Papers by Author: Jun Zhao

Abstract: Multi-axis ball end milling experiments of H13 die steel have been conducted with four different cutter postures, and the 3D topographies of machined surfaces are acquired by optical profiler. The multi-scale analysis is carried out on the machined surfaces based on 2D dual-tree complex wavelet transform (2D DT-CWT). The topographies are decomposed into different frequency bands, and the roughness in every frequency band is calculated. The results reveal that the roughnesses of the machined surfaces under different cutter postures are quite different, the roughness is the lowest when the lead angle is 12° and the tilt angle is-22°. The roughnesses under different cutter postures exhibit similar changing trend with the variation of frequency bands: the roughnesses in the high and low frequency bands are relatively lower, the roughnesses in the medium frequency bands are higher. The 2D DT-CWT provides an ideal comprehensive topography analysis method for performance evaluation of machined surfaces.

Abstract: A good dispersion of powders is crucial to obtain the sintered materials with superior properties. In this paper, the application of different dispersants in fabricating Ti (C,N)-based cermets was investigated. The effect of dispersant on the microstructure and mechanical properties of cermet materials was studied by observing the microstructure, indention cracks and fracture morphology using scanning electron microscope (SEM), and by measuring the transverse rupture strength (TRS), Vickers hardness (HV) and fracture toughness (K_IC). The results reveal that the dispersant oleic acid is helpful for the dispersion of Ti (C,N)-based cermet powders, and can contribute to the homogeneous microstructure and stable mechanical properties of the cermet materials. Compared with the cermets fabricated with polyethylene glycol (PEG), the microstructure of cermets fabricated with oleic acid shows more homogeneously distributed Ti (C,N) particles embedded into more evenly spread binder phases without agglomerations of Ti (C,N) grains. The cermets fabricated with oleic acid possess better comprehensive mechanical properties, with higher transverse rupture strength, hardness and a bit lower fracture toughness than the cermets fabricated with PEG.

Abstract: Optical profiler is employed to measure the surface topography of H-13 die steel machined by ball end cutter. By using 2D discret wavelet decomposition, the topographies of different frequency bands are obtained and the influence of cutter posture on varying frequncy bands topography is studied. The results show that cutter posture has a great influence on the whole frequency band roughness. The changing trend of roughness is roughly the same under different cutter postures which increase firstly then decrease with the decrease in frequency. A surface with a small high frequency roughness may have a large median frequency roughness.

Abstract: In order to acquire high machining quality and minimum machining time, cutting forces are usually modeled to understand the milling process, simulate or predict cutting forces, and optimize the machining parameters. In this paper, side milling tests were conducted on superalloy Inconel 718 with a solid carbide end mill, and the cutting forces vs. cutting time were measured. The average cutting forces were extracted from the measured instantaneous cutting forces under different feed rates of experiments, and the components of the shear forces and edge forces were determined by using the linear regression of the experimental data. The cutting force coefficients, including shear force coefficients and edge force coefficients, were identified. In addition, the algorithms of the mathematical model were implemented in Matlab. The predicted cutting forces were in good agreement with the experimentally measured forces, and the validation of the cutting force model was demonstrated.

Abstract: The tool-workpiece contact zone is an important issue in the ball end milling process. This paper investigated the effects of tool inclination angles on the tool-workpiece contact zone, and variations of the cutting section area and perimeter with the increasing tilt and lead angles were also analyzed by geometrical modeling and measurement method for ball end milling process. The appropriate tool inclination angles, which could avoid the extrusion and friction between tool tip and the uncut materials, shorten the loading time on the cutting flute, and decrease the maximum cutting forces, could be preferentially selected according to the distribution characteristics of the tool-workpiece contact zone and the variations of the cutting section area and perimeter corresponding to various tool postures.

Abstract: The chemical compositions and microstructures of several different cemented carbides were checked by XRD, XRF, SEM, metallographic microscope and stereomicroscope. The influences of phase compositions and microstructures on mechanical properties were investigated. The results show that improving the Co content and refining WC grain properly are effective means to acquire the excellent mechanical properties of cemented carbide. A series of friction and wear tests were carried out in a high-speed tribometer to explore the tribological behavior of three cemented carbides with different Co content and WC grain size against aluminum alloy 7050-T7451. It was found that the tribological behaviors of cemented carbides are largely dependent on their microstructures and mechanical properties. The enhancement of flexural strength and fracture toughness can be ascribed to deformation buffering effects of Co binder, but the improved hardness depends on the rigid skeleton of WC-WC grains.

Abstract: A Si₃N₄ ceramic tool material with high mechanical properties was fabricated by hot-pressing sintering process. The high speed machining of Inconel 718 tests were carried out with round ceramic inserts. The failure surface and microstructure were analyzed by scanning electron microscopy (SEM) to reveal the ceramic tool failure mechanisms. The results showed that the main failure mechanisms of the Si₃N₄ ceramic tool were flaking, micro-chipping, abrasive wear and adhesive wear in the turning process. On the other hand, chipping, flaking and adhesive wear were the main failure reasons in the milling process. Meanwhile, some small flaking along the cutting edge and step-shaped flaking on the rake face closed to the cutting edge were found on the failure surfaces, which was a typical self-sharpening failure characteristic of the ceramic tool in the high-speed cutting process. This tool failure evolution characteristic of the ceramic tool can be attributed to its higher flexural strength and fracture toughness, which was beneficial to improve the tool life and was constrained by cutting conditions.

Abstract: Cutting tool is the executor and core element of machining, and tool material is of decisive significance for the development of high-speed machining technology. For a particular workpiece material, to select the appropriate tool material is an important prerequisite to increase the machining productivity and to improve the machined surface quality.In this paper, a weighting method of subjective judgment was utilized to conduct criteria weighting in the tool materials selection when machining Al-Si piston alloy. The analytic hierarchy process (AHP) method was used to subjectively judge the significance of the material properties and determine their criteria weight. The hierarchical structure model, with the objective layer, the criterion layer, and the alternative layer, was established. The performance of tool materials was set as the objective layer. Seven property indexes, such as density, Young's modulus, hardness, fracture toughness, thermal stability, thermal conductivity, thermal expansion coefficient, were chosen as the criterion layer in comparative judgment of the alternatives and criteria. And five kinds of tool materials, YG6X Cemented carbide, YG8 Cemented carbide, PCD, CBN, and Ceramic were chosen as the alternative layer. Then the pairwise comparison matrix of the material property index was constructed to compare a set of criteria pairwise according to their relative importance weight. The comparative weights were derived by finding the largest eigenvector with respective of the matrix. Finally, the consistency index and consistency ratio were calculated to ensure the consistency of the subjective perception and the accuracy of comparative weights. Then the weights of the material properties were determined, and the synthesis scores of the different tool material candidates were obtained.Results revealed that PCD was found as best materials for Al-Si piston alloy machining, and YG8 is slightly more suitable than YG6X for machining Al-Si piston alloy. In short, the qualitative problems in tool material selection were realized systematically and hierarchically by quantitatively calculation and analysis. This provides a new thinking in tool materials selection for high efficiency machining of difficult-to-machine materials.

Abstract: Optical profiler is employed to acquire topography height data of ball-end milled die steel surface under different spindle speeds ranging from 2000rpm to 12000rpm with lead angle of 20° and tilt angle of-10°. By multi-scale wavelet analysis, measured height data are decomposed and then been reconstructed, meanwhile 3D topography and 3D roughness in different frequency bands are obtained. The results show that the changing trend of roughness with frequency band under different spindle speeds is not the same. In the high frequency bands, roughness has a tendency to increase with the increasing spindle speed. In the median frequency band, the roughness of the surface machined under low spindle speed 2000 rpm is the largest and the roughness of the surface machined under high spindle speed 12000 rpm is the lowest. In the low frequency bands, the roughness of the surface machined under low spindle speed 2000rpm is much larger than those obtained under other spindle speeds, and with the increasing spindle speed, the changing trend of roughness increases firstly then decreases.

Abstract: A systematic and parametric study of the effect of grain size and volume fraction of secondary phase on crack propagation behavior of Al₂O₃ based ceramic tool materials was carried out. Two-dimensional centroid V toughness oronoi tessellations were generated with random grain orientations. Cohesive Zone Method (CZM) was utilized to simulate crack propagation behavior. Zero-thickness cohesive elements were embedded on grain boundaries and inside grains. Crack initiation and propagation in ceramic tool materials microstructure were simulated without predefined crack. Simulation results revealed that crack initiated at the maximum stress position and propagated along the direction perpendicular to external load. Decreasing the grain size or increasing the volume fraction of secondary phase can improve the fracture stress of Al₂O₃ ceramic tool materials.