CHAPTER 5 SUPERPLASTIC FORMING OF 7075 Al 5.pdfCHAPTER 5 SUPERPLASTIC FORMING OF 7075 Al-ALLOY ... (up to the first 8 mm formation), 0.45 MPa (up to the next 4 mm ... plus software

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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

    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

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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 w