2017 Vol.37(6)

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2017, 37(6): 0-0.
[Abstract](105) [PDF 888KB](3)
Present Status and Prospect of Ultra High Strength Steel Applied to Aircraft Landing Gear
Bo ZHAO, Guangxing XU, Fei HE, Xu YANG
2017, 37(6): 1-6. doi: 10.11868/j.issn.1005-5053.2017.001005
[Abstract](440) [FullText HTML] (108) [PDF 1123KB](28)
The paper presents the domestic and overseas current status of the steel applied to aircraft landing gear in combination of the design concept and requirements for aircraft landing gear. The application features and concept of the steel used for landing gear are summarized and the domestic and overseas status are compared. For the moment, the low-alloy ultra-high strength steel and high-alloy ultra-high strength steel are all being used in the material system for aircraft landing gear steel, and the complete technical system for its anti-fatigue manufacturing is built. At present, China's development and application of high strength steel applied to aircraft landing gear is at the world advanced level. At last, the prospect for future development is analyzed.
High Purity Smelting Technology for Ultra-high Strength Steels
Zhouhua JIANG, Wei GONG, Cheng WANG, Dongping ZHAN, Rui WANG
2017, 37(6): 7-15. doi: 10.11868/j.issn.1005-5053.2017.000147
[Abstract](316) [FullText HTML] (63) [PDF 1815KB](17)
Ultra-high strength steel with high tensile strength, good toughness, high specific strength, modulus and other characteristics are widely used in aviation, aerospace and national defense and other fields. Ultra-high strength steel is preferred material for aircraft and aero-engines and other aviation equipments. The application of ultra-high strength steel represents a country's highest level of steel research and production, and it is also an important symbol of the development of national science and technology and national defense industry. The development and application of high purity smelting technology for manufacture of ultra-high strength steels at domestic and overseas is briefly reviewed in the paper, and then the control ability about the impurity elements such as S, P, O and N in typical ultra-high strength steels, and the research status and development trend of non-metallic inclusions control are discussed. The progress in research work of high purity smelting technology for ultra-high strength steels carried out by the authors in recent years has been introduced, it shows that the control level of impurity elements and non-metallic inclusion has been greatly improved, and it also creates a new route for China to manufacture the ultra-high strength steel with high alloy, especially with high purity for ultra-high strength stainless steel, bearing steel and gear steel. Finally, the development direction of high purity smelting technology of ultra-high strength steel in China is pointed out.
Topics on Applied Basic Theory Research of 23Co14Ni12Cr3MoE(A-100)Steel
Zhi LI, Lixin GU, Huiqu LI, Shaojun MA, Wei SHENG
2017, 37(6): 16-24. doi: 10.11868/j.issn.1005-5053.2017.001006
[Abstract](485) [FullText HTML] (58) [PDF 3632KB](12)
The basic features of A-100 steel, such as relationship between cogging process and mechanical properties, relationship among the forging process, grain size and mechanical properties, secondary hardening, and fatigue properties were discussed. The high-temperature homogenization and high deformation at first step technique were developed on the foundation of multiple upsetting and stretching, high forging ratio technique used for 300M steel, and became the technique foundation of cogging process in A-100 steel. The fracture toughness of A-100 steel was tended to be influenced by hot working process. The grain size grew heavily, and mixed grain structure was appeared after heating at 1140℃ and above with deformation amount below 20%, the fracture toughness was also decreased. The secondary hardening performance of A-100 steel was changed after the deformation at low temperature. The tensile strength peak temperature was changed to 468℃, the tensile strength was decreased slowly when over aging. A-100 steel was cyclic hardened, and its fatigue crack growth properties were better than 300M steel. The high cycle fatigue property was heavily deteriorated when tested in 3.5%NaCl solution.
Surface Integrity Characteristics and Fatigue Failure Mechanism of Carburized M50NiL Steel
Yanzhi LOU, Chunzhi LI, Xudong LI, Xiaochang XIE, Zhenye ZHAO
2017, 37(6): 25-33. doi: 10.11868/j.issn.1005-5053.2017.000127
[Abstract](316) [FullText HTML] (76) [PDF 5255KB](8)
The surface integrity of carburized M50NiL steel was studied by optical microscopy, scanning electron microscopy (SEM), atomic force microscopy (AFM), microhardness tester and residual stress tester. The fatigue properties of the two specimens were measured by the rotational bending fatigue test, and the fatigue test results were simulated and analyzed. The results show that the rotation bending fatigue of carburized M50NiL steel is originated in the sub-surface in the ideal case without considering the surface processing defects. The surface stress concentration factor produced by general grinding causes the fatigue source to be moved from the surface to the sub-surface. Precise grinding improves the surface quality by optimizing the grinding process, effectively restrains the stress concentration of the working surface, and returns the fatigue source from the surface to the sub-surface. The maximum rotary bending fatigue life can be increased by 30 times and the average is 15 times.
Failure Mechanism of Contact Fatigue of Surface Super-hardened M50NiL Steel
Qinghong LUO, Zhenye ZHAO, Ziqiang HE, Zhiming LI
2017, 37(6): 34-40. doi: 10.11868/j.issn.1005-5053.2017.000108
[Abstract](294) [FullText HTML] (79) [PDF 4916KB](9)
Hardness gradient, residual stress gradient and the organization structure of surface super-hardened M50NiL steel roller and M50 steel roller were studied, the rolling contact fatigue life was measured, and the failure process was analyzed and compared by using surface profiler, Vickers hardness tester, X ray residual stress determinator, optical microscope, scanning electron microscope (SEM), rolling contact fatigue test machine and other equipment. The fatigue failure mechanism of surface super-hardened M50NiL steel was revealed. The results show that the fatigue failure mechanism of surface super-hardened M50Nil steel returns to the Hertz theory and shows typical contact fatigue characteristics. The high surface hardness, residual compressive stress and a good organization structure can completely inhibit the initiation of surface cracks; therefore, the failure mechanism is returned to the Hertz theory.
Numerical Simulation Study on Fatigue Life of Notched Specimens with Gradient Surface Strengthening Layer
Jijia XIE
2017, 37(6): 41-49. doi: 10.11868/j.issn.1005-5053.2017.000094
[Abstract](313) [FullText HTML] (70) [PDF 6252KB](4)
A modified Tanaka-Mura model is carried out to derive the equivalent stress amplitude at stress ratio R=-1 of complex fatigue stress and the effect of compress residual stress on fatigue life. Then, the fatigue of notched specimens with gradient surface strengthening layer were investigated by means of numerical simulation. The results indicate that the fatigue initiation life and the initiation site of notched specimen are related with the thickness of the strengthening layer, the surface-to-internal hardness ratio and the residual stress. There is a critical thickness. If the strengthening layer thickness is less than the critical value, fatigue crack is initiated at the interface of the matrix and the strengthening layer, otherwise at the surface of the notch root. The critical thickness value is increased with the increase of surface-to-internal hardness ratio. Residual compress stress has little effect on the fatigue initiation life, but the residual tensile stress decreases the fatigue initiation life obviously.
Formation Mechanism of Surface Metamorphic Layer on Turning End Face of GH4169 Superalloy
Changfeng YAO, Xuehong SHEN, Dinghua ZHANG
2017, 37(6): 50-58. doi: 10.11868/j.issn.1005-5053.2017.000112
[Abstract](306) [FullText HTML] (58) [PDF 6601KB](9)
Turning experiments and Deform-3D finite element methods were used to study the formation mechanism of surface metamorphic layer on turning GH4169 processed. The investigations were carried out by analyzing the changing of cutting force, the temperature and strain field, the residual stress, micro-hardness, microstructure, as well as the distribution of the above all along the direction of the depth under various process parameters. The results show that the surface metamorphic layer is formed due to the thermal-mechanical coupled effects on the microstructure of the material in the machining process. The cutting force, heat and strain of surface material are increased with the increase of machining intensity. Besides the higher strength of machining, the greater changes of plastic deformation, metallographic and grain deformation are acquired. In the range of processing parameters, the temperature layer is 130-200 μm, the strain layer is 100-220 μm, the residual stress layer is 80-110 μm, the depth of hardening layer is 50-80 μm, and the depth of surface metamorphic layer is 2.5-5μm.
Effect of Surface Integrity of Turned GH4169 Superalloy on Fatigue Performance
Daoxia WU, Dinghua ZHANG, Changfeng YAO
2017, 37(6): 59-67. doi: 10.11868/j.issn.1005-5053.2017.000100
[Abstract](282) [FullText HTML] (70) [PDF 5931KB](7)
Through turning and rotary bending fatigue test, the effect of turning feed on GH4169 superalloy surface integrity, and the effect of surface integrity on fatigue life were studied. The results show that the surface roughness Ra decreases from 1.497 μm to 0.431 μm when the turning feed decreases from 0.2 mm/r to 0.02 mm/r. The surface residual stresses are changed from tensile stress to compressive stress. The depth of plastic deformation layer decreases from 8 μm to 2 μm. The surface stress concentration factor has the most significant effect on the fatigue life of GH4169. With the increase of stress concentration factor, the fatigue life decreases significantly. When f is 0.13 mm/r, the surface stress concentration factor Kst is 1.166; the surface micro-hardness is 405.27HV0.025; the surface residual stress is 82.08MPa; and the average fatigue life is 6.98×104 cycles. The multiple cracks are initiated at the machined surface defects of GH4169 superalloy specimen.
A New Analysis Method Using Influencing Factors of Anti-fatigue Manufacture Based on Fatigue Life Double-parameter Model
Gang RAN, Xiaowei YI, Shichao ZHANG, Hong WANG
2017, 37(6): 68-74. doi: 10.11868/j.issn.1005-5053.2017.000110
[Abstract](254) [FullText HTML] (55) [PDF 1589KB](7)
A new concept of intrinsic S-N curve was proposed. Then influencing factors caused by manufacture were introduced based on fatigue life double-parameter model and the effects of influencing factors caused by manufacture on S-N curve were discussed. GH4149 was employed to obtain S-N curves with different stress concentrations. The specimens had three surface states resulting from different manufacturing processes which are traditional machining process, machining process with surface integrity and advanced surface strengthening technology. The sensitivity of fatigue resistance coefficient(Mf) and theoretical fatigue limit (Sc) to Kt was analyzed. The results show that the fatigue properties of the smooth specimens and notched specimens both are improved after surface strengthening. When the stress concentration is relatively low, the increase of fatigue resistance coefficient(Mf) plays more important role for the improvement of fatigue life after surface strengthening. Nevertheless, the improvement of theoretical fatigue limit (Sc) becomes more and more important with the increase of Kt. Moreover, it is helpful to analyze the changes of S-N curve using influencing factors caused by manufacture for deepening the understanding of anti-fatigue manufacture.
Effect of High-Speed Milling Parameters on Surface Metamorphic Layer of TC17 Titanium Alloy
Liang TAN, Dinghua ZHANG, Changfeng YAO
2017, 37(6): 75-81. doi: 10.11868/j.issn.1005-5053.2017.000101
[Abstract](281) [FullText HTML] (71) [PDF 2562KB](6)
In order to provide the relatively accurate experimental basis for optimizing parameters and controlling surface metamorphic layer, ball end high-speed milling experiments of TC17 titanium alloy were carried out utilizing one of experimental design techniques based on the response surface methodology. The surface roughness prediction model was built, variance analyses were applied to check the significances of surface roughness model and input parameters, the effect of parameters on surface roughness was analyzed. Meanwhile, the residual stress, microhardness and microstructure under the condition of high, medium and low level of parameters were investigated. Results indicate that the model can predict the surface roughness effectively and feed per tooth and radial depth of cut have an obvious effect on surface roughness. Compressive residual stresses are detected on all milled surfaces and surface residual stresses are increased with the increase of the level of the milling parameters. The compressive residual stress layer is approximately 20 μm regardless of milling parameters level used. The process of thermal softening, then work hardening, and finally tending to stabilize are observed in the microhardness profiles. Grains of the surface layer are broken and bent, the thickness of plastic deformation layer is approximately 10 μm.
Effect of Cold Expansion on Fatigue Life of Hole Structure of TC17 Titanium Alloy
Yingjun AI, Xin WANG, Yinggang SONG, Qiang WANG, Xuekun LUO, Zhihui TANG, Zhenye ZHAO
2017, 37(6): 82-87. doi: 10.11868/j.issn.1005-5053.2017.000102
[Abstract](265) [FullText HTML] (61) [PDF 3200KB](13)
In order to improve the fatigue performance of TC17 central hole specimen, the hole wall surface integrity was characterized by scanning electron microscopy (SEM), roughness meter, X-ray diffraction, and the influence of hole cold expansion(HCE) parameter on the fatigue performance of the hole specimens was investigated. The results show that the minimum fatigue life (14718 cycle) of the HCE specimens with an expansion value of 0.18 mm is higher than the original specimen (13965 cycle). Compared to the specimens with the expansion values of 0.28 mm and 0.38 mm, the fatigue life dispersion is less, no obvious stress concentration phenomenon is found, and it has the best fatigue performance than the other two. The surface roughness value of hole wall is the lowest after the HCE with 0.18 mm, the inner hole wall forms a certain depth of strengthened layer, the residual compressive stress produced in the hole side effectively suppresses the produce of fatigue cracks in the inner hole wall, so the fatigue performance of hole structure is improved.
Effect of Double Cold Expansion of Hole on Fatigue Property of TB6 Titanium Alloy
Xuekun LUO, Yingjun AI, Xin WANG, Qiang WANG, Yinggang SONG, Zhihui TANG, Zhenye ZHAO
2017, 37(6): 88-94. doi: 10.11868/j.issn.1005-5053.2017.000123
[Abstract](296) [FullText HTML] (78) [PDF 2252KB](8)
To improve the fatigue resistance of the bolt connecting hole, the effect of double cold expansion (DCE) of the hole on the fatigue life of TB6 titanium alloy was investigated. The fatigue fracture, surface roughness, residual stress, hardness and microstructure of the hole wall were characterized by scanning electron microscopy (SEM), roughmeter, X-ray diffraction (XRD), microhardness tester and optical microscopy. The mechanism of DCE on the fatigue life of the hole was also investigated. The results show that the mean value of the fatigue life of DCE specimen is much higher than that of the interference fit specimen. The surface integrity of the hole wall is improved after DCE. The roughness decreases remarkably. The deep surface-strengthen-layer with high hardness and deep residual compressive stress field are formed around the hole through severe plastic deformation of the microstructure of the hole wall. It is considered that the improvement of surface integrity plays an important role on the enhancement of fretting fatigue life.
Microstructure and Texture in Surface Deformation Layer of Al-Zn-Mg-Cu Alloy Processed by Milling
Yanxia CHEN, Yanqing YANG
2017, 37(6): 95-101. doi: 10.11868/j.issn.1005-5053.2017.000109
[Abstract](401) [FullText HTML] (66) [PDF 3144KB](7)
The microstructural and crystallographic features of the surface deformation layer in Al-Zn-Mg-Cu alloy induced by milling were investigated by means of transmission electron microscopy (TEM) and precession electron diffraction (PED) assisted nanoscale orientation mapping. The result shows that the surface deformation layer is composed by the top surface of equiaxed nanograins/ultrafine grains and the subsurface of lamellar nanograins/ultrafine grains surrounded by coarse grain boundary precipitates (GBPs). The recrystallized nanograins/ultrafine grains in the deformation layer show direct evidence that dynamic recrystallization plays an important role in grain refining process. The GBPs and grain interior precipitates (GIPs) show a great difference in size and density with the matrix due to the thermally and mechanically induced precipitate redistribution. The crystallographic texture of the surface deformation layer is proved to be a mixture of approximate copper{112}<111>, rotated cube{001}<110> and F {111}<112>. The severe shear deformation of the surface induced by milling is responsible for the texture evolution.
Effect of Shot Peening on Fatigue Limit Stress Concentration Sensitivity of 3 Kinds of Typical Materials for Aeroengine
Xin WANG, Xudong LI, Yinggang SONG, Qiang WANG, Xuekun LUO, Yingjun AI, Zhihui TANG, Zhenye ZHAO
2017, 37(6): 102-107. doi: 10.11868/j.issn.1005-5053.2017.000097
[Abstract](270) [FullText HTML] (70) [PDF 1832KB](10)
Fatigue curves of C250 steel, TA29 titanium alloy and FGH96 powder metallurgy (PM) superalloy with stress concentration coefficients Kt=1 and Kt=1.7were investigated, and the effect of shot peening on the fatigue curve under the stress concentration condition was also studied. The results show that the 107 conditional fatigue limits of C250 steel, TA29 titanium alloy and FGH96 PM superalloy decrease from 757 MPa, 366 MPa and 566 MPa to 526 MPa, 240 MPa and 465 MPa respectively while the stress concentration coefficients increase from Kt=1 to Kt=1.7, indicating that the three kinds of high-strength materials have the stress concentration sensitivity of fatigue limit obviously. Moreover, after shot peening, fatigue limits rise to 597 MPa, 297 MPa and 530 MPa respectively when the Kt is 1.7, which indicates that shot peening can mitigate fatigue limit stress concentration sensitivity of high-strength alloys from a technological point of view. On the other hand, the 105-cycle and 107-cycle strength difference of titanium alloy and PM superalloy is reduced with the increase of stress concentration coefficient, showing that shot peening can reduce the dispersion of fatigue test data.

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