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Ben Project 3
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CREATIVE DESIGN OF PATTERN FOR SAND
CASTING OF TURBINE BLADE
A THESIS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technolgy
In
Mechanical Engineering
By
BENAKNAIK S G
Department of Mechanical Engineering
National Institute of Technology, Rourkela
2009
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CREATIVE DESIGN OF PATTERN FOR SAND
CASTING OF TURBINE BLADE
A THESIS SUBMITTED IN PARTIAL FULFILLMENT
OF THE REQUIREMENTS FOR THE DEGREE OF
Bachelor of Technology
In
Mechanical Engineering
By
BENAKNAIK S G
Under the Guidance of
PROF. S. K. SAHOO
Department of Mechanical Engineering
National Institute of Technology, Rourkela
2009
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National Institute of Technology
Rourkela
CERTIFICATE
This is to certify that the thesis entitled Creative design of pattern for sand casting of
turbine blade Submitted by Benaknaik S G, Roll No: 10503041 in the partial fulfillment of
the requirement for the degree ofBachelor of Technology in Mechanical Engineering,
National Institute of Technology, Rourkela , is being carried out under my supervision.
To the best of my knowledge the matter embodied in the thesis has not been submitted to
any other university/institute for the award of any degree or diploma.
Prof. S. K. Sahoo
Date: Department of Mechanical Engg
National Institute of Technology
Rourkela-769008
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ACKNOWLEDGEMENT
We avail this opportunity to extend our hearty indebtedness to our guide Prof. S.K.
Sahoo , Mechanical Engineering Department, for their valuable guidance, constant
encouragement and kind help at different stages for the execution of this dissertation
work.
We also express our sincere gratitude to Dr. R.K.Sahoo, Head of the Department,
Mechanical Engineering, for providing valuable departmental facilities.
Submitted by:
Benaknaik S G
Roll No: 10503041
Mechanical Engineering
National Institute of Technology,
Rourkela
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CONTENTS
Page No
ACKNOWLEDGEMENT 3
CERTIFICATE 4
ABSTRACT 6
CHAPTER 1 - INTRODUCTION
INTRODUCTION 7
HISTORICAL BACKGROUND 8
PRESENT TREND 9
Chapter 2 EXPERIMENTAL METHODOLOGY
DESIGN OF PATTERN 11
EXPERIMENTAL PROCEDURE 14
INVESTMENT CASTING 16
CONCLUSION 21
REFERENCE 21
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ABSTRACT
The manufacturing of turbine blades is often outsourced to investment casting
foundries by aerospace companies that design and build jet engines. Aerospace companies have
found that casting defects are an important cost driver in the price that they pay the foundries for
the turbine blades. Defect types include porosity, stress, grain, fill, and mold-related defects. In
order to address the defect problem, aerospace companies have adopted a design for manufacture
approach to drive the cost of the turbine blades down. The principal research objective of this
thesis was to discover how the critical part features on the turbine blade drive the number of
manufacturing defects seen in the casting process. In the experiment, the dimensions of the
critical part features were varied in order to quantify how the critical part features relate to
manufacturing defects.
A short holding time will yield a more accurate pattern, but too
short a holding time will cause distortion when removing it from the mould, as it is too soft. Too
long a holding time will cause more shrinkage. For the silicone mould, only the injection
temperature has an effect on the dimensions of the wax patterns. The dimensional errors incurred
during dipping are also measured and found that generally, there is a reduction of 0.20.4% in
dimension. These studies will help the investment caster to estimate the allowance required in
the initial CAD drawings to produce a final casting with minimal dimensional inaccuracy.
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CHAPTER 1
INTRODUCTION
BECAUSE turbine blades play a key role in the performance of
BECAUSE turbine blades play a key role in the performance of
BECAUSE turbine blades play a key role in the performance of advanced turbine engines, a
number of critical conditions must be satisfied in order to ensure adequate operation at working
temperatures (Ref 1). These conditions include high-temperature creep strength and thermal and
mechanical fatigue strength. Since the efficiency of turbine engines increases with temperature,
considerable effort has been directed toward the development of advanced alloys for stable
operation under extreme conditions. Wind turbine (W/T) blades, while in operation, encounter
very complex loading sequences, due to the stochastic nature of wind conditions on wind
turbines sites. The suitability of a particular W/T blade to operate on a specific site is assessed
through a certification procedure
which entails the conduction of a series of static and fatigue laboratory tests on the W/T blade.
The purpose of such tests is to ascertain that the blade can survive the applied (static and fatigue)
loads as per the applicable design standards [1], [2], while the applied static loads aim to
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simulate the 1-in-50-years gust (and is applied on the blade for ten seconds during testing),
followed by fatiguing the same blade for an accelerated 20-years fatigue lifetime test. EarEarly
attempts were based on the use of single y-phase
HISTORICAL BACKGROUND
In the last few years, the principles of good design of filling systems have been defined by
research at the University of Birmingham using mainly sand block moulds [11,12]. The new
designs work well, avoiding the generation of defects such as porosity and inclusions, etc.
However, the task was to use these newly established principles to see if they could be applied
to the design of new bottom-filled investment castings. The production of defect-free vacuum
castings was the aim of the study. Most turbine blades for the aerospace industry are now
produced predominantly by directional solidification. However, in other industries using turbine
power for ships or rail, equiaxed blades are commonly used for ease of manufacture and cost
effectiveness. Normally, the relative proportion of blades produced by equiaxed and directional
solidification is approximately 50:50 at this time. However, the principle of good filling system
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design is appropriate to both techniques, because each is susceptible to the creation of defects
during the filling process.
While wind turbines have been in use for a very long time, only
recently has there been a commercially significant interest by individuals for using wind turbines
to generate power for their homes. In part, this increased interest is due to rising energy costs,
environmental concerns, and lower costs for wind turbines. As a result of this increased interest
in wind power, many new designs of wind turbines have been created. Notwithstanding this
innovation, wind turbines can still be generally classified as either horizontal axis wind turbines
("HAWTs") or VAWTs.
PRESENT TREND
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Investment casting is a method of producing high quality casting. It is especially useful for
providing casting in
geometrys which could not be forged or machined, or where machining would be too wasteful
of material. Investment casting is especially prevalent in the production of turbine blades for
both aerospace and land based turbines. Due to the nature of the casting of the required alloys,
there has to be a high degree of confidence in the shell itself. This is because the cost incurred
when the shell fails during casting can be unacceptable both in terms of furnace down time and
lost materials. In this study the combination of stereolithography (SLA) process and QuickCast
techniques are used for building the prototype pattern and investmentcasting shells.
QuickCastTM combines the SLA prototyping technique with investment casting. A SL
QuickCast pattern differs from a normal SL pattern. These are built in a honeycomb-like fashion
with a strong external skin to reproduce the required shape. If the pattern were solid, the ceramic
shell would be cracked in the burning-out process due to large differences in the thermal
expansion coefficients between ceramic and SL materials. Fig. 1 shows the schematic diagram
of how CAD system integrates with SL process to form a QuickCastTM in an investment
casting process. The investment casting process or what used to be known as lost wax
process is not a new process. Archaeological investigation can be traced back to 4000 B.C. It
was used to produce art castings until the early 20th century. By 1930, investment casting ranked
as a useful specialised casting method, but was of little relevance to mainstream engineering.
The start of World War 2 changed this situation as the demand for finished components for
aircraft and armaments increased and cannot be met by the machine tools industry. The attention
is turned to investment casting to produce precision components . As mentioned by Beeley and
Smart , the traditional process needs to address four requirements to meet this challenge.
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These essential requirements are reproducibility of castings
within close dimensional limits, production of castings in high melting point alloys, high
standard of metallurgical quality and cost savings over parts produced by alternative
manufacturing techniques. Investment casting is classified as a precision casting process. It
lends itself well to rapid prototyping and manufacturing because of its abilities to produce an
accurate and complex casting. As the industries grow, the demand for functional metal working
prototypes increases. Other RPM techniques like SLA can only be used to determine the form
and fit but not the functionality of the prototypes. The latter can only be accomplished by using a
metal prototype, which can be produced using investment casting. Therefore the ability to
control and improve the accuracy getting more attention as the need for more accurate metal
prototype rises. The areas which affects the dimensional accuracy are wax system (pattern wax,
wax press, injection parameters),mould system (type, material, dewax method, wrapping,
backing material), and gating system (alloy, pouring temperature, placement of gates and risers,
positioning of casting on sprue, mould filling method). Therefore, in order to improve the overall
accuracy of the casting, it is essential to improve the accuracy of the individual stages. The
logical place to start improving the accuracy is the wax system since various defects such as wax
pattern composition, wax preparation, injection characteristics, mould filling and temperature,
sprue size, wax temperature and die design affect the wax pattern .
CHAPTER- 2
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EXPERIMENTAL METHODOLOGY
One of the primary objectives of this work is to minimize the dimensional inaccuracies in
producing the wax patterns by using either hard or soft tooling. In this present work attempts
have been made to optimise the injection parameters to achieve better dimensional accuracy of
the product. In addition the dimensional accuracy of the wax patterns made from a hard and soft
tooling are compared.
Design of pattern
In order to design the specific shape of the product for this study several issues were considered.
Some of the key issues considered for the design of shape of the product are as follows: (a)
complexity of the shape for the easy removal of the wax pattern from the mould; (b) complexity
of features, which can distort the shape easily; (c) should have both constrained and
unconstrained dimensions so that the variation between soft and hard tooling can be compared.
By considering all these issues an shape as shown in Fig. 1 is selected, since it satisfies most of
the criterion discussed earlier. Investment casting is classified as a precision casting process. It
lends itself well to rapid prototyping and manufacturing because of its abilities to produce an
accurate and complex casting. As the industries grow, the demand for functional metal working
prototypes increases. Other RPM techniques like SLA can only be used to determine the form
and fit but not the functionality of the prototypes. The latter can only be accomplished by using a
metal prototype, which can be produced using investment casting. Therefore the ability to
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control and improve the accuracy getting more attention as the need for more accurate metal
prototype rises.
The areas which affects the dimensional accuracy are wax system
(pattern wax, wax press, injection parameters), mould system (type, material, dewax method,
wrapping, backing material), and gating system (alloy, pouring temperature, placement of gates
and risers, positioning of casting on sprue, mould filling method). Therefore, in order to improve
the overall accuracy of the casting, it is essential to improve the accuracy of the individual
stages. The logical place to start improving the accuracy is the wax system since various defects
such as wax pattern composition, wax preparation, injection characteristics, mould filling and
temperature, sprue size, wax temperature and die design affect the wax pattern.
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FIG. 1
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Fig.2 Side view Fig.3 Top view
EXPERIMENTAL PROCEDURE
Investment casting
In investment casting, the pattern is made of wax, which melts after making the mold to produce
the
mold cavity. Production steps in investment casting are illustrated in the figure:
Advantages:
Arbitrary complexity of castings
Good dimensional accuracy
Good surface finish
No or little additional machining (net, or near-net process)
Wax can be reused
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Disadvantages:
Very expensive process
Requires skilled labor
Area of application:
Small in size, complex parts such as art pieces, jewelry, dental fixtures from all types of
metals.
Used to produce machine elements such as gas turbine blades, pinion gears, etc. which do
not require or require only little subsequent machining.
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Fig. 4
Thus, an investment casting mould consists of individual layers of fine refractory
material and granular refractory material held together by a binder that has been set to a rigid
gel. Flexibility exists in changing the composition of each layer. Different methods can be used
to remove the wax pattern, normally steam autoclave, leaving a hollow shell. Shells are fired and
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filled with molten metal that solidifies inside the shell. After casting, the ceramic shell is
removed through mechanical or chemical methods to obtain the parts.
The investment casting process has increasingly been used to produce components for the
aerospace industry and it has been particularly successful for the production of single crystal
turbine blades. Problems associated with ceramic shell materials have been exacerbated
following the introduction of the Environmental Protection Act.
The investment casting process involves the production ofengineering castings using an
expendable pattern . The principles can be traced back to 5000 BC .when Early Man employed
the method to produce rudimentary tools. This was followed by centuries of use OF jewellery
and artistic products before the advent of the 2nd World War saw the development of aerospace
and subsequently engineering components.
The term investment casting derives from the characteristic use of
mobile ceramic slurry, or investments, to form a mould with an extremely smooth surface.
These are replicated from precise patterns and transmitted in turn to the casting. Investment
casting allows dimensionally accurate components to be produced and is a cheaper alternative
than forging or machining, since waste material is kept to a minimum . Production of the
investment casting ceramic shell mould is a crucial part of the whole process. The basic steps in
the production of an investment cast component using a ceramic shell mould are shown in . First,
multicomponent slurries are prepared composed of a fine meshrefractory filler system and a
colloidal binder system. A pattern wax is then dipped into the slurry, sprinkled with coarse
refractory stucco and dried.
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FIG. 5
CONCLUSIONS
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It is becoming imperative that the investment casting industry improves current casting quality,
reduces manufacturing costs and explores new markets in order to remain competitive.
Optimisation of the mechanical and physical properties of the material cast will be fundamental
to achieving these aims. Production of the mould is time consuming, currently taking between 24
and 72 h depending upon the component, due to the need to use controlled moisture removal.
Drying and strength development are the most significant rate-limiting factors in the reduction
of lead times and production costs for the industry.
REFERENCES
BOOKS
1. Production technology, HMT publication.
2. Elements of workshop technology, S K Hajra Choudhury, S K Bose, A K Hajra choudhury,
Niranjan Roy, VolII, Media promoters and media publications.[[
WEBSITES
[1] http://en.wikipedia.org/wiki/turbine
[2] http:// sciencedirect.com
[3] http:// www.scopus.com
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