20 August 2019, Volume 43 Issue 8
    

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  • Effect of Fiber Orientation and Milling Parameters on Milling Surface Quality of T300/5222A Composite Laminate
    YANG Zhenchao, LI Yan, HOU Xiaoli, SONG Danlong, TAN Liang
    Materials For Mechanical Engineering. 2019, 43(8): 1-6. https://doi.org/10.11973/jxgccl201908001
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    Milling experiments were carried out on unidirectional and bidirectional laminates of T300/5222A carbon fiber reinforced epoxy resin composite with coated cemented carbide tool. The effects of fiber orientation as well as milling speed, feed per tooth and milling width on the milling surface quality were studied. The results show that with the increase of the angle between fiber orientation and workpiece feed direction, the burr on milling surface of the unidirectional laminate became more and more serious, and even the chipping appeared. With the increase of the milling speed, milling width and feed per tooth, the amount of the protruding fibers and holes formed by fiber pull-out on milling surface of the bidirectional laminate increased, and the height difference between peaks and troughs was also improved. The difference between the two-dimensional surface roughness and three-dimensional surface roughness was large. Using three-dimensional surface roughness can more accurately evaluate the milling surface topography of carbon fiber composites. The three-dimensional surface roughness of the bidirectional laminate increased with the increase of the milling speed, feed per tooth and milling width, and the influence of the milling speed and feed per tooth was relatively significant.
  • Effect of Solid Solution and Aging Process Parameters on Microstructure and Tensile Property of TA19 Titanium Alloy
    ZHOU Ye, DU Jianping, HAN Moliu, LIANG Yu
    Materials For Mechanical Engineering. 2019, 43(8): 7-11. https://doi.org/10.11973/jxgccl201908002
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    TA19 titanium alloy was treated by solid solution at different temperatures (930, 960, 990℃) for 2 h and then aging at different temperatures (550, 590, 630℃) for 8, 16 h. Effects of process parameters on microstructure and tensile property were studied. The results show that after solid solution at different temperatures and then aging at 590℃ for 8 h, the equiaxed α phase content in the test alloy decreased and the tensile strength was improved with the rise of solid solution temperature. After solid solution at 960℃ for 2 h, the microstructure of the test alloy was composed of equiaxed α phase and α' martensite. During the subsequent aging at 550℃ for 8 h, α' martensite decomposed unsufficiently, so the content of granular α phase was relatively small and therefore the tensile strength of the alloy increased limitedly. When the aging temperature rose to 590℃ and the aging time was 8 h and 16 h, fine and diffuse granular α phase precipitated in the microstructure, resulting in the increase of tensile strength. When the aging temperature continuously increased to 630℃, the α phase was coarsened and the tensile strength decreased.
  • Microstructure and Properties of SIMP Steel Joint by Three Step Heating Transient Liquid Phase Diffusion Welding with Different Process Parameters
    HAO Lili, CHEN Sijie, DING Guangzhu, LI Bao, LI Shihui, ZHAO Pifeng
    Materials For Mechanical Engineering. 2019, 43(8): 12-17,50. https://doi.org/10.11973/jxgccl201908003
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    With BNi2 and Fe78Si9B13 amorphous alloy foils as intermediate layer, SIMP steel tubes were joined by three step heating transient liquid phase (TLP) diffusion welding. Microstructure and mechanical properties of the joint welded with different isothermal solidification process parameters (temperature of 1 230, 1 240℃; time of 180, 240 s; pressure of 8, 9 MPa) were studied. The results show that increasing isothermal solidification temperature or time was beneficial to the diffusion of melting elements and reducing formation of brittle hard phases in the weld microstructure, thereby improving the tensile strength of the joint and reducing the microhardness of the weld. The relatively optimal isothermal solidification parameters were listed as follows:temperature of 1 240℃, time of 240 s, and pressure of 9 MPa. After welding with these parameters, the microstructure of the joint was composed of uniform martensite, and the difference between the weld microstructure and the base metal microstructure was small; the tensile strength of the joint reached the largest value of 794 MPa, and the tensile fracture mode was ductile fracture.
  • Micromorphology and Chemical Composition of Carbon Nanotubes after Pretreatment and Electroless Copper Plating
    GUO Li, ZHAN Hao, YOU Yuping, YUE Wei, FENG Min
    Materials For Mechanical Engineering. 2019, 43(8): 18-22,34. https://doi.org/10.11973/jxgccl201908004
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    Carbon nanotubes (CNTs) were pretreated by purification, sensitization and activation in turn, and then electroless copper plated in plating solution at different pH values (7-13) with NaBH4 as reductant. Micromorphology and chemical composition of the CNTs treated at different stages were studied, and influence of plating solution pH value on copper plating effect was analyzed. The results show that after purification treatment, the impurity particles on the surface of CNTs disappeared, and the phase was pure CNTs. After sensitization treatement, a sensitized layer composed of continuous particles was formed on the surface of CNTs. These particles were mainly Sn(OH)2, Sn(OH)Cl and SnO particles. After activation treatment, the activation layer consisted of continuous and relatively fine particles, and these particles were mainly palladium and SnO particles. With increasing pH value of the plating solution, the reducing ability of NaBH4 increased, so the amount of copper particles on the surface of CNTs increased. When the pH value reached 11, a continuous uniform copper plating was formed. But when the pH value increased to 13, discontinuous copper particles with relatively large size were formed on the surface of CNTs.
  • Continuous Cooling Transformation Behaviors of Q500q Bridge Steel under Different Cooling Modes
    FENG Lulu
    Materials For Mechanical Engineering. 2019, 43(8): 23-26,34. https://doi.org/10.11973/jxgccl201908005
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    Cooling transformation behaviors of Q500q bridge steel by two cooling modes were studied with Gleeble-2000 thermal simulation machine. One cooling mode method (constant cooling) was cooling from 900℃ to 300℃ at constant cooling rates (1, 4, 8, 16, 32℃·s-1) and the other cooling mode (subsection cooling) was cooling from 900℃ to 650℃ at constant cooling rates (ditto) and then to 300℃ at halving cooling rates. The results show that under constant cooling conditions, the steel had a structure of polygonal ferrite+pearlite at the cooling rates of 1-4℃·s-1, a structure of bainite at the cooling rates of 8-16℃·s-1, and a structure of martensite at the cooling rates of 32℃·s-1. Under subsection cooling conditions, the ferrite and pearlite transformation occurred at the cooling rates in the intermediate temperature range (650-300℃) of 0.5-4℃·s-1, and the amount of polygonal ferrite was relatively large; the bainite transformation occurred at the cooling rates in the intermediate temperature range (650-300℃) of 8-16℃·s-1. Compared with those by constant cooling method, the bainite transformation end temperature increased, the transformation region was reduced, and the grain refinement degree of bainite decreased.
  • Electrochemical Corrosion Behaviors of D36, F460 and F690 Steels for Offshore Engineering in Simulated Seawater
    ZHANG Zhen, WANG Qi, ZHANG Caiyi, LU Xiaohui, HU Zhengfei, GAO Shan
    Materials For Mechanical Engineering. 2019, 43(8): 27-34. https://doi.org/10.11973/jxgccl201908006
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    Microstructures and electrochemical corrosion behaviors in simulated seawater (3.5wt%NaCl solution) at 20, 50℃ of domestic D36, F460 and F690 steels for marine engineering were studied. The results show that the microstructure of D36 steel rolled by thermo-mechanical control process consisted of ferrite, pearlite and a little widmanstatten structure. The microstructures of F460 and F690 steels after quenching and tempering treatment were both composed of fine bainite lath. In the NaCl solution, the corrosion resistance and pitting corrosion resistance of D36, F460 and F690 steels increased in turn. Compared with those at 20℃, the free corrosion current density at 50℃ of the three tested steels increased significantly, and the corrosion resistance decreased. After long-term immersion in the NaCl solution, the thicknesses of surface corrosion product film on D36, F460 and F690 steel decreased in turn, and the element type in the corrosion product film increased in turn. The three tested steels showed overall uniform corrosion characteristics.
  • Effect of Vibratory Stress Relief Treatment on Fatigue Property of 7055-T7751 Aluminum Alloy
    MA Yongbo, CUI Tengfei, PAN Yifan, CHENG Shumin, LIU Daoxin
    Materials For Mechanical Engineering. 2019, 43(8): 35-40. https://doi.org/10.11973/jxgccl201908007
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    7055-T7751 aluminum alloy pre-stretching plate (pre-strains of 1%-3%) was subjected to vibratory stress relief treatment, and then to fatigue tests under different maximum stresses (300, 350, 400, 500 MPa). The effects of vibratory stress relief treatment on microstructure, microhardness, surface residual stress and fatigue property of the alloy were studied. The results show that the vibratory stress relief treatment had little effect on the microstructure and microhardness; the microstructure after vibratory stress relief was still the texture orientation structure of the rolled plate. After the vibratory stress relief treatment, the longitudinal and transverse residual tensile stresses in the alloy were obviously relaxed, and the longitudinal residual stress distribution was homogenized. The vibratory stress relief treatment improved the fatigue performance of the longitudinal and transverse specimens significantly. Under the maximum stress of 350 MPa, the fatigue lives of the longitudinal and transverse specimens increased by 219.0% and 29.1%, and the fatigue limits were improved by 9.9% and 5.1%, respectively.
  • Microstructure and Mechanical Properties of Imported EA4T Steel Axle for High-Speed Railway
    RAN Xu, JIANG Mingkun, HAN Ying
    Materials For Mechanical Engineering. 2019, 43(8): 41-45,74. https://doi.org/10.11973/jxgccl201908008
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    Microstructure and mechanical properties of imported EA4T steel axle for high-speed railway were investigated, and uniformity of the microstructure and mechanical properties was analyzed. The results show that non-metallic inclusions, including CaS, Al2O3 and a small amount of silicate, were found in the axle, and the number of inclusions increased from the surface to the core. The microstructures at different positions in the axle all consisted of tempered martensite, bainite and a small amount of proeutectoid ferrite; the size and number of the proeutectoid ferrite increased from the surface to the core. The microstructure was uneven. The tensile properties and hardness of the axle met the standard requirements. The tensile strength at the core was obviously lower due to higher content of the proeutectoid ferrite. Banded structure was observed in the longitudinal direction of the axle, and therefore the transverse impact toughness was relatively low. The tensile fracture of the axle sample showed relatively small and deep dimples, indicating good plasticity; the tensile fracture mode was ductile fracture. The impact fracture had obvious river patterns and dimples, and the fracture mode was brittle-ductile mixed fracture.
  • Microstructure and Properties of 5083/6063 Unequal-Thickness Aluminum Alloy Joint by Twin-Wire CMT Fillet Welding
    HAN Shanguo, CHEN Yongcheng, WANG Jinzhao, LUO Ziyi
    Materials For Mechanical Engineering. 2019, 43(8): 46-50. https://doi.org/10.11973/jxgccl201908009
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    Twin-wire cold metal transfer (CMT) fillet welding was conducted on 3 mm thick 5083 aluminum alloy and 16 mm thick 6063 aluminum alloy. The optimal parameters, including torch angle, welding speed and back wire welding current, were determined by analysis of the weld formation and tensile resistance. The microstructure and hardness of the joint welded with the optimal parameters were studied. The results show that the optimal process parameters were listed as follows:torch angle of 60°, welding speed of 2.0 m·min-1 and back wire current of 140 A. The obtained weld surface had a smooth transition and no defects such as undercuts and hump; the tensile force per unit length reached the largest value of 548.7 N·mm-1. The boundary between the fusion zone and the heat affected zone at 5083 aluminum alloy side was distinct, and the impurity phase in the heat affected zone was enriched at the grain boundary. The fusion zone at 6063 aluminum alloy side was narrow, and the weld structure was composed of dendrites and had a strip structure with different grain sizes. The weld zone had the highest hardness, and the fusion zone and heat affected zone were neither hardened nor softened.
  • Microstructure and Mechanical Properties of HR3C Steel after Service at High Temperature for 50 000 h
    LI Xinmei, ZHANG Zhongwen, DU Baoshuai, LI Wen, WEI Yuzhong, YIN Zhiyi
    Materials For Mechanical Engineering. 2019, 43(8): 51-54. https://doi.org/10.11973/jxgccl201908010
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    Microstructures and mechanical properties of HR3C steel in as-received state and after 50 000 h service at high temperature were studied and compared. Main factors influencing the mechanical properties during service were analyzed. The results show that compared with the microstructure in as-received state, the microstructure of HR3C steel after service still consisted of austenite matrix and precipitates, but the amount and size of the precipitates increased. The precipitates on grain boundary and inside grain of austenite were mainly M23C6 carbide. After service, the tensile strength of HR3C steel changed little while the plasticity decreased; during bending, the steel had only a small amount of plastic deformation before brittle fracture. After service, the impact energy of HR3C steel was reduced and the impact fracture showed a typical intergranular fracture feature. The intergranular precipitation and growth of M23C6 carbide was the root cause of the decline of plasticity and toughness of HR3C steel.
  • Effect of Heat Source Offset on Temperature Field and Residual Stress of Plasma Arc Welded Joint of Steel/Aluminum Dissimilar Materials
    LIU Fang, GUO Yulong, ZHOU Guangtao, HAN Xue, CHEN Meifeng, LI Huachen
    Materials For Mechanical Engineering. 2019, 43(8): 55-59,68. https://doi.org/10.11973/jxgccl201908011
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    Effects of heat source offset distance from steel and aluminum interface (heat source offset distance) on the welding temperature field and residual stress of plasma arc welded joint of steel/aluminum dissimilar materials were studied by finite element simulation and verified by experiments. The results show that the temperature field of the welded joint of steel/aluminum dissimilar materials was asymmetrically distributed, and the temperature gradient on the steel side was lower than that on the aluminum side. With increasing heat source offset distance, the temperature and fusion width on the steel side of the joint increased, and those on the aluminum side had the opposite trend. The residual stress on the aluminum side of the joint was mainly tensile stress. On the steel side, the residual tensile stress near the interface decreased with increasing heat source offset distance, and finally changed into compressive stress; the residual stress far away from the interface was mainly compressive stress. The relative error between the experimental residual stress and the simulation was lower than 8%, and the residual stress situation was in line with the simulation.
  • Cause of Speckled Defects on Surface of High-Speed Train Transmission Gear
    WANG Rong, FU Yangyang
    Materials For Mechanical Engineering. 2019, 43(8): 60-63,68. https://doi.org/10.11973/jxgccl201908012
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    After running for a period of time, speckled defects with strong regularity were formed on transmission gears of high-speed trains. The speckled defects were located at root of the large cylindrical gear and at top of the small bevel gear. Causes of the speckled defects were analyzed by macroscopic observation, chemical composition analysis, microhardness test, microstructure and microscopic observation. The results show that the speckled defects on the transmission gears were caused by electrical erosion. Due to poor insulation of the train circuit and insufficient grounding insulation of the rails, some stray currents broke through the thin oil film between the tooth surfaces and caused spark discharge, leading to electric erosion. When the degree of the electric corrosion was light, the oil film was carbonized; when the degree was high, local melting was produced and then caused melting beads. Thus the speckled defects were formed.
  • Cause of Fracture on Special Equipment Wire Rope
    GUI Xu, YANG Jinyan
    Materials For Mechanical Engineering. 2019, 43(8): 64-68. https://doi.org/10.11973/jxgccl201908013
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    Broken cause of wire rope for special equipment was analyzed by macroscopic inspection, fracture analysis, microstructure observation, chemical composition analysis, dismantling test, and so on. The results show that the wire rope failed by fatigue fracture. During use, the wire rope was subjected to complex alternating stresses such as tension, bending, torsion, extrusion and vibration, leading to local stress concentration of the outer strand wires and core wires of the rope; meanwhile, the rising temperature during the extrusion and friction process led to the surface phase transition. Therefore microcracks were initiated and propagated, resulting in the fracture. The core wires wore faster than the outer strand wires, and first failed. After the failure of the rope core, the outer strand wires wore up quickly, resulting in the overall failure of the wire rope.
  • Fracture Cause of Marine Diesel Engine Crankshaft
    HOU Shuaishuai, ZENG Xianchen, SHI Chao, CHI Yuandi, HU Jiakun, FU Chunxia
    Materials For Mechanical Engineering. 2019, 43(8): 69-74. https://doi.org/10.11973/jxgccl201908014
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    A marine diesel engine crankshaft fractured after running for less than 3 a. The failure analysis was conducted on the crankshaft by mechanical property test, chemical composition analysis, metallographic analysis, fracture observation and finite element analysis. The results show that the fracture property of the crankshaft was fatigue fracture under low stresses. The protective slag involved in the electroslag casting process was the direct reason for fracture of the crankshaft. The substandard tempering and nitriding treatment further reduced the fatigue strength of the crankshaft, and accelerated the propagation of the fatigue cracks, eventually leading to the fracture of the crankshaft.
  • Burst Cause of Jacketed Heat Pipe in an Economizer
    ZHANG Peipei, HUANG Jibo, WANG Weize, TU Shantong
    Materials For Mechanical Engineering. 2019, 43(8): 75-78. https://doi.org/10.11973/jxgccl201908015
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    Jacketed heat pipe of a five-stage economizer bursted during the operation, leading to leakage. The burst cause of the jacketed heat pipe was analyzed from the aspects of chemical composition, microstructure, hardness, fractography, etc. The results show that at the first start of operation, the heat pipe worked at an overheating temperature about 400℃. The steam pressure exceeded the burst pressure, causing the cracking of the outer pipe. After the outer pipe cracked, the water in the pipe and the working medium between the inner and outer pipes flew out, reducing the cooling effect; the high temperature flue gas directly contacted the inner pipe, increasing the possibility of sulfuric acid dew point corrosion. Consequently, the burst of the inner pipe occurred after continuously running for three months. In actual operation, the operation process should be strictly controlled and the operation management should be strengthened to prevent the pipeline from overheating, thus preventing the similar failure from happening again.
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