Bulk Deformation Processes

Bulk-Deformation Processes

TABLE 6.1 General characteristics of bulk deformation processes.

PROCESS GENERAL CHARACTERISTICSForging Production of discrete parts with a set of dies; some finishing operations usually

necessary; similar parts can be made by casting and powder-metallurgy techniques;usually performed at elevated temperatures; dies and equipment costs are high;moderate to high labor costs; moderate to high operator skill.

RollingFlat Production of flat plate, sheet, and foil at high speeds, and with good surface finish,

especially in cold rolling; requires very high capital investment; low to moderate laborcost.

Shape Production of various structural shapes, such as I-beams and rails, at high speeds;includes thread and ring rolling; requires shaped rolls and expensive equipment; low tomoderate labor cost; moderate operator skill.

Extrusion Production of long lengths of solid or hollow products with constant cross-sections,usually performed at elevated temperatures; product is then cut to desired lengths;can be competitive with roll forming; cold extrusion has similarities to forging and isused to make discrete products; moderate to high die and equipment cost; low tomoderate labor cost; low to moderate operator skill.

Drawing

Swaging

Production of long rod, wire, and tubing, with round or various cross-sections; smallercross-sections than extrusions; good surface finish; low to moderate die, equipmentand labor costs; low to moderate operator skill.Radial forging of discrete or long parts with various internal and external shapes;generally carried out at room temperature; low to moderate operator skill.

[1] Forging Operations

Forging can be carried out as:

a. cold working (T < 0.3Tm)

b. warm working (0.3Tm < T < 0.5Tm )

c. hot working (T > 0.6Tm)

Three basic types:

1. open-die forging (also known as upsetting)

2. Impression die forging

3. Closed-die forging

Ideal (homogeneous) Deformation

FIGURE 6.1 (a) Ideal deformation of a solid cylindrical specimen compressed between flat frictionless dies. This process is known as upsetting. (b) Deformation in upsetting with friction at the die-workpiece interfaces.

Equations for Open-Die Forging

Reduction in height =

 

and

 

and

%100*0

10

h

hh

0

101 h

hhe

1

01 ln

h

h

01 h

ve

11 h

v

Grain Flow Lines

FIGURE 6.2 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 with 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.

Assumptions: no friction at the die-workpiece interface, and material is perfectly plastic with a yield stress (Y), initial height (h0), initial area

(A0), instantaneous height (h1), and instantaneous area (A1).

 

 

Deformation force and work calculations

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hAA

1

0

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dvolumeWorknK 1

0

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Yd

11

n

KY

n

1.AYF

Average Flow Stress

Impression-Die Forging

FIGURE 6.14 Schematic illustration of stages in impression-die forging. Note the formation of flash, or excess material that is subsequently trimmed off.

AnalysisSimple shapes, without flashSimple shapes, with flashComplex shapes, with flash

3-55-88-12

F = (Kp)(Yf)(A)

TABLE 6.2 Range of Kp values in Eq. (6.21) for impression-die forging.

Orbital Forging Process

FIGURE 6.16 Schematic illustration of the orbital-forging process. Note that the die is in contact with only a portion of the workpiece surface. This process is also called rotary forging, swing forging, and rocking-die forging and can be used for forming bevel gears, wheels, and bearing rings.

Heading

FIGURE 6.17 Forging heads on fasteners such as bolts and rivets. These processes are called heading.

Piercing Operations

FIGURE 6.18 Examples of piercing operations.

Cogging Operation

FIGURE 6.19 Schematic illustration of a cogging operation on a rectangular bar. With simple tools, the thickness and cross-section of a bar can be reduced by multiple cogging operations. Note the barreling after cogging. Blacksmiths use a similar procedure to reduce the thickness of parts in small increments by heating the workpiece and hammering it numerous times.

Roll Forging Operation

FIGURE 6.20 Schematic illustration of a roll forging (cross-rolling) operation. Tapered leaf springs and knives can be made by this process with specially designed rolls. Source: After J. Holub.

Manufacture of Spherical Blanks

FIGURE 6.21 Production of steel balls for bearings by the skew-rolling process. Balls for bearings can also be made by the forging process shown in Fig. 6.22.

FIGURE 6.22 Production of steel balls by upsetting of a cylindrical blank. Note the formation of flash. The balls are subsequently ground and polished for use as ball bearings and in other mechanical components.

Defects in Forging

1. Surface Cracking: due to excessive stresses and poor die design

2. Internal Cracks: due to oversized billets

3. Cold Shuts: due to small corner radii in the mold

4. Exposed grains: poor material flow and original blank orientation

Forgings are generally anisotropic due to various directions in the metal flow.

Defect Formation In Forging

FIGURE 6.25 Effect of fillet radius on defect formation in forging. Small fillets (right side of drawings) cause the defects. Source: Aluminum Company of America.

Forging A Connecting Rod

FIGURE 6.26 Stages in forging a connecting rod for an internal combustion engine. Note the amount of flash that is necessary to fill the die cavities properly.

[2] Rolling Operations

Rolling is the process of reducing the thickness (flat rolling) or changing the cross-section (shape rolling) of a long workpiece by compressive forces applied through a set of rolls.

Rolling Operationsa. Flat Rollingb. Shape Rolling

Plates: thickness>0.25Sheets: 0.004<thickness<0.25Foils: thickness<0.004

Ingot rolling is replaced by continuous casting/rolling with a higher efficiency and lower cost.

Rolling can be performed as hot, warm, or cold forming operations.

Flat-And-Shape-Rolling Processes

FIGURE 6.29 Schematic outline of various flat-and-shape-rolling processes. Source: American Iron and Steel Institute.

Mechanics of Flat-Rolling

FIGURE 6.31 Schematic illustration of the flat-rolling process. A greater volume of metal is formed by rolling than by any other metalworking process.

wo and ho : initial width and thickness

wf and hf : final width and thickness

Vo and Vf : entry and exit velocities

Vr : tangential velocity of roll

R: roll radius

L: roll gap

Neutral (No-Slip) Point

FIGURE 6.32 Relative velocity distribution between roll and strip surfaces. Note the difference in the direction of frictional forces. The arrows represent the frictional forces acting on the strip.

Vo : entry velocity of the workpiece

Vf : exit velocity of the workpiece

Vr : tangential velocity of roll

: angle of acceptance

Forward Slip = r

rf

V

VV

Notes

At the “no-slip” point: Vr = Vworkpiece (neutral point)

Before the neutral point: Vr > Vworkpiece

After the neutral point: Vr < Vworkpiece

Forces and Power

Draft = h = ho – hf = 2 . R µ = tan (angle of acceptance)

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Notes on Force Calculations

As friction increases, the draft, forces, power and the damage to surface finish increase.

Roll forces can be reduced by:–reducing friction–Reducing rolls size–Reducing draft–Applying tension forces (back, front, or both)

Diverse effects of roll forces:–Rolls deflection–Rolls flattening

Problems in Rolling Operations

1. Rolls Deflection: forces bend rolls elastically, this results in thicker parts at the center (CROWN), which can be reduced by:

• Grinding rolls thicker at the center (CAMBER)~0.01“, or

• Bending rolls by applying moments at the end

2. Roll Flattening: results in larger roll radius, and hence larger contact area, which results in larger rolling forces for the same rolling draft.

3. Plastic deformation of rolls (Thermal Camber)

4. Stretch of roll stand (low rigidity)

5. Spreading, especially with high thickness/width ratio. Solution: edger mills.

Roll Deflection and Spreading

FIGURE 6.37 (a) Bending of straight cylindrical rolls (exaggerated) because of the roll force. (b) Bending of rolls, ground with camber, that produce a sheet of uniform thickness during rolling.

FIGURE 6.38 Increase in the width of a strip (spreading) in flat rolling. Spreading can be similarly observed when dough is rolled with a rolling pin.

Workpiece Defects

1. Wavy edges: due to roll bending

2. Surface cracks: due to poor ductility and/or low rolling temeperature

3. Alligatoring: due to defects in the original cast

4. Residual stresses (only when not desired): the type of these stresses are:

• Tensile in the middle/compressive at surface when using small rolls or draft

• Compressive in the middle and tensile on the surface when using large rolls or draft.

Workpiece Defects In Flat Rolling

FIGURE 6.39 Schematic illustration of typical defects in flat rolling: (a)wavy edges; (b) zipper cracks in the center of strip; (c) edge cracks; (d) alligatoring.

Shape RollingFIGURE 6.44 Stages in shape rolling of an H-section part. Various other structural sections, such as channels and I-beams, are also rolled by this process.

Ring-Rolling

FIGURE 6.45 (a) Schematic illustration of a ring-rolling operation. Reducing the thickness results in an increase in the part’s diameter. (b) Examples of cross-sections that can be formed by ring rolling.

Thread-Rolling Processes

FIGURE 6.46 Thread-rolling processes: (a) flat dies and (b) two-roller dies. These processes are used extensively in making threaded fasteners at high rates of production.

Machined And Rolled Threads

FIGURE 6.47 (a) Schematic illustration of machined or rolled threads. (b) Grain-flow lines in machined and rolled threads. Unlike machined threads, which are cut through the grains of the metal, rolled threads follow the grains and are stronger, because of the cold working involved.

Mannesmann Process

FIGURE 6.48 Cavity formation by secondary tensile stresses in a solid round bar and its use in the rotary-tube-piercing process. This procedure uses the principle of the Mannesmann mill for seamless tube making. The mandrel is held in place by the long rod, although techniques have been developed in which the mandrel remains in place without the rod.

[3] Extrusion Processes

Extrusion Processes

1.     Direct Extrusion

2.     Indirect Extrusion

3.     Hydrostatic Extrusion

4.     Impact Extrusion

5.     Lateral Extrusion

Types Of Extrusion

FIGURE 6.49 Types of extrusion. (a) direct; (b) indirect; (c) hydrostatic; (d) impact.

Extrusion

FIGURE 6.50 Extrusion and examples of products made by sectioning off extrusions. Source: Kaiser Aluminum.

FIGURE 6.51 Schematic illustration of three different types of metal flow in direct extrusion.

(a) Ideal: no-friction at the billet-container-die interfaces

(b) Typical: friction at the billet-container-die interfaces

(c) High container wall-billet friction

Extrusion Parameters

1.     die angle ()2.     extrusion-ratio (R)3.     circumscribing-circle-diameter (CCD): the diameter of the smallest circle into which the extruded cross-section will fit.4.     shape factor (ratio of the perimeter to the cross-

sectional area)5.     billet temperature6.     ram speed7.     type of lubricant

Extrusion Force/Power Calculations

 

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Extrusion pressure (ideal – no friction). For strain-hardening materials, use average flow stress Y

Extrusion pressure (friction between die and billet only)

Extrusion Ratio

Absolute value of the true strain

Energy dissipation per unit volume (for perfectly plastic materials)

Work supplied by the ram force (F), which travels a distance Lo

Extrusion Constant

FIGURE 6.55 Extrusion constant, Ke, for various materials as a function of temperature. Source: After P. Loewenstein, ASTME Paper SP63-89.

P=Ke*ln(R)

Cold and Impact Extrusion

FIGURE 6.56 Examples of cold extrusion. Arrows indicate the direction of material flow. These parts may also be considered as forgings.

FIGURE 6.57 (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.

Notes on Extrusion Practices

1. Cold Extrusion has the following advantages over hot extrusion: better mechanical properties (due to work hardening), better dimensional accuracy (little finishing operations) and surface finish (no oxide film), and high production rates at relatively low cost. However cold extrusion requires more expensive tooling (high hardness, strength, toughness, and fatigue strength), complex lubrication and cooling process, and high capacity presses.

2. Hydrostatic extrusion requires much lower forces due to the increased ductility of the material (suitable for brittle materials), and the low friction. Also, it is possible to use low die angles as well as high extrusion ratios.

3. Metal flow in the die influences the quality and mechanical properties of the parts.

4. Extrusion ratio (R) can go up to 400:1

5. Coaxial extrusion (cladding) is possible when the strength and ductility of both metals are compatible.

6. Hot extrusion: low forces, excessive die wear (can be reduced with preheated dies), non-uniform deformation (can be reduced with preheated dies), oxide film on the billet (reduced by a smaller ram than the container), poor surface finish due to surface oxidation.

7. Dead-metal zones produce extrusions with bright finishes

8. Die materials are generally hot-work die steels, coated with Zicronia to extend the die life.

Defects in ExtrusionThree principal defects in extrusion:

(1) Surface CrackingCause: too high extrusion temperature, friction or speed (intergranular)or at low temperature due to periodic sticking of the extrudate and the die land (knows as bamboo defect) especially during hydrostatic extrusion. Can be reduced by increasing the extrusion speed.

(2) Extrusion defects (pipe, tailpipe, and fishtailing)Cause: metal flow on the container wall draw surface oxides and impurities towards the center of the billet. Can be reduced by modifying the flow pattern to more homogeneous by reducing friction and minimizing the temperature gradient.

(3) Internal Cracking (chevron cracking or centerburst)Cause: hydrostatic (secondary) tensile stresses at the centerline of the deformation zone.Can be reduced reduced by adjusting the extrusion parameters (die angle, extrusion ratio, friction) so the the deformation zones around the die overlap.

Chevron Cracking

FIGURE 6.59 (a) Deformation zone in extrusion, showing rigid and plastic zones. Note that the plastic zones do not meet, leading to chevron cracking. The same observations are also made in drawing round bars through conical dies and drawing flat sheet plate through wedge-shaped dies. Source: After B. Avizur. (b) Chevron cracking in round steel bars during extrusion. Unless the part is inspected properly, such internal detects may remain undetected and possibly cause failure of the part in service.

Extrusion of Seamless Tube

FIGURE 6.60 Extrusion of a seamless tube. The hole in the billet may be prepunched or pierced, or it may be generated during extrusion.;

[4] Drawing Operations

FIGURE 6.62 Variables in drawing round rod or wire.

Drawing is similar to extrusion, except “pulling” through a die is used instead of pushing. It can be used to make wires as small as 0.001 in diameter.

Process Parameters

Ao , Af initial and final diameters

= the die angle

F = Drawing Force

d = Drawing Stress

= coefficient of friction

Maximum Reduction per passThe maximum allowed drawing stress is the yield stress of the existing material. Therefore, there is a maximum strain (reduction) per pass.

For a perfectly plastic material, the maximum reduction in cross sectional area is 63% (where ε1=1).

For a strain hardening material, the maximum reduction in cross sectional area can be calculated as:

11 ne

Tube Drawing

FIGURE 6.67 Various methods of tube drawing.

Defects is Drawing Operations

1. Center cracking is caused by inhomogeneous plastic deformation which increases by increasing the die angle, decreasing the reduction per pass, increasing friction, and the presence of inclusions.

2. Seams, longitudinal scratches/folds in the material which can open up during subsequent forming operations.

3. Residual stresses (transverse, longitudinal, radial) due to inhomogeneous deformation. Very light reductions leaves compressive residual stresses on the workpiece surface which improves fatigue strength.

Rotary Swaging

FIGURE 6.71 Schematic illustration of the swaging process: (a) side view and (b) front view. (c) Schematic illustration of roller arrangement, curvature on the four radial hammers (that give motion to the dies), and the radial movement of a hammer as it rotates over the rolls.

Also known as Rotary Forging, where a solid rod or a tube is reduced in diameter by the reciprocating radial movement of two or four dies driven by a set of rollers in a cage.

Rotary Swaging (continued)

Mandrels are used to control the internal diameter and/or shape (example gun/rifle barrels).

The workpiece diameter is limited to 2 inches, while the length is limited to the length of the mandrel.

Generally performed at room temperature.

FIGURE 6.72 Reduction of outer and inner diameters of tubes by swaging. (a) Free sinking without a mandrel. The ends of solid bars and wire are tapered (pointing) by this process in order to feed the material into the conical die. (b) Sinking on a mandrel. Coaxial tubes of different materials can also be swaged in one operation.

Cross-Sections Produced By Swaging

FIGURE 6.73 (a) Typical cross-sections produced by swaging tube blanks with a constant wall thickness on shaped mandrels. Rifling of small gun barrels can also be made by swaging, using a specially shaped mandrel. The formed tube is then removed by slipping it out of the mandrel. (b) These parts can also be made by swaging.

Die Failures

Failure of dies in metal forming operations results from one or more of the following causes:

1. Improper die design2. Defective die materials3. Improper heat treatment and finishing operations4. Improper installation, assembly, and alignment5. Overheating and heat checking6. Excessive wear7. Overloading, misuse, and improper handling

Dies can fail by cracking, chipping, wear, heat checking (from thermal cycling), or deformation (especially in hot working)