Low Cycle Fatigue and Strengthening Mechanism of Cold Extruded Large Diameter Internal Thread of Q460 Steel

large diameter internal thread of high-strength steel（LDITHSS） manufactured by traditional methods always has the problems of low accuracy and short life. Compared with traditional methods, the cold extrusion process is an effective means to realize higher accuracy and longer life. The low-cycle fatigue properties of LDITHSS are obtained by experiments, and the initiation and propagation of fatigue cracks are observed by scanning electron microscope（SEM）. Based on the mechanical properties, surface microstructure and residual stress, the strengthening mechanism of cold extruded large diameter internal thread（LDIT） is discussed. The results show that new grains or sub-grains can be formed on the surface of LDIT due to grain segmentation and grain refinement during cold extrusion. The fibrous structures appear as elongated and streamlined along the normal direction of the tooth surface which leads to residual compressive stress on the extruded surface. The maximum tension stress of LDIT after cold extrusion is found to be 192.55 k N. Under low stress cycling, the yield stress on thread increases, the propagation rate of crack reduces, the fatigue life is thus improved significantly with decreasing surface grain diameter and the average fatigue life increases to 45.539×10~3 cycle when the maximum applied load decreases to 120 k N. The low cycle fatigue and strengthening mechanism of cold extruded LDIT revealed by this research has significant importance to promote application of internal thread by cold extrusion processing.

Overall Evaluation of the Effect of Residual Stress Induced by Shot Peening in the Improvement of Fatigue Fracture Resistance for Metallic Materials

Before 1980s,the circular suspension spring in automobile subjected to torsion fatigue load,under the cyclic normal tensile stresses,the majority of fatigue fracture occurred was in normal tensile fracture mode（NTFM）and the fracture surface was under 45°diagonal.Because there exists the interaction between the residual stresses induced by shot peening and the applied cyclic normal tensile stresses in NTFM,which represents as＂stress strengthening mechanism＂,shot peening technology could be used for improving the fatigue fracture resistance（FFR）of springs.However,since 1990s up to date,in addition to regular NTFM,the fatigue fractures occurred of peened springs from time to time are in longitudinal shear fracture mode（LSFM）or transverse shear fracture mode（TSFM）with the increase of applied cyclic shear stresses,which leads to a remarkable decrease of FFR.However,LSFM/TSFM can be avoided effectively by means of shot peening treatment again on the peened springs.The phenomena have been rarely happened before.At present there are few literatures concerning this problem.Based upon the results of force analysis of a spring,there is no interaction between the residual stresses by shot peening and the applied cyclic shear stresses in shear fracture.This;means that the effect of＂stress strengthening mechanism＂for improving the FFR of LSFM/TSFM is disappeared basically.During shot peening,however,both of residual stress and cyclic plastic deformed microstructure are induced synchronously like＂twins＂in the surface layer of a spring.It has been found for the first time by means of force analysis and experimental results that the modified microstructure in the＂twins＂as a＂structure strengthening mechanism＂can improve the FFR of LSFM/TSFM.At the same time,it is;also shown that the optimum technology of shot peening strengthening must have both＂stress strengthening mechanism＂and＂structure strengthening mechanism＂simultaneously so that the FFR of both NTFM and LSFM/TSFM can be improved by shot peening.

NICKEL-BASE ALLOY SHEET ALLOYS USED IN AEROSPACE APPLICATIONS

Many gas turbine components are made from nickel alloy sheet. Most are used for directing or containing gases at high temperatures and pressures where metal temperatures can be as high as 1090℃ (2000°F). These applications included combustor systems, casings and liners, transition and exhaust ducting, afterburners, and thrust reversere. Light weight components and sub-assemblies call for alloy sheet with high levels of stength and oxidation resistance. Complex component design calls for excellent ductility and ease of fabrication.The wide range of nickel alloy sheet alloys presently used in aircraft and land-based gas turbines is briefly described and typical properties presented. New sheet alloy developments, involving INCONEL￣* alloys 625LCF, 718SPF and MA754, are presented including the process routes involved and material properties.

What is going on in magnesium alloys？

China has been developed into one of the most active regions in terms of both fundamental and applied research on magnesium （Mg） and its alloys in the world from a solid base laid by its prominent metallurgist and materials scientists over the past decades. Nowadays, a large number of young-generation researchers have been inspired by their predecessors and become the key participants in the fields of Mg alloys, which consequently led to the establishment of China Youth Scholar Society for Magnesium Alloys Research in 2015. Since then, the first two China Youth Scholars Symposiums on Mg Alloys Research had been held at Harbin （2015） and Chongqing （2016） China, respectively. A number of crucial research inter- ests related to fundamental and applied Mg research were discussed at the conferences and summarized in this short perspective, aiming to boost far-reaching initiatives for development of new Mg-based materials to satisfy the requirements for a broad range of industrial employments. Herein, four main aspects are included as follows： i） Plastic deformation mechanism and strengthening strategy, ii） Design and development of new Mg-based materials, iii） Key service properties, and iv） New processing technologies.