Laser Shock Processing of Ni-base Superalloy and High Cycle Fatigue

Properties

L. Zhoua*, Y.H. Li, W.F. He, X.D. Wang and Q.P. Li

Engineering Institute of Air Force Engineering University, Xi’an 710038, Shanxi, China

a*Corresponding author: zhoulei8012@yahoo.com.cn

Keywords: Lsaer shock processing, High cycle fatigue life, Ni-base superalloy

Abstract. A plasma sound wave detection method of laser shock processing (LSP) technology is

proposed. Speciments of Ni-base superalloy are used in this paper. A convergent lens is used to

deliver 1.2 J, 10 ns laser pulses by a Q-switch Nd:YAG laser, operating at 1 Hz. The influence of the

laser density to the shock wave is investigated in detail for two different wavelength lasers. Constant

amplitude fatigue data are generated in room environment using notch specimens tested at an

amplitude of vibration 2.8 mm and first-order flextensional mode. The results show that LSP is an

effective surface treatment technique for improving the high cycle fatigue performance of Ni-base

superalloys having a factor of 1.62 improvement in fatigue life.

Introduction

Ni-base superalloy is a key material for turbine engine blades, and further important of its anti-high

cycle fatigue (HCF) ability has been a very important subject in recent decades.

Laser shock processing (LSP) is a surface treatment technology, which consists of irradiating a

metallic target with a short and intense laser pulse in order to generate, through high-pressure surface

plasma, a plastic deformation and a surface strengthening. Particular benefit is achieved for

improving the fatigue behavior[1]. By now, the theoretical aspects of LSP are well elaborated and are

widely presented in many publications, which describe physical processes of laser driven shock wave

generation[2], models of pressure generation[3]. For the past few years, current studies have focused

on improvement of the general system design of LSP equipment for yieding the most reliable

performance.

In this paper the results are presented on LSP technology application to improve the high fatigue

life in Ni-base superalloy. LSP shock waves were measured with a PVDF sensor and microphone in

order to monitor the process, which allows detailed analysis of the generated waves.

Experiments

Material. The materials used in this work was a K417 superalloy, its chemical compositions

contain(mass,%) Ni-balance ， Co-14~16 ， Cr-8.5~9.5 ， Al-4.8~5.7 ， Ti-4.5~5.0 ， V-0.6~0.9,

Zr-0.05~0.09. The sample was a 120mm×20mm plate with thickness of 7mm.The treated area was

40mm×20mm as shown in Fig. 1.

R20

Fig. 1 Geometry of sample

Materials Science Forum Vols. 697-698 (2012) pp 235-238Online available since 2011/Sep/21 at www.scientific.net© (2012) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.697-698.235

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Principle and experimental procedure of LSP. The experimental setup of LSP process is shown in

Fig.2. The sample surface was mechanically polished with no particular treatment. The sample was

immersed in a water jacket with 2-5 mm water thckness. As a laser source the Nd:YAG Q-swithed

oscillator and amplifier were used to produce a laser pulse of 10 ns pulse duration with an energy of

1.2J at a repetition rate of 1 Hz. The diameter of the sigle shot area was 1 mm.

Fig. 2 The experimental setup of LSP

Measurements of shock wave pressure. A scheme of the experimental set-up is shown in Fig.3. In

our experiments a polyvinylidene flouride (PVDF) pressure sensor was used to measure shock wave

pressure. Laser

50Ω

Oscilloscope

Confining

layerAbsorbing

layer

Bed platePVDF

Pressure

sensor

Fig. 3 Schematically principle of PVDF pressure test

Plasma sound wave detection method of LSP. A plasma sound wave, generated during the laser

shock processing, was observed, as shown in Fig.4. The microphone passed sloping to the sample

surface at 35 mm from the surface, and was connected to an oscilloscope. The waveform from the

oscilloscope was collected by a PC via a GPIB interface card. For the time of generation of the wave

we set the onset of the laser pulse, wheres the time of arrival was the time of transition of the wave

front through the probe beam. We took the arrival of the first signal peak as the time of arrival of the

wave.

1111

2222

33334444

6666

5555

77778888

9999 10

1-Robot 2-Sample 3- Absorbing layer 4-Probe beam 5-Laser 6- Plasma sound wave 7-Microphone

8- Oscilloscope 9-Data acquisition controller 10-Computer

Fig. 4 The experimental setup of plasma sound wave test

236 Advances in Materials Manufacturing Science and Technology XIV

Fatigue test. Fatigue testing of the samples was carried out unden first-order flextensional vibration

tests, constant amplitude (CA) method loading in servo-hydraulic fatigue test machines, as shown in

Fig.5. The test frequency for all CA tests was 300 Hz in a laboratory air environment.

Fig. 5 The experimental setup of vibration fatigue test

Results and Discussion

It is found that for shock wave surge pressure shows a nonlinear behavior with laser power density. As

shown in Fig.6, the shock wave surge pressure first increases slightly and sturates at greater-than

match 7GW/cm2 laser power density. The nonlinear growth of the shock wave surge pressure with

rising laser power density is caused by the water transparent confining layer. The water transparent

confining layer is used to prevent the laser generated plasma from expanding rapidly away from the

surface. The hydrodynamic expansion of the heated plasma in the confined region between the sample

and the water transparent confining layer creates a high-amplitude and short-duration pressure

pulse.The energy generated due to the expansion of the plasma partially propagates as a shock wave

into sample[5]. The shock wave surge pressure, that depends also on the applied laser fluence, grows

first, tends to oscillate and keeps constant after a specific laser power density. In the

linear-slope-range wave pressure generated with a lower laser power density, which agrees very well

with the R Fabbro’s theory(Eq. 1)[6].

IscmgZGPaP ⋅⋅⋅+

=−− )(

301.0)( 12

α

α （1）

When the laser power density is greater than 7 GW/cm2, the shock wave pressure is not increased.

Because in this power density water transparent confining layer is breakdown by laser. Under the

same power density, 532 nm laser-induced shock wave pressure is higher than 1064 nm laser-induced

shock wave pressure.

Fig. 6 Effect of laser power density to shock wave surge pressure

Laser induced shock waves are rapidly attenuated and dispersed as they propagate through water

and air. They turn into plasma sound wave. The time behavior of plasma sound wave and its relation

to the confining layer or absorbing layer is illustrated in Fig.7. It is clear that the waveform is not

controlled by water confining layer. But the signals change considerably throught the absorbing layer.

Mechanical

shaker

Power amlifier

Displacement sensor

Sample

Strain gauge Strain gauge tester

Materials Science Forum Vols. 697-698 237

During the absorbing layer undumage the measured signal is wide, having a symmetrical profile with

only one maximum. Signal from damage absorbing layer considerably is varied. The signal is

triangular, with small amplitude and lots of peaks. If absorbing layer is damaged, LSP system can be

stopped to protect sample by using plasma sound wave detection method.

1.3m 1.4m 1.5m 1.6m 1.7m-1.0

-0.5

0.0

0.5

1.0

Voltage/V

time/ms0.5 0.75 1 1.25 1.5

-1.5

-1.0

-0.5

0.0

0.5

1.0

1.5

2.0

TimeTimeTimeTime////msmsmsms

VV VVoo ooll ll tt ttaa aagg ggee ee// // VV VV

Absorbing layer

undamageAbsorbing layer

damage

Fig. 7 Sound wave signal comparison in two conditions

Average fatigue strength values for baseline and LSP processed samples are summarized in Table

1. Date for fatigue test show a dramtic difference in fatigue life between the two groups. While the

fatigue life of LSP is increased by roughly 62%. Once cracking was observed in the baseline samples,

failure generally resulted in relatively few additional cycles. Crack propagation life was estimated on

the order of a few percent of total life at best. For the LSP samples, however, the crack propagation

life was considerably longer. In fact, it was not uncommom for a crack to initiate in the LSP sample,

grow a short distance, then arrest. Nearly 100% of the sample life was spent propagating the crack.

Table 1 Experimental results of fatigue test

Baseline LSP

Amplitude of vibration[mm] 2.77 2.8 2.78 2.75 2.82 2.84 2.81 2.81

Life cycle[106N] 1.63 0.97 1.99 1.99 2.81 2.64 2.74 2.53

Fatigue life[N] 1.65×106 2.68×10

6

Conclusions

1. The nonlinear growth of the shock wave surge pressure with rising laser power density is caused by

the water transparent confining layer. The threshold value of laser power density for the most

shock wave surge pressure saturation was determined with 7 GW/cm2.

2. Optoacoustic detection method is considered as one of the most effective techniques in terms of its

ability to rapid test for sound wave which can reflect situation of absorbing layer and confining

layer.

3. LSP improved the fatigue life of Ni-base alloy from 1.65×106 to 2.68×10

6.

References

[1] P. Peyre, R. Fabbro: Materials Science and Engineering. Vol. A210 (1996), p.102

[2] P. Molian, R. Molian and R. Nair: Applied Surface Science. Vol. 255 (2009), p.3859

[3] I. Nikitin, B. Scholtes and H. J. Maier: Scripta Materialia. Vol. 50 (2004), p.1345

[4] C. R. Gonzălez, J. L. Ocańa: Mater Science Engineering A. Vol. 386 (2004), p.291

[5] Y. K.Zhang, J.Z.Lu,X.D.Ren: Maters and Design. Vol. 30 (2009), p.1697

[6] R. Fabbro, J. Foumier and P. Ballard: Applied Physics. Vol. 68 (1990), p.775

238 Advances in Materials Manufacturing Science and Technology XIV

Advances in Materials Manufacturing Science and Technology XIV 10.4028/www.scientific.net/MSF.697-698 Laser Shock Processing of Ni-Base Superalloy and High Cycle Fatigue Properties 10.4028/www.scientific.net/MSF.697-698.235

DOI References

[1] P. Peyre, R. Fabbro: Materials Science and Engineering. Vol. A210 (1996), p.102.

doi:10.1016/0921-5093(95)10084-9 [2] P. Molian, R. Molian and R. Nair: Applied Surface Science. Vol. 255 (2009), p.3859.

doi:10.1016/j.apsusc.2008.10.070 [3] I. Nikitin, B. Scholtes and H. J. Maier: Scripta Materialia. Vol. 50 (2004), p.1345.

doi:10.1016/j.scriptamat.2004.02.012 [4] C. R. Gonzălez, J. L. Ocańa: Mater Science Engineering A. Vol. 386 (2004), p.291.

http://dx.doi.org/10.1016/j.msea.2004.07.025 [5] Y. K. Zhang, J.Z. Lu,X.D. Ren: Maters and Design. Vol. 30 (2009), p.1697.

doi:10.1016/j.matdes.2008.07.017