Mech303 - 2011-12 Spring - l06 - Sheet Form Pro

SHEETSHEETSHEETSHEET FORMING PROCESSESFORMING PROCESSESFORMING PROCESSESFORMING PROCESSES

Lectured by:

Dr. Volkan Esat

Course Notes by:

Prof. Dr. S. Engin KILIÇ

Content

� Process Classification

� Material Properties

� Formability

� Yield-Point Phenomena

� Textures (Anisotropy)

� Shearing Process

� Bending Process

� Deep Drawing Process

Courtesy of Daimler-Chrysler & Eregli Demir Celik.

2

Process Classification

Sheet Forming ProcessesSheet Forming Processes

BendingBending

NeckingNecking

Deep DrawingDeep Drawing

HydroformingHydroforming

ShearingShearing

FlangingFlanging

Stretch FormingStretch Forming

SpinningSpinning

3

Formability

Formability in sheet metal working is limited by the following failure modes:

• Orange Peel Effect• Lüder’s Lines or Stretcher-Strain Marks• Localized Necking• Fracture

4

5

Orange Peel Effect

� Grainy appearance is observed.

� Individual grains oriented in different crystallographic directions deform toslightly differing degrees.

� Structural integrity of the material is not effected but aesthetic appearance is not satisfactory.

� To solve this problem, one must use finergrained material .

6

Orange Peel Effect

7

Formation of Lüder’s Lines or Stretcher-Strain Marks

� Structural integrity of the material is not effected but aesthetic appearance is not satisfactory.

� Reason for this is yield-point elongation or negative strain rate sensitivity.

� To solve this problem, one must use non-aging materials or roller levelling sheet.

� Highly localized initial yielding results visible bands (Lüder’s lines). These are successive micro-necking regions.

� Stretcher-strain marks are incomplete Lüder’s lines.

yield pointelongation

8

Lüder’s Lines

9

Stretcher-Strain Marks

10

Localized Necking

� Due to excessive tensile deformations, local necks may form.� Load carrying capacity is reduced and appearance is poor.� High n-value delays onset of necking and high m-value

spreads out necking.

Rupture of the sheet:

� If further deformation occurs after localized necking, fractureoccurs.

� Post-necking strain is a function of the m-value.

11

Yield Phenomena

� Some small solute atoms (such as N and C in iron) can fit into spaces in the crystal lattice of the solvent metal to form interstitial solid solution.

� These generally migrate to dislocation sites to form a condensed atmosphere (Cottrell or Suzuki clouds), which in turn pins the dislocations.

� A larger stress must be applied before dislocations can break away from the condensed atmosphere of interstitial atoms.

� After the dislocations break away, they multiply and move in large groups in the direction of maximum shear stress (45o to applied force) resulting localized yielding (Lüder’s bands).

� Strain bands cover entire surface at a low stress (yield-point elongation). After this strain hardening behaviour becomes evident.

12

Yield Phenomena (2)

• Strain aging: is the recondensation of soluteatoms to dislocation sites.

• Strain aged material has reduced ductility and stillthe yield-point elongation phenomena.

13

Stepwise or Serrated Yielding

� Related to negativestrain-rate sensitivity.

� Example: substitutional aluminum alloys.

� Leads to stretcher-strainmarks.

14

Material Properties: Texture (Anisotropy)

During tensile straining slip planes rotate toward thedirection of straining and duringcompressive straining acrossthe direction of straining.

The alignment of the slip planes in preferred directions is named as texture. Textured materials exhibit an anisotropic behavior.

15

Texture (Anisotropy) (2)

l

wt

r0 r

45

r90

rolling

direction

111000 ll ⋅⋅=⋅⋅ wtwt

Volume constancy requires:

10

1

0

1

0

1 =⋅⋅l

l

w

w

t

t

0=++l

εεε wt

or

=

0

1lnl

l

lε (length

strain)

0)1ln(ln0

1

0

1

0

1 ==

⋅⋅l

l

w

w

t

tTaking thelogarithm:

yields where:

=

0

1lnt

ttε

=

0

1lnw

wwε

(thicknessstrain)

(widthstrain)

16


Definition of the r-value:strainthickness

strainwidth≡=

t

wrεε

Isotropic material: directions1∀=r Anisotropic material: 1≠r

Normal anisotropy:

Planar anisotropy:

190450 ≠== rrr

90450 rrr ≠≠

( )45900 24

1rrrr ⋅++=Mean r-value:

Planar anisotropy:

( )45900 22

1rrrr ⋅−+=∆

17

Interpretation of Planar Anisotropy Parameter*

( )45900 22

1rrrr ⋅−+=∆

[*] Adapted from Lange (1980).

18


REMARK: Annealing does not necessarily restore isotropy.

Material HCP withhigh c/a

HCP withlow c/a

FCC BCC

Plastic strain to produce anisotropy

20 to 30% > 50%

Mean r-value

Very small (typically 0.2)

Very large(but < 6.0)

0.4 to 0.8 0.8 to over 2.0

19


High r-valueLow r-value

20


21

Classification of Shearing

Shearing is the physical process of cutting sheet and strip intoappropriate shapes and involves the following types:

1. Shearing: Cutting a sheet along a straight line

2. Slitting: Cutting a long strip into narrower widths between rotary blades

3. Blanking: Cutting a contoured part between a punch and a die in a press

4. Punching: Cutting out a contoured shape from a part between a punch

and a die in a punch

5. Notching: Cutting out a part of the sheet edge

6. Lancing: Partially cut a hole with no material removed

7. Nibbling: Cutting a contoured part by repeated small cuts

8. Trimming: Finish cutting of the excess material in drawn products

22

Classification of Shearing

23

Sub-processes in Shearing

� Rounding of the edge (roll-over)

� Extrusion-like plastic deformation(burnished zone)

� Crack formation at die

� Crack formation at punch

� Meeting of the cracks

� Pushing out the part over the die land

24

Shearing Operation

a) Blanking with optimum clearance.

b) Small clearance (skirt of torn edge).

c) Excessive clearance (burr is produced).

25

Shearing Operation

(a) Schematic illustration of shearing with a punch and die, indicating some of the process variables.

(b) Characteristic features of (b) a punched hole and (c) the slug.

Note that the scales of the

two figures are different.

26

Shearing Force

a) Crack initiation for soft materials(roughly at half the sheet thickness).

b) Crack initiation for hard materials(earlier than halfthe sheet thickness).

c) Burr produced inblanking reduceselongation attainable in subsequent tensile deformation.

27

Shearing Force (2)

( ) ( )areashearingstressshearing ⋅=sP

⋅≈⋅≈

valueaverage an as 0.70

materials ductile lessfor 0.65

materials ductilefor 0.85

UTSUTSstressshearing 1C

n

annealede

nK

≈UTS

Shearing force:

28

Types of Shearing

Ps

h

dp

hd p ⋅⋅= πareashearing

Ps

l

h

Circular Punching: Straight Cutting (no blade angle):

h⋅= lareashearing

29

Straight Cutting with Angle

hareashearing ⋅= l

αtan

h=lwith:

α

Ps

h

l

30

Shearing Areas

Shearing force can be reduced by having an angle between the edges (rake or shear).

Guillotine Blanking Punching (piercing)

31

Shearing Energy

curvent displaceme-forceunder Area =sEShearing energy:

hPCE ss ⋅⋅≈ 2

=materials hardfor 35.0

materialssoft for 50.02C

Or, approximately:

with

32

Finish Blanking & Punching

The standard shearing process produces a fracture surface which is not perfectly perpendicular to the sheet surface and exhibits some roughness. However, there is a great demand to produce very clean-cut edges,

perpendicular to the sheet surface and of a surface finish sufficiently smooth.

Negative-clearanceblanking

Precision (fine)blanking

Counter-blanking

Shaving

Impingingement

ring

33

Example

A carbon steel sheet metal of 2 mm thickness is to be cut along

its length of 1.5 m with a shearing machine. The annealed UTS

of the material is 700 MPa.

a) Find the shearing force of a 0o blade angle.

b) Find the shearing force of a 10o blade angle.

Solution:

a)

tons150N101.47

m1.5m002.0MN/m7007.0

UTS

6

2

1

≈⋅=

⋅⋅⋅=

⋅⋅⋅= lhCPs

ton1N 11,116

10tanm002.0MN/m7007.0

tanhhUTSChUTSCP

o222

11s

≈=

⋅⋅=

α⋅⋅⋅=⋅⋅⋅= lb)

34

Bending

axis neutral of length =NAl

fiberouter of length =OFl

h/2h/2

Rb

Rb

h

αb

β

b

hRR b

OF

hb

NAb +

=+

=ll

2

α

Bending Radius:

Bending Strain:

12

1

+⋅

=−

=

h

RbNA

NAOFtensile

OFl

llε

35

Bending (2)

Rb: radius of bending dieαb: bending angleβb: included die angle

Rf: radius of bent partαf: final bend angleβf: final included

part angle

o180=+ bb βαo180=+ ff βα

Note:

Sheet Metal (Part)

Punch

Die

During Bending After Bending

36

Remarks for Bending

If Rb is generous, the neutral line is in the center.

If Rb is tight, the neutral line shifts towards the compressive side.In this case the centerline is elongated and volume constancy is preserved by thinning! The increased length of the centerline is taken into account for Rb < 2h by assuming that the neutral lineis located at one-third of the sheet thickness.

When the sheet is relatively narrow ( w/h < 8 ), there is also a contraction in width w.

37

Limitations in Bending

1) Orange Peel

3) Minimum Bend Radius:

1

12

1, −≈<

+⋅

= n

uengb

tensile

eng e

h

RεεFor no necking:

( ) he

eR

n

n

b ⋅−

−>

12

2

For no fracture: 2.0for 12

1<⋅

−> qh

qRb

( )2.0for

2

12

2

≥⋅

−−

> qhqq

qRb

(less ductile materials)

(more ductile materials)

2) Crushing on inside surface

38

Springback in Bending

b

f

R

Rb

f

h/2

h/2

f

bαα

..

b

f

b

f hR

hR

αα ⋅

+

+=

2

2

3

431

⋅⋅+

⋅⋅−≈E

Y

h

R

E

Y

h

R

R

R bb

f

b

For a gentle bend and a high Y/E:

From constancy ofneutral axis length:

< 1 (always)

39

Factors Influencing Springback

↑∴↓⇒↑ springbackf

bb

R

R

h

R

↑∴↓⇒↑ springbackf

b

R

RY

↓∴↑⇒↑ springbackf

b

R

RE

40

Residual Stresses in Bending

a) Bending stresses b) Residual stresses

41

Reduction of Springback in Bending

a) Overbending c) Counterpunchb) Indentation

d) Stretching e) Warm/hot bending

42

Bending Force

b

bW

hwP

UTS2 ⋅⋅=

opening die theofh widt:

strength tensileultimate :UTS

knesssheet thic :

part ofdth wi:

bW

h

w

Bending force for free bending:

where

Wb

h

Pb

43

Bending Force

b

bW

hKwP

UTS2 ⋅⋅=

opening die theofh widt:

strength tensileultimate :UTS

knesssheet thic :

part ofdth wi:

factorgeometry :

bW

h

w

Kwhere

Common types of punch and die arrangements

44

Deep Drawing

Converting a circular sheet metal of diameter d0 into a flat-bottomed cup of depth several times the thickness of the sheet metal through a draw die with the aid of a punch of diameter Dp is named as deep-drawing (cupping).

45

Stress States in Deep Drawing*

[*] Adapted from Schey (1987).

46

Parameters of Deep Drawing

0

0Reduction

d

Dd p−≡

pD

d0 RatioDrawing ≡

( )pD

max0d LDR Ratio DrawingLimiting =

47

Common Defects in Deep Drawing

Wrinklingin flange

Wrinkling in wall

Tearing Earing Surfacescratches

48

Common Defects (Cont’d)

49

Deep Drawing: Modes of Operation

Mode I: Drawing without a blankholder

Wrinkling can be avoided for shallow draws:

2.10 <pD

d Thick blanks give higherdrawing ratios:

50

Deep Drawing: Modes of Operation (2)

Mode II: Drawing with a blankholder

Y⋅≈ 015.0 pressurer blankholde

−⋅⋅⋅⋅= 7.0UTS 0

p

pdD

dhDP π

Drawing force:

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Forming Limit Diagram*

[*] Adapted from Goodwin (1968).

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FLD (Cont’d)

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Limiting Drawing Ratio (LDR)

The draw force is composed of the forces required to• compress the sheet in the flange circumferentially• overcome friction between blank and blank holder and die surfaces• bend and unbend the sheet around draw radius• overcome friction around draw radius

Factors effecting the limiting drawing ratio:• High n strengthens the cup but also increases draw force.• High m strengthens an incipient neck while not affecting the draw force• High r-value resists thinning. Also the yield locus is improved.• Tight punch and die radii impose severe bending. But too large radii cause puckering(wrinkling between punch and die). Therefore, optimal radii are: R > 4 h for thick (h > 5 mm) and R > 8 h for thin (h < 1 mm) sheet.

• Friction between blank holder, die and flange surfaces adds to the draw force.• Friction on the punch decreases force which is supported by the cup base.• For thin sheet (d0/h > 50) friction forces dominate, so that with increasing d0/h LDR

drops.

54

Effect of r-value on LDR*

Planar anisotropy leads to earing.

[*] Adapted from Schey (1987).

55

Further Drawing

Redrawing Ironing Reverse redrawing

Mech303 - 2011-12 Spring - l06 - Sheet Form Pro

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