| Inconel718 superalloy is often used as the key material of high strength butt bolt which are used to connecting the airframe for its excellent mechanical properties in 600 ?high temperature and the high strength grade at room temperature which is up to 1500 Mpa.Because the butt bolts bear complex fatigue loads during the flight,so the materials used need to meet not only the requirements of high strength,but also the requirements of high fatigue life to ensure the safe operation of the aircraft.At present,there are two methods to improve the fatigue limit of materials,one is through heat treatment to improve the overall material performance,and the other is by local surface strengthening to improve the materials surface properties.Among them,the research results on the mechanical properties of Inconel718 superalloy show that although the optimized heat treatment process has significant effect on improving the tensile strength while not obvious on improving the fatigue life.At the same time,the research results on the surface deformation strengthening of the material are only limited to the characterization and analysis of the surface characteristic parameters changes after surface deformation strengthening and the results of improving fatigue performance.It is lack of systematic research on the distribution characteristics of residual compressive stress introduced by surface deformation strengthening treatment,and the influence mechanism of residual compressive stress field on fatigue performance has also not been thoroughly analyzed.In particular,after the introduction of higher residual compressive stress and deeper residual compressive stress layer on the surface of the material by the advanced high-energy surface strengthening technology,the influence on the fatigue performance of the material remains to be further studied.It can be seen that the study on the characteristic distributionlaw of residual compressive stress field of Inconel718 superalloy strengthened by surface deformation and its influence law on high cycle fatigue performance has important guiding significance and practical value for improving the fatigue life of aircraft butt high-strength bolts,shortening its research and development cycle and reducing its research and development cost.Under this background,starting from the factors affecting the fatigue performance of the material,such as the strength,the distribution of residual compressive stress on the surface,the surface morphology and its roughness,surface microstructure and micro-hardness distribution,etc.and combining with OM,SEM,TEM,HRTEM and EBSD detection and analysis methods,the quantitative relationship of Inconel718 superalloy between the high-cycle fatigue limit at room temperature and the tensile strength of it under different tensile strengths was explore,the high cycle fatigue limits at room temperature with different residual compressive stress field distributions was compared and finally the effect and its mechanism of residual stress field on its high cycle fatigue limit at room temperature was analyzed.The high cycle fatigue life of Inconel718 superalloy butt bolt head and its fatigue fracture characteristics treated by R ultrasonic rolling and conventional rolling were compared,providing a new technical path for further improving the fatigue performance of the butt bolt.The main research contents and conclusions are as follows:In this context,the relationship between high cycle fatigue limit and tensile strength of Inconel 718 superalloy for aerospace engine butt bolts is discussed based on the characteristics of high cycle fatigue behavior of Inconel 718 superalloy.Different surface strengthening methods and different surface strengthening parameters were used to strengthen the surface of workpieces with different heat treatment states,shapes and sizes.The distribution characteristics of Three-dimensional residual stress,surface morphology and roughness,surface hardness and microstructures of Inconel 718 Alloy Surface Strengthening were analyzed comprehensively.The high cycle fatigue limits of different surface states were compared.Finally,the high cycle fatigue life and fatigue fracturecharacteristics of Inconel 718 Alloy butt bolt under R ultrasonic rolling and traditional rolling are compared,which provides a new technical path for further improving the fatigue performance of butt bolt.The main research contents and conclusions are as follows:(1)The quantitative relationship between high cycle fatigue limit and tensile strength of Inconel 718 Alloy under different strength was studied for the first time.The results show that,at room temperature,the tensile strength of solid solution and aging Inconel 718 alloy is 940 MPa and 1560 MPa respectively,and the high cycle fatigue limit(107 cycles)is 492 MPa and 461 MPa respectively.The tensile strength of a solid solution is 65.9%lower than that of aging state,but the high cycle fatigue limit is 6.73% higher than that of aging state.The relationship between high cycle fatigue limit and tensile strength at room temperature satisfies the quadratic function.(2)Based on X-ray diffraction three-dimensional stress tester,the distribution characteristics of three-dimensional residual stress of Inconel 718 alloy after different surface deformation strengthening were studied for the first time.The results show that the residual stress state of shot peening surface is three-dimensional stress state,plane stress state and uniaxial stress state from surface to subsurface.The surface residual stress state of ultrasonic rolling is the transition from the surface plane stress state to the subsurface uniaxial stress state.The axial surface residual compressive stress of shot peening and ultrasonic rolling is almost the same,about 1000 MPa,but the depth of residual compressive stress layer of ultrasonic rolling is 1.5 times that of shot peening.(3)Based on the theory of contact mechanics,the ultrasonic rolling coverage of workpieces with different surface shapes was theoretically studied for the first time.Mathematical models of ultrasonic rolling coverage of cylindrical end surface,cylindrical surface and a planar workpiece were constructed.(4)The deformation surface characteristics and tensile properties of ultrasonic rolling with different surface deformation strengthening methods and processing parameters were studied.The results show that the surface deformation characteristics of shot peening andultrasonic rolling are basically the same,and the surface roughness of shot peening is 4.44 times of that of ultrasonic rolling.The plastic deformation degree and depth of the surface of ultrasonic rolling increase with the increase of impact times and rolling times per unit area,and the effect of rolling times is greater than that of impact times per unit area.The surface nano-crystallization of solid solution Inconel 718 alloy appears on the condition of larger processing parameters,but the depth of nanostructure layer is shallow,which has little effect on the tensile properties.(5)The room temperature high cycle fatigue behavior of shot peening and ultrasonic rolling treated specimens,specimens of different sizes and different heat treatment status after ultrasonic rolling treatment were studied comparatively.The results showed that the fatigue limit of aging Inconel 718 alloy samples with small size(diameter is 3.6 mm)increased by 3.2% after shot peening and decreased by 24.8% after ultrasonic rolling,while that of solid solution Inconel 718 alloy samples with small size decreased by 25.8%after ultrasonic rolling.However,the fatigue limit of aging Inconel 718 alloy specimens with large size(diameter is 8mm)increased by 10.8% after ultrasonic rolling.Fracture analysis results show that the fatigue cracks of both solid solution and aging alloy specimens with small size are initiated at the center of the specimen and the fatigue strength decreases,but the fatigue cracks of shot peening specimens with small size and ultrasonic rolling specimens with large size are initiated at the sub-surface of the specimen,and the fatigue strength increases.Thus,for the first time,the theory that the effect of the residual stress field of ultrasonic rolling on the fatigue performance under axial tension and compression has a size effect was outputted.Based on the statistics of fatigue crack initiation positions under different load stresses and the distribution characteristics of the residual compressive stress field,the reasons for the reduction of the fatigue limit in ultrasonic rolling of small size specimens were explained.According to the internal fatigue limit theory of triaxial stress degree,the internal fatigue limit distribution model of axial tension-compression fatigue specimens was constructed,and the initiation diameter ratio of size effect was put forward,and the mechanism of the influence of residual compressivestress on the fatigue limit was obtained.These results could provide a key theoretical basis for the fatigue strength design of components with different sizes under the condition of high energy modification and strengthening.(6)The high cycle fatigue life of butt bolt head after R-ultrasonic rolling and traditional rolling under different ultimate load ratios was compared and studied.The results show that the fatigue life of bolts with 40% load ratio is 20 times of that of traditional rolling,and that of bolts with 50% load ratio is 7 times of that of traditional rolling.Therefore,ultrasound rolling has remarkable strengthening effect on R under concrete butt bolt head. |
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