Forming and Shaping

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					Lesson Objectives:

 Today’s lesson objectives:
 • Introduction to forming and shaping
   processes for metal
 • Principles of rolling process
 • Parameters involved in rolling process

                    BMM 3643          Page 2
• Metal forming is a shape production
  process (similar to casting)
• But it uses the fundamental of
  deformation processes.
• These processes is designed to exploit a
  property of engineering materials
  (mostly metals) known as plasticity, the
  ability to flow as solids without
  deterioration of their properties.
• Since all processing is done in the solid
  state, there is no need to handle molten
                    BMM 3643          Page 3
Introduction (cont.)

• The material is simply moved (or
  rearranged) to produce shape, then the
  amount of waste is reduced
  substantially unlike cutting away
• But, the forces required are often high.
• Machinery and tooling can be expensive
  and it is suitable for mass production.
                   BMM 3643          Page 4
Classification of Metal Forming
• In metal forming operations, workpiece
  temperature can be one of most
  important process variables.
• In general, an increase in temperature
  brings about a decrease in strength, an
  increase in ductility, and a decrease in
  the rate of strain hardening – the
  effects that would tend to promote the
  ease of deformation.
• Therefore, forming processes can be
  classified based on working
                    BMM 3643          Page 5
Classification of Metal Forming
Processes (cont.)
• It can be classified as hot working, cold
  working or warm working.

                   BMM 3643           Page 6
 Hot Working
• Defined as the plastic deformation of metals at
  a temperature above the recrystallization
• About 0.6 times the melting point of the
  material on absolute temperature scale (Kelvin
  or Rankine)
• However, the recrystallization temperature
  varies greatly with different materials.
• e.g. Tin is near hot-working conditions at room
  temperature, Steels require temperatures near
  1093°C, Tungsten has hot working temperature
  about 2204°C.           BMM 3643            Page 7
Cold Working
• Cold working is the plastic deformation
  of metals below the recrystallization
• Usually less than 0.3 times the
  workpiece melting temperatures.
• Normally it is performed at room
  temperature, but mildly elevated
  temperature may be used to provide
  increased ductility and reduced

                    BMM 3643          Page 8
Advantages of Cold Forming vs.
Hot Working
 • From a manufacturing viewpoint, cold
   working has a number of distinct
   advantages from hot working include:
    – No heating is required
    – Better surface finish
    – Better accuracy and flatness
    – Better reproducibility
    – Strength, fatigue and wear properties
      are improved
    – Contamination problems are
      minimized        BMM 3643       Page 9
Disadvantages of Cold Forming vs.
Hot Working
  • Some disadvantages associated with
   cold working:
    – Higher forces are required
    – Heavier and more powerful of
    – Less ductility
    – Undesirable residual stresses
    – Metal surfaces must be clean

                    BMM 3643          Page 10
 Warm Working
• Deformation produced at temperatures
  intermediate to hot and cold working.
• Performed at temperatures above room
  temperature but below recrystallization
• Dividing line between cold working and
  warm working often expressed in terms of
  melting point:
   – 0.3Tm < T < 0.6Tm where Tm = melting
     point (absolute temperature) for metal
                       BMM 3643         Page 11
Advantages of Warm Working
• Lower forces and power than in cold
• More intricate work geometries possible
• Need for annealing may be reduced or

                   BMM 3643          Page 12
Classification of Metal Forming
 • Example of hot working processes:
    – Forging
    – Rolling
    – Extrusion
    – Forming of tubes and pipes, etc.
 • Examples of cold working processes:
    – Sheet metal working
    – Piercing
    – Drawing
    – Embossing, etc. BMM 3643       Page 13
Rolling of Metals
• Rolling is the process of reducing thickness
    (or changing the cross section) of a long
    workpiece by compressive forces applied
    through a set of rolls.
•   Rolling accounts for about 90% of all
    metals produced by metalworking
•   The basic operation is flat rolling, where the
    rolled products are flat plate and sheet.
•   Plates are regarded as having a thickness
    greater than 6mm
•   Sheets are less than 6 mm thick.
                        BMM 3643          Page 15
Flat- and Shape-Rolling

                           Schematic outline
                           of various flat-
                           and shape-rolling
                           Source: American
                           Iron and Steel

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The Rolls
The rotating rolls perform two main
• Pull the work into the gap between
  them by friction between workpart and
• Simultaneously squeeze the work to
  reduce cross section

                  BMM 3643         Page 17
Types of Rolling
• By geometry of work:
  – Flat rolling - used to reduce thickness of a
    rectangular cross-section
  – Shape rolling - a square cross-section is
    formed into a shape such as an I-beam
• By temperature of work:
  – Hot Rolling – most common due to the
    large amount of deformation required
  – Cold rolling – produces finished sheet and
    plate stock
                       BMM 3643             Page 18

• Rolling is first carried out at elevated
  temperatures (hot rolling)
• During this phase, the coarse-grained,
  brittle and porous structure of cast
  metal is broken down into a wrought
  structure having finer grain size and
  enhanced properties.
• Cold rolling is carried out at room
                     BMM 3643           Page 19
      Grain Structure During
      Hot Rolling
Changes in the grain structure of cast or of large-grain wrought
metals during hot rolling. Hot rolling is an effective way to reduce
grain size in metals, for improved strength and ductility. Cast
structures of ingots or continuous casting are converted to a
wrought structure by hot working.

                                BMM 3643                  Page 20
Flat Rolling

 • A strip of thickness h0 enters the roll
   gap and is reduced to thickness hf by a
   pair of rotating rolls.
 • Each roll being powered through its
   own shaft by electric motors.

                     BMM 3643           Page 21

(a) Schematic illustration of the flat-rolling process. (b) Friction
forces acting on strip surfaces. (c) The roll force, F, and the torque
acting on the rolls. The width w of the strip usually increases
during rolling, as is shown in Figure above.

                                 BMM 3643                   Page 22
The Flat-Rolling Process
• Roll force, F (N) = LwYavg
   – L = roll-strip contact length
   – w = width of the strip
   – Yavg = average true stress
• This calculation is for a frictionless situation,
  actual force maybe 20% more.
• Power (for 2 rolls), P (kW) = 2πFLN / 60000
   – N = rpm of the roll
• Torque = Fa
  – a = L/2
                            BMM 3643            Page 23
The Flat-Rolling Process (cont.)

 • Example
 An annealed copper strip, 250 mm wide
  and 25 mm thick, is rolled to a
  thickness of 20 mm in one pass. The
  roll radius is 300 mm, and the rolls
  rotate at 100 rpm. Calculate the roll
  force and the power required in this
                   BMM 3643         Page 24
The Flat-Rolling Process

              BMM 3643     Page 25
The Flat-Rolling Process

              BMM 3643     Page 26
Lesson Objectives:

 Today’s lesson objectives:
 • Shape rolling and other rolling
 • Introduction to forging processes for
 • Introduction to forging operations

                    BMM 3643          Page 27
Rolling of Metals
Shape-rolling Operations
• In addition to flat rolling, various
  shapes can be produced by shape
• Straight and long structural shapes,
  such as solid bars (with various cross-
  sections), channels, I-beams, railroad
  rails are rolled by passing the stock
  through a set of specially designed rolls.

                     BMM 3643            Page 29
Shape Rolling

                   Stages in the shape
                   rolling of an H-
                   section part.
                   Various other
                   structural sections,
                   such as channels
                   and I-beams, are
                   also rolled by this
                   kind of process.

        BMM 3643              Page 30
Ring Rolling
• A thick ring is expanded into a large
  diameter ring with a reduced cross-section.
• The ring is placed between two rolls closer
  together, one of which is driven and its
  thickness is reduced by bringing the rolls
  closer together as they rotate.
• Applications: ball and roller bearing races,
  steel tires for railroad wheels, and rings for
  pipes, pressure vessels, and rotating
  machinery .           BMM 3643         Page 31
Ring Rolling (cont.)
• Advantages: material savings, ideal grain
 orientation, strengthening through cold

                    BMM 3643          Page 32
Ring rolling used to reduce the wall thickness and
           increase the diameter of a ring:
     (1) start, and (2) completion of process

                      BMM 3643                Page 33

                    (a) Schematic illustration of
                    a ring-rolling operation.
                    Thickness reduction results
                    in an increase in the part
                    diameter. (b) Examples of
                    cross-sections that can be
                    formed by ring rolling.

               BMM 3643                  Page 34
Thread Rolling
• It is a process in a cold-forming process by
  which straight or tapered threads are formed on
  round rods, by passing them between dies.
• Advantages over thread cutting (machining):
  –   Higher production rates
  –   Better material utilization
  –   Stronger threads due to work hardening
  –   Better fatigue resistance due to compressive stresses
      introduced by rolling

                          BMM 3643                Page 35
                        processes: (a) and
                        (c) reciprocating
                        flat dies; (b) two-
                        roller dies.
                        Threaded fasteners,
                        such as bolts, are
                        made economically
                        by these processes,
                        at high rates of

             BMM 3643          Page 36

(a) Features of a machined or rolled thread. (b) Grain flow in
machined and rolled threads. Unlike machining, which cuts
through the grains of the metal, the rolling of threads causes
improved strength, because of cold working and favorable grain

                             BMM 3643                  Page 37
Defects in Flat Rolling
• Few defects occurred in flat rolling
  process may not only degrade surface
  appearance but may adversely affect
  the strength, formability and etc.
• Examples of defects in flat rolling are:

                   BMM 3643              Page 38
  Defects in Flat Rolling

Wavy edges – the strip is       Cracks – result of poor
thinner along its edges that    material ductility at the
its centre, thus the edges      rolling temperature
elongate more than centre.
                          BMM 3643               Page 39
Defects in Flat Rolling

                       Alligatoring – complex
Edge cracks – poor
                       phenomenon caused by
quality of materials
                       nonuniform bulk deformation
at the edges
                       process or presence of defects
                       in the original materials
                        BMM 3643            Page 40
Forging of Metals

• It is a process in which the workpiece is
  shaped by compressive forces applied
  through various dies and tools.
• Oldest metalworking operation
• Simple forging operations can be
  performed with a heavy hand hammer,
  traditionally done by blacksmiths.

                    BMM 3643          Page 42

• Most forgings, require a set of dies and
  a press or a forging hammer.
• Forging operations produce discrete
• Typical forged products are bolts and
  rivets, connecting rods, shafts for
  turbines, gears, hand tools, aircraft
  components, etc.
                    BMM 3643          Page 43
(a)     (b)

        Figure 14.1 (a) Schematic illustration of the steps
        involved in forging a bevel gear with a shaft. Source:
        Forging Industry Association. (b) Landing-gear
        components for the C5A and C5B transport aircraft,
        made by forging. Source: Wyman-Gordon Company.

              BMM 3643                              Page 44
         Characteristics of Forging
TABLE 14.1
Process                              Advantages                                        Limitations
Open die            Simple, inexpensive dies; useful for small      Limited to simple shapes; difficult to hold close
                    quantities; wide range of sizes available;      tolerances; machining to final shape necessary;
                    good strength characteristics                   low production rate; relatively poor utilization of
                                                                    material; high degree of skill required
Closed die          Relatively good utilization of material;        High die cost for small quantities; machining
                    generally better properties than open-die       often necessary
                    forgings; good dimensional accuracy; high
                    production rates; good reproducibility
Blocker type        Low die costs; high production rates            Machining to final shape necessary; thick webs
                                                                    and large fillets necessary
Conventional type   Requires much less machining than blocker       Somewhat higher die cost than blocker type
                    type; high production rates; good utilization
                    of material
Precision type      Close tolerances; machining often               Requires high forces, intricate dies, and provision
                    unnecessary; very good material utilization;    for removing forging from dies
                    very thin webs and flanges possible

                                                       BMM 3643                                       Page 45
 Classification of Forging

• Cold vs. hot forging:
  – Hot or warm forging – most common, due to
    the significant deformation and the need to
    reduce strength and increase ductility of work
  – Cold forging - advantage is increased strength
    that results from strain hardening

                        BMM 3643            Page 46
Classification of Forging
Operations (cont.)

• Impact vs. press forging:
   – Forge hammer - applies an impact load
   – Forge press - applies gradual pressure

                      BMM 3643            Page 47
Lesson Objectives:

 Today’s lesson objectives:
 • Types of forging dies
 • Force calculation in forging operation

                    BMM 3643          Page 48
Forging of Metals
Types of Forging Dies

• Three types of forging dies:
• 1. Open-die forging - work is compressed
  between two flat dies, allowing metal to
  flow laterally without constraint
• 2. Impression-die forging (closed) - die
  surfaces contain a cavity or impression
  that is imparted to workpart, thus
  constraining metal flow - flash is created
                    BMM 3643           Page 50
Types of Forging Dies (cont.)
• Flashless forging (precision forging) -
  workpart is completely constrained in
  die and no excess flash is produced

                    BMM 3643           Page 51
Open-die Forging
• The forging process can be depicted by
  a solid workpiece placed between two
  flat dies and reduced in height by
  compressing it.
• Common names include upsetting or
  upset forging

                  BMM 3643         Page 52
              Open-die Forging

 (a) Solid cylindrical billet upset between two flat dies.
 (b) Uniform deformation of the billet without friction.
(c) Deformation with friction. Note barreling of the billet
caused by friction forces at the billet-die interfaces.
                              BMM 3643                 Page 53
Forging Force
• Forging force, F, in an open-die forging
  operation can be estimated as:
           F = Yfπr2 (1 + (2μr / 3h))
  – Yf = flow stress (true stress)
  – μ = coefficient of friction between the
    workpiece and the die
  – r = radius of the workpiece
  – h = height of the workpiece

                       BMM 3643               Page 54
• A solid cylindrical slug made of 304
 stainless steel is 150 mm in diameter
 and 100 mm high. It is reduced in
 height by 50% at room temperature by
 open-die forging with flat dies.
 Assuming that the coefficient of friction
 is 0.2, calculate the forging force at the
 end of the stroke.
                   BMM 3643           Page 55
Forging Force

                BMM 3643   Page 56
Closed-die Forging
• The workpiece acquires the shape of
  the die cavities while being forged
  between two shaped dies.

                   BMM 3643             Page 57
     Impression-Die Forging

(a) through (c) Stages in impression-die forging of a solid round billet. Note the
formation of flash, which is excess metal that is subsequently trimmed off (d)
Standard terminology for various features of a forging die.
                                      BMM 3643                      Page 58
Impression-Die Forging
Advantages and Limitations

• Advantages compared to machining
 from solid stock:
  – Higher production rates
  – Conservation of metal (less waste)
  – Greater strength
  – Favorable grain orientation in the
                   BMM 3643         Page 59
Impression-Die Forging
Advantages and Limitations (cont.)

  • Limitations:
    – Not capable of close tolerances
    – Machining often required to achieve
      accuracies and features needed, such
      as holes, threads, and mating
      surfaces that fit with other

                    BMM 3643          Page 60
Forging Force
• Forging force, F, in an impression-die
  forging operation can be estimated as:
           F = kYfA
• k = multiplying factor
• A = projected area of the forging, including

                     BMM 3643           Page 61
• Assume that the flow stress of a
 material at the forging temperature is
 700 MPa, and a part has a projected
 area of 38 000 mm2. Taking a value of
 k = 10, calculate the forging force
 required to do the operation.

                  BMM 3643           Page 62
Precision Forging (flashless)
• Special dies used to produce parts having
    greater accuracies.
•   Little excess material on the forged part
•   Starting workpart volume must equal die
    cavity volume within very close tolerance.
•   Process control more demanding than
    impression-die forging.
•   Best suited to part geometries that are simple
    and symmetrical.
                        BMM 3643             Page 63
  Closed-Die Forging Versus
  Flashless Forging

Comparison of closed-die forging with flash (left side of each
illustration) and precision or flashless forging (right side) of a
round billet. Source After H. Takemasu, V. Vazquez, B.
Painter, and T. Altan.           BMM 3643                  Page 64
Lesson Outcomes:
After today’s lecture, students are
  expected to:
• Differentiate a variety of forging
• Analyze the quality of products
  produced by different forging
• Differentiate a variety of forging
  machines and their characteristics
                   BMM 3643            Page 65
Heading (upset forging)
• Forging process used to form heads on
  nails, bolts, and similar hardware
• Wire or bar stock is fed into machine,
  end is headed, then piece is cut to
• For bolts and screws, thread rolling is
  then used to form threads.

                     BMM 3643          Page 66

(a) Heading operation to form heads on fasteners, such as
nails and rivets. (b) Sequence of operations to produce a
typical bolt head by heading.
                           BMM 3643               Page 67
Cogging / Smithing
• Cogging also called drawing out, is
  basically an open-die forging operation.
• It is a process of reducing the thickness
  of a bar by successive forging steps at
  specific interval.
• Blacksmiths perform such operations
  with a hammer and anvil using hot
  pieces of metals.

                    BMM 3643            Page 68
              Two views of a
              operation on a
              rectangular bar.
              Blacksmiths use
              this process to
              reduce the
              thickness of bars
              by hammering
              the part on an
              anvil. Note the
              barreling of the

   BMM 3643         Page 69

• It is an operation consisting of shallow
  or moderate draws, made with male
  and female matching dies.
• Embossing is used principally for the
  stiffening of flat panels and for
  purposes of decoration.

                   BMM 3643           Page 70
                 An embossing
                 operation with two
                 dies. Letters,
                 numbers, and designs
                 on sheet-metal parts
                 and thin ash trays can
                 be produced by this

      BMM 3643               Page 71
• It is essentially a closed-die forging
  process typically used in minting coins,
  medallions and jewellery.

                     BMM 3643              Page 72
                                              The Coining


(a) Schematic illustration of the coining process. The earliest coins were
made by open-die forging and lacked precision and sharp details. (b) An
example of a modern coining operation, showing the workpiece and
tooling. Note the detail and superior finish that can be achieve in this
process. Source: Courtesy of C & W Steel Stamp Co., Inc.
                                  BMM 3643                       Page 73
• It is a process of indenting (but not
  breaking through) the surface of a
  workpiece with a punch in order to
  produced cavity.
• Piercing may be followed by punching,
  to produce a hole in the part.

                    BMM 3643          Page 74
Grain Flow Pattern of Pierced
Round Billet

                         A pierced round billet,
                         showing grain flow
                         pattern. Source:
                         Courtesy of Ladish
                         Co., Inc.

              BMM 3643                   Page 75
Forging Defects
• In addition to surface cracking, other
  defects developed in forging operation
  – Web buckling (laps formation)
  – Internal defects

                    BMM 3643         Page 76
  Defects in Forged Parts

Examples of defects in forged parts. (a) Laps formed by web
buckling during forging; web thickness should be increased to
avoid this problem.

                            BMM 3643                Page 77
  Defects in Forged Parts

Examples of defects in forged parts. (b) Internal defects
caused by an oversized billet. Die cavities are filled
prematurely, and the material at the center flows past the
filled regions as the die closes.
                             BMM 3643                 Page 78
Forging Machines
• A variety of forging machines, with a
  range of capacities, speeds and speed-
  stroke characteristics.
• These machines are generally classified:
• 1. Presses
• 2. Hammers.

                   BMM 3643          Page 79
Forging Machines (cont.)
- 1. Hydraulic press –
  - these presses operate at constant speeds
    and load limited.
  - Large amounts of energy can be
    transmitted to a workpiece by a constant
    load throughout a stroke

                     BMM 3643            Page 80
 Principles of Various
 Forging Machines

Schematic illustration of the principles of various forging
machines. (a) Hydraulic press.

                             BMM 3643                   Page 81

       (c) general view of a 445 MN
       (50,000 ton) hydraulic press.
       Source: Wyman-Gordon

      BMM 3643                Page 82
Forging Machines (cont.)
 2. Mechanical Presses
 • These presses can be either the crank
   or the eccentric type.
 • The energy is generated by a large
   flywheel to an eccentric shaft.

                   BMM 3643         Page 83
Principles of Various
Forging Machines

                  Schematic illustration of the
                  principles of various forging
                  machines. (b) Mechanical press
                  with an eccentric drive; the
                  eccentric shaft can be replaced
                  by a crankshaft to give the up-
                  and-down motion to the ram.

             BMM 3643                 Page 84
Forging Machines (cont.)

• Another type of mechanical press is knuckle-
• Because of the linkage design, very high
  forces can be applied in this type of press.

                       BMM 3643             Page 85
Principles of Various Forging
Machines (cont.)

                  Schematic illustration of the
                  principles of various forging
                  machines. (c) Knuckle-joint

               BMM 3643                  Page 86
Forging Machines (cont.)
• 3. Screw presses
   – Derive the energy from a flywheel
   – The forging load is transmitted through a
     vertical screw.
   – The ram comes to a stop when the
     flywheel energy is dissipated.
   – Suitable for small production quantities and
     precision parts such as turbine blades
   – Capacities ranges 1.4 MN to 280 MN.
                       BMM 3643            Page 87
Principles of Various Forging
Machines (cont.)

              Schematic illustration of the
              principles of various forging
              machines. (d) Screw press.

               BMM 3643                   Page 88
Forging Machines (cont.)

 - Derive the energy from the potential energy
  of the ram which is converted to kinetic
  - Operate at high speed (gravitational
  - Most commonly used for impression-die
  - Advantages:
      - high speed resulting low forming time,
  thus minimize the cooling of hot forging.
      - Low cooling rate allows the forging of
  complex shape
                      BMM 3643             Page 89
Forging Machines (cont.)

 - Disadvantage: impact energy
 transmitted through anvil into floor of
 - Hammer are available in variety
 - 1. Gravity drop hammer
     - drop forging where the energy is
 derived from free-falling hammer.
     - the energy of the hammer depends
 on the ram’s weight and height of its
                  BMM 3643         Page 90
Diagram showing details of a drop hammer for impression-die

                          BMM 3643                   Page 91
Forging Machines (cont.)

• 2. Power drop hammer
  – the ram’s downstroke is accelerated by
   steam, air, hydraulic.
  – Ram weights range from 225 kg to as
   mush as 22, 500 kg.
  – Energy capacity ranging up to 1150 kJ.

                  BMM 3643         Page 92
Lesson Outcomes:
After today’s lecture, students are
  expected to:
• Analyze the principles in various
  extrusion operations
• Calculate the extrusion force and its
  influencing factors
• Analyze the proper practice and design
  to avoid defects in extrusion

                  BMM 3643         Page 93
Extrusion of Metals
Extrusion Process
• In the extrusion process, a billet
  (generally round) is forced through a
  die, in a manner similar to squeezing
  toothpaste from a tube.

                    BMM 3643           Page 95
 Extrusion (cont.)

Schematic illustration of the extrusion process.

                          BMM 3643                 Page 96
Extrusion (cont.)
• Almost any solid or hollow cross-section
  may be produced by extrusion, which
  can create essentially semi-finished
• Because the die geometry remains the
  same throughout the operations,
  extruded products have a constant
                   BMM 3643          Page 97
Extrusions and Products Made
from Extrusions

                 Extrusions and examples
                 of products made by
                 sectioning off extrusions.
                 Source: Courtesy of
                 Kaiser Aluminum.
             BMM 3643                Page 98
Extrusion (cont.)
• It has numerous important applications,
  including fasteners and components for
  automobile, bicycles, motorcycles,
  heavy machinery and etc.
• Typical products made by extrusion are
  railing for sliding doors, door and
  window frames, and etc.
• Commonly extruded materials are
  aluminium, copper, steel, magnesium
  and lead.
                   BMM 3643         Page 99
Extrusion Process
• Types of Extrusion Processes.
1. The basic extrusion process is called
  direct, or forward extrusion.
  – A round billet is placed in a chamber and
    forced through a die opening by a
    hydraulically-driven ram or pressing stem
    as shown in the figure.
  – The die opening may be round or it may
    have various shapes.

                     BMM 3643             Page 100

Schematic illustration of the direct extrusion process.

                          BMM 3643                    Page 101
Extrusion Process (cont.)

2. Another type is Indirect Extrusion.
  - It is also known as reverse, inverted
  or backward extrusion.
  - The die moves toward the billet.

                   BMM 3643           Page 102
Types of Extrusion

 Types of extrusion: (a) indirect;

                  BMM 3643           Page 103
Extrusion Process (cont.)

3. Another type is Hydrostatic Extrusion
  - The chamber which is filled with a
  - The pressure is transmitted to the
  billet by a ram.
  - Unlike in direct extrusion, there is no
  friction to overcome along the container

                    BMM 3643          Page 104
Types of Extrusion

  Types of extrusion: (b) hydrostatic;

                 BMM 3643                Page 105
Extrusion Process (cont.)

4. Another type is lateral, or side
  - Billet is prepared vertically and
  - The extruded parts will come out
  horizontally through the dies.

                    BMM 3643            Page 106
Types of Extrusion

   Types of extrusion: (c) lateral.

                 BMM 3643             Page 107
  Process Variables in Direct

Process variables in direct extrusion. The die angle,
reduction in cross-section, extrusion speed, billet
temperature, and lubrication all affect the extrusion pressure.
                              BMM 3643                Page 108
Extrusion Force
• Extrusion force, F, can be calculated as,
          F = Aok ln (Ao/Af)
  – k = extrusion constant
  – Ao = billet area
  – Af = extruded product area
  – (Ao/Af) = also known as extrusion ratio, R

                     BMM 3643            Page 109
Extrusion Force

                   Extrusion constant k for
                   various metals at different
                   temperatures. Source:
                   After P. Loewenstein

        BMM 3643                    Page 110
• A round billet made of 70-30 brass is
 extruded at a temperature of 675°C.
 The billet diameter is 125 mm and the
 diameter of the extrusion is 50 mm.
 Calculate the extrusion force required if
 extrusion constant, k, is 250 MPa.

                   BMM 3643          Page 111
Hot Extrusion
• Hot extrusion is carried out at elevated
  temperature to reduce the forces
• The material’s ductility is increased
  once heated but it carries some

                    BMM 3643          Page 112
Cold Extrusion
• It denotes a combination of operations
  such as direct and indirect extrusion
  and forging.
• Widely used for automobile
• The force, F, in cold extrusion is
  calculated as:
           F = 1100AoYavgε
  – Ao = original area, Yavg = flow stress, ε = true
                       BMM 3643                Page 113
Cold Extrusion Examples

   Two examples of cold extrusion. Thin
   arrows indicate the direction of metal flow
   during extrusion.    BMM 3643                 Page 114
 Cold-Extruded Spark Plug

Production steps for a
cold-extruded spark
plug. Source:                A cross-section of the metal part in
                             Fig 15.12 showing the grain-flow
Courtesy of National         pattern. Source: Courtesy of
Machinery Company.           National Machinery Company.
                         BMM 3643                    Page 115
Design of Extruded Cross-

Poor and good examples of cross-sections to be extruded. Note the
importance of eliminating sharp corners and of keeping section
thicknesses uniform. Source: J.G. Bralla (ed.); Handbook of Product
Design for Manufacturing. New York: McGraw-Hill Publishing
Company, 1986. Used with permission.
                                BMM 3643                   Page 116
Extrusion Defects
• Few defects happen in extrusion
• 1. Surface cracking
  – Cracking or tearing at the surface
  – Due to high extrusion temperature, friction
    or speed of extrusion
  – May occur at lower temperature when the
    extruded product temporarily sticks to the
    die land

                    BMM 3643             Page 117
Extrusion Defects (cont.)
• 2. Pipe
   – During metal-flow pattern products tend to
     draw surface oxides or impurities toward
     the centre of the billet
   – Occur as much as one third of the
     extruded product length

                     BMM 3643            Page 118
   Types of Metal Flow in
   Extrusion with Square Dies

Types of metal flow in extruding with square dies. (a) Flow pattern obtained at low friction or
in indirect extrusion. (b) Pattern obtained with high friction at the billet-chamber interfaces.
(c) Pattern obtained at high friction or with coiling of the outer regions of the billet in the
chamber. This type of pattern, observed in metals whose strength increases rapidly with
decreasing temperature, leads to a defect known as pipe (or extrusion) defect.

                                             BMM 3643                             Page 119
Extrusion Defects (cont.)
• 3. Internal cracking
   – Cracks developed at the centre of extruded
   – Names called centre cracking, centre burst,
     arrowhead fracture or chevron cracking

                     BMM 3643             Page 120
Chevron Cracking

 (a) Chevron cracking (central burst) in extruded round steel bars. Unless the
 products are inspected, such internal defects may remain undetected and later
 cause failure of the parts in service. This defect can also develop in the drawing
 of rod, of wire, and of tubes. (b) Schematic illustration of rigid and plastic
 zones in extrusion. The tendency toward chevron cracking increases if the two
 plastic zones do not meet. Note that the plastic zone can be made larger either
 by decreasing the die angle or by increasing the reduction in cross-section (or
 both). Source: After B. Avitzur.
                                       BMM 3643                            Page 121
Lesson Outcomes:
After today’s lecture, students are
  expected to:
• Analyze the characteristics of drawing
• Calculate the drawing force and its
  influencing factors
• Analyze the proper practice and design
  to avoid defects in drawing

                  BMM 3643         Page 122
Wire Drawing
• It is an operation in which the cross
  section of solid rod, wire, or tubing is
  reduced and changed in shape by
  pulling it through a die.
• The major variables in drawing are
  similar to those in extrusion:
  – reduction in cross-sectional area
  – die angle
  – friction along the die-workpiece interfaces
  – drawing speed
                      BMM 3643             Page 123
  Process Variables in Wire

Process variables in wire drawing. The die angle, the
reduction in cross-sectional area per pass, the speed of
drawing, the temperature, and the lubrication all affect the
drawing force, F.             BMM 3643                 Page 124
Drawing Force
• The drawing force, F, under ideal and
 frictionless condition can be calculated as:
           F = Yavg Af ln (Ao/Af)
  – Yavg = true stress
  – Ao and Af = Initial and final area
• Drawing force under friction:
     F = Yavg Af [(1 + μ / α) ln (Ao/Af) + 2α/3]
  – α = die angle

                         BMM 3643          Page 125
• A round wire made of a perfectly plastic
 material with a yield stress of 200 MPa
 is being drawn from a diameter of 2.5
 to 1.5 mm in a draw die of 15°. Let the
 coefficient of friction be 0.1. Estimate
 the drawing force required for both
 friction and frictionless conditions.

                   BMM 3643         Page 126
Drawing Practice
• Successful drawing requires the careful
  selection of process parameters such
  – Reductions in the cross-sectional area
    should be less than 45%
  – Drawing speed may be in the range 1-2.5
    m/s to 50 m/s (depends on the size)
  – Different materials are drawn at different
    temperatures (depends on the application)

                    BMM 3643            Page 127
Drawing Defects
• Typical defects in a drawn rod are
  similar to those in extrusion.
• Other major defects include:
  – Seams – longitudinal scratches or folds in
    the material
  – Residual stresses – due to non-uniform

                     BMM 3643            Page 128