| Titanium alloys have high specific strength,good corrosion resistance,and excellent high temperature mechanical performance.Therefore,these alloys have been used increasingly in aircraft engine fan blades,compressor disks,aircraft landing gears,and fasteners.However,these components are prone to plain fatigue(PF)and fretting fatigue(FF)damages during service due to the low hardness,poor wear resistance and high notch sensitivity of titanium alloys.Shot peening(SP)can effectively improve the PF and FF performance of metal materials.Double shot peening(DSP)treatment with reasonable parameters can achieve better strengthening effect than SP.In addition,gradient nanostructure reveals good prospects in improving anti-wear and fatigue resistance performance.Ultrasonic surface rolling process(USRP)is a new type of surface deformation strengthening technology,which combines static extrusion and dynamic ultrasonic impact.USRP can effectively improve the surface integrity and prepare a gradient nanostructured(GNS)surface layer on the metallic materials.Therefore,USRP has outstanding advantages in improving the fatigue resistance of metallic materials.Plasma surface alloying(PSA)technology can effectively enhance the wear resistance of titanium alloys,but it will significantly reduce the fatigue performance of titanium alloys in the same time.Based on the above background,this article selected TC17 and TC4 titanium alloys as research objects.The influence laws and effect mechanisms of SP and DSP on PF performance of titanium alloy were comparatively studied.Then,the influence laws of USRP and surface gradient nanocrystallization on the PF and FF performance of titanium alloy were explored.Meanwhile,the internal correlation mechanisms of surface integrity and fatigue performance were revealed.Finally,the influence laws and mechanisms of the duplex treatment of PSA(Mo and Zr)with SP and DSP to improve the FF resistance of titanium alloy were also explored systematically.The specific results and conclusions are as follows:(1)The influence laws and mechanisms of SP and DSP treatment on the PF behavior of TC17 titanium alloy were comparatively studied.The results demonstrated that the SP treatment with medium intensity could obtain optimal fatigue resistance.SP intensity is too low or too high to achieve the desired strengthening effect.The reason is that the SP treatment with medium intensity can achieve the best surface integrity of TC17 titanium alloy.DSP treatment is better than SP treatment for improving the fatigue resistance of TC17 titanium alloy.This is attributed to the best synergistic effects of the surface microstructure refinement,work hardening,compressive residual stress and surface morphology,which effectively suppresses the initiation and propagation of fatigue cracks.(2)The influence law and meschanisms of USRP on the surface integrity and PF behavior of TC17 titanium alloy were researched.The results showed that the PF performance of TC17 titanium alloy was enhanced significantly by USRP with different processing numbers(1,5,10,and 20).Moreover,enhancement degree increases gradually with increasing processing numbers.This is attributed to the compressive residual stress field with larger numerical value and deep distribution,which has synergistic effects with microstructure refinement,work hardening and low surface roughness.Thereinto,the surface work hardening and microstructure refinement are dominant factors in determining the PF performance.Simultaneously,compressive residual stress and low surface roughness play a secondary role in enhancing the PF performance.(3)The influence laws and effect mechanisms of USRP on the surface integrity and FF behavior of TC17 titanium alloy were investigated.The results demonstrated that the FF performance of TC17 titanium alloy was enhanced significantly by USRP with different processing numbers(1,5,10,and 20).Among them,the optimum processing number for obtaining a high FF performance is found to be twenty processing numbers.The enhancement of the FF performance can be ascribed to the synergistic effects of compressive residual stress,microstructure refinement and work hardening.The compressive residual stress is a dominant factor in improving the FF performance.Simultaneously,surface work hardening and microstructure refinement play a secondary role in enhancing the FF performance.Those strengthening mechanism is different with the PF.(4)The influence laws and effect mechanisms of USRP on the surface integrity and PF behavior of TC4 titanium alloy were investigated.The results indicated that the PF performance of TC4 titanium alloy was markedly improved by USRP with different processing numbers(1,3,6,and 12).The optimum processing number for obtaining a high PF performance is found to be one processing number.The enhancement mechanism of PF performance can be attributed to the comprehensive effects of compressive residual stress,microstructure,work hardening and reduction of surface roughness.Among these,the compressive residual stress is the primary factor.The influence of USRP processing number on the PF behavior of the TC4 titanium alloy is different with that of the TC17 titanium alloy,which can be attributed to the different surface integrity of two titanium alloys.(5)The influence laws and effect mechanisms of USRP on the FF behavior of TC4 titanium alloy were studied.The results suggested that the FF performance of TC17 titanium alloy was significantly enhanced by USRP with different processing numbers(1,3,6,and 12).Among them,the optimum processing number for obtaining a high PF performance is found to be three processing numbers.This can be attributed to the synergistic effects of compressive residual stress,microstructure refinement and work hardening.The compressive residual stress field with large numerical value and deep distribution mainly contributes to enhance the FF performance,which can effectively suppress FF crack initiation and early propagation.The surface work hardening has a secondary impact on improving the FF resistance.The influence law of USRP processing number on the PF behavior of the TC4 alloy is different with that of FF behavior,which can be attributed to the different fatigue mechanisms of two titanium alloys.(6)A GNS surface layer of 335 μm was synthesized on the surface of the TC4 titanium alloy by means of the USRP with tweenty-four processing numbers.The mean grain sizes of the USRP sample on the surface layer are 45.8～87.5 nm.The PF enhancement mechanism of the TC4 titanium alloy with a GNS surface layer induced by the USRP was studied systematically.The results suggested that the PF performance was obviously enhanced both the low cycle fatigue regime and high cycle fatigue resistance for the specimens with a GNS surface layer.The PF limit shows an increase of 22% in comparsion with untreated specimens,which can be ascribed to the comprehensive effects of the compressive residual stress with large numerical value and deep distribution,GNS surface layer and surface work hardening.Thereinto,the compressive residual stress is a key factor in improving the PF performance.The surface damage caused by USRP gradient nano-crystallization has an adverse effect on fatigue resistance.(7)The influence laws and mechanisms of TC4 titanium alloy with a GNS surface layer on the FF behavior were investigated.The results indicated that the FF performance was markedly improved for the specimens with a GNS surface layer.The PF limit shows an increase of 113.6%,which can be ascribed to the comprehensive effects of the compressive residual stress,GNS surface layer and surface work hardening.The compressive residual stress is a dominant factor in determining the FF performance.Meanwhile,the GNS microstructure can alleviate the fretting wear.(8)The influence laws and mechanisms of plasma surface Mo or Zr alloying and its duplex treatment with SP and DSP on the FF behavior of TC4 titanium alloy were studied.The results indicated that the surface hardness of TC4 titanium alloy with plasma surface Mo or Zr alloying layer was improved,but the FF resistance was reduced because of the low inherent toughness of the alloyed layer and the degradation of the mechanical properties of the substrate.The surface toughness of Mo and Zr alloyed layer is improved by SP with ceramic shots,which significantly improve the FF resistance of TC4 titanium alloy,and the improvement degree is greater than that of TC4 titanium alloy specimens treated by SP.The reason for the improvement of FF performance can be attributed to as following.The compressive residual stress introduced by SP hinders crack initiation and early propagation.The increase of surface hardness relieves fretting wear and hinders crack initiation.The increase of surface roughness can alleviate the fretting damage under the FF condition.For Mo and Zr alloyed specimens,the FF improvement level of the duplex treatment with DSP is less than that of the duplex treatment with SP of ceramic shots.The main reason is that the glass bead shots were easily broken during the DSP treatment,which caused surface damage of related specimens and relaxation of surface compressive residual stress. |
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