super plastic forming- rubber pad forming

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4.3- Super plastic forming-
Rubber pad Forming

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Membrane: polyurethane
Polyurethanes are used widely because of their
abrasion resistance, fatigue life, and resistance to
cutting or tearing.

• The pressure over the rubber membrane is controlled
throughout the forming cycle with a maximum pressure of
up to 100MPa.
• This process control the wrinkling or tearing.
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• Electro hydraulic forming is a type of metal forming in
which an electric arc discharge in liquid is used to convert
electrical energy to mechanical energy and change the
shape of the workpiece.
• A capacitor bank delivers a pulse of high current across
two electrodes, which are positioned a short distance
apart while submerged in a fluid (water or oil).
• The electric arc discharge rapidly vaporizes the
surrounding fluid creating a shock wave.
• The workpiece, which is kept in contact with the fluid, is
deformed into an evacuated die.
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• SPF conducted at high temperature and under
controlled strain rate.
• It increases the elongation.
• So that the sheet can stretch and fill the die cavity.
• NOTE:
• Fine grained microstructure and strain rate
sensitivity of flow stress are important for
superplastic deformation.
• EX: Alloys of titanium, stainless steel and
aluminium.
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• w/p placed in sealed in die.
• Inert gas pressure is then applied at a
controlled rate.
• The die heats up the workpiece.(upto 1000
0C)
• Inert gas pressure upto 50bar.
• The material at the super plastic
temperature can allow upto 500%
elongation. YoucaN

• Advantage:
• One step process
• Minimizes amount of scrap
• Higher material elongation possible
• Application:
• Forming of– automotive body panels, aircraft frames
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• Forming parts dimensions below 1mm.
• Product miniaturization
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The mechanics of all sheet forming basically consists of
stretching and bending, certain parameters significantly
influence the overall operation. These are:
1. Elongation
2. Yield point elongation
3. Anisotropy
4. Grain size
5. Residual stress
6. Springback
7. wrinkling
Sheet Metal - Characteristics
and formability

Elongation :
• When a specimen is subjected to tension it is first
undergoes uniform elongation up to the UTS, after which it
begins to neck. This elongation is then followed by further
non-uniform elongation until the specimen fractures.
• Because the sheet will be stretched during forming process,
high uniform elongation is thus desirable for good formability.
Characteristics of Sheet-Metal
Forming Processes

Anisotropy– different yield strength along different
directions
Another important factor influencing sheet-metal forming is
anisotropy (preferred grain orientation) , or
directionality, of sheet metal.
Anisotropy is acquired during the thermo mechanical
processing history of the sheet.
There are two types of anisotropy:
1. Crystallographic anisotropy (preferred grain orientation)
2. Mechanical fibering ( alignment of impurities, inclusions
and voids.
These behaviors are particularly important in deep drawing
of sheet metals.
Forming Processes

Grain size
 Grain size of the sheet metal is important for two reasons:
1. Because of its effect on mechanical properties of the
material [smaller grain size exhibit more formability
than larger grain size]
2. Because of its effect on surface appearance of the
formed part
The coarser the grain, the rougher the surface appears
(orange peel)
An ASTM grain size of No. 7 is typically preferred for
general sheet-metal forming
Forming Processes

Residual stresses
Residual stresses can develop in sheet-metal parts because
of the non-uniform deformation that the sheet undergoes
during forming. (Residual stresses are stresses that
remain within a part after it has been deformed
plastically non uniformly and all external forces
have been removed).
Disturbing the equilibrium of residual stresses, such as by
bending or stretching of it, the part may distort.
We should remove residual stresses prior to forming.
Forming Processes

Springback
• Because they are generally thin and are subjected to
relatively small strain during forming, sheet-metal parts are
likely to experience considerable springback
• Because all materials have a finite modulus of elasticity,
plastic deformation is always followed by elastic recovery
upon removal of the load. In bending, this recovery is known
as springback.
• This effect is particularly significant in bending and other
forming operations where the bend radius-to-sheet-
thickness ratio is high, such as in automotive body parts.
Forming Processes

Wrinkling
In the sheet metal forming the metal is typically
subjected to tensile stresses, the method of forming
may be that compressive stresses are developed in
the plane of the sheet.
An example in sheet metal forming is the wrinkling of
the flange in deep drawing because of the
circumferential compressive stresses that develop in
the flange.
Other terms used to describe this phenomena are
folding and collapsing.
Forming Processes

Wrinkling
• The tendency for wrinkling in sheet metals increases
with:
1. Decreasing thickness
2. Non-uniformty of the thickness of the sheet
3. Increasing length or surface area of the sheet that is
not constrained or supported.
4. Lubricants that are trapped or are not distributed evenly
at the die-sheet metal interfaces can also contribute to
the initiation of wrinkling
Forming Processes

• Plasticity- Ability of a metal to be deformed extensively
with out rupture.
• Toughness- combination of high strength and medium
ductility. Ability of a material to resist the fracture after the
damage has begun.
• Hardness- ability of a material to resist penetration and
wear by another material
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• Corrosive resistance- resistance to eating away or
wearing by the atmosphere moisture or other such as
acid.
• Compressive strength- the ability of a material to
withstand pressures acting on a given plane.
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Sheet metal formability is generally defined as
the ability of a sheet to undergo the desired shape
change without failure such as necking and
tearing.
Three factors have a major influence on
formability:
1. Properties of the sheet metal
2. Friction and lubrication at various interfaces in
the operation
3. Characteristics of the equipment, tools, and dies
used

Several techniques have been developed to test
the formability of sheet metals, including the
ability to predict formability by modeling the
particular forming operation
1. Cupping test
2. Tension test
3. Bulge test
4. Forming-limit diagrams
Formability of sheet metals

The earliest tests developed to predict formability of sheet
metal were cupping tests, namely, the Erichsen tests
1. In the Erichesn test, a sheet-metal specimen is clamped over a
flat die with a circular opening and a load of 1000 kg
2. A 20 mm diameter steel ball is then hydraulically pressed into the
sheet until a crack appears on the specimen
3. The distance d, in mm, is the Erichsen number
Erichsen test
The greater the value of d the greater is the formability
d = Erichsen number

In this test, a circular blank is clamped at its periphery and
is bulged by hydraulic pressure, thus replacing the punch.
The operation is pure biaxial stretching, and no friction is
involved, as would be the case in using a punch.
The bulge limit (depth penetrated prior to failure) is a
measure of formability
Bulge-test results on steel sheets of various widths.
The specimen farthest left is subjected to, basically, simple tension.
The specimen farthest right is subjected to equal biaxial stretching.

Procedure:
 Blank sheet is marked with a grid of circles (2.5-5mm).
 The blank is then stretched over a punch, until the grid
pattern deforms where necking and tearing occur.
 The deformed circles are measured in the failed region,
that is the major strain, and miner strain are obtained:
that is after stretching, the original circle has
deformed into ellipse shape
 typically 10 data points taken.
 A series of tests on a certain metal produces the FLD.
Major Strain: (5-4)/4 * 100 = 25%
Minor strain: (3.2-4)/4 * 100 = -20%
Example:
Before punch stretch test:
Original circle diameter: 4mm
After punch stretch test:
Major ellipse axis : 5mm
Minor ellipse axis :3.2mm

• During stretching in sheet
metal, volume is constant:
l + w + t = 0
• Major strain always larger
than minor strain
If the surface area of ellipse after stretching is larger than the original circle, we know
the thickness of the sheet has changed, its thinner due to stretching.
Although the major strain is always positive (because forming sheet metal takes
place by stretching in at least one direction), the minor strain may be either
positive [strain occur in the transvers direction greater than the original] or
negative or shrinking [strain occur in the transvers direction smaller than the
original] or zero [ No strain occur in the transvers direction ]in the transverse
direction

#1
+ε1
-ε2
+ε1
+ε2
+ε1
ε2=0
#3
#4
#2
Courtesy of Roy A. Lindberg,
1983
After sheet metal deformation,
the major and minor axes of the
circles on the grid pattern are
used to determine the coordinates
on the forming limit diagram.
ε1 Major strain
ε2 Miner strain
because the
minor planar
strain is zero
When normal isotropy R=1 ( that is, the width and thickness
strains are equal. ϵw = -05. ϵl )

The most basic and common test used to evaluate formability.
It determines important properties of the sheet metal such as:
- total elongation of the sheet specimen at fracture.
- strain hardening exponent, n.
- the normal anisotropy (R) and
-the planar anisotropy (delta R).

If the thickness of the sheet as it enters the die cavity is more
than the clearance between the punch and die, it has to be
reduced by a deformation called ironing.
By controlling the clearance, C, ironing produce a cup with a
constant wall thickness.
Because of the volume constancy, an ironed cup will be
longer than a cup produced with a large clearance.
Thus, ironing can correct earing that occurs in deep drawing.
Ironing
Ironing to achieve a more uniform wall thickness in a drawn cylindrical cup.

super plastic forming- rubber pad forming

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super plastic forming- rubber pad forming