Materials Science Forum Vols. 704-705

Abstract: Aiming at improving the anti-melting-loss property of the 8407 die steel, room-temperature hard anodization treatment was carried out for aluminized 8407 steel in order to form an oxide film on the surface. The influences of current density and oxidation time on the thickness and hardness of the film were discussed, and the optimum oxidation condition was obtained. The influence mechanism of oxidation condition on thickness and hardness of the film was analyzed. Furthermore, the sectional microstructure and surface morphology of the film were observed by means of OM and SEM, and the phases in the layer were determined by using XRD. The results show that a uniform, compact and tightly united Fe₃O₄·Al₂O₃ film has been obtained on the 8407 steel surface after aluminization and subsequent room-temperature hard anodization treatment. Its thickness is 24~67 μm and hardness is 335~382 HV. The optimum oxidation condition is as follows: current density being around 2.5 A/dm², oxidation time being around 60 minutes and oxidation temperature being around 25 ^°C.

Abstract: The sucrose was used as a carbonaceous precursor to fabricate composite alloy powder of Fe-Cr-C-TiFe by the precursor carbonization-composition process using the powder matirial of chromium, iron, tungsten, nickel and ferrotitanium. And the powder of Fe-Cr-C-TiFe was used to form a high-chromium iron-base composite coating on substrate of Q235 steel by plasma cladding process. The microstructure and hardness of the coating were investigated by scanning electron microscope (SEM), energy disperse spectroscopy (EDS), microhardness tester. Wear resistance of the coating was tested on wear tester at room temperature and high temperature 600°C compared with the base material Q235 steel and bearing steel. Results show that the coating consists of TiC, (Cr,Fe)₇C₃ and austenite and the hardness of the coating is 3.4 times as high as the base body Q235 steel. The wear resistance of the coating at room temperature is 11-15 times as high as the base body Q235 steel. The wear resistance of the coating at high temperature 600°C is 2.45 times as high as Q235 steel and is 1.5 times as high as bearing steel. The composite coating has excellent wear resistance because the reinforce phase TiC and (Cr,Fe) ₇C₃ in the coating have high hardness and good wear resistance. They can play key roles in process of friction and wear.

Abstract: A kind of TiB₂/Mo₂FeB₂ composite powders for reactive thermal spraying (RTS) and atmospheric plasma spraying (APS) were prepared using Mo powder, Fe-B alloy powder, Fe powder and TiB₂ powder. The TiB₂/Mo₂FeB₂ composite ceramic coatings were synthesized and deposited by APS and RTS on Q235 steel. Microstructure and phases were investigated by X-ray diffraction (XRD) and scanning electrical microscopy (SEM). The thermal shock resistance, wear resistance and corrosion resistance were characterized with the relevant lab testing equipment. The result indicates that the coatings are composed of the major phases Mo₂FeB₂, TiB₂ and α-Fe, a little of Fe₂O₃, FeO, and B₂O₃.The thermal shock times of RTS-sprayed coatings is 44, the corrosion resistance is 6.07 times than those of the substrate, respectively, while those of APS-sprayed coatings are 55 and 15.5, respectively. The mass loss of Q235 steel is 138.6g/m², while the mass loss of the coatings with 30% TiB₂ which were prepared by RTS is 28.0952g/m², the mass loss of the coatings with 30% which were prepared by APS is 15.4028g/m², when the testing load was 400N. Therefore, the coatings with 30%TiB₂ prepared by APS possess better wear resistance than the Q235 steel substrate.

Abstract: This paper was directed at improving the wear resistance of titanium alloy Ti6Al4V by double-glow plasma surface alloying (DPSA) technology. A molybdenized layer was successfully formed on Ti6Al4V by the DPSA treatments. The microstructure, composition distribution, phase structure and hardness were analyzed by SEM/EDS, XRD and vickers micro-hardness tester. The molybdenized layer was composed of Mo-deposited layer and Mo-diffusing layer. The average surface micro-hardness of the surface modified layer was 1170.8 HV_0.1, which was much higher than that of the Ti6Al4V substrate. Tribological characterization was carried out by the ball-on-disc tribometer at room temperature and 500°C. The molybdenized layer significantly reduced the wear rate of the Ti6Al4V alloy, which is only about 6.6% and 4.9% of those of the untreated specimens at room temperature and 500°C, respectively. Due to the good self-lubricating effect of Mo elements at high temperature conditions, the friction coefficient and wear rate of the molybdenized layer were both reduced at 500°C, compared to at room temperature.

Abstract: Experimental system of CCD real-time acquisition is developed according to high temperature property of plasma jet. Mathematical model of gray image and jet temperature distribution is established and basic principles of the CCD collection is analyzed. It realizes acquisition by a common image acquisition card in LabVIEW environment and real-time display of jet. Jet is collected on the computer image processing by using IMAQ Vision, and it intuitive analyzes the shape and temperature distribution according to pseudo-color image processing results of the jet temperature field. Experimental results deepen understanding of the complex nature of plasma jet, which have a guiding significance on the plasma technology in materials processing application.

Abstract: In this paper a molecular dynamics calculation model for the Nafion 117 membrane is constructed by Materials Studio (MS) software platform to study its micro-structure and transport properties. Based on the calculation model, cell structures of different water content of Nafion 117 membrane are obtained and the predicted density values of simulated cell are in good agreement with experimental data. Meanwhile, the diffusion processes of water molecules and hydrogen ions in the membrane are studied, respectively. The predicted diffusion coefficients of both water molecules and hydrogen ions increase with the water content, which agrees well with the variation trend of experimental data. The reasons for the deviation between numerical results and the experiment values in literature are analyzed.

Abstract: In micro-arc oxidation process, ceramic coating had a rapid growth all along by the way of constant current oxidation, and ceramic coating had a low roughness by the way of constant voltage oxidation. But few research focus on the mixed control process of constant current oxidation and constant voltage oxidation. In this paper we propose a variable parameter process that can combine the advantages of constant current and constant voltage oxidation for the first time. The growth kinetics of different technics was analyzed according to the change law of current and voltage. Surface topographs of ceramic coating were observed using scanning electron microscopy (SEM). The friction tests were carried out using a self-made friction tester. The results show that ceramic coating has an upper growth rate and a low roughness by the process of constant current+constant voltage oxidation. The ceramic coating has a high growth rate by process of constant voltage+constant current oxidation. The results of friction test indicate that the wear rate and roughness of ceramic coating are positive correlation at early stage of friction. While the ceramic coatings treated by different technics have the close wear rate at stable friction stage, which embodies the inner layer of ceramic coating has a well antiwear behavior.

Abstract: In current research, a series of visualization experiments simulating the action of ultrasonic vibration (UV) in metal slurry preparation process on fluid flow, grain nucleation and growth as well as its interaction with viscosity of fluids were conducted. In these visualization experiments, the metal slurry maker was substituted by a transparent cup while the liquid and semisolid slurry of metal were replaced by other fluids or mixture system with similar characteristics. Scaled-up UV was applied to the liquid or mixture systems. The simulation shows that UV can roll up the particles at the bottom of the cup and make the liquid convection intense below the radiating surface of sonotrode while weak above it. UV can break dendrites rapidly and distribute them in melt. High viscosity reduces the actual power transmitted into liquid, and higher viscosity requires higher inception power of UV.

Abstract: An analytical model based on multilayer structure with thermal expansion mismatch caused by temperature gradients was established to predict the residual stress in the system. The solution obtained from the model is independent of the number of layers. Three simplified models: bi-layer structure, coating system and film system with great compatibility are developed considering different engineering application. And the bilayer structure is verified by Stoney’s equation under the same conditions. Tri-layer coating system ZrO₂/ Al₂O₃/1Cr₁₈Ni₉Ti is established in order to research the effect of temperature variations on the residual stress between different layers. The results suggested the stress has obvious mutation in coating interface with different temperature variation. And the residual stress with different temperature variation in different layers is larger than that with identical temperature variation. Key words: multilayer structures; residual stress; analytical model; thermal expansion mismatch; temperature variation

Abstract: The creep of materials makes it difficulty to determine the limit load of component at high temperature. In this paper, limit load was obtained by finite element simulation according to isochronous stress versus cumulative strain data and creep failure criterion at high temperature. Firstly, isochronous stress versus cumulative strain data of P91 steel was generated. In finite element analysis code ABAQUS, isochronous stress versus cumulative strain data was replaced by equivalent elastic-plastic constitutive relation. Then, sustained load versus cumulative strain curves at high temperature during service was obtained after finite element simulation. At last, limit load at high temperature during given working hours was determined based on the restriction of total strain at key point of specific component. The restriction of total strain which could also be regarded as long-term fracture strain was discussed in this paper. Finite element simulation of limit load of component at high temperature is simple and reasonable. Limit load of complex component at high temperature during given working hours can be obtained by this method. Using this method, limit loads of a pipe with local wall thinning defect and a branch junction at high temperature during given working hours were obtained as examples.