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<p>Laser shock peening is a high strain rate plastic deformation process, and it has been widely used in automobile, aerospace, and nuclear industries for surface enhancement. Lots of new developments of the laser shock peeing process have been studied to expand its new applications such as cryogenic laser shock peeing, warm laser shock peening, laser shock peening without coating, laser shock peening without confinement. There are still some issues that has not been addressed: 1) interaction between laser shock wave and layer structured composite material has not been studied. 2) investigation on microstructure and mechanical properties of intermetallic phase strengthened composite material processed by warm laser shock peening is rare. 3) preheating method for warm laser shock peening needs improvement.</p> <p>In this study, thermal and temporal modulated laser shock peening process is developed to control microstructure and mechanical properties. 1) Laser shock peening and cryogenic laser shock peeing was applied to copper graphene heterostructure. Hardness, yield strength were measured and microstructures were characterized. Shock wave propagation and its interaction with monolayer graphene was studied by finite element analysis. Results showed that the yield strength of laser shock peeing and cryogenic laser shock peeing processed copper graphene samples increased by 40%, and 76% respectively. It was found that shock wave could pass through long-distance to generate dislocation transportation from one layer to another graphene with the shock wave interaction between graphene layers separated very far away. 2) Warm laser shock peening with different preheating temperature was performed on lightweight steel. Effect of temperature on mechanical properties, precipitates and dislocation distributions are investigated. A coupled phase field-dislocation dynamics model was developed to study the precipitates and dislocation generation mechanism. The yield strength of the lightweight steel after warm laser shock peeing reaches 2030Mpa, which is the highest for lightweight mid-carbon steel (70% Fe, 1%C). Experiment results have confirmed high density dislocations and precipitates are generated by warm laser shock peeing process. And we find a new mechanism, avalanche multiplication of dislocations and precipitates, during the warm laser shock peeing: I) Dislocations assist precipitates formation. II) Precipitates boost dislocation generation. 3) A novel dual pulse laser shock peening process was developed which combines preheating and laser shock peening process.The effect of modulating pulse width and pulse duration on processing temperature and material microstructures were studied. Results showed that single pulse laser processing could successfully remelted the second phase and had much smaller grain (500nm) due to fast cooling, and dual pulse with appropriate pulse duration resulted in high density nanosized (30nm) intermetallic phase. High hardness 59 HV and yield strength 547MPa could be achieved due to the combination of grain size refinement, hard second phase and dislocations.</p>

Cette thèse vise à déterminer l’influence de deux traitements mécaniques, le grenaillage SMAT effectué avec plusieurs types de billes (en WC, en alumine et en verre) et le choc laser, sur la résistance à l’oxydation haute température de deux alliages de titane : un alliage alpha commercialement « pur » (Grade 1) et un alliage aéronautique béta métastable (TIMETAL-21S).Une fois traitées, les pièces sont oxydées avec différentes conditions : de durée (entre 5 heures pour étudier les premier instants de l’oxydation et 3000h pour se rapprocher d’un essai type en aéronautique), de température (600°C à 700°C) et d’atmosphère (air sec et oxygène).Les pièces sont analysées avant et après oxydation à l’aide de plusieurs techniques d’analyses : mécaniques (dureté, mesures de contraintes), chimiques (DRX, microsonde nucléaire, …) ou structurales (EBSD, texture).Les résultats obtenus montrent que les traitements perturbent fortement les pièces avant leur exposition à haute température d’un point de vue morphologique, structural, mécanique et chimique. Ces traitements mécaniques amènent une réduction de l’oxydation des pièces étudiées. Il semble qu’ils modifient la vitesse de diffusion des espèces (azote, oxygène, aluminium, molybdène) mais aussi la microstructure (recristallisation, morphologie de grain ou texturation) au cours de la mise en température. L’azote joue un rôle essentiel dans les phénomènes observés.Néanmoins, la détermination des conséquences de ces traitements sur la résistance à l’oxydation reste encore complexe de par les observations de ce travail, qui révèlent une contribution simultanée de plusieurs facteurs : chimiques, mécaniques et structuraux. / The aim of this thesis is to determine the influence of two mechanical surface treatments, the shot- peening performed with several type of balls (WC, alumina and glass) and the laser shock peening, on the high temperature oxidation resistance of two titanium alloys : alpha alloy with commercially purity (Grade 1) and aeronautical beta metastable alloy (TIMETAL-21S).After different treatments, the pieces are oxidized with different conditions: of time (between 5 hours to study the firsts times of oxidation and 3000 hours to compare with a classical aeronautical test), of temperature (600°C to 700°C) and atmosphere (dry air or oxygen).The pieces are analyzed before and after oxidation exposure with several mechanical (micro-hardness, strain measurements), chemicals (XRD, nuclear microprobe) and structural (EBSD, texture) techniques. The results show a large surface perturbation before the high temperature exposure in term of morphological, mechanical, structural and chemical point of view.This mechanical treatments lead up to an oxidation rate reduction for all the different titanium alloys. This treatments modified the diffusion rate of several elements (nitrogen, oxygen, molybdenum or aluminum) but also the microstructure (recrystallization, grain morphology or texturing) during high temperature exposure. Nitrogen element plays an important role in the observed phenomena.However, the determination of consequences after mechanical treatment on the titanium oxidation resistance is again difficult with the observations noted in this work. Actually, there is a simultaneous contributions of several factors: chemical, mechanical and structural.

Surface treatments used to improve the life of a material known as peening are already extensively used in industry. The main aim of peening is to introduce compressive resiudal stress to the surface and subsurface of a metallic material, however literature also includes a number of microstructural and mechanical effects that peening introduces to a material when the compressive residual stress is established. The aim of this dissertation is compare and contrast the mechanical and microstructural effects of a current industrial peening method called shot peening, with three new increasingly competitive surface treatments. These are laser shock peening, ultrasonic impact treatment and water jet cavitation peening. The surface finish, and changes in microstructure, hardness depth profile, residual stress depth profile and plastic work depth profile of the four surface treatments are analysed. The effect of the peening parameters on the material is also determined, such as length of time of treatment, shot size, step size, direction of treatment, and irradiance per centimetre squared. The effect of peening on the residual stress depth profile of a gas tungsten eight pass grooved weld is also determined. Welding is a known region of early failure of material, with one of the factors affecting this being the introduction of tensile residual stress to the surface and near surface of the weld. An analysis to determine if peening the welded region alters the residual stress was carried out. In all experiments in this dissertation, the material that was used was austenitic stainless steel, as this material is highly used, especially within the nuclear industry. The results of this dissertation show that different peening types and peenign parameters produce a variety of surface, microstructural and mechanical effects to austenitic stainless steel. Peening of an aaustenitic stainless steel welded region results in teh near surface tensile residual stress to alter to ccompressive residual stress.