Papers by Author: Ji Wen Li

Abstract: The solidified microstructure and the modified treatment have been systematic studied for the low carbon high boron Fe-C-B steel. The cast solidified microstructures of the low carbon high boron steel are consisted of the matrix and the boride phases. The matrix phase is consisted of the ferrite, pearlite and a few martensite phases. The boride phases of the hypoeutectic steel are based mainly on the Fe2B phase. With the increasing of the boron content, the Fe2B phase is decreased. When the boron content is excess 2.6wt.%, the boride phases are changed from the single Fe2B phase to the compound structures. Meanwell, the morphologies of the boride phases are transformed from the long strip-shaped and the fish-bone reticular structure to the rosette type. The boride phases of the eutectic steel are mainly the M2B phase. However, for the hypereutectic steel, it are composed of the M23(C, B)6 and B0.7Fe3C0.3 phases. The suitable modifier for the hypoeutectic low carbon high boron steel is the compound modifier 0.2%RE+0.2%Ti. After modified, the reticular boride phases are disconnected. Moreover, there existed some of isolated massive borides. The appropriate modifier for the hypereutectic steel is the compound modifier 0.9%Nb+0.4%RE. After modified, the primary borides are fine refined and tend to round.

Abstract: In this study, beryllium bronze/steel composite plates were fabricated through explosive welding process using different ratios of explosive. Microstructures of the joint were examined, and then shearing strength, peeling strength, Bending tests and hardness measurements were carried out on the bonded specimens. Experimental studies show that, beryllium bronze and steel could be bonded with a good quality. The interface is wavy texture changing in turns from flat - wavelet - large wave - stable large wave beginning with initiation point. Grains near the interface are elongated parallel to the explosion direction. As the ratio of explosive increase, the amplitude and wavelength of wave are increased, which leads to the increasing of shearing strength and bonging strength. No shearing in the interface is seen from the tensile-shear tests and fracture take place in the low strength material. The bended specimens show that defects such as separation and tearing were not observed. Hardness is increased with increasing explosive ratio and the highest hardness values are obtained near the bonding interface.

Abstract: In recent years, high chromium cast irons have been widely applied in many fields because they have high hardness and abrasion resistance. However, high chromium cast irons are also expensive because much alloying elements, such as chromium, molybdenum and nickel, are added into them. In order to resolve above question, a new abrasion-resistant steel with high boron content was developed in this paper. The new high boron steel, with 0.6%~0.8%B and 0.65% C, was prepared using sand casting method. The microstructure and mechanical properties were researched. The results show that the solidification microstructure of as-cast high boron steel consists of boride (FeB) and matrix composed of pearlite, ferrite, and bainite. And the borides distributes along grain boundary in the form of network. After quenching at 980°C and tempering at 250°C, the FeB transforms to Fe₂B, and the matrix transforms lath martensite. The hardness of as-cast high boron steel is 43HRC, and its impact toughness is 5J/cm². After heat treatment, they increase to 56 HRC and 7J/cm², respectively, approximating that of high chromium cast irons. The new high boron cast steel have a potential in stead of high chromium cast irons

Abstract: To enhance the wear resistance of molybdenum alloys, Mo-based composites reinforced by Al2O3 ceramic particles were developed. Using Al(NO3)3 aqueous solution and MoO3 as raw materials, the Molybdenum powders mixed with Al2O3 particles were prepared by liquid-solid incorporation, drying, pyrolysis of Al(NO3)3 and deoxidation of MoO3 by H2. And then the Mo-based composites with 3-10vol.% Alumina were prepared by compaction and sintering at 1840°C. The morphology of the Molybdenum powder and microstructure of the composites were analyzed by SEM and XRD. The micro-hardness, density and wear property of composites were researched. The results showed that α-Al2O3 ceramic particles and Molybdenum matrix compose the composites. With the increase of Alumina content, the molybdenum powders become fine and rule, the grains of composites become fine, the microhardness of Molybdenum matrix increases, and the density first increases and then decreases. The friction coefficient of composite is scarcely affected by the alumina content. While the wear weight of the composite decrease with the increase of Alumina content. There are obvious plow furrow and abrasive dust on the worn surface, showing the abrasive wear characterization.

Abstract: In the present work, rapidly solidified Al-21Si-0.8Mg-1.5Cu-0.5Mn alloys strips was prepared by melt-spinning method. The microstructures, phase and morphology characteristics of the experimental alloy were characterized by means of scanning electron microscopy, transmission electric microscopy. The results show that the microstructures are changed obviously compared with conventional condition. The nucleation and growth of primary silicon are suppressed and primary silicon can not deposited, meanwhile, α-Al phase is nucleated which prior to eutectic. The microstructures of the rapidly solidified alloys are composed of primary micro-nanostructure α phase and feather-needles-like (α+Si) eutectic which set in the α phase. The mechanism of formation for microstructures of melt-spinning Al-Si alloy have also been discussed.

Abstract: A new wear resistance material named the hypoeutectic high carbon Fe-B cast steel with fine hard carbides dispersive distributed in the matrix have been investigated. The results show that the solidified structures of high carbon Fe-B steel consist of ferrite, pearlite and boride, and borides were distributed along grain boundary in interconnected network. After heat treatment, the metallic matrix changes into martensite and retained austenite. The eutectic borides are appeared to be less continuous network and isolated particles. The increasing of the quenching temperature leads to the improvement of hardness. Quenching at 980°C, impact toughness is increased with the increasing of the tempering temperature. The optimum heat treatment is quenching at 980°C(oil cooling) and tempering at 330°C(air cooling). The wear resistance of modified high carbon Fe-B cast steel is corresponding to Cr26 alloy. The impact wear mechanism is mainly plastic deformation and fatigue spalling.

Abstract: A rapid-solidified hypereutectic Al-21Si-0.8Mg-1.5Cu-0.5Mn alloys strips has been prepared by single roller melt-spinning. The effects of solution technology on the microstructures and properties of the strips have been investigated by SEM, TEM. The results showed that the nucleation and growth of primary silicon are suppressed and α-Al is nucleated which prior to eutectic. The Cu, Mg and Mn are all supersaturated solution in α-Al. The major part of Si solution in α-Al, and the rest are precipitated by the micro-nanocrystals eutectic silicon. The metastable microstructures of micro-nanocrystals hypoeutectic are formed. And in the solution process, Si are precipitated from the matrix and gradually aggregate up to form small particles distributing in the matrix with the heating time. At 480°C for 100 minutes followed by quenching in water and a natural aging for 96h, the finer and more roundness of silicon particle and the maximum Vickers hardness are formed.

Abstract: Brazing with active filler alloys containing some active elements, which promotes wetting of ceramics surfaces, is one of the most widely methods for joining ceramics to metals. The joints formed by brazing A12O3 to metal by using copper-titanium-nickel (Cu-Ti-Ni) as brazing filler were investigated. The metals/ceramics joints were produced at a vacuum level of 10-2-10-3 Pa at 1273K, using a constant holding time of 10 min. The surfaces were studied both morphologically and structurally using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS) and X-ray diffraction analysis (XRD). In the brazing process, copper and titanium in the braze alloy form a series of reaction products. The formation of Ti3Cu3O and Ti2Ni at the interface is characteristic of these joints. The estimated free energies of formation of the Ti3Cu3O and Ti2Ni are -119kcal/mol and -245.92 kJ ~-263.78kJ/mol at 1200~1288K respectively. The highly negative values for the free energies of formation suggest that these compounds are thermodynamically stable.

Abstract: In order to improve the wear resistant properties, WC ceramic particles were used to reinforce Hadfield steel. WCp/Hadfield steel composites were fabricated by optimized solid state sintering process of powder metallurgy. Interface structure, constituent phase and the forming mechanism of the composites were investigated systematically. The results show that the WCp/Hadfield steel composites have uniformly distributed particles and well bonded interface between WC particles and Hadfield steel. In the WCp/Hadfield steel composites, the interface between WC particles and Hadfield steel matrix is of shell shape, in which W, Fe and Mn elements diffuse between the two phases. The interface is of metallurgical bond, in which a new phase, namely Fe3W3C is formed. The micro-hardness of the interface layer is between those of WC and the steel matrix, which can provide a guarantee for the property transition between WC particles and Hadfield steel matrix. The diffusion reaction mechanism of the interface was also systematically studied.

Abstract: Rapidly solidified hypereutectic Al-21Si was prepared by the single roller melt-spinning technique. The microstructure morphology characteristics and phase structures of the alloy were characterized using SEM, TEM and XRD technique. The results showed that the grains were refined and the micro-nano composite structures were formed under rapid solidification. The microstructure of the Al-21Si alloy was composed of micro-nanostructured α-Al phase and feather-needle-like eutectic α-Al+β-Si phase. The α-Al phase was the leading phase in the eutectic α+Si phase. The nucleation and growth of primary silicon are suppressed and primary silicon could not be precipitated. The hypereutectic Al-21Si alloy showed the hypoeutectic solidification microstructure. Wear resistance was improved obviously when the rapidly solidified and was five times higher than that of the traditional casting alloys.