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No description available.

Shot peening is a cold-working surface treatment, basically consisting in pelting the surface of the to-be-treated component with a high number of small hard particles blown at relatively high velocity. This causes the plasticization of the surface layer of the substrate, and the generation of a compressive residual stress field beneath the component surface. The surface topology modification can be beneficial for coating adhesion, and the work hardening enhances the fretting resistance of components, but the most commonly appreciated advantage of the process is the increased fatigue resistance in the treated component, due to the compressive residual stress which inhibits the nucleation and propagation of fatigue cracks. In spite of its widespread use, the mechanisms underlying the shot peening process are not completely clear. Many process parameters are involved (material, dimension, velocity of the shots, coverage, substrate mechanical behavior) and their complex mutual interaction affects the success of the process as well as the jeopardizing of any beneficial effect due to the increased surface roughness. Experimental measurements are excessively expensive and time-costly to deal with the wide variability of the process parameters, and their feasibility is not always granted. The effect of shot peening is indeed particularly effective where geometrical details (e.g. notches or grooves) act as stress raisers and where the direct measurement of residual stresses is very difficult. Nonetheless, the knwoledge of the effects of the treatment in this crictical locations would be extremely useful for the quantitative assessment of the effect of shot peening and, ultimately, for the optimization fo the process as well as its complete integration in the design process. The implementation of the finite element method for the simulation of shot peening has been studied since many years. In this thesis the simulation of shot peening is studied, in order to progress towards a simulation approach to be used in the industrial practice. Specifically, the B120 micro shot peening treatment performed with micrometric ceramic beads is studied, which has proven to be very effective of aluminum alloys, such as the aeronautical grade Al7075-T651 alloy considered in this work. The simulation of shot peening on a flat surface is addressed at first. The nominal process parameters are used, to include stochastic variability of the shot dimensions and velocity. A MatLab routine based on the linearization of the impact dent dimension, on the shot dimension and velocity is used to assess the coverage level prior to the simulation and predict the number of shots to full coverage. To best reproduce the hardening phenomena of the substrate material under repeated impacts, the Lemaitre-Chaboche model is tuned on cyclic strain tests. Explicit dynamic finite element simulations are carried out and the statistical nature of the peening treatment is taken into account. The results extracted from the numerical analyses are the final surface roughness and residual stresses, which are compared to the experimentally measured values. A specific novel procedure is devised to account for the effect of surface roughness and radiation penetration in the in-depth residual stress profile. In addition, a static finite element model is devised to assess the concentration effect exerted by the increased surface roughness on an external stress. The simulation of shot peening on an edge is then addressed as a first step towards more complex geometries. Since the true peening conditions are not known in this locations, a synergistic discrete element - finite element method approach is chosen for the correct modelization of the process. A discrete element model of the peening process on a flat surface is used to tune the simulation on the nominal process parameters, i.e. mass flow rate and average shot velocity, and to assess the nozzle translational velocity. Discrete element simulations are used to simulate the process when the nozzle turns around the edge tip. To lower the computing cost, the process is linearized into static-nozzle simulations at different tilting angles. The number of impacting shots and their impact velocity distribution are used to set up the finite element simulations, from which the resulting residual stress field is obtained. In addition to the realistic simulation, two simplified simulation approaches for the practical industrial use are devised. The resulting residual stress fields are compared with the reference residual stress field computed using thermal fields in a finite element simulation, tuned with experimental XRD measurements. The effect of the dimension of the fillet on the edge tip is studied by modifying the finite element model of shot peening on an edge. 3 different fillet radii (up to 40 um) are considered, on the basis of experimental observations. The resulting residual stress field are compared to analyze the effect of the precise geometry of the substrate. Lastly, the simplified simulation approach devised in the case of the edge is used to simulate shot peening on the root of a notch. The resulting residual stress field is again compared to the reconstructed reference one.

This thesis aims to investigate the manufacturability of nickel-based superalloys, IN625 and IN718, using the laser powder bed fusion (LPBF) process. The study provides a better understanding of the process-structure-property of nickel-based superalloys, their fatigue life, and subsequent post-processing. First, the process-structure-property was investigated by selecting a wide range of process parameters to print coupons for IN625 and IN718. Next, a subset of process parameters was defined that would produce high relative density (>99%), low surface roughness (~2 μm), and a low tensile RS. Second, a multi-scale finite element model was constructed to predict the temperature gradients, cooling rates, and their effect on RS. At constant energy density, RS is affected by scan speed, laser power, and hatch spacing, respectively. Third, the optimum set of parameters was used to manufacture and test as-built and shot-peened samples to investigate the fatigue life without costly heat treatment processes. It was found that shot peening resulted in a fatigue life comparable to wrought heat-treated unnotched specimen. Additionally, IN625 had a better fatigue life compared to IN718 due to higher dislocations density as well as the absence of γ´ and γ´´ in IN718 due to the rapid cooling in LPBF. Finally, the effect of post-processing on dimensional accuracy and surface integrity was investigated. A new approach using low-frequency vibration-assisted drilling (VAD) proved feasible by enhancing the as-built hole accuracy while inducing compressive in-depth RS compared to laser peening, which only affects the RS. These favorable findings contributed to the scientific knowledge of LPBF of nickel-based superalloys by determining the process parameters optimum window and reducing the post-processes to obtain a high fatigue life, a better dimensional accuracy, and improved surface integrity. / Thesis / Doctor of Philosophy (PhD)

No description available.

No description available.

<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>

Compressive residual stresses (CRS) are beneficial for enhancing the fatigue life of metal components. Shot Peening (SP) is an industrial cold working process that is applied to induce a field of CRS and modify the mechanical properties of the metal component. The SP process involves impacting a surface with tiny shots with forces sufficient to create plastic deformation. The process is governed by a number of important parameters such as the shot size, angle of attack, initial velocity, mass flow rate and the distance from the shot nozzle to the surface being peened. The relationship between the optimal peening outcome, particularly the residual stress distribution of the treated surface, and the peening parameters is still unknown and needs to be investigated further. Manufacturers are interested in producing a uniform peening process for complex geometries which optimises the SP parameters. Modelling the process is complex as it involves the interaction of a metallic surface with a large number of shots of very small diameter. Conventionally, such problems are solved using finite element software to predict stresses and strains of a single shot impact then applying superposition. At the moment there are no Finite Element Method (FEM) modelling solutions involving more than tens of shots. The number of shots and elements required for such a modelling process made the approach unfeasible prior to the work described herein. The objective of this work is to develop an appropriate numerical modelling approach that can better simulate the real SP process. The model will be provided by combining Discrete Element Method (DEM) with FEM. The DEM is employed to get a distribution of impact velocities over space and time which are then implemented into a FEM analysis. A discrete element model with randomly distributed steel shots bombarding a steel component at various velocities has been developed as benchmark example. With this model the SP shot - shot interaction, the shot - target interaction, the surface coverage, angle of impingement, shot size, impact velocity and the overall shot flow can be parametrically studied in details and with little computational effort. The novel approach also proposes a new method to dynamically change the coefficient of restitution for repeated impacts during the simulation and predicts the CRS more effectively. The effects of SP on different materials of relevance to gas turbine engine components will be investigated in order to improve the understanding of the interaction between the shots and the targeted material. Initially, an uncoupled analysis was peforned, in order to assess the capabilities of the two modelling systems, DEM and FEM, to delivery an improved solutuion when combining two commercially available codes. This parametric analysis is performed using the state-of-the-art Discrete Element (DE) application EDEM. In the subsequent part of this work, a dynamic Finite Element (FE) application Abaqus will be used to investigate single shot impacts and to obtain the residual stress distribution. This gives us a prescribed residual stress distribution and peening coverage. A Combined DEM/FEM tool (DEST) is proposed that eliminates any manual pre-processing required for linking/coupling, eliminating the use of two different applications and provide an integrated solution for the simulation of the Shot Peening process. In the subsequent chapter, the implementation of essential tools for the enchanced modelling of Shot Peening process functionalities, such as the nozzle, bounding box, coverage and intensity is described. A number of computational improvements are also implemented to reduce the computation time. The existing binary search is enhanced to self-balancing search tree and further improved to allow insertion and deletion of elements. A bounding box feature which removes shots that move out of the domain during the course of the simulation is also implemented. Experiments featuring single shot impacts are performed to gain better understanding the deformation process in the target material subjected to impact conditions to those occurring in the production peening. The single shot impacts are experimentally examined using SEM and EBSD. During final chapter, case studies are performed to compare the results of the simulations with large-scale experimental work. The coverage of peening of single and multiple nozzles with different angle of impingements are assessed. Finally, possible directions for further research concerning the accurate quantification of material responses to SP are identified in the report.

O objetivo deste trabalho foi a avaliação do comportamento à fadiga de materiais compósitos de matriz metálica (CMM) obtidos pela rota da metalurgia do pó, tendo como variáveis a fração volumétrica de reforços e o tipo de tratamento superficial mecânico utilizado. Foram obtidos materiais compósitos com matriz de alumínio AA 1100 reforçados com partículas de carboneto de silício (SiC) nas frações volumétricas de 5, 10 e 15%. Uma quantidade de material de controle, constituído unicamente pelo material da matriz também foi produzida para fins de comparação. Os materiais obtidos foram caracterizados física, mecânica e microestruturalmente. Os resultados mostraram, de maneira geral, uma distribuição homogênea das partículas de reforço e melhoria do limite de resistência dos compósitos com relação ao material de controle. Entretanto, alguns defeitos como porosidades e veios de alumínio puro foram detectados esporadicamente. Em outra etapa, foram realizados ensaios de fadiga do tipo axial nos materiais em modo tensão-tensão, com razão de tensões R = 0,1 e frequência de 15 Hz. Os tipos de tratamentos superficiais utilizados na confecção dos corpos de prova de fadiga foram: usinagem e jateamento. As variáveis de usinagem foram: taxa de avanço e tipo de ferramenta. Os materiais jateados não apresentaram melhorias significativas de vida à fadiga com relação ao material de controle. Os usinados grosseiramente apresentaram a pior vida em fadiga e as marcas de usinagem, nestes casos, funcionaram como fortes concentradores de tensão. Os reforçados com 5% de SiC, diferentemente daqueles reforçados com 10 e 15%, apresentaram vida à fadiga inferior à do material de controle, ou por causa do menor limite de escoamento ou devido à menor fração volumétrica de reforços. Os materiais usinados com metal duro (MD) não apresentaram diferenças de vida à fadiga com relação aos usinados com PCD, provavelmente devido à classe do metal duro utilizado. Os materiais reforçados com 5% de SiC e jateados apresentaram resultados de fadiga com os maiores desvios padrões. Os materiais reforçados com 5% de SiC apresentaram as menores sensibilidades à fadiga com a variação da carga. / The objective of this work was the evaluation of the fatigue behaviour of aluminium metal matrix composites (MMC) obtained by powder metallurgy. The testing variables were the volumetric fraction of reinforcements and the type of mechanical surface treatment used. Initially, the composite materials were obtained from aluminium AA 1100 matrix, reinforced with silicon carbide (SiC) particles in the volumetric fraction of 5, 10 and 15%. An amount of control material (unreinforced) was produced for comparison purposes. The obtained materials were physically, mechanically and microstructurally characterised. The results showed a homogeneous distribution of the reinforcement particles and an improvement of the ultimate tensile strength of the composites with relation to the control material. However, some defects such as porosity and streaks of pure aluminium were detected. In a second stage, the fatigue tests of the composites were accomplished. The types of surface treatments used in the fabrication of the fatigue test specimens were machining and shot peening. For machining the variables were feed rate and tool type. The shot peened materials did not present a significant fatigue life improvement when compared to the control material. The coarse machined materials presented the worst fatigue life and the machining marks worked as strong stress concentrators. The material reinforced with 5% of SiC, differently of those reinforced with 10 and 15% showed inferior fatigue life when compared to the control material, probably because of a lower yielding strength, or lower reinforcement volumetric fraction. The material machined with hard metal (MD) did not present differences of fatigue life with relation to the machined with PCD, probably due to the class of the hard metal used. The material reinforced with 5% of SiC and shot peened, presented fatigue results with the largest standard deviations. The materials reinforced with 5% of SiC presented the smallest fatigue sensibility with the load variation.

Os tratamentos de cementação, têmpera, revenimento e jateamento por granalhas são muito utilizados na produção seriada de peças da indústria automobilística, como engrenagens e eixos. Variações destes processos podem influenciar a vida em fadiga por flexão dos componentes descritos. O objetivo deste trabalho foi estudar a influência de variações no tempo de jateamento por granalhas, variações na temperatura de revenimento e de diferentes profundidades de oxidação intergranular, no crescimento de trincas curtas por fadiga na camada cementada do aço DIN 20MnCr5. Para isso foram feitos ensaios de fadiga por flexão em quatro pontos, utilizando corpos de prova de seção retangular e sem entalhe. Para o acompanhamento do crescimento de trincas curtas foi utilizado o método de réplicas de acetato. Foram estudados seis níveis de jateamento, quatro níveis de revenimento e dois níveis de oxidação intergranular. Como resultados principais, foram obtidas curvas de comprimento de trinca por número de ciclos e taxa de crescimento de trinca por tamanho médio da trinca. Amostras jateadas entre trinta minutos e uma hora, revenidas à 160&#176C e com oxidação intergranular por volta de dez mícrons, apresentaram os melhores resultados em relação à vida em fadiga. / Carburizing, quenching, tempering and shot peening treatments are often used in mass production of automotive parts, such as gears and shafts. Changes in these treatments can influence the bending fatigue life of these parts. The purpose of this study was to measure the influence of different shot peening times, different tempering temperature and different internal oxidation depths, on the short fatigue crack growth in case carburized DIN 20MnCr5 steel. Four-point-bend fatigue tests were carried out in rectangular section specimens without notch. The surface short crack growth was monitored by means of acetate replication technique. Six levels of shot peening, four levels of tempering temperatures and two levels of internal oxidation were studied in this work. Crack length versus number of cycles and crack growth rate versus mean crack length were obtained as principal results. Shot peening from 30 minutes to one hour, tempering at 160#176C and a lO~m depth of internal oxidation showed the best results in fatigue life.