Papers by Author: Jiu Hua Xu

Abstract: This paper mainly deals with the reconstruction and fractal analysis of wear topography of brazed polycrystalline cubic boron nitride (PcBN) grain in grinding process. On the basis of fractal analysis, the uncertainty in shape and complexity in wear mechanism are analyzed by means of the construction of micro fracture of PcBN grain after producing the brazed abrasive tool with PcBN grains and carrying out grinding test. The main results are summarized as follows: (1) Actual behavior of self-sharpening phenomenon due to micro fracture in the grinding process can be grasped and reconstructed using SEM and 3D video microscope. The reconstruction model can clearly express the complicated wear features and characteristics of the grain. (2) The fractal dimension can relate the performance of PcBN grain to its micro fracture, and then the self-sharpening phenomenon due to micro fracture can be evaluated quantitatively on the basis of fractal analysis.

Abstract: Following the picking characteristics of Tianmu Mountain and aiming at the technique problem and safety problem existing in the picking task of hickory nuts, new scheme of efficient picking technology adapted to the hilly area site condition is proposed and the study of miniature motive power picking device is accomplished.

Abstract: This article focuses on the drilling of carbon fiber reinforced plastics (CFRP) laminate with brazed diamond core drill under no cooling condition. The effect of grit size, wall thickness of core drill and feed speed on machining quality at the hole exit is studied. The experimental results indicate that the grit size has a significant influence on the diameter of the hole exit. The expanding factor of diameter at hole exit increases with the decrease of grit size. Both large wall thickness and high feed speed lead to serious drilling-induced defects of the hole exit. The drilling-induced defects at hole exit are transformed from broken-edge and burr to local tear.

Abstract: Alumina (Al2O3) bubble particles were added into the mixture of CBN abrasive grains, Cu-Sn-Ti alloy and graphite particles to prepare the composite blocks for porous CBN abrasive wheels. The specimens were sintered at the temperature of 920°C for the dwell time of 30 min. The bending strength of the composite blocks was measured by the three-point bending tests. The fracture surface of the blocks was characterized. The results show that, the content of alumina bubble particles does not take significant effect on the mechanical strength of the composite blocks. Even the lowest strength of the composite blocks, 98 MPa, is higher than that of the vitrified CBN abra-sive wheels. Cu-Sn-Ti alloy has bonded firmly alumina particles and CBN grains by means of the chemical reaction and corresponding products. Finally, the chip space was formed through the re-moval of the ceramic wall of the alumina bubble particles within the CBN abrasive wheel during dressing.

Abstract: A three dimension heat transfer model, using a commercial code Fluent software which was based on FEM, was developed to describe the grinding process of heat pipe grinding wheel in this paper. Temperature field of heat pipe grinding wheel was analyzed by this model to identify the effects of different conditions on the temperature of the work surface of grinding wheel including the heat flux of contact zone, cooling parameter and rotation speed of grinding wheel. The heat exchange performance of heat pipe grinding wheel also was studied based on the temperature of the work surface of grinding wheel. Model calculations showed that the high heat flux and unsatisfactory cooling capacity could cause a high temperature of the work surface of grinding wheel as well as the temperature of the work surface of grinding wheel was not affected by the rotation speed of grinding wheel. A significant amount of the input heat could be taken away from the heat pipe grinding wheel.

Abstract: Particle reinforced metal matrix composites (PMMC) possess many outstanding properties and are increasingly applied in automobile, aerospace, electronics and medical industries. However, PMMC is a typical difficult-to-machining material due to the rapid tool wear rate and excessive machining induced defects. Although large amount of investigations have been done on the conventional machining of PMMC, merely several researchers have dedicated themselves to the study of milling, especially high speed milling of this material. Within the milling studies, most researchers have selected the carbide coated or uncoated solid carbide tools whose tool life was not satisfactory for engineering application. The literatures review indicates that most researchers limited their study to sintering or casting SiCp/Al composites at the low or moderate cutting speed. Material produced by the in-situ reaction method or titanium matrix composites was seldom selected as the research object. The research content was limited to the effect of cutting parameters on the machined surface quality or cutting forces. It is suggested that high-speed milling with PCD tool should be conducted in order to improve the machined surface quality and material removal rate and decrease the machining cost. Tool life modeling, surface roughness prediction, cutting parameters optimization and high-speed milling data base and the expert system should be greatly noticed by the researchers.

Abstract: High-speed milling tests were performed on SiC_p/2009Al composites in the speed range of 600-1200m/min using PCD tools to investigate the cutting temperatures and the influence factors. The results showed that the cutting temperature could reach 580°C under the given cutting conditions. Graphitization took place on the PCD tools under the high cutting temperature coupled with the effects of abrasive wear of SiC particles and catalysis of copper in the 2009 aluminum matrix. Cutting parameters, tool materials, workpiece materials and tool wear condition had significant effect on the high speed milling temperature while tool geometries had the minor effect. Among these influence factors, cutting speed was the most significant factor. Reinforcement volume fraction was the less significant factor and followed by radial depth of cut, feed rate and tool materials.

Abstract: The dressing methods of monolayer diamond tool have recently been developed increasingly because a substantial improvement of the ground surface roughness could be achieved with the dressed monolayer diamond tools. In this paper, a new dressing method was proposed, namely chemical-mechanical dressing of the diamond grits. Dressing experiments were carried out on the monolayer brazed diamond grinding wheel. The grit-tip distances from the base of wheel substrate were measured before and after dressing. Grinding experiments were conducted on K9 optical glass after each dressing interval. The roughness parameters of the ground surfaces were measured. The outcome of this attempt appeared highly encouraging, and the dressing of monolayer brazed diamond grinding wheel is effective with the chemical-mechanical dressing.

Abstract: Nickel-based superalloy plays an extremely important role in gas turbine engines in aerospace industry. To understand the high and super high speed grinding mechanism of nickel-based superalloy, the single-grit grinding mechanism is needed to be investigated. Previous studies on single grit were all carried out by mean of cutting or scratching. In this work, a novel experimental set-up of single-grit grinding had been developed to meet the reality of the complex kinematic grinding process. Grooves and collected chips were investigated in grinding of a typical nickel-based alloy GH4169 with wheel speed up to 150 m/s. The groove integrity is improved by increasing the grinding speed even though the chip thickness is kept constant. The typical serrated chips were observed and the frequency of chip segmentation increases linearly with the increasing of the grinding speed. Under high and super high grinding, the thermal softening due to the high temperature in adiabatic shear zone contributes to accelerate the chip formation and leads to decrease the grinding force.

Abstract: Self-lubrication CBN abrasive composite blocks and corresponding grinding wheels were made through the sintering process of CBN grains, graphite particles and Cu-Sn-Ti alloy at 920° for 30 min. The mechanical strength of the composite blocks was measured by means of the three-point bending experiments. Scanning electron microscope (SEM), energy dispersive spectrometer (EDS) and X-ray diffraction (XRD) were employed to characterize the fracture morphology and the interfacial microstructure of the composite blocks. Dressing experiments were carried out and the graphite film on the CBN grain surface was observed. The results obtained show that the bending strength of the composite blocks with 5 wt.% graphite particles reached 116 MPa, which met the mechanical requirements of the working layer of the grinding wheels. Chemical joining has taken place at the interface of CBN/Cu-Sn-Ti and graphite/Cu-Sn-Ti during the sintering process. Graphite film has been formed and spread to the grain surface after dressing.