20 August 2020, Volume 44 Issue 8
    

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  • Research Progress on Microstructure and Properties ofSelective Laser Melted TC4 Titanium Alloy
    ZHU Dong, ZHANG Liang, WU Wenheng, LU Lin, NI Xiaoqing, SONG Jia, ZHU Wenhua, WANG Jiaqi
    Materials For Mechanical Engineering. 2020, 44(8): 1-4,9. https://doi.org/10.11973/jxgccl202008001
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    Selective laser melting technique is a widely used laser additive manufacturing technology in preparation of precise and complex parts, which could realize the near net forming of complex parts. Through the forming process parameters, and solution and aging treatment methods, the research progress of microstructure and mechanical properties control of TC4 alloy formed by selective laser melting in the current stage is reviewed, and the future development direction is prospected.
  • Effect of Hot Sintering Process on Electrical and Mechanical ComprehensiveProperties of Printed Flexible Silver Wire
    XU Zhang, SUN Quan, SONG Chao, WANG Yinggang, ZHANG Yijie, CHEN Zhen, LI Chao
    Materials For Mechanical Engineering. 2020, 44(8): 5-9. https://doi.org/10.11973/jxgccl202008002
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    The direct coriting printed flexible electronic silver wire was hot sintered. The effects of sintering temperatures (120,140,160,180, 200 ℃) and times (2,4,6,8,10 h) on conductivity, ductility and resistance to bending fatigue were studied with the self-made flexible electronic mechanics reliability test platform. The results show that with the increase of sintering temperature and time, the conductivity of the silver wire was improved first and gradually stabilized, the ductility was also improved, but the bending fatigue life was significantly reduced. The printed silver wire could obtain good conductivity when the sintering temperature was above 160 ℃ and the sintering time was more than 8 min.
  • Effect of Grain Size on Quasi-static Mechanical Properties of Low StackingFault Energy Fe-Mn-Si-Al Austenitic Alloy Steel
    LIU Wenjin, YANG Weitao, YANG Qi, ZHAN Ke
    Materials For Mechanical Engineering. 2020, 44(8): 10-16. https://doi.org/10.11973/jxgccl202008003
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    The cold rolled Fe-29.8Mn-5.0Si-1.7Al austenitic steel with low stacking fault energy was annealed at different temperatures (700,730,800,900,1 000,1 100,1 200 ℃). The influence of original austenite grain size on quasic-static mechanical properties and phase transformation behavior during deformation of the steel was studied. With the annealing temperature (above 730 ℃) increasing, obvious static recrystallization occurred on the alloy steel, the grain size increased, and the microstructure was single austenite. Recrystallized annealed alloy steel underwent ε-martensitic transformation during tensile deformation, the fine-grained structure (austenite grain size was less than 21 μm) was conducive to the alloy steel to obtain high yield strength and high tensile strength, and the coarse-grained structure (austenite grain size was greater than 90 μm) was beneficial to improve the plasticity, because of evenly distributed and intersecting multiple variants ε-martensite formed in coarse austenite grains.
  • Microstructure and Hardness Changes of High Temperature Furnace Tubes during Service and Damage Evaluation
    QIN Jiayi, GUAN Kaishu
    Materials For Mechanical Engineering. 2020, 44(8): 17-22. https://doi.org/10.11973/jxgccl202008004
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    Microstructure and hardness changes at different positions on axial section and at certain depths from outer surface by grinding HP40Nb furnace tube after service for different times were compared, and then the feasibility of cladding metallographic technique and on-site hardness test in evaluating the damage of furnace tube in service was analyzed. The results show that after service of the furnace tube at high temperature for a long time, the microstructure of different positions on axial section, especially carbide morphology, was significantly different, and even holes appeared because of overheating, which led to the early failure. The hardness at different positions on the axial section varied greatly, and the closer to the wall thickness center, the higher the hardness, which was consistent with the changes of microstructure. The surface of furnace tube was ground, and the hardness at certain depth was measured; when the measured hardness continued to be stable, the microstructure at this depth could characterize most of the microstructure of the furnace tube. This verified the feasibility of cladding metallography and on-site hardness test for evaluating the microstructure demage of HP40Nb furnace tubes in service.
  • Effect of Diffusion Bonding Process on Connection Interface Bonding Propertyof SP-700 Titanium Alloy Hot Rolled Sheet
    LIU Jixiong, WANG Wenjun, HUANG Tuo, WANG Xin, WANG Xiaoxiang, WANG Dingchun, GAO Qi
    Materials For Mechanical Engineering. 2020, 44(8): 23-26. https://doi.org/10.11973/jxgccl202008005
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    Diffusion bonding test was performed on SP-700 titanium alloy hot rolled sheet, and the effects of diffusion bonding temperature (720, 750, 780, 810, 840 ℃) and time (5, 15, 30, 45 min) on the connection interface morphology and bonding strength were studied. The results show that with the increase of diffusion bonding temperature and time, the unbonded area of the alloy decreased until disappeared, and the interface bonding strength increased. The higher the temperature, the shorter the time interval required for fully integrating of the interface. When the bonding temperature was higher than 750 ℃ and time was 30 min, the interface was basically fully integrated. After diffusion bonding at 840 ℃ for 30 min, many equiaxed α phases existed on the bonding interface, and the interface bonding strength was the highest, reaching 400 MPa.
  • Phase Composition of Nickel Free Titanium Based Alloys withDifferent Composition and Heat Treatment
    ZHOU Zhengcun, ZHU Xiaobin, DU Jie, YAN Yongjian, GU Suyi, YANG Yifei
    Materials For Mechanical Engineering. 2020, 44(8): 27-31. https://doi.org/10.11973/jxgccl202008006
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    Ti-Mo and Ti-Nb alloys with different composition were melted by using vacuum arc furnace. The effects of aluminum, iron, tin, niobium and molybdenum elements on the phase composition of quenched and annealed alloys were studied. The results show that with increasing content of molybdenum, the α-phase content in annealed Ti-Mo alloy decreased, the β phase content increased, and α',α″ and βM phases were formed sequentially in quenched Ti-Mo alloy. Aluminum and tin elements could increase the content of α phase in quenched Ti-12Mo and Ti-33Nb alloys, which acted as α stabilizer. In annealed Ti-12Mo and Ti-33Nb alloys, tin could suppress the transformation of β to ω, showing the tendency of β stabilizer. Aluminum increased α phase content in annealed Ti-12Mo alloy, but showed the tendency of β stabilizer in annealed Ti-33Nb alloy. Iron was a strong β stabilizer, which could increase the β or βM phase in Ti-33Nb alloy and quenched Ti-12Mo alloy. However, due to the eutectic decomposition of β phase, α phase content in annealed Ti-12Mo-2Fe alloy increased. Niobium and molybdenum were also β stabilizers, which showed weaker effects than iron.
  • Superplastic Deformation Behavior and Microstructure Evolution of(TiB+La2O3)/IMI834 Titanium-Based Composite
    LAI Xiaojun, QIU Peikun, Lü Weijie, HAN Yuanfei
    Materials For Mechanical Engineering. 2020, 44(8): 32-37,43. https://doi.org/10.11973/jxgccl202008007
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    Superplastic deformation of (TiB+La2O3)/IMI834 titanium-based composite was carried out. The effects of different temperatures (850, 900, 950, 1 000 ℃) and initial strain rates (0.000 5, 0.001 0, 0.005 0 s-1) on its superplastic deformation behavior and microstructure were studied. The results show that the composite was composed of α-Ti, TiB, La2O3 and dispersed (TiZr)xSi particles. The composite exhibited perfect superplasticity, and showed the largest elongation of 505% at 900 ℃ and 0.001 0 s-1. The strain rate sensitivity coefficient of the composite was higher than 0.30. The flow stress and deformation activation energy increased with the strain rate. With the increase of the deformation degree, the α phase of lamellar layer was gradually equiaxed, the small-angle grain boundary changed to the high-angle grain boundary, while the hole defects increased. Grain boundary sliding, grain rotation and dynamic recrystallization were the main deformation mechanisms of the superplastic deformation of (TiB+La2O3)/IMI834 titanium-based composite.
  • Effect of Carbon Content on Microstructure and Thermal Stability of 4Cr5Mo2V Hot Work Die Steel
    SUN Liguo, ZHOU Jian, YIN Junwei, CHI Hongxiao, QI Yong
    Materials For Mechanical Engineering. 2020, 44(8): 38-43. https://doi.org/10.11973/jxgccl202008008
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    Quenching and tempering heat treatment of 4Cr5Mo2V steel with different carbon contents were carried out, and the effect of carbon content (0.35%, 0.47%, 0.71%, mass fraction) on its microstructure, carbide evolution and thermal stability was studied. The results show that There was no carbide in test steel with carbon mass fraction of 0.35% after quenching at 1 030 ℃, while granular vanadium-rich MC carbides existed in test steels with carbide mass fraction of 0.47% and 0.71%. M23C6 carbides were precipitated in test steels after quenching and tempering treatment, and the M23C6 carbides increased with the carbon content. After thermal stability test, a large amount of carbides were precipitated and coarsened, and with the thermal stability holding time decreasing and the carbon content increasing, the hardness and thermal stability of test steels increased. The increasing carbon content could improve the tempering transition temperature of test steels, the solid solution strengthening effect of carbon element and the precipitation strengthening effect of MC carbide were the main reasons for the enhancement of high temperature thermal stability of the steel.
  • Effect of Temperature and Stress Ratio on Crack Growth Behavior of High Strength and Toughness 7055 Aluminum Alloy Section Material
    CAI Ling
    Materials For Mechanical Engineering. 2020, 44(8): 44-46,51. https://doi.org/10.11973/jxgccl202008009
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    Crack propagation rate tests of 7055-T76511 aluminum alloy section material under different temperatures (150 ℃, room temperature, -70 ℃)and stress ratios (0.1, 0.5) were carried out. The effect of temperature and stress ratio on crack propagation rate and threshold of the alloy was studied. The results show that with decreasing temperature and stress ratio, the threshold for crack propagation of 7055-T76511 aluminum alloy gradually increased, and the crack propagation rate gradually decreased. At -70 ℃, the crack growth rate when the alloy fractured was smaller, and the crack growth threshold was larger, showing the best crack growth resistance.
  • Welding Cold Crack Sensitivity of EH36-Z35 Steel inLow Temperature Environment
    ZHUO Xiao, LI Lixin, LI Di, LIU Yiyang, KONG Haijun, ZHAO Dan
    Materials For Mechanical Engineering. 2020, 44(8): 47-51. https://doi.org/10.11973/jxgccl202008010
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    The welding cold crack sensitivity of 50 mm thick EH36-Z35 marine engineering steel in a low temperature environment was studied by empirical formula and inclined Y-groove welding crack tests. The results show that the test steel had a certain welding cold crack sensitivity, and the safety of the structure could be guaranteed only when the preheating temperature before welding was about 114 ℃, according to the calculation by the empirical formula. The preheating temperature before welding should be higher than 125 ℃ at environmental temperature of 15 ℃ and should be higher than 150 ℃ at environmental temperature of 5 ℃, according to the inclined Y-groove welding crack tests. The test results were close to the theoretical calculated values. At environmental temperatures of -18 ℃ to 15 ℃ and with similar absolute humidity and same preheating temperature, the crack rate at the weld root of the test steel increased, and the maximum hardness of the heat-affected zone increased with decreasing environmental temperature. The maximam calculated critical constraint stress was 577 MPa at environmental temperatures of -18 ℃ to 15 ℃, and the constraint stress introduced by inclined Y-groove welding crack tests was 588-1 695 MPa, which satisfied the condition of crack formation and led to crack generation and propagation.
  • Microstructure and Mechanical Properties of 550 MPa Grade High Strengthand High Ductility Automobile Square Tube Steel
    HUI Yajun, WU Kemin, XIAO Baoliang, LIU Kun, XU Yongxian, XU Kehao
    Materials For Mechanical Engineering. 2020, 44(8): 52-56. https://doi.org/10.11973/jxgccl202008011
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    A 550 MPa grade high strength and high ductility steel for automotive square tubes was developed. The microstructure and mechanical properties of hot rolled steel strip and plane, fillet and weld of square tube were studied. The results show that the microstructure of hot rolled steel strip was composed of uniform and fine ferrite and a small amount of pearlite, and a large number of (Nb,Ti)C phases with size of 1-110 nm were precipitated in the matrix. The microstructures of the square tube plane and fillet were basically the same as that of the hot rolled steel. The dislocation density in the steel increased after tube forming, dislocation plugging occurred at the grain boundary and the second phase, and dislocation walls and cells were formed at the fillet. Work hardening occurred in the steel after tube forming. The yield strength, tensile strength and yield ratio of the square tube plane, fillet and weld were increased in sequence, while the elongation decreased sequentially, and the toughness at the weld was relatively poor.
  • Microstructure and Mechanical Properties of a Precipitate-HardenedFe-C-Mn-Ni Austenitic Alloy Steel
    BU Linsen, WANG Min, HAO Qingguo, YANG Qi, LI Wei
    Materials For Mechanical Engineering. 2020, 44(8): 57-62. https://doi.org/10.11973/jxgccl202008012
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    Fe-C-Mn-Ni-X (where X stands for Cr, V, etc.) austenitic alloy steel forgings were solid solution and aging treated. The effects of aging temperature (650, 700, 750 ℃) and aging time (0-25 h) on the microstructure and mechanical properties of the alloy steel were studied. The results show that the microstructure of solid solution treated and aging treated steels were similar. After aging treatment, a large amount of nano-VC phases in co-lattice or semi-co-lattice orientation with the austenite matrix were precipitated in the alloy steel. The solid solution treated alloy steel showed a strong age hardening capability, the aging time to reach the peak hardness was shorted with increasing aging temperature, and the peak hardness decreased. After aging treatment, the yield strength and tensile strength of the alloy steel increased significantly, the elongation and work hardening index decreased, and the tensile failure mode changed from ductile fracture to ductile-brittle mixed fracture. With the aging temperature increasing and the aging time extending, the strength of the alloy steel was reduced, but the work hardening ability was enhanced.
  • Creep Deformation Simulation of Notched Specimens of ZSGH4169Alloy Based on θ-projection Method
    LI Di, WEI Dasheng
    Materials For Mechanical Engineering. 2020, 44(8): 63-68. https://doi.org/10.11973/jxgccl202008013
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    With creep test data of ZSGH4169 alloy, the θ-projection method with 16 parameters was used to analyse the creep curves, and the equation parameters were obtained. The creep behaviour and stress relaxation of different notched specimens were simulated and calculated with the net section stress as unified standard. The results show that the θ-projection method could describe the creep behaviour of ZSGH4169 alloy accurately through the obtained parameters. The higher the degree of stress concentration, the more obvious the creep deformation of the specimen notch root, and the higher the degree of stress relaxation. When the stress concentration factor was small, the creep deformation of notched specimens approached that of non-notched specimens. The stress concentration factor (or maximum stress) and stress gradient influence factor were introduced to modify the traditional creep model, and the general expression for calculating the creep deformation of notched specimens was given.
  • Optimization of Controlled Rolling and Cooling Process for 20CrMnTi Gear Steel Bar
    GAO Yubo, XU Leqian, MENG Xiaoling
    Materials For Mechanical Engineering. 2020, 44(8): 69-73. https://doi.org/10.11973/jxgccl202008014
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    The microstructure and hardness in different positions of 20CrMnTi gear steel bar produced by actual controlled rolling and cooling process were studied. The controlled rolling and cooling process of the bar was simulated by finite element method, and then the temperature and equivalent strain distribution of the bar was obtained. The optimal measures of controlled rolling and cooling process were put forward, and verified by tests. The results show that the microstructures of surface and 1/2 radius of bar consisted of ferrite and pearlite with actual controlled rolling and cooling process, while the microstructure of center consisted of ferrite, pearlite and a larger amount of bainite; the hardness of the center was significantly higher than that of other parts. The deformation of the bar during controlled rolling and cooling process mainly occurred at the surface and 1/2 radius, and the equivalent strain decreased with increasing distance from the surface of the bar. The temperature of the center of the bar was the highest, and the temperature of 1/2 radius and surface decreased in turn. The temperature of the center increased with the accumulation of rolling processing. The controlled rolling effect of the surface of the bar was the best, followed by that of 1/2 radius and center. When the final rolling temperature decreased from 880 ℃ to 840 ℃, the microstructures in different positions of the bar after controlled rolling and cooling process all consisted of ferrite and pearlite, and the hardnesses of surface, 1/2 radius and center were 182,188,190 HBW, respectively, indicating the uniformity of microstructure and hardness was improved obviously.
  • Finite Element Analysis on Creep Damage for Hot Outlet Manifold ofSteam Reformer Based on Damage Mechanics
    WANG Bin, NAN Ningning, GUO Xiaofeng, GONG Jianming, CHAO Ke
    Materials For Mechanical Engineering. 2020, 44(8): 74-77. https://doi.org/10.11973/jxgccl202008015
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    Creep damage analysis of hot outlet manifold of steam reformer under the ideal operating condition was carried out by finite element methods based on improved Liu-Murakami constitutive model. The results show that the improved Liu-Murakami model could predict the uniaxial creep curve of the manifold under different stress levels accurately. Under ideal conditions, the maximum Mises stress and the maximum principal stress in initial service stage appeared at the junction of the pigtail pipe and the manifold. During the service process, stress redistribution occurred in the manifold, the maximum Mises stress and the maximum principal stress were significantly reduced. After serving for 105 h, the maximum damage value of the manifold was only 0.071.
  • Simulation of Uniaxial Compression Mechanical Properties ofSiC Ceramics with Elliptical Microvoids
    JIANG Shengqiang, HE Mingxue, LI Xu, TANG Chao
    Materials For Mechanical Engineering. 2020, 44(8): 78-83. https://doi.org/10.11973/jxgccl202008016
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    Elliptical microvoid flaws with different sizes were defined and introduced into the calibrated discrete-element model of SiC ceramics. Effects of inclination angle, aspect ratio and area of the microvoid on the compression properties of SiC ceramics were studied by uniaxial compression simulation. The results show that the presence of microvoids reduced the compressive strength and elastic modulus of the sample and improved the Poisson's ratio. With a relatively small inclination angle, the larger the aspect ratio of the microvoid, the smaller the compressive strength and elastic modulus of the sample, and the higher the Poisson's ratio. With increasing inclination angle, the effect of the aspect ratio on the compressive strength, elastic modulus and Poisson's ratio gradually decreased. When the inclination angle increased above 60°, the compressive strength, elastic modulus and Poisson's ratio were close to those of the non-defective samples. With decreasing microvoid area, the effect of the aspect ratio and inclination angle on the compressive strength of the sample gradually decreased. When the aspect ratio of the microvoid was 1 or the inclination angle was 90°, increasing the void area had little effect on the compressive strength.
  • Fracture Reasons of Guide Wheel Shaft of Linear Motor Pumping Unit
    SONG Chengli, ZHAO Mifeng, XING Xing, LIU Xinbao, KUANG Xianren, LIU Yangqin, CONG Shen
    Materials For Mechanical Engineering. 2020, 44(8): 84-87. https://doi.org/10.11973/jxgccl202008017
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    The guide wheel shaft of a linear motor pumping unit in an oil field fractured. The failure reasons were analyzed by macroscopic inspection, non-destructive test, chemical composition analysis, metallographic inspection, scanning electron microscope, tensile test and impact test. The results show that there were brittle Widmanite and martensite structures in the fractured wheel shaft, and the crack was easy to initiate at the welding joint with the web effected by the welding thermal stress. Under the alternating load caused by the reciprocating movement of the pumping unit, the cracks continued to propagate, and eventually led to the fatigue fracture of the wheel shaft. It was recommended to strictly control the heating temperature and cooling speed during heat treatment of the wheel shaft, optimize the welding process, and strengthen the quality supervision and delivery inspection of the wheel shaft.
  • Optimization Design and Fracture Analysis of 60Si2Mn Steel Front Stabilizer Bar for Automobile
    ZHANG Defa, LAI Jiexiu
    Materials For Mechanical Engineering. 2020, 44(8): 88-92. https://doi.org/10.11973/jxgccl202008018
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    A 60Si2Mn steel front stabilizer bar for automobile suffered fatigue fracture during bench endurance test. The fracture reasons were analyzed macroscopic morphology observation, chemical composition analysis, hardness test and metallographic inspection. Two optimization schemes were proposed, and stress analysis, fatigue life test, bench endurance test, subjective driving evaluation and road reliability test were carried out on the optimized front stabilizer bar. The results show that the reverse change of the shape between the left and right limit rings caused line stiffness increasing of the front stabilizer bar, and fatigue fracture occurred under the action of alternating stress. Changing the material to 55Gr3 steel and reducing the diameter and bore diameter to 21, 20.5 mm could effectively reduce the linear stiffness and roll stiffness of front stabilizer bar. After optimization, the stress distribution, fatigue life, subjective driving evaluation, bench endurance and road reliability test of the front stabilizer bar all met the requirements.
  • Early Fracture Reason of Leaf Spring of Heavy Duty Truck
    JIANG Songwei, MA Mingtu, WANG Jianbin, FENG Yi, GONG Yu, JIN Xuejun, ZHANG Junping, ZHOU Jia
    Materials For Mechanical Engineering. 2020, 44(8): 93-96,102. https://doi.org/10.11973/jxgccl202008019
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    A 51CrMnVA leaf spring of heavy duty truck broke when placed in winter after being preloaded. The early fracture reasons were analyzed by fracture analysis, microstructure observation, mechanical performance testing, and comparison with the fractured leaf spring after normal service. The results show that the high strength and hardness and low toughness of the leat spring due to a relatively low tempering temperature, and high diffusible hydrogen content in steel and high tension rates during presetting caused the brittle fracture of the leaf spring at low temperatures. Reducing the diffusible hydrogen content in steel, increasing the tempering temperature, extending the tempering time, increasing the number of hydrogen traps, and achieving appropriate matching of strength and toughness could effectively avoid early failure and hydrogen-induced delayed fracture of the leaf spring.
  • Causes of Delamination of 07Cr18Ni11Nb Stainless Steelduring Diagonal Rolling Piercing Process
    WANG Xiaoping, DONG Yimiao, ZHU Xiongming, SHEN Weijie
    Materials For Mechanical Engineering. 2020, 44(8): 97-102. https://doi.org/10.11973/jxgccl202008020
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    A batch of 07Cr18Ni11Nb stainless steel exhibited delamination during the piercing process. The causes of delamination were analyzed through macroscopic and microscopic inspection, chemical composition analysis, metallographic examination, scanning electron microscopy and energy spectrum analysis. The results show that the chain-like and rod-like primary Nb(C,N) segregation phase and non-uniform deformation during the piercing process were the main causes of delamination. During the piercing process, the stress concentration caused by the deformation incompatibility between primary Nb(C,N) phase and austenite matrix reduced the local fracture strength of the tube billet. Under the effect of additional tensile stress caused by non-uniform deformation, cracks occurred in the stress concentration area and continued to expand in the subsequent rolling process, resulting in the delamination. It was recommended to control the content of carbon and niobium in smelting process, improve the remelting and solidification conditions, adjust the piercing process parameters and reduce the roll speed appropriately to effectively prevent the occurrence of piercing delamination.
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