Laser Shock Peening of Thin Open-hole Aluminium Specimens · V. Ristori LSP of Thin Open-hole Aluminum Specimens 25/37. Introduction Research rogrammep LSP treatment Results Conclusions

Laser Shock Peening of Thin Open-hole Aluminium

Specimens

Vjola Ristori (MSc Student)

University of Pisa, Italy

European Laser Shock Peening MeetingBertinoro, Italy, 16-18 February 2011

IntroductionResearch programme

LSP treatmentResults

Conclusions

Contents

1 Introduction

2 Research programme

3 LSP treatment

4 Results

5 Conclusions

V. Ristori LSP of Thin Open-hole Aluminum Specimens 2 / 37



Conclusions

Research motivationInvolved institutions

Contents

1 IntroductionResearch motivationInvolved institutions


3 LSP treatment

4 Results

5 Conclusions




Conclusions


Research goals

To evaluate the potentiality of the LSP process, by comparing the

experimental results with the performances of the more

conventional techniques for residual stress introduction around

holes, such as cold-working and stress-wave methods.




Conclusions


Open-hole specimens and residual stresses

Previous research done on the same specimen geometry using

conventional techniques

The possiblity of using LSP on thin open-hole specimens is

investigated

This activity intends to point out the importance of the

sequence of operations that are performed on a lamina, as well

A possible advantage of LSP is that it can treat a larger

portion of material, pontentially in�uencing also the

propagation of a crack

L. Boni, A. Lanciotti, C. PoleseFatigue crack propagation in open hole specimens with cold worked holesProceeedings of 24th ICAF Symposium, edited by L. Lazzeri, Volume 2, (2007); pp. 832-845.




Conclusions


International collaboration research

Involved institutions

University of Pisa, Italy

University of Bologna, Italy

Polytechnic University of Madrid, Spain

University of the Witwatersrand, South Africa

The South African Nuclear Energy Corporation

Elettra Synchrotron Laboratory Trieste, Italy




Conclusions

SpecimensPlanned tests

Contents

1 Introduction

2 Research programmeSpecimensPlanned tests

3 LSP treatment

4 Results

5 Conclusions




Conclusions


Specimen geometry

Al 7075-T73, dog-bone specimens, 2.3 mm thick lamina




Conclusions


Additional specimens

The available material was insu�cient for a complete test

campaign in order to get all the result wanted

The use of 6082-T6 was a compromise solution in order to get

also results about the e�ect of the order of treatment on the

residual stresses distribution

The comparison between the residual stresses distributions to

be made is of a qualitative nature




Conclusions


Planned tests

Material Specimen N◦ Treatments RS Measurements Tests7075-T73 Open hole 13 LSP Synchrotron Fatigue7075-T73 Open hole 3 LSP X-ray Crack

propagation6082-T6 Open hole 4 LSP Synchrotron, X-ray Fatigue6082-T6 Plain 6 LSP + hole Synchrotron, X-ray Fatigue




Conclusions


Planned tests

Four more open hole specimens (2 in 7075-T73 and 2 in 6082-T6)

are dedicated to residual stress measurement




Conclusions

Selection of parametersLaser settingsResidual stress measurementSpecimen �xing

Contents

1 Introduction


3 LSP treatmentSelection of parametersLaser settingsResidual stress measurementSpecimen �xing

4 Results

5 Conclusions




Conclusions


Optimization of the LSP set-up

When it comes to LSP treatment of thin aluminium specimens, a verylimited number of published works exists

High energy lasers that are generally used for the LSP treatment are notsuitable for treatment of thin alumimium sheets without previousoptimization of the process

It is important to chose carefully the setup of the process

Due to limited time and number of available specimens, a single set ofparameters was identi�ed for this research activity, based on previousexperience of the scientists from Centro Laser at UPM with treatment ofAl specimens

J.-M. Yang, Y.C. Her, N. Han, A. H. Clauer

Laser shock peening on fatigue behavior of 2024-T3 Al alloy with fastener holes and stopholes

Mater. Sci. Eng. A298 (2001) 296-299.

W. Zhang, Y. L. Yao

Micro Scale Laser Shock Processing of Metallic Components

J. Manuf. Sci. Eng. 124 (2002) 369-378.




Conclusions


LSP parameters

Main parameters that in�uence LSP results

Laser power density

Laser wavelength

Peen size

Pulse Duration

Number of layers

Coating




Conclusions


Energy, wavelength, pulse duration

Laser currently in use by Centro Laser for LSP treatment

Laser type Wavelength Output energy Pulse duration Laser frequency[nm] [J] [ns] [Hz]

Nd-YAG 1064 2.8 (10% loss) 9 10




Conclusions


Peen size

The spot size chosen was 1.5 mm in diameter

In order to obtain relatively high laser power densities

necessary for successful LSP treatment, small beam spot

surface is required, given the relatively low laser energy.




Conclusions


Overlapping

Given the relatively small peen diameter, the amount of

residual stress is not as high as it would be with a larger one

It was necessary to set the appropriate density of laser peens

(overlapping rate)

The overlapping rate was based on the results of other

experimental activities on other Al alloys

Two di�erent settings were investigated: 625 spots per cm2

and 900 spots per cm2




Conclusions


Overlapping

Spot densities under 625 spots per cm2 would not introduce

signi�cant residual stresses in the material, while increasing the

spot density over 900 spots per cm2 could cause excessive

deformation of the specimen or even signi�cant damage to the

surface of the specimen




Conclusions


Hole drilling

The experimental set-up of the hole drilling measurement

(a) LSP treated specimen

with attached strain gauges

(b) Detail: strain gauge

rosette used

(c) Hole drilling measure-

ment




Conclusions


Results

The results of the residual stresses measurement




Conclusions


Results

The hook-shaped pro�le of residual stresses could be explained by

The LSP treatment was realized in a direct ablation mode,

where no thermal protective material is used

The spot dimension and its circular shape a�ect the

distribution of residual stresses

The accuracy of hole drilling method is limited when applied

on thin specimens, therefore the measurement could be

a�ected by this inaccuracy

Additional measurements using X-ray and synchrotron are to

be performed in order to verify these preliminary results




Conclusions


Fixing

When a thin panel is laser peened, it is usually �xed in order to preventthe unwanted wave re�ection on the back side of the plate

Thin specimens are subjected to deformation in the peened zone, so theymust be �xed very rigidly to the backing plate in order to avoid vibrations

This rigid �xing can cause undesirable local variation in residual stressesintroduced in the specimen

In order to avoid �xing problems without encountering undesired wavere�ection, no backing plate was used in combination with a short impulsetimes that in fact ensure less shock re�ection

G. Ivetic,3-D FEM Analysis of Laser Shock Peening of Aluminium Alloy 2024-T351 Thin SheetsSurface Engineering, DOI: 10.1179/026708409X12490360425846




Conclusions


Fixing

(a) Front view (b) Top view




Conclusions


Specimen deformation

(a) One side peening

(b) Two side peening




Conclusions

Roughness measurementFatigue testsResults interpretation

Contents

1 Introduction


3 LSP treatment

4 ResultsRoughness measurementFatigue testsResults interpretation

5 Conclusions




Conclusions


Roughness

The thermal e�ect could indeed ruin the surface causing

premature fatigue crack initiation and this e�ect is certainly to

be avoided

Thanks to a confocal microscope at UPM, it was possible to

obtain a three-dimesional map of the roughness of the

specimen in the LSP treated zone and in the not treated one




Conclusions


Roughness

Roughness comparison

Material condition Ra [µm] Rt [µm] Reference

7075 as milled 0.6 5.2

7075 LSP 3 shots 1.3 117075 SP 125% 5.7 42

7075 as milled 1.1 7.9 Present experimental7075 LSP 900 shots/cm2 3.6 15.6 activity

P. Peyre, R. Fabbro, P. Merrien, H.P. LieuradeLaser shock processing of aluminium alloys. Application to high cycle fatigue behaviourMater. Sci. Eng. A210 (1996) 102-113.




Conclusions


Specimen preparation and testing

(a) Specimen with tabs (b) Testing




Conclusions


Results

Fatigue tests, R=0.1, σmax=160 MPa





Conclusions


Results






Conclusions


Specimen modi�cation

Polishing of the inner surface of the hole

(a) Fracture surface, non polishedhole

(b) Polished hole

Inner surface was polished with a diamond pasteFatigue life at σmax=160 MPa showed practically no di�erence(34579 polished vs. 34782 as peened)




Conclusions


Specimen modi�cation

Polishing of the LSP treated area

(a) Hole edge as peened 5x (b) Polished hole edge 5x

LSP treated area was polished to as machined state (Ra = 0.4µm)

Again, fatigue life at σmax=160 MPa showed practically nodi�erence (32094 polished vs. 34782 as peened)




Conclusions


Possible causes of fatigue life decrease

Increased roughness

Laser power density of a too high magnitude

The sequence of operation open hole + LSP might have

caused peaks in tensile stresses at the edge of the hole (to be

con�rmed by synchrotron measurements)

The choice of using open hole + LSP because of suspected

release of residual stresses in case of LSP + open hole




Conclusions

Completed workWork to be doneFurther research

Contents

1 Introduction


3 LSP treatment

4 Results

5 ConclusionsCompleted workWork to be doneFurther research




Conclusions


Completed work

7075-T73

16 open hole specimens LSP treated

2 open hole specimens LSP treated on three di�erent areas

Residual stress measurement using hole-drilling

Roughness test on base material and LSP treated material

Fatigue testing on 9 LSP treated open hole specimens




Conclusions


Work to be done

7075-T73

Residual stress measurement at the synchrotron facility in

Trieste of LSP treated open hole specimens

Residual stress measurement with X-ray technique of LSP

treated open hole specimens to be carried out at South African

Nuclear Energy Corporation, Pretoria

Crack propagation tests on LSP treated open hole specimens

to be done at University of the Witwatersrand, Johannesburg

(South Africa)




Conclusions


Additional tests

6082-T6

LSP treatment on open hole and plain specimens

Residual stress measurement at the synchrotron facility

Fatigue tests

Goal

Determining the importance of the sequence of operations: open

hole + LSP vs. LSP + open hole


Acknowledgements

European Science Foundation - short research visit grant to

Polytechnic University of Madrid

Future publications

G. Ivetic, E. Troiani, I. Meneghin, G. Molinari, J.L. Ocaña, M. Morales, A.Porro, A. Lanciotti, V. Ristori, C. Polese, A. Venter

Fatigue and crack propagation in open hole specimens with Laser ShockPeened holesICAF 2011 conference, Montreal, Canada

Documents

Laser Shock Peening of Thin Open-hole Aluminium Specimens · V. Ristori LSP of Thin Open-hole Aluminum Specimens 25/37. Introduction Research rogrammep LSP treatment Results Conclusions