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Metal Forming Defects

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Defects in Rolled Plates and Sheets

Defects may be present on the surfaces of rolled

plates and sheets, or there may be internal

structural defects.

Defects are undesirable not only because they

compromise surface appearance, but also

because they may adversely affect strength,

formability, and other manufacturingcharacteristics.

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Defects in Rolled Plates and Sheets

1. Wavy edges

2. Zipper cracks

3. Edge cracks

4. Alligatoring

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(a) Wavy edges (b) Zipper cracks in the

centre of the strip

(c) Edge cracks (d) Alligatoring

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1. Wavy edges

Wavy edges on sheets are the result of roll

bending. The strip is thinner along its edges than

at its center ; thus, the edges elongate more than

the center.

Consequently, the edges buckle because they are

constrained by the central region from expanding

freely in the longitudinal (rolling) direction.

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2., 3. Zipper & Edge cracks

The cracks shown in Fig are usually the result of

poor material ductility at the rolling temperature.

Because the quality of the edges of the sheet

may affect sheet-metal-forming operations, edge

defects in rolled sheets often are removed by

shearing and slitting operations

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4. Alligatoring

Alligatoring is a complex phenomenon and

typically is caused by non uniform bulk

deformation of the billet during rolling or by the

presence of defects in the original cast material.

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Extrusion Defects

There are three principal extrusion defects:

1. Surface cracking

2. Pipe

3. Internal cracking

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1. Surface cracking

If extrusion temperature, friction, or speed is too

high, surface temperatures can rise significantly,

which may cause surface cracking and tearing.

These defects occur especially in aluminum,

magnesium, and zinc alloys.

This situation can be avoided by lowering the

billet temperature and the extrusion speed.

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1. Surface cracking

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1. Surface cracking

Surface cracking also may occur at lower

temperatures, where it has been attributed to

periodic sticking of the extruded product along

the die land.

Because of the similarity in appearance to the

surface of a bamboo stem, it is known as a

bamboo defect.

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2. Pipe

The type of metal-flow pattern in extrusion shown inFig tends to draw surface oxides and impuritiestoward the centre of the billet-much like a funnel.

This defect is known as pipe defect, tailpipe, orfishtailing.

As much as one-third of the length of the extrudedproduct may contain this type of defect and thus has

to be cut off as scrap. Piping can be minimized by modifying the flow

pattern to be more uniform, such as by controllingfriction and minimizing temperature gradients.

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2. Pipe

Another method is to machine the billetssurface

prior to extrusion, so that scale and surface

impurities are removed.

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3. Internal Cracking

The centre of the extruded product can develop

cracks, called centre cracking, centre-burst,

arrowhead fracture, chevron cracking.

The tendency for centre cracking (a) increases

with increasing die angle, (b) increases with

increasing amount of impurities, and(c)

decreases with increasing extrusion ratio andfriction.

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3. Internal Cracking

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Forging Defects

Defects can develop during forging as a result of

the material flow pattern in the die.

If there is an insufficient volume of material to fill

the die cavity completely, the web may buckle

during forging and develop laps

If the web is too thick, the excess material flows

past the already formed portions of the forgingand develops internal cracks.

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Laps

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Laps formed

by web

buckling

during

forging; web

thickness

should be

increased toavoid this

problem

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Internal

cracks

Internal defects

caused by an

oversized billet.

Die cavities arefilled

prematurely,

and the

material at the

centre flows

past the filled

regions as the

dies close.

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DEFECTS IN DRAWING

Sheet-metal drawing is a more complex

operation than cutting or bending, and more

things can go wrong.

A number of defects can occur in a drawn

product, some of which we have already alluded

to.

Following is a list of common defects, withsketches in Figure.

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DEFECTS IN DRAWING

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DEFECTS IN DRAWING

a) Wrinkling in the flange . Wrinkling in a drawn

part consists of a series of ridges that form

radially in the undrawn flange of the workpart

due to compressive buckling.

b) Wrinkling in the wall. If and when the wrinkled

flange is drawn into the cup, these ridges

appear in the vertical wall.

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DEFECTS IN DRAWING

c) Tearing .Tearing is an open crack in the verticalwall, usually near the base of the drawn cup,due to high tensile stresses that cause thinning

and failure of the metal at this location. Thistype of failure can also occur as the metal ispulled over a sharp die corner.

d) Earing . This is the formation of irregularities

(called ears ) in the upper edge of a deep drawncup, caused by anisotropy in the sheet metal. Ifthe material is perfectly isotropic, ears do not

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DEFECTS IN DRAWING

e) Surface scratches . Surface scratches can occur

on the drawn part if the punch and die are not

smooth or if lubrication is insufficient.

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