Download - E2 Deep Drawing
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Deep Drawing, Formability
Deep drawing
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Principle
Deep drawing of metal sheet is used to form containersA flat blank is formed into a cup by forcing a punchagainst the center portion of a blank that rests on thedie ringTooling: punch, blankholder ring, blank, die ring
punch, blankholder ring, blank, die ring
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Redrawing
Final shape of a container can be reached by more drawing steps, these operations are called asredrawings
blankholder
blank
workpiece
die ring
punch for redrawing
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Combined drawings
More drawing operations can be combined into onetool using multiple ram system in a hydraulic press
First drawing
Second drawing
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Other characteristics of deepdrawing
• Easiest way is to draw cylindrical partsfrom circle disc, but…
• The process is capable of forming box(rectangular) shapes or shell-likecontainers
• Special variants are liquid pressureforming and rubber die forming
• These processes result in near netshapes for many purposes.
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Deep drawing ratio
• Limiting Drawing Ratio (LDR) –β = Do/dn
• where Do is the diameter of the first (largest) blankand dn is the smallest cup diameter that can be successfully drawn
• Drawing Ratio in general:βi = di/di+1
• First drawing β1 =2,2…1,8redrawing βi =1,4…1,1 (copper, aluminium, mild steel)
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Calculation of blank diameter
• In case of cup-likecomponents the blankis circular
• Area of blank equal tothe area of cup:
Do
h
d1
hddD
hddD
1210
1
21
20
444+=
+= πππ
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Calculation of drawing force
Drawing force: Fd,max = n*π*d*t*UTSExample:Cup diameter: d= 45,7 mmSheet thickness: t = 0,5 mmUltimate Tensile Strength: UTS = 320 MPaDrawing coefficient: n = 0,7…0,95
Fd,max = 0,9*π*45,7*0,5*320 = 20 674 N
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Drawing toolBélyeg
Munkadarabalakításközbeniállapotban
Húzógyűrű
Ráncgátló
punch
blankholder
workpiece
Die ring
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Drawing errors (1)
Earing Stress corrosion cracks
Other errors: Buckling and wrinklingFracturing
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Drawing errors (2)
Buckling and wrinkling, causing fracturing
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Drawing errors (3)
Fracturing
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Deep Drawing
Example
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Analysis of a cup drawing• The cup to be drawn:
– Diameter: 60,3 mm– Height: 104,8 mm
• Blank diameter (earing is eliminated):
mmDD
hddD nnn
1,1708,1043,6043,60
4
0
20
20
=⋅⋅+=
+=
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Calculation of drawing ratio
• Suggested drawing ratio– First drawing ~2– Second drawing 1,2…1,25– Third drawing 1,15…1,18– Further drawings 1,1…1,12 etc.
• Calculations by approximative(iterative) way
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Results
1,1660,3104,83. Redrawing
1,2369,985,72. Redrawing
1,9885,765,11. Cup drawing
-170,1-0. blank
Drawingratio
Diameterin mm
Heightin mm
Nr. ofoperations
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Drawing force• Cup drawing (first drawing)
– Diameter: 85,7 mm– Thickness: 2 mm– UTS: 320 MPa
kNNFF
UTStdnF
d
d
d
15515507932027,859,0
max
max
max
≅=⋅⋅⋅⋅=
⋅⋅⋅⋅=π
π
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Fluid forming„hidromec” process
Fluid forming uses only one solid die half.
Forming pressure is applied by the action of hydraulic fluid, whichforces the blank to assume the shape of the rigid tool
Sometimes a flexibile membran is used to separate blank and thefluid
Punch, blankholder
Seal, fluid, fluid holder
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Strech drawing
Used for drawing car body panels
Blank is clamped, rigid die gives the shape of sheetmetal
Change in thickness: s1 < s0
clamping
workpiece
die
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Formability Testing of SheetMetals
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Representation of Strain
• True or logarithmic strain: ϕ• The integral of the incremental change
in length dL, divided by the actuallength L:
LdLd =ϕ
0L1Lln
LdL1L
0L=∫=ϕ
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Equivalent strain• Strain in the direction of 3 axes:
φ1= φL φ2= φw φ3= φth
• Equivalent strain:
2)31(2)32(2)21(3
2ϕϕϕϕϕϕϕ −+−+−=e
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True stress-strain(flow stress) curve
• Flow stress curve: σf = σf (φe) if temperature (T) and strain rate,is constant.
• In general(n: strain hardening exponent; m: strain rate sensitivity)
dtd
eϕϕ =&
( )
.)(
.);(
;;
constTifc
constTifc
T
me
nef
enef
eeff
=⋅⋅=
=⋅=
=
ϕϕσ
ϕϕσ
ϕϕσσ
&
&
&
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Plastic strain ratio (r)Measurement
• Tensile test:
φL= ln(L1/L0); φw= ln(w1/w0); φth = - (φL + φw) ( as φL+ φw + φth = 0 )
L0
w0
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b = w; s = th
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Plastic strain ratio (r)Evaluation
• Calculation: r = φw / φth
• Definition: ratio of the true width straindevided to the true thickness strain
• The r value frequently changes withdirection in the sheet
• Test specimens should be machinedparallel (0o), perpendicular (90o), and(45o) related to the rolling direction
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Measure of anisotropyTest
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Measure of anisotropy
• Average normalanisotropy:
• Planar anisotropy:
42 90450 rrrrm
++=
2900
45rrrr +
−=∆
The value rm determines the limiting drawing ratio, and ∆r is in correlation with the extent of earing.
A combination of high rm and low ∆r providesoptimal drawability.
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Average normal anisotropy andhardening exponent of metals
3,0-5,00,05Titanium0,6-0,80,2-0,3Aluminium0,6-0,90,35-0,5Copper0,9-1,20,4-0,55Austenitic steel1,4-1,80,22-0,26Drawing steel1,0-1,40,2-0,5Mild steel
rn
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Forming Limit Diagram (1)Major engineering strain, %
Minor engineering strain, %
Safe
Safe
Failure
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Forming Limit Diagram (2)
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Forming Limit Diagram (3)• Sheet metal can be deformed only to a
certain level – before local thinning(necking) and failure occur
• FLD shows the limit of necking (orfailure) as function of minor and major strain
• Strains can be evaluated from thedeformation of circle grids plotted onthe surface of sheet metal
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Other formability tests
• Ball punch test (Erichsen test)• Hydraulic bulge test• Hemispherical dome test• Cup drawing test (drawing test)
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Ball punch testWell knownas Olsen orErichsen test
The cupheight atfracture is used as themeasure ofstrechability
Die
Specimen
Punch
Blank-holderring
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Ball punch testExamples
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Hydraulic bulge test
Hydraulic fluid
Base plate
Die ring
Lock bead
Materialcharacterisationin biaxialstreching
Testing to muchhigher strainlevels than thoseachievable intensile testing
Research inplasticity theory
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Hemispherical dome testLubricatedpunch is usedfor deformationof sheet metal
The domeheight atfracture is measured
The test yieldsreproducibleresults
Die ring
Blankholderring
Lock beadPunch
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Cup drawing testCircular blanks ofvarious diametersare usedTooling is standardisedLimiting drawingratio (LDR) is theratio of thediameter of thelargest blank thatcan be successfullydrawn to thediameter of thepunch
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Cup drawing testExamples