Kabilraj Thesis

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CHAPTER 1

INTRODUCTION TO SUPERPLASTIC FORMING

Superplastic materials show very high ductility, i.e. maximum elongation of about

500% or even more in some cases, even if they are lowly stressed. This is due to both

peculiar process conditions and material intrinsic characteristics. The aerospace industry has

shown that, in order to produce complex parts requiring large tensile elongations that cannot

be formed by conventional processes, superplastic forming can be used. A detailed design of

technological process is necessary so as to exploit at best the peculiar potentialities of

superplastic forming. When deformed in particular conditions some metallic materials are

characterised by exceptional ductility, i.e. maximum elongation of above 150%, even if they

are lowly stressed. This characteristic, known as superplasticity, is due to both peculiar

process conditions and material intrinsic characteristics. In detail, the forming temperature

should be greater than about half the material absolute melting point and constant during the

whole process. Moreover, the strain rate should be very low: it ranges generally between 10-3

and 10-5 s-1. Finally, the material should have a fine and stable grain size, usually of about few

microns.

The aerospace industry has demonstrated that superplastic forming can be employed

to produce components characterised by complex forms requiring large elongation that

cannot be formed by means of conventional processes. In this way it is possible to decrease

both the complexity of the manufacturing process and the use of structural connections, such

as welded or mechanical joints, which are weak points internal to an assembled part. The

traditional approach to characterise superplastic materials submits suitably shaped samples to

tensile tests. Superplastic forming (SPF) is a near net-shape forming process which offers

many advantages over conventional forming operations including low forming pressure due

to low flow stress, lower die cost, greater design flexibility, and the ability to shape hard

metals and form complex shapes. However, low production rate due to slow forming process

and limited predictive capabilities due to lack of accurate constitutive models for superplastic

deformation, are the main obstacles to the widespread use of SPF. This factor has restricted

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the growth of applications of Superplastic alloy to low volume production industries like the

aerospace industry.

Conventional metals and alloys typically exhibit tensile elongations in the range of

10% to 30%. By producing sheet materials with ultra fine grain size, and performing the

deformation at low strain rates and elevated temperature, elongation can exceed 100% and

may be as high as 200% to 300%. This super plastic behaviour can be used to form materials

into large, into complex shaped products with compound curves. Deep or complex shapes can

be made as single piece, single operation pressings rather than multi step conventional

pressings or multi-piece assemblies.

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CHAPTER 2

REVIEW OF LITERATURE

I STRAIN RATE SENSITIVITY

Chan (2004) have investigated the deformation and cavitations behavior of AL-4.4Cu-

1.5Mg/21SiCw composite under bi-axial stress states with variable strain rate paths where

enquired in this paper. The strain distribution of the composite diagram deformed under this

two-stage variable strain rate path in different dies was determined. It was found that the

distribution similar to that deformed at constant strain rate path. The cavitation behavior and

the limit strains of the composite under the two-stage strain rate path were also determined

and compared with that under a constant strain path.

Ho Sunglee et al (2006) have carried out a series of tensile test on Ti-6Al-4V- ELI

(Extra Low Interstitials) at the strain rate of 10 -4 to 10-2 S-1 and temperature range of 1073-

1223K. The maximum elongation of 1898 % was obtained at the strain rate of 10 -3 at 850˚

C. Based on this result diffusion bonding process of super plastic Ti-6Al-4V-ELI(Extra Low

Interstitials) sheet metals was developed.

Mimaroglu et al (2003) have developed a systematic approach using ANSYS finite

element code to carry out the forming analysis at constant strain rate. In this approach

experimentally obtained material data at different cross head speeds were used to model and

the process was analyzed at constant strain rate.

Yenihayat et al (2004) have developed a systematic approach to control the variation

in thickness of the product by carrying out forming analysis at constant strain rate. In this

approach, experimentally determined material properties data at 525˚C temperature and at

different cross head speeds were used to model and analyze processes at constant strain rates.

II BLOW FORMING

Chung et al (2004) investigated the Superplastic deformation behavior of fine-grained

AZ61 magnesium alloy sheet during equi-biaxial tensile deformation. Thin circular

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diaphragms were successfully deformed into the hemispherical domes at 673K in applied gas

pressure range of 0.46–1.20MPa. In this pressure range, average shell stress in range of 7–

23MPa and average deformation rate in range of 2 × 10−4 to 5 × 10−3 s−1 were imposed on the

deforming hemisphere. The thickness profile of the resulting shape, which is sensitive to

strain rate sensitivity (m) and the extent of deformation, was examined and compared with

the analytical model. The extent of uniformity in thickness distribution is less than predicted

by the theoretical model.

Guo et al (1989) have developed a model for the bulge forming of domes. Which is

based on the observation of stress related hole growth, has been developed. The variation in

dome thickness with time (and height) has been analytically expressed as a function of the

strain rate sensitivity parameter m, the grain size d and the substructure related constant in the

constitutive equation for superplasticity. Both constant pressure and constant strain-rate

forming can be simulated using the model. The predictions of the analysis show good

agreement being obtained over previous models.

Han Wenbo et al (2006) have investigated the superplastic forming/ Diffusion bonding

with gas pressure control for honeycomb structure. The effects of microstructure were carried

out with the bonded samples by using the optimal parameters such as bonding temperature,

bonding pressure, bonding time. The distribution of thickness after SPF/DB was investigated.

III THICKNESS DISTRIBUTION

Hwang et al (2003) have carried out the simulations and calculate the pressurization

profile and the sheet thickness distribution during the blow forming process. A pressure

control algorithm is proposed to keep the maximum strain rate in the deformation zone of the

sheet equal to the target value, which corresponds to the highest m-value of the material

being Superplastic formed. The thickness distribution of the formed product using constant

pressure control and keeping target strain rate control are compared. Experiments using 8090

Al-Li sheets on superplastic blow-forming in a rectangular closed-die are also carried out.

The theoretical predictions of thickness distribution of the product are compared with

experimental results.

Kalaichelvan et al (2005) Studied the eutectic Pb–Sn superplastic sheet materials. Cast

sheet blanks were thermo-mechanically treated to obtain superplastic properties. A

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sophisticated PC-controlled loading system was prepared for the bulge-forming process. An

optimum pressure–time profile based on variable pressure was proposed. In addition, the

optimum pre forming required for improving the uniform thickness distribution and dome

height was also analyzed. The thickness distribution after preforming effect was compared

with optimum pressure–time performance. This combined method based on variable pressure

and preforming gives good results than the variable strain rate method. This work reduces the

system complications on the variable strain rate method. The cavitations in the various

process samples were also analyzed. The combined method based on optimum variable

pressure paths and the preforming gives more reduction in cavitations in formed parts.

Senthil kumar et al (2006) prediction of the final thickness distribution and the strain-

rate necessary to maintain the superplasticity. This paper attempts to explore the superplastic

deformation behavior of the AA7475 aluminium alloy during blow forming into a circular die

by a simple theoretical model and by a numerical simulation using standard finite element

code ABAQUS. The numerical results obtained by a finite element code (ABAQUS) and the

values obtained by the model are compared with the existing experimental values to verify

the validity of both the models. The theoretical prediction of the thickness distribution, strain-

rate and time to form the required bulge has been analyzed with the existing experimental

values.

Tan et al (2007) have studied the cavitations and grain growth during superplastic

forming. In this work, a finite element method is developed, which considers the grain

growth and the effect of material damage. The effects of material parameters and deformation

damage on the superplastic deformation process are numerically analyzed, and the means to

control cavitations growth is discussed. The micro structural mechanism of grain growth

during superplastic deformation is also studied. A new model considering the grain growth is

proposed and applied to conventional superplastic materials. The relationships between the

strain, the strain rate, the test temperature, the initial grain size, and the grain growth

respectively in superplastic materials are discussed. The effect of variation of strain rate

sensitivity (m value) on the strain limit of the superplastic deformation is investigated, and

the theoretically calculated values are compared with the experimental results

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CHAPTER 3

SUPERRPLASTIC FORMING MATERIAL CHARACTERISTICS

The determination of the superplastic properties of a metallic sheet material is

important for the observation, development and comparison of superplastic materials. It is

also necessary to predict the correct forming parameters during an SPF process. SPF tensile

testing has peculiar characteristics compared to conventional mechanical testing, which

distort the true values of stress, strain, strain hardening, and strain rate at the very large

elongations encountered in an SPF pull test, consequently conventional mechanical test

methods cannot be used. This test method addresses those characteristics by optimizing the

shape of the test coupon and specifying a new test procedure.

The evaluation of a superplastic material can be divided into two parts. Firstly, the

basic superplastic-forming (SPF) properties of the material are measured using the four

parameters of stress, temperature, strain, and strain rate. These are obtained using conversions

from the raw data of a tensile test. Secondly, derived properties useful to define an SPF

material are obtained from the basic properties using specific equations.

This test method describes the procedure for determining the superplastic forming

properties (SPF) of a metallic sheet material. It includes tests both for the basic SPF

properties and also for derived SPF properties. The test for basic properties encompasses

effects due to strain hardening or softening.

This test method covers sheet materials with thicknesses of at least 0.5 mm but not

greater than 6 mm. It characterizes the material under a uni-axial tensile stress condition.

Most industrial applications of superplastic forming involve a multi-axial stress condition in a

sheet; however it is more convenient to characterize a material under a uni-axial tensile stress

condition.

This method has been used successfully between strain rates of 10 -5 to 10-1 per

second. This method has been used successfully on Aluminum, Magnesium and Titanium

alloys. The use of the method with other metals should be verified. The values stated in SI

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units are to be regarded as standard. No other units of measurement are included in this

standard.

3.1 STRAIN RATE

All materials will undergo some change in their dimensions when exposed to stress.

Strain is a measure of the amount of deformation that occurs when an object is placed under

stress. Strain rate is defined as the change in strain over the change in time. The deformation

caused by stress can be fully reversible or permanent, depending on the amount of stress

applied. Strain rate is a function of the geometry of the specimen, it is different from the

deformation rate which may be defined as the speed at which a tension test is being carried

out.

3.2 STRAIN RATE HARDENING

Work (Strain) Hardening is when a metal is strained beyond the yield point.

An increasing stress is required to produce additional plastic deformation and the metal

apparently becomes stronger and more difficult to deform. If true stress is plotted against true

strain, the rate of strain hardening tends to become almost uniform, that is, the curve becomes

almost a straight line. The gradient of the straight part of the line is known as the strain

hardening coefficient or work hardening coefficient, and is closely related to the shear

modulus (about proportional). Therefore, a metal with a high shear modulus will have a high

strain or work hardening coefficient. Grain size will also influence strain hardening. A

material with small grain size will strain harden more rapidly than the same material with a

larger grain size. However, the effect only applies in the early stages of plastic deformation,

and the influence disappears as the structure deforms and grain structure breaks down. Work

hardening is closely related to fatigue.

3.3 STRAIN RATE SENSTIVITY

The most important characteristic of superplastic materials is the high sensitivity of

flow stress to deformation rate. In general, a constant strain rate sensitivity index value is

usually used for calibrating models describing superplastic deformation. However,

experimental results indicate that the strain rate sensitivity index depends on strain rate, strain

and does not remain constant during deformation. In this work, the effects of strain rate

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sensitivity variation on the stability of deformation during superplastic forming are examined.

For superplastic materials, the strain rate sensitivity index should be greater than or equal to

0.3 and for the majority of superplastic materials, it lies in the range of 0.4–0.8. Strain rate

sensitivity represents the capacity of the material to resist necking and influences the overall

deformation and stability during superplastic deformation. Therefore, in order to capture the

deformation characteristics of superplastic materials, the strain rate sensitivity has to be

determined accurately.

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CHAPTER 4

MAGNESIUM ALLOY

4.1 INTRODUCTION

Magnesium is the lightest engineering metal available and also has good vibration

damping characteristics. The mechanical properties of pure magnesium are very low,

prohibiting its use in engineering also it is very inflammable and burns with a dazzling flame

developing a great deal of heat. However, the alloys of magnesium posses much better

mechanical properties which ensure applications in structural and non-structural applications

where weight is of primary importance. Magnesium is also an alloying element in various

nonferrous metals.

Magnesium alloys have high strength to weight, readily machinable and good

resistance to atmospheric exposure. Typical uses of magnesium alloys include aircraft and

missile components, material handling equipment, portable power tools such as drills and

sanders, ladder, bicycles, sporting goods and general light weight components

Magnesium alloy developments have traditionally been driven by aerospace industry

requirements for lightweight materials to operate under increasingly demanding conditions.

Magnesium alloys have always been attractive to designers due to their low density, only two

thirds that of aluminium. This has been a major factor in the widespread use of magnesium

alloy castings and wrought products. A further requirement in recent years has been for

superior corrosion performance and dramatic improvements have been demonstrated for new

magnesium alloys. Improvements in mechanical properties and corrosion resistance have led

to greater interest in magnesium alloys for aerospace and speciality applications, and alloys

are now being specified on programmes such as the McDonnell Douglas MD 500 helicopter.

Magnesium alloy forgings are also used in aerospace applications including critical

gearbox parts for the Westland Sea King helicopter and aircraft wheels. Forged magnesium

parts are also used in aero engine applications. In the future, magnesium forgings are most

likely to be used in higher temperature applications. Magnesium alloys are used in light

weight engineering applications. Magnesium-zirconium alloys tend to be used in relatively

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low volume applications where they are processed by sand or investment casting, or wrought

products by extrusion or forging. Zirconium-free alloys, principally AZ91 but also other

alloys, are used in automotive and various other high volume applications.

Wrought magnesium alloys have a special feature. Their compressive proof strength

is smaller than tensile proof strength. After the forming, wrought magnesium alloys have

string texture in the deformation direction and increases tensile proof strength. In

compression the proof strength is smaller because of twinning, which happens more easily in

compression than in tension in magnesium alloys because of the hexagonal lattice structure.

4.1.1 Properties

The following key properties of magnesium alloys as follows,

Light weight

Low density (two thirds that of aluminium)

Good high temperature mechanical properties

Good to excellent corrosion resistance

4.1.2 Material Data

Table 4.1 Material Data

VARIABLES VALUE

Material AZ31B Magnesium alloy

Melting Temperature 6300C

Density 17*103Kg/m3

Young’s modulus 45*103N/m2

Poisson’s ratio 0.35 (No unit)

Shear strength 130MPa

Strain rate sensitivity 0.51 (No unit)

Co-efficient of friction 0.1 (No unit)

Annealing Temperature 3450C

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CHAPTER 5

METHODOLOGY

Materials are being widely used in engineering field according to their applications. In

order to avoid the wastage and to reduce the cost of materials they were optimized. Now a

day’s number of tests has been carried out in order to enhance the material properties. In this

project work magnesium alloys has been consider for study. Different steps like tensile

testing, blow forming and FEA validation has been carried out to enhance the Magnesium

alloy properties. The methodology in which the work has been carried out is shown in figure

5.1.

The superplasticity of a material depends on its strain rate sensitivity. Hence the

material characteristics such as Engineering stress, Engineering Strain, % elongation, Yield

stress are determined through hot tensile test. With this data, the temperature required for

maximum plastic deformation is determined. Then to conduct the bow forming test, a

hemispherical die, furnace is designed. Under various argon pressure the time required for

bow forming into a hemispherical die is determined.

For numerical simulation and for experimental validation, a code is being generated

using Abaqus.

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Figure 5.1Flow chart of methodology

Literature Review

Problem Definition

Material Selection

Material Properties

Composition

Application

Determination of Strain Rate Sensitivity Die Design ABAQUS

Furnace Design Furnace Design Modelling

Hot Tensile Test Selection of Gases Constraints

Grip Design Pressure

Temperature

Load

Application

Load

Elongation

Engineering Stress

Blow Forming Study of Various Parameters

True Stress

True Strain

Strain Rate Sensitivity

Strain in the Specimen Pressure Vs Thinning

Result and Discussion

Conclusion

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CHAPTER 6

DETERMINATION OF STRAIN RATE SENSITIVITY

6.1 EXPERIMENTAL SETUP

The Experimental setup for determining strain rate sensitivity index consists of designing a furnace, gripper, heating coil, control unit. The complete assembly is shown in figure 6.1.

Figure 6.1 Experimental Setup of Tensile test

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6.2 DESIGN OF GRIPPER

The selection and design of gripper can have a great influence on the test, since it

affects the selection of the specimen geometry, and has a direct significant impact on the

most controversial issue of heating/holding time. The BS ISO 20032 standard hardly

describes the clamping device. It merely indicates that the gripping device should impose

pressure on the specimen surface to provide the gripping action. The main problem with the

hole gripper is the hole of the test sample got deformed and it propagated as a crack, thereby

making the material to fail at the earlier stage. The ASTM E2448 standard proposes and

describes in detail a fairly simple clamping device. Pro-e model showing an assembled view

of the gripper and specimen are shown in figure 6.2.

Figure 6.2 Pro-E model of old gripper for hot tensile test

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Figure 6.3 Old Gripper design for Tensile Test

The different views of old gripper for hot tensile test are shown in figure 6.3 .The

gripper will be clamped to the tensile specimen with the help of the Allen screws.

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Figure 6.4 New Gripper Design for Tensile Test

Figure 6.5 Isometric view of the new gripper

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The hole of the test sample starts deforming and it is propagated as a crack there by making material to fail at the earlier stage. So the design was changed to without hole in the gripper as mentioned in figure 6.4. The new gripper design in isometric view is shown in figure 6.5.But compared to old gripper, setting time of the specimen in the new gripper was more but the end results were more satisfactory.

6.3 DESIGN OF FURNACE

A furnace is a device used for heating. It consists of a temperature controller and an

electrical furnace. The electrical furnace contains 2 ceramic heating elements of 2.3 Amps

each. The furnace is capable of producing temperatures in the range of 0°C to 630°C. The

temperature control varies the temperature by increasing and decreasing the control supply.

The furnace is cylindrical type and it is fixed with separate stand. Its electrical leads are

connected to the temperature control unit from where the temperature inside the furnace can

be varied. The unit also consists of a thermocouple which senses the temperature inside the

furnace and sends the data as feedback to the temperature control unit, which then varies the

temperature accordingly. The schematic diagram of the complete furnace unit is shown in the

Figure 6.6.

Figure 6.6 Furnace Design for hot Tensile test

http://en.wikipedia.org/wiki/Heat

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6.4 PARAMETER OF HOT TENSILE TEST

The specimen for the hot tensile test as per ASTM E2448 standard is shown in figure 6.7. The required dimension specimen was machined from rectangular workpiece by milling and wire cutting EDM process.

Figure 6.7 Specimen for hot tensile test as per ASTM Standards

6.4.1 Milling

The plate which we obtained was initially of a thickness of 6mm and dimensions 300

x150mm, this was milled down to 3mm thickness which was the required ASTM E2448

standard as specified in the above figure 6.7.

6.4.2 Wire Cutting Process

Preceding the wire cutting operation the 300x150x3 plate was sized into smaller ones

of size 35x75x3mm for the purpose of economical feasibility. These smaller plates of the

foresaid dimensions were stacked on top of each other and the wire cutting process was

performed on them.

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6.5 STRAIN RATE SENSITIVITY CALCULATIONS

The superplasticity of a material depends on its strain rate sensitivity. So the strain rate sensitivity calculation is very important role in this work. The strain rate sensitivity calculations are as follows.

6.5.1Engineering Strain Vs True Strain

For many engineering applications, the use of 'engineering stress and strain' values for

material stress-strain curves will be sufficient for obtaining correct answers in a plasticity

analysis. Engineering stress and strain are commonly represented in 2D by these equations:

= L / Lo ----(6.1)

= F / Ao ----(6.2)

However, engineering strain is a small strain measure which is invalid once the strain

in your model is no longer 'small' (approximately 5%). True strain, which is a nonlinear strain

measure that is dependent upon the final length of the model, is used for large strain

simulations. True stress and strain are commonly represented in 2D by these equations:

= ln(L / Lo) ----(6.3)

= F / A ----(6.4)

As engineers are often supplied with engineering stress and strain test data, a

conversion to true stress and strain (also called log strain) is needed before inputting these

material properties into Abaqus. For stress-strain data, engineering stress and strain can be

converted to true stress - log strain by:

l = ln (1+) ----(6.5)

= (1+) ----(6.6)

The strain rate sensitivity calculations has beeen carrieed out in order to find the materials superplastic behaviours.The calculations as follows,

έ = strain rate

έ = V/LO(1+e) ----(6.7)

where,

v = Machine cross head velocity

LO=original length (gage length)

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e= Engineering strain

m = ln ∆σ/ ln ∆έ (Or) m= log (σ2/σ1)/log (έ2/έ) ----(6.8)

According ASTM E646 standard, the strain hardening coefficient and strain hardening exponent

log σ = log K + log E ----(6.9)

N N N

n = N ∑ (log εi log σi) – (∑log εi log σi) / N(log εi) 2 – (∑ log εi)2

i=1 i=1 i=1

where, ----(6.10)

y=log σ;

x=log ;

b= log K ;

n= N ∑xy - ∑x ∑y / N ∑X2 - (∑X)2 ----(6.11)

b=∑y - n ∑x /N = ----(6.12)

K=exp[b] ----(6.13)

where,

n = strain rate sensitivity index

CHAPTER 7

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SUPERPLASTICITY THROUGH BLOW FORMING

7.1. Superplasticity

Superplasticity is the ability of certain materials to undergo large elongation at the

proper temperature (> 0.5 Tm) and strain rate (0.0001s-1 to 0.01s-1). Under proper conditions

these materials can be strained several times their original length. Superplastic materials are

characterized by large neck free elongation under low-stress (0.2 MPa to 2 MPa), when they

are formed at temperature exceeding about one half the melting point (0.5Tm). Superplastic

forming is carried out essentially under isothermal conditions with very low stain rates

(0.0001s-1 to 0.01s-1).

7.1.1 Superplastic Forming

Conventional metals and alloys typically exhibit tensile elongations in the range of

10% to 30%. By producing sheet materials with ultra fine grain size, and performing the

deformation at low strain rates and elevated temperature, elongation can exceed 100% and

may be as high as 200% to 300%. This super plastic behavior can be used to form materials

into large, into complex shaped products with compound curves. Deep or complex shapes can

be made as single piece, single operation pressings rather than multi step conventional

pressings or multi-piece assemblies.

7.1.2 Requirements of Superplasticity

For materials exhibiting superplastic behavior, the high ductility will be observed only

under certain condition, thus the basic requirements for superplasticity are:

1. Very fine grain size material (less than 10µm)

2. Relatively high temperature (greater than about one-half the absolute melting point)

3. A controlled strain rate, 0.0001 to 0.01s-1

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Because of these requirements, only a limited number of commercial alloys exhibit

superplastic behavior, and these materials are formed using methods and conditions that are

different from those used for conventional metals.

7.1.3 Characteristic of Superplastic Metals

The parameter that is commonly selected as measure of superplastic formability is the

tensile elongation at the optimum temperature i.e. creep, and strain rate. Because,

superplasticity is a highly strain rate sensitive property and real component can experience

significant variation in strain rate during forming, tensile elongation is measured as a function

of strain rate.

The forming temperature is just as important a variable in superplastic forming as the

strain rate. Temperature variation in a forming die is a primary source of localized thinning.

Characterization of material behavior should therefore include not only determination of the

optimum superplastic temperature but also the sensitivity of flow stress and elongation to

temperature. A large temperature sensitivity of flow stress is not desirable, because local hot

spots will lead to severe strain localization. The modes of failure in superplastic forming are

strain localization and necking. Therefore fracture occurs in most superplastic materials of

engineering application.

Grain size has a profound influence on the superplasticity of metals. When the grain

size is fine, the flow stress is low, the value of m (strain rate sensitivity index) is generally

high, and the tensile elongation is greater. Characterization of grain size is therefore

important in the overall characterization of superplasticity. A few coarse grain in an

otherwise fine grain structure can control the strain rate range over which m is high, and may

in some cases cause the appearing of a threshold stress. The important effect of grain size

distribution in real materials is to produce a relatively high m (m>0.5).

7.1.4 Variables in Superplastic Forming

Superplastic flow may be described by an equation

σ = Kέm, -----(7.1)

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where,

‘σ’ is effective flow stress (N/m2)

‘έ’ is strain rate, (S-1)

‘K’ is constant depends upon temperature and grain size

‘m’ is strain rate sensitivity index

The value of ‘m’ has a controlling influence on the stability of superplastic flow. The

value of ‘m’ lies between 0.3 and 0.9 for most of the superplastic materials. The high value of

‘m’ imports to the material resistance to localized deformation such as necking and thinning

so that the specimen can undergo large deformation without failure. A high value of ‘m’

causes the flow stress to be highly sensitive to the strain rate. An ideal value of m=1 would

correspond to Newtonian viscous flow which leads to complete neck-free tensile

deformation.

7.1.5 Advantages of Superplastic Forming

Superplastic forming is used to produce a complex shape.

Forming pressure is drastically reduced. In some cases enough to make feasible, the

use of economical, novel or more light weight, forming equipment.

Considerable cost savings may be mainly because extremely close tolerance can be

guaranteed which reduces machining cost and process therefore, labor intensive.

Wastage is minimized so that there is maximum utilization of materials, which is an

important in energy intensive materials.

A uniform microstructure is produced which leads to uniform reproducible,

mechanical properties throughout the body of finished product.

Service properties are generally improved over conventionally formed materials

because a, fine uniform grain size is obtained which leads to better strength, ductility,

and fatigue resistance.

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7.1.6 Applications of superplastic forming

The major applications of superplastic forming as follows,

1. In automotive body panel.

2. In forming of aircraft frames and skins.

3. Diaphragm forming of plastics.

4. Complex shape parts, window frame.

7.2 EXPERIMENTAL SETUP

The Experimental setup for performing blow forming consists of Hydraulic press,

specially designed die, Argon gas, Cylinders, gas regulation and pipe linings. The complete

assembly of the Experimental setup is shown in figure 7.1

Figure 7.1 Experimental Setup for Blow forming

The press unit available has a hydraulic power press of 60 tons capacity and a main motor of

10 horse power. The fixture is bolted on to the top die using Allen screws. The top die is

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made to seat on top of the bottom die with the help of the grooves provided. A stainless steel

rod is attached to the L-shaped hole in the top die. On to the top die, a circular pipe for the

passage of argon is welded and the other end is connected to the argon gas hose pipe. The

compressed argon gas is passed into the top die

7.3 DIE DESIGN

Figure 7.2 Two Dimensional view of Top Die

The Die Design for performing hemispherical cup is shown below. It consists of two

part- male and female part. On the female part, the workpiece is placed and above which the

male part is placed. Atmost care has to be taken in such a way, there should not be any

clearance at the interface between the mating parts, or otherwise, the required pressure set at

the gas regulator will not be given to the workpiece. To the male part a pipe is welded for the

passage of Argon Gas. The complete die assembly is shown in figure 7.1.The top die in two

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dimensional view is shown in figure 7.3. The bottom die in two dimensional view is shown in

figure 7.4.

Figure 7.3 Two Dimensional view of Bottom Die

7.4 FURNACE DESIGN

It consists of a temperature controller and an electrical furnace. The electrical furnace

contains 4 ceramic heating elements of 3.6 Amps each. The furnace is capable of

temperatures in the range of 0°C to 1000°C. The temperature control varies the temperature

by increasing and decreasing the voltage supply. The furnace is of hinged type and can be

opened to accommodate the die unit in its central cavity. Its electrical leads are connected to

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the temperature control unit from where the temperature inside the furnace can be varied. The

unit also consists of a thermocouple which senses the temperature inside the furnace and

sends the data as feedback to the temperature control unit, which then varies the temperature

accordingly. The schematic diagram of the complete furnace unit is shown in the following

Figure.7.4. The schematic diagram of the complete experimental setup is shown in figure 7.5.

Figure 7.4 Schematic Diagram of Furnace Unit Setup

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Figure 7.5 Schematic Diagram of Experimental Setup

CHAPTER 8

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NUMERICAL ANALYSIS OF BLOW FORMING USING ABAQUS

8.1 STUDY OF ABAQUS SOFTWARE:

A complete ABAQUS/Standard analysis usually consists of three stages:

preprocessing, simulation, and post processing. These three stages are linked together by files

as shown below in Figure.8.1.

Figure 8.1 ABAQUS FLOW CHART

8.1.1 Preprocessing (ABAQUS/CAE)

In this stage you must define the model of the physical problem and create an

ABAQUS input file. The model is usually created graphically using ABAQUS/CAE or

another preprocessor, although the ABAQUS input file for a simple analysis can be created

directly using a text editor.

8.1.2Simulation (ABAQUS/Standard)

30

The simulation, which normally is run as a background process, is the stage in

which ABAQUS/Standard solves the numerical problem defined in the input file. Examples

of output from a stress analysis include displacements and stresses that are stored in binary

files ready for post processing. Depending on the complexity of the problem being analyzed

and the power of the computer being used, it may take anywhere from seconds to days to

complete an analysis run.

8.1.3 Post processing (ABAQUS/Viewer)

We can evaluate the results once the simulation has been completed and the

displacements, stresses, or other fundamental variables have been calculated. The evaluation

is generally done interactively using ABAQUS/Viewer or another postprocessor.

ABAQUS/Viewer, which reads the neutral binary output database file, has a variety of

options for displaying the results, including Colour contour plots, animations, deformed

shape plots, and X–Y plots.

8.2 COMPONENTS OF AN ABAQUS ANALYSIS MODEL

An ABAQUS model is composed of several different components that together describe the

physical problem to be analyzed and the results to be obtained.

8.2.1 Discretised geometry

Finite elements and nodes define the basic geometry of the physical structure being

modeled in ABAQUS. The Each element in the model represents a discrete portion of the

physical structure, which is, in turn, represented by many interconnected elements. Elements

are connected to one another by shared nodes. The coordinates of the nodes and the

connectivity of the elements i.e., which nodes belong to which elements, comprises the model

geometry. A part model of 72.8 mm diameter and 3mm thickness was created. The part

model is shown in Figure 8.2.

The collection of all the elements and nodes in a model is called the mesh. The

element type, shape, and location, as well as, the overall number of elements used in the

mesh, affect the results obtained from a simulation. The greater the mesh density (i.e., the

greater the number of elements in the mesh), the more accurate are the results. As the mesh

31

density increases, the analysis results converge to a unique solution, and the computer time

required for the analysis increases. The solution obtained from the numerical model is

generally an approximation to the solution of the physical problem being simulated. 65,000

four noded elements were used in the mesh. The discretised geometry of the model is shown

in Figure.8.3.

Figure 8.2 Part Model of the Sheet

Figure 8.3 Discretised geometry of the model

8.2.2 Material data

32

Material properties for all elements must be specified. While high-quality material

data are often difficult to obtain, particularly for the more complex material models, the

validity of the ABAQUS results is limited by the accuracy and extent of the material data.

The data already presented in Table 4.1.

8.2.3 Loads and boundary conditions

Loads distort the physical structure and, thus, create stress in it. An uniform pressure

of 0.6 Mpa was applied on the surface and temperature was kept at 420 0C. Boundary

conditions are used to constrain portions of the model to remain fixed (zero displacements) or

to move by a prescribed amount (nonzero displacements).The sheet is allowed to move freely

only along Z axis. The boundary conditions and loads applied on the model are shown in

Figure.8.4.

Figure 8.4 Application of Boundary Conditions and Loads

8.3 OUTPUT REQUESTS

An ABAQUS simulation can generate a large amount of output. To avoid using

excessive disk space, you can limit the output to that required for interpreting the results.

Generally preprocessor such as ABAQUS/CAE is used to define the necessary components

of the model. The output is shown in Figure 8.4.

33

Figure 8.5 Simulation of Superplastic Forming

34

CHAPTER 9

RESULTS AND DISCUSSIONS

9.1 HOT TENSILE TESTS

The hot tensile test tests are carried out under different loads and at different

temperatures. By observing the graphs from different tests, the values of strain rates and

strain rate sensitivities are obtained as shown below. The Engineering Stress strain plots and

true stress strain plots for 2mm thickness specimens and 3 mm thickness specimens are

shown in fig. 9.1 to 9.4 respectively. The true stress and true strain are calculated based upon

the relationship expressed in chapter 6. Theses curves are plotted to study the behaviour of

the material (i.e) the flow stress at various temperatures.

. Figure 9.1 Engineering stress Vs Engineering Strain

35

Fig. 9.2 True Stress Vs True Strain ( 2 mm thickness)

36

Fig. 9.3 Engineering Stress Vs Engineering Strain ( 3mm thickness)

Fig. 9.4. True Stress Vs True Strain ( 3 mm thickness)

From the above graph, it is clearly depicted that as the temperature increases the yield strength (i.e) the flow stress of the material decreases and the curve shifts towards the right hand side. Hence it is inferred that the magnesium alloy on heating above the recrystallisation temperature, elongates more and has more plastic deformation. Based on the above conclusion, it is inferred that even though magnesium alloy exhibits hexagonal closed pack crystal structure, on heating it behaves more plastically. The temperature at which the maximum elongation before failure is also noted and this temperature is taken as a reference for superplastic forming.

9.1.1 TEMPERATURE Vs % ELONGATION

The temperature at which maximum elongation is obtained is determined from the following bar chart shown in figure. 9.5 to figure. 9.7. from these chart it is inferred that the maximum % elongation of 40 % was obtained for 4000 C. This temperature is taken as a reference while performing the super plastic forming experiment for magnesium alloy. The maximum elongation at 4000 C might be due to the fact that at this temperature the material recrystallises itself and grains get refined giving a eauiaxed structure.

37

Figure 9.5 Temperature Vs Elongation (%) for 2mm thickness

38

Figure 9.6. Temperature Vs Elongation (%) for 3mm thickness

Temperature Vs % Elongation in different temperatures

0

10

20

30

40

50

60

1

Temperture

% E

lon

gat

ion

300 cross head velocity 1.25



39

Figure. 9.7 Temperature Vs Elongation (%) at different temperatures (3 mm thickness)

Strain rate hardening and Strength coefficients Calculations

9.1.2 LOG TRUE STRESS Vs LOG TRUE STRAIN

The logarithmic true stress strain curve for Magnesium AZ31B at various temperatures is shown below to determine the strain rate sensitivity of the given material at various conditions.

Specimen No 1: 300oC @ 1.25mm/min

Figure 9.8 lnἑ Vs ln of sample 1

40

Figure 9.9 True stress Vs strain rate of sample 1

From the above graphs the strain rate sensitivity (m) was found to be 0.57 for the

given conditions.


41

Figure 9.10 ln ἑ Vs ln of sample 2


From the above graphs the strain rate sensitivity (m) was found to be 0.72 for the given

conditions.

Sample No3: 350oC @ 1.25mm/min

42




conditions.


43




conditions.

Specimen No 5: 300°C @ 2.5mm/min

44




conditions.


45


From the above graphs the strain rate sensitivity (m) was found to be 0.54 for the

given conditions.


The strain rate sensitivity of AZ31B alloy at various temperatures ranges from 0.3 to

0.72. The maximum value of the strain rate sensitivity occurred at the temperature of 3000C

and at a strain rate of 1.7x10-5

46

9.2 SUPERPLASTIC FORMING

Out of 8 experiments only 3 experiments were successful with different forming

heights and thicknesses variations. The other 5 experiments failed due to the following

reasons:

Due to thermal losses.

Defects in sheet metal

Hardening of Sheet Metal

Clearance between contact dies

Oxide layer formation

Due to misalignment of work piece

Gas leakage in hosepipe and between the contact dies

Due to time constraints all the above problems could not be rectified. The problem

of clearance between the contact dies was solved by using a copper ring between the contact

dies. Copper ring also acts as a sealing and this avoided the gas leakage in the die.

A nipple of appropriate size was used to connect the hose pipe from the cylinder to

the connecting pipe of top die. The nipple was braced to the connecting pipe. The Magnesium

sheet was cut according to specific dimensions which fit exactly in the groove of the bottom

die and this avoided the misalignment of work piece during the forming process.

The thermal losses were controlled by using extra insulating material (glass wool),

but still 100-120 oC were lost during the process.

Oxide layer formation could have been avoided by vacuum conditions. The two

successful experiments with different process parameters such as sheet thickness, forming

time, applied pressure and temperature.

The results of all the 8 experiments are tabulated from Table 9.1

EXPERIMENT RESULT TABLE -9.1

Exp. No

Thickness(mm)

Forming Time

Forming temp

Forming height

Forming Pressure

Remarks

47

(Min) (0 C) (mm) (Mpa)

1 3 - 350 - 0.2 Sheet not formed

2 3 - 350 - .3 Sheet not formed

3 3 8 400 18 0.4 Sheet partially

formed.

4 3 10 400 - 0.6 Sheet not formed.

5 3 12 450 7 0.3 Sheet formed.

6 3 15 500 16 0.5 Sheet partially

formed.

7 3 10 550 18 0.5 Sheet formed. But

not completely

ignited.

8 3 20 600 - 0.5 Sheet ignited.

48

CHAPTER 10

CONCLUSIONS

Superplastic properties of AZ31B have been observed in the tensile tests conducted.

The results of the tensile test have been recorded and plotted.

The Magnesium alloy AZ31B of true stress, true strain, strain rate and strain rate

sensitivity index, was found by tensile test.. The strain rate sensitivity is found to be in the

superplastic range.

Forming trials for AZ31B magnesium sheet were successfully carried out in a

hemispherical die. The forming process was numerically simulated using finite element

package ABAQUS.

Tensile tests were conducted at different temperatures and load values. The value of

strain rate was calculated from the steady state region of the creep curve. The value of strain

rate sensitivity index was obtained from log strain rate Vs log stress graph in the linear

region. Both the values of strain rate and strain rate sensitivity index correspond to the

requirements for superplastic behavior. The results obtained experimentally and analytically

were compared.

From the study of theoretical model, it is possible to establish a simple mathematical

relation which governs the superplastic forming of simple die geometries like hemispherical.

At a given forming temperature, as the forming pressure increases, the forming time

decreases.

At a given temperature, regardless of initial blank thickness, the amount of thinning at

the pole is increased with increasing forming pressure.

The strain-rate decreases rapidly in the initial stage of forming which is about 8% of

the total forming height is mainly due to the large deformation of the sheet initially, as

compared to the later stages of forming.

49

APPENDIX 1

Shape obtained from blow forming

Experimental setup for Blow forming Tensile test machine

50

APPENDIX 2

( I ) ( II)

Experimental results obtained from Tensile specimen ( I ) with hole at different temperature ( II ) without hole at 300 0 C

51

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53

PUBLICATIONS

1.Kabilraj.S And Rameshbabu.S., (2010), “Superplastic Behaviour Of AZ31B Magnesium Alloy Using Abaqus”Proceedings of National Conference on April at Sastha Engineering

College