Papers by Author: Hong Wei Liu

Abstract: Metal fluid flow in weld pool would influence final quality of forming part in GMAW welding deposition-based rapid forming process. To numerical study fluid flow in weld pool, heat and force effects on weld pool surface must been made clear firstly. A three-dimensional numerical model has been built to study arc behavior in GMAW welding deposition-based rapid forming process. Solving the model, heat flux and pressure distributions on the cathode were derived. Calculated results show that heat flux from the arc to the cathode is related to arc temperature nearly above the cathode, and is not monotonous about radial distance within 2 mm distance away from arc axis. A maximum pressure with a value of 800 Pa happens at 1mm away from arc axis.

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 TiC_0.3N_0.7 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.

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.

Abstract: The multiphase coatings with major phases Al2O3-AlxCuy were prepared by the reactive flame spray technology. The melting and reactive behavior of the Al-CuO agglomerated particles and the forming process of the coatings were studied by means of water-quenching experiments and the methods of XRD and SEM. It was shown that during the spray distance between 60 and 150 millimeters, CuO in the agglomerated particles was heated to decompose into Cu2O, Cu and O2. During the flying course, a little of Al reacted with Cu2O and produced Al2O3 and Cu. The reduced Cu mutually dissolved with Al and formed liquid Al-Cu alloy. After the spray particles bumped into the substrate, Cu2O reacted with Al richly, then lots of Al2O3 was produced and Cu was reduced at the same time. These products were wallowed up by the liquid Al-Cu alloy soon. When the temperature of the system dropped quickly, the structure transforming process began. The intermetallics of Cu9Al4 and Al2Cu3 were deposited from the liquid Al-Cu alloy via complicated eutectic reaction and eutectoid reaction.

Abstract: Based on the Ti-B4C-sucrose(precursor of carbon) -5wt.% Al sprayed system, the self-reactive spray formed multi-phased structural ceramic preforms tests were done. And the organization, structure and performances of the products were studied. The results show that the products are composed of continuous matrix phase TiC0.3N0.7, dispersed micro/nanometric rodlike grains TiB2, some of Al2O3, pores and intermetallic phase Ti3Al which is hard to identify by SEM. Aluminum is benefit to improve organization, structure and performances of the products. The average density of the products is 97.7%, the Vickers hardness is 20.6GPa, and the flexural strength is 425MPa. There are crack deflection and grain bridging toughening mechanisms owing to the rodlike grains of TiB2 in the products, which makes their fracture toughness is as high as 7.3MPa•m1/2.

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.

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.

Abstract: Based on SHS reactive flame spray technology, Al2O3 multiphase ceramics coatings were produced. Chemical copper-plating technology was used to produce the Al-CuO powders of copper cladding, which became individual SHS reactive units respectively. The phenomena and principles of the SHS reactive flame spray of the Al-CuO powders of copper cladding forming Al2O3 multiphase ceramics coatings were investigated. It was found that copper cladding of Al-CuO powders were necessary for the SHS reaction during the spray process. The better the cladding was, the more sufficient the SHS reaction, and the higher the transformation rate was. The Cu and Al2O3 fine molten drops produced by the SHS reaction bumped the substrate and flatted to form layer-shaped Al2O3-based multiphase ceramics coatings. The binding strength between the coating and substrate was as high as 19.8MPa, and the micro-hardness of the coatings was Hv712. The overall properties of the coatings were better than those of the ceramics coatings sprayed by the traditional flame spray.