Papers by Author: Jun Ting Luo

Abstract: On the basis of the uniformly distributed load assumption, powder flexible cavity forming of cup shell is analyzed by MSC.Marc. The stress and strain parameters for different deformation zone are obtained. The technology is compared with rigid die deep drawing by using of point tracer method. The results prove that the forming limit can be improved and defects can be prevented when parts are formed by powder flexible cavity forming technology, which present the theoretical basis for practical application for powder flexible cavity forming technology.

Abstract: The paper mainly focused on the two issues that restricted the practical application of superplastic ceramics, which were the low strain rate in superplastic forming as well as resulted severely cavitation in deformed materials. The alumina-based composites Al₂O₃-ZrO₂ (3Y) and Al₂O₃-30mol%ZrO₂(3Y)-30mol%MgAl₂O₄ (AZ30S30) were selected as research materials. The nano-sized composite powders were synthesized by heating of ethanol-aqueous salt solutions method. The superplastic forming tests under the compressive stress state were carried out to evaluate the superplastic formability of the as-sintered materials. The results demonstrate that the following conditions are the essentials for attaining high-strain-rate superplastic forming in alumina based ceramic composites: reduction in the initial grain size by second phase dispersion and insurance of a homogeneous microstructure, enhanced diffusivity by co-doped certain elements, suppressed dynamic grain growth in deformation, as well as provide new rate-controlling accommodation process in superplastic forming. The results also indicate during the superplastic forming the cavitation damage was eliminated because of compression stress state, which ensured the mechanical properties after deformation. Therefore, the postdeformation mechanical properties after superplastic forming were enhanced in some extent.

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: The MoSi2 nanocrystal was prepared by mechanical alloying (MA) large particle-sized starting powders, in which the milling time is much longer than usual MA time. It was found that the Mo-Si powder mixture mixed at stoichiometry proportion forms α-MoSi2 and β-MoSi2 in the MDR mode rather than pure α-MoSi2 in the SHS mode. The grain size of MoSi2, calculated using Scherrer′s formula, is 18nm when milled for 96h, and decreases to 12nm when further milling to 144 h. This is because that the milling balls provide enough energy to refine most of the rough crystal grain. The average grain size increased to 15nm when milled for 192 h, which indicates that further expand time could not refine the crystal grain while cause the growth of a part of the crystal grain. The particle size of MoSi2 is about 0.5μm when milled for 96 h and the agglomerating phenomenon is severe. The particle size of MoSi2 decreases to 0.4μm and releases the agglomerating phenomenon with the milling time for 144 h.

Abstract: Y2O3 nano-phase powders were prepared by polyacrylamide gel method. The mechanism of polyacrylamide gel process and its calcinations procedure were analyzed. The high purity yttria powders can be produced by polyacrylamide gel method at the calcination temperature of 850°C, which is about 100°C lower than prepared by precipitation method. The microstructure of yttria powders prepared in different proportion of monomer and network agent was investigated. When the mole proportion of monomer and network agent is 5:1, the average size of grains is less than 20nm. Some grains grow abnormally if the proportion deviate from this value and there are a lot agglomerates. Development of such an inexpensive and easy technique for synthesis of high purity yttria nano-phase powders can be of commercial significance.

Abstract: The precision forming technology developed rapidly during passing two decades, however technologies of precision plastic forming the parts with deeper hole are far behind developed countries. The warm backward extrusion-ironing forming technology was presented for precision forming of non-circular hole joint in this paper. The forming process and parameter variable trend were simulated by finite element method, which the software MSC.Marc was applied. The forming die was designed and the forming experiment was finished. The products were deserved with good quality and performance. The feasibility of the forming technology is proved by experimental results and numerical simulation.

Abstract: The cold extrusion forming of copper/aluminum clad composite based on the low pressure casting billet is presented in this paper. The technology was studied by using the experimental investigation and the finite element method. The drop-in phenomenon occurred in aluminum during the extrusion forming process. The product will be having good quality when the extrusion ratio is 5.45 and the extrusion modular angle is 30°. The crack appeared on the head of product when the extrusion ratio and extrusion angle is large than the aforementioned values.

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: The Si3N4- Si2N2O composites were fabricated with amorphous nano-sized silicon nitride powders by the liquid phase sintering(LPS) method. The sintering temperatures ranged from 1500°C to 1700°C. Microstructure and component of the composites were performed by scanning electron microscopy (SEM) and X-ray diffraction (XRD). Results show that sintered body consists of Si2N2O and β-Si3N4, with an average grain size about 1μm. The maximum value of flexural strength of the material is 680MPa when sintered at 1700°C. Transcrystalline cracking is the main fracture mechanism of the composites.

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.