| Titanium alloys are commonly used materials for the manufacture of some large-scale critical complex curved components,such as the aeroengine impellers,blades,casings,and rotors.Suffering from the thermal load,aerodynamic load and other inertial loads,the fatigue failure is a key issue for the critical components.As an effective way to improve the fatigue performance by means of improving the surface integrity,burnishing strengthening technique has aroused widespread concern and attention.Its remarkable advantages are of high strengthening efficiency,uniform stress distribution and stable performance of the strengthened layer.At present,the study of burnishing technology mainly focuses on the improvement of surface integrity.However,there are quite few literatures concerning the active control of burnishing strengthening.In addition,the basic theory of improving the fatigue resistance of burnished components is relatively insufficient.Based on the research on the surface integrity of TA2 titanium alloy after burnishing,this dissertation focuses on the influence and mechanism of burnishing strengthening on fatigue properties.The aim of the present paper is to provide some foundational researches on the anti-fatigue manufacturing technology of the critical components of titanium alloy.Specific contents of this paper are as follows:(1)Experimental study on burnishing process.The effects of different burnishing process parameters on the surface roughness and microhardness are studied.In order to actively control the surface roughness and microhardness,the relational models of the roller burnishing process parameters and the surface roughness as well as microhardness are established respectively by means of analysis of variance and multiple linear regression analysis.The effects of key process parameters of low plasticity burnishing have also been studied.The results show that the low plasticity burnishing has both the finishing effect and the surface strengthening effect.The finishing effect is obvious if the feed rate is lower.Similarly,increasing the hydraulic pressure and reducing the burnishing speed as well as increasing the number of burnishing passes are beneficial for improving the surface strengthening effect.Finally,the microstructural characteristics of the surface strengthened layer at different depths are analyzed by means of X-ray diffraction and metallographic microscope.(2)Distribution of residual stress induced by low plasticity burnishing.a burnishing force model is built by a combination of theory and experiment method.Then,with the help of the finite element simulation software,a three-dimensional finite element model of low plastic burnishing is established by using the explicit dynamic finite element method.And the distributions of residual stress under different burnishing process parameters are systematically analyzed.The results show that the depth of residual compressive stress keeps increasing with the increase of the number of burnishing passes.However,as the hydraulic pressure increases,the location of the maximum residual compressive stress is gradually transferred to the surface.Increasing the number of burnishing passes,the maximum residual stress and its depth as well as the maximum depth of residual compressive stress are increasing.But increase amplitude is declining.In order to quantitatively study the degree of plastic deformation induced by burnishing strengthening,the residual stress and strain area are used to characterize.The relationship models between the burnishing process parameters and the residual stress and the strain area are established,according to the similarity theory and orthogonal test results.(3)Fatigue crack propagation behavior of TA2 titanium alloy treated by burnishing strengthening.The influence mechanism of burnishing strengthening on the fatigue crack propagation properties(stress intensity factor and crack propagation rate)is studied by utilizing the theory of fracture mechanics.A fatigue crack growth life equation of burnished specimens is built using the Forman formula.This equation takes into account the effect of external load and residual stress on the stress intensity factor.In order to verify the validity of the equation,the crack propagation properties of TA2 titanium alloy under different residual stress distributions are investigated by carrying out the fatigue crack growth rate tests under constant amplitude load.Compared the experimental value of crack propagation life with the theoretical value,it can be found that the theoretical value is conservative.But it is safe to estimate the actual fatigue life of the specimen.Finally,the macroscopic and microscopic fatigue fracture topographies of compact tensile specimens before and after burnishing are analyzed by scanning electron microscopy.Analysis results reveal that the fatigue crack growth rate of the burnished specimen is obviously lower than that of the original specimen at the early and middle stage of crack propagation.(4)High cycle fatigue performance of TA2 titanium alloy treated by burnishing strengthening.Dimensional Analysis is firstly carried out to determine the key factors influencing the fatigue properties of the burnishing strengthened components.Based on the dimensional analysis,aiming at the shortage of calculating the gradient by using the differential method,a relationship model between the fatigue strength of unburnished and burnished specimens is built by introducing the influence factors of stress and strain.The relationship model has been taken into account the positive effects of residual stress,microhardness,surface roughness,stress gradient,microhardness gradient and specimen size.Then,fatigue tests are carried out to study the effects of burnishing on tensile fatigue strength and three-point bending fatigue strength of TA2 titanium alloy.Comparing the experimental value with the theoretical value,it is found that the the measured values is close to the theoretical value.It can be concluded that the relationship model can well reveal the effect of burnishing strengthening process on the fatigue strength. |
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