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Ultrasonic Shot Peening - MTU Aero Engines

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<strong>Ultrasonic</strong> <strong>Shot</strong> <strong>Peening</strong> (USP) on Ti-6Al-4V and Ti-6Al-2Sn-4Zr-6Mo<br />

<strong>Aero</strong> Engine Components<br />

Paper_ICSP11.doc<br />

Inga Stoll 1 , Dietmar Helm 1 , Holger Polanetzki 1 , Lothar Wagner 2<br />

1 <strong>MTU</strong> <strong>Aero</strong> <strong>Engines</strong> GmbH, Munich, Germany<br />

2 Department of Materials science and engineering, Technical University of Clausthal,<br />

Clausthal, Germany<br />

Abstract<br />

Bladed disks (Blisks) have been developed and applied in the compressor sections of modern<br />

aero-engines to reduce the component weight and improve efficiency. Overtime, the blisk<br />

design has been further optimized with respect to weight reduction resulting in thinner<br />

geometries in the web area of the disk. Consequently, precautions have to be taken into<br />

account to avoid stress induced distortion in thin walled areas during the peening process.<br />

<strong>MTU</strong> has therefore implemented ultrasonic shot peening as an alternative to the conventional<br />

shot peening process. For safety reasons detailed knowledge about the effect of this process on<br />

the properties of the blisk material, especially the fatigue behavior is a necessity.<br />

Most known is the conventional shot peening process, where the peening media is accelerated<br />

by pressurized air. A new method of this mechanical strengthening process is shot peening<br />

using bearing balls accelerated by an ultrasonic vibrating plate. The bearing balls are<br />

surrounded by a closed chamber that fits the geometry of the part to be peened. Only a very<br />

small amount of bearing ball peening media is needed. To understand the mechanics of this<br />

rarely researched process, extensive experiments were performed at <strong>MTU</strong>.<br />

The effect of <strong>Ultrasonic</strong> <strong>Shot</strong> <strong>Peening</strong> compared to conventional shot peening on the near<br />

surface area – surface roughness, residual stress - of Ti-6Al-4V (Ti64) and Ti-6Al-2Sn-4Zr-6Mo<br />

(Ti6246) was investigated using common methods. The resulting fatigue behavior including<br />

fracture characterization for both materials was studied in detail comparing both mechanical<br />

strengthening processes.<br />

Keywords<br />

<strong>Ultrasonic</strong> <strong>Shot</strong> <strong>Peening</strong>, Ti-6Al-4V, Ti-6Al-2Sn-4Zr-6Mo, residual stress, surface roughness,<br />

fatigue behavior, fracture characterization, BLISK<br />

Introduction<br />

In the compressor section of jet engines at operating temperatures


shot peening are able to shift the crack initiation from the surface to sub-surface regions which<br />

leads to significantly higher fatigue life of the component [4, 5]. <strong>Shot</strong> peening has been used for<br />

many years as the standard process in the final surface treatment of rotating and static engine<br />

components [6].<br />

For specific reasons (weight reduction, elimination of assembly costs) in modern aero-engines,<br />

blisks (bladed disks) made from Titanium alloys are commonly used in the compressor section.<br />

blisks are integral components of rotor blades and disks (see Figure 1).<br />

Paper_ICSP11.doc<br />

Figure 1. Picture of a BLISK part [<strong>MTU</strong> <strong>Aero</strong> Engine].<br />

These components are either produced by milling from the solid or by linear friction welding [7].<br />

For further weight and cost savings, the BLISK should be manufactured exhibiting thinner wall<br />

thickness in the disk area. During the conventional shot peening process of such a blisk<br />

distortion may occur due to the directionally focused beam of peening media and the reduced<br />

wall thickness of the part. <strong>MTU</strong> has therefore implemented ultrasonic shot peening as an<br />

alternative to the conventional shot peening process. In addition to some other benefits of<br />

ultrasonic shot peening, lower distortion of thin-walled parts are expected as a result of the<br />

randomly distributed peening media during the ultrasonic shot peening process.<br />

<strong>Ultrasonic</strong> <strong>Shot</strong> <strong>Peening</strong><br />

<strong>Ultrasonic</strong> waves are generated by means of<br />

a piezoelectric excited oscillator and<br />

transmitted by an acoustic amplifier on the<br />

sonotrode. The acceleration of the peening<br />

balls occurs by kinetic energy transfer during<br />

collision of the shot with the continuously<br />

vibrating sonotrode. In case of ultrasonic shot<br />

peening the peening media consists of<br />

perfectly spherical bearing balls having a<br />

diameter several times larger than that of<br />

conventional shot peening media. <strong>Peening</strong><br />

media and the component sealed off in the<br />

peening chamber. Due to interflection of<br />

peening media on the component, chamber<br />

walls and the sonotrode, randomly distributed<br />

collisions between the peening media and<br />

the part, e.g. the blisk can be realized.<br />

Figure 2 shows the operating principle of a<br />

ultrasonic shot peening facility.


Experimental Methods<br />

To measure the effects of shot peening using different peening processes on the near surface<br />

zone, two different experimental methods were employed: Roughness measurement and<br />

residual stress measurement by means of X-ray diffractometry.<br />

Fatigue tests were carried out using smooth LCF and HCF tests under axial loads well as LCF<br />

rotating bending tests for Ti6246.<br />

The surface of all samples were modified by conventional or ultrasonic shot peening with a low<br />

Almen intensity (0,10mmA) and a high Almen intensity (0,23mmA for Ti64 / 0,2mmA for Ti6246)<br />

for LCF tests. HCF tests representing the blade area of a blisk were conventionally shot peened<br />

using glass beads as peening media or ultrasonic shot peened using bearing balls with Almen<br />

intensities of 0,18N or 0,27N. The coverage was 100% in all cases.<br />

Experimental Results<br />

Roughness<br />

Comparing the surface roughnesses of ultrasonic shot peened and conventionally shot peened<br />

specimens peened with the same Almen intensity and coverage, it is obvious that conventional<br />

shot peening results in surface roughness values much higher than ultrasonic shot peening.<br />

This behavior is observed on both Ti64 (Figure 3) and Ti6246 (Figure 4). For Ti64, the absolute<br />

value of surface roughness is higher.<br />

Figure 3. Roughness measurements Ti64. Figure 4. Roughness measurements Ti6246.<br />

[µm]<br />

25<br />

20<br />

15<br />

10<br />

5<br />

0<br />

conventional 0,1A<br />

Ra [µm]<br />

Rmax [µm]<br />

Rz [µm]<br />

USP 0,1A<br />

Paper_ICSP11.doc<br />

Ti-64<br />

conventional 0,23A<br />

USP 0,23A<br />

[µm]<br />

25<br />

20<br />

15<br />

10<br />

5<br />

0<br />

conventional 0,1A<br />

Ra [µm]<br />

Rmax [µm]<br />

Rz [µm]<br />

USP 0,1A<br />

Ti-6246<br />

conventional 0,2A<br />

Residual stress measurements<br />

For both alloys, the resulting compressive residual stresses at the surfaces for conventional<br />

shot peened specimens are higher than for ultrasonic shot peened ones. For a given alloy, both<br />

methods result in a similar location of the subsurface compressive residual stress maximum as<br />

well as comparable penetration depths. For Ti6246, the penetration depth is lower than for Ti64<br />

as shown in Figure 5 and Figure 6<br />

Figure 5. Residual stress measurements Ti64.<br />

residual stress [MPa]<br />

200<br />

0<br />

-200<br />

-400<br />

-600<br />

-800<br />

-1000<br />

Ti64<br />

conventional peened, 0,23mmA<br />

USP, 0,23mmA<br />

0 20 40 60 80 100 120 140 160 180 200 220<br />

distance from surface [µm]<br />

USP 0,2A<br />

Figure 6. Residual stress measurements.<br />

residual stress [MPa]<br />

200<br />

0<br />

-200<br />

-400<br />

-600<br />

-800<br />

-1000<br />

Ti6246<br />

conventional peened,<br />

0,2mmA<br />

USP, 0,2mmA<br />

0 20 40 60 80 100 120 140 160 180 200 220<br />

distance from surface [µm]


LCF Fatigue Life<br />

For Ti64, ultrasonic shot peening and<br />

conventional shot peening lead to higher LCF<br />

life compared to the unpeened condition<br />

indicated by the fatigue results at 0.9% strain<br />

range. The peening benefit of ultrasonic shot<br />

peened specimens is lower than for<br />

conventional shot peened specimens if both<br />

are tested at high strain ranges. For lower<br />

strain ranges, the resulting LCF life is<br />

comparable (see Figure 7). SEM<br />

investigations show surface failures for the<br />

unpeend (Figure 8) and the ultrasonic shot<br />

peened specimens at high strain ranges<br />

(Figure 9). At lower strain ranges,<br />

conventional shot peening (Figure10) and<br />

Paper_ICSP11.doc<br />

strain range [%]<br />

1,1<br />

1<br />

0,9<br />

0,8<br />

0,7<br />

0,6<br />

Figure 7. LCF test results Ti64.<br />

Ti64 axial LCF tests at RT, R=0<br />

<strong>Ultrasonic</strong> shot peened 0,23 mmA<br />

conventionell 100% 0,23 mmA<br />

unpeened<br />

0,5<br />

1,E+04 1,E+05 1,E+06 1,E+07<br />

cycles to failure<br />

ultrasonic shot peening (Figure 11) result in a shift of the crack initiation site from the surface to<br />

subsurface regions.<br />

R-16746 0609A03084<br />

Figure 8:<br />

unpeened<br />

Figure 9: USP<br />

(high strain rate)<br />

R-16488 0605A01309<br />

For Ti6246, conventional shot peening leads<br />

to LCF lives under rotating bending<br />

significantly higher than that of the unpeened<br />

condition. <strong>Ultrasonic</strong> shot peening still gives a<br />

life benefit compared to the unpeened<br />

condition but exhibits significantly lower LCF<br />

lives than conventional shot peening (Figure<br />

12). Crack intitiation sites at the surface were<br />

detected by SEM investigations for all<br />

unpeened specimens (Figure 13). For<br />

ultrasonic shot peening, crack inititation sites<br />

were observed at both locations, surface and<br />

subsurface (Figure 14, 15). Conventional shot<br />

Figure 10: conv.<br />

shot peened<br />

R-16383-16385 0603A02261<br />

Stress range [MPa]<br />

Figure 11: USP<br />

(low strain rate)<br />

R-16552 0606A01201<br />

Figure 12. LCF rotating bending at RT<br />

Test results Ti6246.<br />

900<br />

800<br />

700<br />

Ti6246 - rotating beending tests at RT, R=-1<br />

unpeened<br />

conventionell peened; 100%; 0.2A<br />

Ulrasonic shot peened; 100%; 0.2A<br />

600<br />

1,E+03 1,E+04 1,E+05 1,E+06 1,E+07 1,E+08<br />

cycles to failure<br />

peening (Figure 16) generally shifted the crack initiation to subsurface regions.


R-18114 0901A00907<br />

Figure 13:<br />

unpeened<br />

Paper_ICSP11.doc<br />

Figure 14: USP<br />

surface failure<br />

R-18288 0904A00636<br />

Figure 15: USP<br />

subsurface failure<br />

R-18288 0904A00646<br />

Figure 16: conv. shot<br />

peened<br />

R-18244 0903A02651<br />

HCF Fatigue Life – Comparison of glass peening and ultrasonic shot peening<br />

<strong>Shot</strong> peening of Ti64 gives similar HCF life for Figure 17. HCF Test results Ti64.<br />

conventional glass peening and <strong>Ultrasonic</strong><br />

RT; 600MPa, R=0<br />

shot peening. In case of <strong>Ultrasonic</strong> shot<br />

<strong>Ultrasonic</strong> shot peened 0,27N<br />

peening with a high Almen intensity value, the<br />

<strong>Ultrasonic</strong> shot peened 0,18N<br />

conventional glass peened 0.27N<br />

scatter can be reduced.<br />

conventional glass peened 0,18N<br />

Compared to the unpeened surface condition,<br />

unpeened<br />

all tested peening processes and Almen<br />

intensities show an increase of the HCF life<br />

about one magnitude.<br />

SEM investigations indicate surface failures<br />

for the unpeened condition and subsurface<br />

failures for all peened specimens (Figure 18,<br />

19, 20).<br />

R-16691 0608A02861<br />

Figure 18:<br />

unpeened<br />

peening parameter<br />

Figure 19: conv.<br />

glass peened<br />

R-16383-16385 0603A02279<br />

<strong>Shot</strong> peening of Ti6246 also leads to similar<br />

resulting HCF life for conventional glass<br />

peening and <strong>Ultrasonic</strong> shot peening.<br />

Compared to the unpeened surface condition<br />

specimens peened with the higher Almen<br />

intensity show a slightly improved HCF life<br />

whereas peening with low Almen intensity<br />

leads to HCF life which is more or less<br />

comparable to unpeened surfaces (Figure<br />

21).<br />

SEM investigations indicate surface failures<br />

for the unpeened condition and subsurface<br />

failures for all peened specimens (Figure 22,<br />

23, 24).<br />

peening parameter<br />

1,E+04 1,E+05 1,E+06 1,E+07<br />

cycles<br />

Figure 20: <strong>Ultrasonic</strong><br />

shot peened<br />

R-16525 0605A02960<br />

Figure 21. HCF Test results Ti6246.<br />

<strong>Ultrasonic</strong> shot peened 0.27N<br />

<strong>Ultrasonic</strong> shot peened 0.18N<br />

conventionell glass 0.27N<br />

conventionall glass 0.18N<br />

unpeened<br />

RT, 800MPa, R=0<br />

1,E+05 1,E+06 1,E+07<br />

cycles to failure


R-17907 0810A00057<br />

Paper_ICSP11.doc<br />

Figure 22:<br />

unpeened<br />

Figure 23: conv.<br />

glass peened<br />

R-18069 0812A01165<br />

Figure 24: <strong>Ultrasonic</strong><br />

shot peened<br />

R-18069 0812A01171<br />

Discussion and Conclusions<br />

Conventional shot peening leads to higher surface roughness and higher resulting compressive<br />

residual stresses at the surface than <strong>Ultrasonic</strong> shot peening using same Almen intensity. This<br />

characteristic can be shown for Ti64 as well as for Ti6246. The location of the compressive<br />

residual stress maximum under the surface as well as the penetration depth is comparable for<br />

both methods.<br />

The lower surface roughness and penetration depth for Ti6246 compared to Ti64 can be<br />

explained by the lower deformation resistance of Ti64.<br />

The resulting LCF life of peened surfaces depends on the pening process. Conventional shot<br />

peening leads to higher LCF life than for the unpeened condition of the same alloy for all tested<br />

almen intensities. SEM investigation show that conventional shot peening is able to shift the<br />

crack initiation from the surface to the subsurface region. Therefore crack initiation occurs under<br />

vacuum giving a substantial contribution to life improvement. Additionally, life extension due to<br />

crack growth under vacuum must be considered. For ultrasonic shot peened Ti64 specimens<br />

this behavior takes place only at low strain rates, while at higher strain rates crack initiation<br />

occurs again at the surface results in lower LCF life. Ti6246 specimens with ultrasonic shot<br />

peened surface condition exhibits lower LCF life compared to specimens with conventional<br />

peened surface.<br />

The resulting HCF life for peened specimens is comparable for both investigated peening<br />

processes. The life benefit of peened Ti64 specimens is nearly one magnitude. Ti6246<br />

specimens hardly show a life improvement for both shot peening methods. This difference to<br />

Ti64 may be explained by less sensitivity of Ti6246 to crack initiation at laboratory air.<br />

References<br />

[1] Boyer, R. R., An Overview on the Use of Titanium in the <strong>Aero</strong>space Industry, Materials<br />

Science and Engineering A; 213, 1996, p. 103-114.<br />

[2] Williams, J. C., Titanium Alloys: Production, Behaviour and Application, High Performance<br />

Materials in <strong>Aero</strong>space. London: Chapman & Hall, 1995, p. 85-134<br />

[3] König, G. Life Enhancement of <strong>Aero</strong> Engine Components by <strong>Shot</strong> <strong>Peening</strong>: Opportunities<br />

and Risks, Wagner, L. (Ed.) <strong>Shot</strong> <strong>Peening</strong>. Weinheim: Wiley-VCH 2003, p. 13-22.<br />

[4] Wagner, L. Der Einfluss von Kugelstrahlen auf das Ermüdungsverhalten hochfester<br />

Titanlegierungen. Bochum, Ruhr-Universität Bochum, Dissertation, 1981.<br />

[5] Wagner, L. Mechanical Surface Treatments on Titanium Alloys: Fundamental Mechanisms,<br />

1996 TMS meeting Symposium on Surface Performance of Titanium, TMS, 1997, p. 199-<br />

215<br />

[6] Carek, G.A <strong>Shot</strong> <strong>Peening</strong> for Ti-6Al-4V Alloy Compressor Blades, NASA Technical Paper<br />

2711, 1987, p. 1-7<br />

[7] Wilhelm, H.; Schneefeld, D.; Helm, D. Lineares Reibschweißen von Verdichterläufern aus<br />

Titan-Legierungen, Moderne Werkstoffe im Luft- und Raumfahrzeug. Düsseldorf: DVS<br />

Verlag, 2000, p. 42-47.


ISCSP agrees to publish:<br />

Paper_ICSP11.doc<br />

International Scientific Committee for <strong>Shot</strong> <strong>Peening</strong><br />

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__________________________ ______________________________<br />

in the conference proceedings of the 11 th International Conference on <strong>Shot</strong> <strong>Peening</strong> (ICSP-11)<br />

and its affiliated web site http://www.shotpeening.org.<br />

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