Case Study on Advanced Manufacturing and Quality Control of Compressor Blades

International Journal of Emerging Technology and Advanced Engineering

Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 10, October 2014)

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Case Study on Advanced Manufacturing and Quality Control of

Compressor Blades Sisir Sagar

1, D. Sarath Chandra

2

1Scholar,

2Assistant Professor, Department of Mechanical Engineering, VNRVJIET, JNTUH

AbstractManufacturing of aero-engine components has been improvised over the years, commensurate with

technological developments in the aeronautics sector.

Compressor blade plays a vital role in an aircraft engine and

flaws in its production can have adverse impacts on the

performance of an aero engine. Despite technological

advancements, many defects arise in these single crystal

blades, which are manufactured by investment casting. This

paper presents the developments in the manufacture of

compressor blades and quality control inspection checks on

them, by performing Non Destructive Tests (NDT).

Metallurgical observations furnish better explanations on the

developments in blade manufacture from conventional casting

to additive manufacturing

Keywords Defect analysis, Super alloy, Dye penetration,

Metallurgical inspection, Quality control, Additive

manufacturing

I. INTRODUCTION

The development of the gas turbine engine led to its

incorporation in various disciplines. One such area is the

aircraft manufacturing sector which employs cutting edge

technology in producing gas turbine engines. Today turbine

engines power majority of the aircrafts. Heat energy is

converted into mechanical work which is then expended by

the turbine to generate thrust. A mixture of compressed air

and fuel is ignited and allowed to expand through the

annular combustion chamber by impinging high pressure

gas on the turbine blades.[1] The turbine blades are

instrumental in generating the required thrust in an aircraft.

Compressor blades are often subjected to acute stresses and

temperatures up to 600C making them most susceptible to

failure, second to turbine blades. The high operating

temperatures, particularly in the high pressure compressor

section tend to reduce the service life of the blades in the

long run. An estimated 40 percent of failure in gas turbine

engines can be attributed to balding problems. [2]

The interior of a gas turbine is a demanding environment

where the temperatures and pressures are skyrocketing and

can go well beyond the limits of conventional metals.

Accordingly, specially developed Heat Resistant Super

Alloys commensurate with the increasing operating

temperatures are required for components which are

integral in a gas turbine. Titanium super-alloys are

characterized by a combination of high strength to weight

ratio, corrosion resistance and low thermal conductivity,

which make them ideal for many gas turbine applications.

Pure Titanium undergoes allotropic transformations at

different temperatures and the most popular of them is the

alpha structure ( alloy). It has alpha stabilizer elements and possesses excellent creep resistance which makes it

instrumental in the manufacture of compressor blades. The

Titanium grade generally used in aircraft blades

corresponds to Titanium Beta alloys which are fully heat

treatable and weldable. They are characterized by high

hardenability, high strength and exhibit excellent

formability in the solution form. [8] and [6]

The Titanium used in aerospace is usually forged or cast.

This frequently results in the development of a forging skin

on the metal, whose removal is extremely difficult. In this

case, the compressor blades are cast. Titanium alloys are

generally solution treated and subject to ageing to increase

the overall strength of the end product and simultaneously

relieve them of stresses. [1] and [4]

Figure.1 Example of an aircraft compressor blade profile



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II. DEVELOPMENTS IN BLADE MANUFACTURING

Gas turbines are highly dependent on efficiency to

produce the necessary thrust. Methods of increasing

efficiency are limited by metallurgical properties of the

turbine components. This is achieved by incorporating

components that can withstand extreme working

temperatures. The development of directionally solidified

airfoils was a significant advancement in aircraft engines.

[2]It provided for increases in operating temperatures

and higher rotor speeds. In the conventional casting

technique, the molten metal is poured into a ceramic

mould. By controlling metal pouring and surrounding

conditions, the molten metal solidifies from the surface to

the centre of the mould, creating an equiaxed structure. In

Directional Solidification, planar solidification occurs in

the blade and the part is solidified by moving the planar

front longitudinally. This produces a blade with an oriented

grain structure that runs along the major axis and devoid of

transverse grain boundaries unlike the former. The

elimination of transverse grain boundaries adds on to the

creep resistance and rupture strength of the alloy and the

orientation provides a favourable modulus of elasticity to

enhance fatigue life. In addition to the above, these blades

possess more thermal fatigue resistance when compared to

equiaxed blades. Later developments included single

crystal blades which eliminate all grain boundaries

(longitudinal and transverse). A single crystal with

controlled orientation is produced in an airfoil shape. A

substantial increase in the melting point of the alloy and an

increase in high-temperature strength can be achieved by

eliminating the grain boundaries. The transverse creep

resistance and fatigue strengths of a single crystal blade are

higher when compared to equiaxed and Directionally

Solidified blades. [3] and [4]

Figure.2 Different grain structures of blades

The latest advancement in blade production is additive

manufacturing. Titanium alloy blades are being developed

by using metal-melting electron guns. The component is

first drafted in a three dimensional space from a stockpile

of molten Titanium alloy powder. It is then sintered by an

electron beam in an Electron Beam Melting machine. Many

of the additive manufacturing processes can produce the

expendable patterns required directly from design data,

bypassing the cumbersome process of injection mould

tooling. Part complexity does not affect the cost and the

possibility of shell cracking when the pattern material is

melted out of the ceramic shell, is minimized. [4]

Table I

Figure.3 Electron Beam Melting apparatus

III. EXPERIMENTAL PROCEDURE

In this experiment, the Titanium investment-cast blade

was analyzed to detect probable manufacturing defects that

would have been generated during the casting process.

Specification Test Procedure: ASTM E 1476 1994

Material

Identification

Mo Nb Z

r

Ti Fe Sn

TURBINE

BLADE

0.86

1.13

4.12

90.63

0.25

3.02



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Key features of interest are internal pockets, T-joints and

varying thicknesses. All the quality control checks

performed on the test specimen are Non Destructive Tests

which help in locating defects without damaging the blade.

As a part of the investigation on the aircraft compressor

blade, the following Non Destructive Tests have been

carried out to detect the presence of anomalies or defects.

XRF Spectrometry

Radiography Testing

Penetrant Testing

A. XRF Spectrometry

[7] The constituent elements in the blade alloy have

been found out by Positive Material Identification (PMI)

using a handheld X-Ray Fluorescence gun. This is a

metallurgical inspection technique which eliminates the

need to cut the test specimen and supplements conventional

metal grade verification techniques. The Non Destructive

methods also have the potential for monitoring grade

during batch production. X Ray Fluorescence spectrometry

is characterized by the emission of an x-ray excitation onto

a sample test specimen. During this process, if the incident

x-ray has sufficient energy, electrons are ejected from inner

shells, creating vacancies and instability. As the atom

regains its stability, electrons from the outer shells are

transferred to the inner shells and simultaneously irradiate a

characteristic x-ray whose energy is the difference between

the two binding energies of the corresponding shells. Each

element produces x-rays at a unique set of energies because

each one has a unique set of energy levels. This allows the

non-destructive measurement of elemental composition of

the sample. X- Ray Fluorescence process example:

Titanium Atom (Ti=22)

Figure.4. An electron in the K shell is ejected from the atom by an

external primary excitation x-ray, creating a vacancy.

Figure.5 An electron from the L or M shell jumps in to fill the

vacancy. In the process, it emits a characteristic x-ray unique to this

element and in turn produces a vacancy in the L or M shell.

Figure.6 When a vacancy is created in the L shell by the primary

excitation x-ray or by a previous event, an electron from the M or N

shell jumps in, to occupy the vacancy. In this process, it emits a

characteristic x-ray unique to this element and in turn, produces a

vacancy in the M or N shell.

B. Radiography testing

X-rays are used to produce images of the blade specimen

using a film that is sensitive to radiation. The blade is

placed between the radiation source and detector. The

thickness and the density of the material that the X-rays

must penetrate affect the magnitude of radiation reaching

the detector. This variation in radiation produces an image

on the detector that depicts the internal features of the test

specimen. [10],[11] and [12]



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Figure.6 The lighter portions shown in the x-ray symbolize hollow

sections in the blade and are not to be mistaken as casting defects.

Table II

C. Penetrant Testing

The blade is pre cleaned following which penetrant

solution is applied to the surface of the component. The

liquid is pulled into the surface breaking defects by

capillary action. Excess penetrant material is carefully

cleaned from the surface.

A developer is applied to pull the trapped penetrant back

to the surface where it is spread out and forms an

indication. This indication is relatively easier to spot and

gives an account of where the defect has occurred. [12].[13] and [14]

Table III

IV. DISCUSSIONS

The above performed tests assure blade quality up to a

certain extent. For better inspection and quality control

checks, techniques like Computerized Tomography are

incorporated due to the complex structures involved. The

CT cross-sectional image facilitates the detection of highly

precise geometries. Computerized Tomography

measurement permits an exact inspection verdict to be

reached on good/poor quality in conforming to the

regulations set by the quality control department. Inspite of

improvements in creep strength of single crystal alloys, the

expectant increase in temperature resistance is not beyond

1140- 1150 C. The best alternative is to develop single

crystal blades in conjunction with thermal barrier coatings

to facilitate operation at relatively higher temperatures.

Plasma spraying is widely used to deposit thermal barrier

coatings. However, its usage is restricted and cannot be

applied to turbine blades. The Electron Beam Physical

Vapour Deposition (EBPVD) technique is mostly

incorporated in critical components. Its merits far outweigh

the plasma spraying technique by improvised thermal cycle

life, higher erosion resistance and improved surface finish.

In order to avoid all these complexities, a new dimension in

blade production is slowly coming into picture. Additive

manufacturing using EBM (Electron Beam Machining) can

produce components in a fraction of the time taken by

conventional casting.

Specification Procedure Acceptance

Criteria

Solvent Removable ASTM E 165 2010

ASME SEC VIII DIVISION. I

Pre-Clean Time

Penetrant Time

Dwell Time Developer Time

1 MIN 5 MIN 10 MIN 10 MIN

TEST DETAILS

Sample No Identification Observation Result

1 Compressor Blade

No surface defects

Acceptable

PROCESS PARAMETERS

Source: X-RAY

Voltage: 100 KV

Current: 3 mA

Focal Spot Size: 1.1 X 1.1

MM

Type of Joint: CASTING

Density: 2-4

Image Quality Indicator: ASTM 7

Technique: SINGLE WALL

SINGLE IMAGE

SFD: 60 CM

Lead Screen: FRONT: 0.02,

BACK: 0.1 CM

Film Type: D7

Exposure Time: 60 SEC

TEST DETAILS

Identification: Compressor Blade Test Procedure: ASME

SEC-V, ARTICLE 2

Accepted Standard: ASTM-E 155

Sample No

Identification Film Size

Sector Observation Result

1 Compressor Blade

7X15 cm

A No significant Defect

Acceptable



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Titanium Aluminide is a light weight inter-metallic alloy

which is resistant to heat stress and oxidation, which makes

it an ideal substitute to the current alloys used in blade

manufacturing. Titanium Aluminide considerably boosts

the engines thrust to weight ratio but tends to shrink and crack as it cools in a conventional wax-mould, leading to

higher production wastage.

V. CONCLUSIONS

The single crystal compressor blade was investment cast

and subject to various non destructive tests to detect the

presence of any casting defects or anomalies. The tests

performed on the blade account for a majority of the defect

analysis checks. Some tests like Ultrasonic Testing could

not be performed because the specimens thickness was less than 6mm, which is a mandatory for performing the

test. The radiography test illustrates hollow sections at the

root and tip which are allowed in the blade for fixing it to

the compressor disc. The inspection concludes that there

are no defects in the blade and the part is good to go for

further processing and usage in the aircraft compressor

section. Additive manufacturing offers a number of

benefits to the aerospace sector. It completely eliminates

the tooling phase and promotes greater speed, lower costs

and rapid production. It offers huge potential cost savings

in and enables designers to develop innovative designs that

are not too main stream, using advanced lattice structures.

Additive manufacturing comes handy where production

volumes are relatively low, part geometries are complex

and materials used are expensive and difficult to process by

conventional means.

REFERENCES

[1] http://continentalsteel.com/titanium/titanium-grades/.

[2] DEGARMO'S MATERIALS AND PROCESSES IN MANUFACTURING Y E. PAUL DEGARMO, J. T. BLACK, RONALD A. KOHSER.

[3] MEHERWAN P. BOYCE GULF PROFESSIONAL PUBLISHING (2002)- GAS TURBINE ENGINEERING HANDBOOK, SECOND EDITION, PP. 403-

404.

[4] http://gizmodo.com/this-electron-gun-turns-titanium-powder-into- turbine-bl-1623144300.

[5] Joanna R. Groza, James F. Shackelford CRC Press (2007) , Materials Processing Handbook.

[6] Brian Cantor, H Assender, P. Grant CRC Press (2001), Aerospace Materials, pp. 81-86.

[7] By Rainer Kurz, Solar Turbines Inc., San Diego, CA, Klaus Brun, Southwest Research Institute, San Antonio, TX, and Saeid Mokhatab, Contributing Editor | September 2011, Vol. 238 No. 9

[8] Thermal Analysis of an aero gas turbine compressor blade and vane using temperature sensing thermal paints.

[9] Defence Science Journal, Vol. 52, No. 4. Octoba 2002, pp. 363-367 0 2002, DESlDOC

[10] Y. Li, P. Gu, Free-form surface inspection techniques state of the art review, Journal of Computer-Aided Design, 36 (13) (2004), pp. 13951417.

[11] Volume 34 No. 5, ISSN: 0271-5333; eISSN: 1527-1323

[12] New Potentials of PenetrantTesting , ECNDT 2006 - Th.1.8.1

[13] Liquid Penetrant Testing: Industrial Process, Riccardo Fazio, Gennaro Caturano, Giovanni Cavaccini, Antonio Ciliberto, Vittoria

Pianese

[14] Human Factors and Ergonomics in Dye Penetrant and Magnetic Particles Nondestructive Inspection Methods, Engineering Letters, 15:1, EL_15_1_25

[15] Journal of Engineering physics and Thermo physics, vol.82 no.4, 2009

Case Study on Advanced Manufacturing and Quality Control of Compressor Blades

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