Water cavitation peening for aerospace materials
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Conventional shot peening has been used for almost a century as a means to mechanically improve the fatigue and corrosion resistance of metals. It employs spherical shots of different size and materials to introduce compressive residual stress in the surface layers of metals, thereby improving their mechanical integrity when subjected to cyclic loading and/or a corrosive environment. However, this technique is also characterized by substantial surface roughening, high consumables cost and workpiece contamination. Over the past few decades, a number of alternative peening techniques have been developed to overcome these limitations. Among them, water cavitation jet peening (WCP) has showed promising preliminary results in further extending fatigue life, reducing surface modification and lowering equipment and operating costs. Nevertheless, there is limited scientific understanding of this process at present. Consequently, there is a need for further studies aimed at developing detailed fundamental understanding of the cavitation jet peening process, exploring its potential and adequately characterizing the process to satisfy the demanding requirements set by industry, especially for aerospace applications. This thesis presents an investigation of a novel co-flow water cavitation jet peening (WCP) system that expands the limited understanding of the process and addresses the limitations of the currently available peening processes. The basic concept involves creating cavitation by injecting a high speed jet into a low speed jet in a concentric (or co-flow) configuration, and placing the co-flow nozzle at an optimum distance from the workpiece surface. By suitably controlling the flow parameters and nozzle dimensions, it is possible to produce a sufficiently aggressive cavitation cloud capable of plastically deforming metal surfaces and introducing beneficial compressive residual stresses. The main focus of this thesis is on the development of a WCP peening and the fundamental characterization of the cavitating flow for peening applications. The flow characterization is carried out by means of accelerated erosion tests and strip curvature tests. Accelerated erosion tests are commonly used in the field of cavitation erosion to evaluate cavitation intensity and rank the erosion resistance of common engineering materials. Strip curvature tests are an established practice in the peening field used to evaluate the effectiveness of peening treatments. The results indicate that cavitation intensity and peening capability can be substantially increased by adopting suitable flow conditions, and that the optimum outer flow velocity Vout and normalized standoff distance sN are essentially independent of the inner flow velocity Vin within the range adopted for this study. Peening tests carried out on Aluminum 7075-T651 give higher and deeper residual stresses compared to shot peening while lowering the surface roughening, potentially leading to a substantial increase in the fatigue life of components treated by WCP compared to conventional shot peening. Nozzle geometry in co-flow configuration is investigated following the same approach developed in the previous study. Among other aspects, the scalability of the process is investigated by testing three nozzles with constant geometry ratios but increasing in size. Peening time is found to diminish by 30% as a consequence of a 12% increase of the nozzle diameter. Interestingly, optimum flow parameters previously identified are found to apply to different nozzles as well. In addition to erosion and strip curvature tests, high speed video imaging analysis is introduced to investigate the effect of nozzle geometry on the cavitation cloud. A correlation is found between cavitation intensity, cavitation cloud width and its power spectral density. Finally, pitting tests are carried out on mirror finished Aluminum 7075-T651 samples under several flow conditions and impact loads exerted by single cavitation phenomena onto the material surface are calculated. The pitting tests are found in good agreement with the flow characterization performed in chapter 4, showing larger pit diameters and impact loads for the flow conditions leading higher mass loss and strip curvature. The stress generated by the cavitation phenomena and the pit diameter observed during the pitting tests are used as input in a dynamic, 2D axisymmetric finite element model solved in ABAQUS/Explicit. The purpose of the model is to obtain an estimate of the longitudinal residual stresses introduced in the material surface by a single cavitation phenomenon using the information collected through the stress-strain analysis. Although the actual peening process is characterized by thousands of cavitation impacts per unit time and area, the pitting tests, along with the model of a single cavitation bubble collapse, reveal encouraging results for the future development of a semi-empirical model capable of predicting the residual stress state in WCP. In summary, this thesis describes the investigation of an innovative water cavitation peening process capable of introducing compressive residual stresses in aerospace materials such as Aluminum 7075-T651 while limiting surface roughness and contamination typical of conventional shot peening. The characterization of the process and nozzle serve as a basis for users to select optimal peening conditions and create a cost effective alternative to the conventional shot peening technology.