Papers by Author: Zhi Ming Du

Abstract: Mg-Zn-Zr-Y billets with different mass fraction of Y (wt%(Y)=0.2%、0.5%、1.0%、1.5%、2.0%) was prepared by permanent mold casting. The increase in Y content has shown grain refinement effects on the microstructure morphologies of Mg–Zn–Y–Zr alloys. When the content of Y achieves 1.0 wt%, the grain refinement effect of the Y is most obvious than any more contents of the Y content. In the test result of XRD, the type of precipitated phase in Mg-Zn-Zr-Y alloys is related to atomic ratio of Y/Zn. With content of Y increases, atomic ratio of Y/Zn increases,the precipitated phase in alloy is changed from Phase I to Phase W. Tensile strength and extension rate of alloy increase with the increasing of Y content; When Y content reaches 1.0%, mechanical property reaches maximum value.When content of Y exceeds 1.0%, with the increasing of Y content, mechanical property of alloy declines gradually.

Abstract: Plasma spray zirconia gradient thermal barrier coating (TBC) and the traditional double-layer structure thermal barrier coating were both studied in this paper. NiCoCrAlY alloy powder was used as bottom bonding materials, 8YSZ (8mol% yttrium stabilized zirconia) was used as top layer materials and the different proportions mixed powder of 8YSZ and NiCoCrAlY was used as gradient layers materials. Bonding strength of thickness gradient coating was measured by tensile experiment. Thermal cycle experiment was carried on to test the thermal shock resistance of TBC. The microstructure after thermal shock experiment was observed to analyze the failure mechanism of TBC. The research result shows that the bonding strength and thermal shock resistance of gradient TBC are much better than the traditional double-layer structure TBC. With the increasing of top layer thickness, thermal shock life of TBC substantially reduces. With the decreasing of ZrO₂ composition proportion in gradient layer, thermal shock life of TBC significantly rises. This result has important significance for the future study of thick TBC.

Abstract: The sintering temperature of 8YSZ (8mol% yttrium stabilized zirconia) is very high, usually above 1500°C. BAS (BaO-Al2O3-SiO2) microcrystalline glass can be used as sintering aids to reduce the sintering temperature of 8YSZ. In this research, large proportion (30-50wt%) BAS was added in 8YSZ to observe the influence of the sintering aids. The change rules of mechanical properties such as density test, bending strength and toughness in different material component and sintering process were researched. The results show that physical and mechanical performance improvement with the increase of sintering temperature and sintering time, density and bending strength decreases with the increase of BAS mass fraction. The optimal mechanical properties are obtained by sintering temperature 1300°C, sintering time 0.5h and 30wt% BAS.

Abstract: Wearability of SiC_p reinforced aluminum matrix composites creeper treads was studied. Wear resistance of these creeper treads was tested by sliding friction pin-disk tester at room temperature. Friction morphology was observed and the friction mechanism of these creeper treads was investigated. The experiment parameters such as SiC_p particle size, volume fraction, mixing temperature, mixing time were adjusted and the best parameters were found through the tests. The results showed that the creeper tread had the most excellent wearability, high strength and hardness with 875r/min mixing speed, 40min~60min mixing time, 660°C~700°C mixing temperature, 7μm SiC_p particle size and the 10vol% SiC_p volume fraction. There were no microscopic holes and other defects in these dense composite materials. The interface between SiC_p and Al matrix was clean and straight, where Al₄C₃ brittle phase wasn't observed.

Abstract: Microstructure of creeper treads is inhomogeneous when they are produced by way of common casting, which lead to lower mechanical properties. Compound loading and feeding are introduced into the technology of liquid forging. Defects of filling incompletely, shrinkage porosities and cavities are usually appeared in raised part of creeper treads. With feeding amount increased above 5mm, plastic deformation also would be increased, so the microstructure and mechanical properties of raised part were improved and those defects disappeared meanwhile. SiC_P/2A50 composite materials were used in order to enhance abrasive resistance and extend their service life. Creeper tread produced by compound loading and feeding could obtain homogeneous and fine microstructure. Average tensile strength and hardness of specimen reached 360MPa and HV115.97 respectively after (500°C×3h) solution treatment, (20min) quenching and (160°C×10h) artificial aging. The results show that inhomogeneous character of microstructure and mechanical properties of creeper treads could be overcome by compound loading and feeding.

Abstract: The 6061Al matrix composites reinforced with SnO₂-coated aluminum borate whisker were fabricated using squeeze casting method. The composite were extruded successfully at 350°Cwith different die angles. The experiment results illustrate that the die angle have important influences on the extrusion process. The surface quality of extruded composites was improved and the extrusion load was reduced with die angle increasing from 30 to 60 degree. The <111> and <100> textures for aluminum matrix were formed in all extruded composites. The dispersion degree of the <111> texture, the alignment degree and the average length of whiskers increases with die angle increasing. There is a suitable die angle for the optimization of the extrusion process of the present composite.

Abstract: The 6061Al matrix composites reinforced with SnO₂-coated aluminum borate whisker were fabricated using squeeze casting method. The composites with different SnO₂ coating contents were extruded successfully at 350°C with the extrusion rate of 18mm/s. The results show that the Sn particles at the interface as liquid state at extrusion temperature result in the reduction of the extrusion load and the decrease of the probability of whisker fracture during hot deformation process. Moreover, unlike many surface cracks can be seen in the extruded composite without whisker coating, the surface crack-free extruded rods can be obtained for composites with whisker coating. The ABOW/SnO/6061Al composites tensile properties increased obviously after extrusion.

Abstract: By semi-solid aluminum alloy compression test, deformation mechanism of semi-solid aluminum alloy was investigated, the thixotropic strength existing in semi-solid aluminum alloy was verified and the influences of temperature and strain rate on the deformation mechanism and thixotropic strength were discussed. The results show that there are 4 stages in the process of compression deformation for semi-solid aluminum alloy: stress swift growth stage, stress drop stage, steady stress stage and stress gradual growth stage. The semi-solid billet has a thixotropic strength, which is sensitive to temperature and strain rate. The higher the temperature or the higher the strain rate is, the smaller the thixotropic strength of the semi-solid alloy.

Abstract: In this paper, thixoforging of a magazine plate made of AZ91D magnesium alloy were investigated by means of numerical simulation and experiments. Numerical simulation results show that with increasing punch displacement, local bending, formation of a concave shell part and bulk plastic deformation occurs in billet continuously. Equivalent strain and stress increase and the temperature of the semi-solid billet decreases. When the temperature of the semi-solid billet or the die temperature is elevated, equivalent stain and stress decrease. Optimal technological parameters such as a billet temperature of 545°C, die temperature of 450°C and punch velocity of 15 mm/s were obtained by numerical simulation. Experimental results demonstrate that magazine plates with high mechanical properties such as tensile strength of 316.8 MPa, yield strength of 228.3 MPa and elongation of 12.6 % can be manufactured successfully when the optimal technological parameters selected according to the results of numerical simulation are applied.

Abstract: High quality magnesium alloy sheets with non-dendritic microstructure can be manufactured by the technique which incorporates semi-solid processing with continuous roll stripcasting. In particular, the process is believed to be cost-effective in mass production of magnesium sheet products. The processing parameters, such as roll speed and temperature, have significant influence on roll strip-casting process. In this paper, the commercial software, DEFORM-3D, for deformation simulation of bulk materials was used to carry out the simulation of roll strip-casting process for AZ91D magnesium alloy in semi-solid state, and the distribution of equivalent stress, equivalent strain and temperature was obtained. Based on the results, the optimization of processing parameters was discussed.