Papers by Author: Hai Rong Le

Abstract: Being examined by tensile tests at 820°C with initial strain rates of 2.5×10-4 s-1, 5.0×10-3 s-1and 1.0×10-1 s-1, the hot-rolled spay formed high speed steel (SF-HSS) had superplastic properties. With έ=2.5×10-4 s-1and 5.0×10-3 s-1, the σ-ε curves indicate that there has occurred dynamic recrystallization at the later stage of deformation.The tensile elongation decreases monotonously with strain rate increasing. The strain rate sensitivity m =dlogσ/dlog έ is about 0.23. In the sub-surface of fractured SF-HSS samples with έ=2.5×10-4 s-1,, most of holes on the subsurface are observed to distribute near the carbides and arranged along the tensile axis direction. The interface between coarse carbides and matrix is very weak sites where the holes are easy to nucleate and connect to cracks during deformation. In the necked region, fine carbide particles on the grain boundary have pinned the slip of dislocations and formed dislocation wall inside the grain. Dynamic recrystallization and some climb of dislocations has occurred. The superplastic deformation mechanism with έ=2.5×10-4 s-1 was dislocation creep mechanism controlled by dynamic recrystallization. During deformation, the role of some carbide in the materials was to retard the grain growth and keep SF-HSS having fine equiaxed grain size and remain stable.

Abstract: The parameters of atomizer were obtained from the experiment. Based on the obtained parameters, a mathematical model was proposed to simulate the growing profile of billet during spray forming. The model included some process parameters which relate to the shape profile such as nozzle data, eccentric distance, rotation speed, withdraw speed and so on. After being compared with the billet shape of experiment, we got good consistent results between the simulation and experiment, it was found that the results of the simulation is in good consistent with that of the experiment.

Abstract: It is commonly considered that the character of atomizing gas-flowing field would affect breaking method of molten metal, liquid droplet size and mass distribution. It is one of the important factors which determine the microstructure and metallurgical quality of products. In this paper, three types of atomizers were designed and Pitot tube was used to investigate the steady state gas flow field generated by a close-coupled spray deposition atomizer. Through the study of atomization gas velocity distribution by simulation and experiment, the construction and gas flow dynamical property were introduced in this paper.

Abstract: The CuNi15Sn8 alloy has quite potential applications in the field of the electrical appliance industry. However, this alloy is prone to generate the segregation of alloying elements at the grain boundaries because of the high content of Sn element with the low melting point by conventionally processing route. Although the CuNi15Sn8 alloy can be obtained by powder metallurgy or mechanical mixing techniques, it is hard to avoid the formation of oxidation in the alloy. As compared with these techniques, spray forming technique has obvious advantages in preparing the CuNi15Sn8 alloy. The spray formed CuNi15Sn8 alloy has indicated that the elements of Sn and Ni were very uniformly distributed in matrix with the fine equiaxed grains. Influence of aging treatment on the microstructure and properties of spray formed CuNi15Sn8 alloy was studied.

Abstract: Through the probability simulation method, the statistical model of mass metal liquid droplets during metal spray forming process was developed and the ejecting process of molten steel was studied. The distribution of metal liquid droplets, their different initial velocity and the original appear location during spray forming were obtained based on the above computation. After made statistic and analyzed on large number of metal liquid droplets, the forming and motion of liquid drop in whole space were defined in detail, which provided the precondition and reference for further study of liquid droplets deposition process on substrate.

Abstract: The simulation software of compressible gas flow process in spray forming equipment was developed under three-dimensional cylindrical coordinates. When the gas flow strands sprayed from some definite angles into the atomizer chamber, they converged at the definite height in axial line then expanded outside which showed the jet current characteristic. The substrate stagnated the divergence jet strands of gas and then separated the flow strand into two big recycle zones in whole space. One of the recycle flow zone directed downwards, the other recycle flow zone, directed upwards and reached the inlet area, which is the main reason caused the bonding of small liquid drop in the delivery hole.

Abstract: A structural steel, 35CrMoV steel, has been attempted firstly by explosive powder compaction followed by sintering (EPC-sintering). The nitrogen content of the steel was 0.15wt%, which was accordant with the definition of high nitrogen steel (HNS). The final density of the EPC-sintering steel was only about 6.9g/cm3, which indicated that the processing parameters must be modulated further. In the sample of this steel, some radial cracks were found around the center of the cross-section of the steel, resulting in no mechanical tests carrying out. Observing the majority of the rim region of the sample of this steel, the microstructures were very tight, suggesting that it was possible and successful to manufacture HNS through EPC-sintering. The characteristics of the EPC-sintering high nitrogen 35CrMoV steel were that the cementites in the pearlites were found to be extremely fine. There were many (Cr,Mo)23(C,N)6 carbonitrides precipitates in the matrix. Some precipitates were round and others were needle-like. Some were distributing orderly in matrix and crossing over the dislocations. The dislocation density in the EPC-sintering steel remained high.