Papers by Keyword: Self-Reactive Spray Forming

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Authors: Hong Wei Liu, Long Zhang, Jian Jiang Wang, Xin Kang Du
Abstract: A new near-net-shape technology, namely, self-reactive spray forming, to prepare ceramic preforms with low cost was proposed by combining the self-propagating high-temperature synthesis (SHS) with the metal spray forming. The feasibility of the technology was illustrated. And TiC-TiB2 -based structural ceramic was prepared by the new technology. The microstructure of the self-reactive spray formed preforms was analyzed. It was shown that the self-reactive spray formed preforms are composed of four kinds of structure, which takes on the characteristics of rapid solidification. They are griseous continuous base phase TiC0.3N0.7, black columnar grain TiB2 with the size of 100nm-1μm, white by-product phase TiO2 distributing along the boundary of the base phase, and a few of black anomalous pores respectively.
1126
Authors: Jian Jiang Wang, Wen Bin Hu, Hong Wei Liu, Xin Kang Du
Abstract: With Ti-B4C-C as self-reactive spray forming system, the flying combustion process of the sprayed particles was studied by means of water-quenching experiments and numerical simulation. It was found that after the particles have been heated in the oxyacetylene flame for a short time, Ti in the particles melts first and then infiltrates B4C and C. The SHS reaction of the sprayed particles takes place subsequently. Then the liquid ceramic beads appear and crystallize into ceramic grains finally. By the ANSYS finite element analysis, it can be known that the SHS reaction of the sprayed particles starts after they have left the muzzle for about 9.5×10-4s and lasts about 1.45×10-3s before the ceramic beads solidify. The calculated optimal melting distance for the spray particles is about 116mm, which is consistent with the experimental results on the whole.
1686
Authors: Hong Wei Liu, Jian Jiang Wang, Xiao Feng Sun, Ji Qiu
Abstract: With graphite, 45 steel and copper as substrates respectively, Ti(C,N)-TiB2 composite ceramic preforms with micro/nanometric grains were prepared by self-reactive sprayed forming technology. The cooling rate of spray particles deposited on different substrates was calculated by finite element method. The influence of cooling rate on morphology of micro/nanometric grains of Ti(C,N)-TiB2 composite ceramic preforms was studied by means of SEM, XRD and EDS. The results showed that the average cooling rates of particles deposited on the three kinds of substrates were 7.0×107°C/s, 8.1×107°C/s and 10.7×107°C/s respectively. The extremely quick cooling rate was the essential reason why the spray formed preforms were composed of micro/nanometric grains. The TiC0.3N0.7 grains in preforms deposited on three kinds of substrates all took on anomalous equiaxed grains. Quicker the cooling rates of the deposited particles were, smaller the grains were. The grain size of them was all less than 3μm. Whereas the influence of cooling rate on the morphology of the TiB2 grains was great. When with graphite as substrate, TiB2 took on rod-like grains with big length to diameter ratio. When with 45 steel as substrate, it took on near equiaxed grains. And when with copper as substrate, it took on lamina grains with thickness of about 100nm due to the extremely quick cooling rate and the extremely large degree of supercooling. That’s because with the change of the cooling rates, the remaining time of the liquid phases is different, so as to the growing time of the grains along the habit plane is also very different.
348
Authors: Hong Wei Liu, Jian Jiang Wang, Xiao Feng Sun, Ji Qiu
Abstract: The temperature change process of the single sprayed composite powder during the self-reactive spray forming process for preparing the Ti (C,N)-TiB2 ceramic preforms was numerically simulated by means of finite element analysis. The results show that after the sprayed composite powder with grain size of 50μm has entered the flame field for 0.35ms, the surface temperature of it will reach the igniting temperature and the self-propagating high-temperature synthesis (abbr. SHS) reaction will take place. The heating rate of the particle in this period is about 2.82×106°C/s. After the SHS reaction has taken place, the heating rate becomes quicker because of the double function of the flame and the reactive heat release. When the temperature of the sprayed particle is higher than that of the flame, the heat exchange process will turn into heat absorption from heat release, which leads to the great drop of the heating rate (about 1.20×106°C/s). The composite powder completes the reaction in 0.88ms and reaches the highest temperature of 2920°C, which makes it become a ceramic droplet. After the reaction has finished, the droplet cools down quickly from exterior to interior, and the surface temperature of it descends to the theoretic eutectic melting point of the composite ceramics (2620°C) after 0.34ms. Then the droplet begins to solidify at some degree of supercooling and becomes ceramic particle. The numerically simulated results before, during and after the reaction match the water-quenching experiments of the sprayed particle with particle size of 50μm during the corresponding period. It indicates the heat process of the sprayed composite powder on the whole, which is composed of being heated, heat releasing, cooling and solidifying.
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