Qi Lehua

Dominant Factors of Metal Jet Breakup in Micro Droplet Deposition Manufacturing Technique

Abstract For the solutions of random variations of metal jet breakup and difficulties in controlling and predicting the process parameters (e.g. jet length) in micro droplet deposition manufacturing technique, experimental methods combining with theoretical analyses have been developed. The jet formation, jet length and their dominant factors (oxygen concentration and disturbance frequency, etc.) are discussed. The statistical law of jet length is found that the probability density function (PDF) of jet length is a log-normal distribution. The results show that the formation and size accuracy of metal jet breakup are improved by adjusting the gas pressure and optimizing the disturbance frequency. Under this circumstance, the jet length and morphological deviation can be minimized, which provides a stable droplet stream for the subsequent manufacturing process.

An imaging technique using rotational polarization microscopy for the microstructure analysis of carbon/carbon composites.

A novel image analysis technique was proposed for microstructure investigation of carbon/carbon (C/C) composites. The rotational polarization microscopy was developed to meet the special imaging requirements. The samples of C/C composites were observed in reflection polarized light microscope, where the analyzer was rotated instead of the stage, and the polarizer was taken out. The bireflectance of like‐graphite negative uniaxial crystal was analyzed. It was the theoretic foundation of image collection and data processing. The analyzer was rotated through 36 × 10° intervals without any movement of the specimen. The polished cross‐section of C/C composites took micrographs at each analyzer orientation. All image data collected from the same field of view were processed by image registration and image fusion. The synthesized images were obtained by calculating the maximum and minimum gray values and their differences at each point of the million pixels at 18 orientations of the analyzer. They are unique and quite reliable to be applied to analyze the microstructure of C/C composites. Subsequently, image segmentation was performed, and the feature parameters of each component were calculated. Good agreement was found between the results from image analysis and experimental data. Microsc. Res. Tech., 2012.

3D Dynamic Simulation Analysis of Thermal-Mechanical Coupling during 7075 Aluminum Alloy Micro-droplet Deposition Manufacture

Abstract In aluminum alloy droplet deposition manufacture process, thermal warping, layering and hot cracking of formed 3D components are the most common defects, which have been found to be associated with the large temperature gradient and thermal stress concentration. To obtain insight into the common defects formation mechanism in metal micro-droplet deposition manufacture, a 3D transient finite element (FE) simulation model has been developed by the APDL(ANSYS) code and element birth-death technique. The distribution and variation of time-dependent temperature and thermal stress fields were predicted and the thermo-mechanical behaviors were analyzed in fabricating aluminum alloy 3D components. And then, a series of deposition experiments were conducted using 7075Al alloy droplets under the setting process parameters (same to the initial and boundary conditions). The results show that the experimental results basically agree with the simulation results of thermo-mechanical behaviors. The reliability and correctness of the simulation model were verified experimentally by the measured temperature field and the observation of thermal deformation and hot cracking of formed 3D component. The work provides a useful theoretical and experimental guide for optimizing metal droplets deposition manufacture.

Three-dimensional printing technology based on uniform metal droplet ejecting

As a novel type of three-dimensional (3D) printing technology, the metal micro-droplet deposition manufacturing technique was developed in the 1990s based on the principle of ink printing technology. In the process, uniform metal droplets are generated as the basic building blocks, and 3D structures are fabricated by sequentially depositing droplets on the substrate layers. Because of its advantages, including a wide range of material selection, free-form fabrication, and no need for expensive equipment, the technology has great potential applications in directly fabricating complex metal parts, electrical circuits, and the structure/function of integrating components. Because of the high melting temperature point, high viscosity, great surface tension, and good oxidability, metal droplet ejection is complex and completely different from the ejection of non-metallic material. In the present work, different modes and principles of droplet ejection are analyzed. Then, the key issues of the technology, such as development of a droplet generator and optimization of process parameters, are discussed based on state-of-the-art and research results of the authors team. Lastly, technology development trends are proposed.

Fabrication of PyC/SiC Compound Coating on Carbon Fiber Preform and Its Effect on the Properties of Cf/Al Composite

Abstract PyC/SiC compound coating was deposited on carbon fiber preform by chemical vapor deposition (CVD) process for improving the wettability and reducing the chemical reaction between molten aluminum alloy and carbon fibers. Cf/Al composites were fabricated through the extrusion directly following vacuum pressure infiltration process. Effects of coating deposition parameters on the quality of carbon fiber coating were studied, and the influences of the coating on the infiltration quality and the mechanical properties of composites were also investigated by means of the microstructural analysis and mechanical property tests. The results show that the deposition rate of the coating is mainly controlled by the deposition temperature, and the uniform thickness of PyC/SiC compound coating can be obtained by controlling the deposition temperature or deposition time. The coating on carbon fiber preform can improve the infiltration quality and mechanical properties greatly based on the suitable interface bonding strength and good wettability. The optimized PyC/SiC compound coating thicknesses are 0.068 µm (PyC coating) and 0.257 µm (SiC coating) for better improving the composite properties.

Simulation and Experiment Research of the Uniform Drolet Spray Process

It is difficult to adjust the process parameters of material increase manufacturing based on droplet spray and generally these experiments are hard to implement.For solving this problem,a flow modal of uniform droplet spray based on volume of fluid(VOF) two phase flow model is developed.The pattern of droplet stream,the pressure field and the velocity field in the droplet spay process are studied by using the numerical simulation method.The inner low of uniform droplets formation is revealed and the optimal frequency of uniform droplet is obtained.On the basis of the simulation results,a uniform droplet spray device is built and equipped with a corresponding high speed camera.The jet pattern,droplet spray process and jet velocity are studied experimentally. The results show that the jet velocity depends on the jet pressure,which presents a periodical change,and uniformity of droplet stream depends on disturbance frequency and amplitude.The simulation results agree well with the experimental results,which proves the feasibility of the flow modal.The researches find a useful method for simulating the jet flow with different parameters, and lay the theoretical foundation for the application of material increase manufacturing based on droplet spray.

The stress states of a workpiece in the process of liquid-metal forging

Abstract In this paper the authors analyze the stress states of a workpiece in the process of liquid-metal forging, on the basis of numerous experiments. Schematic diagrams of the stress state in every stage for two conditions (i.e., insufficient pressure and sufficient pressure) are given, which reveal that the stress state in a workpiece changes continuously in order that plastic deformation can continue. The results of the analysis have been examined by carrying out engineering calculations, which has indicated that the results are practical and correct for engineering purposes.

Eliminating defects of ZnAl alloy by squeeze casting or liquid extrusion

Abstract In the paper, the two effective methods to eliminate the casting defects of ZnAl alloy, the squeeze casting process and the liquid extrusion process, have been expounded. By using ZA27 alloy as the experimental objective, the microstructures and the properties and the formability by permanent mould casting, squeeze casting and liquid extrusion have been compared. The results show that both the two kinds of processes can not only eliminate the casting defects, but also raise the properties and improve the microstructure of the ZA27 alloy. The effect of the liquid extrusion process is especially good.

Effect of magnetic field on microstructure of carbon nanotube reinforced Mg matrix composites

CNTs have been used as reinforcements due to its excellent mechanical properties. However, the effect of magnetic field on the microstructure of Mg composites reinforced with nano-magnetic particles has not been well understood. For this reason, Mg matrix nanocomposites containing 0.5 wt.% CNTs coated with Fe3O4 was fabricated using stirring casting method by applying a unilateral magnetic field and the microstructure of the composites was investigated. The magnetic field is effective for the movement of CNTs coated with Fe3O4 in molten Magnesium alloy. More shrinkage cavities were found in the region closed to the magnetic source. Meanwhile, the magnetic response of CNTs coated with Fe3O4 in liquid magnesium alloy was validated.