Papers by Author: Le Hua Qi

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: Liquid-solid microextrusion is one type of microplastic forming processes at elevated temperature and can be used in the forming of pins, screws, shafts, and gears in micro-scale. Microextrusion setup operated by use of ball screw was designed and fabricated by authors. Microshaft of diameter 1 mm was extruded in the liquid-solid state at different forming temperature from Al-Mg alloy ER5356 billet of 4 mm in diameter. Heating temperature in the furnace for billet were set 650, 700, 750, and 800 degree C which was corresponding to the forming temperature range from 475 to 631 degree C because of temperature drop during transfer from furnace to mold. Forming load ranged from 4kN to 8kN. Microstructural observation shows that the grain size was reduced greatly compared to the original billet material. Microindention hardness shows that the extruded pin was strengthened which caused by small grain size.

Abstract: Liquid-solid extrusion following vacuum pressure infiltration technique (LSEVI), which integrates melting, pouring, infiltration, and liquid-solid forming under high infiltration pressure, is a promising technique for the fabrication of metal matrix composite. LSEVI technology combines the advantages of both squeeze casting and gas pressure infiltration method. In this study, 2D carbon fiber reinforced AZ91D matrix composites (2D-C_f/Mg composites) were fabricated by LSEVI. Pyrolytic carbon (PyC) coating was deposited on surface of T700 carbon fiber by chemically vapour deposited (CVD) before fabrication. SEM observation indicated that the composites were well fabricated by LSEVI. The ultimate tensile strength of 2D-C_f/Mg composites fabricated by LSEVI was 390-410 MPa. Two kinds of failure behavior were found during tensile test: abrupt failure and progressive failure. The abrupt failure was characterized by a complete failure after the ultimate tensile strength (UTS) was reached. The progressive failure was a unique failure behavior with gradual damage after the UTS. In the case of progressive failure, the remaining strength after the UTS was 79% of the UTS. There was a remaining strength of 200 MPa under the strain of 0.1. Fracture surface morphology indicated that the remaining strength was attributed to the gradual breakage of the fiber bundles.

Abstract: Thermal accumulation in micro droplet deposition manufacturing (MDDM) has a significant influence on geometric profile and microstructure of the fabricated metal micro-parts. In this paper, thermal behavior of a new aluminum droplet on the deposit surface was investigated using one-dimensional heat transfer model. Then several thin-walled aluminum cubic pipes were fabricated by MDDM to verify the numerical analysis result. The result shows that the thermal accumulation would increase gradually with the increase of the deposit height. It associated with thermal input and output on the top surface of the deposit, which could be controlled or eliminated by optimizing processing parameters such as deposition frequency.

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: Droplet deposition and solidification is vital to dimensional accuracy and mechanical properties of components prepared by uniform droplet spray (UDS) forming. In this paper, a volume-of-fluid (VOF) based model was developed to study the deposition and solidification processes of a 1 mm Al-4.5%Cu droplet generated by drop-on-demand jetting. The effects of droplet falling velocity (0.6-0.8 m/s), initial temperature (933-973 K), and substrate temperature (300-473 K) were investigated. The results show that the final morphology of the deposited droplet is largely dependent on falling velocity and substrate temperature. The solidified droplet obtained from an UDS experiment validates the numerical simulation.

Abstract: A metal rapid prototyping technique, uniform droplet spraying, is utilized to fabricate 2024 Al alloy objects. Molten droplets are generated by drop-on-demand mode and deposited onto a zigzag moving substrate, which produces objects with different side surfaces. The tensile strength of the deposited specimen is 180.5 MPa, with elongation to fracture of ~8.2%. The fracture surface of deposited specimen presents two regions, namely, rough region and smooth region. The smooth region initially cracks under tension load, and the rough region presents many elongated dimples, indicating a ductile shear fracture. This unique tensile behavior could be attributed to weak metallurgical bonding between droplets and the special movement of substrate.

Abstract: Studying on the mechanism and device for high-melting-point metals droplets deposition technology are imperfect. In our research, a pneumatic driven droplet generator was developed to generate mono-sized copper droplets. Experiments were conducted to investigate the influence of spray pressure and pulse width on the size of droplets. Droplet was stably ejected while spray pressure was between 30kPa and 35kPa and pulse width was between 600μs and 900μs. The droplet size increased as the spray pressure increased. However the pulse width had no significant impact on the droplet size. At last a copper column was fabricated to certify the stability of the system and the reproducibility of the parameters.

Abstract: To control the length and morphology of carbon nanotubes(CNTs) is difficult in its producing process. Influencing factors of length and morphology of carbon nanotubes have been analyzed in the catalytic chemical vapor deposition (CCVD) process. According to the growth characteristic of carbon nanotubes, the diffusion model was developed. Over time, the diffusion potential decreases due to a reduction in the carbon atom concentration gradient. With the model, the shape of catalytic particle is analyzed. From theoretical derivation, it is demonstrated that with the temperature ranging from 1173K to1373K, the growth rate raises up sharply with the increasing of temperature. Growth rate of CNTs increases with increasing layers at the same temperature.