Papers by Author: Jing Liu

Abstract: The weld toe surface and its nearby area of welded cruciform joints were treated by ultrasonic impact. Under the same stress concentration and after heat treatment to eliminate residual stress, the effect of residual stress on the fatigue life of joint was researched. The fatigue tests are performed on the joints of 16MnR both for the un-treated and treated joints by using EHF-EM200K2-070-1A type fatigue tester when the load ratio is 0.1, frequency is 10Hz. The experimental results indicate that the severe plastic deformation in the vicinity of weld toe surface was formed by impact treating for 2 minutes, the thickness of the plastic deformation layer is about 60μm. Residual tensile stress in the weld toe surface can be changed to residual compressive stress by impact treatment. The fatigue life of welded joint is 0.260×10⁶ cycle, and the fatigue life of treated joint is 0.499×10⁶ cycle. Compared to the un-treated joint, the fatigue life of treated joint has been increased by 91.92%. The residual stress contributed to fatigue life is about 16%. Residual stress has great effect on the fatigue life of welded cruciform joint.

Abstract: The weld seam and weld toe surface were treated by ultrasonic impact method. The contrast corrosion tests were performed in 3.5% NaCl aqueous solution both for the un-treated and treated joint. The experimental results indicate that the severe plastic deformation on the surface of weld seam and weld toe were formed by ultrasonic impact for different time, the maximum depth of the plastic deformation layer is about 300μm. Residual tensile stress in the surface of weld seam and weld toe can be changed to residual compressive stress by impact treatment, and the grain in the surface of welded joint could be refined. Compared to the un-treated joint, when the impact current is 1.2A and 1.5A, respectively, the corrosion rate of treated joint was reduced by 22.67%, 54.59%, 43.99% and 33.92%, 56.54%, 49.29%, respectively for 10, 20 and 30 min. treating. The corrosion resistance of welded joint has a certain relationship with the residual stress on the surface of welded joint. The ultrasonic impact treatment has distinct effect on the corrosion resistance of 16MnR welded joint.

Abstract: With increasing of the speed of the train, the requirement of the performance of the braking friction materials is more and more higher. It is urgent to find new braking materials to satisfy the rigorous using environment. Cf/SiC has the broad application prospects when it is used as a new kind of braking material. On this article, in order to shorten preparation cycle and reduced production costs, short carbon fiber is substituted continuous carbon fiber felt. Cf/SiC brake materials were prepared by the way of molding compression-pressureless sintering with resin binder. The effect of short carbon fiber dispersion on bending properties of brake materials was researched. The experimental results show that the bending strength of Cf/SiC brake materials, which manufactured by dispersing short carbon fibers, is higher than the composites manufactured by using fiber bundle. When the distribution of carbon fiber in composites is in single fiber state, the interfacial strength between carbon fibers and matrix were increased. At the experimental condition, compared to the sample with fiber bundle, the bending strength of the specimen with dispersed fiber is increased 40.79%. The toughening mechanism of carbon fiber debonding and fiber pull were generated in the process of bending fracture. The fracture model of composites is pseudo-ductile type.

Abstract: Using the surface self-nanocrystalline technology can produce dense nano layer without the defects such as pores. The surface self-nanocrystalline technology can improve the micro hardness, abrasive resistance, fatigue durability, corrosion resistance and other combination properties. It is with wide application prospect in the industrial area. In this article, the application investigation of the surface self-nanocrystalline technology such as the surface mechanical polishing method, ultrasonic impacting method, ultra sound peening ect. improving the prehensive properties of the materials are summed up. And the study work of the surface self-nanocrystalline technology in the future is prospected here.

Abstract: The ultrasonic impact treatment on U70 raill steel was carried out under different technology by using HJ-II ultrasonic impact machine. The electric current was 1.5 A, the amplitude was 24 µm and the impact frequency was 10000 HZ during the treatment, the peening duration was 60 min. The influence of impact on microstructure was observed with the scanning electron microscope of 6360LA type and high resolution transmission electron microscope of JEM-2100 type, and the hardness of impacted surface was also researched before and after impact treatment. The wear resistance was tested by using M-2000 type wearing machine. The experimental results indicate that the surface strength of rail steel can be improved evidently by using ultrasonic impact treatment. The surface microstructure of the rail steel could be refined to nanoscale and the hardness and wear resistance of impact layer were significantly improved through the ultrasonic impact technology. Compared to the specimen without treatment, its surface hardness was increased 26.4%, and the wear resistance was increased 40.63%. The depth of plastic deformation zones of the specimen with 60 min treatment is nearly 100µm. In the plastic deformation zone, ferrite is refined to nanoscale grains so that the matrix is strengthened.

Abstract: Porous ceramics composite was prepared with mixed powders (Al, TiO₂ and B₂O₃) by SHS (Self-propagating High-temperature Synthesis) method. The effects of the particle size of aluminum power, the additives of SiC and the compacting pressure on the product properties and the pore size, as well as the combustion process in the Al₂O₃-TiB₂ system were researched. The effects of all factors on properties of the product were tested in this paper. The microstructure and mechanical properties (density and compressive resistance strength) were reported. It is learnt from the study that the relation between the open porosity and the pore size does not exist, the former depends on the green density, and the latter depends on the particle size of the aluminum powder. The porous ceramic composite with porosity is ranged from 45% to 68%, pore size is ranged from 1um to 400um, and the compressive strength is about 9.5 MPa. SEM observations of fracture surface suggest that the fracture mode is brittle fracture, and the microstructures of the porous of ceramic composite is very homogenous.

Abstract: With increasing of the speed of the train, the requirement of the performance of the braking friction materials is more and more higher. It is urgent to find new braking materials to satisfy the rigorous using environment. C_f/SiC has the broad application prospects when it is used as a new kind of braking material. On this article, in order to shorten preparation cycle and reduced production costs, short carbon fiber is substituted continuous carbon fiber felt. Cf/SiC brake materials were prepared by the way of molding compression-pressureless sintering with resin binder at 1800°C, 1850°C and 1900°C, respectively. The effect of sintering temperature on bending property of brake material was researched. The experimental results show that with the increase of sintering temperature, the bending strength of Cf/SiC brake materials rose first and then fell. If sintering temperature is too high, which will lead the crystal grain to grow up and the crack is easy to produce. Finally make the bending strength of composites reduced. When sintering at 1850°C, the bending strength of the specimen reached maximum. At the same experimental condition, the bending strength of Cf/SiC brake material sintered at 1850°C is 18.81MPa, compared to the sample sintered at 1800°C and 1900°C, the bending strength of the specimen is increased 14.56% and 5.26%, respectively. The toughening mechanism of carbon fiber debonding and fiber pull were generated in the process of bending fracture. The fracture model of composites is pseudo-ductile type.

Abstract: It’s a simple technology to manufacture big trimming die by hardfacing on the die insert of cast iron. Here, the chemical composition, microstructure, micro-hardness, wear resistance and wear mechanism of the hardfacing layer with a newly developed electrode have been studied. The experimental results indicate that the hardfacing technology has much more advantages and cheaper than that of traditional process. The chemical composition, microstructure, micro-hardness of the hardfacing metal are all satisfied for manufacturing the big trimming die by welding on die insert of cast iron. The wear resistant test of hardfacing layers were carried out by using M2000 type tester with two different electrodes. The wear resistance of new electrode hardfacing layer is higher than that of normal hardfacing electrode D322. The wear mechanism of hardfacing layer belongs to abrasive wear model. The service life of hardfacing die insert is as long as tool steel insert which was fully heat-treated. More than 25000 workpieces can be carried out by using the hardfacing die insert. The life of repaired cold die with new kind of hardfacing electrode bar can be raised up to 30~50% than that with the electrode D322 under the same condition.

Abstract: Because of the high-speed, high transport capacity, low power consumption and a lot of technical and economic advantages, high-speed rail way are universal importance. High-speed railway with the speed of more than 300k/h has been run in China. But fatigue cracking of wleded bogie structure is markedly increased with increasing the train speed. How to avoid fatigue destroy of wleded bogie structure and ensure the safety of transportation are urgent problems to be soved in engineering. A lot of research works have been done at home and abroad. Comparing with traditional surface engineering method, there are many advantages of ultrasonic impact , for example, simple operation, less power consumption, high efficiency, adapt to a wide range, easy to achieve automate production and so on. It is an effective way to surface strengthening of metallic materials. Plastic flow and grain refinement on the metal surface can be obtained by using ultrasonic impact method, and the residual compressive stress on the surface can also be formed. So the mechanical properties of metal surface can be greatly improved. It is new method used in the area of welded structure, especially in the welded bogie structure. It is a new research direction to research the surface nanocrystallization mechanisium of ultrasonic impact, the effect of ultrasonic impac on the fatigue properties and failure mechanisium of wled joint of bogie.

Abstract: CrZrCu is extensively used in spot welding due to their high electrical, thermal and mechanical performance. But when CrZrCu electrode is used in spot welding of galvanized steel sheet, the abrasion, corrosion and oxidation shorten the electrode life rapidly. The electrode is not able to meet the needs of spot welding of galvanized steel sheet .In the paper, double glow plasma discharge surface titanizing was carried out on CrZrCu alloy. The processes of double glow plasma discharge titanizing, surface alloying layer structure were also analyzed elementary. The wear resistance property of glow discharge plasma titanizing layer was also researched. The experimental results indicated that in the plasma titanizing alloying layer, the diffusion of titanium element toward the interior of the CrZrCu alloy substrate forms the intermetallic compound named Cu4Ti, which is beneficial to the reinforcement of CrZrCu substrate. The experimental results confirmed that double glow plasma discharge titanizing could improve the wear resistance of CrZrCu alloy greatly. Under the external load of 300N and 500N, the wear resistance of double glow plasma discharge titanizing layer is7 times and 8.5 times than that of CrZrCu.