Xiaoxiao Chen

Research on the machined surface integrity under combination of various inclination angles in multi-axis ball end milling:

It is well known that the multi-axis machining technology has been developed into a key technique applied in the manufacturing field. The machined surface integrity is one of the most important factors influencing the performance of the produced components. Hence, this research concentrated on the machined surface integrity induced by the multi-axis milling operation with different inclination angle combinations. In this research work, the cutting conditions of the conventional or up-milling process with tool orientations were divided into eight different types, and the machining characteristics corresponding to different cutting strategies were discussed. The varying conditions of surface topography, texture and other machining features induced in machining process which corresponds to different inclination angle combinations were analyzed by geometrical analysis, and the surface roughness and linear profile along specific directions on the machined surface were also investigated. There is a special corresponding relationship between the rotation angle and tool tilt and lead angles. Better surface roughness could be achieved when rotation angles are 0° (positive lead), 60° (combination of positive tilt and positive lead), 90° (positive tilt) and 330° (combination of negative tilt and positive lead). Then, the samples used for study of the metamorphic layer were produced by lapping and polishing process, and the macro hardness (HL) and micro-hardness (HV) of the top machined surface were investigated. According to the measured results of both Leeb hardness and micro-hardness, higher surface hardness could be obtained under rotation angles of 60° (combination of positive tilt and positive lead), 120° (combination of positive tilt and negative lead) and 210° (negative tilt and negative lead). Moreover, the variations of the micro-hardness along the depth direction of the samples were studied. Finally, discussions on the machined surface residual stresses in both feed and cross-feed direction were carried out. Compressive surface residual stress in both feed and cross-feed direction could be generated when rotation angles are 210° (combination of negative tilt angle with smaller value and negative lead angle with larger value) and 330° (combination of negative tilt angle with smaller value and positive lead angle with larger value). Generally, the evaluation indicators of surface integrity are not all very satisfying under one special cutting condition with a certain inclination angle combination, and the optimal cutting parameters should be selected based on the specific application requirements. Finally, the optimal tool orientations and related cutting parameters were recommended, and further studies of the related topics were also presented.

SURFACE INTEGRITY OF HIGH-SPEED FACE MILLED Ti-6Al-4V ALLOY WITH PCD TOOLS

This study is focused on the machined surface integrity of Ti-6Al-4V alloy using polycrystalline diamond (PCD) tools under wet milling condition. The surface integrity in terms of surface roughness, surface topography, microhardness, microstructure, and metallurgical alternations is investigated. The observations and conclusions are primarily focused on the effect of cutting speed (250–2,000 m/min) on the surface and subsurface of the machined Ti-6Al-4V. Experimental results show that machined surface integrity of Ti-6Al-4V alloy is sensitive to the variation of cutting speeds. Obvious machining (feed) marks can be found on the machined surfaces. Micro hardness examinations showed 5–20% hardening of the top machined surfaces than the bulk material. The analyses of microstructure and metallurgical alternations reveal that slight subsurface microstructure alteration such as plastic deformation on the subsurface and no phase transformation were observed. The evolution of crystallographic texture induced by the intense plastic deformation of the machined surface should be responsible for the modifications of the peak intensity radios in XRD patterns as well as higher peak broadening crystal structures.

Investigation on the chip formation concerning tool inclination angles in ball end milling of H13 die steel

Widespread application of ball end milling operation is an outstanding characteristic in the field of manufacturing of die and molds. Tool inclination angles, which could improve the cutting performance of ball end mill, are critical factor in the ball end milling process, and also chip formation is one of the most important phenomena in the machining process. Numerical simulation, geometric analysis, observation by optical microscope and scanning electron microscope, and energy-dispersive spectroscopy analysis were adopted to study the chip formation during ball end milling of H13 die steels involving tool inclination angles in this work. The theoretical uncut chip geometry and tool–work contact zone were analyzed by computer-aided modeling technology. Finite element modeling of chip formation process involving tool inclination angles was performed, and variations of the maximum chip temperature which could provide assistant understanding of practical chip formation were analyzed. This article also investigated the practical chip morphologies, chip color, and the cutting characteristics under different process conditions and tool inclination angles. The optical microscope and scanning electronic microscope were used to capture the micro-photos of the chips under different process parameters, and the chip color and chip morphologies were discussed together with the cutting characteristics with regard to various process parameters. Energy-dispersive spectroscopy analysis of the chip free surface and back surface was also carried out for some special tool inclination angles. Deep understanding of the chip formation in multi-axis milling process was enhanced, and the research work could provide support for the selection of process parameters to some extent.

Microstructure level modelling for properties prediction of WC–Co cemented carbides

Abstract A random distribution micromodel was developed in order to predict the properties of WC–Co cemented carbides. MATLAB and VC++ were hybrid programmed to extract the necessary information for modelling the microstructure from scanning electron microscopy images. By analysing the distribution regularity of the microstructural parameters, the randomness characterisation of the microstructure of WC–Co cemented carbides was accomplished using probability density function. The parameterised model based on ‘random method’ was developed, in which the random distribution characteristics of microstructural topology parameters, such as average grain diameter, major axis, minor axis, centroid and grain orientation, were considered, with Co volume fraction freely controlled. The representative volume element (RVE) size was determined using the moving window method, with average grain diameter distribution as the evaluation criterion. It was found that the RVE should contain ∼120 grains. The RVE was directly imported in the finite element software ABAQUS, and the finite element simulation of microindentation tests and uniaxial tensile tests were accomplished to calculate the hardness and elastic properties of cemented carbides. The predicted results are in good agreement with the experimental dates, which reveals that the microstructure level finite element model can effectively predict the properties of WC–Co cemented carbides.