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CHAPTER 5

SUPERPLASTIC FORMING OF 7075 Al-ALLOY

5.1 SUPERPLASTIC FORMING OF RECTANGULAR BOX

WITH DOME IMPRESSION

The superplastic forming technique is used to form a near-net shape

in superplastic materials, with tremendous cost savings and weight reduction

potential over conventional forming processes. Blow forming of superplastic

sheets uses a single die surface, rather than the matched dies used in typical

sheet metal forming operations. The superplastic sheet material is usually

formed into a fixed die cavity, shaped to the geometry of the desired part,

using gaseous pressure in one single step. To have this capability, a computer

controlled pneumatic operated bulge forming setup was designed and

fabricated. The setup consists of the forming die assembly and the software

module for control.

5.2 EXPERIMENTAL SETUP

5.2.1 Superplastic Forming Die Assembly and Accessories

The experimental setup consists of an air compressor, a split type

electric furnace, sensors and control units. The forming die consists of the top

and bottom parts, and a space is provided in the bottom part to hold the

forming sheet. The top part of the die is a complex shape (combination of the

rectangular and the dome shape). The complex die assembly was placed

inside the furnace, and the die temperature was maintained by the temperature

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controller at the forming temperature. The schematic of the setup is shown in

Figure 5.1. An LVDT (Linear Variable Differential Transducer) sensor was

used to measure the dynamic height variation of the formed component. The

data acquisition card received the signal from the LVDT, which was input to

the computer, that monitors the motion of the motor, thereby controlling the

forming process. The interface between the sensor, computer and stepper

motor is through the data acquisition card (NIDAQ 6009).

The cross-sectional detail of the top and bottom pressure-forming

die set is shown in Figures 5.2 and 5.3 respectively.

Figure 5.1 Schematic diagram of the Experimental setup

(all dimensions in mm)

Figure 5. 2 The cross-sectional detail of the top pressure-forming die

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Figure 5.3 The cross-sectional detail of the bottom pressure-forming die

5.2.2 Stepper Motor

A stepper motor is an electromechanical device which converts

electrical pulses into discrete mechanical movements. This stepper motor is

operated based on the control signals applied to the driver circuit. The

specification of the used stepper motor are, type- unipolar, Voltage - 6V,

Torque - 10Kg cm, step angle- 1.8 deg.

In the experimental setup the stepper motor is coupled with the

pressure regulator knob. So, according to the movement of the stepper motor

the pressure applied to the furnace varies.

5.2.3 LVDT Arrangement

The linear variable differential transformer (LVDT) is a type of

electrical transformer used for measuring the linear displacement (position).

The specifications of the LVDT used are, Range 0-20mm, and output 0-5V.

The LVDT operates only in alternating signals. So, an oscillator is

used to produce an oscillating signal. The output alternating signals are

compared by a phase detector, and amplified and filtered to be converted into

DC output signals of 0-5V. In the experimental setup, the LVDT is connected

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to the top of the specimen through the clamp with a stand; according to the

specimen formation, the LVDT is used to identify the forming height.

5.2.4 Pressure Sensor

A pressure transmitter senses the pressure, and outputs a

proportional current signal. The piezo-resistive type pressure sensor has a

diaphragm element, over which the strain gauges are placed. So the pressure

sensor converts 0-10 bar pressure into a 4-20 mA current signal.

5.2.5 Data Acquisition and Interfacing

Data acquisition is the process of real world physical conditions,

and conversion of the resulting samples into digital numeric values, that can

be manipulated by a computer. The National Instruments Data Acquisition

card 6009 has a direct USB interface with the computer. It has 8 analog input

lines, 2 analog output lines and 12 configurable digital input/output lines that

enable easier data acquisition and control.

The signals from the LVDT and pressure sensor are connected to

two analog input lines, and the stepper motor is controlled via four digital

lines configured as outputs.

5.3 SOFTWARE MODULE

The Laboratory Virtual Instrument Engineering Workbench (Lab

VIEW) developed by national instruments, is a powerful analysis

programming language. Lab VIEW is a highly productive graphical

development environment with the performance and flexibility of a

programming language, as well as high-level functionality and configuration

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utilities, designed specifically for measurement and automation application.

Lab VIEW integrates data acquisition, analysis and presentation in one

system, making programming simple and manageable.

Lab VIEW programs are called virtual instruments or VIs, having

three parts:

The front panel

The block diagram

The icon/connector

The front panel is the user interface of the virtual instruments. The

front panel contains controls and indicators, which are the Interactive input

and output terminals of the VI, respectively. The block diagram contains this

graphical source code, also known as the G code or block diagram code.

Figure 5.4 shows the block diagram of the control program of the

process with the algorithm. Figure 5.5 shows the front panel of the software

program having text boxes for getting various level limits, and their

corresponding pressure values. It also has indicators for the measured

pressure and dome height values.

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Figure 5.4 Algorithm for the control program of the SPF

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Figure 5.5 Front panel of lab - view program

5.4 EXPERIMENTAL PROCEDURE

The experimental work has been divided into the following four

segments.

(i) First segment - only one sample was considered.

(ii) Second segment - two samples were considered.

(iii) Third segment - four samples were considered.

(iv) Fourth segment- twenty samples were considered.

5.4.1 Superplastic Forming Under Constant Pressure

In the first segment, sample I was formed under a constant forming

pressure of 0.5 MPa and temperature of 530 C. The deformed sample was

taken out from the die setup, and the thickness distribution was measured,

using a Digital micrometer, and the cavitation effect was measured in the

formed part.

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5.4.2 Superplastic Forming Under Variable Pressure

In the second segment, sample II was formed under two different

forming pressures of 0.5 MPa (up to the first 8 mm formation) and 0.45 MPa

(up to the next 8 mm formation), and sample III was formed under three

different forming pressures of 0.5 MPa (up to the first 8 mm formation), 0.45

MPa (up to the next 4 mm formation), 0.4 MPa (up to the last 4 mm

formation). The forming processes of samples II and III were performed at

530 C. The deformed samples were taken out from the die setup, and the

distribution of thickness was measured, using a Digital micrometer, and the

cavitation effect in the formed part was also measured.

5.4.3 Superplastic Forming Under Various Temperatures

In the third segment, three different forming pressures of 0.5 MPa

(up to the first 8 mm formation), 0.45 MPa (up to the next 4 mm formation)

and 0.4 MPa (up to the last 4 mm formation) were chosen as constants for all

the samples. The constant forming temperature was changed from 500 C to

540 C; for sample IV the forming temperature was 500 C, for sample V

510 C, for sample VI 520 C, and for sample VII the forming temperature was

540 C; the superplastically formed die setup is shown in Figure 5.1. The

thickness distribution and the cavitation effect were measured in the

superplastically formed parts.

Optical microscopy and SEM were used to inspect the cavitation in

the samples. The specimens were cut, so as to obtain a flat surface for the

metallographic examination, mechanically polished, and then etched with

Keller’s reagent, which has a composition of 2 ml HF(48%), 3 ml HCl

(conc.), 5 ml HNO3 (conc.), 190 ml H2O, and the etching time was 15

seconds. From the digitized images, taken with a CCD camera through an

optical microscope, model MM 25 IS, year 2005-2006 at 400X and High

Resolution Scanning Electron Microscope, the grain size variation and cavity

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volume fractions were measured and calculated, using the Biovis material

plus software for material science and metallography, and the results were

confirmed with the SEM images.

5.4.4 Superplastic Forming Under Various Sheet Thicknesses and

Annealing Times

Three different forming pressures of 0.5 MPa (up to the first 8 mm

formation), 0.45 MPa (up to the next 4 mm formation) and 0.4 MPa (up to the

last 4 mm formation) were chosen as constants for all the samples. A constant

forming temperature of 530 C was selected. The experiment was divided into

four segments.

In the first segment, five 1.25 mm thick sheet samples, namely, A,

B, C, D and E were considered. The annealing time was changed from 60 min

to 180 minutes. For sample A the annealing time of 60 minutes was selected;

for sample B, 90 minutes; for sample C 120 minutes, for sample D 150

minutes, and for sample E 180 minutes was selected. In the second segment,

five 1.50mm thick sheet samples, namely, F, G, H, I and J were considered.

The annealing time was changed from 60 min to 180 minutes. For sample F

the annealing time of 60 minutes was selected; for sample G, 90 minutes, for

sample H 120 minutes, for sample I 150 minutes, and for sample J 180

minutes was selected. In the third segment, five 1.75mm thick sheet samples,

namely, K, L, M, N and O were considered. The annealing time was changed

from 60 min to 180 minutes. For sample K the annealing time of 60 minutes

was selected; for sample L, 90 minutes, for sample M 120 minutes, for

sample

N 150 minutes, and for sample O 180 minutes was selected. In the fourth

segment, five 2.00 mm thick sheet samples, namely, P, Q, R, S and T were

considered. The annealing time was changed from 60 min to 180 minutes. For

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sample P the annealing time of 60 minutes was selected; for sample Q, 90

minutes, for sample R 120 minutes, for sample S 150 minutes, and for sample

T 180 minutes was selected. The 7075 Al alloy sheet samples were

superplastically formed in the die setup as shown in Figure 5.1. The forming

time was measured. The superplastically formed part was taken out of the die

setup, and the cavitation effect and thickness distribution were measured,

after the specimens were cut off. The thickness distribution was measured

directly, using a digital micrometer.

5.5 RESULTS AND DISCUSSION

5.5.1 Thickness Distribution and Cavitation Study under Constant

and Variable Pressure

Strain is accumulated during superplastic deformation, primarily as

a result of grain boundary sliding, rather than by the elongation of the grains

themselves. The thinning factor, that is, the ratio of the thickness at the pole

to the average thickness of the formed part, was also calculated for each

sample. Superplastic materials fail during deformation due to an unstable

plastic flow, or as a consequence of the growth and interlinkage of the

internally nucleated cavities. All Al alloys are prone to cavitation during

superplastic flow (Nieh 1997). Sample I, formed under a constant high

forming pressure, took a less forming time, and had a comparatively lower

thinning factor and high cavity volume fraction; due to the high forming

pressure, the dynamic recrystallisation of the initial formation took place;

(Pilling and Ridley 1989) it controlled the grain growth because it offered

high resistance to the deformation in the initial stage, which was the reason

for the high cavity volume fraction. Sample II was formed under two variable

pressures: the thinning factor was high and the cavity volume fraction was

less, but the forming time increased, due to the dynamic recrystallisation of

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the initial stage formation, and in the next stage, the grains slide in a

controlled manner and occupied the voids in a slow and steady formation.

Sample III was formed under three different forming pressures; it took high

forming time, but gave a high thinning factor and less cavity volume fraction,

due to the dynamic recrystallisation of the initial stage to get refined grains; in

the next stage, the forming pressure enhanced the thinning factor, because the

grains slid in a controlled manner, and in the last stage of formation, the

pressure was very less, so, the grains very readily occupy the cavities,

reducing the micro voids, but slightly increasing the grain size, due to the

long forming time. Figures 5.6 and 5.7 show the formed component and the

different locations of the thickness measurement in the components

respectively. Table 5.1 shows the thinning factor of samples I, II and III.

Table 5.2 shows the cavity volume fraction of samples I, II and III.

Figure 5.6 Superplastically formed component of complex shape

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Figure 5.7 Locations of thickness measurement in the component

Table 5.1 Thinning factor of samples I, II and III

Sample

Average

thickness, tavg

(mm)

Thickness at

Pole, tpole

(mm)

Thinning

factor,

[tpole/tavg]

I 1.350 1.10 0.8148

II 1.461 1.23 0.8419

III 1.456 1.28 0.8791

Figure 5.8 Thickness distribution along the transverse cross- section of

samples I, II and III

94

Table 5.2 Cavity volume fraction of samples I, II and III

SampleForming time

(minutes)Grain size (µm)

Cavity volume

fraction ( %)

I 20 11 12.64

II 40 12 8

III 72 13 5

Figure 5.8 shows the thickness distribution of the completely

formed samples I, II and III along the transverse cross- section. Figures 5.9,

5.10 and 5.11 show the Microstructures of samples I, II and III respectively,

and Figures 5.12, 5.13 and 5.14 show the SEM pictures of the samples I, II

and III respectively. On continuous control, the forming pressure reduced the

cavity volume fraction, and increased the thinning factor.

Figure 5.9 Microstructure of Figure 5.10 Microstructure of

Sample I Sample II

Figure 5.11 Microstructure of Figure 5.12 SEM picture of

SampleIII Sample I

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Figure 5.13 SEM picture of Figure 5.14 SEM picture of

Sample II Sample III

5.5.2 Thickness Distribution and Cavitation Study at Different

Forming Temperatures

The flow stresses would generally be lower at slower strain rates

and higher temperatures; however, the effect of changing the strain rate or

temperature on cavitation during superplastic deformation, would vary

widely, depending on the material. The differences which arise, are often the

result of changes in the microstructure of the material, particularly, the grain

size. In the aluminium alloys, increasing the temperature and decreasing the

strain rate, result in a decrease in the overall level of cavitation. Samples IV

and V were formed at temperatures of 500 Cand 510 C respectively, and took

a long forming time; due to the low forming temperature, the thinning factor

was also low. Even dynamic recrystalisation occurred during the initial stage

formation, and it was not stable for a long time; there was less formability,

thinning factor, and the cavity volume fraction was very high, due to the

instability of the grains in the long forming time at a low forming

temperature. Sample VI was formed at a temperature of 520 C, and it took a

comparatively medium forming time for the high temperature, and the

thinning factor was less due to the instability of grains; the grains moved

randomly, and it automatically reduced the thinning factor and increased the

cavity volume fraction. Sample VII was formed at a temperature of 540 C,

96

and took less forming time due to the high temperature; the grains lost their

stability, and this increased the grain growth, increasing the cavity and the

thinning factor, due to the high temperature at which the grains moved fast

and in a random manner. Table 5.3 shows the thinning factor of samples IV,

V, VI and VII. Table 5.4 shows the cavity volume fraction of samples IV, V,

VI and VII.

Table 5.3Thinning factor of samples IV, V, VI and VII

Sample

Average

thickness,

tavg (mm)

Thickness

at Pole, tpole

(mm)

Thinning factor,

tpole/tavg

IV 1.452 1.23 0.8471

V 1.371 1.17 0.8534

VI 1.414 1.12 0.7921

VII 1.383 1.19 0.8604


samples III, IV, V, VI and VII

97

Table 5.4 Cavity volume fraction of samples IV, V, VI and VII

SampleForming time

(minutes)

Grain size

( µm)

Cavity volume

fraction ( %)

IV 185 13 10.25

V 113 13 9.31

VI 85 13 11.69

VII 43 14 13.09

Figure 5.15 shows the thickness distribution of the completely

formed samples of III, IV, V, VI and VII along the transverse cross - section.

Figures 5.16, 5.17, 5.18 and 5.19 show the microstructures of the samples IV,

V, VI and VII respectively.


Sample IV Sample V


Sample VI Sample VII

98

5.5.3 Thickness Distribution and Cavitation Study for Various

Annealing Times and Sheet Thicknesses

The thinning factor, which is a ratio of the thickness at the pole to

the average thickness of the formed part, was calculated for each sample and

the cavity volume fraction was calculated in the dome of each sample.

Sample A took a high forming time of 48 minutes and a thinning factor of

0.8562. Due to the variable pressure method, the initial pressure is high, and

dynamic recrystallisation took place, reducing the grain size. After 60

minutes’ annealing, the grains were stable; so, it took a high forming time and

grains slide in a controlled manner, producing a less cavity volume fraction.

Sample B took a forming time of 34 minutes and the thinning factor of 0.884;

and the cavity volume fraction of 5% was high, due to the instability of the

grains after 90 minutes’ annealing. It affected the grain boundary strength;

the grains slide rapidly in a controlled manner, reduced the forming time and

increased the thinning factor due to the instability of the grains during the

process results high cavity volume fraction. Sample C and D took a less

forming time and the thinning factors were less. The high cavity volume

fraction due to the high annealing time affected the grain boundary strength;

the grains slide rapidly and randomly, so, automatically the forming time and

thinning factor reduced and the cavity volume fraction increased. Sample E

took a forming time of 32 minutes, and the thinning factor of 0.8333 was less;

the 180 minutes’ annealing time affected the grain boundary strength and the

grain growth also took place; because of their instability, the grains showed a

high cavity volume fraction, and a less thinning factor. Table 5.5 shows the

thinning factor of samples A, B, C, D and E. Table 5.6 shows the cavity

volume fraction of samples A, B, C, D and E. Figure 5.20 shows the thickness

distribution of the completely formed samples A, B, C, D and E along the

transverse cross- section. Figure 5.21 shows the microstructure

of the samples A, B, C, D, E and Figure 5.22 shows the SEM picture of

sample A.

99

Table 5.5 Thinning factor of samples A, B, C, D and E

Sample

Average

thickness

tavg (mm)

Thickness at

Pole,

tpole (mm)

Thinning

factor,

[tpole/tavg]

A 0.911 0.78 0.8562

B 0.837 0.74 0.8841

C 0.877 0.72 0.8210

D 0.845 0.70 0.8284

E 0.876 0.73 0.8333


samples A, B, C, D and E

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1 2 3 4 5 6 7 8 9 10

Th

ick

ne

ssin

mm

Measuring Position

Sample A

Sample B

Sample C

Sample D

Sample E

100

Table 5.6 Cavity volume fraction of samples A, B, C, D and E

Sample sheetForming time

(minutes)

Grain size

(µm)

Cavity volume

fraction ( %)

Sample A 48 11 1.65

Sample B 34 12 5.0

Sample C 29 12 6.076

Sample D 30 12 5.8

Sample E 32 13 8.0

Figure 5.21 Microstructure of samples A, B, C, D and E

Figure 5.22 SEM picture of sample A

101

Sample F took a high forming time of 65 minutes, and the thinning

factor of 0.8677 was also high; due to the variable pressure method, dynamic

recrystallisation took place and after 60 minutes’ annealing the grains were

highly stable, and resisted deformation; so the cavity volume fraction was

also high. Sample G took a forming time of 50 minutes and the thinning

factor of 0.8670 was high. Due to the variable pressure method, dynamic

recrystallisation took place and after 90 minutes’ annealing, the grains were

stable, took a high forming time, and were sliding in a controlled manner,

because a less cavity volume fraction was obtained. Sample H took a forming

time of 29 minutes and the thinning factor of 0.8738was very high, and the

cavity volume fraction was also high. The 120 minutes’ annealing time

affected the grain boundary strength; the grains slid rapidly in a controlled

manner, reduced the forming time and increased the thinning factor, due to

the instability of the grains during the process, resulting in a high cavity

volume fraction. Samples I and J took a less forming time, low thinning

factor, and high cavity volume fraction; and the high annealing time affected

the grain boundary strength. The grains slide rapidly and randomly; due to

that the forming time and thinning factor were reduced and the cavity volume

fraction was also high; but compared to sample H here the cavity volume

fraction was low due to the grains adjusting the cavity with their random

movement. Table 5.7 shows the thinning factor of samples F, G, H, I and J.

Table 5.8 shows the cavity volume fraction of samples F, G, H, I and J.

Figure 5.23 shows the thickness distribution of the completely formed

samples F, G, H, I and J along the transverse cross- section. Figure 5.24

shows the microstructure of the samples F, G, H, I, J and Figure 5.25 shows

the SEM picture of sample G.

102

Table 5.7 Thinning factor of the formed samples F, G, H, I and J

Sample

Average

thickness

tavg (mm)

Thickness at

Pole tpole

(mm)

Thinning

factor

[tpole/tavg]

F 1.141 0.99 0.8677

G 1.115 0.97 0.8670

H 1.133 0.99 0.8738

I 1.066 0.90 0.8443

J 1.011 0.85 0.8408


samples F, G, H, I and J

Table 5.8 cavity volume fraction of the formed samples F, G, H, I and J

Sample sheetForming time

(minutes)

Grain size

(µm)

Cavity volume

fraction ( %)

Sample F 65 12 8

Sample G 50 12 2.135

Sample H 29 12 11.812

Sample I 30 12 4.21

Sample J 31 13 5.8

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1 2 3 4 5 6 7 8 9 10

Th

ick

ne

ssin

mm

Measuring Position

Sample F

Sample G

Sample H

Sample I

Sample J

103

Figure 5.24 Microstructure of samples F, G, H, I and J

Figure 5.25 SEM picture of Sample G

Sample K took a high forming time of 70 minutes and the thinning


recrystallisation took place, and after 60 minutes’ annealing, the grains were

highly stable, and able to resist deformation because the cavity volume

fraction is also high. Sample L took a forming time of 63 minutes; and the 90

minutes’ annealing time affected the grain boundary strength; the grains slid

rapidly, and it reduced the thinning factor and forming time, due to the

104

instability of grains during process, resulting in a high cavity volume fraction.

Sample M took a forming time of 52 minutes and the thinning factor of

0.8740 was very high; and the 120 minutes’ annealing time affected the grain

boundary strength; the grains slide rapidly, reduced the forming time and

increased the thinning factor, producing a very less cavity volume fraction.

Here, the grains moved rapidly in a controlled manner and occupied the

cavity. Samples N and O gave a low thinning factor ; and the high annealing

time affected the grain boundary strength; the grains slide randomly, and

reduced the forming time and thinning factor; the cavity volume fraction is

increased but compared to sample L, it is small due the grains adjusting the

cavity during the random movement. Table 5.9 shows the thinning factor of

samples K, L, M, N and O. Table 5.10 shows the cavity volume fraction of

samples K, L, M, N and O. Figure 5.26 shows the thickness distribution of the

completely formed samples K, L, M, N and O along the transverse cross-

section. Figure 5.27 shows the microstructure of the samples K, L, M, N, O

and Figure 5.28 shows the SEM picture of sample M.

Table 5.9 Thinning factor of the formed samples K, L, M, N and O

Sample

Average

thickness

tavg (mm)

Thickness

at Pole tpole

(mm)

Thinning factor

[tpole/tavg]

K 1.29 1.11 0.8600

L 1.209 1.00 0.8271

M 1.27 1.11 0.8740

N 1.274 1.05 0.8242

O 1.302 1.11 0.8525

105


samples K, L, M, N and O

Table 5.10 Cavity volume fraction of the formed samples K, L, M, N

and O

Sample

sheet

Forming

time

(minutes)

Grain size

(µm)

Cavity

volume

fraction

( %)

Sample K 70 12 6.075

Sample L 63 12 10.25

Sample M 52 12 3.923

Sample N 50 13 4.664

Sample O 53 13 7.062

0.5

0.6

0.7

0.8

0.9

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

1 2 3 4 5 6 7 8 9 10

Th

ick

ne

ssin

mm

Measuring position

Sample K

Sample L

Sample M

Sample N

Sample O

106

Figure 5.27 Microstructure of samples K, L, M, N and O

Figure 5.28 SEM picture of Sample M

Sample P took a high forming time of 72 minutes, and the thinning


recrystallisation took place and after 60 minutes’ annealing, the grains were

highly stable, and resisted deformation because the cavity volume fraction is

also high. Sample Q took a forming time of 64 minutes; and the 90 minutes’

annealing time affected the grain boundary strength; the grains slide

randomly, and it reduced the thinning factor, due to the instability of the

grains during the process, resulting in a high cavity volume fraction. Sample

107

R took a forming time of 54 minutes and the thinning factor of 0.8690 was

high; and the 120 minutes’ annealing time affected the grain boundary

strength; the grains slide rapidly in a controlled manner, and occupied the

cavity, thus obtaining a very less cavity volume fraction, reduced forming

time and increased thinning factor. Sample S and T gave a low thinning

factor; and the high annealing time affected the grain boundary strength; the

grains slide randomly; it reduced the forming time and thinning factor, and

increased the cavity volume fraction. Table 5.11 shows the thinning factor of

samples P, Q, R, S and T. Table 5.12 shows the cavity volume fraction of

samples P, Q, R, S and T. Figure 5.29 shows the thickness distribution of the

completely formed samples P, Q, R, S and T along the transverse cross-

section. Figure 5.30 shows the microstructure of the samples P, Q, R, S, T and

Figure 5.31 shows the SEM picture of sample R.

Table 5.11 Thinning factor of the formed samples P, Q, R, S and T

Sample

Average

thickness

tavg (mm)

Thickness

at Pole tpole

(mm)

Thinning

factor

[tpole/tavg]

P 1.456 1.28 0.8791

Q 1.431 1.21 0.8456

R 1.518 1.32 0.8690

S 1.484 1.23 0.8310

T 1.421 1.13 0.7952

108


samples P, Q, R, S and T

Table 5.12 Cavity volume fraction of the formed samples P, Q, R, S and T

Sample

sheet

Forming

time

(minutes)

Grain size

(µm)

Cavity

volume

fraction ( %)

Sample P 72 13 5

Sample Q 64 13 4.448

Sample R 54 13 1.973

Sample S 53 13 6.1974

Sample T 53 14 11.4563

1

1.1

1.2

1.3

1.4

1.5

1.6

1.7

1.8

1.9

2

2.1

1 2 3 4 5 6 7 8 9 10

Th

ick

ne

ssin

mm

Measuring position

Sample P

Sample Q

Sample R

Sample S

Sample T

109

Figure 5.30 Microstructure of samples P, Q, R, S and T

Figure 5.31 SEM picture of Sample R

In general, cavities nucleate at the grain boundaries, and their

subsequent growth and coalescence invariably lead to premature failure. The

presence of cavities in the superplastically formed components could

adversely affect their mechanical properties. Under conditions of

homogeneous compression, cavitation is not observed, and cavities which are

produced during superplastic tensile flow are removed, during the subsequent

compressive flow.

110

Most of the results reported, that the cavity is high and the thinning

is low, or that the cavity is low and the thinning is high; without considering

the initial sheet thickness. In the first segment, sample A had a lesser cavity

volume fraction of 1.65%, while sample B had a high thinning factor of

0.8841 and less forming time; so the annealing time strongly affected the

micro grains. In the second segment, sample G had a lesser cavity volume

fraction of 2.135%, while sample H had a high thinning factor of 0.8738 and

less forming time; here, the thickness increased, so the annealing time was

high. In the third and fourth segment, samples M and R had a lesser cavity

volume fraction and high thinning factor and less forming time. So,

120minutes’ annealing time was the best. Sample P had a high thinning factor

of 0.8791, and high forming time due to the high stability of the grains after

60 minutes’ annealing. In all the segments, after 120 minutes’ annealing did

not produce any adverse effect in the forming time.

5.6 SUMMARY

In the constant pressure method the forming time was 20 minutes,

and the grain growth was less due to the high strain rate; dynamic

recrystalisation took place, but the cavity volume fraction was 12.64%,

because the grains initially gave more resistance to the deformation, and the

thinning factor was 0.8148.

In the two variables pressure method, the forming time was 40

minutes, and the small grain growth took place in a high forming time, but the

cavity volume fraction was 8%; here the grains adjusted the cavity during

slow forming and the thinning factor was 0.8419.

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In the three different forming pressures method, the forming time

was 68 minutes, and the small grain growth took place in a high forming time,

but the cavity volume fraction was 5%, and the thinning factor was 0.8791,

due to the very slow and controlled formation.

Continuous control over the forming pressure enhanced the

thinning factor and reduced the cavity volume fraction, due to the controlled

and steady deformation. In the cavity and temperature study, the forming

temperature of the 530 C sample gave a better result of the cavity volume

fraction of 5%; the forming time was 68 minutes. The combination of this

temperature and forming time gave good stability to the grains.

The superplastic forming temperature of the Al 7075 alloy is

530 C; below this temperature, the micro grains gave high resistance to the

deformation; above this temperature structural instability occurred, because

both these conditions gave high cavity volume fraction.

For a high thickness sheet (1.75 and 2mm) a less cavity volume

fraction and a high thinning factor have been obtained at the same annealing

time (120 minutes). For a low thickness sheet (1.25 and 1.5mm), the

annealing required for a high thinning factor is high, compared to that of a

less cavity volume fraction.

For a sheet of less thickness, when the annealing time was

increased, the cavity volume fraction was also high, whereas in the sheet of

higher thickness, the grains were accommodated easily at a high annealing

time, because the cavity volume fraction was low. The Optical micrograph

and SEM investigation of a high initial sheet thickness, show that during

superplastic deformation, grain boundary sliding accommodated by

intragranular slip is the main mechanism of the deformation process. When

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the annealing time is increased, the cavity volume fraction alters more

compared to the grain size.

After 2 hours’ annealing, irrespective of the thickness (1.25, 1.5,

1.75 and 2mm), the forming time does not affect the forming of the

component, and grain growth was identified only in the less initial sheet

thickness (1.25mm), because the amount of grains were less, compared to that

of the high thickness sheet.