Papers by Author: Dong Ying Ju

Abstract: As a novel surface strengthening technique, Water-jet cavitation peening has been applied to improve the fatigue life by inducing the compressive residual stress in the near surface layer of mechanical components. Compared with conventional shot peening, one advantage of WCP is that the complicated and tiny surface can be peened more easily. In this study, the small holes with various diameters and depths in the SUS304 stainless steel specimens were treated by WCP. In order to estimate its strengthening capability to the small holes, the surface residual stress and the depth distributions in the near surface layer of the small holes were measured by X-ray diffraction method. The experimental results show that WCP can successfully improve the near surface compressive residual stress of small holes. The maximum of surface compressive residual stress of WCP state is up to around –450 MPa. The dept of compressive residual stress zone is up to around 125 μm.

Abstract: Experimental study on downstream rolling process for twin roll casting magnesium alloy strip was carried out. The microstructure of the casting and rolling strip were observed by OM, SEM, and TEM. The effects of homogenizing process, the parameters of rolling process and annealing after rolled on secondary rolling of AZ31B casting strip were analyzed. The results show that the twin roll casting strip are suitable for rolling at 380-420 °C after homogenizing at 400 oC for two hours, the reduction rate of each pass should be controlled lower than 18%. Mg alloy sheets with thickness from 0.3-1 mm can be produced in once annealing period in this way. By controlling the pass reduction and rolling temperature, the fabricated AZ31B Mg alloy sheets can achieve both high strength and good plasticity by grain refining below 10μm. The mechanical properties of rolled strip after annealing at 400 oC for 0.5h-1h could get tensile strength 220MPa and meantime elongation rate 20%.

Abstract: Recently, the surface mounting devices (SMD) have been rapidly developed for miniaturisation of electronic applications such as cellular phones, cameras, computers, etc. Low temperature sintering NiCuZn and MnCuZn ferrite was employed at most cases due to its co-firability with Ag (below 960OC).The purpose of this study is to fabricate NiCuZn and MnCuZn ferrite sintered body with high-strength and high-frequency magnetic properties. Following is the procedure: firstly, NiCuZn and MnCuZn ferrite powder were synthesized under CO2 atmosphere at 500 OC from the mixed doxalate synthesized by liquid phase deposition method; then a small amount of boric acid [H3BO3] was added to the powder, and the NiCuZn and MnCuZn ferrite powder compact were prepared with Newton press and CIP methods; finally, NiCuZn and MnCuZn ferrite sintered body was fabricated by sintering at 900 OC under CO2 atmosphere. The effect of boric acid additives on growth of particle and sintering temperature were discussed. The high frequency magnetic properties and density as well as bending strength of the NiCuZn and MnCuZn sintered magnet with various adding of H3BO3 as well as at various sintered temperatures were evaluated by using Impedance analyzer, TMA (Thermo Mechanical Analyzer), TEM (Transmission Electronic Microscopy) and 3-point bending test. From the evaluated results, the most suitable producing conditions were determined and NiCuZn ferrite with wider scope of high frequency and high bending strength were obtained.

Abstract: Water cavitation peening (WCP) with aeration is a novel surface enhancement method. A new ventilation nozzle with aeration is adopted to improve the process capability of WCP by increasing the impact pressure induced by the bubble collapse on the surface of components. In this study, in order to investigate the process capability of the WCP with aeration, a standard N-type almen strips of spring steel SAE 1070 was treated by WCP with various process conditions, and the arc height value and the residual stress in the superficial layers were measured by X-ray diffraction method. The optimal fluxes of aeration and the optimal standoff distances were achieved.

Abstract: A study of a catalyst for NOx reduction used for a diesel engine of cars is done frequently currently. In the recent studies, Ag/Al2O3 catalyst system is considered to be a candidate for practical use. The ceramics molding body (ceramics honeycomb etc.) is used as a border brim of these catalysts usually. However, there is a problem. Strength to thermal shock is low. Moreover, it is not easy to go up in the center part until the catalyst operating temperature because thermal conductivity is bad and is obstructed working of the catalyst to the first stage of the reaction for that. In this research, the Ag/Al2O3 catalyst system was coated to stainless steel (SUS304) substrate that forming it is easy and heat conduction rate is good. The aluminum nitrate was used as a binder. After the binder is dissolved in the ion exchange water, the stainless steel substrate coated Ag/Al2O3 catalyst is sintered. In order to optimize coating conditions, the heating temperature, the sintering time, the density of the binder and the amount of the coating powder addition as well as the dip coating frequency were changed, respectively.

Abstract: Magnetite nanoparticles were obtained by liquid phase precipitation method in which the pH value of [FeCl2⋅4H2O], [FeCl3⋅6H2O] and [NaOH] solution were controlled. Then the magnetite nanoparticle were scattered in water solution and put in [Na2SiO3] and [HCl], the resultant of reaction SiO2 can be coated on magnetite nanoparticles surface. The morphology and magnetite properties of the coated nanoparticles were evaluated by XRD, TEM, FTIR and VSM. The SiO2 thin film with nanometer size was coated on surface of nanoparticle, so that the magnetic value is decreased with the coating thickness increasing.

Abstract: Water cavitation peening (WCP) with aeration is a recent promising method in the surface enhancement technique, which can induce compressive residual stress in the near surface of mechanical components by the bubble collapse on the surface of components in the similar way as conventional shot peening. In this paper, the effect of WCP on fatigue crack growth behavior was investigated in single-edge-notched flat tensile specimens of S45C steel. The notched specimens were treated by WCP, and the compressive residual stress distributions in the near surface layer were measured by X-ray diffraction method. The tension-tension (R = Smin/Smax = 0.1, f = 10 Hz) fatigue tests were conducted. A Shimadzu servo-hydraulic fatigue test machine with in-situ observation by JSM-5410LV scanning microscope was used for all testing. Compared with those without WCP treatment, WCP can induce the residual compressive stress in the near surface layer, and delay the fatigue crack initiation, and decrease the rate of fatigue crack growth.

Abstract: Strategy and function of a new developed FEM code COSMAP(COmputer Simulation of MAterial Process) for surface hardening during of thermo-mechanical processing, including heat treatment, carbonizing and nitriding, is briefly introduced in this paper. The simulation code is developed based on the metallo-thermo-mechanical theory considering the coupled equations of diffusion, heat conduction, inelastic stresses and kinetics of phase transformation. Some examples of simulation and the experimental verification for carbonized quenching, carbonizednitrided- quenching process of a cylinder and ring as well as a gear are illustrated, and comparison of the simulated values of distortion, residual stresses and profile of induced phases with the experimental data is made with some discussions.

Abstract: Mechanism of transformation plasticity (TP) is discussed from continuum mechanics viewpoint, and derivation of TP law from the unified thermo-mechanical and transformation plasticity constitutive equation. Result of identified TP coefficient for a chromium steel (JIS SCr420) by use of multi-functional testing machine is introduced as one of the material data together with other data to the simulation of a quenching process by use of newly developed code COSMAP. The simulated distribution of temperature, phases and stress/distortion are compared with the experimentally measured values to verify the accuracy.

Abstract: As one of the activities carried out by our group of IMS-VHT (Virtual Heat Treatment tool for monitoring and optimising HT process), results of a benchmark project on the simulation of carburized quenching process is summarized. Several programs available for taking into account the metallo-thermo-mechanical coupling had been employed for the simulation for a cylinder, ring as well as a helical gear by use of common data of material properties and cooling characteristics. Comparison of the simulated values of distortion, residual stresses and profile of induced phases with the experimental data is made with some discussions.