Laser Shock Peening of Thin Open-hole Aluminium Specimens

Transcription

1 Laser Shock Peening of Thin Open-hole Aluminium Specimens Vjola Ristori (MSc Student) University of Pisa, Italy European Laser Shock Peening Meeting Bertinoro, Italy, February 2011

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

3 Contents Research motivation Involved institutions 1 Research motivation Involved institutions V. Ristori LSP of Thin Open-hole Aluminum Specimens 3 / 37

4 Research goals Research motivation Involved institutions 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. V. Ristori LSP of Thin Open-hole Aluminum Specimens 4 / 37

5 Research motivation Involved institutions 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 inuencing also the propagation of a crack L. Boni, A. Lanciotti, C. Polese Fatigue crack propagation in open hole specimens with cold worked holes Proceeedings of 24th ICAF Symposium, edited by L. Lazzeri, Volume 2, (2007); pp V. Ristori LSP of Thin Open-hole Aluminum Specimens 5 / 37

6 Research motivation Involved institutions 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 V. Ristori LSP of Thin Open-hole Aluminum Specimens 6 / 37

7 Contents Specimens Planned tests 1 2 Specimens Planned tests V. Ristori LSP of Thin Open-hole Aluminum Specimens 7 / 37

8 Specimen geometry Specimens Planned tests Al 7075-T73, dog-bone specimens, 2.3 mm thick lamina V. Ristori LSP of Thin Open-hole Aluminum Specimens 8 / 37

9 Additional specimens Specimens Planned tests The available material was insucient 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 eect 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 V. Ristori LSP of Thin Open-hole Aluminum Specimens 9 / 37

10 Planned tests Specimens Planned tests Material Specimen N Treatments RS Measurements Tests 7075-T73 Open hole 13 LSP Synchrotron Fatigue 7075-T73 Open hole 3 LSP X-ray Crack propagation 6082-T6 Open hole 4 LSP Synchrotron, X-ray Fatigue 6082-T6 Plain 6 LSP + hole Synchrotron, X-ray Fatigue V. Ristori LSP of Thin Open-hole Aluminum Specimens 10 / 37

11 Planned tests Specimens Planned tests Four more open hole specimens (2 in 7075-T73 and 2 in 6082-T6) are dedicated to residual stress measurement V. Ristori LSP of Thin Open-hole Aluminum Specimens 11 / 37

12 Contents Selection of parameters Laser settings Residual stress measurement Specimen xing Selection of parameters Laser settings Residual stress measurement Specimen xing 4 5 V. Ristori LSP of Thin Open-hole Aluminum Specimens 12 / 37

13 Optimization of the LSP set-up Selection of parameters Laser settings Residual stress measurement Specimen xing When it comes to of thin aluminium specimens, a very limited number of published works exists High energy lasers that are generally used for the are not suitable for treatment of thin alumimium sheets without previous optimization 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 of parameters was identied for this research activity, based on previous experience of the scientists from Centro Laser at UPM with treatment of Al 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) W. Zhang, Y. L. Yao Micro Scale Laser Shock Processing of Metallic Components J. Manuf. Sci. Eng. 124 (2002) V. Ristori LSP of Thin Open-hole Aluminum Specimens 13 / 37

14 LSP parameters Selection of parameters Laser settings Residual stress measurement Specimen xing Main parameters that inuence LSP results Laser power density Laser wavelength Peen size Pulse Duration Number of layers Coating V. Ristori LSP of Thin Open-hole Aluminum Specimens 14 / 37

15 Energy, wavelength, pulse duration Selection of parameters Laser settings Residual stress measurement Specimen xing Laser currently in use by Centro Laser for Laser type Wavelength Output energy Pulse duration Laser frequency [nm] [J] [ns] [Hz] Nd-YAG (10% loss) 9 10 V. Ristori LSP of Thin Open-hole Aluminum Specimens 15 / 37

16 Peen size Selection of parameters Laser settings Residual stress measurement Specimen xing The spot size chosen was 1.5 mm in diameter In order to obtain relatively high laser power densities necessary for successful, small beam spot surface is required, given the relatively low laser energy. V. Ristori LSP of Thin Open-hole Aluminum Specimens 16 / 37

17 Overlapping Selection of parameters Laser settings Residual stress measurement Specimen xing 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 dierent settings were investigated: 625 spots per cm 2 and 900 spots per cm 2 V. Ristori LSP of Thin Open-hole Aluminum Specimens 17 / 37

18 Overlapping Selection of parameters Laser settings Residual stress measurement Specimen xing Spot densities under 625 spots per cm 2 would not introduce signicant residual stresses in the material, while increasing the spot density over 900 spots per cm 2 could cause excessive deformation of the specimen or even signicant damage to the surface of the specimen V. Ristori LSP of Thin Open-hole Aluminum Specimens 18 / 37

19 Selection of parameters Laser settings Residual stress measurement Specimen xing Hole drilling The experimental set-up of the hole drilling measurement LSP treated specimen (b) Detail: strain gauge (c) Hole drilling measurewith attached strain gauges rosette used ment (a) V. Ristori LSP of Thin Open-hole Aluminum Specimens 19 / 37

20 Selection of parameters Laser settings Residual stress measurement Specimen xing The results of the residual stresses measurement V. Ristori LSP of Thin Open-hole Aluminum Specimens 20 / 37

21 Selection of parameters Laser settings Residual stress measurement Specimen xing The hook-shaped prole of residual stresses could be explained by The was realized in a direct ablation mode, where no thermal protective material is used The spot dimension and its circular shape aect the distribution of residual stresses The accuracy of hole drilling method is limited when applied on thin specimens, therefore the measurement could be aected by this inaccuracy Additional measurements using X-ray and synchrotron are to be performed in order to verify these preliminary results V. Ristori LSP of Thin Open-hole Aluminum Specimens 21 / 37

22 Fixing Selection of parameters Laser settings Residual stress measurement Specimen xing When a thin panel is laser peened, it is usually xed in order to prevent the unwanted wave reection on the back side of the plate Thin specimens are subjected to deformation in the peened zone, so they must be xed very rigidly to the backing plate in order to avoid vibrations This rigid xing can cause undesirable local variation in residual stresses introduced in the specimen In order to avoid xing problems without encountering undesired wave reection, no backing plate was used in combination with a short impulse times that in fact ensure less shock reection G. Ivetic, 3-D FEM Analysis of Laser Shock Peening of Aluminium Alloy 2024-T351 Thin Sheets Surface Engineering, DOI: / X V. Ristori LSP of Thin Open-hole Aluminum Specimens 22 / 37

23 Fixing Selection of parameters Laser settings Residual stress measurement Specimen xing (a) Front view (b) Top view V. Ristori LSP of Thin Open-hole Aluminum Specimens 23 / 37

24 Specimen deformation Selection of parameters Laser settings Residual stress measurement Specimen xing (a) One side peening (b) Two side peening V. Ristori LSP of Thin Open-hole Aluminum Specimens 24 / 37

25 Contents Roughness measurement Fatigue tests interpretation Roughness measurement Fatigue tests interpretation 5 V. Ristori LSP of Thin Open-hole Aluminum Specimens 25 / 37

26 Roughness Roughness measurement Fatigue tests interpretation The thermal eect could indeed ruin the surface causing premature fatigue crack initiation and this eect 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 V. Ristori LSP of Thin Open-hole Aluminum Specimens 26 / 37

27 Roughness Roughness measurement Fatigue tests interpretation Roughness comparison Material condition R a [µm] R t [µm] Reference 7075 as milled LSP 3 shots SP 125% as milled Present experimental 7075 LSP 900 shots/cm activity P. Peyre, R. Fabbro, P. Merrien, H.P. Lieurade Laser shock processing of aluminium alloys. Application to high cycle fatigue behaviour Mater. Sci. Eng. A210 (1996) V. Ristori LSP of Thin Open-hole Aluminum Specimens 27 / 37

28 Specimen preparation and testing Roughness measurement Fatigue tests interpretation (a) Specimen with tabs (b) Testing V. Ristori LSP of Thin Open-hole Aluminum Specimens 28 / 37

29 Roughness measurement Fatigue tests interpretation Fatigue tests, R=0.1, σ max =160 MPa Fatigue tests, R=0.1, σ max =160 MPa V. Ristori LSP of Thin Open-hole Aluminum Specimens 29 / 37

30 Roughness measurement Fatigue tests interpretation Fatigue tests, R=0.1, σ max =160 MPa Fatigue tests, R=0.1, σ max =160 MPa V. Ristori LSP of Thin Open-hole Aluminum Specimens 29 / 37

31 Specimen modication Roughness measurement Fatigue tests interpretation Polishing of the inner surface of the hole (a) Fracture surface, non polished hole (b) Polished hole Inner surface was polished with a diamond paste Fatigue life at σ max =160 MPa showed practically no dierence (34579 polished vs as peened) V. Ristori LSP of Thin Open-hole Aluminum Specimens 30 / 37

32 Specimen modication Roughness measurement Fatigue tests interpretation 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 (R a = 0.4µm) Again, fatigue life at σ max =160 MPa showed practically no dierence (32094 polished vs as peened) V. Ristori LSP of Thin Open-hole Aluminum Specimens 31 / 37

33 Roughness measurement Fatigue tests interpretation 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 conrmed by synchrotron measurements) The choice of using open hole + LSP because of suspected release of residual stresses in case of LSP + open hole V. Ristori LSP of Thin Open-hole Aluminum Specimens 32 / 37

34 Contents Completed work Work to be done Further research Completed work Work to be done Further research V. Ristori LSP of Thin Open-hole Aluminum Specimens 33 / 37

35 Completed work Completed work Work to be done Further research 7075-T73 16 open hole specimens LSP treated 2 open hole specimens LSP treated on three dierent 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 V. Ristori LSP of Thin Open-hole Aluminum Specimens 34 / 37

36 Work to be done Completed work Work to be done Further research 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) V. Ristori LSP of Thin Open-hole Aluminum Specimens 35 / 37

37 Additional tests Completed work Work to be done Further research 6082-T6 Goal on open hole and plain specimens Residual stress measurement at the synchrotron facility Fatigue tests Determining the importance of the sequence of operations: open hole + LSP vs. LSP + open hole V. Ristori LSP of Thin Open-hole Aluminum Specimens 36 / 37

38 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 Shock Peened holes ICAF 2011 conference, Montreal, Canada