mech303 - 2011-12 spring - l06 - sheet form pro
TRANSCRIPT
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
Texture (Anisotropy) (3)
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
Texture (Anisotropy) (5)
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
Texture (Anisotropy) (6)
High r-valueLow r-value
20
Texture (Anisotropy) (6)
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:
51
Forming Limit Diagram*
[*] Adapted from Goodwin (1968).
52
FLD (Cont’d)
53
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