20 May 2020, Volume 44 Issue 5
    

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  • Microstructure and Tensile Properties of Titanium Matrix Composite TIG Welded Joint
    PAN Yuwei, MAO Jianwei, ZHANG Lixin
    Materials For Mechanical Engineering. 2020, 44(5): 1-5,53. https://doi.org/10.11973/jxgccl202005001
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    Discontinuously reinforced titanium matrix composite was welded by tungsten inert gas welding (TIG) technique. The microstructure and tensile properties of the welded joint were studied. The results show that TIG welding can well realize the connection of titanium matrix composite. The weld had the good formation with an even and clean surface, and no welding defects such as microcracks and pores were found. The joint consisted of weld zone, heat-affected zone and base metal zone. The TiB reinforcements distributed on the boundaries of β phase in the weld zone and the heat affected zone near the weld in the joint had high aspect ratios with a high degree of refinement, and there were a large number of acicular matensite α' phase in the two regions. The tensile strength of the joint was 1 137 MPa, 92% of that of the base metal, and the percentage elongation after fracture was 2.20%. The joint fractured at the base metal zone during tensile; the tensile fracture of the joint was mainly characterized by ductile fracture, and local areas showed intergranular fracture features.
  • Preparation of Mn3O4/Vertically Aligned Carbon Nanotube Composite Electrode Material and Its Electrochemical Performance
    WANG Weibo, SUN Gan, ZHAO Bin
    Materials For Mechanical Engineering. 2020, 44(5): 6-11. https://doi.org/10.11973/jxgccl202005002
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    Vertically aligned carbon nanotubes (VACNTs) were prepared by water-assisted chemical vapor deposition method. The manganese precursor was loaded on VACNTs surface by supercritical carbon dioxide assisted impregnation, and then the Mn3O4/VACNTs composite electrode material was prepared by vacuum annealing at different temperatures (250-350 ℃). The microstructure and electrochemical performance of the composite electrode material were studied. The results show that in the composite electrode material Mn3O4 nanoparticles were loaded on the surface of carbon nanotubes. After vacuum annealing at 300 ℃, Mn3O4 nanoparticles with diameter of 6-10 nm were evenly distributed on the surface of carbon nanotubes. Compared with pure VACNTs, specific capacitance of the composite electrode material increased by 3-4 times, and Warburg impedance, equivalent series resistance and charge transfer resistance at the electrode/electrolyte interface were relatively small. The composite electrode material by vacuum annealing at 300 ℃had the best electrochemical performance; the maximum specific capacitance was 168 F·g-1, and the specific capacitance retention was 56% with the charge and discharge current density increasing from 1 A·g-1 to 10 A·g-1; the equivalent series resistance was the smallest (about 2.5 Ω).
  • Effects of CaO Addition on Magnetic Properties of High Permeability Mn-Zn Ferrite
    MO Caisong, MA Li
    Materials For Mechanical Engineering. 2020, 44(5): 12-15,48. https://doi.org/10.11973/jxgccl202005003
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    After fixing the ratio of basic raw materials of MnO, ZnO and Fe2O3, and then adding CaO with mass fractions of 0-1.0%, the Mn-Zn ferrite was prepared by traditional two-step synthesis process. The effect of CaO on microstructure and magnetic properties of Mn-Zn ferrite was studied. The results show that with increasing CaO addition content, the average grain size and sintering density of Mn-Zn ferrite increased first and then decreased; the elastic permeability at the frequency of 2.2 MHz increased first, then sharply decreased, and then slightly increased; the saturation magnetization at the magnetic field intensity of 1 T increased first and then decreased; the coercivity decreased first and then increased. Adding proper amount of CaO could improve microstructure of Mn-Zn ferrite and decrease core loss. When the content of CaO was 0.4wt%, the Mn-Zn ferrite had the largest saturation magnetization at the magnetic field intensity of 1T of 46 A·m2·kg-1, the relatively large elastic permeability at the frequency of 2.2 MHz of 86.7, the relatively low loss power of about 20 kW·m-3 and the relatively low coercivity of 183.4 kA·m-1, and the Mn-Zn ferrite had the good comprehensive magnetic properties.
  • Effect of Artificial Defects on Fatigue Limit of S38C Axle Steel
    BU Weijie, GAO Jiewei, DAI Guangze, HAN Jing, LI Hengkui, LI Yabo, XU Zhongxuan, CHEN Cheng
    Materials For Mechanical Engineering. 2020, 44(5): 16-20. https://doi.org/10.11973/jxgccl202005004
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    Two kinds of artificial defects including indentation and electro-discharge machined crater were introduced into the fatigue specimen of S38C axle steel by Brinell hardness tester and electric spark ablation. The fatigue limits of specimens with different sizes of artificial defects were studied and compared with calculation by Murakami equation. The results show that the fatigue crack originated from the bottom of the artificial defects. The fatigue limit of specimens with artificial indentation and electro-discharge machined crater showed a linear decreasing trend with the increase of the defect projection in the study scope. Under the same projection area condition on the cross section of defects, the tested fatigue limit of the specimen with the electro-discharge machined crater was about 30 MPa smaller than that of the specimen with the artificial indentation; the tested fatigue limit of the specimen with the artificial indentation was about 50 MPa larger than the calculated value; the tested fatigue limit of the specimen with the electro-discharge machined crater was only 20 MPa higher than the calculated value, indicating the fatigue limit of specimen with such defect could be predicted approximately by Murakami equation.
  • Effect of Al-5Ti-1B Refiner Addition Amount on Microstructure and Tensile Properties of Cast Aluminum Alloy Containing Ce
    WANG Bo, LIU Rangxian, YU Hongwei, LUO Weichao, ZHAO Wenlong
    Materials For Mechanical Engineering. 2020, 44(5): 21-27. https://doi.org/10.11973/jxgccl202005005
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    On the basis of adding 0.1wt% Ce, the effect of the Al-5Ti-1B refiner content (0wt%-0.4wt%) on the microstructure and tensile properties of A356 cast aluminum alloy was comparatively studied. The results show that with increasing content of Al-5Ti-1B refiner, the average secondary dendrite spacing of α-Al dendrite decreased first and then increased, and reached the smallest value of 37.3 μm when the content of refiner was 0.2wt%. The proper amount of refiner promoted the eutectic silicon shape changing from long and narrow fibrous to thick and short rods, and resulted in the reduction of equivalent diameter and aspect ratio of eutectic silicon; but the excessive amount of refiner caused the sharpening of eutectic silicon. After adding the refiner, the tensile strength and percentage elongation after fracture of the aluminum alloy all increased, and increased first and then decreased with increasing refiner content. The comprehensive modification effect of the aluminum alloy after adding 0.2wt% refiner was the best, and the tensile properties were the best; the tensile strength and percentage elongation after fracture increased by 20.7% and 66.7% compared with those without adding refiner, respectively.
  • Microstructure and Mechanical Properties of Girth Joint for X80 Pipeline SteelPipe Welded with Self-shielded Flux Cored Wire
    ZHANG Zhanwei, WANG Hao, XIAO Cunyong, LIANG Kan, GUO Xuming
    Materials For Mechanical Engineering. 2020, 44(5): 28-32. https://doi.org/10.11973/jxgccl202005006
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    X80 pipeline steel was girth welded with self-made self-shielded flux cored wire. The weld formation, microstructure and mechanical properties of the joint at different welding positions were analyzed. The results show that the weld at different welding positions had good formation, indicating that the wire had good all position welding adaptability. The microstructures of the weld at different welding positions were almost the same. The cap layer consisted of lath bainite and acicular ferrite; the fine granular bainite and quasi-polygonal ferrite were in the filling layer. The fine-grained heat-affected zone had almost the same microstructure of fine-grained ferrite and acicular ferrite. The coarse lath bainite and granular bainite were in the coarse-grained heat-affected zone, and the grain size of this area at vertical position was smaller than that at flat and overhead positions. The average hardnesses of weld and heat-affected zone were 226, 227 HV, respectively, which were slightly lower than that of the base metal (233 HV). The average impact absorbing energies at -10 ℃ of weld and heat-affected zone at vertical position were higher than that at flat position. The tensile strength of joints at different welding positions was similiar, and the tensile fracture of the joints showed ductile fracture characteristics.
  • Effect of Compression and Semi-solid Isothermal Heat Treatment on Microstructure and Properties of Mg-3Al-0.8Gd Alloy
    XUE Haitao, LI Xiaoni, DU Wei, YANG Shuo
    Materials For Mechanical Engineering. 2020, 44(5): 33-37,43. https://doi.org/10.11973/jxgccl202005007
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    Compression deformation and semi-solid isothermal heat treatment of Mg-3Al-0.8Gd alloy were carried out. The effects of the compression deformation (10%,15%,20%), isothermal temperature (530,540,550,560,570 ℃) and holding time (3,5,10,15 min) on the microstructure and hardness of the alloy were studied. The tensile and impact properties of as-cast and heat-treated Mg-3Al-0.8Gd alloys were compared. The results show that the microstructures of Mg-3Al-0.8Gd alloy after compression deformation and isothermal heat treatment under different conditions were all composed of α-Mg matrix and β-Mg17Al12 phase. With the increase of compression deformation, isothermal temperature and holding time, the α dendrites were gradually transformed into equiaxed grains, the grains were refined, the structure uniformity was improved, and the microhardness increased. The tensile strength, elongation after fracture, section shrinkage and shock absorption energy of Mg-3Al-0.8Gd alloy with compression deformation of 20% after isothermal treatment at 550 ℃ for 15 min were 11.3%, 32.6%, 3.8% and 23.3% higher than those of the as-cast alloy, respectively.
  • Effect of Volumetric Energy Density on Microstructure and Properties ofHastelloy X Alloy Manufactured by Selective Laser Melting
    LI Yong, XU Hejun, LI Kai, BA Fahai, HE Beibei
    Materials For Mechanical Engineering. 2020, 44(5): 38-43. https://doi.org/10.11973/jxgccl202005008
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    Hastelloy X alloy was manufactured by selective laser melting (SLM) technique, and the effect of volumetric energy density(18.3,29.8,30.3,44.9,46.3,50.9,58.8,61.7,88.4 J·mm-3) on micromorphology, microstructure, density and hardness of the alloy was investigated. The results show that the longitudinal section morphology of the Hastelloy X alloy manufactured by SLM was fish-scale-line morphology. Equiaxed crystals, dendrite and columnar grains spanning multiple deposited layers were present in the molten pool area, and the width of grains was about 0.6-1.2 μm. When the volumetric energy density was 18.3-46.3 J·mm-3, there were pore defects in the alloy. As the volumetric energy density continued to increase, the pore gradually decreased and disappeared, and microcracks emerged and gradually increased. The density and hardness of the alloy increased first and then tended to be stable with the volumebric energy density increasing. The alloy with volumetric energy density of 50.9 J·mm-3 had the least porosity and cracks, and the formability was the best.
  • Research Progress on Effect of Surface Texturing on Tribological Properties
    LI Tiantian, SUN Yaoning, ZHANG Li, WANG Guojian
    Materials For Mechanical Engineering. 2020, 44(5): 44-48. https://doi.org/10.11973/jxgccl202005009
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    Surface texture technique is a new type surface modification technique, which can effectively improve the tribological properties of materials through machining patterns with a certain shape and size, and with regular arrangement on the surface of materials by micro-fabrication technique. Surface texture plays an important role in mechanical friction pairs because of its outstanding advantages in improving the tribological characteristics of materials. The common texture preparation methods are introduced; the wear reducation mechanism of surface texture under different working conditions is described; the influence of surface texture topography and its geometric parameters on the wear resistance is summarized. The development direction of surface texture technology is prospected.
  • Effect of Vacuum Environment and Substrate Preheating on Microstructure and Properties of Laser Cladding WC Reinforced Nickel-Based Alloy Coating
    YAO Yongqiang, LIN Chen, SHEN Jingyi, LIU Jia, XU Huanhuan
    Materials For Mechanical Engineering. 2020, 44(5): 49-53. https://doi.org/10.11973/jxgccl202005010
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    Under the condition of vacuum and substrate preheating at 200 ℃, WC reinforced nickel-based alloy coating was prepared on the surface of 45 steel substrate by laser cladding technique. The macro- and micro-morphology, phase composition, hardness and wear resistance of the coating were studied, and compared with those under the condition of direct nitrogen blowing protection in atmospheric environment and substrate unpreheating. The results show that under the condition of vacuum and substrate preheating, there were no cracks and pores in the coating; the structure was uniform and dense, and mainly consisted of cell-like crystals. The phases in the coating were mainly composed of Cr23C6, Cr3C2, γ-Ni and FeNi, and the content of Cr23C6 hard phase was lower than that under the condition of direct nitrogen blowing protection in atmospheric environment and substrate unpreheating. The average microhardness of the coating was 751.14 HV, about 85.43 HV lower than that under the condition of direct nitrogen blowing protection in atmospheric environment and substrate unpreheating. The stable friction coefficient of the coating was 0.50, and the cross-sectional area of the surface wear scar was about 0.4×10-3 mm2, which were about 23% and 58% lower than those under the condition of direct nitrogen blowing protection in atmospheric environment and substrate unpreheating, respectively, indicating that the wear resistance of the coating was significantly improved.
  • Microstructure and Mechanical Properties of Laser Cladding Carbon NanotubesToughened Fe-based Amorphous Coating
    WANG Tiancong, ZHU Yanyan, YAO Chengwu, LI Zhuguo
    Materials For Mechanical Engineering. 2020, 44(5): 54-59,65. https://doi.org/10.11973/jxgccl202005011
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    After the nickel-plated carbon nanotubes and iron-based amorphous powder were mixed by ball milling process, the coating was prepared on the steel plate by coaxial powder laser cladding method. The optimal ball milling time was determined, and the effect of the mass fraction of nickel-plated carbon nanotubes (0-1.00%) on the micro structure and mechanical properties of the coating was discussed. The results show that the proper ball milling time for mixing iron-based amorphous powder and nickel-plated carbon nanotubes was not longer than 30 min. The coatings all consisted of amorphous region, equiaxed dendritic region and columnar dendritic region; with increasing mass fraction of nickel-plated carbon nanotubes, the area fraction of amorphous phase in the coating decreased, and the size of the precipitated nanocrystals increased. The content of nickel-plated carbon nanotubes hardly affected the hardness of the columnar dendrite and equiaxed dendrite regions in the coating. With increasing mass fraction of nickel-plated carbon nanotubes, the average hardness of the amorphous region in the coating decreased from 1 615.0 HV to 1 464.3 HV, and the fracture toughness increased from 5.75 MPa·m1/2 to 7.67 MPa·m1/2.
  • Research Progress on Porosity Reduction in Plasma Sprayed Coatings
    LI Chenglong, LI Wenge, ZHAO Yuantao, ODHIAMBO Gerald, XIA Yu
    Materials For Mechanical Engineering. 2020, 44(5): 60-65. https://doi.org/10.11973/jxgccl202005012
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    Plasma sprayed coatings have been widely used in surface protection engineering due to the excellent properties such as wear resistance, high temperature resistance and corrosion resistance. Pores are an important structural defect of plasma sprayed coatings. Excessive pores can lead to premature detachment failure of the coatings and shorten the service life, and therefore porosity is an important indicator to evaluate the coating quality. The pore formation mechanism, influencing factors, and the effect of the pores on the coating performance are briefly discribed. The research status of the porosity reduction of plasma sprayed coatings is reviewed from the aspects such as spraying process parameter optimization, laser remelting treatment, and modification of spraying materials. Moreover, the problems and development directions in the development of plasma sprayed coatings are summarized.
  • Effect of Nanosecond Laser Machining and Heat-Treatment on Wettability of Pure Aluminum Surface
    MA Linxu, WANG Lina, LI Chengying, GUO Jian, DENG Cui, ZHAO Jingnan
    Materials For Mechanical Engineering. 2020, 44(5): 66-71. https://doi.org/10.11973/jxgccl202005013
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    The micro-nano structure was prepared on the surface of pure aluminum plate by nanosecond laser machining technique, and then was heat-treated at 150 ℃ for 2 h. The effect of laser scanning interval (0.005-0.020 mm), scanning speed (100 -1 700 mm·s-1) and heat treatment on the wettability of the pure aluminum plate surface after laser ablation was studied. The results show that the regular micro-nano grid structure was formed on the surface of pure aluminum plate after laser ablation with different process parameters. The pure aluminum plate surface after laser ablation was super hydrophilic, and after heat treatment changed to be hydrophobic or even super-hydrophobic. With increasing scanning speed and scanning interval, the contact angle of pure aluminum plate surface after laser ablation and heat treatment changed little, and the sliding angle increased; the surface showed different degrees of wettability. After laser ablation with laser scanning speed of 100 mm·s-1 and scanning interval of 0.005 mm and heat treatment, the pure aluminum plate surface had dense micro-nano structure, and the best super-hydrophobicity with contact angle of 155.6° and sliding angle of 4°.
  • Microstructure and Corrosion Resistance of Magnetron Sputtering Aluminum Film on Magnesium Alloy Surface Treated by High Energy Shot Peening
    ZHANG Conghui, LIU Ying, ZHAO Xu, ZENG Xiangkang, WANG Jing
    Materials For Mechanical Engineering. 2020, 44(5): 72-76,81. https://doi.org/10.11973/jxgccl202005014
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    AZ91D magnesium alloy substrate surface was treated by high energy shot peening (HESP) for different times (0, 20,30,40 min), and then aluminium film was plated on the HESP substrate surface by magnetron sputtering technique. The microstructure and corrosion resistance of the aluminium film were studied. The results show that the surface structure of the aluminium film on the HESP substrate surface was uniform and dense, and the film was relatively thick; with increasing HESP time, the structure was finer and more uniform, and the film was thicker. After HESP for 40 min, there was a significant transition layer between aluminium film and substrate, and the bonding force was 16 N, which was 3.2 times that without HESP. Compared with those without HESP, the free corrosion current density of the aluminium film on the HESP substrate surface in NaCl solution and the etch pit depth formed in salt spray for 24 h were relatively small. With increasing HESP time, the free corrosion current density and the etch pit depth both decreased, indicating that the electrochemical and salt spray corrosion resistance of the aluminium film were significantly improved.
  • Microscale Quasi-Static Contact Mechanical Properties of Plasma Molybdenum Gradient Modified Layer on Pure Titanium Surface
    YANG Zhao, DONG Yaqian, WANG Shuaikang, MA Yong, YU Shengwang, WU Yucheng, TANG Bin
    Materials For Mechanical Engineering. 2020, 44(5): 77-81. https://doi.org/10.11973/jxgccl202005015
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    The molybdenum gradient modified layer was prepared on the surface of TA2 pure titanium substrate by double glow plasma surface alloying technique. The cross section morphology and element distribution of the modified layer were analyzed, and the microscale quasi-static contact mechanical property of the modified layer were studied. The results show that the molybdenum gradient modified layer was uniform and compact with the thickness of 12 μm, and was composed of the deposition layer with 2.7 μm thickness and the diffusion layer with 9.3 μm thickness. The content of Mo and Ti element changed in gradient along the thickness direction, and the modified layer had a good metallurgical bond with the substrate. The hardness and elastic modulus of the modified layer were 13.82, 142.96 GPa, which were 10.87, 142.38 GPa larger than those of the substrate, respectively, indicating that the modified layer had high strength and good plasticity. When the test load was 15 N, the micro-indentation depth was nearly 15 μm, and the composite hardness and elastic modulus were close to those of the substrate, indicating that the strengthening effect of the modified layer was completely ineffective.
  • Effect of Laser Quenching Process on Surface Hardness and Hardened LayerDepth of QT700-2 Ductile Cast Iron
    TANG Liang, WANG Wenjian, ZHANG Yalong, SUN Wei, ZHAN Ke
    Materials For Mechanical Engineering. 2020, 44(5): 82-86. https://doi.org/10.11973/jxgccl202005016
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    QT700-2 ductile cast iron was treated by single-pass and asymmetric multi-pass laser quenching processes, and then the surface hardness and hardened layer depth of the ductile cast iron under different laser quenching processes were studied. The results show that after single-pass laser quenching, the surface hardness of the ductile cast iron was 52-59 HRC; the surface hardness increased with increasing laser power, and increased first and then decreased slightly with increasing scanning speed; with laser power of 1 300 W and scanning speed of 8 mm·s-1, the surface hardness of the ductile cast iron evenly distributed at 55-58 HRC, and the hardened layer depth was about 1 mm. After asymmetric multi-pass laser quenching with different process parameters, the surface hardness of the ductile cast iron was greater than 52 HRC; with the previous pass laser power of 1 200 W and scanning speed of 6 mm·s-1, and the latter pass laser power of 1 300 W and scanning speed of 8 mm·s-1, the surface hardness of the ductile cast iron was relatively high, and the fluctuation of hardness was relatively small; the width of the softened zone at 0.3 mm from the surface was about 4.0 mm, and the hardened layer depths in the non-softened zone and in the softened zone were about 1.0, 0.5 mm, respectively.
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