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
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
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
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
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
The front panel
The block diagram
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.
Figure 5.4 Algorithm for the control program of the SPF
Figure 5.5 Front panel of lab - view program
5.4 EXPERIMENTAL PROCEDURE
The experimental work has been divided into the following four
(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
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
Kellers 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
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
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 w