This lecture describes fabrication processes for superplastic forming, i.e. female and male die forming, and the criteria for selecting the correct process. General background in production engineering and material science is assumed.

1. TALAT Lecture 3803
Process Technologies
15 pages, 21 figures
Advanced Level
prepared by K. Siegert and T. Werle, Institut für Umformtechnik,
Universität Stuttgart
Objectives:
− to describe fabrication processes for superplastic forming, i.e. female and male die
forming, and the criteria for selecting the correct process
Prerequisites:
− General background in production engineering and material science
Date of Issue: 1994
© EAA – European Aluminium Association
TALAT 3803 1

2. 3803 Process Technologies
Table of Contents
3803 Process Technologies ...................................................................................2
3803.01 Superplastic Fabrication Methods ............................................................ 2
3803.02 Female Die Superplastic Forming ............................................................. 4
3803.03 Superplastic Forming with Male Dies....................................................... 7
3803.04 Criteria for the Choice between Male Die and Female Die Forming... 11
3803.05 List of Figures............................................................................................ 15
3803.01 Superplastic Fabrication Methods
Figure 3803.01.01 defines in general terms the application of superplastic forming of
sheets metals.
Superplastic Forming of Sheets
Manufacturing complex shaped components
in small and medium series
using simple tools and alloys
having a special fine-grained microstructure
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Superplastic Forming of Sheets 3803.01.01
In principle, superplastic forming of metals can be subdivided into bulk metal and sheet
metal forming, see Figure 3803.02.02. Hot die forging, precision forging and isothermal
forging are examples for bulk superplastic forming. Bulk forming, however, shall not be
treated in this lecture.
TALAT 3803 2

3. Forming Superplastic Materials
Superplastic Superplastic
sheet forming bulk forming
Air injection Precision forging
process process
-Female die forming - Isothermal forging
-Male die forming - Hot die forging
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Forming Superplastic Materials 3803.01.02
Training in Aluminium Application Technologies
Processes for superplastic forming of sheet metals are subdivided further into three main
pneumatic forming processes: die forming, patrizen forming and membrane forming,
see Figure 3803.01.03. The economy of female die forming and male die forming can
be increased by using a number of tool inserts assembled in one tool so as to produce a
number of parts simultaneously.
Superplastic Forming Processes for Sheet
Male Die Forming
Female Die Forming
Membrane Forming
Female Die Forming Male Die Forming
with Inserts with Inserts
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Superplastic Forming Processes for Sheet 3803.01.03
Fields of application for the different manufacturing processes are shown in Figure
3803.01.04. Superplastic forming can best be applied for producing medium series of
parts and for complicated shaped components. The field of application of superplastic
forming is limited by conventional process technologies. Small series of complicated
parts generally have to be produced as single-parts. Classical sheet forming technologies
are suitable for parts which are not so complicated. The relatively expensive pressure
die casting is most suitable for producing large series of complicated parts.
TALAT 3803 3

4. Fields of Application for the Different
Manufacturing Processes
3
1 Single-piece production
4
2 Deep drawing and stretch forming
Complexity
3 Pressure die casting
4 Superplastic forming
1
2
100 101 102 103 104 105
Series size
alu Fields of Application for the Different
3803.01.04
Training in Aluminium Application Technologies
Manufacturing Processes
3803.02 Female Die Superplastic Forming
Female die forming utilises a concave form, see Figure 3803.02.01. During the whole
forming process, the flange of the sheet blank is clamped tight between the top and
lower pressure chambers. A flow-in of the material into the tool interior is thus not
possible. The forming zone, therefore, consists of the blank membrane area within the
tool. The pressure difference applied in the first phase, causes free forming (pneumatic
sinking) of the blank membrane in the direction of the lower pressure. In the second
phase, the gradually increasing doming leads to contact between tool and blank. The gas
membrane then divides into sub membranes, leading to varying flow rate conditions and
consequently to non-homogeneous material flow. Furthermore, the locally varying
frictional conditions result in an unfavourable wall thickness distribution.
TALAT 3803 4

5. Female Die Forming
po
po
1st process step
Process start
Free forming of
Tightly clamped blank pu pu
a dome
2nd process step
End of process
Hindering of po
material flow Formed part
due to
tool contact starts
pu
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Training in Aluminium Application Technologies
Female Die Forming 3803.02.01
Figure 3803.02.02 illustrates the forming steps during female die forming, described in
Figure 3803.02.01, using a photographic technique. The first phase of undisturbed free
forming can be seen clearly. In the second phase, two sub membranes exist.
Forming Steps During Female Die Forming
Source: Photographic Art, R. Häselich
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Forming Steps During Female Die Forming 3803.02.02
Training in Aluminium Application Technologies
Figure 3803.02.07 shows a tool insert for a Landrover front part. The complicated
internal design with very narrow webs is clearly visible. The use of die inserts reduces
the cost significantly.
TALAT 3803 5

6. Tool for a Landrover Front Part
Source: Superform Metal Ltd.
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Tool for a Landrover Front Part 3803.02.03
Training in Aluminium Application Technologies
Figure 3803.02.04 shows a complete equipment for female die forming. One sees a
rather simple machine construction which is, however, equipped with a complicated
control and regulation system for gas and heat flow.
Equipment for Female Die Forming
Source: Superform Metal Ltd.
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Equipment for Female Die Forming 3803.02.04
Training in Aluminium Application Technologies
The rear wheel housing of an ASTON MARTIN LAGONDA is a typical component
produced with female die forming, see Figure 3803.02.05
TALAT 3803 6

7. Superplastically Formed Wheel Housing
with the Female
Die Forming Process
Rear Wheel Housing
- ASTON MARTIN -
- LAGONDA -
Source: Superform Metals Ltd.
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Superplastically Formed Wheel Housing 3803.02.05
Training in Aluminium Application Technologies
.
3803.03 Superplastic Forming with Male Dies
The male forming process works with a convex form, see Figure 3803.03.01. During
the whole forming process, the flange of the sheet blank is clamped tight between the
top and lower pressure chambers. A flow-in of the material into the tool interior is thus
not possible. The forming zone, therefore, consists of the blank membrane area within
the tool. The pressure difference applied in the first phase, causes free forming
(pneumatic sinking) of the blank membrane in the direction of the lower pressure. In the
second phase, when the doming is large enough, the punch with the positive form is
moved in and the material pressed on to the form by changing the pressure direction.
Frictional contact and local thinning of the membrane occurs much later than in female
die forming. As a result, the wall thickness distribution is more favourable than in
female die forming.
TALAT 3803 7

8. Superplastic Male Die Forming
Process Start 1st 2nd End
Process Step Process Step of Process
pO pO pO
pU
pU
Tightly Form punch moved
clamped blank Free forming Formed part
in and air pressure
direction reversed
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Superplastic Male Die Forming 3803.03.01
Training in Aluminium Application Technologies
Figure 3803.02.11 shows the complete equipment used for superplastic male die
forming: preheating furnace, machine and regulation and control systems for gas and
heat flow.
Equipment for Superplastic Male Die Forming
Source: Superform Metal Ltd.
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Equipment for Superplastic Male Die Forming 3803.03.02
Training in Aluminium Application Technologies
Figure 3803.03.03 shows an opened malew die forming machine where the material
flanges are visible. The punch is at the top-end of the stroke.
TALAT 3803 8

9. Machine for Male Die Forming
Source: Superform Metal Ltd.
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Machine for Male Die Forming 3803.03.03
Training in Aluminium Application Technologies
Figure 3803.03.04 shows the tool for male die forming. One can clearly see the crimped
seam, running around the periphery, used to seal the gas pressure cavities as well as the
connections for heating the tool. The punch has moved to the top-end of the stroke.
Tool for Male Die Forming
Source: Superform Metal Ltd.
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Tool for Male Die Forming 3803.03.04
Training in Aluminium Application Technologies
Figure 3803.03.05 illustrates the different stages in the manufacturing of a component
using the male die forming process. The three intermediate stages of the left show the
TALAT 3803 9

10. initial phase of free forming. The last three stages show the steps of forming after
pressure reversal.
Stages in the Manufacturing of a Component
Using the Male Die Forming Process
Source: Superform Metals Ltd.
alu Stages in the Manufacturing of a Component 3803.03.05
Training in Aluminium Application Technologies Using the Male Die Forming Process
Figure 3803.03.06 shows a tool set, consisting of a punch, punch holder plate and blank
holder, for male die forming of an external rear view mirror housing for a car together
with an untrimmed and a trimmed component.
Tool Set for Manufacturing a Car External
Rear View Mirror Housing
Tool for Manufacturing a Car External
Rear View Mirror Housing
Process:
Male Die Forming
Source: Superform Metal Ltd.
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Training in Aluminium Application Technologies Rear View Mirror Housing
TALAT 3803 10

11. Figure 3803.03.07 shows a head rest for a pilot seat. The characteristically high form of
the part makes it most suitable for male die forming. A sectional view is shown on the
left. A good wall thickness distribution exists above the trimming line.
Wall Thickness Distribution in a Head Rest
- Male Die Process -
Starting sheet thickness s = 3 mm
Section A - A Sectional line A-A
1,13mm 1,20mm
1,45mm
37,7%
1,45mm 48,3% 40,0% 48,3%
1,38mm
46,0%
48,0% 1,44mm
1,79mm 59,7%
Cutting line
3,00mm
3,00mm
Wall thickness along part
Part: Head rest of a pilot seat
Source: Schneider, K.E.
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Wall Thickness Distribution in a Head Rest 3803.03.07
Training in Aluminium Application Technologies
3803.04 Criteria for the Choice between Male Die and Female Die
Forming
Figure 3803.04.01 shows the criteria required in principle for choosing the male die
forming process. The male die forming process allows an accurate replication of the
interior contours. Due to local differences in material flow, the external contour do not
exactly conform to the required geometry. Since the male die forming process exploits
the material volume better than die forming, it is possible to work with thinner starting
sheets in the former case. Secondary form elements protruding out of the basic form in
the blowing direction have a positive effect on the male die forming. The maximum
ratio for the surface increase is 0.6.
TALAT 3803 11

12. Criteria for Choosing Male Die Forming
b
l The ratio of surface area increase A0/A1
is greater than 0.4.
A0 = l ⋅ b
h A0 = l ⋅ b + 2 ⋅ h(b + l )
z Exact replication of contours on inside of part.
External surface of part free from
z
Actual contour manufacturing blemishes (scratches etc.).
Wavy external surface of part due to wall
Required contour
thickness variations.
Weight of part and sheet thickness required to be
s as low as possible.
Secondary Secondary form elements make female die
form element forming very difficult.
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Criteria for Choosing Male Die Forming 3803.04.01
Principle criteria for choosing the female die forming process are discussed in Figure
3803.04.02. The maximum ratio for the surface increase is 0.4.
Criteria for Choosing Female Die Forming
An exact form replication on the outside
surface of the part is essential.
z
Part interior surface free from manufacturing
blemishes (scratches etc.). Wavy interior
Required contour
surface of part due to variations in wall Actual contour
thickness.
Ribs divide the part.
Secondary form elements make male die
forming difficult.
The blank size necessary is not conducive
to male die forming.
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Criteria for Choosing Female Die Forming 3803.04.02
Figure 3803.04.03 compares the component requirements, blank dimensions,
geometrical and dimensional requirements, economy and the controlling parameters as a
help for selecting the right process for superplastic forming processes.
TALAT 3803 12

13. Choosing a Process for Superplastic Forming
Part requirements Dimensional and geometrical Economy Control possibilities
requirements
Criterion Part size Defect-free Dimensional Part wall Economic Forming Control of
(possible) surface accuracy thickness aspects ratio log. strain
rate
< 2300 x 1300
< 900 x 1300
Process
Constant
surfacce
surfacce
External
External
Multiple
Internal
forming
Internal
surface
surface
fk < 0,4
fk < 0,6
> 900
costs
Tool
ϕ
Female die
process -- + + -- --- +++ --
Female die
process -- + + -- --- ++ --
with insets
Male die + - - + + + +
process
Male die
process + - - + + + +
with insets
Source: Superform Metals Ltd.
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Choosing a Process for Superplastic Forming 3803.04.03
Figure 3803.04.04 gives a method for estimating the required gas pressure for forming
which can be calculated using the formula p = 2 ⋅ kf ⋅ s/r. This formula is based on the
equilibrium principle of the strength theory: p ⋅ Aproj. = kf ⋅ Astressed
Estimating the Required Gas Pressure
dA cosα
Hemisphere dA
σl
σu
σr
dFn
α σu
p
σl
α
F s
r = average sphere radius p = internal pressure
kf = flow stress s = sheet thickness
Equilibrium conditions p Aproj. = kf Astressed
p = kf 2s
r
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Estimating the Required Gas Pressure 3803.04.04
In order to obtain kf from the flow curves it is necessary to estimate the logarithmic
strain rate, ϕ ,as outlined in Figure 3803.04.05, where vwzg is the tool velocity (see also
!
TALAT 3300, Figure 3303.02.04).
TALAT 3803 13

14. Estimating the Log. Strain Rate (or True Strain Rate)
Elongation of a tensile specimen element
l
ϕ = ln
l0
l0 dl
" dϕ dl vwzg
ϕ= = =
dt ldt l
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Estimating the Log. Strain Rate (or True Strain Rate) 3803.04.05
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TALAT 3803 14

15. 3803.05 List of Figures
Figure No. Figure Title (Overhead)
3803.01.01 Superplastic Forming of Sheets
3803.01.02 Forming Superplastic Materials
3803.01.03 Superplastic Forming Processes for Sheet
3803.01.04 Fields of Application for the Different Manufacturing Processes
3803.02.01 Female Die Forming
3803.02.02 Forming Steps during Female Die Forming
3803.02.03 Tool for a Landrover Front Part
3803.02.04 Equipment for Female Die Forming
3803.02.05 Superplastically Formed Wheel Housing
3803.03.01 Superlastic Male Die Forming
3803.03.02 Equipment for Superplastic Male Die Forming
3803.03.03 Machine for Male Die Forming
3803.03.04 Tool for Male Die Forming
3803.03.05 Stages in the Manufacturing of a Component Using the Male Die
Forming Process
3803.03.06 Tool Set for Manufacturing a Car External Rear View Mirror Housing
3803.03.07 Wall Thickness Distribution in a Head Rest
3803.04.01 Criteria for Choosing Male Die Forming
3803.04.02 Criteria for Choosing Female Die Forming
3803.04.03 Choosing a Process for Superplastic Forming
3803.04.04 Estimating the Required Gas Pressure
3803.04.05 Estimating the Log. Strain Rate (or True Strain Rate)
TALAT 3803 15