| TC17 titanium alloy has been widely used for manufacturing key components such as aero-engine fans,compressor discs and blades due to its high specific strength,good welding performance,outstanding thermal processing properties,and excellent fatigue strength.Key components of aero-engines always serve in severe working environments,therefore,it is easy to generate wear,distortion,pits,cracks and other defects.The re-manufacturing of repairable components is one of the key technologies in the operation and maintenance of aero-engines,which helps to extend their service lives and significantly reduce costs.Laser additive manufacturing technology and electron beam welding technology have been used to repair microscale damaged and moderate damaged components by virtue of their unique technical advantages.When using homogenous powders to repair damaged titanium alloys,the hardness of the repaired region is often higher than that of the matrix.Therefore,in order to match the mechanical properties of the repaired region and the matrix,it is better to choose heterogeneous powders for laser additive repairing(LAR).In addition,further surface strengthening method should be used to improve the fatigue strengths and prolong the service lives after repairing process.In order to solve the problem of fatigue life extension for aero-engine repaired components,based on the repairing and surface strengthening research of aero-engine blade material TC17 titanium alloy,in this paper,TA15/TC17 titanium alloy laser additive repaired(LARed)joints and TC17 titanium alloy electron beam welded(EBWed)joints were taken as the research objects,the surface microstructural evolution and residual stress distribution of repaired joints after laser shock peening(LSP)were investigated,the fatigue crack propagation characteristics,high-cycle fatigue performance and surface strengthening fatigue life extension mechanism of the repaired joints were systematically explored.The main work is as follows:1.Research on surface morphologies,surface microstructural evolution and strengthening mechanism of LARed joints after LSPAfter LSP,the surface morphologies and roughness of the LARed joints were observed using a three-dimensional surface topography instrument;the crystallographic properties(grain size,grain boundary distribution and texture)and the dislocation evolution were studied by using the microstructural characterization technologies;the load-displacement curves were obtained by a nano-indenter;the Xray diffraction spectrums on the surface were obtained by using an X-ray diffractometer.It is shown that LSP has little effect on the surface morphologies of the joints,it can induce high-density dislocation proliferation on the surface of the LARed joints and improve their ability to resist foreign damages;dislocation proliferation,twin formation and grain refinement works together to strengthen the LARed joints.2.Residual stress gradient distribution and high-cycle fatigue life extension mechanism of LARed joints after LSPThe residual stress distributions on the surface and depth direction of the joints before and after LSP was measured by X-ray diffraction method and drilling method.The fatigue crack propagation test and the rotational bending fatigue test under a specific stress level were carried out.The results show that LSP can induce a residual compressive stress layer with a depth of 0.7 mm on the surface of the LARed joints,it can also improve their fatigue resistance and reduce the fatigue crack growth rates.The mathematical statistics results confirm that the fatigue lives of the LARed joints is significantly improved by LSP.3.Microstructural evolution and grain refinement mechanism of EBWed joints after LSPThe crystallographic properties and dislocation evolution of EBWed joints under the effect of laser shock waves were studied.The residual stress distributions on the surface and depth of joints before and after LSP were measured by X-ray diffraction and drilling methods.The results show that laser shock wave can induce a residual compressive stress layer with a depth of about 0.6 mm on the surface of the joints,and the residual compressive stress amplitude in different regions is related to the yield strengths.The microstructural evolution mainly originates from the dislocation proliferation mechanism and the twinning nucleation mechanism.4.Study on the fatigue crack propagation behavior of EBWed joints after different LSP methodsThree kinds of LSP methods with different positions and different areas were carried out on EBWed joints and base metal.Residual stress distributions under different LSP methods were extracted by means of finite element simulation method.The fracture morphologies of the EBWed joints and the base metal were characterized to study the effects of LSP on their fatigue crack propagation behavior.The results show that the maximal residual compressive stress can be obtained by impacting along the width direction of the stable fatigue crack propagation zone of the specimen,so that the life extension effect is the best.The difference in microstructures between the EBWed joints and the base metal leads to different fatigue crack growth characteristics.5.Evaluation of high-cycle fatigue life of EBWed joints and study on the combined effect of internal defects and LSP-induced residual compressive stress on high-cycle fatigue lifeRotating bending fatigue tests under different stress levels were carried out on the joints.Combined with micro-fracture defect statistics and fatigue band analysis,the correlation between defect characteristics and fatigue lives was studied.The results show that the existence of porosity defects in the EBWed joints weakens the life extension effect of LSP,and the influence of the defect position on the fatigue life is more significant than that of the defect size;based on the principle of sample information aggregation,P-S-N curves can provide a safer and more reliable fatigue loading range for EBWed joints. |
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