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CHAPTER 6 Bulk Deformation Processes ( 부피성형가공법)

Bulk Deformation Processes - Manufacturing

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Page 1: Bulk Deformation Processes - Manufacturing

CHAPTER 6

Bulk Deformation Processes(부피성형가공법)

Page 2: Bulk Deformation Processes - Manufacturing

Forging(a) (b)

Figure extra (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.

Page 3: Bulk Deformation Processes - Manufacturing

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

(c)

Page 4: Bulk Deformation Processes - Manufacturing

Grain Flow Comparison

Figure extra A part made by three different processes, showing grain flow. (a) casting, (b) machining, (c) forging. Source: Forging Industry Association.

VIDEO

Page 5: Bulk Deformation Processes - Manufacturing

Outline of Forging and Related Operations

Page 6: Bulk Deformation Processes - Manufacturing

Homologous Temperature Ranges for Various Processes

TABLE 3.2 Process T/Tm Cold working Warm working Hot working

< 0.3 0.3 to 0.5 > 0.6

T/Tm > 0.5 Recrystallization

VIDEO

Page 7: Bulk Deformation Processes - Manufacturing

Characteristics of Forging ProcessesTABLE 6.1 Process Advantages Limitations Open die Simple, inexpensive dies; useful for small

quantities; wide range of sizes available; good strength characteristics

Limited to simple shapes; difficult to hold close tolerances; machining to final shape necessary; low production rate; relatively poor utilization of material; high degree of skill required

Closed die Relatively good utilization of material; generally better properties than open-die forgings; good dimensional accuracy; high production rates; good reproducibility

High die cost for small quantities; machining often necessary

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 type; high production rates; good utilization of material

Somewhat higher die cost than blocker type

Precision type Close tolerances; machining often unnecessary; very good material utilization; very thin webs and flanges possible

Requires high forces, intricate dies, and provision for removing forging from dies

Page 8: Bulk Deformation Processes - Manufacturing

Upsetting

Figure 6.1 (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.

(Open-die forging)

Page 9: Bulk Deformation Processes - Manufacturing

Figure 6.2 Schematic illustration of grid deformation in upsetting: (a) original grid pattern; (b) after deformation, without friction; (c) after deformation, with friction. Such deformation patterns can be used to calculate the strains within a deforming body.

VIDEO

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Figure 6.3 Grain flow lines in upsetting a solid steel cylinder at elevated temperatures. Note the highly inhomogeneous deformation and barreling. The different shape of the bottom section of the specimen (as compared to the top) results from the hot specimen resting on the lower cool die before deformation proceeded. The bottom surface was chilled; thus it exhibits greater strength and hence deforms less than the top surface. Source: J. A. Schey, et al., IIT Research Institute.

Page 11: Bulk Deformation Processes - Manufacturing

h xσ

h

Figure 6.4 Stresses on an element in plane-strain compression (forging) between flat dies. The stress is assumed to be uniformly distributed along the height of the element. Identifying the stresses on an element (slab) is the first step in the slab method of analysis for metalworking processes.

Method of analysis: Slab method

Page 12: Bulk Deformation Processes - Manufacturing

Slab analysis:

).()2()(

).1(

].1[

.,

.2

.

.3

2

.02

02)(

'

/)(2''

/)(2'

/2'

/2

'

widthapFhaYp

eYY

eYpeYC

Ceordxh

d

dd

YY

dxh

d

hdxhd

av

av

hxayx

hxay

ha

hxy

y

y

xy

xy

yx

xyxx

=

+≅

−=−=

==

=

=−=

=

==−

=+

=−++

µ

σσ

σ

σµσσ

σσ

σσ

µσσ

σµσσσ

µ

µ

µ

µ

Force equilibrium

Page 13: Bulk Deformation Processes - Manufacturing

ρ

'Y

Figure 6.5 Distribution of die pressure p in plane-strain compression with sliding friction. Note that the pressure at the left and right boundaries is equal to the yield stress in plane strain, Sliding friction means that the frictional stress is directly proportional to the normal stress.

hxay eYp /)(2' −== µσ

•Friction hill The pressure is higher than it is without friction

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Figure 6.6 Normal stress (pressure) distribution in the compression of a rectangular workpiece with sliding friction under conditions of plane stress using the distrotion-energy criterion. Note that the stress at the corners is equal to the uniaxial yield stress of the material,

.Y

•Corners: Uniaxial compression The pressure is Y

•Edges: Friction Friction hill along the edges

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Figure 6.7 Increase in contact area of a rectangular specimen (top view) compressed between flat dies with friction. Note that the length of the specimen has increased proportionately less than its width. Likewise, a specimen in the shape of a cube acquires the shape of a pan-cake after deformation with friction.•Length 40% increase

•Width: Less resistance to material flow 230% increase

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Figure 6.8 Stresses on an element in forging of a solid cylindrical workpiece between flat dies. Compare with Fig. 6.4. Source: (a) After J. F. W. Bishop, J. Mech. Phys. Solids, vol. 6, 1958, pp. 132-144. (b) Adapted from W. Schroeder and D. A. Webster, Trans. ASME, vol. 71, 1949, pp. 289-294.

Forging of a solid cylindrical workpieces: Slab method

Page 17: Bulk Deformation Processes - Manufacturing

Y ⎟⎠⎞

⎜⎝⎛ +≅→= −

hr

YpYep averagehxr

32

1/)(2 µµ

Y

Figure 6.9 Ratio of average die pressure to yield stress as a function of friction and aspect ratio of the specimen: (a) plane-strain compression; and (b) compression of a solid cylindrical specimen. Note that the yield stress in (b) is , and not as in plane-strain compression in (a).

⎟⎠⎞

⎜⎝⎛ +≅→= −

hrYpYep average

hxr

321/)(2 µµ

⎟⎠⎞

⎜⎝⎛ +≅→= −

haYpeYp average

hxa µµ 1'/)(2'

'Y

Page 18: Bulk Deformation Processes - Manufacturing

Figure 6.10 Normal stress (pressure) distribution in the compression of a rectangular specimen in plane strain and under sticking condition. The pressure at the edges is the uniaxial yield stress of the material in plane strain, .

'Y

Forging under sticking condition •Sticking: µp=k (shear yield stress)

⎟⎠⎞

⎜⎝⎛ −+=

hxaYp 1'

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Figure 6.11 Plastic deformation in forging as predicted by the finite-element method of analysis. Source: After T. Altan, et al.

•Finite Element Analysis

•Solid cylindrical workpiece

•Quarter of the specimen

•Microstructural change

•Temperature distribution

•Onsets of defects

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Figure 6.12 Die pressures required in frictionless plane-strain conditions for a variety of metalworking operations. The geometric relationship between contact area of the dies and workpiece dimensions is an important factor in predicting forces in plastic deformation of materials. Source: After W. A. Backofen, Deformation Processing, (Fig. 7.1, p. 135), © 1972, Addison-Wesley Publishing Company, Inc. Reprinted by permission of Addison Wesley Longman, Inc.

Deformation- zone geometry

Frictionless forging: slip-line analysis

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Lh /

Figure 6.13 Examples of plastic deformation processes in plane strain showing the ratio. (a) Indenting with flat dies. This operation is similar to cogging, shown in Fig. 6.19. (b) Drawing or extrusion of strip with a wedge-shaped die, described in Sections 6.4 and 6.5. (c) Ironing. See also Fig. 7.57. (d) Rolling, described in Section 6.3. As shown in Fig. 6.12, the larger the ratio, the higher the die pressure becomes. In actual processing, however, the smaller this ratio, the greater is the effect of friction at the die-workpieceinterfaces. The reason is that contact area, hence fiction, increases with decreasing ratio.

Lh /

Lh /

Lh /

Deformation-zone geometry vs. process

Page 22: Bulk Deformation Processes - Manufacturing

Toccata, Bach

Namche, 3550m, gateway to Everest, Nepal

Page 23: Bulk Deformation Processes - Manufacturing

Impression-Die Forging

Figure 6.14 Stages in impression-die forging of a solid round billet. Note the formation of flash, which is excess metal that is subsequently trimmed off.

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Range of k Values for Equation F=kYfA

TABLE 6.2 Simple shapes, without flash 3–5 Simple shapes, with flash 5–8 Complex shapes, with flash 8–12

•F: Impression-die forging load

•k: pressure-multiflying factors

•Yf: flow stress of the material

•A: projected area of forging (including the flash)

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Figure 6.15 Typical load-stroke curve for impression-die forging. Note the sharp increase in load after the flash begins to form. In hot-forging operations, the flash requires high levels of stress because it is thin, that is, small h, and cooler than the bulk of the forging. Source: After T. Altan.

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Comparison of Forging With and Without Flash

Figure extra Comparison of impression-die forging to closed-die forging (precision or flashless forging) of a cylindrical billet. Source: H. Takemasu, V. Vazquez, B. Painter, and T. Altan.

Impression-die

Closed-die

Page 27: Bulk Deformation Processes - Manufacturing

Orbital Forging

Figure 6.16 (a) Various movements of the upper die in orbital forging (also called rotary, swing, or rocking-die forging); the process is similar to the action of a mortar and pestle. (b) An example of orbital forging. Bevel gears, wheels, and rings for bearings can be made by this process.

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CoiningFigure extra (a) Schematic illustration of the coining process. the earliest coins were made by open-die forging and lacked sharp details. (b) An example of a coining operation to produce an impression of the letter E on a block of metal.

VIDEO

Miscellaneous forging operations

Page 29: Bulk Deformation Processes - Manufacturing

Heading/Upset Forging

Figure 6.17 (a) Heading operation, to form heads on fasteners such as nails and rivets. (b) Sequence of operations to produce a bolt head by heading.

VIDEO

Page 30: Bulk Deformation Processes - Manufacturing

Grain Flow Pattern of Pierced Round Billet

Figure 6.18 A pierced round billet, showing grain flow pattern. Source: Courtesy of Ladish Co., Inc.

•Piercing

•Cavity

•Impression

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Cogging

Figure 6.19 Two views of a cogging 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 workpiece.

Drawing out

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Roll-Forging

Figure 6.20 Two examples of the roll-forging operation, also known as cross-rolling. Tapered leaf springs and knives can be made by this process. Source: (a) J. Holub; (b) reprinted with permission of General Motors Corporation.

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Production of Bearing Blanks

Figure 6.21 (a) Production of steel balls by the skew-rolling process. (b) Production of steel balls by upsetting a cylindrical blank. Note the formation of flash. The balls made by these processes are subsequently ground and polished for use in ball bearings.

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Defects in Forged Parts

Figure 6.23 and 6.24 Examples of defects in forged parts. (a) Labs formed by web buckling during forging; web thickness should be increased to avoid this problem. (b) Internal defects caused by oversized billet; die cavities are filled prematurely, and the material at the center flows past the filled regions as the dies close.

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Figure 6.25 Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects. Source: ALCOA.

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Figure 6.26 Mechanical properties of five tensile-test specimens taken at various locations and directions in an AZ61 magnesium alloy forging. Note the anisotropy of properties caused by inhomogeneous deformation during forging. Source: After S. M. Jablonski, Modern Matals, vol. 16, 1963, pp. 62-70

Page 37: Bulk Deformation Processes - Manufacturing

Forging a Connecting Rod

Figure 6.27 (a) Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash required to ensure proper filling of the die cavities. (b) Fullering, and (c) edging operations to distribute the material when preshaping the blank for forging.

Desugn of forging dies

Page 38: Bulk Deformation Processes - Manufacturing

Figure 6.28 Intermediate stages in forging a crankshaft. These intermediate stages are important for distributing the material and filling the die cavities properly.

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Trimming Flash from a Forged Part

Figure 6.29 Trimming flash from a forged part. Note that the thin material at the center is removed by punching.

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Impression-Forging Die and Die Inserts

Figure extra Die inserts used in dies for forging an automotive axle housing. Source: Metals Handbook, Desk Edition. ASM International, Metals Park, Ohio, 1985. Used with permission.

Figure 6.29 Standard terminology for various features of a typical impression-forging die.

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Classification of Metals in Decreasing Order of Forgeablilty

TABLE extra Metal or alloy Approximate range of hot

forging temperature (°C) Aluminum alloys Magnesium alloys Copper alloys Carbon and low–alloy steels Martensitic stainless steels Austenitic stainless steels Titanium alloys Iron-base superalloys Cobalt-base superalloys Tantalum alloys Molybdenum alloys Nickel-base superalloys Tungsten alloys

400–550 250–350 600–900

850–1150 1100–1250 1100–1250

700–950 1050–1180 1180–1250 1050–1350 1150–1350 1050–1200 1200–1300

Forgeability: capability to undergo deformation by forging without cracking

Page 42: Bulk Deformation Processes - Manufacturing

Speed Range of Forging Equipment

TABLE extra Equipment m/s Hydraulic press Mechanical press Screw press Gravity drop hammer Power drop hammer Counterblow hammer

0.06–0.30 0.06–1.5 0.6–1.2 3.6–4.8 3.0–9.0 4.5–9.0

Page 43: Bulk Deformation Processes - Manufacturing

Principles of Various Forging Machines

Figure 6.30 Schematic illustration of the principles of various forging machines. (a) Hydraulic press. (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. (continued)

Page 44: Bulk Deformation Processes - Manufacturing

Figure 6.30 (continued) Schematic illustration of the principles of various forging machines. (c) Knuckle-joint press. (d) Screw press. (e) Gravity drop hammer.

Principles of Various Forging Machines (cont.)

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Unit Cost in Forging

Figure extra Typical unit cost (cost per piece) in forging; note how the setup and the tooling costs per piece decrease as the number of pieces forged increases, if all pieces use the same die.

Page 46: Bulk Deformation Processes - Manufacturing

Relative Unit Costs of a Small Connecting Rod

Figure extra Relative unit costs of a small connecting rod made by various forging and casting processes. Note that, for large quantities, forging is more economical. Sand casting is the more economical process for fewer than about 20,000 pieces.

Page 47: Bulk Deformation Processes - Manufacturing

Tannhaeuser, Wagner

Hillery school, Khumjung, 3900m, Nepal

Page 48: Bulk Deformation Processes - Manufacturing

Rolling of Metals

Page 49: Bulk Deformation Processes - Manufacturing

Flat- and Shape-Rolling Processes

Figure 6.31 Schematic outline of various flat- and shape-rolling processes. Source: American Iron and Steel Institute.

VIDEO

Page 50: Bulk Deformation Processes - Manufacturing

Grain Structure During Hot RollingFigure 6.32 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.

T/Tm > 0.6 Recrystallization

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Flat-Rolling

Figure 6.33 and 6.34 (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 Fig. 6.40.

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Four-High Rolling Mill

Figure extra Schematic illustration of a four-high rolling-mill stand, showing its various features. The stiffnesses of the housing, the rolls, and the roll bearings are all important in controlling and maintaining the thickness of the rolled strip.

Page 53: Bulk Deformation Processes - Manufacturing

Figure 6.35 Stresses on an element in rolling: (a) entry zone and (b) exit zone.

Page 54: Bulk Deformation Processes - Manufacturing

Figure 6.36 Pressure distribution in the roll gap as a function of the coefficient of friction. Note that, as friction in crease, the neutral point shifts toward the entry. Without friction, the rolls slip and the neutral point shifts completely to the exit.

H

ff

HHf

ehhYp

zoneexit

ehh

Yp

zoneentry

µ

µ

'

)(

0

'

:

:

0

=

= −

•Neutral point: shifts toward the exit as friction decreases

Page 55: Bulk Deformation Processes - Manufacturing

Figure 6.37 Pressure distribution in the roll gap as a function of reduction in thickness. Note the increase in the area under the curves with increasing reduction in thickness, thus increasing the roll-separating force.

•Increase of reduction

•Increase of contact length

•Increase of peak pressure

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Figure 6.38 Pressure distribution as a function of front and back tension. Note the shifting of the neutral point and the reduction in the area under the curves with increasing tension.

•Front tension: controlled by the torque of the coiler (delivery reel)

•Back tension: controlled by a braking system of the uncoiler (payoff reel)

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Roll Bending

Figure 6.39 (a) Bending of straight cylindrical rolls, caused by the roll force. (b) Bending of rolls ground with camber, producing a strip with uniform thickness.

the diameter at the center is slightly larger than at the edges: less than 0.25 mm on radius

Page 58: Bulk Deformation Processes - Manufacturing

Spreading of a Strip

Figure 6.40 Increase in the width (spreading) of a strip in flat rolling. Similarly, spreading can be observed when dough is rolled with a rolling pin.

•Width-to-thickness ratio

•Friction

•Ratios of roll radius-to-strip thickness

Page 59: Bulk Deformation Processes - Manufacturing

Roller Leveling and Defects in Flat Rolling

Figure 6.41 Schematic illustration of typical defects in flat rolling: (a) wavy edges; (b) zipper cracks in the center of the strip; (c) edge cracks; and (d) alligatoring.

VIDEO

Page 60: Bulk Deformation Processes - Manufacturing

Residual Stresses in Rolling

Figure 6.42 (a) Residual stresses developed in rolling with small rolls or at small reductions in thickness per pass. (b) Residual stresses developed in rolling with large rolls or at high reductions per pass. Note the reversal of the residual stress patterns.

large deformation

Page 61: Bulk Deformation Processes - Manufacturing

Figure 6.43 A method of roller leveling to flatten rolled sheets. See also Fig 6.74.

Page 62: Bulk Deformation Processes - Manufacturing

Rolling Mill

Figure extra A general view of a rolling mill. Source: Inland Steel.

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Backing Roll Arrangements

Figure 6.44 and 6.45 Schematic illustration of various roll arrangements: (a) two-high; (b) three- high; (c) four-high; (d) cluster (Sendzimir) mill.

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Tandem Rolling

Figure extra A tandem rolling operation.

Page 65: Bulk Deformation Processes - Manufacturing

Shape Rolling

Figure 6.46 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.

Page 66: Bulk Deformation Processes - Manufacturing

Ring-Rolling

Figure 6.47 (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.

Page 67: Bulk Deformation Processes - Manufacturing

Thread-Rolling Figure 6.49 (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 flow.

Figure 6.48 Thread-rolling 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 production.

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Mannesmann Process

Figure 6.50 Cavity formation in a solid round bar and its utilization in the rotary tube piercing process for making seamless pipe and tubing. (The Mannesmann mill was developed in the 1880s.)

•Compression on round bar: tensile stress at the center

•Cycling compressive stress: cavity at the center

Page 69: Bulk Deformation Processes - Manufacturing

Tube-Rolling

Figure extra Schematic illustration of various tube-rolling processes: (a) with fixed mandrel; (b) with moving mandrel; (c) without mandrel; and (d) pilger rolling over a mandrel and a pair of shaped rolls. Tube diameters and thicknesses can also be changed by other processes, such as drawing, extrusion, and spinning.

Page 70: Bulk Deformation Processes - Manufacturing

Spray Casting (Osprey Process)

Figure extra Spray casting (Osprey process), in which molten metal is sprayed over a rotating mandrel to produce seamless tubing and pipe. Source: J. Szekely, Scientific American, July 1987.

UO Process: VIDEO

Page 71: Bulk Deformation Processes - Manufacturing

Hungarian dance, Brahms

A typical trekker lodge in Nepal, Khumjung, 3900 m

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Extrusion and Drawing of Metals

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Direct Extrusion

Figure extra Schematic illustration of the direct extrusion process.

Wall friction between billet and container

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Types of Extrusion

Figure 6.51 Types of extrusion: (a) indirect; (b) hydrostatic; (c) lateral.

•No wall friction between billet and container: (a), (b)

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Extrusions

Figure 6.52 Extrusions, and examples of products made by sectioning off extrusions. Source: Kaiser Aluminum.

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Types of Metal Flow in Extruding With Square Dies

Figure 6.53 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 cooling 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.

VIDEO high-shear area

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Process Variables in Direct Extrusion

Figure 6.54 Process variables in direct extrusion. The die angle, reduction in cross-section, extrusion speed, billet temperature, and lubrication all affect the extrusion pressure.

⎟⎟⎠

⎞⎜⎜⎝

⎛+=

of

o

DL

AAYp 2ln7.1

L

Page 78: Bulk Deformation Processes - Manufacturing

Circumscribing-Circle Diameter

Figure extra Method of determining the circumscribing-circle diameter (CCD) of an extruded cross-section.

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Figure 6.55 Schematic illustration of typical extrusion pressure as a function of ram travel: (a) direct extrusion and (b) indirect extrusion. The pressure in direct extrusion is higher because of frictional resistance in the chamber as the billet moves toward the die.

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Figure 6.56 Schematic illustration of extrusion force as a function of die angle: (a) total force; (b) ideal force; (c) force required for redundant deformation; and (d) force required to overcome friction. Note that there is an optimum die angle where the total extrusion force is a minimum.

•Optimum die angle:minimum force

• Difficult to determine

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Figure 6.57 Schematic illustration of the effect of temperature and ram speed on extrusion pressure. Compare with Fig. 2.11.

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Extrusion Constant k for Various Metals

Figure 6.58 Extrusion constant k for various metals at different temperatures. Source: P. Loewenstein.

⎟⎟⎠

⎞⎜⎜⎝

⎛==

f

o

AARkp ln

Page 83: Bulk Deformation Processes - Manufacturing

Figure 6.59 Examples of cold extrusion. Arrows indicate the direction of material flow. These parts may also be considered as forgings.

•Cold extrusion: combination of processes: Direct/indirect extrusion + forging

•Improved mechanical properties, surface finish, ……

•High production rates and relatively low cost

Miscellaneous extrusion processes Cold Extrusion

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Cold Extruded Spark Plug

Figure extra Production steps for a cold extruded spark plug. Source: National Machinery Company.

Figure extra A cross-section of the metal part, showing the grain flow pattern. Source: National Machinery Company.

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Impact Extrusion

Figure extra Schematic illustration of the impact-extrusion process. The extruded parts are stripped by the use of a stripper plate, because they tend to stick to the punch.

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Figure 6.60 (a) Impact extrusion of a collapsible tube (Hooker process). (b) Two examples of products made by impact extrusion. These parts may also be made by casting, forging, and machining, depending on the dimensions and materials involved and the properties desired. Economic considerations are also important in final process selection.

tubular sections

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Figure 6.61 Extrusion pressure as a function of the extrusion ratio for an aluminum alloy. (a) Direct extrusion, = 90 deg. (b) Hydrostatic extrusion, = 45 deg. (c) Hydrostatic extrusion, = 22.5 deg. (d) ldeal homogeneousdeformation, calculated. Source: After H. LI., D.Pugh and K. Ashcroft.

α

αα

Hydrostatic extrusion

- Some fluid on the die surface

- Ductility increase

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Chevron Cracking: internal defects

(a) (b)

Figure 6.62 and 6.63 (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 part 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 hte plastic zone can be made larger either by decreasing the die angel or by increasing the reduction in cross-section (or both). Source: B. Avitzur.

DefectsSurface crackingExtrusion defect

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Figure 6.64 Extrusion of a seamless tube. The hole in the billet may be prepunchedor pierced, or it may be generated during extrusion.

Extrusion Practice

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Extrusion Temperature Ranges for Various Metals: Hot Extrusion

°CLead 200–250Aluminum and its alloys 375–475Copper and its alloys 650–975Steels 875–1300Refractory alloys 975–2200

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Extrusion-Die Configurations

(a)

(b)

(c)

Figure extra Typical extrusion-die configurations: (a) die for nonferrous metals; (b) die for ferrous metals; (c) die for T-shaped extrusion, made of hot-work die steel and used with molten glass as a lubricant. Source for (c): Courtesy of LTV Steel Company.

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Components for Extruding Hollow Shapes

Figure 6.65 (a) An extruded 6063-T6 aluminum ladder lock for aluminum extension ladders. This part is 8 mm (5/16 in.) thick and is sawed from the extrusion (see Fig. 15.2). (b)-(d) Components of various dies for extruding intricate hollow shapes. Source: for (b)-(d): K. Laue and H. Stenger, Extrusion--Processes, Machinery, Tooling. American Society for Metals, Metals Park, Ohio, 1981. Used with permission.

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Cross-Sections to be Extruded

Figure extra 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.

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Hydraulic-Extrusion Press

Figure extra General view of a 9-MN (1000-ton) hydraulic-extrusion press. Source: Courtesy of Jones & Laughlin Steel Corporation.

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Process Variables in Wire Drawing

Figure 6.66 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.

VIDEO

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Figure 6.67 Variation in strain and flow stress in the deformation zone in drawing. Note that the strain increases rapidly toward the exit. The reason is that when the exit diameter is zero, the true strain reaches infinity. The point Ywirerepresents the yield stress of the wire.

⎟⎟⎠

⎞⎜⎜⎝

⎛=

f

od A

AY lnσ

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Figure 6.68 Stresses acting on an element in drawing of a solid cylindrical rod or wire through a conical converging die.

⎥⎥⎦

⎢⎢⎣

⎡⎟⎟⎠

⎞⎜⎜⎝

⎛−⎟⎟

⎞⎜⎜⎝

⎛+=

αµ

µασ

cot

1tan1o

fd A

AY

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Figure 6.69 Variation in the drawing stress and die contact pressure along the deformation zone. Note that as the drawing increases. This condition can be observed from the yield criteria, described in Section 2.11. Note the effect of back tension.

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Figure 6.70 The effect of reduction in cross-sectional area on the optimum die angle increases with reduction. Source : After J. G. Wistreich.

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Examples of Tube-Drawing Operations

Figure 6.71 Examples of tube-drawing operations, with and without an internal mandrel. Note that a variety of diameters and wall thicknesses can be produced from the same initial tube stock (which has been made by other processes).

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Figure 6.72 Residual stresses in cold-drawn AISI 1045 carbon steel round rod: T = trans-verse, L = longitudinal, and R = radial direction, Source: After E. S. Nachtman.

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Die for Round Drawing

Figure 6.73 Terminology of a typical die used for drawing round rod or wire.

Figure extra Tungsten-carbide die insert in a steel casing. Diamond dies, used in drawing thin wire, are encased in a similar manner.

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Roll Straightening

Figure 6.74 Schematic illustration of roll straightening of a drawn round rod (see also Fig. 6.43).

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Cold Drawing

Figure extra Cold drawing of an extruded channel on a draw bench, to reduce its cross-section. Individual lengths of straight rod or of cross-sections are drawn by this method. Source: Courtesy of The Babcock and Wilcox Company, Tubular Products Division.

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Multistage Wire-Drawing

Figure extra Two views of a multistage wire-drawing machine that is typically used in the making of copper wire for electrical wiring. Source: H. Auerswald.

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Swaging

Figure 6.75 (a) Schematic illustration of the rotary-swaging process. (b) Forming internal profiles on a tubular workpiece by swaging. (c) A die-closing type swaging machine, showing forming of a stepped shaft. (d) Typical parts made by swaging.

VIDEO

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Swaging of Tubes With and Without a Mandrel

Figure 6.76 and 6.77 (a) Swaging of tubes without a mandrel; not the increase in wall thickness in the die gap. (b) Swaging with a mandrel; note that the final wall thickness of the tube depends on the mandrel diameter. (c) Examples of cross-sections of tubes produced by swaging on shaped mandrels. Rifling (spiral grooves) in small gun barrels can be made by this process.