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Figure 3.22 The methods of bending tubes.
Figure 3.23 (a) The bulging of a tubular part with a flexible plug. (b) Production of
fittings for plumbing by expanding tubular blanks under internal pressure.
3.3.4 DIMPLING, PIERCING AND FLARING
1. In dimpling, a hole first is punched and then expanded into a flange.
2. Flanges also may be produced by piercing with a shaped punch.
3. As the ratio of flange diameter to hole diameter increases, the strains
increase proportionately.
4. Depending on the roughness of the edge, there will be a tendency for
cracking along the outer periphery of the flange.
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5. To reduce this possibility, sheared or punched edges may be shaved off
with a sharp tool to improve the surface finish of the edge.
3.3.5 HEMMING AND SEAMING
1. In the hemming process (also called flattening), the edge of the sheet is
folded over itself.
2. Hemming increases the stiffness of the part, improves its appearance,
and eliminates sharp edges.
3.3.6 BULGING
1. This process involves placing a tubular, conical, or curvilinear part into a
split-female die and then expanding it, usually with a polyurethane plug as
shown in Figure 3.24.
2. The punch then is retracted, the plug returns to its original shape (by total
elastic recovery), and the formed part is removed by opening the split
dies.
3. The major advantages of using polyurethane plugs is that they are very
resistant to abrasion, wear, and lubricants; furthermore, they do no
damage the surface finish of the part being formed.
3.3.7 SEGMENTED DIES
1. These dies consist of individual segments that are placed inside the part
to be formed and expanded mechanically in a generally radial direction.
2. They then are retracted to remove the formed part.
3. Segmented dies are relatively inexpensive, and they can be used for large
production runs.
Example 3.3 Manufacturing of bellow
Explain the process of manufacturing of bellow.
Solution
Bellows are manufactured by a bulging process, as shown in Fig. 4.24. After the tube is
bulged at several equidistant locations, it is compressed axially to uniformly collapse the
bulged regions, thus forming bellows. The tube material must be able to undergo the large
strains involved during the collapsing process without developing cracks.
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Figure 3.24 Process of manufacturing of bellow.
3.3.8 STRETCH FORMING
1. In stretch forming, the sheet metal is clamped along its edges and then
stretched over a male die (form block or form punch).
2. Fig. 3.30 shows the Schematic illustration of a stretch-forming process.
Aluminum skins for aircraft can be made by this method.
3. In most operations, the blank is a rectangular sheet clamped along its
narrower edges and stretched lengthwise, thus allowing the material to
shrink in width.
4. Most applications require little or no lubrication.
Figure 3.25 Schematic illustration of a stretch-forming process.
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3.3.9 DEEP DRAWING
1. Numerous parts made of sheet metal are cylindrical or box shaped, such
as pots and pans, all types of containers for food and beverages,
stainless-steel kitchen sinks, canisters, and automotive fuel tanks.
2. Fig 4.26 shows the metal-forming processes involved in manufacturing a
two-piece aluminum beverage can.
3. The process generally is called deep drawing because of its capability for
producing deep parts.
4. It also is used to make parts that are shallow or have moderate depth and
is one of the most important metalworking processes because of the wide
use of products made.
5. Figure 4.27 shows Illustration of the deep-drawing process on a circular
sheet-metal blank and the process variables in deep drawing.
EXERCISE 3.3
Identify the factors that influence the deep-drawing force, F, in figure below, and explain
why they do.
Figure: Deep drawing
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Fig 3.26 Metal-forming processes involved in manufacturing a two-piece aluminum
beverage can.
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Figure 3.27 (a) Illustration of the deep-drawing process on a circular sheet-metal
blank. (b) Process variables in deep drawing.
3.4 SHEET METAL FORMING
1. In stamping, drawing, or pressing, a sheet is clamped around the edge and formed
into a cavity by a punch.
2. The metal is stretched by membrane forces so that it conforms to the shape of the
tools.
3. The membrane stresses in the sheet far exceed the contact stresses between the
tools and the sheet, and the through-thickness stresses may be neglected except at
small tool radius.
4. Figure 4.28 shows a stamping die with a lower counter-punch or bottoming die, but
contact with the sheet at the bottom of the stroke will be on one side only, between
the sheet and the punch or between the die and the sheet.
5. The edge or flange is not usually held rigidly, but is allowed to move inward in a
controlled fashion.
6. The tension must be sufficient to prevent wrinkling, but not enough to cause splitting.
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Figure 3.28 A schematic section of a typical stamping die. The sheet contacts only the
punch or the die at any point.
3.4.1 RUBBER FORMING & HYDROFORMING
1. Rubber forming is a process of bending and embossing of sheet metal using
rubber pad (Polyurethanes) as a replacement die (as female die).
2. Polyurethanes are used widely because of their:-
a. Resistance to cutting or tearing by burrs or other sharp edges on the sheet
metal.
b. Long fatigue life.
3. Fig 4.29 shows the examples of the bending and the embossing of sheet metal
with a metal punch and with a flexible pad serving as the female die.
Figure 3.29 Bending and embossing of sheet metal
4. In the hydroform or fluid-forming process, the pressure over the rubber membrane is
controlled throughout the forming cycle with a maximum pressure of up to 100 MPa.
5. Fig 4.30 shows the the hydroform (or fluid-forming) process. Note that in contrast to
the ordinary deep drawing process, the pressure in the dome forces the cup walls
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against the punch. The cup travels with the punch; in this way, deep drawability is
improved.
Figure 3.30 Process of hydroform (or fluid-forming).
6. In tube hydroforming, metal tubing is formed in a die and pressurized internally by a
fluid.
7. Fig 3.31 (a) shows the schematic illustration of the tube-hydroforming process. (b)
Example of tube-hydroformed parts. Automotive-exhaust and structural components,
bicycle frames, and hydraulic and pneumatic fittings are produced through tube
hydroforming.
Figure 3.31 (a) Schematic illustration of the tube-hydroforming process. (b) Example
of tube-hydroformed parts.
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8. When selected properly, rubber-forming and hydroforming processes have the
advantages of :
(a) the capability to form complex shapes;
(b) forming parts with laminated sheets of various materials and coatings;
(c) flexibility and ease of operation;
(d) the avoidance of damage to the surfaces of the sheet;
(e) low die wear;
(f) low tooling cost.
3.4.2 SPINNING
Spinning is a process which involves the forming of axisymmetric parts over a mandrel by
the use of various tools and rollers—a process that is similar to that of forming clay on a
potter’s wheel.
a. Conventional spinning
1. In conventional spinning, a circular blank of flat or preformed sheet metal is placed
and held against a mandrel and rotated while a rigid tool deforms and shapes the
material over the mandrel.
2. Fig 3.32 (a) shows the schematic illustration of the conventional spinning process. (b)
Types of parts conventionally spun. All parts are axisymmetric.
3. Although most spinning is performed at room temperature, thick parts and metals
with high strength or low ductility require spinning at elevated temperatures.
Figure 3.32 (a) The schematic illustration of the conventional spinning process.
(b) Types of parts conventionally spun.
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b. Shear spinning
1. Also known as power spinning, flow turning, hydrospinning, and spin forging, this
operation produces an axisymmetric conical or curvilinear shape, reducing the
sheet’s thickness while maintaining its maximum (blank) diameter.
2. Fig 3.33 (a) shows the schematic illustration of the shear-spinning process for
making conical parts. (b)The mandrel can be shaped so that curvilinear parts can be
spun.
3. The spinnability of a metal in this process generally is defined as the maximum
reduction in thickness to which a part can be subjected by spinning without fracture.
4. For metals with low ductility, the operation is carried out at elevated temperatures by
heating the blank in a furnace and transferring it rapidly to the mandrel.
Figure 3.33 (a) The schematic illustration of the shear-spinning. (b)The mandrel
can be shaped so that curvilinear parts can be spun and (c) Schematic illustrations of
the tube-spinning process.
c. Tube spinning
1. In tube spinning, the thickness of hollow, cylindrical blanks is reduced or shaped
by spinning them on a solid, round mandrel using rollers.
2. The reduction in wall thickness results in a longer tube.
3. Tube spinning can be used to make rocket, missile, and jet engine parts,
pressure vessels, and automotive components, such as car and truck wheels.
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EXERCISE 4.4
Explain why the drawability of a material is higher in the hydroform process than
in the deep-drawing process?
3.4.3 SUPERPLASTIC FORMING
1. Superplastic alloys can be formed into complex shapes by superplastic forming—
a process that employs common metalworking techniques—as well as by
polymer-processing techniques (such as thermoforming, vacuum forming, and
blow molding.
2. The very high ductility and relatively low strength of superplastic alloys offer the
following advantages:
a. Complex shapes can be formed out of one piece, with fine detail, close
tolerances, and elimination of secondary operations.
b. Weight and material savings can be realized because of the good
formability of the materials.
c. Little or no residual stresses develop in the formed parts.
d. Because of the low strength of the material at forming temperatures, the
tooling can be made of materials that have lower strength than those in
other metalworking processes, hence tooling costs are lower.
1. Diffusion bonding / superplastic forming
1. Fabricating complex sheet-metal structures by combining diffusion
bonding with superplastic forming (SPF/DB) is an important process,
particularly in the aerospace industry.
2. Fig 3.34 shows the types of structures made by diffusion bonding and
superplastic forming of sheet metals. Such structures have a high
stiffness-to-weight ratio.
3. The SPF/DB process improves productivity by eliminating mechanical
fasteners, and it produces parts with good dimensional accuracy and low
residual stresses.
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Figure 3.34 The types of structures made by diffusion bonding and superplastic
forming of sheet metals.
Example 3.5
Explain what are the applications of superplastic forming/diffusion bonding.
Solution:
The majority of applications for SPF/DB involve titanium parts for military aircraft,
such as the Tornado and the Mirage 2000. The components made include fuselage
bulkheads, leading-edge slats, heat-exchanger ducts, and cooler-outlet ducts. The
nozzle fairing of the F-15 fighter aircraft also is made by this process. In civilian
applications, the Airbus A340 has its water-closet, drain, and freshwater maintenance
panels (made of Ti-6Al-4V) manufactured by this process.
The superplastic forming process usually is carried out at about 900°C for titanium
alloys and at about 500°C for aluminum alloys; temperatures for diffusion bonding
are similar. However, the presence of an oxide layer on aluminum sheets is a
significant problem that degrades the bond strength in diffusion bonding. To illustrate
cycle times, 74 nickel-alloy sheets 2 mm in thickness were, in one application,
superplastically formed in ceramic dies at 950°C using argon gas at a pressure of 2
MPa. The cycle time was four hours.
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3.4.4 MANUFACTURING OF METAL HONEYCOMB STRUCTURES
1. A honeycomb structure consists basically of a core of honeycomb or other
corrugated shapes bonded to two thin outer skins.
2. Fig 4.35 shows the methods of manufacturing honeycomb structures: (a)
expansion process; (b) corrugation process; (c) assembling a honeycomb
structure into a laminate.
3. Because of their light weight and high resistance to bending forces, metal
honeycomb structures are used for aircraft and aerospace components, in
buildings, and in transportation equipment.
Fig 3.35 Methods of manufacturing honeycomb structures: (a) expansion process; (b)
corrugation process; (c) assembling a honeycomb structure into a laminate.
4. In the expansion process, which is the more common method, sheets first are
cut from a coil, and an adhesive is applied at intervals (node lines) on their
surfaces.
5. The corrugation process is similar to the process used in making corrugated
cardboard.
6. The sheet metal first passes through a pair of specially designed rolls, becoming
a corrugated sheet; it is then cut into desired lengths.
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EXERCISE 3.5
Many axisymmetric missile bodies are made by spinning. What other methods could you
use if spinning processes were not available?
SUMMARY
In this chapter we have studied that:
1. shearing operations process principally involves punch, die and slug;
2. major processing parameters in shearing are the shape of the punch and die, the
speed of punching, lubrication and the clearance between the punch and the die;
3. metohds of shearing operations include die cutting, fine blanking, slitting, steel
rules and niblling;
4. type of shearing dies include compound die, progressive die and transfer die;
5. characteristics of sheet metal forming includes elongation, yield point elongation,
anistropy, residual stresses, spring back, wringkling and quality of sheraed
edges;
6. formability test for sheet metal includes cupping tests and forming limit diagrams.
7. sheet metal bendingoperations principally invlove air bending, four slide machine,
roll bending, beading and flanging;
8. another method in sheet metal bending such as roll forming, tube bending and
forming, dimpling, piercing and flaring, hemming and seaming, bulging,
segmented dies and stretch forming;
9. process involve in deep drawing and parameters involve in deep darwing such
as deep drawability, earing, draw beads, ironing, redrawing, embossing;
10. sheet-metal forming processes are among the most versatile of all operations.
They generally are used on workpieces having high ratios of surface area to
thickness;
11. for general stamping operations, forming-limit diagrams are very useful, because
they establish quantitative relationships among the major and minor principal
strains that limit safe forming;
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12. for deep-drawing operations, the important parameter is the normal or plastic
anisotropy of the sheet (the ratio of width strain to thickness strain in tensile
testing).
REFERENCES
th
1. S. Kalpakjian, S.R. Schmid, Manufacturing Engineering and Technology 5 edition,
Prentice Hall, 2005.
2. Steve F.Krar Technology of Machine Tools, Mc Graw Hill, 2005.
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CHAPTER 4 : BULK DEFORMATION PROCESSES
4.1 INTRODUCTION
Metallic components can be shaped in a manner similar to the molding of pottery. The raw
material of a fundamental simpler shape is provided by a primary process like casting,
powder consolidation, earlier forming processes, or even by electric deposition. Metals
deform very much like soft clay or wax. Even in the solid state, permanent changes in shape
can be forced upon them by displacement of relative positions between neighboring material
points. To enforce these changes, external forces are applied. While soft plasticine can be
molded by tiny toddler’s fingers, for metal forming, specially constructed tooling, usually of
hard materials, are manipulated, sometimes by colossal machinery. A variety of bulk
deformation process, the equipment and tooling, and the concepts involved will be
discussed in this course. This will provide an understanding of the typical processes (rolling,
forging and extrusion) involved.
Figure 4.1 Some products of bulk defromation processes.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. define the rolling, forging and extrusion process;
2. identify the range of rolled, forged and extruded products;
3. describe the suitable materials and design for rolling, forging and extrusion process;
4. differentiate the types of defects in rolled, forged and extruded plates and sheets;
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5. analyze the suitable parameters for rolling, forging and extrusion process.
4.2 ROLLING OF METALS
1. Rolling is the process of reducing the thickness or changing the cross section of long
workpiece by compressive forces applied thought a set of rolls.
2. The metal is compressed between two rotating rolls for reducing its cross sectional
area into desired thickness.
3. This is one of the most widely used of all the metal working processes, because of its
higher productivity and low cost.
4. Rolling would be able to produce components having constant cross-section
throughout its length.
5. Many shapes such as I, T, L and channel sections are possible, but not very complex
shapes.
6. It is also possible to produce special section such as railway wagon wheels by rolling
individual pieces.
7. Rolling is normally a hot working process unless specifically mentioned as cold
rolling.
8. The metal is taken into rolls by friction and subsequently compressed to obtain the
final shape as shown in Figure 5.2.
9. The thickness of the metal that can be drawn into rolls depend on the roughness of
the roll surface.
10. Rougher rolls would be able to achieve greater reduction than smoother rolls as
shown in Figure 5.2. But, the roll surface gets embedded into the rolled metal thus
producing rough surface.
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Figure 4.2 Rolling processes.
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4.2.1 RANGE OF ROLLED PRODUCT
1) Structural shapes or section
Round, square bar, angle, channel, H and I beam and special section (rail). Some
structural shapes or section produced by rolling are shown in Figure 5.3.
Figure 4.3 Cross structures and shapes of rolled products.
2) Plate and sheets
1. Plates and sheet are produced in plate and sheet mills for the hot rolling of metal and
in cold reduction mill for the production of cold-rolled coils.
2. Plates, which generally having thickness 6mm – 300mm are used for structural
application such as ship hull, boiler (150mm), bridge, tanks (100-125mm) etc.
3. Sheet, generally less than 6mm thick and used for automobile bodies (0.3 mm),
appliance, container for food beverages (0.28mm), etc.
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3) Special-purpose rolled shapes
This group includes special shape, one-piece rolled wheels, rings, balls, ribbed tube and die
rolled section.Typical applications of rolled shape are gearwheel rims, roller bearing race,
flanges, etc. as shown in Figure 5.4.
Figure 4.4 The production of a railway car wheel; (left) sequence of stages; (right) wheel
in final stag in mill.
4) Seamless tubes
This hot working process for making long, thick wall seamless tubing and pipe. It is based
on the principle that when a round bar is subjected to radial compressive force, tensile
stress develop at the center of the bar; then when subjected to cyclic compressive stress, a
cavity begin to form at the center of the bar (Figure 5.5).
Figure 4.5 The production of seamless tubes by rolling.
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4.2.2 DEFECTS IN ROLLED PLATES AND SHEETS
Some defects appearance in rolled products shown in Figure 5.6.
Figure 4.6 Defects in rolled products.
Example 4.1
Rolling is one of the importance processes in manufacturing. In your own words, briefly
define the meaning of rolling.
Solution:
- A process of reducing the thickness or changing the cross-section of a long
workpiece.
- By compressive forces.
- Applied through a set of rolls.
EXERCISE 4.1
Identify four typical defects in flat rolling.
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4.3 FORGING OF METALS
Figure 4.7 Forging process.
1. Forging is the oldest of the metalworking processes.
2. The forging process consists of hammering or pressing metal in to a desired shape
as shown in Figure 5.7. This can be done with or without dies.
3. Parts can be forged "hot" (the metal is heated to just below the melting point) or
"cold" (the metal is at 1/3 the melting point temperature).
4. In today's forging industry most parts are hot forged.
5. Forging is the operation where the metal is heated and then a force is applied to
manipulate the metal in such a way that the required final shape is obtained.
6. This is the oldest of the metal working processes known to mankind since the copper
age. Forging is generally a hot working operation though cold forging is used
sometimes.
7. Figure 5.8 shows some of the forging products.
Figure 4.8 Some of the forging products.
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4.3.1 FORGING TYPES
There are four types of forging methods, which are generally used. They are:
1. Smith forging: This is the traditional forging operation done openly or in open dies by the
village blacksmith or modern shop floor by manual hammering or by power hammers.
2. Drop forging: This is the operation done in closed impression dies by means of drop
hammers. Here the force for shaping the component is applied in a series of blows.
3. Press forging: Similar to drop forging, the press forging is also done in closed impression
dies with the exception that the force is a continuous squeezing type applied by the
hydraulic presses.
4. Upset forging (Machine forging): Unlike the drop or press forging where the material is
drawn out, in machine forging, the material is only upset to get the desired shape.
4.3.2 FORGING CATEGORY
The three basic categories of forging are open die, impression die, and closed die.
1) Open Die Forging
1. The simplest form of open-die forging involves placing a solid cylindrical workpiece
between two flat dies and reducing its height by compressing it as shown in Figure
5.9.
2. The die surfaces be shaped therefore forming the ends of the cylindrical workpiece
while compressing it.
3. Figure 5.10 shows the forged product.
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Figure 4.9 Open die forging.
1. Solid cylindrical billet deformed between two flat dies.
2. Uniform deformation of the billet without friction.
3. Deformation with friction.
4. Note barreling of the billet caused by friction forces at the billet-die interface.
Figure 4.10 Forged part.
2) Impression Die Forging
1. The workpiece is forced to conform to the shape of the die cavities while it is being
compressed between the closing dies.
2. The closing of the die cavities occurs at high striking forces.
3. Some of the material flow radially outward and forms a flash.
4. The formation of this flash prevents further material from flowing in the radial
direction in the flash gap.
5. As the length-to-thickness ratio of the flash is high, it is subjected to high-pressure
that result in high frictional resistance to material flow.
6. Further, in the case of hot forging, the flash cools faster than the bulk of the
workpiece.
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7. This increases its resistance to deformation compared to the bulk. Thus the flash
plays a significant role in helping filling the die cavities.
3) Closed Die Forging
1. In closed-die-forging, no flash is formed and the workpiece is completely surrounded
by the dies.
2. Therefore, proper control of the volume of material is essential to obtain a forging of
desired dimensions.
3. Undersized blanks in close-die forging prevent the complete filling of the die, while
oversized blanks may cause premature die failure or jamming of the dies.
4.3.3 ADVANTAGES OF FORGING
1. The grain flow, i.e., the directional pattern that metal crystals assume during plastic
deformation, can be aligned with the directions of the principal stresses that will
occur when the workpiece is loaded in service (Figure 5.11).
2. Higher strength, ductility, and impact resistance are achieved along the grain flow of
the forged material than in the randomly oriented crystals of the cast metal or welded
metal.
3. Structural integrity from piece to piece is better. Good quality forging control makes it
easier to avoid internal pockets, voids, inclusions, laps, etc.
Figure 4.11 Forged parts have greater strength due the re-alignment of the grain pattern.
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4.3.4 DISADVANTAGES OF FORGING
1. The forged parts often need to be machined before use.
2. Forging tooling for complicated geometry may be expensive and require multiple
passes on the same workpiece.
4.3.5 FORGING MATERIAL
Following is a table of materials and their 'forgability' ranking. The ranking indicates the
material suitability for the forging process.
Material Ranking Material Ranking
Cast Iron 50 Titanium & Alloys 80
Carbon Steel 80 Nickel & Alloys 50
Alloy Steel 80 Refractory Metals 50
Stainless Steel 80 Thermoplastics 0
Aluminum & Alloys 100 Thermosets 0
Copper & Alloys 100 Ceramics 0
Zinc & Alloys 50 Photopolymers 0
Magnesium & Alloys 50 Wood (dry) 0
A value of zero means that the corresponding material is never used with this process, a
ranking of 100 means that it is excellent for use with this process.
5.3.6 OTHER FORGING PROCESS
1. Coining.
2. Piercing.
3. Roll Forging.
4. Skew Forging.
5. Drop Forging.
6. Press Forging.
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Example 4.2
Briefly explain, why is control of the volume of the blank important in closed-die
forging.
Solution:
1. Accurate control of the blank volume and proper die design are essential in order
to produce a forging with desired dimensional tolerance.
2. Undersized blanks produce uncompleted filling of the die cavity.
3. Oversized blanks generate excessive pressures and may cause die fail.
EXERCISE 4.2
In impression-die forging, during deformation, some of the material flows
outward and forms a flash. Explain the functions of flash in impression-die
forging.
4.4 EXTRUSION OF METALS
1. Extrusion is the process by which a block of metal is reduced in cross section by
forcing it through a die orifice under high pressure (Figure 5.12).
2. Extrusions have numerous applications in the manufacture of continuous as well
as discrete products from a wide variety of metals and alloys.
th
3. Metal extrusion was developed in the late 18 century for making lead pipe. The
basic process of forcing a round billet through a shaped die is still used today.
4. Modern variants can produce clad products in one go - e.g. copper clad with
silver.
5. In the extrusion process, the material is forced through a deforming die.
6. Drawing is related to extrusion but is used for smaller (round) where the cross-
section of solid rod, wire, or tubing is reduced or changed in shape by pulling it
through the die rather than pushing.
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Figure 4.12 Extrusion process.
4.4.4 GENERAL ASPECTS OF EXTRUSION PROCESS
1. Process where a solid plastic (resin), in the form of pellets, is continuously fed to
a heated chamber and carried along by a feed-screw within.
2. The feed-screw is driven via drive/motor and tight speed and torque control is
critical to product quality.
3. As it is conveyed it is compressed, melted, and forced out of the chamber at a
steady rate through a die.
4. The immediate cooling of the melt results in re-solidification of that plastic into a
continually drawn piece whose cross section matches the die pattern.
5. This die has been engineered and machined to ensure that the melt flows in a
precise desired shape.
6. Figure 4.13 shows the example of machine for extrusion
7. Three basic types of extrusion:
direct or forward extrusion;
indirect extrusion;
hydrostatics extrusion.
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Figure 4.13 The example of machine for extrusion.
a) DIRECT EXTRUSION PROCESS
1. Billet is placed in a chamber (container) and force through a die opening by a
hydraulically driven ram (pressing stem or punch) as shown in Figure 4.14.
2. The die opening may be round, or it may have various shapes, depending on the
extrusion shape desired.
3. The function of dummy block is to protect the tip of the pressing stem, particularly
in hot extrusion.
4. Figure 4.15 shows the direct extrusion for producing solid and hollow object.
5. Main problem about this type of process is that frictional forces are created
between the walls of the container and the metal (billet; this requires more force
and power to run the extrusion process).
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Figure 4.14 Illustrations of direct extrusion process.
Figure 4.15 Direct extrusion for producing solid (left) and hollow object (right).
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b) INDIRECT EXTRUSION PROCESSES
1. Also called reverse, inverted or backward extrusion.
2. The dies move toward the un-extruded billet is shown in Figure 5.16 and Figure
5.17.
3. The frictional forces between the container walls and the billet are lower and the
power required for extrusion is less than for direct extrusion.
4. In practice, a hollow ram carries a die, while the other end of the container is
closed with a plate.
5. Frequently, for indirect extrusion, the ram containing the die is kept stationary,
and the container with the billet is made to move.
Figure 4.16 Illustrations of indirect extrusion process.
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(a)
(b)
Figure 4.17 Indirect (backward) extrusion for producing solid (a) and hollow object (b)
c) HYDROSTATICS EXTRUSION PROCESSES
1. In this process, the chamber is filled with a fluid that transmits the pressure to the
billet, which is then extruded through the die as shown in Figure 4.18.
2. The billet is smaller in the diameter than a chamber and the pressure is transmitted
to the billet by the ram.
3. Unlike in direct extrusion, there is no friction to overcome along the container walls
because the billet is stationary with respect to the container.
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Figure 4.18 Illustrations of hydrostatics extrusion process.
4.4.5 HOT EXTRUSION
Figure 4.19 Hot extrusion products.
1. This type of process is used to minimize the forces seen by the tools and by the die.
2. Basically hot working process.
3. Done at high temperature (50-75% of melting point of the metal) and high pressure
(35-700 MPa).
4. High cost process.
5. Die wear can be excessive.
6. Cooling of the surface of the hot billet and die can result in highly non-uniform
deformation.
7. To reduce cooling of the billet and to prolong die life, extrusion die maybe preheated.
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8. Die wear can be excessive.
9. Cooling of the surface of the hot billet and die can result in highly non-uniform
deformation.
10. To reduce cooling of the billet and to prolong die life, extrusion die maybe preheated.
11. Figure 4.19 shows some of the hot extrusion products.
4.4.6 COLD EXTRUSION
1. General term that often denoted a combination of operations such as direct and
indirect extrusion and forging.
2. The process done at room temperature or slightly elevated temperatures.
3. Process can be used for most materials-subject to designing robust enough tooling
that can with stand the stresses created by extrusion
4. Cold extrusion process is done below the re-crystallization temperature of the
material used.
5. This process is typically used to make parts with hollow cross-sections as shown in
Figure 4.20.
6. Used widely for components in automobiles, motorcycles, bicycles.
7. Examples of the metals that can be extruded are lead, tin, aluminum alloys, copper,
titanium, molybdenum, vanadium, and steel.
8. Figure 4.21 shows example of cold extrusion.
Figure 4.20 Some components produced by cold extrusion.
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Figure 4.21 Examples of Cold Extrusion. The arrows indicate the direction of metal flow
during extrusion.
4.4.7 THE ADVANTAGES OF COLD EXTRUSION
1. No oxidation takes place, and provided that lubrication is effective.
2. Good mechanical properties due to cold working, as long as the temperatures
created are below the re-crystallization temperature.
3. Good surface finish with the use of proper lubricants.
4. Good control of dimensional tolerances, reducing the need for subsequent machining
or finishing operations.
5. Production rates and costs that is competitive with those of other methods of
producing the same part.
6. Some machines are capable of producing more than 2000 parts per hours.
4.4.8 PRINCIPAL VARIABLES, WHICH INFLUENCE THE FORCES REQUIRED
TO CAUSE EXTRUSION
1. The type of extrusion.
2. The extrusion ratio.
3. The working temperature.
4. The speed of deformation.
5. The frictional condition.
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4.4.6 EXTRUSION FORCE
Controlled by:
1. The strength of the billet materials.
2. The extrusion ratio.
3. Friction between the billet and the chamber and die surfaces.
4. Process variables such as the temperature of the billet and the speed of extrusion.
Extrusion force, F can be estimated from the formula:
F=A k ln (Ao/Af)
0
where:
k = extrusion constant (Figure 5.22)
A = billet area
0
A = extruded product area
f
Figure 4.22 The K values for several metals for a range of extrusion.
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4.4.7 CRITERIA OF DIE DESIGN
1. The die is usually a piece of steel with the shape of the desired part machined into it.
2. Very tight tolerance.
3. Correct die geometry from the beginning.
4. Carefully finish the land surface.
5. Proper design (giving maximum die life).
6. Rational production resulting in low die manufacturing cost.
4.4.8 DIFFERENCE BETWEEN GOOD AND POOR DIE
1. Figure 4.23 shows several characteristics that explain the differences between poor
and good die.
2. Some of the criterias required for a good dies are eliminating sharp corners and
keeping section thicknesses uniform.
Figure 4.23 The differences between poor and good die.
5.4.9 TYPICAL DIE OF EXTRUSION
Typical dies for extrusion processes are shown in Figure 4.24. Some of them are:
a) Die for nonferrous metals.
b) Die for ferrous metals.
c) Die for T-shaped extrusion.
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Figure 4.24 (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for a T-shaped
extrusion.
4.4.10 CRITERIA OF DIE MATERIAL
1. High strength.
2. High stability.
3. High resistance to softening.
4. High resistance to cracking.
5. Medium resistance to wear.
6. High toughness.
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Example 4.3
Explain why cold extrusion is an important manufacturing process and give its advantages
over hot extrusion.
Solution:
Cold extrusion is an important because:
Cold extrusion is capable of economically producing discrete parts in various shapes
with good mechanical properties and dimensional tolerances.
The advantages of cold extrusion over hot extrusion are:
i) Improved mechanical properties resulting from work hardening.
ii) Good control of dimensional tolerances.
iii) Improved surface finish.
iv) Production rates and costs that is competitive with those other
methods of producing.
EXERCISE 4.3
Identify the factors that control the efficiency of metal extrusion.
SUMMARY
In this chapter we studied that:
1. rolling is the process of reducing the thickness or changing the cross-section of a
long strip by compressive forces applied through a set of rolls;
2. in addition to flat rolling, shape rolling is used to make products with various cross-
sections;
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3. products made by rolling include: (a) plate, sheet, foil, rod, seamless pipe, and
tubing; (b) shape-rolled products, such as I-beams and structural shapes; and (c)
bars of various cross-section;
4. rolling may be carried out at room temperature (cold rolling) or at elevated
temperatures (hot rolling).;
5. forging process is deformation of the workpiece is carried out by compressive forces
typically applied through a set of dies;
6. forging is capable of producing a wide variety of structural parts with favourable
characteristics, such as strength, toughness, dimensional accuracy, and reliability in
service;
7. the forging process can be carried out at room, warm, or high temperatures;
8. extrusion is the process of forcing a billet through a die to reduce its cross-section or
to produce various solid or hollow cross-sections. This process generally is carried
out at elevated temperatures in order to reduce forces and improve the ductility of
the material;
9. important factors in extrusion are die design, extrusion ratio, billet temperature,
lubrication, and extrusion speed.
REFERENCES
1. Kalpakjian S. And Schmid S. (2006). Engineering And Technology Fifth Edition in SI
Units, Pearson Education, Inc., Upper Saddle River, New Jersey.
2. Steve F.Krar Technology of Machine Tools, Mc Graw Hill, 2005.
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CHAPTER 5 : JOINING PROCESSES
5.1 INTRODUCTION
When we inspect the vast numbers of product around us, we soon realize that almost all of
them are assemblages of components that were manufacture as individual parts. Joint two
or more elements to make a single part is termed as fabrication process. There are large
number of industrial component are made by joining process. Common examples are
machine components, ship bodies, cars, etc. One of the best joining process that applied
widely in industries is welding, brazing, soldering and adhesive bonding.
Figure 5.1 Large number of industrial componentS are made by joining process.
LEARNING OBJECTIVES
The objectives of this unit are to :
1. define the welding process;
2. describe the type of welding and joining;
3. identify the factors affecting the welding and joining processes;
4. select the right welding process to join or cut the right product.
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5.2 FUSION WELDING
1. Fusion welding is defined as melting together and coalescing materials by means of
heat.
2. Filler metals (which are metals added to the weld area during welding) may be used.
3. Fusion welds made without the use of filler metals are known as autogenous welds.
4. Table 5.1 shows the characteristics of several fusion welding processes.
Table 5.1 General characteristics of fusion welding processes.
5.2.2 SHIELDED METAL ARC WELDING (SMAW)
1. SMAW is based on providing an electric circuit comprising the electric current source
the feed and return path, the electrode and the workpiece.
2. The arc welding process involves the creation of a suitable small gap between the
electrode and the workpiece.
3. When the circuit is made a large current flows and an arc is formed between the
electrode and the workpiece. The resulting high temperatures (6,000°C) causing the
workpiece and the electrode to melt.
4. The electrode is consumable. It includes metal for the weld, a coating that burns off
to form gases, which shield the weld from the air and flux, which combines with the
nitrides and oxide generated at the weld. When the weld solidifies a crust is formed
from the impurities created in the weld process (slag). This is easily chipped away.
5. Localized melting is obtained using the heat from an electric arc established between
a consumable metal electrode and the work material.
6. Electrode, which is coated, act as filler material. The coating decomposes in the arc,
providing a gas shield around it, as well as forming a protective slag that prevents
oxidation and other contaminant of the weld.
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7. Figure 5.2 shows the schematic illustration of SMAW.
Figure 5.2 Schematic illustration of SMAW.
Example 5.1
Describe the characteristics of Shield Metal Arc Welding (SMAW).
Solution:
Process characteristic:
1. Uses a consumable rod electrode.
2. Deposits slag on the weld bead.
3. Provides shielding by vaporization of the flux coating on the electrode.
4. Supplies constant welding current.0
5. Weld appearance and quality depend on operator skill in maintaining a constant arc
length and travel speed.
5.2.3 SUBMERGED ARC WELDING (SAW)
1. Submerged arc welding uses a bare metal wire electrode.
2. The arc is protected by a separate supply of loose flux, some of which melts and
forms a slag cover over weld.
3. During welding, the bare metal wire electrode is fed automatically into the blanket of
flux.
4. There are minimal fumes, no visible arcing and the equipment is easy to use.
5. Submerged arc welding is popular in the heavy metal fabrication industry because it
gives welds of high quality.
6. It is used to make pipes, pressure vessels, boilers, tankers and other structures that
require welding in a straight and continuous line.
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7. The heat from the arc melts and fuses the base and filler metals.
8. The arc is protected from atmospheric contamination by the blanket of flux that also
prevents weld spatter, arc noise and fumes.
9. The speed of the process helps to keep distortion to a minimum.
10. Figure 5.3 shows the equipment of SAW while Figure 5.4 shows the SAW process.
Figure 5.3 Submerged arc welding equipment.
Figure 5.4 Submerged arc welding process.
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5.2.4 GAS METAL-ARC WELDING (GMAW)
1. Gas Metal-Arc Welding (GMAW) also known as metal inert gas (MIG). Figure 5.5
shows the illustration of GMAW process and basic equipment used in gas metal-arc-
welding operations.
2. The Metal Inert Gas Process uses a consumable electrode of wire form and an inert
gas shield of carbon dioxide when welding carbon steel.
3. The wire electrode provides a continuous feed of filler metal allowing welds of any
length without stopping. This consumable electrode is in the form of continuous filler
metal, which is fed through special gun.
4. This process is an electric arc welding process in which is an arc is struck between a
consumable wire electrode and the workpiece to be welded.
5. The inert gas shield eliminates slag and allows cleaner and stronger weld, an inert
gas surrounds the arc and shields it from the ambient to prevent oxidation.
6. The following gases are used: Argon (Ar), helium (He) and carbon dioxide. CO2 is
used extensively to weld steel, as it is the cheapest gas. Ar and He are mainly used
in the welding of aluminium, magnesium and stainless steel.
7. Carbon steels, low alloy steels, stainless steels, most aluminium alloys, zinc based
copper alloys can be welded using this process.
8. MIG welding causes a lot of spatter that needs to be sanded or filed, if cosmetically
objectionable. Thus, it is best to avoid MIG welding on exterior surfaces if cosmetics
are important.
9. The industries served include shipbuilding, general engineering (pressure vessel
tanks, pipes) and automotive industries.
10. This process is used widely for automated welding using robots.
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(a)
(b)
Figure 5.5 (a) Gas metal-arc-welding process, formerly known as MIG welding (for metal
inert gas). (b) Basic equipment used in gas metal-arc-welding operations.
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5.2.5 GAS TUNGSTEN-ARC WELDING (GTAW)
1. Gas Tungsten-arc Welding (GTAW) also called Tungsten Inert Gas (TIG) welding.
Figure 5.6 shows the GTAW process and equipment for gas tungsten-arc-welding
operations.
2. The Tungsten Inert gas (TIG) system uses a non-consumable electrode of tungsten
and also provides an inert gas shield of argon or helium.
3. An arc is truck between a tungsten electrode (non-consumable) and the sheet metal
to be welded. The tungsten electrode is not consumed because of its extremely high
melting point.
4. An inert gas, Typically Ar or He shields the arc from the ambient to prevent oxidation
and contamination on molten metal.
5. A filler material is optional; depending on the type of weld and the thickness of weld,
generally bare wire, and electrode size range from 1.6 mm to 5.0 mm. However, it is
not usually required for thin material.
6. Carbon steels, low alloy steels, stainless steels, most aluminium alloys, zinc based
copper alloys can be welded using this process.
7. TIG is quite suitable for welding dissimilar materials, but usual cautions of galvanic
corrosion still apply.
8. The TIG process is a slower process compared to the MIG process, but the quality of
weld is cosmetically better.
EXERCISE 5.1
Describe the characteristics of Gas Tungsten Arc Welding (GTAW).
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(a)
(b)
Figure 5.6 (a) Gas tungsten-arc-welding process, formerly known as TIG welding (for
tungsten inert gas). (b) Equipment for gas tungsten-arc-welding operations.
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5.3 RESISTANCE WELDING
1. Resistance welding is a process in which contacting metal surface are joined by the
heat obtained from resistance to electric current flow.
2. Equipped with water-cooled to keep the temperature low.
3. This process forms a small nugget at the interface of the two plates (Figure 5.7 (a))
4. The maximum thickness is about 3-4 mm.
5. Welding quality is dependent on the cleanliness of the material, free of dirt, scale and
other contaminants.
5.3.2 PROCESS SCHEMATIC
There are three stages in making spot weld (Figure 5.7 (b)):
1. Electrodes a brought together again the metal and pressure is applied.
2. Current is tuned on momentarily.
3. Hold stage, in which current is tuned off but pressure is continued, this hold time
forges the metal while cooling.
(a)
(b)
Figure 5.7 Welding small nugget area (a) Resistance welding process (b).
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Example 5.2
Describe the process characteristics of resistance welding.
Solution:
Process characteristics:
1. Requires no filler metal or fluxes.
2. Can achieve high speed production.
3. Is easily automated.
4. Does not require skilled operators.
5. Is used primarily on sheet metal.
6. Uses non-consumable, low resistance, copper allow electrodes.
5.4 BRAZING
Figure 5.8 Brazing process.
1. Brazing is the coalescence of a joint with the help of filler metal (Figure 5.8).
2. Braze welding joins two pieces of metal (may be dissimilar) where the base metals
are not heated to the melting point. A brazing rod of lower melting temperature is
used to bond the two base metals.
3. Filler metal liquidus temperature is above 450°C and is below the solidus
temperature of the base metal.
4. The filler metal is drawn into the joint by means of capillary action (entering of fluid
into tightly fitted surface).
5. This process where base metal is not melted, but joint is obtained by means of a filler
metal.
6. Sequence of brazing process shown in Figure 5.9
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