CREATIVE DESIGN OF PATTERN FOR SAND

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1


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

BENAKN
AIK S G





Department of Mechanical Engineering

National Institute of Technology
,

Rourkela

2009

2


CREATIVE DESIGN OF PATTERN FOR SAND
CASTING OF TURBINE BLADE

A THESIS SUBMITTED IN PART
IAL 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

3



Na
tional 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 f
or the degree of
Bachelor 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 u
niversity/institute for the award of any degree or diploma.




Prof. S. K. Sahoo


Date:

Department of Mechanical Engg


National Institute of
Technology


Rourkela
-
769008


4


ACKNOWLEDGEMENT




We avail this opportunity to extend our hearty indebtedness to our guide
Prof. S.K.

Saho
o
, 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 E
ngineering, for providing valuable departmental facilities.


Submitted by:


Benaknaik S G

Roll No: 10
5
0
3
0
4
1

Mechanical

Engineering

National Institute of Technology
,

Rourkela






5


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






6





ABSTRACT




The manufacturing of turbine blades is often outsourced to in
vestment 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 poro
sity, 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 wa
s 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
lo
ng 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 r
eduction of 0.2

0.4% 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.






7






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 opera
tion, 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)
8


loads as per the applicable design standards [1], [2], while the applied s
tatic loads aim to
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.

Ear

Early
attempts were based on the use of single y
-
ph
ase









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 generat
ion 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 man
ufacture and cost
9


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
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 inte
rest 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.













10


PRESENT TREND


Investment casting is a method of p
roducing high quality casting. It is especially useful for
providing casting in

geometry’s 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 turbi
ne 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 t
he 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 a
n 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 centu
ry. 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

increase
d and cannot be met by the machine tools industry. The attention
11


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.





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 manufacturi
ng 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 functi
onality 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, placeme
nt 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 a
ccuracy 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 .



12


CHAPTER
-

2



EXPERIMENTAL ME
THODOLOGY


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 achi
eve 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 ea
rlier.

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 met
al working
13


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 investmen
t 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

(pat
tern 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). The
refore, 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.

14






FIG
. 1

15










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 sur
face finish


No or little additional machining (net, or near
-
net process)


Wax can be reused

16



Disadvantages:


Very expensive process


Requires skilled labor


Area of application:


Small in size, complex parts s
uch 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.


17





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. Flexibili
ty exists in changing the composition of each layer. Different methods can be used
18


to remove the wax pattern, normally steam autoclave, leaving a hollow shell. Shells are fired and
filled with molten metal that solidifies inside the shell. After casting, t
he 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
e
xpendable 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 dev
elopment 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 ke
pt 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 a
re 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.

19







FIG. 5












20


CONCLUSIONS

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
ca
st 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 s
ignificant rate
-
limiting factors in the reduction

of lead times and production costs for the industry.




R
EFERENCES


BOOKS

1. “Production technology”, HMT publication.


2. “Elements of workshop technology”, S K Hajra Choudhury, S K Bose, A K Hajra choudh
ury,
Niranjan Roy, Vol

II, 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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