HIGH PRECISION MANUFACTURING OF AN ULTRA

MINIATURE CERAMIC GAS TURBINE IMPELLER

K. Liu, T. Waumans, J. Peirs, D. Reynaerts

Div. PMA, Department of Mechanical Engineering,

Katholieke Universiteit Leuven, Leuven, Belgium

Abstract: Precise manufacturing of an ultra miniature ceramic composite turbine impeller (Ø 20mm), which is

a key component of a Belgian micro fuel-based power generation system, is finalized. The EDM (Electrical

Discharge Machining) process is optimized which not just preserves the surface integrity with a satisfied strength

but also increases the dimensional and geometrical accuracy. Furthermore, with the help of a universal clamping

device, a production chain is carefully designed to allow one-time clamping for all processes and reduce the

systematic error. The grinding of the bearing-connected surfaces as a post-processing step after EDM of ceramic

composite not just meets the requirements on accuracy but also elevates the material performances.

Key words: PowerMEMS; micro gas turbine impeller; ceramic composite; micro-EDM

1. INTRODUCTION

The innovative Belgian micro fuel-based power

generation system has a compact design with an

overall volume less than 1 dm3 and electrical power

output up to 1 kW [1]. To attain the required output

and obtain a positive cycle, the turbine impeller is

expected to endure extreme working conditions for

example a rotational speed of at least 500,000 rpm,

operating temperature of more than 1200 K and a

maximum centrifugal stress of 580 MPa.

To satisfy these requirements, materials with a low

density-to-strength ratio, resistance to oxidation and

ability to work at elevated temperature are essential for

manufacturing the gas turbine impeller. Conventional

alloys are exposed to strength degradation at higher

temperature and are no longer suitable. Novel

technical ceramics, especially Si3N4 and its derived

ceramic composite, embrace outstanding physical and

mechanical properties meeting those requests and are

therefore selected. However, to ensure the high

geometric accuracy of the turbine impeller, traditional

ceramic processing techniques such as casting and

injection moulding are ditched because of the

difficulties in producing complex shapes and accurate

components, especially at micro/meso-scale. By

adding 30-40 vol. % of a secondary TiN phase, the

electrical resistivity of the ceramic composite is largely

reduced and a non-conventional machining method

such as Electrical Discharge Machining (EDM) can be

applied for precise three dimensional manufacturing.

Furthermore, the TiN phase also improves the fracture

toughness, the flexural strength and the wear resistance

of the ceramic composite.

At the PowerMEMS workshop 2007, a preliminary

research on the production of this mesoscopic ceramic

composite turbine impeller was presented [3]. There

still existed major problems such as surface non-

integrity after EDM process. It not just leads to

strength degradation and geometric inaccuracy, but

also is not able act as functioning connecting surfaces.

In this year, a modified EDM technique is developed,

which not just improves surface topography on the

turbine blades but also preserves a promising flexural

strength. Grinding of the bearing connected surfaces as

a post-processing technique after EDM process is also

investigated. Thanks to the carefully designed turbine

impeller production chain, high overall geometrical

and dimensional accuracy is obtained.

2. PRODUCTION

2.1 µEDM of ceramic composite

Micro-EDM of Si3N4-TiN ceramic composite

material to obtain even surface was a problem

irrelevant to the EDM machines and dielectrics [4]. A

foamy surface layer is formed over ceramic composite

matrix due to chemical reactions at high temperature:

the decomposition of Si3N4 and TiN, and the oxidation

of TiN, both which lead to the generation of a large

amount of nitrogen gas bubbles and prevent the

formation of irregular sparking craters. The

topography and the cross sections after EDM are

shown in Fig. 1, respectively. Even with reduced EDM

discharge energy to the limits of the machines, this

phenomenon persists and the minimum surface

roughness obtained is 0.72 µm Ra. Furthermore,

reduced discharge energy also means the lowered

machining speed, while the tool wear is also increased

which is not preferable for preserve the geometrical

accuracy.

Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)

389

Topography

10 µm

Cross-section

Fig. 1: Topography and cross-section views after

EDM using relaxation pulse.

However, it is discovered that this phenomenon

exists when applying relaxation discharge pulses

which are normally equipped on micro-EDM

machines. On the contrarary, using a particular type of

iso energetic discharge pulses with high peak

discharge current and short discharge duration often

used on macro machines to provide higher power,

material removal mechanisms in the form of melting

and evaporation are enhanced and the foamy layer is

eliminated (Fig. 2). On the other hand, since iso

energetic pulse always has higher energy input than

relaxation pulses, micro cracks are left on the surface

and lead to a strength degradation. Though the three-

point flexural strength degrades, still it satisfies the

requirement of the application as a high temperature

micro gas turbine impeller. The surface roughness

which is lower than 0.8 µm is easily obtained. Most

importantly, machining speed used for obtaining this

surface quality is 100 times faster than using a

relaxation pulse as mentioned in previous paragraph,

despite the very high tool wear ratio. The comparison

of machining performances using these two types of

pulses is listed in Table 1.

Topography

10 µm

Cross-section

Fig. 2: Topography and cross-section views after

EDM using iso energetic pulse.

2.2 Production Chain

Due to the different functions of the surfaces on

the gas turbine impeller, variable machining

procedures are required. Though all the features can be

manufactured by EDM, further surface optimisation is

still essential especially on those which are acting as

bearing contacting and the coupling surfaces with the

compressor. Thus it is decided to using grinding as a

post-processing step to give extra surface finish to

maximise the material strength and provides precise

geometries. Therefore, a universal clamp with some

added reference features which can be used on all the

manufacturing steps is produced. Since only one-time

clamp is necessary, the systematic error because of

unclamping and reclamping is avoided.

The schematic views of all the manufacturing

steps are illuminated in Fig. 3. The ceramic blank is

first preshaped by Wire EDM with an added T-shape

connecting feature. Then the blank is mounted on the

universal clamp. Necessary features such as blends and

chamfers are also EDMed to ensure the grinding

quality. The grinding process using diamond tools are

performed on a micro-milling machine Kern. An air-

bearing rotary table with nano meter error at rotation

axis is developed and applied for holding the turbine

piece. According to the working surface and

requirements on surface finish, different shapes and

grit sizes of grinding tools are applied. The detailed

grinding strategies and process parameters are listed in

Table 2. Finally the coupling features and the turbine

cavities are manufactured by die-sinking EDM on a

Charmilles machine.

Fig. 3: A schematic view of the production chain for

manufacturing the ceramic composite turbine impeller.

Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)

390

Table 1: EDM performance comparison with variable discharge pulse.

Pulse\

Regime

Material Removal

Rate (mm3/min)

Tool Wear

Ratio (%)

Ra

(µm)

Rt

(µm)

3-point flexural

strength (MPa)

Relax. Roughing 13.78 3.29 2.91 22.73 708.8 ± 25.65

Relax. Finishing 0.0318 25.5 1.54 13.12 773.32 ± 21.08

Iso energetic Finishing 1.794 22.39 0.93 7.66 729.12 ± 60.97

Table 2: Grinding tools and strategies used for grinding the bearing and coupling surface for ceramic turbine

impeller.

Turbine Drawing Tool Dimension Grit

size

Grinding

surfaces Regime

Ø 6 mm × 6 mm D90 B, C, D Rouging

Ø 4 mm × 6 mm D90 A Roughing

A

B

C

D E

F

Ø 3 mm × 6 mm D15 B Finishing

Ø 8 mm D30 C, D Finishing

Ø 5 mm × 1.5 mm D30 B Semi

Finishing

Ø 5 mm × 1.5 mm D30 A Finishing

Machining Settings: � Spindle rotational speed:

20000–40000 rpm

� Feed rate: 25–100 mm/min

� Depth of cut:

o 1–2 µm, roughing

o 0.25–1µm; finishing

Ø 3 mm D30 E, F

2.3 Quality Control

The surface and dimensional quality control is

done on a Taylor-Hobson Talysurf surface

profilometer and a Mitutoyo CMM (Coordinate

Measurement Machine), respectively. In Table 3, the

shape and dimensional accuracy and surface roughness

for bearing and coupling surfaces after EDM and after

grinding are compared. As can be seen, the surface

quality and accuracy is dramatically improved and all

the features are within in the tolerance after the

grinding.

As for the turbine cavities, a probe with a stylus Ø

0.7 mm is applied during the. 17 probe postures in

total are used in order to correctly engage the cavities

without collision and 1240 points on each cavity are

collected. The results are compared with the original

CAD model. In Fig. 4, the deviation maps of variable

views after 6 degree-of-freedom fitting are plotted. As

shown in these figures, all the turbine cavities are fully

symmetrical. The maximum deviation of 45 µm is

found at the most curvature part of the turbine blade,

and it is very repeatable on all the blades. Among the

statistic results, more than 90% of the points lie in the

deviation of ± 30 µm. The final product and an

enlarged view of the cavities are shown in Fig. 5.

Table 3: Dimensional and geometrical accuracy of the

bearing and coupling surfaces after EDM and after

grinding processes.

After

EDM

After

Grinding

Shaft External

(Bearing radial)

Diameter (mm) 8.156 7.999 Cylindricity (mm) 0.018 0.005

Ra (µm) 2.450 0.110

Shaft Internal

(coupling hole)

Diameter (mm) 5.738 6.000

Cylindricity (mm) 0.009 0.006 Ra (µm) 2.057 0.080

Thrust plane

Flatness (mm) 0.025 0.005 Ra (µm) 2.571 0.060

Turbine outer

diameter

Diameter (mm) 20.162 19.999 Cylindricity (mm) 0.023 0.007

Proceedings of PowerMEMS 2008+ microEMS2008, Sendai, Japan, November 9-12, (2008)

391

a) Bottom View b) Backside View c) Right Side View

-0.028

+0.036

-0.045

-0.043

+0.041

Fig. 4: The deviation maps of the measured manufactured ceramic turbine impeller comparing to the CAD model

in variable views.

Fig. 5: The finished turbine impeller (on the clamp)

and an enclosed view of turbine cavities.

3. CONCLUSION

The key component of the Belgian innovative

micro fuel-based power generation system, a ceramic

composite turbine impeller, has been successfully

produced by Electrical Discharge Machining and the

requirements on geometry and shape tolerances and

surface quality are met. The realization of

manufacturing this prototype in Si3N4-TiN ceramic

component expands the possibilities of using

alternative materials for high temperature application

as turbine. Meanwhile, accurate and complex three-

dimensional turbine impeller manufacturing is also

proved.

ACKNOWLEDGEMENT

This research is sponsored by the Institute for the

Promotion of Innovation by Science and Technology

in Flanders, Belgium, project SBO 030288, and by the

Belgian program on Interuniversity Poles of Attraction

(IAP5/06: AMS). Also, this work was carried out

within the framework of the EC Network of

Excellence "Multi-Material Micro Manufacture:

Technologies and Applications (4M)".

REFERENCES

[1] www.powermems.be

[2] Peirs J et al 2007 Proceedings of PowerMEMS,

2007 (Freiburg, Germany, 28-29 Nov 2007) 355-

358

[3] Ferraris E, Liu K, Peirs J, Bleys P and Reynaerts D

2007 Proceedings of PowerMEMS 2007

(Freiburg, Germany, 28-29 Nov 2007) 229-232

[4] Liu K, Ferraris E, Peirs J, Lauwers B, Reynaerts D

2008 International Journal of Manufacturing

Research vol.3 27-47

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