Creative Design of Pattern for Sand Casting of Turbine Blade

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

Mechanical Engineering

BENAKNAIK S G

Department of Mechanical Engineering

National Institute of Technology,Rourkela

2009

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

Mechanical Engineering

BENAKNAIK S G

Under the Guidance of

PROF. S. K. SAHOO

Department of Mechanical Engineering

National Institute of Technology, Rourkela

2009

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 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 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

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

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

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. Thedimensional errors incurred during dipping are also measured and found that generally, there is a reduction 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.

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 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 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 Universityof 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 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

Investment casting is a method of producing 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 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 armamentsincreased and cannot be met by the machine tools industry. The attention is turned to investment castingto 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 closedimensional 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, whichcan be produced using investment casting. Therefore the ability to control and improve the accuracy getting moreattention 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

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 waxpatterns 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 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.

FIG. 1

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

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.

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 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 industryand it has been particularly successful for the production of single crystal turbine blades. Problems associatedwith 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 . Theprinciples 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 Warsaw 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 basicsteps 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.

FIG. 5

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 cast will be fundamental to achieving these aims. Production of the mould is timeconsuming, currently taking between 24 and 72 h depending upon the component, due to the need to use controlledmoisture 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, Vol‐II, Media promoters and media publications.

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WEBSITES

[1]

[2] sciencedirect.com

[3]

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