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Bulk Deformation Processesin Metalworking

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Rolling

Rolling - deformation process with thickness reduced by compressive forces exerted by two opposing rolls.

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Rolling AnalysisConservation of material

Continuity of volume flow rate

Forward slip

Rolling force, F

fffooo vwtvwt

fffooo LwtLwt

r

rf

v

vvs

wLYpdLwF f

L

0

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Rolling AnalysisWhere the deformation strain

and the average flow stress

The torque required for the deformation process

The power required by the process is

f

o

t

tln

n

KY

n

f

1

FLT 5.0

FLP 2

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Rolling Mechanics

The rolling process is governed by the frictional force between the rollers and the workpiece. The frictional force at the entrance side is higher than that at the exit side. This allows the roller to pull the workpiece towards the exit end.

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Frictionvo<vr<vf

Maximum draft, which is the thickness reduction, is given as 2R.

Coefficient of friction depends on lubrication, typically:

cold working 0.1

warm working 0.2

hot working 0.4

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Material and Process Parameters

Material Parameters– ductility– coefficient of friction– strength, modulus and Poisson’s ratio

Process Parameters– roller speed– power– draft– lubrication

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Shape Rolling

In addition to the material and process parameters, the rollers will acts as a set of dies and have to be pre-formed to take the negative shape of the cross-section.

There may be more than one set of rollers required to reduce the workpiece to the appropriate shape.

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Rolling Mill Configurations

a) two high b) three high c) four high

d) cluster mill e) tandem rolling mill

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Ring Rolling

• To make a larger and thinner ring from the original ring

• Usually a hot rolling process for large rings and cold rolling for small rings

• Typical applications: bearing races, steel tires, rings for pressure vessel.

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Thread Rolling

• Production of external thread

• Cold rolling

• High and competitive production rate (up to 8 parts per second)

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Gear Rolling

• Similar to the screw thread.

• Typically for helical gears

• Shares the same advantages:– better material usage– smoother surface– stronger thread due to work hardening– better fatigue resistance due to compression

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Roll Piercing

• Hot working process

• Production of Seamless thick-wall tubes

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Forging

Open-die forging

Impression-die forging

Flashless forging

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Mechanics of Forging

Under ideal condition:

h

holnAYFf

Where F = forging force

Yf = flow stress

A = cross-section of part

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Forging

In open-die forging, barreling occurs.

But with hot forging, the issue is complicated by the thermal distribution within the workpiece and the associated flow of metal.

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Shape factorThe actual forging force is

greater than the ideal case.

The shape factor is to cover the effect of barreling and the friction effect.

Open-die forging is not a net-shape process and will require subsequent machining to dimension.

Load-stroke curve

AYKF ff

h

DK f

4.01

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Open-die Forging

Fullering Edging

Cogging

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Open-die Forging

• FulleringReducing workpiece cross section to prepare for

subsequent shaping action. Dies with convex surface cavity are used.

• EdgingSimilar to Fullering, but the dies have concave

surface cavitiy.

• CoggingOpen dies with flat or slightly contoured surfaces

to reduce cross-section and to increase length.

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Impression-die Forging

Dies containing the inverse of the shape of the part. Flash is allowed on the parting surface. The flash serves as a constraint for metal flow in the die and help to fill the intricate details of the cavity.

Higher forging forces are required in this process than open-die forging. The shape factor generally will have a higher value.

AYKF ff

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Impression-die Forging

• The forces are largest at the end of the process when the projected area of the blank and the friction is largest.

• Again, progressive dies are needed to transform the starting blank into a final desired geometry.

• Machining is needed to produce the fine tolerance needed.

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Impression-die Forging

Pros:• high production rate• conservation of metal• greater strength• favorable grain orientation

Forging Machining

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Shape Factor

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Flashless Forging

Conventional forging part Precision forging part

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Flashless Forging

The volume control is important and the outcome is precision re-production of inverse of cavity geometry. Typically for aluminum and magnesium alloy.

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Corning

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Drop Hammer and Dies

Dies are normally made from tool steel with high impact strength and high wear resistance.

Webs - Thin section parallel to parting line.Ribs - thin section perpendicular to parting lineGutter - area for containing flash

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Upsetting and Heading

Upsetting and Heading

The leading section of the stock is forged to form a head section using closed-die forging.

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Upsetting and Heading

Upsetting is used to form heads of screw and bolt with different geometric forms.

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SwagingSwaging used to reduce the cross-sections of forged rods or tubes using a set of rotating dies. A mandrel is sometimes used to control the internal form of the tube.

Radial forging rotates the stock rather than the die.

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Roll Forging

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Orbital Forging

Small contact area reduce the forging force required substantially.

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Hobbing

To press the die against the softer blank to form the final shape.

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Trimming

Trimming is a shearing process to remove the flash from the workpiece.

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Design Considerations

• Material

• Die design

• Machine– Machine processing range– Machine process setting

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Design Considerations

Material– Ductility– Strength– Plastic deformation law (constitutive

relationship)– Coefficient (Die/workpiece)– Variation of properties at processing

temperature range

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Design Considerations

Die Design– Number of die stations (progressive die)– Geometric complexity of the part– Die geometric details

• Draft angle, fillet, radii

• Webs and ribs

• Flash

– Parting surface and parting direction– Die material– Die life

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Design Considerations

Machine processing range– Maximum forging force– Maximum power– Maximum speed– Maximum die size

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Design Considerations

Machine process setting– No. of stations – Velocity profile– Temperature / time profile– Force