| Nanocrystalline materials have many special and novel properties compared to the coarse-grained counterparts,thanks to their nano-scale structure with high volume fraction of grain boundaries.However,the production of bulk nanocrystalline materials with large size and uniform structure was still a challenge for now.Considering that materials failures initially occur on the surface in most cases,generating a nanostructured surface layer on coarse-grained substrates had been put forward to achieve the similar properties like bulk nanocrystalline materials.Kinds of synthesis techniques have been developed for surface nanocrystallzation.Among them,ultrasonic shot peening(USP)is considered as one of the most effective mechanical surface treatment to achieve surface nanocrystallization(SNC),due to the advantages of low energy consumption,short process time,without pollution,the strong controllability of process parameters,wide applicability for workpieces with different materials and shapes,and so on.In recent years,owing to the good mechanical properties(such as high strength weight ratio,high heat proof),chemical stability,and biocompatibility,titanium and its alloys are widely used in aerospace,military,marine,medical and other many fields.Thus,theoretical and experimental studies about the surface modification and forming generated by USP were carried out to reveal the mechanisms of USP forming and surface modification and grasp the influence of various USP process parameters on the surface quality and forming,which have important theoretical and practical significances for the optimization of USP forming scheme and the improvement of theoretical development and engineering application of USP forming and surface modification.In this paper,industrial pure titanium(TA2)plate with hexagonal close-packed(HCP)crystal structure was selected as the research object.The mechanisms of USP forming and surface modification were studied via experiments,numerical simulation and theoretical analysis.The influence of the USP process parameters on the surface microstructures,surface properties and bending forming of TA2 were analyzed in detail.A mathematical model of the "whole physical process of USP was established.The main contents and conclusions in this work are as follows.(1)The influence of the USP process parameters,such as peening duration,shot diameter,sonotrode amplitude and peening distance,on the thickness of the nanocrystalline-amorphous(NC-A)mixed layer.grains size and amorphization were studied by the self-designed USP devices.The mechanism of the grain refinement and amorphization of TA2 during the USP process were discussed and a relationship e between the thickness of the NC-A mixed layer and the USP process parameters was proposed.(2)The influence of the USP process parameters,such as peening duration,shot diameter,sonotrode amplitude and peening distance,on the surface hardness,surface roughness and hydrophilicity were studied and the relation between the surface roughness and hardness was analyzed.Then the predictive models of the surface hardness,surface roughness and hydrophilicity of the USP-treated TA2 were established.Based on the classical Wenzel model,the influence of surface roughness and Young angle ? on the changes of hydrophilic ity after USP was also discussed.(3)The influence rules and mechanism of the NC-A mixed layer with different thickness obtained by USP on tensile properties including tensile strength(UTS).yield strength(YS).fracture elongation(FE)of TA2 were studied by the observation of the microstructure and the fracture morphology.After the discussion about the relation between the surface hardness and the UTS,a model was established to predict the UTS of USP-treated pure titanium plate only using the process parameters and the accuracy of the model was verified.(4)Combining the motion law of individual shot in the USP process and the influence of ultrasonic vibration obtained by finite element simulation on the collision recovery coefficient,a mathematical model of the average impact velocity was established.Based on the Hertz contact model and the elastic-plastic spherical shell expansion model,the impact process of an rigid ball with a certain velocity on the elastic-plastic plate was systematically studied,and an analytical model of the impacting process between a rigid shot and a elastic-plastic plate was established.Combining the established the average impact velocity model and the shot impacting model,an overall mathematical model of the motion and impact of individual shot was obtained.(5)According to the analysis of the stress superposition caused by multiple impacts at the same position and simultaneous collision of multiple shots,the overall mathematical model of the motion and impact of individual shot was corrected and extended to the actual situation of multiple impacts of multiple shots.Finally,an analytical model that could directly estimate the saturated residual stress distribution after multiple collisions of multiple shots with USP process parameters was obtained and was verified by experiments.(6)The bending forming of the TA2 plate was realized by USP and the influence of the USP process parameters,such as plate thickness,fixing distance peening duration,shot diameter,sonotrode amplitude and peening distance,on the bending angle were studied.Based on the results of the experiments,the established USP analytical model,the finite element simulation and the ARGUS strain measurement,the residual stress distribution of the USP-bended TA2 specimen were analyzed and discussed. |
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