Papers by Author: Ji Ming Zhou

Abstract: In this paper, continuous carbon fiber reinforced aluminum matrix (C_f/Al) composite plate with complex curved surface is fabricated by liquid-solid infiltration extrusion. The influence of fabrication parameters, such as the melting temperature, extrusion temperature and extrusion pressure on the C_f/Al composite plate is studied. The experimental results show that the liquid-solid infiltration extrusion process can be used to fabricate the C_f/Al composite with curved surface. The C_f/Al composite exhibits good infiltration and well-formed quality when the extrusion pressure is 50MPa, the extrusion temperature is 670°C and the melting temperature is 690°C. According to the microstructure observation, the carbon fiber uniformly distributes in the aluminum alloy matrix without damage and obvious defects. The density of the composite is decreased by 17.9%, from 2.71g/cm³ to 2.29g/cm³, which is lower than that of the matrix alloy, and the ultimate tensile strength is increased by 120% compared with the matrix alloy, from 127MPa to 279MPa.

Abstract: CNTs-grafted carbon fiber reinforced magnesium composites (CNTs-C_f/Mg) were successfully prepared by liquid-solid infiltration extrusion process, in which the carbon fiber cross-ply preform were grafted with carbon nanotubes (CNTs) by using the injection chemical vapor deposition (ICVD) technique. The mechanical properties of AZ91D matrix alloy and magnesium matrix composite at different states were tested and compared. The results show that the dendrites of the as-cast AZ91D alloy are transformed into granular grains after liquid-solid forming. The composite reinforced by carbon fiber with grafting nanotubes is beneficial to the one reinforced with carbon fiber but without grated CNTs. The nanotubes grafted on carbon fiber improve the bonding property of the fiber-matrix interface and protect the carbon fiber from degradation more effectively. The tensile strength of the CNTS-C_f/Mg composites is 28.3% higher than the C_f/Mg composites.

Abstract: Threshold pressure is a very important parameter for melt alloy successfully infiltrating into the porous preform. However, the precise measurement for threshold pressure is very difficult for the reason that infiltration process is undertaken very fast under extreme elevated temperature and high pressure without effective measuring devices to monitor it. A totally new measuring device was proposed and fabricated, which can be used to monitor the infiltration process “visually” and measure the threshold pressure directly at the same time. The infiltration speed can be controlled by adjusting the gas flow speed. The infiltration behavior of melt AZ91D alloy in Al2O3sf preform was researched at temperature of 800°C and pressure of 0.6 MPa. The optimized gas velocity was controlled at 25L/min. The degree of vacuum of the infiltration cavity was set 30kPa in experiments. The volume fraction of Al2O3sf was 10%. Under these conditions, the threshold pressure of melt AZ91D alloy into porous Al2O3sf preform was found to be related with vacuum degree in infiltration chamber, and it was about 30 kPa

Abstract: Magnesium matrix composites are attractive for weight critical application, such as automotive and aerospace components, because of its high specific strength and stiffness. Extrusion process directly following vacuum infiltration (EVI) can eliminate the porosity and obtain the well-aligned and uniform fiber distribution during the fabrication of Csf/AZ91D composite. This process combines the advantages of gas pressure infiltration, squeeze casting, and semi-solid extrusion. The mechanical properties of the magnesium are improved greatly by introducing the carbon fibers into the magnesium matrix through the EVI process. In the present study, the carbon short fiber reinforced magnesium matrix composites Csf/AZ91D were fabricated by EVI process. The microstructure and tensile property of Csf/AZ91D composites were investigated. The results showed that the microstructure of the composite presented a uniform distribution of carbon short fibers in the matrix and good interfacial integrity. The yield strength and stiffness of the composites increased with increasing carbon short fiber content, but at the cost of ductility. Nonetheless, Csf/AZ91D can keep relatively high ductility during the improvement of strength compared with reported composites in the literatures. Increasing carbon fiber content in the composite was not always beneficial to the ultimate tensile strength at the same magnitude. When the fiber content exceeds 10%, the matrix was not strengthened as greatly as under 10% fiber content. The yield strength improvement was attributed to (i) load-bearing effects due to the presence of carbon short fiber reinforcements; (ii) grain size refinement due to the large extrusion deformation; (iii) generation of dislocations to accommodate CTE mismatch between the matrix and the particles.

Abstract: Extrusion directly following vacuum infiltration is a special forming technique that combines the advantages of liquid metal infiltration and semisolid extrusion. The major advantages of this process are elimination of porosity and shrinkage, good surface finish, good dimensional accuracy, high strength to weight ratio and near net shaping. Magnesium matrix composites are fabricated usually through stirring casting, powder forming, injecting deposition, liquid metal infiltration or die casting at present time. However few investigations on magnesium matrix composite are conducted for the specific characteristics of magnesium alloy, such as high chemical activity and easy oxidation. The present paper is focused on Csf/Mg composites obtained via infiltration of porous short carbon fiber preform by liquid Magnesium. The complete experiment setup is designed and fabricated by ourselves, which include the forming molds, the unit for melting the magnesium, the unit for vacuuming and the monitoring and collecting system of forming process parameters. In this method the whole experiment setup is vacuumed firstly. Then the pressurized nitrogen is used to infiltrate the magnesium melt through a porous preform of short carbon fibers. After the infiltration completed, the punch of the press extrude the magnesium-infiltrated preform out of the forming die to form the tubes or bars. X-ray diffraction (XRD), optical and SEM microscopes were used to characterize the infiltration and the microstructure of fabricated composites. The compression test was used to characterize the mechanical properties of fabricated composites. The results show that the preform was infiltrated thoroughly by melt magnesium and the fabricated Csf/Mg composites have excellent mechanical properties compared with the magnesium alloys. Csf/Mg composites should be very promising candidates for automobile parts and portable electronic appliance parts in the future.

Abstract: Many process parameters are involved during the fabrication of Csf/Mg composites using extrusion directly following vacuum infiltration. The selection of suitable process parameters is important for the successful fabrication of composites. This will require a continuous monitoring and collecting system of process parameters. This paper describes how this is performed. The monitoring and collecting system is developed to monitor and control the forming process successfully. The hardware was built with data acquisition (DAQ) card based on PCI and various sensors for temperature, pressure, displacement. The industrial computer is used to process the data collected from the sensors. The data acquisition card is the bridge between the computer and sensors. In order to reduce the signal noise from sensors, the hardware filter circuit is designed. The data acquisition card can not work by connecting the computer and sensors through it simply. It must be operated through the self-developed software. The data colletcing software is developed in this paper. It can realize the parameter monitoring and collecting easily by setting up the hardware through the user friendly interface. The curves of parameters can be displayed on the computer screen and the data can be saved into the database for post-processing. The software also supplies the warning function. When the parameters (for example the temperature of mold) arrives the set value, the computer can sound a note of warning to tell the worker to operate the press. It is demonstrated that the main parameters, such as temperature of mold and liquid metal, the loaded pressure and the displacement of punch, can be monitored and collected in real-time by use of this system. This paper found the base for the further selection of optimized process parameters.

Abstract: On the basis of thermo-mechanical coupled FEM model which characterized the composites liquid-solid extrusion process, the constant-velocity and non-constant-velocity extrusion process were simulated by 3D thermo-mechanical FEM. Their influence on forming temperature, forming quality and deforming force was analyzed and compared. In the constant-velocity extrusion process, the results show the over-high and low forming temperature in earlier and terminal extrusion stage were responsible for the surface annular cracks and inner fibers breakage. In the new non-constant-velocity extrusion process, however, the range of forming temperature can be shortened in a reasonable range and the over-high and low forming temperature were thus eliminated. Therefore the forming quality and its consistency can be improved. In addition, the deforming force can also be reduced. The simulation and the mechanisms analysis coincide with the experiment results reported in previous literatures. The study provides the theoretical gist and a new feasible technical scheme to favor the consistency of the extruded product quality in composites liquid-solid extrusion process.

Abstract: The pressure infiltration process of porous preforms by molten metals was investigated numerically in this paper. The finite element model of heat and mass transfer of the infiltration in liquid infiltration extrusion process was founded by the introduction of a new continuum model of fluid in porous medium and a distribution resistance concept. The proposed model can describe the transient flow behavior of semisolid materials qualitatively. Numerical simulations were developed in particular for non-isothermal infiltrations which take into account the thermal aspects (the mould, the fibres and the metal are initially preheated at different temperatures). The temperature distribution, infiltration front and infiltration depth in the infiltration area were gained by the simulation of ANSYS／FLOTRAN code. It is shown that the fiber volume fraction and initial temperature have a strong effect on the infiltration process. The simulation results of axisymmetric infiltration have a good agreement with their experimental ones. In addition, the infiltration time was predicted to get the effective infiltration depth based on the simulation results.

Abstract: This paper describes a new integration algorithm for hyperbolic sine constitutive equation (HSCE) used in semi-solid forming. An intermediate variable Lambda ranged from zero to one is introduced to replace the inelastic strain rate in HSCE so that the inelastic strain rate can be solved indirectly from the Lambda. The proposed integration algorithm is based on the stress update concept and the effect of normal stress updating on the material compression is also discussed thoroughly in this paper. The investigation results show that the new algorithm can integrate the HSCE efficiently and the normal stress should keep constant as the deviatoric stress updating. An example of semi-solid extruding was given in the paper at last to illustrate the implementation of new algorithm and effect of normal stress updating on the compression of material.

Abstract: The numerical simulation of liquid-solid extruding is completed with ABAQUS code in this paper. The hydrostatic fluid element is used to simulate the deformation of liquid phase and the continuum element is used to the solidified layer. The combination of fluid element and continuum element avoids the mesh distortion of liquid phase during extruding. The dynamic transition from liquid to solid is implemented by sequential coupled thermal-mechanical analysis. One illustrated example of liquid-solid extruding is presented in this paper at last to show the whole modeling process of the dynamic transition from liquid to solid in detail.