open forging

forging coining

closed forging

forging lubricants

forging pressures

radial forging