Papers by Author: Hong Bo Li

Abstract: Aluminum nitride (AlN) is a stoichiometric compound with the hexagonal wurtzite structure. AlN has excellent thermal conductivity and good properties as electronic insulator. It displays good mechanical resistance up to elevated temperatures and is resistant against corrosion by molten metals. Bulk AlN may therefore be used as a refractory structural material as well as a substrate for high power microelectronic devices. However, it is very difficult for sintering high-density AlN at lower temperature than 1800°C. Nano-sized AlN powders were sintered by hot press sintering at low temperature of 1500~1700°C and mechanical properties were investigated. β-AlN and β-Al2O3 were detected when the sintering temperature is 1600°C. The phase transition β-AlN to α-AlN was discovered at a 1700°C sintering temperature. Relative density and average grain size were increasing with the increasing of sintering temperature, and fracture form is intercrystalline crack in 1500°C and transcrystalline crack in 1700°C. 97.3% relative density and 850nm average grain size were deserved at 1700°C.

Abstract: Both experimentation and calculation of the Yada model modulus of casting AZ31 magnesium alloy are provided in this paper. Based on revised Yada model, the microstructure simulation of precision forming inner gear is performed using Superform software. On the basisi of result analysis, the microstructure transformation of the casting magnesium alloy in the hot-forming process is forecasted in terms of revised Yada model.

Abstract: Si3N4-Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering. The mass loss, relative density and average grain size increase with increasing sintering temperature. The average grain size is less than 500nm when the sintering temperature is lower than 1700°C. Friction coefficient is from 0.35 for sintering temperature 1650°C to 0.74 for 1600°C when the composites were worn by silicon nitride bearing ball. High hardness of 21.5GPa and relative wear resistance of 32 were observed at a sintering temperature of 1600°C. The wear surface are very smooth and no grooving and subsurface fracture, which indicates that they are worn slightly.

Abstract: Nano-structured Si2N2O-Sialon composites were prepared by liquid phase sintering method with the amorphous nano-sized Si3N4 and nano-sized AlN powders. Nano-sized Al2O3 and Y2O3 powders synthesized by polyacrylamide gel method were introduced as additives. SEM examination shows the grain size of sintered body to be less than 80nm. Superplastic extrusion can be undertaken at 1550°C with a high velocity of 0.5mm/min and a big extrusion ratio of 3.57. There are a lot ring-shaped structure just like “annual ring” of trees in radial fracture and clear flow trace in axial fracture, all which is very similar to deformed metal materials and approve good superplastic deformation capability of Si2N2O-Sialon composites.

Abstract: The superplasticity of magnesium alloy is important in industrial application. However the superplastic deformation of casting magnesium alloy is hard to be realized. In this paper, the stress–strain behaviors of casting AZ31 magnesium alloy with various strain rates at different deformation temperatures were investigated. The alloy was tested in the tensile condition with initial grain size of 25μm. It was found that the elongation of the alloy at 400°C with ε& = 4.25×10-4 s-1 is almost 200%. According to the results of uniaxial tensile experiment, the alloy exhibited superplastic deformation behavior with the slow stain rate in a temperature range of 350 to 450°C. The microstructures deformed and undeformed samples were observed with aid of optical microscope.

Abstract: Based on the two-stage forming technology, the casting AZ31 Magnesium alloy bar was forged into cylindrical straight inner gear between the temperature 250°C-400°C. At 250°C, the teeth of the inner gear are almost formed. But there are some cyclic cracks on the surface of the sample. When improving the temperature above 300°C, the surface quality of the sample has greatly improved. According to the result of this experiment, the best temperature range for forging AZ31 magnesium gear is 280°C to 380°C.The forming load gradually reduced with the temperature improved. At 250°C, the forming load is 93t. At the 400°C, the forming load reduces to 80t.The initial grain size of AZ31 magnesium alloy bar is 22μm. The microstructure evolution during the warm deformation was observed by optical microscopy (OM). It is demonstrated that the grain refinement happened during the deformation process.