Papers by Author: Xuan Hui Qu

Abstract: Characterizations of a liquid polycarbosilane used as SiC matrix precursor were investigated by TG-DTA, FTIR, XRD, etc, which indicated the feasibility of using LPCS as precursor for CLVD process to prepare C/SiC composites. The results show that the inorganic conversion of LPCS to SiC is almost completed at 900 °C, and the crystallization of β-SiC appears at 855 °C approximately. As the temperature increases, the deposit becomes more pure and the crystallinity of β-SiC also increases. The atomic ratio of C/Si in the deposit attained at 1200 °C is near-stoichiometric, the crystallite size of β-SiC is about 33.4 nm.

Abstract: For producing higher density PM parts a new method, High-Velocity Compaction process with additional upper relaxation assist (URA) device is presented in the paper. Using zinc stearate as a die wall lubricant, iron powder was pressed with and without upper relaxation assist device of mass 0.14 Kg focusing to investigate the density and mechanical properties. To explain a compaction process in loading stage a conservation of momentum principle has been introduced during collision of hammer, upper piston and upper relaxation assist device in die compaction. To observe the morphological characteristics and mechanical properties, a scanning electron microscopy (SEM) and computer controlled universal testing machine were used. The experimental results showed that the samples compacted with URA device had an improved green density and mechanical properties compared to the samples compacted in the absence of URA device.

Abstract: Ti-24Nb-4Zr-7.9Sn alloy was prepared by Powder Metallurgy (PM) and Spark Plasma Sintering (SPS) using titanium hydride powder, niobium powder, zirconium powder and tin powder as raw materials. The effect of sintering process on microstructure and mechanical properties was investigated by mechanical measurement and SEM. The results showed that the best sintering process by PM was at 12500C for 2 h. The relative density, tensile strength and elongation of the alloy reached 97.2%, 705MPa and 6.2%, respectively. The microstructure was a typical Widmannstatten microstructure, which possessed β-matrix and α-precipitation. The best process by SPS was at 12500C. The relative density, tensile strength and elongation of the alloy sintered by SPS reached 99.4%, 788.5MPa and 6.4%, respectively. The grain size was about 100µm and the microstructure was uniform. The fracture morphology of the alloy was ductile rupture. Compared to PM, Ti-24Nb-4Zr-7.9Sn alloy fabricated by SPS exhibited better comprehensive properties and more uniform microstructure.

Abstract: Micro powder injection molding has been accepted as a potential forming technology for large-scale production of miniature components in industries. However, mold filling capability is often poor due to the blind hole structure of the die cavity, which restricts the widespread application of this technique. In this paper, ZrO₂ micro gears with addendum circle ranging from 900μm to 200μm were studied. Instead of using traditional PW-HDPE-SA binder as in powder injection molding, thermoplastic binder system is optimized here for better replication of micro gear. It is found that the dimensional accuracy and demold after injection molding can be improved by utilizing proper binder system. ZrO₂ feedstock with a powder loading of 46vol.% and the binder system consisting of BW, HDPE and SA was successfully injection molded and demolded, which indicates good mold filling capability and sufficient demolding strength. The binder system was successfully removed by debinding process in two steps, solvent debinding followed by thermal debinding. Debound components free of defects were obtained. Keywords: Micro powder injection molding, ZrO₂ feedstock, Binder system, Molding filling capability, Demolding strength

Abstract: The powder injection molding (PIM) combines the thermoplastic and powder metallurgy technologies to manufacture intricate parts to nearly shape. The powder segregation is a special effect arising in PIM different from than the pure polymer injection. The two-fluid flow model is used to describe the flows of binder and powder so as to realize the prediction of powder segregation effect in PIM injection. To take into account binder–powder interaction, the mixture model of inter-phase exchange term is introduced in the two-fluid model. The two-fluid equations largely resemble those for single-fluid flow but are represented in terms of the mixture density and velocity. The volume fraction for each dispersed phase is solved from a phase continuity equation. As the key to calculate the phase exchange term, the drag coefficient is defined as a function of mixture viscosity. The effective viscosity of binder and powder are agreed with the additive principle. The volume fractions of binder and powder give directly the evolution of segregation during the injection course. Segregation during PIM injection was simulated by software CFX and results were compared with experimental data with good agreement. The basic reasons that caused segregation are identified as boundary effect, differences in density and viscosity of binder and powder. The segregation zones are well predicted. This showed that the two-fluid model is valid and efficient for the prediction of the segregation effects in PIM injection.

Abstract: A binder comprising of 60% PW (paraffin wax), 10% HDPE (high-density polyethylene), 12% PS (polystyrene), 13% PPR (random-polypropylene), and 5% SA (stearic acid) has been selected and successfully employed to fabricate the diamond preforms possessing high strength and well shape retention capability. Various effects of processing methods on the properties of the diamond preforms have been investigated. The results indicate that the binder consisting of PW-HDPE-PS-PPR-SA possesses excellent comprehensive rheological properties and is more appropriate as the organic vehicle for fabricating porous diamond preform. It has been evaluated that the mixing of 140m size diamond particles with 20m size diamond particles in the weight proportion of 3:1 provides the best particle size distribution for preparation of diamond preforms. It is also observed that impregnation of preform with 1.7wt (SiO2)% water glass and subsequently heat-treated in vacuum at 800°C for 40min yields in high strength (with the strength of 7.1 MPa) diamond preform for infiltration. The strengthened preforms have been observed to possess an excellent shape maintaining capability. It is concluded that the diamond-copper composites fabricated by utilizing this high strength preform through hot-press infiltration method have high relative density of 96.4% and thermal conductivity of 300W.m-1.K-1.

Abstract: Metal powders behave high strain rate, viscous effect and first hardening then softening deformation characteristics during the forming process of high velocity compaction. The characteristics of high strain rate and viscous effect are described by composite nonlinear viscoelastic body which consists of non-linear spring, linear spring and high strain rate Maxwell element. The deformation characteristics of first hardening then softening can be described by changing the degree of the term of nonlinear spring from greater than 1 to less than 1. Constitutive relation of metal powder in high velocity compaction is established. The degree of the term of nonlinear spring is considered as a function of strain. The function is approximated by linear, quadratic and cubic polynomial and the stress-strain curves are analyzed respectively. Analysis results indicate that the constitutive equation can describe the deformation characteristics of metal powder in high velocity compaction.

Abstract: A novel Sn-2.5Ag-2.0Ni alloy has been developed for soldering of SiCp/Al composites substrate with various types of Ni coatings. An investigation about electroplated Ni layer, electroless Ni-5 wt.% P, Ni-10wt.% P and Ni-B layers has been carried out. It is found that the solder joints possess a single intermetallic compound (IMC) Ni3Sn4, which coarsens with an increase in aging time. The formation of Ni2SnP has been observed to significantly affect the reliability of the solder joints. But the formation of Ni2SnP can be suppressed by lowering the P contents in as-deposited Ni coatings. It has been also found that the thermal stresses generated in solder joint increases with the decrease of P contents in Ni-P layer. Furthermore, the concentration of thermal stresses in the electroplated Ni solder joint is found to be higher than that in other three electroless Ni layers. Out of four as-deposited Ni coatings, the Ni-B layer exhibits good wettability with solder and low IMC growth rate during aging. Also, the shear strength of solder joint decreases with an increase in aging time and Ni-B solder joint demonstrates the highest shear strength after long term aging.

Abstract: In this study, the effect of various binders’ compositions on the feedstock of pre-alloyed tungsten heavy alloys (WHAs) powders has been investigated. Four kinds of wax-based binders were prepared from paraffin wax (PW), high-density polyethylene (HDPE), polypropylene (PP) and stearic acid (SA), and the characteristic of each feedstock was investigated at various temperatures and shear rates. It was found that all the feedstocks exhibited pseudoplastic flow behavior. Feedstock having multi-polymer components showed better rheological properties than those having mono-polymer because of good wettability between powder and binder, and less sensitivity to temperature and shear rates. This could be due to the molecular weight and length of molecular chain of PP and HDPE.

Abstract: The high temperature mechanical properties of dual phase heavy metal of 95W-3.5Ni-1.5Fe alloy were investigated in tension. The specimens were prepared by liquid phase sintering. Yield strength decreased and ductility increased as the testing temperature was increased to 300°C, reached a plateau at between 300 and 500°C and then decreased considerably. The fracture modes of alloys when deformed at room temperature were a mixture of intergranular fracture and transgranular cleavage. As the temperature was increased, the percentage of intergranular cleavage increased, although transgranular fracture also remained. At higher temperatures, substantial reduction in ductility and in yield strength was a result of loss of bonding strength between tungsten grains and matrix phase.